MLS’iD-1241A31 March 1967

SUPERSEDINGMIL-STD- 124130 September 1960

MILITARYSTANDARD

OPTICALTERMSANDDEFINITIONS

MIL-STD-1241A31 Mtwcb 1947

DEPARTMENT OF DEFENSE

WASHINGTON, D.C. 20301

Optical Terms and Definitions 31 ?tbBCH 1967

MILSTD-1241

1. This Military Standard is mandatory for use by all Departments and Agencies of theDepartment of Defense.

2. Recommended corrections, additions, or deletions should be addressed b Command-ing Officer, Frankford Arsenal, Philadelphia, Pa. 19137.

ii

.M!I.–VD-1 241A31 Murch 1967

FOREWORD

This Military standard has been arranged alph~betically according to the noun name ofthe term defined. Terms italicized in the text indicate that the terms are defined elsewherein the document.

iii

MIL-5TD-I 241A31 March 1967

1.

1.1

1.1.1

1.1.2

1.2

2.

3.

1.

2.

3.

4.

5.

6.

7.

8.

9.

CONTENTS

P*

SCOPE . . . . . . . .. ......................... .

General .. .. . ... .. ....... . ...... ....... .. .. . . . ............... ........ ........ .. ....... ...........................................................

Photographic optics ....... .. .. .... .... ................... .... ..... . ...... . ... .. ............... .. . ........................... .................

Physiolo~cal and ophthalmic optics ... . ... . .. . . .. . ...... ........ ........ .. .................................................

Application ........ ............................................ .............. .. .... ... ............. .... ....... ............ ..............................................

REFERENCED DOCTJMENTS .. . ...................... ... ....... .......................................................

OPTICAL TERMS AND DEFINITIONS ...... ..... ..... ........... ...... .... . ... ........................................-

FIGURES

1

1

1

1

1

2

9

Diagram illustrating spherical aberration ...... ........... ..... . .. ........... .............................................. 65

A typical component ................................................ ..... ...... ... ..._ _..-..~ ... . ............................................. 66

Images formed by a lens ...... ... ...... .. ....... ....................................................... .............................................. 67

Focal lengths, principal points, and principal planes ............ .............................................. 66

Illustration of nodal points ................................................... .. ..... .. ...... ....... .. .............................................. 69

Illustration of a pechan prism ...... ..... ............................. ............................ .............................................. 70

Measuring aperture ratio ... . . ................... .................... ..... ......................................................................... 71

Illustration of angle of reflection ................ . ...... ... . . .. ... .. . .............................................. 72

Illustration of angle of refraction . . . .. ... . . ........... ...... . ..... ..... .. . ..... ........ .. .... .. 73

MIL-STD-1241 A31 March 1967

1. SCOPE

L1 GENERAL. This standard establishesdefinitions for the words, terms, and expres-sions peculiar to the general field of optics,except as detailed in 1.1.1 and 1.1.2 below.

1.1.1 Photographic optics. The uniformdefinitions, nomenclature, classification ofdefects, methods of testing, and measure-ments pertaining to the field of photographiclenses are contained in MIIA3TD-160.

1.1.2 Phytilogical and ophthalmic optics.The terms and definitions related to the fieldsof physiological and ophthalmological opticsshould be derived from medical text books orWebster’s International Dictionary,

1.2 APPLICATION. It is anticipated thatthis standard be used primarily, but not ex-clusively, by those nondesigning personnelwho must be cognizant of and conversantwith optical terminology.

MlL–STI&l 241A31 Merdl 1*%

2. REFERENCED DOCUMENTS

(Not applicable)

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MIL–STD–1241A31 Morch 1967

3. OPTIC.4L TERMS AND DEFINITIONS

AAberration. Generally, any systematic

deviation from an ideA path of the imagc-forming ?wys passing through an optical sys-tILm, which causes the image to i.)e imperfect.Specific a~erratlons are spiwricaJ atkwrm-tion, coma, curvature of jieid, astigmat%8m,longitudinal chromatic aberration, lateralchromatic aberration., and distortion.

Aberration, chromatic. Image imperfec-tion caused by light of different wavelengthfollowing different paths through an opticaltqfstem due to dispersion in the optical ele-ment8 of the system.

Aberration, chromatic, lateral. A varia-tion in the size of image8 for light of differ-ent colors or wtzvelenghts produced by anoptical sy8t em. It is measured as the radialdisplacement of the image in the first color,from the image of the same point in the8econd color. A direction away from theaxial image point is taken as positive, and adirection towards the image point is nega-tive. In the case of tehxwopes, lateral chro-matic aberration is a difference in magnifi-cation for light of different colors, and ismeasured as the angulur Reparation in ap-parent jield between the images of the samepoint in two colors.

Aberration, c h r o m a t i c, longitudinal.The distance between the joci for light ofdifferent colors measured along the opticalaxia.

Aberration% least circle of. A synonymfor confusion, lead, circk of.

Aberration Bpherical. A symmetrical op-tical defect of lewe8 and spherical mtworain which light rags that come from a com-mon axial point, but strike the lens at differ-ent distances from the optical axis, do not.come to a common few. Spherical aberra-tion of an uncorrected or undercorr=tedlens is of the type illustrated in figure 1A.

overcorrection is possible, ana is illustrateciin figure lE. The radius of a zone is ordi-narily specified as ,the height, h. The dis-tance o-y 1s ca]ied the kdfTa/ Sphf?rica; cdwr-

?U(1OK, o-z the longitu.dina[ 8phefl”ca.! aber-ration.

Aberration, spherical, lateral. T h e d i s-tance o-y from the axis of a ray a-y whoseheights is h, as shown in figure 1A. Theaxis Y and Z have the paraxial point o astheir origin, and are positive in the direc-tions indicated by the arrows. The lateralaberration y, is negative with undercorrec-tion and positive with overcorrection.

Aberration, spherical, longitudinal. T h edistance o-z along the axis for a ra~ a-y ofheight h, as shown in figure lB. The longi-tudinal sperical aberration, z is negativewith undercorrection, and positive withovercorrection.

Aberration, spherical, speciileation of. Aterm denoting the magnitude of 8phericafaberration given by one of the following al-ternatives: (1) the Jm?eral 8pherictd aberra-tion is plotted or listed as a function of thebight, h; (2) the longitudtnul 8phericaJaberration is plotted or listed as a functionof the height, h, as shown in figure 1.

Aberration, spherochromqtic. The varia-tion in 8pheric(d aberration for light of dif-ferent wauekmghts or co10r8. Often shownas a graph of the spherical aberration forseveral specific wavekngthu, usually definedby the Fraunhofer lines of the solar spee-trum.

Abrasive, A material such as silica, sili-con carbide, emew, cerium oxide, or rougewhich is used in the optical industry forgrinding or polbhing optical elements.

Abaorptance. The ratio between the fhaabsorbed by a body and the incident flux.

MlL–STD-l 241A31 March 1967

This term, and its specifications, are appliedto radiant flux and to luminous flux,

Absorptance, internal. The ratio of thefZuz absorbed between the entrance andemergent surfo.ees of the medium, to the fluxthat has penetrated the entrance surface.The effects of interretlections between thetwo surfaces are not included. Internal ab-sorptance is numerically equal to unity,minus the intervud tnmunnittance.

Accommodations, limits of. The distancesof the nearest and farthest points which canbe focused clearly by the eyes of an observer.Usually varies from 4 to 6 inches to injinitySPP accommodation).

Achromat. A contraction of the term lew,achromatic.

Achromatic. Having the quality of beingfree of chromatic aberration.

Absorptance, spectral. The ab80rptance Achromatism. The absence of chromutic

evaluated at one or more wavelength. Spec- aberration,

tral absorptance is numerically the same forradian t and luminou8 f luz. Acuity, visual. The degree of sharpness of

vision.

Absorption, light. The conversion of lightmto other forms of energy upon traversing Acutance Edge sharpness and high edge

the continuous portion of a medium. This contrast.

conversion weakens the transmitted lightbeam. Energy reflectance R, tran8nLittance

Adaptation, dark. The ability of the hu-

T, and absorption A, obey the law of the con-man eye to adjust itself to low levels of illu-

servation of energy, R + T + A = 1.mination.

Absorption, selective. The act or procesgby which a substance “takes up” or “soaksup” all the color8 contained in a beam ofwhite light, except those colors which itreflects or transmits. Some substances aretransparent to light waves of certain fre-quencies, allowing them to be transmitted,while absorbing waves of other frequencies.

Some reflecting w-faces will absorb light ofcertain frequencies and reflect others. Thecolor of a transparent object is the color ittransmits, and the color of an opaque objectis the color it reflects.

Absorptivity y. The internal ab80rptanceper unit thickness of the medium. Numeri-cally, absorptivity is unity minus the trurz8-mi981vat y,

Accommodation. A function of the humaneve, whereby its total refracting power isvaried in order to clearly see objects at dif-ferent distances.

Adaptation, light. The ability of the hu-man ewe to adjust itself to a change in theintensity of light.

Adjuster, image, height of. A glass platewith plane surfs.ee8, which is tipped one wayor the other in the line of tight, in one of theinternal telescopes of a range$nder orhetghtfinder. It displaces the image upwardor downward so that its height in the fieldcan be made equal to that of the other tele-scope, In some rangefinders instead of aglass p!ate, the height adjuster tilts the op-tical bar about a front and rear horizontalaxis. In Navy usage, this item in a stereo-scopic rangefinder is called a “height ad-juster,” and in a coincidence range finder a“halving adjuster.”

Adjustment, interpupiliary. The adjust-ment of the distance between the e~epiece8of a binocdar instrument to correspond tothe distance between the pupils of the ob-server’8 eye8.

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MlL-STD-l 241A31 March 1967

Afocal. An optical 8@em whose object at the object by the ba8e kngth of a range-and image point are at infinity. $nder.

Aluminizing. The application of a film ofaluminum to a surface, usually by evapora-tion.

Analyzer. A polarizing element which canbe rotated about its axis to control theamount of transmission of incident planepokrized light, or to determine the plane ofpolarization of the incident light.

Anamorphic. A term used to denote dif-ferent magnification along mutually perpen-dicular radii. The term is also applied to anoptical uystmn that produces this condition.

Anastigxcmt. A W8 in which the astig-matic difference is zero for at least one off-exis zone in the inwge plane. In such a lensthe other abewationu are sufficiently wellcorrected for the intended use.

Angle The amount of rotation of a linearound the point of its intersection with an-other necessary to bring the line into coin-cidence with a second line.

Angle, critical. The angle of incidence ina denser medium, at an interface betweenthe denser and less dense medium, at whichall of the light is refracted along the inter-face, i.e., the angle of refraction is 90°.When the critical angle is exceeded, the lightis totally reflected back into the denser me-di um. The critical angle varies with the in-dices of refnwtwn of the two media with the

#relationship, sin 1== ~ where Ic is the criti-

cal angle; n’ the index of refraction of theless dense medium; n the index of refractionof the denser medium.

Angie, paraliactic. The angular di:?erencein direction to an object as seen from twopoints of observation, The angle ‘subtended

Angstrom. A unit of measurement of thewavekngth of light equal to 10-~ centimeter.

Angular. Composed of, or measured by,angle8.

Anieotropic. A term used to denote a sub-stance which exhibits different propertieswhen tested along axes in different direc-tions.

Annealing. The process of relieving un-wanted stresses by means of suitable healtreating.

Aperture. An opening or hole throughwhich light or matter may pass. In an opt~cal 8M8tem, it is equal ta the diameter of thelargest entering beam of light which cantravel completely through the system. Thismay or may not be equal to the aperture ofthe objective. See aperture, clear; aperture,relative; aperture, operating, f rent; aper-tire, operating rear.

Aperture, clear. Abbreviated CA. Theopening in the mount of an optical u@emor any component thereof, that limits theextent of the bundle of rayu incident on thespecific 8urface. It is usually circular andspecified by its diameter. Clear aperture issometimes referred to as “free aperture,”or “objective aperture.”

Aperture, effective. Equivalent to thediameter of the largest bundle of ray8 thatcan be imaged by the opti.d 8~8tem.

Aperture free. A term sometimes used asa synonym for apertwre, clear.

Aperture, numerical. Abbreviated NA.The sine of the half-angle of the widest bun-dle of ray8 capable of entering a lem, multi-plied by the index of refraction of themedium containing that bundle of rays.

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MlL-STD-l 241A31 hkwch 1967

Aperture, objective.Yynonym for aperture,

Aperture, operating,

Sometimes used ak aclear.

front. The limitingaperture at the front of the lens. Usuall~rspecified as the maximum diameter of theentrance cone at the front vertex. for thespecified field of view at inlinit~ focw?.

Aperture, operating, rear. The limitingopening at the rear of a kna or @em. Usual-1y quantitatively specified as the maximumdiameter of the emergent cone for the speci-Fied field of view, at infinity focw.

Aperture, relative. The diameter of theentrance pu~”l of a Len8 or optical~8temmeasured in terms of the e@mhrnt focalkgth of that lens or pystem. It is writtenas a fraction in which f the equivalent f wallength, is the numerator, and it is syxnbol-~‘Ledby f\ followed by a numerical value. Forsxample, f/2 signifies that the diameter ofthe entrance pupil is equal to +!! the equiva-hmt focal length. For an object at infinttv,the denominator of the relative aperture,and the second member N of aperture ratio,Ire identical, providing the bnuge is formedin air. Relative aperture is applicable fordetermining exposure time only when theobject is at tnjinity.

Apochromat. An q.darmtic&ns in whichihe 8econda~ apectrwm has been reduced orin which three colors have been brought to acommon foew, by the use of special glasses,

Apostilib. A unit of luminance equal to7..caruf&8 per square meter./(

Arc. A part of the circumference of a. !rcle,

.Aspheric. Nonspherical. Aapheric eurfaceame frequently, but not necessarily, surfaces:f revolution.

Aspherical. A term used to characterize adeparture from the spherical shape.

Astigmatism. Abbreviated ASTIG. Ar,aberatton which causes an off-axis point tobe imaged as a pair of lines at right anglestn each other. Each line is at a differentdistance from the image forming elementaloqg the chief ray of the image formingbundle of rays. The image forming elementthus has two foci, one radial and the othertangential to the optical azti. A sharp imageof a point cannot be obtained. A comrm-mise image po8ition must be eelected at apoint betwexm the two line imagee at whichthe image blur is smallest (circle of leadconf U8iOn). bnue6, h??u381/8teW, ?)Li?’TtW8, ormirror systems, or combinations of the two,whose curveE are not symmetrical about theaxis can produce “axial ~tigmatfsm.” “ofl.axis astigmatism” results naturally withcentered spherical surfaces. Axial astigma-tism, unless deliberately introduced, is to beconsidered a defect of workmanship, and notan aberration. It should be noted that indealing with lenses, astigmatism arises fromoblique refractwn, whereas, in the humaneye it ariaes from a departure of a surfacefrom a truly spherical form.

Aatigmatizer. A Wlindhal late that cmbe introduced into the line of sight of arangefinder to form line imafles of point-likesources.

A- crystalline. The axis of -ymme@in a c~utal structure.

Axis cylinder. The meridian perpndic-u]ar to that in which the cylindrical powerfunctions.

AxA optical. The line formed by the coin-ciding principal aze8 of a series of optialeiemente comprising an optid u~utem. It isthe line paasing through the centers of cur-vatures of the optical rurfacea. The opticalcenterline.

Axi% principal. A straight hne connect-ing the centers of curvature of the ref ratt-ing surfaces of a km. In a mechanical

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MIL–STD–1241 A31 Murch 1967

semw, a !ine joining the centers ~i a lens as object through the nods/ poin( of the cuc toit 1s placed in a mount. The principle axis the fovea, or point of sharpest retina] acuity.is the optical axis of a lens.

Azimuth, angle of. The angle measuredAxis, secondary. k. 1ine formed by the clockwise in a horizontal plane, usually from

ciuej rail of an oblique bundle of ra?ls. north (may be true north, l’-nortL, gridnorth, or magnetic north ).

Axis, visual. An imaginary line from the

MIL–STD-1241 A31 March 1967

B

13alsam, canada. .An adhesive used tocement optical elerncnts.

Barrel, lens. The mechanical structureholding a complete lens.

Base, virtual. The actual base or baseline of a rangefimier or heightfinder, multi-plied by the power or ?rmgrzification of theinstrument.

Baume. The designation of scale for hy-drometers which are used to measure thespecific gravity of a polishing suspension.

Beam. A shaft or column of light; abundle of ra~s. It may consist of parallel,converging, or diverging rays.

Beamsplitter. An optical device for divid-ing a 1ight beam into two separated beams.A simple beamsplitter is a plane parallelplate, with one surface coated with a die-lectric or metallic coating, which reflects aportion and transmits a portion of the inci-dent beam; i.e., part of the light is deviatedthrough an angle of 90° and part is un-changed in direction. A beamsplitter is oftenmade by coating the hypotenuse face of oneof two 45” —90” Przkns and cementing thehypotenuse faces together. The thicknessof the metallic beamsplitting interface willdetermine the proportions of the light re-flected and transmitted. However, in all me-

, tallic beamsplitters, an appreciable amountof light is lost by absorption in the metal.It may be necessary to match the reflectedand transmitted beam not only for brightn-ess, but for color. In these cases it will benecessary to use a material at the interfacewhich gives the same color of light, both bytrawmtision and reflection. Where colormatching at the surface or interface cannotbe accomplished, a correcting color filtermay be placed in one of the beams.

Beer, law of. The radiunt fk transmit-ted by a solution of absorbing solute in anonabsorbing solvent is an exponential func-tion of the product of the concentration andthickness of the solution,

BeveL The shape of the edge of a lens orprism. Bevels are used to prevent chipping,or to achieve mechanical fits.

Bevel, controlled. A bevel, also known as“true bevel,” whose width and angle are con-trolled to specified dimensions.

Bevel, cup. A bevel formed by grindingwith a cup-shaped tool.

Bevel, machine. A synonym for bevel,controlled.

Bevel, protective. The slight removal ofa sharp edge of glass, without a specificationof the diameter. Protective bevel is oftencalled “breaking the edge.”

Bevel, sagged. The shape of the edge ofa concave surface when the depth of theplane of the bevel to the vertez of the sur-face is controlled to a specified distance,

Binocular. Vision with both e~es. A termapplied to instruments consisting of twotelescopes, thereby using both eyes of theob8erver.

Birefringence. The characteristic of hav-ing two indices of refraction with differentvalues. Birefringence is the characteristicof certain media, which is dependent on theangle between the ray path within the me-dium and the optical azis (or axes) of themedium. See refraction double.

Blackbody. A body which absorbs all therudi.ant energ~ which strikes it; a perfectradiator and a perfect absorber. It is acontraction of the term “ideal blackbody”

8

and is often used synonymously for “idealradiator,” “full radiator,” or “completeradiator. ”

Blackbody, ideal. A synonym for bluck-bod~.

Blacking, optical. The light absorbingmaterial applied to the ground surfaces ofopticai elements. Such material should havean index of refraction as high as that of theunderlying glass and must be in direct con-tact with the glass,

Blank. A pressed-glass mold with theapproximate size and shape of the opticrdelement to be ground and polished.

Blank, flat. A piece of glass with a roughpiano surface on each side.

Blank, molded. A blank with basic sur-face curves attained by heating and form-ing a specific gram weight of raw glass,

Blank, molded, hot. A synonym forpressing.

Blank, rough. A formed piece of glass,neither side of which has been surfaced.

Blank, semifinished. A formed piece ofglass, one su~face of which has been groundand polished to finished curvatures.

Blisters. Elongated bubbles or 8eed$, ellip-tical in shape and longer than onequarterinch,

Block. A supporting member for holdingoptical parts during grinding and polishing.It also denotes the assemblage of opticalparts on the supporting member, i.e., a blockof lenses, or block of prisms.

Block, green. A porous ceramic materialwhich is ground to an optical form, and onwhich a polished plate of glass is sagged bythe application of heat. It is capable of trans.

MIL-STO-1241A31 March 1967

mitting a partial vacuum. The glass con-tacts the green bloc!{ surf ace by the appliedvacuum and gravitation, and the upperpolished surface of the glass is used as anoptical surface, Green block is generallyused for forming aspherw surfaces of me-dium precision.

Blocking. The process of attaching bl.unksto a block in a position for grinding orpoltihing.

Blocking, button. The process of form-ing a block by attaching the optical elementsto a plate by means of individual buttons ofpitch or other thermo-plastic material.

Blocking, contact. The process of form-ing a block by making optical contact witha number of optical element8 to a large op-tical element.

Blocking, mold, soft. Mocking by meansof a pitch ring, pitch button or an entirelypitched block. Pitch, containing a soft filler,is sometimes used between the optical partsto assure no lateral movement.

Blocking, ring. The process of forming ablock by attaching optical element8 to a plateby means of a ring of pitch or other thermo-plastic material.

Blocking, spot. Blocking by means of aspotted tool.

Blocking, transfer. A process for precise-ly controlling thickness and parallelism dur-ing the manufacture of plane parallel plates.A number of elements are cemented to ablocking tool and their upper surface8polished. A second blocking tool is thencemented to the upper polished surface andthe original blocking is released. Thus theoptical elements are transferred from thefirst to the second blocking too] without dig.turbing their relative positions. The sec-ond, or unfinished surface can now bepollshed.

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M!L”-sm-124 IA31 March 1967

Block, lens multiple. A block that willaccept more than one lew or blank.

Body. A piece of glass to which a h-s orprism is cemented, and the unit ground andpolished as a whole, in order to obtain asharp edge on the finished piece.

Boioxneter. An electrical instrument formeasuring radiunt energ~ by measuring thechanges in resistance of a blackened t.em-perature-eensitive device exposed to the ra-diations.

Boreaight. To adjust the line of sight ofthe sighting instrument of’ a weapon parallelto the axis of the bore. Also applied to theprocess of aligning other equipment, such asradar mounts, directors, etc. As a noun, theterm defines an optical instrument for check-ing alignment.

Brewster, law of. When the angle be-tween the refracted and reflected ray is 90”,maximum polarization occurs in both rays.The reflected ray has its maximum polariza-tion in a direction normal to the plane ofincidence,’ and the refracted ray has itsmaximum polarization in the plane of inci-dence.

Brightness. . Themeans of which antinguish differences

effect upon sensation byobserver is able to dis-in luminance.

Bring-in. A term used to denote the finalcorrection of a polished surface or angle tothe required precision.

Bubble. A gaseous incluslon in glass.

Bubble, open. A bubble at a polished8urface, which haa been opened by grindingor poltihtng.

Bump. An unintentional elevation in asurf ace.

Bundle, axial. A cone of TUIMthat eman-ates from an object point which is locatedon the optical ati of the iena 8y8te?n.

Burn. A surface defect caused by a polish-er running dry too long. It occurs with feltor plastic polishers, and may appear as areddish brown stain.

Burnishing. The process of turning athin edge of metal over the bevelled edge ofa lens to hold it in its ceU.

Button. A piece of glass with a highindez of refraction fused to the major blank.

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MIL–STD-1241A31 Morch 1%7

cCalipers. An instrument of the shear type

used for acurate thickness measurements.

Candela. A unit of luminous intensity de-fined such that the luminance of a btu.ckbodyradiutor at the temperature of solidificationof platinum is 60 candelas per square centi-meter.

Candle. A unit of luminous inte~”tg.

Candlepower. A unit of measure of theilluminating power of any light source. Thenumber of candles in the luminous Meneitvof a source of light. A luminous intensity ofone candle produces one lumen of lumtnouflaw per steradian of solid angle measuredfrom the source.

Case-hardened. A m.wjace heat-treutingprocess which produces a highly stressedsurface.

Cell. The mechanical structure holdingan element, component, or member.

Cement. An adhesive used to bond opticalelementu together, or to holding devices.There are three general types of cementused in the optical industry, blocking ce-ment, mounting cement, and optical cement.

Cement, blocking. An adhesive used tobond optical elements to bhcking tools. Itis usually a thermoplastic material such asresin, beeswax, pitch, or shellac.

Cement, mounting. An adhesive used tohold optiad elements in their mounts. Itmay be either a thermoplastic or chemical-hardening material.

Cement, opticaI. A perrmment and trans-parent adhesive, which is capable of with-standing extremes of temperature. Canad.uba.bam is a classic optical cement althoughit is being replaced by such modern syn-

11

khetics as the methacrylates, caprinates, andepoxies.

Cement., thermoplastic. An a d h es i v ewhose viscosity decreases as the tempera-ture is raised to a limit. Canadu bakam,resin, and pitch are common thermoplasticcements.

Cemen& thermoaetting. An “adhesivewhich permanently sets or hardena at a cer-tain high temperature. Methacrylate is acommon thermosetting cement.

Center, cutting. The point on a cuttingline which will become the geonwtriccd cen-te~ of the cut lea.

Center, geometrical. The physical centerof the lens; it is on the axis of the lens, half-way between the front and read vertex. Itis sometimes referred to as the “mechanicalcenter” of the lens to distinguish it from theoptioa.1 center.

Centering. The process of causing thegeometrical center of a lene or miwor tocoincide with ita opticalcentev or the mount-ing of a iirna or mirror such that ite opticalaxia is coincident with the optical or me-chanical axes of other portions of the sys-tem.

Center, mechanical. A term sometimesU81XIas a synonym for center, geometrical.

Center, optical. The point, generallywithin a &me, but sometimes exterior to ikat which the optical azie intereecte the opti-cal path of any ray directed at a nodal point.

Center, to. To mount a he on a chuckso that upon edging the lens, the optical ce%ter will coincide with the geometrical center,

Centex4 The tradelenses having standardcurves.

name referring to(noncorrected) base

MII.-STD-1 241A31 March 1967

Centimeter. A unit of metric measure-ments:

100 centimeters equal one meter.

10 millimeters equal one centimeter.

2.54 centimeters equal one inch.

Centration, errore of. L e n 8 e 8 w i t h~pherical surfaces are usually designed to beao constructed that the center of curvatureof al 1 the surfaces lie on a single straight‘1ne te~ed the axis of the lens, If aapheric

w.irfaces are used their individual axisshould correspond with the axis of the lens..Failure of compliance with these conditionsia termed errors of centration.

Cerium. A poliuhing material (ceriumJxide). It has faster polishing action than~ouge (ferrous oxide) and is cleaner tohandle.

Chamfer. Abbreviated CHAM. T h eprocess of removing sharp edges by @n.d-ing.

Characteristic% optical. T h e qualitlca-tiom! an Optical 8~8tem poaseeaea by reasonof its optical nature, such as fiehf of view,magnification, brightn4x38 of image, imageyudity, correction for aberratimu, etc.

Chart, Foucault. A test target contain-i iag groups of alternate black and white barsspaced at various internals, which is used to. i]easure the resolving power of telescope8and lenues. This chart is placed at a dis-tance from the oite of obeenwation such thatthe aagular separations between the centersof adjacent black bars in the various groups~have precalculated values. The moat closelyspaced group who8e bars as imaged by anoptical instrument can be resolved, deter-mines the resolving power of that instru-mmt.

Chipping. A term sometimes used as a:ynonym for cn”bbing.

Chromaticjty. The combination of thedominant wavelength and purity character-istics, but not the photometric magnitude oflight.

Chuck. A tube to which a lens is fastenedfor centering.

Circlq aiming. An optical instrumentfor measuring angleu in azimuth and site.

C1eared-ouL A term used to denote afinished circular edoe. A decentred hm8 isadjusted on a centering chuck, so that itsimage runs true, and the lens is then edged.If the edge is circular, it has “cleared out.”However, if the recentering is excessive andthe lens is edged down to its specified diam-eter, there can be a portion of the edgewhich never touches the grinding wheel.The finished lens will not have a circularoutline and is said to have not “cleared out.”

Clearing. A grinding and polishing oper-ation on one uurface of a blank to permit amore thorough inspection for quality.

Clips. Small metal pieces placed betweenthe disc and the major to achieve properfu3ing.

Coat, hard. A term applied to the pro-cess, or to the result of the process, of pro-ducing (usually) dielectric coatings that aremore durable under adveree condition8 thanthose produced from other processes.

Coating, antiretlection. A class of singleor multilayer coatings that are applied to a8urface or surfaces of a substrate for thepurpose of decreasing the re@tance of the8urface and increasing the tran+wnti8ion ofthe substrate over a specified wavelengthrange.

Coating, high-re5ecting. A broad class cfsingle or multilayer coatings that are ap-plied to a 8urface for the purpose of increas-ing its reflectance over a spec%?d range of

12

wavelength. Singlesilver are common;least two dielectrics

films of aluminumbut multilayers of

orat

are utilized when lowabsorption is imperative.

Coatings, protective. Films that are ap-plied to a coated or uncoated optical eurfaceprimarily for protecting this Awface frommechanical abrasion, or from chemical cor.rosion, or both. An important class of pro-tective coatings consists of evaporated thintilms of titanium dioxide, silicon monoxideor magnesium fluoride. For example, a thinlayer of sikon monoxide may be added toprotect an aluminized surface.

Coat, soft. A term designating the softcoating applied to coated optia b differenti-ate between the harder and more durablecoating known as hard coat. Certain evaporating coatings are not capable of forminga hard coat and are easily removed by clean-ing. Cryolite is a soft coat material.

Coefficient absorption. The internal a3-sorptanee of an intlnitzsimally thin layer ofmedium, divided by the thickness dx of thelayer. Thus defined, the absorption coefR-cient is the coefficient B (A) in Bouger’s law,that that

–B (A) X

F= Foe

where Fo is the flus, F, at a pointence of x = O.

Coincidence. Agreement as toIn a coincidence rwngefinder, the

of refer-

position.two half

tmages of a distant object are in “coinci-dence” when they are in exact alignment.

Collimati. To render parallel.

Collimation. The process of aligning theoptical azi8 of optical 8gSteWM to the refer-ence mechanical axes or uw-face8 of an in-strument; or the adjustment of two or moreoptical axes with respect to each other. Theprocess of making li@t raV8 parallel,

M

MlL-STD-l 241A31 March 1%7

Collimator. An optical device which ren-ders diverging or converging ravs parallel.It may be used to simulate a distant target,or to align the optical axe8 of instruments.

Colmascope. A pokcriacope for demon-strating 8tmtn existing in a piece of glass.

Color. The sensation produced by light~f different wavelength throughout thevisible spectrum. The color, shape, andnumber of Newton’8 ring8 present, whentwo optical eurf oce8 are placed together.This is a broad term which covers specifi-cations for the above. Color ie a synonymfor aberratioril chromutic.

Calorimeter. An optical instrument usedto compare the coZor of a sample with a syn-thesized stimulus. For example, in a three-color calorimeter, the synthesized stimulusis produced by mixtures of three colors oftied chrornm%ity, but variable luminance.

COUW An abematwn of a lerw whichcauses oblique pen.cila of light ray8 from anobject point to be imaged as a comet-shapedblur.

Compacting. The heat-treatiw processin which the refractive tndez of glass isstabilized near, or at ite maximum value, byhoiding the glass for different periods atsuitable degrees of heat below the range ofunnding temperature.

Comparator. An inspection instrument,usually a projection device, which presentsa composite of a reference contour and theimage of the actual contour for comparison.More commonly called an “optical compara-tor,” or “contour projector.”

Compensator. An optical ekrnent used tocorrect for mechanical or optical ,displace-ment.

MlL-STD-l 241A31 Mu+ 1967

Component. A subdivision of a member.It may consist of two or more parts ce-mented together, or with near and approxi-mately matching surfuces,

Compone@ metal. A precise metalpriem or plane parallel plate which is ce-mented to an optical element and remainswith it during a series of grinding or poltih-zng operations. The metal component con-tains threaded holes for attaching it invarious position to a base plate (see fig. 2).

Component, optical. One or more opticalelements in an optical8petem that singly, oras a group, serve a definite major purposein arriving at the total petiormance of thesystem.

Concave A term denoting a hollowcurved surface,

Concentric. The characteristic of havingthe same center. Circles differing in radius,but inscribed from a single center point.

Conffmion, least circle of. A circlewhose perimeter defines the area, for anypoint in the field of view, covered by thesmallest imuge (usually of a point sourceformed by the lew).

Conjugate image. Synonym for tmugedietance.

Conjugatq object. Synonym for objectdistance.

ConatanL abbe. A mathematical expres-sion for determining the correction forchromatic aberration of an optical 8y8tem.It is often called the “NU value” or “Veevalue” and is usually expressed as,

n13’1vor V=

refractivitynr —W ‘ ‘r dispersion ‘

where n~, np, and n? are the indice8 of re-fraction for light of Me wavelev.,, ‘.: 04 the

D line of sodium, and the F and C lines ofhydrogen respectively.

Contact, optical. A condition in whichtwo sufficiently clean and close fitting eur-faces adhere together without re)lectwn atthe interface. The optically contacted sur-face is practically as strong as the body ofthe glass.

Contact stereoscopic. T h e result ofbringing the target into the same apparentdistance plane as the central measuringmark of the reticle when using a stereo-scopic height)inder. In a stereoscopic tele-scopic instrument such as a stereoscopicrange$nder the object space is perceived asthredimensional with an exaggeratedstereoscopic erect. If each e~epiece is pro-vided with a reticle the observer’s e~ea fusethe pair of reticies to appear as one, locatedat some particular distance ,away in thethree-dimensional jiekl of view. A lateralshift of the image produced by one of theobjectives will cause the entire image field toappear to move toward or away from thespatial position of the reticle thereby bring-ing any particular target into the same dis-tance plane as the reticle. When this is thecase the reticle and the target are in stereo-scopic contact.

Convergence. The bending of light TUVStowards each other, as by a convex or pluslens.

Convergence angle of. The angk formedby the lines of eight of both eyes in focus-sing on any line, corner, surface, or part ofan object. It is also referred to as “conver-gent angle.”

Convex. A term denoting a surface likethe outside of a sphere or ball.

Coolant. A fluid used to reduceperature rise produced by frictioncauses.

the tem-or other

14

Cord. A large stria in glass.

Cornering. The process of removing theslight overlap that may be found on a blankor pressing.

Correct, to. To remove slight errors onan optical surface by hand or by a specialadjustment of a polishing machine. Handcorrection is often required in the making ofhigh precision spherical or aspheric surfacesas well as flats.

Correction. The process of compensatingfor, or adjusting the aberration, in the opti-cal design stage.

Correction, color. The reduction of longi-tudinal, iatera4, and secondary chromaticaberations. The color correction may bespecified in terms of the Fraunhofer linesin the solar spectrum, indicative of the wave-length of ra~s for which the correction hasbeen made; for example, C-F correction.

Countersink. The concave segment of asurface formed on a biank (major), on whichthe disc of higher refractive index glass willbe fused.

Crack% fire. Small clefts or fissurespenetrating the surface of the glass, usuallyin the shape of short-hooked crescents. Firecracks are caused by the sudden heating orchilling of the surface.

Cribbin~. The process of breaking the ex-cess glass from the required shape.

Crush. A term denoting a type ofscratch.

Crust. A stain in a glass surface.

Crystal, A naturai or artificial substance,such as fluorite, quartz, calcite, or lithiumfluoride, used for optical construction.

Crystal, birefringent. A synonym forcrpstal, refracting, doubly.

Crystal, refracting, doubly. A transpar-ent crystalline substance which is anieo-tropic with respect to the velocit~ of li~ht,

MlL-STD-l 241A31 March 1967

Current, dark. The flow of currentpassed bv a photoelectric device when noradiunt energ~ is incident thereon.

Curvature. The amount of departure froma flat surface, as applied to lenses. The re-ciprocal of the radius of curvature.

Curvature, center of. The center of thesphere of which the wrface of a lens ormirror forms a part.

Curvature, field. A synonym for theaberration known as field, ourvature of.

Curve, compensated. A curve computedto attain a desired vertex power consideringthe thickness factor.

Curve, centered. The 8urface curvaturedesigned to minimize the marginal errorfound in the periphery of a lens,

Curve, luminosity. A term so tailed bythe Committee on Calorimetry, Optical So-ciety of America to denote the plot ofluminow rel.a.live eficiencp against wave-length.

Curve, luminosity, absolute. A term SO

called by the Committee on Calorimetry,Optical Society of America to denote theplot of 8pectral luminous e@iency againstwavelength.

cutoff. Refers to fused mu.ltifocais,where the upper portion of the dtic is of thesame refractive index as the major. Mterfw?ing, this upper portion blends with themajor appearing as one pieee of glass. Inthe case of sharp cutoff fitiem, “the term re-fers to the zuavehmgth at which the tranu-mittarwe has fallen to 87 percent. It shouldbe noted that the abbreviation “cut” is usedextensively in some publications, such ascolor filter catalogs.

Cut runner. A term denoting a type of8cratc)L.

Cutting. The process of shaping a Zens toa given pattern, or of severing a piece ofglass along the line of scratch.

15

WIL–STG-I 241A31 March 1967

D

Deblocking. The process of removingop?~ca[ elenl~nts from a bloc~,

T)ecentering. The process of grinding oredging a lens so that the geometm’cal centerand optical center do not coincide.

Deep. A term used to denote a conditionin which a concave surface has too muchnegative power; i.e., its radius of curvatureiS too short. Cutting material from thecenter portion of the polisher corrects thiscondition.

Defects+ beauty. Imperfections on opti-cal surfaces that are not optically critical,tmd that do not appreciably impair per-~ormance.

Definitions. The degree of clarity of re-production of the object that depends upona combination of resolving power and con-trast.

Deflection. A small horizontal (traverse)angle by which a weapon is aimed slightly~way form the target to allow for factorssuch a8 wind or drift.

Density. A term used as a synonym forden8ity, optical.

Density, diffuse. Logarithm to the base10 of the reciproca] of difltue tranmnitttznce.

Density, luminous. Luminous energy per~mit volume.

l!3mdty, optical. Logarithm to the base10 of the reciprocal of tranmnittance.

Density, optical, internal. Logarithm tothe base ten of the reciprocal of the internaltranwnittan.ce (also called transmission fac -lor ).

Density, uniform. A term usied to denote~, layered lene or blank, one layer of which

is clear, and the opposite one of absorptive-type glass. The clear portion is surfaced tothe desired curvature, while the thickness cfthe tinted layer remains constant, which re-sults in a lens with equal shade in the centerand periphery.

Deviation, angle of. The angle throughwhich a ray of light is bent by refZectton orrefm.ction (see figs. 8 and 9).

Deviation, constant. That property ofcertain optical devices, e.g., a penta P+?%that preserves the angular relationship be-tween the entering and emerging rays pas-sing through the device, regardless of theorientation of the device in the plane of devi-ation.

Device, half+hade. A device used toproduce at least two fields of polarized lightin juxtaposition. The angle between theplanes of polarization of these fields is gen-erally small.

Dialyti. A type of compound l-ma inwhich the inner surfaces of the two elementsare ground to different curvatures to correctfor aberrations. The dissimilar faces can-not be cemented together.

Diaphragm. A fixed or adjustable aper-ture in an optical 8@em. Diaphragms areused to intercept scattered light, to limitfield angle8, or to limit image formingbundles of ray8.

Diaphragm, antirellection (or g 1a r e).A diaphr~m for eliminating internal r-flections and glare in the field of view of theinstrument.

Diaphragm, iris. A diaph?ogm whosecircular apertwc is smoothly and continu-ously adjustable, from its minimum to itsmaximum opening. Because it is composedof a number of overlapping leaves, the nameis derived from this iris-like appearance.

16

MlL-STD-l 241A31 March 1967

Dichroic. Exhibiting the (~uaiity of

dtchrowm.

Dichroism. As applied to anisotropic ma-terials, such as certain cr~stah, this termrefers to the selective absorption of light~ays vibrating in one particular plane rela-tive to the crystalline axes, but not those vi-brating in a plane at right angles thereto.As applied to isotrupic materials this termrefers to the aeiective reflectwn and trans-mission of light as a function of wavelengthregardless of its plane of vibration. Thecolor vf such materials, as seen by trans-mitted light, varies with the thickness ofmaterial examined. An alternative term forthis phenomenon might be polychromatism.

Difference, thickne~ edge. The maxi-mum difference in thickness near the edgein a recentered lerM element.

Diffraction. The process by means ofwhich the propagation of radiant waves orlight waves are modified as the wave inter-acts with an object or obstacle. Some of theraws are deviated from their path by diflrac-tiotz at the object whereas other rays remainundeviated by diffraction at the object. Asthe object becomes small in comparison withthe wavelength, the concepts of re)tectianand ?efraction become useless’ and cliffrac-tion plays the dominant role in determiningthe redistribution of the rays following in-cidence upon the object.

Diffusion. The scattering of ligth by re-flection or tranmnission. Diffuse reflectionresults when light strikes an irregular 6ur-face such as a frosted window or the surfaceof a frosted or coated light bulb. Whenlight is diffused, no definite tmuge is formed.

Dig. A short 8cratch whose width is sutTi-cient to be measured.

Diopter. Abbreviated DIOPT. A unit ofrefractive power of a lens or pm”sm. In a

lens or ldns s!~stem, it is nurneric.al~y equaito the reciprocal of tiw J’ocai length meas-ured in meters. For example, if a lens hasa focal length of 25 centimeters, i.e., l/~meter, its power is 4 diopters,

Diopter, prism. A unit of measure of thereerecting power of a ptim. One diopteris the power of a prism that deviates a ravof light by one centimeter at a distance ofone meter from the prism.

Dip. A hollow in an optical surface.

Divergence. The vertical angzd.m dis-parity between the images of a common ob-ject seen through the left and right systemsin a binocw&r instrument. It is defined asplus, when the right image is below the leftimage.

Disc. A piece of glass which eventuallybecomes the bifocal segment as it appearsprior to being fwsed to the major blank.

Disc, airy. When light from a pointsource passes through a circular or annularaperture, and is brought to a focus by meansof a le?w, the image so formed is not a point,but owing to diflracttin by the aperture, isa bright central disc of light surrounded bya series of progressively fainter rings. Thecentral disc is called the Airy Disc by someauthorities; others prefer to apply the termto the entire diffraction pattern.

Dispersion. The process by which raysof light of different wavelength are deviatedangular] y by different amounts as, for ex-ample, with prism-s and diffraction gratings.The term dispersion is also applied to otherphenomena which cause the index of refrac-tion and other optical properties of a med~um to vary with wavelength.

Displacement, object. The movement ofobjects seen through a refracting prism to-wards the apex of a prism.

17

MIL-STZ&1241A31 Morch 1967

Distance eye. The distance from thevertex of the last optical surface of thevisual optical system to the extt Pupii. Alsotermed “eye relief.”

Distance, ●ye, clear. The distance fromthe rear vertex of the egelerw to the frontvertex of the cornea of the eye in telescopicsystems. Conventionally, this distance isequal to the eye distance less Y4 inch.

Distance, focal, flange. Abbreviated FD.The distance measured from the locatingsurface of the lens mount, to the pAwipalfocus in the image space.

Distance, focal, front vertex. Abbrevi-ated FVD. The distance measured fromthe principol focus in the back space, to thevertez of the f rent nurface.

Distance, hyperfocal. The distance be-tween the rear focal point of a lew and theimage plane when the object is not at in-finity.

Distance, image. The axial distance be-tween the image and the 8econd principalpoint of a len8.

Distance, interpupillary. AbbreviatedIPD. The distance between the two evepupils, when the observer is viewing dis-tance objects.

Distance, object. The distance from theobject to the observer’s cornea, or to thejii”stprincipal point of the objective in anoptical system.

Distance, optical. The length of the pathcovered by a ray between two points in anwdium, multiplied by the value of the indexof refraction of that medium,

Distance, overall. The physical distancefrom the objecl point ,.’ the znwge puint,

Distance,’ reading, normal. A synonymfor vision, cfwtwtcf, dt.utance of.

Distance, working, image. Distance fromlast vertez of lend to the image.

Distance, working, objec~ Distance fromthe object to the front vertez of the optical8@em.

Distortion. Also called radial distortion.An aberration of hu s@enu? characterizedby the imaging of an extra-axial straightline as a curved line, without necessarilyaffecting the dqtnition. Unsymmetrical, orotherwise irregular distortions of the imugecan also be caused by imperfect centrationor irregularity of optical 8urface8. (seefig. 3).

Distortion, barrel. A form of dtitortion,radial.

Distortion, pincushion. A form of dis-tortion, radial.

Distortio% radial. A change in magnifi-cation from the center of the field to anyother point in the field, measured in a radialdirection to the center of the field. It is aninherent aberratwn of Iem 8y8tem8, but canbe eliminated or minimized by proper de-sign. “Barrel distortion” results when themagnification decreases with field angle;“pincushion distortion” results when themagnification increases with the field angle.Asymmetry of radial distortion can resultfrom manufacturing errors. The measure-ment of radial distortion in a tebxwope is

tan U’l–Mtinu expressed in percent, where

U is the true fieZd angle, U’ the apparent fiehfangle, and M the central mugnif~ng power.

The measurement in a l-mu is 1Y

—--# ex-

pressed in percent, where Y i: the &ctuaiheight of the image, and y is the idea;height.

T

MlL-STD–l 241A31 Morch 1967

Distortion, tangential. An Image defectresulting in the displacement of imagepoints perpendicular to a radius from thecenter of the field. It is usually caused bycrrcws of ccntration.

Divergence. Thr bending of rays awayfrom each other, as by a concave or minuslens, or by a convez mirror. In a l)tnocu.lurinstrument, divergence is the horizontal an-gu.la? disparity between the image8 of acommon object, as seen through the left andright systems. Divergence is defined as posi-tive when the right image is to the right ofthe left image.

Doublet. A compound lens consisting oftwo element8. If there is an air’$pace be-tween the elements it is called an “air-spaceddoublet .“ If the inner surfaces are cementedtogether, it is called a “~~mented doublet.”

Drilling. The proce~s of making a hole ina lens to permit its attachment to a mount-ing.

Drop. A term used as a synonym for sag.

Dropping. The process of causing ablank or disc to form a desired curvature byheating to a sufficiently high temperature.

Dummy. A lens formed to a desired curveand then used to form a polisher. It mayalso he used to denote a piece of glass in-cluded in a block to fill out the a-rea; h inalso referred to as a “surround,”

Dynameter, optical. A small low-powermie~oacope or magnifir with a micrometerscale, used in the precise measurement ofthe ezit pupil diameter, eve distance, ancmagnification of other optical instruments.

\ .,.

MlL–STD–l 241A31 March 1967

E

Edge. The fiat or angled surface, usuallyfine m-ound, which limits the refracting sur-faces of a /ens or prism.

Edge, broken. A chamfer or protectivebevel.

Edge, dubbed-off. A term used as a syn-onym for edge, rolled.

Edge, rolled. A rapid change of curva-ture near the edge in a len8 or pri8m; alsoreferred to as a “turned down edge.”

Edging. The finishing of the edge of anoptical element on a grinding wheel.

Effect, stereoscopic. The sense of reliefor solidity resulting when an object isviewed by both eye8. It is due to the factthat each eye views the object from aslightly different point of view.

Efficiency, luminou~ relative. The ratioof the radiant flux at wavelength Am to thatrequired at wavelength A for producingequally intense luminous sensations underspecified photometric conditions where Amis t+e wavelength for which the ratio as.

sumes its maximum value of unit. Relativeluminous efficiency is often designated bythe symbol VA or V(A).

Efficiency, luminous spectral. Luminouseflcienc~ of radiation evaluated as a func-tion of wavelength.

Efficiency, radiant. The quotient of theradiant fZuz emitted, divided by the powerconsumed.

Element. A single uncompounded lens;an optical part constructed of a single piece.

Element. optical. An optical part con-structed of a single piece of optical material;I]sua]!v sing]e lenses, p:-;S?7t8, or m2rror8.

Elevation. angle of. The angle betweenthe line of sight (imaginary line fromweapon to target), and the line of elevation(formed by axis of bore when weapon is infiring position).

Emergence. A term referring to thetrigonometric relation between the emergentray and the 8urface of the medium (seegrazing emergence and norrnul emergence).

Emergence, grazing. A condition inwhich an emergent ray makes an angk of90° to the nornwl of the emergent m.wface ofa medium.

Emergent% normal. A condition inwhich a ra~ emerges along the normal to theemergent surf ace of a medium.

Emery. A natural abrtive, preparedfrom corundum in grades ranging fromrough to fine, which is used in the coarseand fine gn”n.ding of glass.

Emissivity. The ratio of the radiantemittance of a source to the radiant emit-tance of a blackbody at the same tempera-ture.

Emittance, luminous. A term appliedto extended sources; the total luminous fluxemitted by a unit area.

Emittance, radiant. The radiant fiuzgiven off per unit area of a source.

Emittance, spectral. A term which usu-ally refers to radtint emittance as a functionof wavelength, Where spectral luminousemittance is intended, it should be so speci-fied.

Energy, luminous The radiunt energgtimes the luminous efliciencg of radiation,

20

Energy, radiant.magnetic wave8.

Erector. A term8y8teni, erecting.

Error, centering.tration, error8 of.

Error, marginal.

The energy of electro-

ueed as a synonym for

A 8ynonym for cen-

That dim%rtion in anophthalmic lea resulting from the refrac-tion of light nzw8 entering the periphery ofthe lens surface. This error increases inproportion to the power of the lens.

Error, oblique. The image errors aris-ing from adigmatium, cmuz, oblique epher-tcat aberration, lateral color, and dietortitm.

Enor, pyramida!. The geometric errorof the lack of parallelism among the edge8formed by the face8 of a -. If thesharp edgee have been removed by grhdino,the above definition refers to the line ofintersection of two faces extended, a8 con-stituting a prism edge.

Error, surface. The departures of anoptiazl mwface from its specified toleranceor figure.

Error, unit of. Abbreviated U.O.E. Aunit of measurement in a rangejinder cor-responding to 12 seconds of arc in the ap-parent field. It is the normally expectederror in range due to human and instru-mental limitations.

Etching. The marking of a 8urface by

MlL-STD-l 241A31 Morch 1967

acid, acid fumes, or a tool. A process exten-sive] y used in the mmmf acture of reticle8.

Eye. The organ of vision. Also a termused in the optical industry as follows: as-sume that a fine ground convez or flut mw-fo.ce is being polished ta make it moresteeply convex. The potishing then will pr-teed from the edges and work toward thecenter, which is not yet polished, but re-mains fine ground. This central area be-comes smaller and smaller as the polishingproceeds, and is called the “eye.” The pol-ishing continues until the eye just disap-peam. In this way the thickness is con-trolled, and by keeping the “eye” centered,the centering h also controlled.

Eyeguard. A shield of rubber, plastic, ormetal used to protect the eyeu of the ob-server from stray light and wind, and tomaintain the proper ege diutawe.

Eyelen~ The &w of an eyepiece whichis nearest to the obsemer’s eye. Varioustypes of lenses are used for this purpose.

Eyepiece. An optical 8V8tern used toform an enlarged virtual image of the imageformed by the objective, and to direct thelight into the We of the observer. Theoptical system of an eyepiece usually con-sists of two lenue8, an e~el.ew and a coUec-tive or fiekf iens, but may consist of onlyone lene or of more than two lenses. Erfle,Ramsden, Huygenian, Kellner, Plod, andBertele are various type8 of eyepieces.

EyeaMeld. A term used as a synonymfor eweguard,

,?A[L-STD-1 241A31 March 1967

F

Falling. A veritcal shift of tne tmag~from its true position. Rotating pnkm. 8v8-tems are employed in instruments used bythe observer to view the entire horizon with-out changing his position. These systemsare used in panoramic sights and in peri-8cope8. The. complete rotating prism sys-tem consists of a right angle prim whichscans the horizon and an image rotatingprism. In order to prevent the viewed im-age from falling when clifferent parts of thehorizon are viewed, the two prisms are so:.;?ared that when the right angle prism isi)otated through an angk 2X the second.>rism is rotated through an angle x,

Feathera Feathery flaws located insidethe body of glass.

Field, apparent. The size of the fieZd ofview in the image space of an optical instru-

,ent, as differentiated from the size of the. .dd of view in the object space. In the ab-sence of distortion, the following relation‘ ‘!Ids :.

tan a’ =M tanaT 7

,vhere a’ is the apparent field, M;Lmtion of the instrument, and ufield.

the m.agn~is the true

Field, curvatur. .1. An aberration of:!ction8 which causes the inmge of a plane tohe f ocusaed into. a curved surface, instead ofx plane.

Yield, linear. The actual width of thejiEld of vko at any distance.

I%id, split. The field of view as seen.vhe~ observing through some types of coin-r~dencc range?nzb-s. It is formed by thej uxtaposition of opposite halves of the. im -

17~s nr~du~pti k)y twn Ob-~”FC?Z’fIeS... . ‘Me hzlf -ima.qm firs separated by tl}e ?Ia!”.r:ng iine.

Field, true. ‘l’he size of the field of vitnoin the object space of an optical instrumentas distinguished from the size of the fieldof view in the image space (see a.pparen tfield), More specifically, it is the maximumcone or fan of rags subtended at the en-trance pupil that is transmitted by the in-strument to form the usable image.

Figure. The geometrical form of an opti-ca4 eurface, usually as defined on a drawing.

Figuring. The process of changing anoptical uwface to improve ita performance.In cases where the glass has areas in whichthe index of refraction varies, “figuring” thecorresponding areas of the surface will com-pensate for the errors in the index of refrac-tion.

Filter. Often referred to as “ray filter.”It is a device with the desired characteristicsof selective tramnnittan.ce and optical homo-geneity, used to modify the spectral com-position of radiunt fk.z. Usually specialglass, gelatin, or plastic optical parts withplane parallel 8urface8, which are placed inthe path of light through the optical 8@emof an instrument to selective y absorb cer-tain wavelengths of light, reduce glare, orreduce light intensity. Colored, ultraviolet,neutral density, and polarizing filters are mcommon use. Filters are provided as sepa-rate elements or as integral devices mountedso that they can be placed in or out of posi-tion, as desired.

Filter, polarizing. A jilter’ that polarizesthe light passing through it.

Filter, ray. A synon~ f[tr fiiter.

P’iriing. A grincting yocess using fine

Fixation, point of. An object on which

the observer’s epe is concentrated.

Flare. A nonimage forming light whichis transmitted through the lens to reach theimage. It is usually caused by reflectionsfrom the lens surfaces, lens barrel or reelect-ing surfaces within the barrel, shutter, orlens mount, Flare may be concentrated ordiffused.

Flat. A glass surface whose radius ofcurvature is infinite. A convex surface withtoo long a radius.

Flat, optical. A test plate having an op-ticall y flat surface.

Flaw. In a m.mdded Munk an inclusionof gas or dirt caused by folding of the glass.

Fluorescence. Luminescence that persistsfor less than about 10-S second after excita-tion.

Fluorite. The optical form of the crys-tal fluorspar, calcium fluoride. This mate-rial is utilized for its low optical dispersion,its low index of refraction, and its trans-parency to infrared and ultraviolet radia-tion.

Flux. A term ud aa a contraction foryadiant flux or Juminow jluz.

Flux luminous. The quantity that spec-ifies the capacity of the radiant flus to pro-duce the attribute of visual sensation knownas brightne88. Luminous flux is radiant fluxevaluated with respect to its lwninoua efl-cienc~ of radiution. Unless otherwise stated,luminous flux pertains to the standard pho-toptic observer.

Flux, radiant. The radiunt emrgy croaa-ing or striking a mwjace per unit time usu-ally measured in watts.

F-number. The ratio o? the equivalentfocal length of an ohjectwe to the diarc,?two?: its ~w.tra~?ce 2J:@i

,...

MlL–STO–l 241A31 March 1%7

Focus. Often used as a synonym forpoint, #ocaL The term is also used to de-scribe the process of adjusting the eyepieceor objective of a tele8cope, so that the imageis clearly seen by the observer. The term isalso used to denote the adjustment of thetens, plate, or film holder of a camera so thata sharp, distinct image is registered. Also,to move the entire microscope body tubewith respect to a specimen, to obtain thesharpest possible image.

Focu& fixed. A term used to denoti in-struments that are not provided with ameans of focussing.

FOCUA principal. A term used as a syn-onym for focus, principal, point of.

FoctI% principal, point of. The point towhich incident parallel raws of light con-verge, or from which they diverge whenthey have been acted upon by a k= or miT-ror. A lens has a single point of principalfocus on each side of the lens. A mirror hasbut one principal focus. A lens or mirrorhas an infinite number of zmafle points, realor virtual, one for each position of theobject.

Fog. A term used to denote the foggyappearance of an incompletely polished sw -face which scatters light, The individuallight scattering centers are too small andclose together to appear as discrete. Theterm is also used to indicate the accumula-tion of moisture on an optical surface.

Fold. A flaw caused by folding the plaa-tic surface of a bkm.k during the formingof the blank.

Foot-candle. A unit of illuminance equalto one lumen incident per square foot. Theiiluminance (former!y called illumirmti m )of a surface placed one foot from 8 li~ktSX:.WWhsving a [?iminoua fntensitv cf Q?%cantie.

MlL-STD–l 241A31 Morch 1967

Frequent:, ‘?’he number of crests of Fringe. .A})L,reviated FFiSG. An inter-uJu?;e,s that pass a fixed point ]n a g]ven unit ference band, such a~ A’ei(’ton’s rinp,of time, in light or other wave motion,

Fusing. The permanent uniting of tt;.oFrequency, spatial. The frequency of pieces of glass by means of a high tempera-

sine wave objects. The reciprocal of the dis- ture process.tance between maxima of the object. l’su-ally expressed in cycles per mm.

24

NJL-STD-I 241A31 March 1%%7

G

Gage angle. A term used to denote ac-curate glass or metal pnkms of preciselyknown angle8.

Gage, brw A measure for determiningthe accuracy of the curvature of a lap orlens surface.

Generating. A term used to denote arapid roughing process for quickly removingglass. It is accomplished by means of coarseemer~ and a regular, or diamond impreg-nated, lap.

Ghostj prism. A term used as a syn~nym for a ghost image formed by a prism.

Glq barium. A term commonly usedin reference to a type of glass one of theingredients of which is barium oxide, whichis added for the purpose of increasing theindez of refraction, while maintaining arelatively low dispersion.

Gla baryta. A type of glass contain-ing lead for increasing the indez of refrac-tion, together with barium which furtherincreases the index, while maintaining arelatively low dispersion.

Glq compensating. Also called “clearglass,” or “clear filter.” Where a Jilter i8used in converging or diverging light, achange of focus would occur upon removingthe filter. To avoid this, a clear glass plateof equivalent optical thickness, called a com-pensating glass, is substituted for the filter.

Gliu% crown. A type of optical gZuss ofthe alkali-lime-silica type. It usually haa anindez of refraction in the 1.5 to 1.6 rangeand an Abbe constant in the 64 to 57 range.Since the positive element of an achromaticlens is almost always made of crown glass,it is often referred to simply as the “crown,”as differentiated from the negative element,“the flint” (see glass, flint).

Glw field. A hand-held binocular tek?-scope, usually of the Galilean type.

G1~ flint. A type of optical gias towhich lead, or other elements are added toprod uce general] y a higher indez of ref rac-tion ( 1.6 to 1.9) and a low Abbe constant(29 to 51).

Glass optical. A glass which duringmanufacture is carefully controlled with re-spect to composition, melting, heat treat-ment, and other processing in order that itsoptical characteristics such as its index ofrefractwn, dtipersion, tranwnittance, spec-tral tranmnittunce, freedom from birefting-ence, permanence, etc., have the values re-quired for the optical application for whichit is to be used.

Glq raw. A term used to denote anysolid state of glass prior to its manuf act ureas an element.

Gl~ reading. A low powered magni-fier, generally of large diameter.

GIu test. A master optical surfacewhich is accurately made to a specified /ig-ure. It is used to measure the fi~re toler-ance of the surfaces of production elements.

Glazing. The process of inserting kaesinto a frame.

Grating, power, resolving, chromatic. Thechromatic resolving power determines theminimum wavelength difference for anyspectral order that can be distinguished asseparate. The chromatic resolving powerfor diffraction gratings is usually stated forcases in which parallel ra~s of light are inci-dent upon the grating and is numericallyequal to the number of lines or ruled spac-ings in the grating (see order, spectral, dif-fraction grating.)

25

Mfl-sTo-1241 AS1 March 1947

Grey. A term used to denote an incom- Grinding. A term used to denote thepletely polished surface showing a grayness process in the fabrication of an opticalarising from the remaina of fine grm.ding. ekm-ent which gives it a specific geometric

shape.

Haze. An aggravated form of fog in apolished surjace caused by light scattering.The defects causing haze are larger thanthose causing fog, but not large enough tobe seen by the unaided eye as separated.

Heightfinder. An instrument used to de-termine the height of altitude of aerial tar-gets by means of optical triangulation. Theinstruments used employ the stereoscopicprinciple.

Heigh~ segment. The vertical measure-ment of distance betwen the uppermostborderline of the bifocal segment and thelower edge of the lens.

MlL–STD–l 241A3t Moreh 1967

H

Height sharp. The distance from thebase to the vertex before edp”ng a ptim.

High. In piano work a high surface isconvez and touches a flat test glass at itscenter.

Ho]% dirt. A hole in an optical surfacefilled with dirt, such as polishing abrasive(see bubble, dig, pit, scratch).

Hole, sand. Rough spots on the polishedeurface produced during coarse grinding,which subsequent fine grinding did not re-move owing, to some exten$ to coarse grainsof grinding aand becoming mixed with finergrades.

27

Mk-5T0-l 241A31 Makh 1967

I

Illuminance. Luminou8 JIuz incident perunit area of a 8urface. Widely known as‘illumination. ”

Illuminated. A m.wface or object is saidto be illuminated whenever luminous fhtx isincident upon’ it.

Image. A representation of an objectproduced by means of light raV8, An image-torming optical element forms an image bycollecting a bundle of light rays divergingfrom an object point and transforming it!nto a bundle of rays which converge to-ward, or diverge from, another point, Ifthe rays converge to a point a real image ofthe object point is formed; if the rays di-wrge without intersecting each other theyappear to proceed from a virtual image.

Image, aspect of. A term denoting theorientation of the image, such as normal,canted, inverted or veverted.

Image brightness of. A term used todenote the apparent brightne8e of the imagewxm through an optical system. This bright-ness depends on the brightness of the object,the tran.emieeion, magnification, distortion,and diameter of the exit pupil of the instru-ment,

Image, double. A term used to denotethe doubling of an image caused by optical\mperfections in the lem u@em.

Image, erect. An inmge, either rest orvirtual, that has the same special orienta-tion as the object, The image obtained atthe retina with the aesietance of an opticaJay8tem is said to be erect when the orienta-tion of the image is the same as with theunaided ege.

alone. The geometrical image is to De dis-tinguished from the diffraction image, whichis determined from considerations of bothphysical and geometrical optics. With com-pletely corrected objectives, the geometricalimage of two points is again two points, butthe actual image or the diffraction imagemay or may not suggest the pre8ence of anobject comprised of two points or two tinyparticles.

Imag~ ghost. Spurious multiple tmagesof objects seen in optical instruments, causedby the reflectwne from optical mw.facea. BYcoating the optical surfaces with low reflec-tion films, the harmful effects of ghosts aregreatly reduced.

Image, real. See image.

Imag~ reflection. An image formed bya reflecting 8urf ace. An unwanted reflec-tion image “is more properly termed a gho8timage.

Image, reverted. An image, the right sideof which appears to be the left side, andvice versa,

Image, virtual. If a bundle of ragg hav-ing a given divergence has no real or physi-cal point of intersection of the rays, then thepoint from which the rays appear to pro-ceed is called the virtual image. The dis-tance of the virtual image is inversely pro-portional ta the divergence of the rays.Since there is no physical intersection ofrays there is no real image that can befocused on a screen. The image of any rea’object produced by a negative lenu or convexmirror is always virtual. The image pro-duced by a positive lens of an object locatedwithin its focal length is also virtual.

Image, geometrical. A term used to re- Imbalance, vertical. The difference infer to the location and shape of the image of base up or down prism power at correspond-? particle, as predicted by geometrical opticu ing points on the two h?maes of a pair.

20

Ineandeacence. The emission of lightthemal radiation in quantities sufficientrender the source of radiation visible.

Incidence. The act of falling upon,affecting, as light upon a 8arface.

byto

or

Incidence, angle of. The angk betweenthe normal to a reflecting or refracting eur-face and the incident rav (see figs. 8 and 9).

Incidence, grazing. A term used to de-note light incident at 900 to the normal.

Incidence, normal. A term used to de-note light incident at 900 to the 8urface.

Inclusion. A term used to denote thepresence, within the body of the glass, ofextraneous or foreign material (see bubble,8eed, 8triu).

Inde& absolute. A synonym for refrac-tion, index of.

Index, refractive. A term used as a syn-onym for refraction, index of.

Index, refractive, absolute. The re@c-tive index of a medium relative to that ofvacuum. F&fractive index and absolute re-fractive index are numerically identical.

Inde% relative. See refraction, index of.

Infinity. In the optical industry, a termused to denote a distance sufficiently great sothat light ra~s emitted from a body at thedistance are practically parallel. Infinity isindicated by the symbol m.

Infrared. The visible electromagnetic ra-diation beyond the red end of the visible~pectrum. The wavelength range from 768?nillimicrons to the region of 30 or 40 mi-crons. Heat is radiated in the infraredregio~

Insert, total. The lateral distance sepa-rating a vertical lme drawn through the

MlL-STD–l 241A31 March 1%7

geomet+aZ center of the distance portion ofa multifocal and a vertical line passingthrough the geometrical center of the 8e9-Tw?lt.

Inset. The horizontal distance from the90 degree meridian of a bifocal len8 to thegeometrical center of the 8egnaent.

Intensity, luminou~ The ratio of theluminom flux emitted by a source, or an ele-ment of the source in an infinitesimallysmall cone about the given direction, to thesolid angle of that cone. Usually stated asluminous flux emitted per solid angle.

Intendty, radiant. Fluz radiated per unitsolid angle about a specified direction.

Intensity, spherical, mean. The averagevalue of intensity of a source with respectto al] directions.

Interference. A term used to denote theadditive process, whereby the amplitudes oftwo or more overlapping wave8 are system-atically attenuated and reinforced. Theterm is applied also to the converse processin which a given wave is split into two ormore waves by, for example, reflection andrefractwn at bea?n-8plitter8.

lnterf erometer. An instrument employ-ing the interference of light waves for pur-poses of measurement, such as the accuracy ‘of optical &urface8 by means of Newton’8rmg8, the measurement of optical path8, andlinear and angzdar displacements.

Interferometer, Twyman-Green. A test-ing device in which the observer sees a con-tour map of the emergent wavefront interms of the wavelength of the light usedin the test.

inverted. Turned over; upsiae - down.Usually refers to the effect of a priem orlerw upon the image. Inversion m the effectof turning upside-down

MX-STD-1241A31 March 1967

Irradiance. Radunt flux incident per Item, fringe. A term used to denoteunit area. ien.aen or bi.imka of 8uch curvuturc, power,

or type, that they are not used in greatIrradiation. The product of irradiance quantity.

and time, i.e. rudiunt energu received perunit area.

MiL-STD-1241 A31 Morch 1967

J

Jump, image. The apparent displacementof an object due to an erroneous prismaticcondition in an optical qmtem.

Ill

MIL–STD-1241A31 Morch 1967

L

Lambert. A unit of luminance, equal to10’

candles per square meter,‘u

Laminated. A term used to denote thatthe product consists of different layers ofmaterial.

Lap. A tool with a known sw-jaa curva-ture which can be imparted to a piece ofglass when used with an abrasive.

Lapping. A term used to refer to thegrinding of a rough piano surface.

Layout. In the laboratory, this term re-fers to the process of positioning and mark-ing a blank or lens preparatory to surfacingor cutting and edging.

Layout. In manufacturing, this termrefers to a quantity of work keyed to atime factor.

Leach. A glass surface from which someof the glass constituents have been removedby chemical action.

Length, base. The distance perpendicularto the l$ne of sight between the centers ofthe two entrance pupi18 in a two pupilaystem.

Length, focal. In a Lens, focal length issynonymous with equivalent focal length.In a mirror or single refracting surface, itis the distance measured from the focalpoint to the mirror or surface. (See fig. 4).

Length, focal, back. Abbreviated BF.The distance measured from the vertez ofthe back surface of the lens to the rearfocal point.

Length. focal, calibrated. AbbreviatedCFL. An adjusted value of the ~y?!lvfil~n’/oflal lmgtii 0! ; 7.*v.s moi:r,ted j,n ?. cam~ra

or cone, so chosen that the extreme positiveand negative values of distortion, are equalin magnitude (over the designated imagearea),

Length, focal, equivalent. AbbreviatedEFL. The distance f ram a principal pointto its corresponding principal focal point,The focal length of the equivalent thin lens.The size of the image of an object is di-rectly proportional to the equivalent focalIen@h of the lem forming it.

Length, front, focal. Abbreviated FFL.The distance measured from the principalfocux located in the front space, to the fiT8tprincipal point.

Lens. A transparent optical element,usually made from optical ghm, having twoopposite polished major surfaces of whichat least one is convex or concave in shapeand usually spherical. The polished majorsurfaces are shaped so that they serve tochange the degree of convergence or diver-gence of the transmitted rays.

Lens, achromatic. A lens consisting oftwo or more elements, usually made ofcrown and flint glws, which has been cor-rected, so that light of at least two selectedwavelengths is focussed at a single axialpoint (see lens, compound).

Len% aplanatic. A lens which has beencorrected for spherical abewation and de-parture from the sine condition freedomfrom coma. It may also be corrected forcolor.

Lens axis of. A term used as a synonymfor azis, principal.

Len% bitoric. A lens, both surfaces cfwhich :ire ground and pdjqhw] in ii tf)ri:

-T‘indr; :al form‘Jr 6. 1

32

MIL–STD–1241 A31 March 1967

Lens, cartesian. A lem, one surface ofwhich is a cartesian oval. It produces anaplanatic condition.

Lens, collective. A lens of positive power

(a field lens, for example) used in an opti-cal system to refract the chief rays ofimage-forming bundles of rays, so that thesebundles will pass through subsequent opti-cal elements of the system. If the entirebundles do not pass through an opticalelement a loss of light ensues, known asvignetting, Sometimeslens is used incorrectlyof positive power.

Len% compound. Atwo or more separateother optical material.

the term collectiveto denote any lens

lem composed ofpieces of glass or

These componentpieces or eZenwnts may or may not becemented together. A common form ofcompound lens is a two element objective,one element being a converging lens ofcrown glass and the other a diverging lemof flint gla88. The combination of suitableglasses or other optical materials (plastics,minerals ) properly ground and polished re-duces aberrations normally present in asingle lens.

Len% concave. A term used as a syno-nym for lens, diverging.

Len% concavo-convex. A term used as asynonym for rneniscw.

Len% concentric. An optical component,usually a single ekrnent, in which the cen-ters of curvature of the 8urface coincide.Concentric lenses thus have a constantradial thickness in all zones.

Len% condensing. A lens or system oflenses of positive power used for condens-ing radian f energy from a source onto anobject.

Lens, converging. Also known as a “con-vergent lens,” ‘“positive lens,” “convexlens,” “collective lens,” and colloquially insome optical shops as a “crown” lens. Alens that adds convergence to an incidentbundle of rays. One surjace of a converg-ing lens may be convexedly spherical andthe other plane (plano+onvex), both mayhe corwez (double-qonvex, biconvex ) or onesurface may be convex and the other con-cave (converging meniscus).

Len% convex, A synonym for lens, con-verging.

Lena, convexo-concave. A synonym formeni8cus.

Lensj corrected. A lens so designed thatit is sufficiently free from one or more aber-ration. Such a lens, for example, may be asimple lens with an aspheric surface, or it

may be a compound lens consisting of sev-eral optical element8 and different glasses.

Len% cylindrical. A lens with a cylin-drical surface. Cylindrical lenses are usedin range firuiers to introduce astigmatism inorder that a point-like source may beimaged as a line of light. By combiningcylindrical and spherical surfaces an opticals~stem can be designed which gives a cer-tain magnificatwn in a given azimuth of theimage and a different magnification at rightangles in the same image plane. Such asystem is designated as being anurnorphic.

Len% dispersive. A synonym for 1en8,diverging.

Lens, divergent-meniscus. A Jew with oneconvex surface and one concave surface, thelatter having the greater curvature orpower. A negative menticus. Also calleda “diverging meniscus lens, ”

33

~~-~~~.m i ~<4! &31 Mo~h 1$:7

izns, divergm:~. Also known as ‘ ~ive:.gent lens, “ ‘“negative lens, ” ‘“coccave iens:”“dispersive iens.” A lene which causesparallel light rays to sp=a out. One zur-fme of a diverging lens may be concavely

spherical and the other plane ( piano-um-cave ), both may be concave (double con-cave 1 or one surface may be concave andthe other convcz (concave-convex, divergent-meniscus). The divergingthicker at the edge than at

Lens doubl~oncave. Aing both curfacea concave.

lens is alwaysthe center.

minus lens hav-

Len% doubl~convex. A PIW lens havingboth surf=es convez.

Lens field. A positive len.a used to col-lect the chief rays (field rays) of imugeforming bundles so that the entire bundlesor sufficient portions of them will passthrough the exit pupil of the instrument. Afield lens is usually located at or near the/ocal point of the objective Jew. The fieldlens increases the size of the field which canbe viewed with any given evelena diameter.

Len% finished. Any len.a having both sur-faces ground and poli8hed to specific diop-tric power or focu8.

Len% meridian of. Any line drawn on alens from edge to edge perpendicular to theoptical azi3m

Lens, minu% A dwe?ging lens. A lens[with negative focal length (focal pointwards object ).

LeW negative. A synonym for lem,verging.

Lena pancratic. See zoom lens.

to-

di-

Len% piano. A lens having no curvedsurface, or whose two curved 8urfmes neu-

34

Jralize eacn other:ref ratting pow 6?.

sc tna. it rJO.SSeSS.2-,:c

kw planoconcava A lev.? Wiiii oii. sw-1ace plane, the other conave.

LeW pianoconvex. A lens wi+~ on: SUr-face plane, the other comw

Len% PIUS. A converging (convex) WM.

Leu positive. See lens, converging.

Lens tapered. A lens whose cross sectiofishows a greater edge thickness on one sidethan on the other.

Len& teiephoto. An objective lerw 8y8temconsisting of a positive and a negative com-ponent separated from each other, havingsuch powers and separation that the backfocal length of the entire system is small incomparison with the equivalent focal length.Such lenses are used for. producing largeimages of distant objects without the neces-sity of a cumbersome length of the instru-ment.

Lew thick. A lens whose axial thicknessis so large that the principal pointi andthe optical center cannot be considered ascoinciding at a single point on the axis,

Len% thin. A lens whose axial thiclmesais sutliciently small that the principalpoints, the optid center, and the verticesof the two surf ace8 can be considered ascoinciding at the same axial point.

Len% tinted. A general term referring toabsorptive lenses (see ab80rptance, spectral,and 8e&ctive).

Len% zoom. An opticul 8y8tem which hascomponent8 that move in such a way as tochange the focat length while maintaininga fixed image position. Thus the image sizecan be varied while leaving the optical sw-te?n in a fixed position. Sometimes called apancratic len8.

Lever, optical. A term applied Lo thtmeans of amplifying small angmlar move-ments by reflecting a bcanl of light from. ai)$i)-?’o(”01 p/”.a//i.

Light, collimated. A light bundle inwhich the rays emanating from any singlepoint in the objec~ are paraliel to one all-~ther. Light from an infinitely distant realsource, or apparent source, suckI as a colli-mator reticle, is collimated light.

Light, parallel. A synonym for light,collimated.

Light, pencil of. A narrow bundle oflight ru~s, diverging from a point source orconverging toward an image point.

Light, polarized. A light beam whoseelectric vectors vibrate along the same di-rection, that is in a single plane containingthe line of propagation, is said to be “planepolarized” (often called linearly polarized ).If each electric vector can be broken intotwo perpendicular components that haveequal amplitudes and that differ in phaseby 14 wavelength, the light is said to be“circularly polarized.” Circular polariza-tion is obtained whenever the phase differ-ences between the two perpendicular com-ponents is any odd, integral number ofquarter wavelengths. If the electric vectorsare resolvable into two perpendicular com-ponents of unlike amplitudes and differingin phase by values other than 1, IA, J/2, ~/A,1, etc., wavelengths, the light beam is saidtO be “elliptically polarized.”

Light, quantity of. A term applied to theproduct of Luminous flux and times.

Light, velocity of. This term usually re-fers to the speed of monochromatic lightwaves, i.e. tO the phase velocity, The velW.ity of light, CO,in vacuum is 299, 792.5 kil-ometers per second, The phase velocity ina medium is cO/n where n is the index of

MlL-STD–l 241A31 MrJrch 1967

r~jract~o?~ at the wav~?engt}i of the lightwave.

Lighi, while. Radlatlun having a SpeCLrtiienergy distribution that produces the sameCO1O;”sensation to the average human eycas average noon sunlight.

Line, hair. A fine line having no apparentwidth on a polished surface,

Line, halving. The link which divides thetw70 half images in a coincidence range-finder. The two halves of the images pro-duced by the two objectives of the instru-ment must be brought to a point where theymatch or coincide above and below thehalving line.

Lines. A term used to denote wave8 whichextend continuously across the glass, so thatthe reflection from the surface appears as aline, or series of lines, extending either thefull width or a considerable distance acrossthe glass.

Lint. In a cemented 8urface the remainsof cloth or paper fibers used in cleaning.

Loupe. See magnifier.

Low. A term used to refer to an opticalsurface which touches the teat g&88 only atits edges,

Lumen. The unit of luminous flux, equalto the flux issuing from one-sixtieth of asquare centimeter of opening of a standardsource, and included in a solid angle of one8teradian.

Lumen-hour. A term used to denote theunit quantity of light equal to one bumen ofluminous flux flowing for one hour.

Lumen-second. A term used to denote theunit quantity of light equal to one lumenof iuminoua flux flowing for one second,

MlL-ST&l 241A31 March 1967

Lumerg. A term used to denote centi-meter gram second unit of luminous energ~,equal to 10-71unten-second.

Luminana. The ratio of the luminousintensit~ emitted in a given direction by aninfinitesimal area of the source, to the pr~jection of that area of the source upon theplane perpendicular to the given direction.Usually stated as luminous intensity perunit area; i.e. luminous flu.z per unit solidangte emitted per unit projected area.

Luminescence. The process whereby mat-ter emits radiation which for certain wave-length, or restricted regions of the 8pec-

h-um, is in excess ofthermal state of thesivtty of its ewrface.

that attributable to thematerial and the emui.The radiation is char-

acteristic of the particular luminescentmaterial.

Luminosity. A term so called by the Com-mittee on Calorimetry, Optical Society ofAmerica, to denote the ratio of luminoudflux to the radiant flux in a sample ofradiant flux, for example, lumen.a per wattof radiant energg.

Lux A term used to denote the lumen persquare meter; a unit of illumiruznce.

!f6

Machine, bowl-feed. A polishing machinein which the rouge slurry is contained in abowl and continuously diverted mechrmical-ly to flow over the work.

Machinq flat. A polii?hing machine con-structed to allow adjustment of the polisherspeed and movement for the control of tlatsurfaces.

Machine, stick. A polishing machine inwhich the lens ie mounted on a woodenstick, allowing a very wide sweep. It isused in the polishing of hemispherical orhyperhemispherical eurfacea.

Magnification. Magnification is best de-fined by the following conditions: (1) Lat-eral magnification is the ratio of the linearBize of the image to that of the object, asused in enlarging limes. (2) Angular mag-nification is the ratio of the apparent sizeof the image seen through an optical ele-nwnt or instrument to that of the objectviewed by the unaided eye, when both theobject and image are at infinit~ (telescopes),or when both the object and image are con-sidered to be at the dtitarwe of diutinctvision (microscope ). Angular magnifica-tion is often used as a synonym for power,magnifying.

Magnification, absolute. The magnifica-tion produced by a kna placed in front of anormal eye and at such a distance from theeye that either the rear focal point of thelens coincides with the center of rotation ofthe eye or else that the front focal point ofthe eye coincides with the 8ewnd @wipdpoint of the lens all under the conditionthat the object is located close to the frontfocal point of the lens. This magnificationis numerically equal to the dtitance of die-tinct vision divided by the equivalent focallength of the lens, with both distances ex-pressed in the same units of length.

MlL-STD-l 241A31 Morch 1967

M

Magnification, normal, individual. Whena magnifier is used by an individual havingmyopia, or hyperopia, the magnification isdifferent from the abaolute magnificationand is called “individual normal magnifica-tion,”

Magnifier. A lene or lene 8@em forminga magnified virtual image of an objectplaced near its front foca4 point. Magnifiersare also referred to as “loupes,” “simplemicroscopes,” or “magnifying glasses.” Themagnifications of magnifiers range fromapproximately, 3x to 20x.

Major. A term used to denote a bkmk towhich a piece of glass of a different imiezof refraction will be fued to make a multi-focal hm8.

Mark, chuck. A term used to denote themarks resulting when, in hand centering,the movement of the len8 over the face ofthe centering chuck abrades the 8urface.

Mark, generating. A term used to denotethe curved marks resulting when, in theprocess of generating, a loose or coarse dia-mond particle from the generating lapscores the work so deeply, that subsequentfine grinding or polishing will not coxn-pletely remove the scoring.

Mark, akuff. A term used to denote dam-age to polished eurface8 due to carelesshandling.

Mark stick. A term used to denote theresulting fine 8cratche8 when, in hand cen-tetino, the forked stick used to move thelens on the chuck marks the rotating lew8urf me.

Match. A term used to denote thetion of identity of visual appearance.

condi -

37

Ilateria!, dichroic. A substance lvhich ex-hibits dichroism in either or both of thesenses listed under that definition.

Material, isotropic. A substance which ex-hibits simila~ properties when tested alongaxis in an}’ direction.

Measure, lens. A mechanical device formeasuring surface curvature in terms ofdioptric power.

Microscope An op!ic?.! in.w-ument +9;producing enlarged images of smali, nearobjects. Microscopes are of two types“simple” and ‘“compound.” A simple micro-scope produces but one image nf an ob-ject, whereas a compound microscope firs’forms an image by the ob.?ecttvt levtA’,arlthis primary image is further magnified ro-an ewepiece.

Microscope, compound. See microscope.

Medium. Tny substance or space through Microscope, simple. See microscope,which light can travel.

Member. In a lem, the group of partsMillidioptera. A unit of metric measure

equal to 0.001 diopters. The power of aconsidered as an entity because of the prox- lens in millidiopters is the reciprocal of ita;mity of the parts, or because it has a dis-tinct, but not always entirely separate

focal kngth in kilometers.

function,Millimeter. A unit of metric measure-

Meniscus. A lens having 8urfaces, one ofment:

which is convex, the other concave. 1000 millimeters equal one meter,

10 millimeters equal one centimeter,Meniscu& concentric. A synonym for le?w,

concentric. 25.400 millimeters is one inch.

Meniscus converging. A term used to Milliznicron (mP). A unit of length in

denote a converging lens, with one convex the metric system equal to 0.001 micron. It

mrface and one concave. is also equivalent to 10 Ano8trOm8.

Meniacu% side, first. A term u8ed to re- Milling. An automatic generating process.

[w to the operation of grinding the concave The abrasion of glass by means of a dia-

surface of a single vision spherical h?n8. mend charged wheel.

Meniscus, side, second. A term used to Mirror. A smooth, highly polished surfacefor reflecting light. It may be plane or.<.*fer to the operation of grinding the con-

~?:G8Wf ace of a convexo-concave menimwa. curved. Usually a thin coating of silver oraluminum on glass constitutes the actual

Meter. A unit of metric measurement: reflecting surf ace. When this surface isapplied to the front face of the glass, the

1000 millimeters equal one meter, mirror is termed a “front surface mirror.”

100 centimeters equal to one meter,Mirror, mangin. Essentially, a negative

one meter is equal to 39.37 inches.—

m enisc~ lens whose second or convex sur-face is silvered, By carefully choosing the

Micron (P). A unit of length in the metric radii, spherical abewation can be corrected3ystem equal to 0.001 miUimeter. for any given position of the imuge.

38

Mirrol, paraboloium. A concal “ m@~”which has the form of a D3rab10id of m~~Iution, Sometimes the Daratdoidal mirrormay consist of oniy a portion of a para-‘bcdoida} surface through which the axisdoes not paw, and is known as ar “off-axis”para’boloida! mirror. A1! tid naral]eiiight rays are focused at the joed point oithe paraboloid withou~ spherical aberraticm,and conversely all light rays emanatingfrom an axiai source at the f oc.d point arereflected as a bundle of parallei rays with-out sDherica] aberration. Paraboloidal mir-rors are free from ehrom.uttc aberration.

Mirror, surface first. A tirm used assynonym for mirror, surface, front.

Mirror, surface, front. An optical mirroron which the reflecting mwf ace is applied tothe front surface of the mirror instead ofto the back, i.e. to the first surface ofincidence.

Mirror, triple. Three retiecting surfaces,mutual] y at right angles to each other, ar-rarmed like the inside corner of a cube.The triple mirror may be constructed ofsolid glass in which case the transmittingface is normal to the diagonal of the cube,or it may consist of the three plane mirrorssupported in a precisely constructed metalframework. The triple reflector haa a con-stant deviation of 180° for all angks ofincidence, hence a ray of light incident fromany angle is reflected back parallel to itself.Such systems are also known as “corner-cube reflectors,” or “retrodirective reflec-tors.”

WU-STD--124 D,F~31 March lWh’

Mod u Iation. A measure of the variat iorof i~~ml~nc~ across an image cf z ,gIr.

wave obiect. De fine2 asM = Imax-Imin

Imax +-lminWhere Imax is the maximum ilhminancein the im~e, and Imin is the minimumU ?tminance in the imoge

Modulation transfer function. The func-tion, usually a graph, describing the modt!-lation of the image of a sinusoidal objectas the frequemw increases. Also called sinewave response, and contrast transfer func-tion.

Monochromabc. Composed of one color.

Monocular. Pertaining to one eye.

Monogramming. The process of puttinga “breathe on” type trademark on a lens.

Mounting, eccentric. A t y p e o f 1e n 8

mounting consisting of eccentric ringa thatmay be rotated to shift the axis of the lensta a prescribed position.

Movexnen& diopter. A term applied to theadjustment of the eyepiece of an instrumentto provide accomm.dztion for eyesight vari-ations of individual observers. The axialdistance through which the eyepiece mustbe moved, to provide a convergence of therays emerging from the eyepiece of one(1) diopter, is calculated from the focallength of the eyepiece. If d is the requireddistance, it can be expressed: 1 diopter = d(in inches).

Multifocal. A lens which is characterizedby two or more foci.

$9

MlL-STD-l 241A31 March 1967

N, n. A symbol used to indicate index ofrefraction. It is usually used with a sub-script to indicate the wavelength of light,e.g., ND or n,, indicates the index of refrac-tion for sodium light of 5893 angstromwavelength.

Neutralization. The process of combining

N

of two lenses of equal and opposite powersto produce a resultant with no power.

Normal. Sometimes called the “perpen-dicular. ” An imaginary line forming rightangles with a swrface or other lines. It isused as a basis for determining angles ofincidence, reflection, and refraction.

-.. ..-.

40

MlL–STD-l 241A31 March 1%7

oObject. The figure viewed through or

imagined by an opticul system. It may con-sist of natural or artificial structures ortargets, or may be the real or vwtual ima9@of an object formed by another optical sys-tem. In the optical field, an object shouldbe thought of as an aggregation of points.

Object, sine wave. An object having asinusoidal variation of luminance. It has

the advantage that the image will have asinusoidal variation of illuminunce and theonly effect of degeneration by the he sys-tem will be to decrease the modulation inthe image relative to that in the object.

Objective. The optical component whichreceives light from the object and formsthe first or primary image, in telescope8and micro8cope8. In cameras, the imageformed by the objective is the final image.In telescopes and microscopes, when usedvisually, the image formed by the objec-tive is magnified by use of an eyepiece.

Objective, aperture of. The diameter ofthat part of the objective which is notcovered by the mounting (see aperture,clear).

Objective, Maksutov. An objective con-sisting of a spherical reflector, or reflectors,and a weak negative menticm havingstrongly curved spherical 8urf aces to cor-rect the sphen”cal abewatwn of the reflec-tor (s) without introducing significant /ong-tttiid chromatic abewatwn. The correc-tor len9 can also concomitantly correctcoma and reduce curvature of field.

Object, self-luminous and nonself-lumi.nous. Self-luminous and nonself-luminousobjects need to be distinguished since iw -a~o formation ctiII h~cor,e q~i:e difle:e:~~

depending upon whether or not the objectmust be regarded as nonse]f-luminous. Aseif-luminous object radiates sufficient lightJIUZ to render its image visible, whereasnonself-luminous objects do not. Image

formation with objects that scatter or dif-fuse the incident illumination markedly, isvery similar to image formation with self-luminous objects.

Occluder. A device which completely orpartially limits the amount of light reach-ing the ewe.

Offset, prism. The term applied to cer-tain telescopes having a characteristic off-set, due to the mounting of the ptim erect-ing system in the body of the instrument.

One-piece. A term referring to multi-focab consisting of one piece of glass, thesegment, or segments, having different our-vature than the main body of the lens.

Opaque. Impervious to light, i.e., haszero luminous trarwmittance. A substancewhich is impervious to light applied totransparent or translucent substances. Tomake impervious to light.

Ophthalmic. Pertaining to the humaneye.

Optical. Pertaining to vision and thephenomena of light.

Optical transfer function. The functiondescribing modulation and 8patial phaaeshift of the image of a sinusoidal objectwith fTequ.en.cy as the independent variabje,

Optics. That branch of physical sciencewhich is concerned with the nature andproperties of eletromagnet.ic radiatiou alttiwith the ~lheiionlena of vision.

41

MW-STD--I 241A31 March 1967

optics, coated. ~~~~cl! I1, i)l~)i~.~ ,]4 L) TTI-

;)inations thereof \vhose refracting or re-flecting sur.faces are coated with one ormore thin layers of dielectric or metallicsubstances. The term is commonly usedxvith reference to antircflect~on coatings.

The optical surfaces of optical elements arecoated with dielectric materials such asmagnesium fluoride, silicon monoxide, sili-con oxide, titanium oxide, or zinc sulfide,for the purposes of reducing or increasingwfkctiona and for protecting the surfaces.

Optics, crystal. The study of the propa-gation of radiant energ?j through crystals,

especially arn%otropic crystals, and theireffects on polarization.

Optic% fiber. A device for relaying animage by means of a large number of trans-parent fibers (filaments), hy multiple totalinternal reflection. The fibers are mostcommonly glass and less often a highlytransparent plastic. Each fiber carriesonly one element of the image, so that theimage is a mosaic in which the cell size isthe fiber cross-section rather than a con-tinuous picture. The image may be trans-formed in almost any desired manner to arandomly scrambled picture, to producemagnification, to produce the familiar op-tical distortions or conversely to correctthem.

Optic% fiber, numerical aperture of. Theproduct of the refractive index NO of themedium in contact with the end of the fiberand sin o., where o,, is haIf of the angle

of the cone of light that is accepted and

:r:. nsmittecl b:.- jn:ernaj VC$rc:inn,~%,.vit~;rthe fiber, For libers \vhose end-face%are perpendicular to the axis of the fiber:Numerical aperture = N,, sin flu =

\ N,’ – X,- where Y, and N, denotes re-spectively the refractive indices of the fiberand the medium surrounding the reflecting\valls of the fiber.

Optics, geometrical. The branch of sci-ence which treats light propagation interms of TCWS,considered as straight linesin homogeneous medta.

Optic& physicaL That branch of sciencewhich treats light as a wave phenomenonwherein light propagation is studied bymeans of wave-fronts rather than rays asin geometrical optics.

Order, spectral, diffraction gratingsWhen, for example, a beam of parallel ra~sof monochromatic 1ight pass through adiffraction grating, the emergent rays thathave remained undeviated belong to thezero spectral order. The light flwr in thefamily of deviated rays that emerge afterdiffraction at the grating exhibit pro-nounced maxima along well defined andenumerable directions, on each side of theundeviated beam. The integers that dis-tinguish these directions mark the so calledspectral orders.

Orthoscopic. Corrected for distortion.

Overcoat. A term used to denote a layerof material applied to a coated surface toprotect it from physical or chemical action.

42

Paraboloid, off.axis. A paraboioidal re-flector whose center is displaced from thepole of the paraboloid. Usually the pole ofthe paraboloid lies entirely outside the re-flector.

Path, optical. The sum of the opticaldmtances along a specified ra~.

Patina. A thin film or coating whichforms on various finished surface8. Onoptical surfaces it is usually a sign of age.

Pattern. A device used to determine theshape of a lens in the cutting or edgingphase of fabrication. The term is alsoused to denote the arrangement of mark-ings on a reticle.

Peel, orange. A term used to describethe unevened or dimpled appearance of alen8 surface which has been improperly, orinsufficiently polished. A polished surfaceshowing a granular appearance under mag-nification.

Peripheral. Near the boundary or edgeof the field of an optical sy8tem; the outerfringe.

Periscope. An optical instrument de-signed to displace the line of sight in a ver-tical direction. It is used to permit obser-vation over the top of a barricade or out ofa tank or submarine,

Phase shift, spatial. The displacementof the image of a sine wave object from itsideal position. Usually measured in degreeswith 360 degrees assigned to a full cycle ofthe image,

Photoelastic. Showing the phenomenonof bircfringm.ce upon the application ofstress.

Photometer. An instrument for compar.

ML-STD-i 24i A31 March 1967

P

ing the luminow intenaitie8 of two sourcesby comparing the illuminance they produce.

Photometer, bench. A photometer inwhich the distance between the comparisondevice, and one or both of the light sourcesis varied to obtain a match,

Picking. The process of breaking thepitch and removing leme8 or blanks fromthe block.

Pipe, light. A channel through whichlight is conducted by means of reflectionsby the walls. The usual shape is cylindri-cal or conical, and usually makes use oftotal internal reflection.

Pit. A term denoting small holes in aglass surface which can be seen as smallred particles by reflected light.

Pitch, A material used to hold the leruto the block during surfacing operations.Frequently used as material for polishinglaps.

Plane. A surface which has no curva-ture; a perfect] y flat surface,

PIan~ focal. A plane through the focalpoint perpendicular to ‘he ptincipal axia ofa len8 or mirror. The film plane in a cam-era f ocussed at infinitp.

Plane, image. The plane in which theimage lies, or is formed. It is perpendicu-lar to the axis of the lerw. A real imageformed by a corwer~in~ tens would be vis-ible upon a screen placed in this plane.

Plane, object. The plane which containsthe object points lying within the field o,iview,

Plane% Gauss. Planes lying nomnal tothe optical axis of a lens or lens systemcontaining the Gaus8 points,

49

MlL-StD-l 241A31 March 1967

Planes, principal. Planes of unit mag-nification, i ,e., a TUB directed at the firstprincipal plane appears to leave the secondprincipal plane at the same height.

Plate, corrector. An optical elementcomputed to correct each zone of a reflec-tor, or refractor, for npherical aberration.

Plate, Schmidt. An aaphenc plateplaced at, or near, the center of curvatureof a spherical reflector for correcting8phen’cal aberration.

Plate, surface. A plate having a veryaccurate plane 8urf ace, used for testingother surfaces, or to provide a true surfacefor accurately locating a testing fixture.

Plus. A term used to denote a convexsurface or a lens having dioptric power orfocus,

Point. A unit of lens thickness measure-ment. Five points are the equivalent ofone millimeter.

Point, aiming. The mark on which agunner sights when aiming a weapon. Thismark may or may not be the target,

Point, foca[. The point at which a bun-dle of rays from a sharp image of an ob-ject; alternatively, the point at which theobject must be placed for a sharp image.The term is also used as a synonym forfocwp principal, point of,

Point, principal, first. The principalpoint related to the object 8pace,

Point, principaj, second. The principalpoint related to image space,

Points, cardinal. In a thlcli lens or sys-tem of lenaea, the two pm”ncipal points, twonodal points, and two jocal points. If theoptical medium is the same in the object

44

and image spaces, the principalthe nodal points are coincident.

points and

Poin@ conjugate. Those pair of pointson the prinm”pal azti of a mirror or lens solocated that light emitted from either pointwill be focused at the other. Related pointsin the object and image are located opticallyso that one is the image of the other.

Poin@ Gausa.cardinal.

Points, nodal.of exterior to it

A synonym for potnt8,

Two points within a Zemsuch that any ra~ aimed

at one will emerge from the lens parallelto itself from the second point (see point8,cardinal and fig. 5).

Point4 principal. The points of inter-section of the principal plane8 and the op-tical axi8 (see points, cardinal).

Polarimetero A polari8cope equippedwith a half-shade device and an angularwale generally attached to the analyzer. Itis used to measure the amount of rotationof the plane of polarization by materialsplaced within it.

Polariscope. A combination of a pokM-izer and an analyzer used to detect bire-fringence in materials placed between themor to detect rotation in the plane of polari-zation caused by materials placed betweenthem.

Polarization, circular. See hght, pcJar-ized.

Polarization, elliptical. See light, polar-ized.

Polarizer. An optical device for con-verting unpolarized or natura] light intopolari:cd /?ght.

Polisher, pressing a. The prccess ofshaping a polisher by pressing it with anop?i~~l ~~(~fa<,e,

MIL-STD-1241A31 Morch 1967

Polishing. The process of putting a high- Power, reeolving. A measure of the abil-ly finished surface on a lens. ity of a J43ne or opticai 8y8tem to form sepa-

rate and distinct images of two object8

Polishing, plastic. The process of po/- close together, Because of diffraction by the

ishimg with a plastic pad. aperture 9top, no optical system can forma perfect image of a point, but produces in-

Polychromatism. See dichrozem.

Power. A measure of the ability to bendor refract light in a mirror or lens. It isusually measured in diopter8. In a teie8-cope, it is the number of times the instru-ment magnifies the object viewed. Forexample, if with a six-power instrument,an object 600 yards away is enlarged sixtimes, it appears as it would to the nakedeve if it were at a distance of only 100yards.

Power, magnifying. Synonymous withmagnification, magnifying power is themeasure of the ability of an opticaI deviceto make an object appear larger than itappears to the unaided eye. For ~mple,if an optical ekment or system haa a mag-nification of 2-power (2X) the object willappear twice as wide and high. The mag-nification of an optical instrument is equalto the diameter of the entrance pupil di-vided by the diameter of the exit pupil,For a telescopic system, the magnificationis also equal to the foca4 length of the ob-jective divided by the focal length of the~epiece. Another expression for the mag-nification of an instrument is the tangentof an angle in the apparent @&f divided

stead a small disk of light (at~ dwk) sur-rounded by alternately dark and bright con-centric rings. When two object points areat that critical separation from which thefirst dark ring of one diffraction patternfalls upon the central disk of the other, thepoints are just “resolved” or distinguishedas separated, and the points are said to beat the limit of resolution.

Power, reaolvtng, chrornati~ Some opti-cal components, ouch as primw and grat-ings, are used, not to resolve two or moreobject points, but rather to separate twowavelen@h8 of nearly equal value, Theability of the instrument to eeparate twosuch wavelengths is called chromatic reaolv.ing power and is specified as the ratio ofthe shorter wavelength divided by the dif-ference between the wavelengths,

Power, resolving, theoretical. The maxi-mum possib]e resolving power determinedby difhction. Frequently meas_ured as anangular reaolwtion determined from

e = 1.22Ad

where # is the limiting resolution in radi-ans, A is the wave&ength of light at whichthe resolution is determined, and d is thediameter of the effective aperture.

by the tangent of the corresponding anglePower, stereoscopic. The gain in stereo.in the true &&i.

scopic eflect afforded by a magnifying bino-cular instrument, as compared with the

Power, prism. The power of a prism, ability of the unaided e~eu. This powerexpreaeed in prism diopter8 is the apparentdisplacement, in centimeter, of an object

will vary with the separation of the ob@-

located one (1) meter distant from thetiue8 and the power of the instrument,

prism. Power, vertex. The refractive power ofa le?w based on the measurement of the dle-

Power, radiant. A synonym for JIuz, tance between the vertex of its rear euF-radiant. face and the p“ncipal foouu,

MIL-STD-1 241A31 *+ 1967

Pressing. A biunk w i t h basic surfacecurves attained as the result of formingheat-softened glass by pressing in a mold.

Prism. A transparent body with at leastleast two polished plane faces inclined withrespect to each other, from which light isreflected or through which light is refract-ed. When light is refracted by a prismwhose refractive index exceeds that of thesurrounding medium, it is deviated or benttoward the thicker part of the prism, See8@em, erecting, Abbe prism; prism, amici;priwn, POTTO; prism, rhomboiiful; primn,right angle; miwor, triple.

Prism, Abbe. A d i r e c t vision primnwhich inverts and reverta the image (erectsthe image).

Prism, Amici. A 1s o called “roof prism”and “right-angle prism with roof .“ A formof roof prism consisting of a roof edgeformed upon the long reflecting face of a~“ght-angk prism. Used as an erecting8y$tem in elbow and panoramic te&8cope8.It erects the image and bends the line ofsight through a 90° angk.

Pn~ apex of. The thin edge of a re-fracting priurn. It is the line of intersec-tion of two refracting faces of a prism.

Prismatic. Pertaining to a prism, or theeffects produced by prisms,

Priam, k ~of. The thick edge of aptim.

Prism, coincidence. A CO]..-conaiating of a system of small prisms ce-mented together, used in a coincidencerange-finder to bring the “v.uge8 from thetwo objective8 to single e?’‘+ece forviewing.

Prism, Dove Also known as “rotatingprism. ” It is used to invert the image inone plane without deviating or displacingthe axis, Used as the rotating prism inthe conventional type of optical 8gstern ofpanoramic teie8cope8. Dove prisms a n ddouble dove prisms are also used in scan-ning systems.

Prism, objective. Usually a right+zngleprism employed in some types of instru-ments to bend light 900 before it enters theobjective alternately, a dispersing priemplaced in f r o n t of an, astronomical tele-8cope objective to produce spectra of allluminous objects in the fieZd of view.

Prism, ocular. The prisms employed in arange finder to bend the lines of 8 i g h tthrough the instruments into the e~epieces,

Priemograph. A graphical device for de-termining pri8m power.

Prism, Pechan. A prism composed of twoair-spaced prism element8 each of whichhas prism angles of 22” 90’, 45° and 112”30’, as shown in figure 6. This prism hasthe property of reverting without invertingan im.uge, and can be used in convergentor divergent light as well as pmul.lel light.

Prism, pen- ‘A five-sided prim used tobend light through a constant 6i@, usual-ly 90°, without producing inveruion. A pen-ta prism c a n be rotated about an axisparallel to its faces without producing achange in its deviation of 90°.

Prism, Porro. A 460—90”460 reflectingpmkm with the 8urface8angle reflecting the light

;i; IZ -.~ l~no.

forming the 90°beam through a

M!L-STD–l 241A31 March 1967

Prism, power, resolving, chromatic. Thechromatic resolving power of a jnism is in-variably stated for the c a s e in whichparallel ra~s of light are incident on theprism, in which the prism is oriented atthe angk of minimum deviation at wave-

length A and in which the entire height ofthe prism is utilized. The correspondingresolving power R, deduced on thebasis of Rayleigh’s criterion is R = A/AA =b dn,l~ where n is the indez of refractionof the prism for the wavelength A and b isthe maximum thickness of prism traversedby the light rays. The quantities dn/dAand b are often called the dispersion andbase-length of the prism, respectively.

t

Prism, rhomboidal. A reflecting prismwhose unpolished side faces are rhomboidalin shape. It has two parallel transmittingfaces, and two parallel reflecting faces; thelatter are oblique to the former (usually butnot necessarily at 45° ). This prism has theproperty of offsetting the optical axis with-out changing the mpect of the image. Byrotating the rhomboidal prism around anaxis normal to the entrance 8urface, the off-set emergent axis can be moved parallel toitself in a circular arc; hence pairs of theseprisms are often used to provide interpu-pillary adjustment of the eyepiece of binoc-ukzr instruments such as stereoscopic range-finders and height finder8.

Prism, right angle. A type of 45°—900A5” ptim used to turn a beam of lightthrough a right angle (90”) with the sur-faces forming the 90° angle acting as trans-mitting faces.

Prism, rotating. See pri3m, Dove.

Prism, roof-angle or roof. A synonym forpri8m, amici.

Propagation, rectilinear. straight 1i n etravel; refers to the fact that light travelsin a straight line while traveling through amedium with a constant index of refraction.

Propertie& optical. ln optical glass, thoseproperties which pertain to the effect of theglass upon light, such as index of refrac-tmn, dispersion, homogeneity, and freedomfrom defects.

Pupil, artificial. A diuphTagm or otherlimitation that confines the beam of lightentering the eye to a smaller cone than doesthe iris of the human eye.

Pupil, entrance. The image of the limit-ing aperture stop formed in the object spaceby all optical element8 preceding the limit-ing aperture stop; also used as a term todenote the aperture of the objective whenthere are no other limiting stops followingit in the system.

Pupil, exit. The image of the limitingaperture stop in an optical sgst em formedby all Jewes following this stop. In photo-graphic objectives this image is virtual andis usually not far from the iris diaphragm.In tele8cope8 the image is reaJ and can beseen as a small bright, circular disc by look-ing at the eyepiece of the instrument di-rected toward an illuminated area or lightsource. In telescopes, its diameter is equalto the diameter of the entrance pupil di-vided by magnificatwn of the instrument.In Galilean telescopes the exit pupil is a vir-tual image between the objective and eye-piece and acts as an out-of-focus field 8top.

Pyramid, error due to. The error in theposition of the image introduced by pyra-midal error in a prism, measured relativeto a chosen line of intersection of two prismfaces.

47

MIL-STD-I 241A31 March 1967

Quality, image. Image quality embracesall the properties of a lens or opticaJ systemaffecting the optical performance such asreaolvmg power; aberratwn.a, image defects,and contrast rendition. Aberrations con-tribute to poor image quality. Errors ofconstruction and defects in materials ad-versely affect image quality. Because of thecharacteristic effects of aberrations onimage quality it is possible to distinguishbetween their effects and those of accidentalerrors of workmanship are found nons-pherical surfaces, poor polish, ucratches,pits, decentermg, defects in cementing, andscattered light, all of which contribute todeterioration of the image. Defects in glasssuch as bubbles, utones, sttie, crystallinebodies, cloudiness, and strain play a part inpoor image quality.

Quality, surface. A means of specifyingallowable flaws by comparison to reference

Qstandards of quality. Two graded WAS ofsurface qualiw standards are used. Thefirst indicates defects of a long nature suchas Ucratcheu, and the second illustrates es-sentially round def acts such as digs. Desiredsurf ace quality is specified by reference tothese in order, e.g., 80-50 or 80/50.

Quartz. A natural mineral composed ofsilicon dioxide (S, 0,) crystallized in thehexagonal crystallographic system. It isuniaxial and optically active. Quartz isused as an optical medium in scientific ap-paratus because of ita transparency over awide interval of the electro-magnetic 8pec-trwm, particularly the ultraviolet.

Quartz, fused. A vitreous (glassy) ma-terial resulting from the fusion of crystal-line quartz. It is optically isotropic, and hasa much lower index of refraction than cvs-talline quartz.

43

Radiance. The rndiunt Intensify per unitprojected area of an extended source.

Radiation, efficiency of, luminous. A syn-onym for luminosity.

Radiation, (or light), monochromatic,This term refers to the flux at one wave-length or frequency.

Radiator, complete. A synonym for black-bod~.

Radiator, full. A synonym for blackbody.

Radiator, ideal. A synonym for black-

body.

Radiator, thermal. The process at emis-sion wherein the radiated energy is ex-tracted from the thermal excitation ofatoms or molecules.

Radiometer. An instrument designed tomeasure radiant intensitp.

Range, environmental. The maximum tominimum range of temperature, pressure,humidity, vibration, and biological condi-tions under which a lens can operate and bestored, constitute its environmental range.The range required depends upon the in-tended use.

Rangeflnder. An optical instrument usedto determine the distance of an object ortarget by triangulation.

Rangefinder, coincidence. A self-containeddistance measuring device operating on theprinciple of triangulation. Two images ofthe same object, simultaneously observedfrom two points a known distance apart,are matched to determine the range.

Ratio, aperture. In general, apertureratio is twice the value ~~ in the equation.

MlL–STD-l 241A31 March 1967

R

P,,, = n s]n LIn,,where n is the index of re.frnction of thezmagr space, and c., is the maximum angu-lar opening of the axial bundle of refractedra~s as illustrated in figure 7. The speed(i.e , energy per unit area of image) of anobjectize is proportional to the square ofits aperture ratio. When the angular open-ing is small, when n= 1, and when theobject distance is great, it is approximately

Dtrue that n sin am = ~, or that

f 1— = f-number = —

1D 2p,,, = aperture ratio

Ratio, telephoto. The ratio of the equiva-lent focal length to the f~ont vertex focal

distance.

Ray. A contraction of the term ray, light.

Ray, chief. The central ray of a bundleof rays.

Ray, emergent. A ray of light leaving,i.e., emerging from a medium as contrastedto the entering or incident ray.

Ray, extraordinary. A ray which has anonisotropic velocity in a doubly refracting

crystal. It does not necessarily obey SneU’alaw upon Refraction at the crgstal surface.

Ray, incidenL A ray of light which fallsupon, or strikes, the surface of an objectsuch as a lens or mirror. It is said ta beincident to the surface.

Ray, light. The term applied to the linesperpendicular to the wavefront8 of wavesof light to indicate their direction of travel.

Ray, ordinary. The ray which has anisotropic velocity in a doubl~ refractingcr@aL It obeys Snelt~ Zuw upon refrac-tion at the cr’@4d mrface.

49

MlL-STO-l 241A31 March 1967

Ray, paraxial. A ray of a bundle of rayswhich approaches the chief ray of thatbundle as its limiting position. More prop-erly, it is a ray in the sense of Gaussianor first order optaca.

Ray, principal. In the object space, theprincipal ray is one directed at the fimtprincipal point, and hence in the im.ugespace this ray, projected backward, wouldintersect the axis at the uecond principaJpoint.

Ray, reflected. The ra~ of light leavinga reflecting tmrface, representing the pathof light after reelection.

Ray, rim. A ray of an image-formingbundle which passes through the rim of theentrance pupal or aperture dep. Usuallyused in connection with meridian rays, an“upper rim ray” being one which passesthrough the top of the pupil or trtop,and a“lower rim ray” one through the bottom,

Ray, skew. In a system having rotationalsymmetry, it is a ray which does not lie ina plane containing the axis of the system.

Ray& diffuse. See reflection.

Ray% field. In the object space, they areray8 which intereect the optical axis at thecenter of the entrance pupal of a symmetri-cal optica4 s@em. In the image space, thesame rays emerge from the ezit pupil. Ina thick lenJL the field ray is the principt-dvay.

RayA marginal. Rags of light passingthrough an optical 8@em near the edge ofthe aperture.

Ream. A nonhomogeneity in flat glassin the form of an approximately planelayer.

Rectilinear. In a straight line. Whenanp!!ec? tc a !erts, it iwiicdiw tbr imuge8OJf s:raight !;nes pled.k~d by LY,i ;::::s are, cd dis’nr:d

Reek, bJock. A type of scratch.

Reflectance, The ratio of the reflectedflux to the inctint flux. This term is ap-plied to radiant and to lumimm.a flux. Un-less qualified, reflectance applies to specuiur(regulw) TOefkctwn.

Reflectance, diffuse. The ratio of fluxreflected diffusely in all directions to thetotal Ilux at incidence (8peculur reflectionexcluded). Also called “Total Diffuse Re-flectance.” Also, the refkctance of a sam-ple relative to a perfectly diffusing, and per-fectly reflecting standard with 45° angk ofincidence. and observation along the per-pendicular to the surface.

Reflectance, dithse, total. see ref&?ct-ance, diffuse.

Rellectanc% spectral. The reflectanceevaluated as a function of wavelength. Itis numerically the same for radiant andluminous flux.

Reflection. When light rav8 strike asmooth, Polighed surface they are bent backinto the medium whence they came. Spec-ular or regular reflection from a polishedsurface, such as a TAwor, will return a ma-jor portion of the light in a definite direc-tion lying in the plane of the incident ravand the norrn.d (eee reflection, angle of).After specular reflection, light can be madeto form a sharp image of the original8ource. Diffuse reflection occurs when thesurface is rough and the reflected light isscattered from each point in the surface.These diffuse raya cannot be made to forman image of the original source, but only ofthe diffusely reflecting surface itself.

Reflection, angle of. The angk betweenthe normal to a reflecting surface and thereflected ray.

Reflection, diffuse, See reflection.

l?sflec%~, direct. S3: ~:f!retio~’, .snectI-f<-

Reflection, internal, total. The reflectionwhich takes place within a substance be-cause the angle oj incidence of light strikingthe boundary surface is in excess of thecm”tical angle.

Reflection, law of. The angle of rejlectlonis equal to the angle of zn.cidence; the inci-dent ray, reflected ray, and normal, all lie inthe same plane.

Reflection, mixed. The simultaneous oc-currence of specular and difiuse reflection.

Reflection, regular. See reflect~on.

Reflection, selective. See absorption, 8elec-tiveo

Reflection, specular. See reflection.

Reflectivity. The reflectance of an opaquematerial or of a layer of that material ofsufficient thickness so that further increasesin thickness do not alter the reflectance.

Reflectivity, spectral. The reflectivityevaluated as a function of wavelength.

Reflector, annuiar. A ring-shaped reflect-or, or a series of ring-shaped reflectors,mounted about a common center.

Reflector, corner-cube. See

Reflector, retrodirective.triple.

mirror, triple.

See mirro~,

Refraction. The bending of oblique in-cident rays as they pass from a medium ofone index of refraction into a medium of adifferent index of refraction.

Refraction, angle of. The acute angle be-tween the normal to a refracting surface atthe point of incidence, and the refracted ray.

F?efrzction, ?odt. !z, ‘!7Fle s8par2ti:)j1 2i’

5;

MlL-STD–l 241A3 T March 1967

unpolarized light into two plane polarizedcomponents by a doubly re~racting crystal.

Refracticm, index of. A Jlumber appliedthe relation between the angle of inctdence

and the angle of refraction when lightpasses from one medium to another. Theindex between two media is called the “rela-tive index,” while the index when the firstmedium is a vacuum is called the “absoluteindex” of the second medium. The index ofrefraction expressed in tables is the “abso-lute index,” that is, vacuum to substance ata certain temperature, with light of a cer-tain wavelength, Examples: vacuum 1.000,air, 1.000292; water, 1,333; ordinary crownglum, 1.516. Since the index of air is veryclose to that of vacuum, the two are oftenused interchangeably as being practicallythe same (see Snell, law of).

Refraction, law of. A synonym for Snell,law of,

Relief. l’he discernment of depth or ap-parent difference in distance that makes theobject stand out from its background due tostereoscopic vkion, The impression of re-lief can be obtained from the arrangementof highlights and shadows when the objectis viewed monocularly.

Relief, eye. A synonym for distance, eve.

Repressing. A synonym for blank, mold-ed.

Resolution. Abbreviated RESOLN. Theability of a lens system to reproduce animage in its finest details (see power, re-solving ).

Resolution, angle of, limiting. The anglesubtended by two points or lines which arejust far enough apart to permit them to bedistinguished as separate. The ability ofan optical device to resolve two points orlines is called resolving power and quanti-tatively is inversely proportional to the Iim-itim: angi~ ~~ ~eso]ution as deillleJ.

MIL-STD-1241Aal March 1967

Reticle. A scale, indicator, or patternplaced in one of the focal planes of anoptical instrument which appear to the ob-server to be superposed upon the field ofview. Retitles, in various patterns, areused to determine the center of the field orto assist in the gaging of distance, deter-mining Ieadq, or measurement. A reticlemay consist of fine wires, or fibers, mount-ed on a support at the ends, or may beetched on a clear, SCXWPU1OUSIYpolishedand cleaned plane parallel plate of glass.In the latter case the entire piece of glassis referred to as the reticle. An altirnatebut less common form of spelling is reti-cule. In England the term “graticule” isgenerally used.

ReticulA See reticle.

Reversibility, law of. If the direction oflight is reversed, it will travel in the oppo-site direction over the same path despitethe number of times it is refraoted or re-flected.

Reverted. Turned the opposite way sothat right becomes left, and vice versa. Itis the effect produced by a minor in re-flecting an image.

Revolution, angle of, quadrant. The anghbetween the horizontal and the line of ele-vation, The angle of ekwattin, plus theangle of site, equals the quadrant angle ofelevation.

Rhqm Newton’* When two polishedsur@ea are cleaned and placed in contactwith a thin air film between them, reflectedbeams of light from the two adjacent sur-faces interfere to form a series of rings orbands k~wn as Newton’s rings or fringes.By counting these bands from the point ofactual contact the departure of one surfacefrom the other is determined, The regular-ity of the fringes maps out the regularityof the distance between the two surfaces.

This is the usual method of determining thefitof a surface under teat to a dumdardsurface of a teat gk188.

Ripplea If a eur~uce is polished withoutan oscillation of the polishing Zup the pol-ished surface contains approximately pa.md-lel wave8 or ripples.

Rotatiom optical. The ungvdar displace-ment of the plane of polarization of lightpassing through a medium. The =imuthaldisplacement of the @Ai of view achievedthrough the use of a ~otating priem.

Rouge. Iron oxide; a very fine abrasiveused for poltihing a &e eurface.

Roughing. A term used to denote a verycoarse grinding process.

Rub. A type of mratch.

Rule, Prentice’s. A means of determin-ing pti power at any point on a hma.Prism power equala dioptric power multi-plied by the distance in centimeter fromthe opticalcenter.

Run-out edge. A term used to denotethe total wobble, measured in unite oflength, resulting when a recentered lens isrotated about its optical aa’is and the edgeof the lens wobbles.

Run+u& Image, total. If a recenteredZeti is rotated in a chuck whose axis ofrotation paaaea through the geometriod cen-ter of the rim of the ha, the image of anobject will wobble. If the light source isat an infinite distance, the total image di~placement is called the “total image run-Out,” and is a measure of the &centering.Conversely, if the light source is at thefocal point of the lens the direction of theemergent parallel bundle changes throughan angle which measures the recentering inangulur ternus,

52

Saddle. A term used to denote a saddle-shaped polished surface, generally an error,whose contours are indicated by the shapeof Newton’8 n’ng8. It is a saddle-shapedimage of a point light source caused byaetigmuttim.

Sag. To cause a sheet of glass to con-form to a ceramic or metal form, by heat-ing the glass to its softening point andallowing it to settle. In the geometricsense, it is also used ae an abbreviationfor “sagitta,” the height of a curve meas-ured from the chord,

%ale brightneea. A graduated series ofstimuli perceived as having equal differ-ences of brightne68.

Scale diopter. A scale usually found onthe focusing nut of the e~epiece of an op-tical instrument. It measures the changein the position of the eyepiece necessary toproduce a confection to compensate thenearsightedness or farsightedness of theindividual observer. Thus if the observerknows his diopter correction, he can presetthe instrument for fooue. .

!kl% stadia. Graduation on a retickwhich @ conjunction with a rod of definitelength can be used to measure distanoes.

Scratch. Any marking or tearing ofthe eurfoce appearing as though it had harmdone by either a sharp or rough instru-ment. Scratchea occur on sheet glass ,in alldegrees from various accidental causes.B&ck reek is a chain-like scratzh producedin poliehitag. A runner-at is u curvedscratch caused by @@n#. A 6buk is ahairline scratch. A crush or rd is a sur-face scratch or series of small scratchesgenerally caused by mishandling.

Seed. A term used to denote a gaseous

MlL-STD-l 241A31 Morch 1967

sinclusion having an extremely small dia-meter in glass.

Seed% heavy. Refers to a conditionwhen the fine and coarse seeds are verynumerous, such as 26 or more to the squareinch.

seed% ecattered. A term used to indi-cate the condition of a few and occasionaleasily visible coarse seeds. Two or threemay be spaced 1 or 2 inches from eachother, but one here and there at muchgreater distances apart is the usual inten-tion of the term.

Segment. A term used to denote glass,with a high index of refraction, ones it hasibeen fused to the major and ground to acurvature which results in added power.

Semifinished. A term used to denote aleru or blank with one surface completelyfinished.

Setup. A term used to describe the ad-justments and changes necessary to adapta machine to perform a different process.

Shading. The sorting of lenses by theirCOlur.

Shadowgraph. A method of demon-stration or inspection, using a point sourceillumination, without the use of any projec-tion lens between the object and its shadow,the shadow being caet on a distant screen.

shallow. Atermused todenoteaecave uurf ace when its radius of curvatureis too long, i.e. Na negative power is toosmall or low.

Shaper, blocking. A convex, coneove, orflat cast iron form used to shape a uo~trnokf block of optical eienwnte.

63

al Mnrch 1967

Sharp. A term used to denote a cvnvexsurface that has too short a radius of cur-vature. To correct this condition, materialis cut from the outer portion of the polish-ing tool,

Sight, angle of. ‘I’he vertical angk be-tween the horizontal and the line of sight(line from weapon to target).

Sight, line of (L.O.S.). The line of vi-sion; the optical axis of a telescope or oth-er observation instrument. The straightline connecting the observer with the aim-ing point; the line along which the sightsare set.

Skim, A term used to denote 8treak0 ofdense seeo% with accompanying small bub-b.b38.

Slab-off. A term used to denote the pro-cess of making an abrupt break in a spher-ical surface so that a new center of curva-;ure is established for a portion. This hasthe effect of adding a pri8m to that portionof the surface.

Sleek. A polishing scratch without visibleconchoidal fracturing of the edge8.

Snell, law of. When iight is passing from.8 ~-ven medium to a denser medium, its pathis deviated toward the normal; when paasinginto a less dense medium, its path is deviatedaway from the normal. Sneh’ law, oftencalled the law of refraction, defines this phe-nomenon by describing the relation between‘M angle of incidence and the angk of re-

]m.ction as follows:

The sine ofthe angle ofincidence.

=The sine ofthe angle ofrefraction,

The index of refraction ofthe medium containing therefracted ray.

The index of refraction ofthe medium containing theincident ray.

sin i h’‘i’his is written =—#rA ~f ~. ,

Sourceg light, luminous efficiency of. Theratio of the tuminous flux emitted to thepower consumed by the source, for example,Lumens per watt applied at the source.

Spar, iceland. A transparent variety ofthe natural crystal calcite (also called calc-spar ) that displays very strong double re-fracticln. It is calcium carbonate crystallizedin the hexagonal rhombohedral crystallo-graphic system, and is uniaxisl.

Spatter. A term used to denote the condi-tion resulting when small chunks of mate-rial may ‘fly from the hot crucible onto theglass twrface, and adhere there, in evapora-tive coatings.

Spectrometer. A s-pectroacope providedwith an angle scale capable of measuring theangtdar deviation of radiation of differentwavehmgtha. In common usage, the dis-persing means may be dispersed with, andthe instrument used for measuring anglesas on or through a prism.

Spectroscope. Any one of a class of in-struments capable of dispersing radiationinto its component wavelengths and observ-ing, or measuring, the resultant 8pectrum.

Spectrum. The visual spectrum is theband of color produced by decomposingwhite light into its components by the pro-cess of dhpera-ion. The rainbow is an ex-ample of a spectrum produced by the dis-persion of white light by water dropleta(see 8pectru7n, ele&om.agnetic).

Spectrum, electromagnetic The entirerange of wavelength, extending from theshortest to the longest or conversely, thatcan be generated physically, This range ofelectromagnetic wavelengths extends almostfrom zero to infinity and includes the visibleportion of the 8pectruna known as light.

Speetrwm secondary. The residual ChW-matu aberration, Parttirlll the Zonm”tu-dinnl chromatic abemation of an achromaticlens. Unlike the p+naW 8pectrwn, itcauses the image formed in one particularcoh to lie nearest the kne, the images inall other colors being formed behind thefirst at distances that increase sharplytowards both ends of the useful waueknpth8pectrum.

Spectrum, visible. The portion of theelectromagnetic spectrum to which theretina is sensitive and by which we see. Itextends from about 400 to about 750 ?niUi-microne in wavekagth of the radiution.

Speed, Ien& That property of a ienewhich affecti the illuminanoe of the image.Lens speed is specified in terms of the fol-lowing expressions: aperture ratio, numer-ical aperture, T-stop, or F-number.

Spherk A term commonly used to des-ignate a kw with two IBperical surfaces.

Sphere-cylinder. A lenu or lens surfacewhich is a combination of a ephiwe and acylinder.

Spherometer. An instrument for theprecise measurement of the radius of curv-ature of eurfacea.

Spindk A part of a uurfacing machine.In the manufacture of lenses it is used asa minimum unit of production.

Spo@ silver. Spots in a polished eur-face of glass which are opaque and have asilvery, metallic, rejlectiom

Steradian. The solid angle subtendedat the center of a ephere by an area on itsewjace numerically equal to the square ofthe m.+ Ius. The un!t of solid angu!ar meas-I?rement.

L5

MIL-SID-l 241A31 kld 1*7

StinL A term used to indicate a quan-tity of work or production.

Stock, bad. Glass having defects whichwere incurred during original manufactum.

Stone. An opaque inclusion in glass con-sisting of undissolved or crystalline ma-terial.

Stone bl~ An edgingrelatively coarse abrauive.

Stop. See atop, aperture;

stone with a

see Etop, Judd.

StopS ●perture. The ditlph~m whichlimits the sise of the aperture.

Stop, field. A diaphragm used to de-limit the usable field. The field stop isused to produce a sharply defined edge tothe field.

strain. Mechanical tension, compres-8i0n or shear in optiea.1 glusu by in.ter?udutrem and brought about by improper cool-ing or an?aea.lin~ during manufacture of theglass or the subsequent weakening of mold-ed parts.

S- internal. The tension, compree-siou or shear stresses within an opticalek?mertt usually caused by cooling or im-proper wwwa&gm

Stria- A defect in optical gkaus consist-ing of a sharply defined streak of trans-parent material having a slightly differentkkx of refraction than the body of thegiaaso

Striae, Internal imperfections of glassappearing as wavy distortion.

Strings Wavy transparent lines appear-ing as though a thread of glass had beenincorporated into the sheet.

stripe. See s!eek.

MIL–~D–l 241A31 March 1%7

Sturm, interval of. Separation betweentwo focal lines of an astigmatic image.

Surface. A term used to denote one ofthe exterior faces of an optical element. Itis also used to describe the process of grind-ing or generating the face of an opticalelement.

Surface, aplanatic. A surface is socalled, if for each of its pointa the sum ofthe optical dtitance to two fixed points onthe surface has a constant value.

Surface, cementing, The surface 01 anelement of a compound lens which will formthe cemented interf aoe. The curves of thisinterface are known as “inside curves. ” Tofacilitate cementing, the two inside curvesshould have a slight mismatch so that thetwo elements strictly contact only at theedges.

Surface, eggy. A defect in the shape ofa spherical surface causing it to be eggshaped.

Su}face, internal. Internal nonopticalsurfaces of lenses and lens mounts, contri-bute largely to jlare by light reflected fromthem into the image apace. Consequentlysuch surfaces, when so located as to con-tribute to flare, are finished with a dullblack light-absorbing material.

Surface ocular. That surface of a ZeWwhich is placed nearest to the eye.

Surface, optical. A reflecting or refract-ing surface that close] y approximates thedesired geometrical suflace (see k?n.s; hat,optical; mirror, paraboloid).

Surface, Petzval. A parahcdoidal 8urftaceon which the image lies in the absence ofa.digmutiem.

Surface, toric. A 8urface swept out byrevolving a circle about an axis that is in

the plane of the circle but does not passthrough ita center. However, the term applied to other asphem”c surfaces means asurface having different curvature8 in dif-ferent sections, the greatest and least pow-ers occurring in meridians perpendicularto each other.

Surround. A term referring to both thecolor and intensity of the immediate en-vironment of the object or image beingviewed. In another sense, the term is asynonym to dummy.

Swing. The length of the oacillatjon of agrinding or polishing lap,

System, erecting. A system of lewe8 orprisms, the function of which is to producean erect image which would otherwise beinverted. An erecting system may consistof a lens or system of lenses to reimage theobject or of one or more prisms.

System, erecting, Abbe prism. See pritwn,Abbe; see also ~8tem, erecting, porro prismtgpe 2.

System, erecting, Porro prism. A prismpair designed by M. Porro, in which thereare four re~ctiorzs to completely erect theimage. Each prism has angles of 45” and90”. The hypotenuse faces are parallel andmay be either air-spaced or cexnentxxl. Theedges at the 90” angle of the two prismsare at right angles to each other. The Jineof sight through this system is laterallydisplaced but not deviated. This system isgenerally used as an erecting 8@enz inbinoculur field glasses.

System, ●recting, Porro prkm type 1. Adirect vision prism system containing twoPorro prism8 with their roof edges at rightangke, and their hypotenuse faces paralleland opposed. This system wiI1 invert andrevert the image (erects the image) andhas the characteristic of displacing the lineof eight laterally and vertically.

66

MIL-5TD-1241A31 Morch 1%7

System, erecting, Porro prism type 2. Alsoreferred b as Abbe’s modification of thePorro prism erecting system, or Abbe prismerecting system. It is a direct vision prismsystem which can he thought of as consist-ing of two pairs of 45 0—90” right angleprims of equal size, in which the reflectingsurfaces in each case are the hypotenusefaces, Each pair is constructed with the45” face of each prism opposed to the 45”face of the adjacent prism, and each hy-potenuse face is rotated to deviate the lineof eight 90 “. Between the pairs, the lineof sight is deviated 180° resulting in a totalsystem deviation of 360”. In contradistinc-tion to the type 1 system, conventionallythis system is manufactured in three pieceswith the second and third prisms made asone porro prism, or in two pieces with thefirst and second pieces paired, and the thirdand fourth pieces paired. This combina-

tion will displace the line of sight onlylaterally or vertically, depending on orienta-tion.

System, len= Two or more len8e8 ar-ranged to work in conjunction with oneanother,

System, len% centered. A le?w 8ystem inwhich the principaJ UXe8 are coincident.

System, optical. A combination of opt~ctd components arranged so as to performone or more optical functions.

SystaU optical, catadioptrk An optical

sI@&n containing both lens (dioptric) andcurved mirror (catoptric ) optid compo-nents. Occasionally, a single componentmay be cntadioptric, serving simultaneous-ly aa a lens or mirror.

s?

MIL-STD-1241A31 March 1967

T

Talbot. A term used to denote the meter-kilo6ram-second unit of luminow energyequal to ten million hmergs; also equal toone lumen-second.

Teleobjective. An objective lens 8@em

consisting of a positive and a negativecomponent separated from each other, hav-ing such power8 and separation that theback focal length of the entire system issmall in comparison with the eqwivak%tfocaZ length. Such lenses are used for pro-ducing large ima~es of distant object8 with-out the necessity of a cumbersome lengthof the instrument,

Telescope. An afocal optical instrumtmtcontaining a 8y8tem of len8e8 or mi7TOT8,usually but not always, having a magnifi-cation greater than unit, which rendersdistant object8 more clearly visible by en-larging their image8 on the retina of theeye. Telescopes have two major uses: ob-serving and pointing as in the measure-ment of anpla?, and in aiming,

Telescope, astronomical. A tele8copewhich produces an inverted image.

Telescope, auxiliary. A small telescope,placed between the eyepk?ce of an opticalinstrument and the observer’s eye, to in-crease the overall m.agntlkztion of theimage. This type of telescope is usuallyof low magnifying power.

Teleecope, Caaeegra inlan. A tehwcope inwhich the ra~8, after repctton from a pri-mary parabo10i4i4d mirror, fall upon a small-er secondary convex hyperboloidal mirrorthat reflects the rays back through an open-ing in the center of the primary where theimage may be examined with an egepiece.This term has been broadened to includeany type of secondary mirror from whichthe light is reflected through an opening inthe primary mirror.

Telescope, collimating. A telescope, themechanical axis of which, referred to theouter cylindrical surface of the tube, iscoincident with its opticcd azis. In thistelescope instead of an eyepiece, a reticleand generally an illuminating system re-places the eyepiece. This telescope providesbundles of parallel light ray8, i.e. it ha.ge8the reticle at infinity. It is generally usedfor optical adjustments where parallel lightis neeessary.

Tel~ elbow. A refracting teZ.e8wpewhich bends the line of m“flht90° by meansof a prism.

Teleecope, Gregorian. A tele8cope inwhich the ra~u, after re~ction from a pri-mary paraboZoi&2 mirror, fall upon a small-er secondary concave ellipsoidal mirror thatreflects the rays back through an openingin the center of the primary where theimage may be examined with an e~epiece.

Teleecopq Newtonian. A teleecope inwhich the 7xw8, after re~ction from a pr-imary paraboloidul mirror, fall upon a small-er plane mirror, from which the light is re-flected laterally outside the tube where itcan be examined with an eyepiece.

Teleaco~ panoramic. A tehwcope so de-signed that the image remains erect andthe position of the e~epiece is unchangedas the line of tight is pointed in any hori-zontal direction,

Teleecope, termetrfal. A te&8cope whichproduces an erect, or natural, huge.

Temperature, color. The temperature ofa b&.ckbody that emits light of the sameCOZOTas the body in question, Color tem-perature is expressed in c?egres Kelvin.

68

MU.-STD-1241A31 Mwch 1947

Temperature, luminance. The tempera-ture of an ideal blackbody that wouldhave the same luminance as the source inquestion for some narrow spectral region.

Temperature, radiation, total. The tem-perature at which a blackbod~ radiates atotal amount of fluz equal to that radiatedby the body under consideration.

TesG Hartman. A test for 8phericalabberation, departure from the sine condi-tion, or coma in which incident rays froma point source are isolated by means ofsmall holes in a disk positioned in front ofthe le~ or m~mcr under test. The focalpoint8 of the rays entering the lens or mir-ror at varying heights are then comparedin the image space.

Test, knife edge, Foucault. A method ofdetermining the errors in an ima~e of apoint source by partia)l y occluding the lightfrom an image by means of a knife edge.The same test may be used to measure theerrors in refracting or reflecting eurf ace8.

Theory, electromagnetic. The theory ofpropagation of energy by combined ekc-tric and magnetic fields embodied in Max-welI’s equations.

Thickne~ center. The thickness of alens measured “at the optical field.

Threshold, luminance. A contraction of?he term thre6ho&i, luminance, absolute.

Threshold, luminane~ abeoluta A termused to indicate the lowest limit of Jumi-nance necessary for vision.

‘l’-number. The equivalent f-number of afictitious lew that has a circular openingand 100 percent tramwnittance, an that

gives the same central illumination as theactual lens under consideration,

T number =E. F. L.

Diameter of T-stop

or

T number =E. ~ L. 1A

v At

where E. F. L. is the eguival.ent focallength, A is the area of the entranee pupiland t is the transmittance of the lew ey8-tern.

Tolerant% figure. The allowable devia-tion from the specified figure or geometricalform. It may be defined in terms of ftirtgeuor wavelength.

Tolerance wedge. A means of specify-ing the allowable edge thic?mem di#erenceor decenteting of a lene.

Tool, blocking. An instrument for sup-porting optical parts to be cemented ta iLor mounted in plaster.

Tool, grinding. Lapa of cast iron orother suitable material used with a slurryor silicon carbide, aluminum oxide, or em-ery, for grinding opticaleurfacee.

Tool, mdiua A metal instrument of con-vex or concave curvature to which &macastings or semifinished lenses are cementied with only their edges in contact withthe instrument.

Tool, spotted. A metal instrument forholding a Mock of Zenueg in which seate forone eurfa.ee of the lenaee have been formed,and to which the lenses are cementad.These tools have the advantage of not re-quiring bkking pkatsi, aiie permanen~and preven. distortion of the finished sur-face,

Toric, +I_, flra~. A term used to referto the pi OO?-IJ-= grinding the tcric eurfa*e

- “-d “...1 :yti~.of -. sin~le vision WIW*v.e:.w,, Y ,.i

69

#JIL-T-r-T-O-~~1A

TorIG sJda second. A term used to tierto the process of grinding the concave 8mr-face of s epm~~~ *“

Tranamiaaioll. The proceea of conduc-tion of radiant energy through a medium.

Tranemieaion, eeloctivm see UIW@’iW8elective.

Tranam Wvity. The internal transmittancefor unit thickneaa of a nondiffusing sub-.j*m2e.

Transmittance. The ratio of the fluskmmitted by an object to the incidentjlw. This term and M specializations areapplied to radiaut ●nd to &mhou8 flux.‘lJn]W qualithd, the term applied b regular!specular) transmieeion.

Tranamlttin~ diffuse. The tranemit-hwtce measured with diffueely incident Puz.Mao, the ratio of the flux diffueely trana-rnitted in all directions to the total incidentflux.

‘lhnnmittancq internal. The ratio ofthe flux transmitted to the eecond surf-of a medium b the corresponding flux thatMM juet paeeed through the tlret surface,i.e. the trancmittunce from the firet cur-face to the second surface. Internal trans-mittances does not include the el?ecta dueto interreflectjon between the two surfacee.

Tranamlttan~ lumino~ The ratio ofthe hmirwuu jk tranmnittid by an objectto the incident luminous flux.

Transmittance mdiark The ratio of ‘thew.diunt flux transmitted by an object to theincident radiant flux.

Tranamittanc% eelective. The property ofvariation of transmittance with wavelen.@h.

Tranemitt8nce, spectral. Trarwmittanceevaluated at one or more wavelengths.

Numerically the came for rodimd andluminous flux.

TranamMancy. The ratio of the trane-nti.ttzmce of a eolution to that of an equalthickneea of the eolvent.

~tioll. Ch8rWb the relativecurves of a ieru without changing ite re-fractive value.

Trawl, horiaontd. Atermuaedtod-note the rotation of an instrument (or thetine of dght of an optical instrument) in ahorizontal plane; traveree.

TreaUnC, h 8at- The proceae of subject-ing glaee to temperature cycling to inducephysico-chemtcal raactione which changethe properties (see ammlingp case harden-ing, compacting) .

lh’iple~ A three-lene component of anoptical euetem which may or may not becemented.

Truing. The proceae of making a surfaceto conform accurately to a required cuma-tureo

T-atQp. The equivalent,mitting, circular openingsuch that

Dau

() T =

perfectly trane-of diameter D

tA

where A ie the area of the entrance pupilof the objective and where t is the tram-rnittunce of the law 8@em.

h, invert. The type of image obeervedin certain coincidence nzngefinderu. Whenin coincidence, the upper half image ap-pearis to be the mirrored re&ctwn of thelower half image.

MIL-STD-1241A31 March 1967

uUltraviolet. Those ra~u of radiant energg Unblocking. The procae of removing

immediately beyond the violet ends of the optical elementu from a block.visible upectrum and in of the order of 390to 100 ?nWt?nierone. Uncut. A term app!ying to leraeu with

both mwfa.ce8 finished but not yet cut to anyshape,

~~.5T~_1241A31 March 1967

vValue, nu. A term for con8tant, Abbe.

Value, VW. A term for con.wkmt, Abbe.

Vector, electric. A term referring to theelectric field associated with an electromag-netic wave and hence with a light wave.The electric vector specifies the directionand amplitude of this electric field.

Vertex, The point of intersection of theOpt?kd az28 with any optical &u?’face,

View, field of. In general, the maximumcone or fan of rays passed by an apertwreand measured at a given vertez. In an in-strument, field of view is synonymous withtrue field,

Vignetting. A term used to denote theloss of light through an optical element dueto the entire bundle not passing through,

VieJon, binocular. The simultaneous useof both eyes in the process of vision.

Vision, distinct, distance of. The near-point distance of the normal eye conven-tionally given the value of 10 inches or 25centim.dcrs. This value is used in calcu-lating the designated magnification of asimple magnifier or eyepiece.

Vidon, double. A malfunction of a binoc-ular instrument causing two hW?8 to beseen separately instead of being fused. It

is caused by the optical aze8 of the two teZe-8cope8 not being parallel. In minor cwws,the ewes will adjust themselves to compen-sate for the error of the instrument untilthe images are superimposed and only oneobject is seen (see divergence and diver-gence).

Vision, stereoscopic. Vision in depth ofthree dimensions due to the spacing of theeye8. This spacing permits the eyes to seeobject8 from slightly different points ofview.

62

MIL-STD-1241A31 March 1%7

wWash. In a cemented surface a streak

appearing as a striation caused by inder ofrefraction variations in the cement,

Watch, lens. A dial depth gage gradu-ated in diopters.

Wave. Vibration; a form of movementby which all radiant energ~ of the electro-magnetic spectrum is assumed to travel. Itis also used to denote a type of surfacedefect, usually due to improper polishing.

Wave front. A surface norrnul to a bundleof rays as they proceed from a source. Thewave front passes through those parts ofthe waves which are in the same phase.For parallel raps, the wavefront is a plane;for rays diverging from or converging to-ward a point, the wavefront is spherical.

Wavelength. The length of a wave meas-ured from any point on one wave to thecorresponding point on the next wave;usually measured from crest to crest.Wavelength determines the nature of thevarious forms of radiant energy which com-prise the electromagnetic 8peCt?Wm; it de-termines the color of light,

Wedge. A prism with a very small anglebetween the refracting surfaces. Wedgesmay be circular, oblong, or square in out-line.

Wedge, correction. In ra%ge@der8 andheightfinders, a rotatable or sliding wedge-shaped element used to divert the line ofsight in a precise manner in order to cor-rect errors in the optical s~stem caused bytemperature variation or any other errorsof collinmtion.

Wedge, measuring. A wedge in a range-finder or height$nder to displace the image

formed by one telescope, so that it coincides

with that formed by the other telescope,thus affording a measurement of the paral-lactic angle between the line of sight of thetwo telescopes. There are two principaltypes of measuring wedges. The slidingwedge is mounted on slides parallel to theoptical axis, between the objective le~ of

one telescope, and its focal plane. It pro-duces an image displacement equal to theproduct of the deviation of the wedge mul-tiplied by its distance from the focal plane.Compensating wedges rotate simultaneouslythrough equal angles in opposite directions.The pair of equal compensating wedges,which is mounted in” front of one telescope,is equal to a wedge having a variable angleof deviution in the plane of triangulation,but none at right angles to it.

Wedge, rotating. A circular optical wedge(primn of small refracting angle) mountedto be rotated in the path of light raV8 todivert the Mae of eight to a limited degree(see wedge, correctti, wedge, waeaswvng).

Width, face. (of a bevel). The actualwidth of a bevel rather than its widthprojec+~d along the aperture of the lew.

Width, segment. The lateral measure-ment of a mdtifocd 8egwmt at ita greatestwidth.

Window. A piece of glass with planeparallel 8zwfaces used to admit light into anoptical instrument, and to exclude dirt andmoisture.

Window, correction. Optics] wedge8 ofvery small ang{es. They admit light, sealout dirt and moisture, and are so mountedthat they may be rotated to compensate forthe accumulated errors in the entire system.Two are used as end windows on ~omerang ejh.der8.

63

MtL–STD-1241A31 March 1967

zZen- Concentric waves in a polished Zylene. Fluid used to aid inspection of

surface which appear as zones in the New- semifinished blunks.

ton’s rings when a test gl.uas is applied tothe wrface

MIL-STD-1241 A31 Morch 1967

A-UNOERCORRECTED

z

PARAXJALFOCUS

OBJECTIVE

Fmuu L DiavmmW41mting 8pherbl aiuwmlion,

65

MIL-STD-I 241Aa] Mmch 1967

GLASS-\

CEMENT

METALSASE

PLATE

1

COMPONENT

/ ‘MmAL]

LJ

MIL-STD-1241A31 mlueh B967

(a) (b)

@

HAS BARREL OR NEGATIVE DISTORTION

tS FREE FROM OISWRTION

W CUSHION OR POSITIVE DISTORTION

Fxawu8. f~sannedbyalmu.

(c)

@

67

M&STD-1 241A31 krch 1947

# PRINCIPAL PLANE

--Y

+ PRINCIPAL PLANE

%

~. ~

PRIMARY EfL g SECOttOARY EfL IFIOVU4. ?ual&nelh%P++=lw,d*-”

68

MlL-STD-l 241AS1 Murch 1%7

NODAL POINT OFINCIDENT RAY

/

N%NODAL POINT OF

EMERGENT RAY

o= op~lcAL CENTER

FmuRa a. Iuwtmlion 0) mdol $&i8t8,

69

)AIL-STD-I 241A31 krch 1967

——— — ---- -A

/“” Floum 0. Iulntmtiono) Wohm *.

70

/l

MlL-STD-l 241A31 March 1%7

LIMITING APERTURE

/

o

.

.

71

MIL-STD-1 241A31 Morth 1967

r NORMAL

AWLE OF tNCIDENCE~

\ I FN”LEo’RE’LEm’O°

OEVIATION

-\

hac58 a Iuwhtivn OJ9n@ 0/ r9$oc&m.

72

—

NORMAL

Flown 9. Ilkdmlim 0} @n#ao0/ Tcjrwtion.

MlL-ST&l 241A31 March 1967

Notice. When Government drawings, specifications, (CoPiea of this otandard for Military use may heor other data are used for any purpose other than in obtained as indicated in the foreword to, or Lhe gen-connection with a definitely related Government pro- eral provisions of, the Index of Military Specifica-curement oneration. the United States Government tions and Standards. )thereby incurs no reepormibility nor any obligationwhatsoever; and the fact that the Government may (The title and identifying symbol should be stipu-

hnve formulated, furnished, or in any way supplied lated when requesting copies of Military standards, )

the said drawings, specifications, or- other date, isnot to be regerded by implication or otherwise m inany manner licensing the holder or any other personor corporation, or conveying uty rights or permis-sion to manufacture, twe, or sell any patmttd inven-tion that may in any WDYbe related thereto.

coetlna..J :

Army—MU

NsTY-SH

Air Forcd2

Re~iew activities:

Army—ME, MU

NaTy4H

AIY Force-S2

Ueer ●ctivities:

Army—AT, AV, EL, MI

Navy—Me

Air Force-None

343-221 (-4-537 )

Preparin~ 8cthity:

Army—MU (FA)

Projeet No. 6650-0178

74

INSTR lJCTt ONS: IF, s mntin~ effofi w nMKe our ●tandardtsatton documen~ bettex, tbe DoD p~tidti W fortn for use in

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?o mire my portion of the referenced docunwnt(s) or to ●mend contmctusl rer@emont&

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