288 PHILIPS TECHNICAL REVIEW VOLUME 20

AUTOMATIC RECORDING OF LUMINOUS INTENSITY DIAGRAMS

The purpose of the instrument described hereis to automatically record the luminance patternof beamed light sources.The luminance pattern is formed of contours of

equal luminous intensity, known as isocandelacurves, drawn on graph paper. In principle, thecurves are obtained in the following manner.The light source to be measured is mounted in

such a way that it can be turned about two axesat right angles to each other; the angles throughwhich it turns on these axes (its azimuth andelevation, for example) serve as coordinates todefine the alignment of the lamp. A calibratedphotocell is fixed a suitable distance away fromthe light source. As the latter turns, therefore, the

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photocell scans through all directions in the coneof light emitted. A certain value of the luminousintensity is now chosen. When the source pointsin a direction such that the illumination of thephotocell corresponds precisely to the chosen lumi-nous-intensity value, the coordinates defining thealignment of the source will then determine onepoint on the relevant isocandela curve. Other pointson the same curve are found by turning the sourceon its two axes such that the illumination of thephotocell remains constant.

The procedure normally employed hitherto con-sists in finding, by trial and error, a sufficientnumber of such points for each luminous-intensityvalue to allow the appropriate isocandela curve

Fig. 1. Diagram and photograph of therecorder portion of the instrument. T isthe recording table and B the carriage,both being free to move at right angles toeach other on guide-rails. Tand Baremoved by the wheel L, which is drivenby a small motor N. Servo-motor Msteers L via rod S. Potentiometers P2 andP, control separate positional servoswhich communicate the displacement oftable and carriage to the mounting of thelight source under measurement.

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1958/59, No. 10 AUTOMATIC RECORDING OF LUMINOUS INTENSITY DIAGRAMS 289

Fig. 2. Light-source holder and actuating mechanism. Servo-motors M. and Me turn thesource about the vertical shaft V and the horizontal shaft H respectively. Ma and Me arecontrolled by Pz and P, (shown in fig. 1) by means of the balancing potentiometers Q2 andQ, respectively.

to be drawn manually with reasonable accuracy.This is clearly a time-consuming business, particular-ly if the number of curves required is large; more-over, a curve drawn thus will only coincide exactlywith the true isocandela curve at the points actuallymeasured.The isocandela-diagram recorder developed in the

Philips Research Laboratories for the PhotometricLaboratory of our lamp factories is open to neitherof these disadvantages. In this instrument the lightsource moves automatically and in such a way that,within a specified margin of error, the illuminationof the photocell always remains at the same, pre-setvalue. Briefly, the design and functioning of theinstrument are as follows.The current from the photocell is compared with

a reference current, the resulting difference signalbeing used to control a servo-motor (M in fig. 1).In the absence of a difference signal, i.e. when theintensity of illumination of the photocell has thepre-set value, the servo-motor remains unenergized.When the illumination is too high, the shaft of themotor turns in one sense; when it is too low, the shaftturns in the opposite direction. The rotation of theshaft is transmitted via a reduction gear to steeringrod S. This carries a holder in which are suspendeda drive wheel L and its motor N. Both the recordingtable T and the servo-motor carriage B can movefreely at right angles to each other on guide-rails.They are moved by the drive wheel as a result ofthe friction between the latter and the recordingtable. Each movement is communicated to themounting of the light source under measurementby positional servo systems (here in the form of

slide-wire potentiometers controlling servo-motors).The two shafts about which the light source turnsare driven by the servo-motors (Ma and Me infig. 2) of the two servo systems. At any instant,therefore, the light source is always pointed in adirection (azimuth and elevation) corresponding tothe position of table and carriage.

For the purpose of tracing an isocandela curvethe drive-wheel motor N is supplied with a constantcurrent, with the result that the drive wheel has aconstant peripheral velocity. This velocity resolvesitself into movements of carriage B and registrationtable T, each running along its respective guide-rails.If at any instant the intensity of illumination on thephotocell has the pre-set value, no current passesthrough the servo-motor that actuates steering rodS; in consequence of this the path of the drive wheelis straight with respect to the table - that is tosay, table and carriage move in such a way that thewheel traces a straight line on the table. The move-ments of carriage and table alter the alignment ofthe light source, and if as a result of this the intensityof illumination of the photocell undergoes a change,then the drive wheel ceases to run straight; at anygiven instant, the current flowing through servo-motor M has a value such that the steering rod anddrive wheel are turned to left or right in accordancewith the shape of the isocandela curve being deter-mined. The intensity of illumination of the photocellis thereby restored to its original value, and in virtueof this continuous correction the steering rod isalways "on course". A stylus fixed to the servo-motor carriage maps out the curve on graph paperfixed on the recording table. (Since all points on the

290 PHILIPS TECHNICAL REVIEW VOLUME 20

carriage generate curves of identical size and shapewith respect to the table except for a constantdisplacement, it is possible to fix the stylus to anyconvenient point on the carriage.)A set of isocandela curves obtained in this manner

appears in jig. 3. As will be clear from the above

interesting problems mainly concerned with stabilityof the movement. Inspection of the differentialequation describing the behaviour of the system

. shows that from the viewpoint of stability the pointof contact of the drive wheel L should not, as mightbe supposed, be placed directly in line with steering

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Fig. 3. Isocandela curves recorded by the instrument described in the text. The curves referto the lamp - a projector for airfield beaconing - shown in the holder in fig. 2.

description and from this example, it is possiblewith the new instrument to trace curves formingclosed loops - the luminance curves ofbeamed lightsources are almost all of this form. The use of theinstrument represents a very great saving of timecompared with plotting and drawing the curves byhand.

In passing, it may be observed that, combinedwith suitable measuring and scanning apparatus, therecorder portion ofthe apparatus (the part in fig. 1)can be used for the automatic tracing of all kindsof other "iso" curves, such as the curves of equalpotential in an electrolytic-tank model, for example.

The designing of the instrument involved various

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rod S (see fig. I); instead, it should have a smalloff-set d from the axis of rod S. Good results areobtained by satisfying the condition

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in which v is the constant peripheral velocity of thedrive wheel, referred to above, and k is a coefficientrelated to the loop gain of the servo system. Thiscondition and a number of constructional detailswill be discussed at greater length in a fuller des-cription of the instrument that will appear in thesepages in due course.

W. BÄHLER.