Factors That Determine Visual Acuity Through Night ... - DTIC

USAARL Report No. 94-16

AD-lA279 39

Factors That Determine Visual AcuityThrough Night Vision Goggles for Emmetropes

DTICmo ELECTE

By ziMAY 1 894

John C. Kotulak

and DTIGQu3. i -I

Stephen E. Morse

Aircrew Health and Performance Division- tD-- I

_)0 April 1994

Approved tr puMbic release; dWbut nl mked.

94 5 17 105United States Army Aeromedical Research Laboratory

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Human use

Human subjects participated in these studies after giving theirfree and informed voluntary consent. Investigators adhered to AR70-25 and USAMRDC Reg 70-25 on Use of Volunteers in Research.

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(U) Factors that Determine Visual Acuity Through Night Vision Goggles for Emetropes12. PERSONAL AUTHOR(S)

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17. COSATI CODES IS. SUBJECT TERMS (Continue on reerset if necessry and identify by block number)FIELD GROUP Sue-GROUP Night vision goggles, visual acuity, esmetropia,

20 06 refractive error23 02 1

19. ABSTRACT (Continue on roevm if necenary and identit by block number)The purpose of this study was to investigate factors which could affect the visual acuityof emmetropes (individuals without clinically significant refractive error) while viewingthrough night vision goggles, namely unaided visual acuity, clinically insignificantrefractive error, and experience as a visual observer. We found that night vision gogglevisual acuity is related to unaided visual acuity for emmetropes when the night visiongoggle eyepieces are focused to infinity, and that this relationship is robust with respectto changes in target contrast. We also found that both the unaided and night vision goggle(infinity focus eyepiece) visual acuity of nominal emetropes is related to uncorrectedrefractive error, and that experience as a visual observer is significantly related tonight vision goggle visual acuity. The implication of these results is that many aviatorswho are medically cleared to fly without glasses will suffer from reduced vision throughany night vision device with a fixed infinity focus eyepiece. Similarly, ground vehicleoperators who have astigmatism and who must use spectacle incompatible night vision goggles

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Acknowleduments

The following personnel from the U.S. Army Aeromedical ResearchLaboratory are thanked for their contributions to this study:SPC Mark A. Kenzie, LTC James M. King, Dr. William E. McLean,LTC Jeffrey C. Rabin, Dr. Robert W. Verona, and Dr. Roger W.Wiley.

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Table 2f contents

List of figures ............................................. 2

List of tables .............................................. 2

Introduction ................................................ 3

Methods ..................................................... 5

Subjects .......... . ....... ..... ...... . ................. 5

Apparatus ................................................. 6

Procedures ................................................ 7

Design and statistical analysis ........................... 7

Results ..................................................... 9

Bivariate relationships ........................................ 9

Multivariate relationships ................................ 10

Fixed infinity focus aided visual acuity ................ 11

Aided visual acuity for high contrast targets ......... 11

Aided visual acuity for low contrast targets .......... 14

User adjusted focus aided visual acuity ................. 14

Discussion .................................................. 14

References .................................................. 17

1. Relationship between resolving power of systemcomponents to that of the combined system ............ 4

2. ANVIS modulation transfer function ...................... 4

3. Distribution of uncorrected unaided high contrastvisual acuities ...................................... 8

4. Distribution of uncorrected unaided low contrastvisual acuities ...................................... 8

5. Relationship between unaided visual acuityand astigmatism for nominal emmetropes ............... 9

6. Relationship between aided visual acuityand sphere for nominal emmetropes .................... 10

7. Relationship between aided and unaided acuityfor eametropes ....................................... 11

8. Relationship between aided and unaided acuity compar-ing observed to predicted results .................... 15

Lit 2f tables

1. Descriptive statistics of subjects ...................... 5

2. Target parameters ....................................... 7

3. List of candidate independent variables to predictaided visual acuity (high contrast) wheneyepiece is focused at infinity ...................... 12

4. List of candidate independent variables to predictaided visual acuity (low contrast) whenthe eyepiece is focused at infinity .................. 12

5. List of candidate indepandent variables to predictaided visual acuity (high contrast) whenthe eyepiece is focused for best vision .............. 13

6. List of candidate independent variables to predictaided visual acuity (low contrast) whenthe eyepiece is focused for best vision .............. 13

2

Introduction

Many factors which could affect visual acuity (VA) withnight vision goggles (NVGs) already have been studied, e.g.,night sky condition and target contrast (Levine and Rash, 1989aand 1989b; Wiley, 1989; Kotulak and Rash, 1992), NVG generation(Miller et al., 1984; Kotulak and Rash, 1992), nuclearflashblindness protection (Levine and Rash, 1989a and 1989b),chemical protective masks (Miller et al., 1989; Donohue-Perry,Riegler, and Hausman, 1990), signal-to-noise ratio (Riegler etal., 1991), interpupillary distance misadjustment (King andMorse, 1992), and instrument myopia (Kotulak and Morse, 1992,1994a, and 1994b; Kotulak, Morse, and Wiley, 1993). Anotherfactor which could influence NVG VA is decreased unaided VA,i.e., VA without NVGs; however, relatively little is known aboutit.

Kim (1982) investigated the influence of astigmatism on NVGVA; however, he did not report the unaided VA of his subjects.Hoover (1983) measured both unaided and aided VA; however, mostof Hoover's subjects suffered from vision loss due to eyedisease. Therefore, it is not certain whether Hoover's resultsare relevant to healthy populations.

In the current report, we present measurements of bothunaided and aided VA on healthy, emmetropic subjects in order todetermine whether there is a correlation between the two. Thetheoretical basis for such an association comes from thefollowing: When two optical systems of unequal resolving powerare combined, the resolution of the combined system can bepredicted by the equation below, in which RH and RL represent theresolving powers of the high and low resolution elementsrespectively, and Rc represents the resolving power of thecombined system (Farrell and Booth, 1984).

1 1 1

RC"1 7 RN'7 RL1.7

An observer viewing through NVGs can be thought of as such asystem, in which the eye is the high resolution element when theobserver is emmetropic. Figure 1 is derived from the aboveequation by holding RL constant at 20 cycles/degree (cpd), theapproximate resolution limit of current NVGs (Figure 2), andvarying RH over a wide range. The equation predicts that theresolving power of the combined system is affected by changes inR., especially in the region at and below the eye's maximumresolution, which is approximately 40 cpd.

3

225Resolving power

E of low resolution element2Is 20 cpd (dotted Ine)

0- .

0

0C.m 15.C

01P

10L : : : ' I " ,

0 20 40 60 80 100 120Resolving power of high resolution element (cpd)

Figure 1. The relationship between the resolving power of thehigh resolution element and the combined system, giventhat the resolving power of the low resolution elementis held constant. This model was derived from experi-ments with photographic systems (Farrell and Booth,1984).

1.0k0-0 9.7 X 1o-3 f.

0.8 - & X. x10-4 f•.a --A 1.7 X 10-4 f."Ea A-A 1.4X 10-4 ftLC

o° 0.6--

0.4-

"00

S0.2 oA-A-A- A

0.0 I I

0 5 10 15 20 25

Spatial frequency (cycles/degree)

Figure 2. The ANVIS spatial modulation transfer function undervarying levels of ambient luminance (Kotulak andMorse, 1994).

4

In this report, we also explore other factors which couldinfluence NVG VA among emmetropes, namely refractive error andexperience as a visual observer (flight experience and NVGexperience). In the strictest sense, refractive error andemmetropia are mutually exclusive. However, emmetropes arecommonly defined clinically as persons who have a distance VA ofat least 20/20 in each eye, a condition which does not precludesmall refractive errors (Hirsch, 1945).

Subjects

Sixteen volunteer subjects, who were either U.S. Armyaviators (n - 12) or flight school students (n = 4), wererecruited for the experiment. All subjects had unaided visualacuities of at least 20/20 in each eye, and were free from eyedisease and other ocular anomalies. All of the subjects werecleared to fly without spectacles. Table 1 gives descriptivestatistics regarding age, flight and NVG experience, andrefractive error for the subjects. The refractive error dataprobably overestimate the degree of myopia by about 0.25 diopters(D) due to instrument myopia elicited by the autorefractor (Miwa,1992).

Table i.Descriptive statistics of subjects.

Variable Mean SD Median Range

Age (years) 27.1 4.9 27.0 22 to

Total flight 988.1 1347.2 300.0 68 tohours 4000

Flight hours 84.7 139.1 21.0 0 towith NVGs 500

Equivalent -0.35 0.37 -0.44 -0.88sphere (D) to 0.50

Sphere (D) -0.16 0.39 -0.25 -0.63to 0.63

Cylinder (D) -0.39 0.26 -0.25 -0.13to_-1.00

5

Apparatus

The NVG used in the study was the AN/AVS-6 Aviator NightVision Imaging System {ANVIS} (Jenkins and Efkeman, 1980). ANVISis a unity-magnification pair of binoculars which electronicallyamplify ambient light and thus provide photopic vision undernight sky conditions. ANVIS consists of two identicalmonoculars, the main components of which are an objective, athird-generation image intensifier, and an eyepiece. The ANVISmodulation transfer function (Figure 2) demonstrates that thephosphor image is spatially lowpass filtered. As a result, VAwith ANVIS under optimum conditions is only 20/35, and it getsworse with decreasing night sky luminance (Kotulak and Rash,1992). The output luminance of ANVIS falls off steadily withdecreases in input luminance when the latter is less than quartermoon, the lower limit of the ANVIS automatic gain control. Thisallows the ANVIS display luminance to be manipulated as anexperimental variable.

The visual stimuli were high (Bailey and Lovie, 1976) andlow (Bailey, 1982) contrast Bailey-Lovie acuity charts. Twoversions of the chart, differing only in letter sequence, wereused at each level of contrast. These charts were chosen becausetheir scale is five times finer than that of Snellen-like charts,and their test-retest reliability is twice as great (Bailey etal., 1991). In addition, Bailey-Lovie charts incorporate anequal-interval scale that permits the use of parametricstatistics (Lovie-Kitchin, 1988).

The contrast of the Bailey-Lovie optotypes was calculatedfrom the equation below, in which LB and LL represent backgroundand letter luminance respectively.

100 (LB - LL)LB

Photometrically-measured luminance was used to calculate targetcontrast, both on the NVG phosphor screen under simulated nightsky conditions (labelled "Aided" on Table 2), and on thecharts themselves under photopic conditions (labelled "Unaided"on Table 2). Table 2 also gives the background luminance (i.e.,the luminance of the white portion of the chart) for the aidedand unaided conditions.

6

Table 2.Target parameters.

Aided UnaidedParameter High Low High Low

Contrast (percent) 62 12 98 21

Luminance (cd/m2) 6.5 6.5

Procedures

VA always was measured under binocular conditions. The samecharts were used for aided and unaided viewing. VA thresholds,which were defined as the common logarithm of the minimum angleof resolution {log MAR} (Bailey and Lovie, 1976), were recordedusing Bailey-Lovie scoring procedures (Bailey and Lovie, 1976),without a time limit, and without reinforcement. Contrastchanges were made by switching between charts. The order ofpresentation of the stimuli was randomized.

Prior to making focus adjustments, the subjects were trainedto reach a most-plus endpoint, i.e., use the most plus (or leastminus) dioptric power that was required for best vision. This isconsistent with established clinical technique for refraction.Eyepiece power was verified with a dioptometer. Refractive errorwas measured objectively with an autorefractor.

Design and statistical analysis

The dependent variables were high and low contrast aided VAmeasured with the NVG eyepieces focused at infinity, and high andlow contrast aided VA measured with the NVG eyepieces focused bythe users for best vision. The design was within subjects. Thecorrelation of the dependent variables with various candidateindependent variables was tested by simple and multiple linearregression. The independent variables were high and low contrastunaided VA, total flight hours, NVG flight hours, and refractiveerror. Three refractive error components were consideredseparately, i.e., sphere, cylinder, and equivalent sphere (one-half the cylinder power plus the sphere power).

7

8

High contrast ' - 16" - -0.095 (20/16)

~ 6 s - 0.088

0 4

Ec2

, 2- -

0._-0.25 -0.15 -0.05 0.05 0.15

Cell midpoints (log MAR)

Figure 3. The distribution of uncorrected unaided visual acu-ities to high contrast letters for nominal emmetropes.

6

Low contrast n - 16

_x - 0.16 (20/29)

9- 0.070

04E 2C oI

. .�2•0E 2

C

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

Cell midpoints (log MAR)

Figure 4. The distribution of uncorrected unaided visual acu-ities to low contrast letters for nominal emmetropes.

8

-, 0.4 50

M 0.3 R 0.5102 p 0=.05

0~

0 0.2 .•"n n16

Ž0.1 0 .

o 3 0D 0.0-- 20 v)2-,5= -0.1 VQ)--D'-• C>

S-0.2 Contrast= 98% c C

-0.3 Luminance = 6.5 cd/m 2 10-0.4I I I I 8

0.00 0.25 0.50 0.75 1.00 1.25

Astigmatism (diopters)

Figure 5. The relationship between uncorrected unaided VA andthe absolute value of the astigmatic component of therefractive error for nominal emmetropes. The rela-tively low chart luminance, which was intended tomatch that of the NVG display, resulted in elevatedacuity thresholds.

Bivariate relationships

Figures 3 and 4 give the distributions of unaided VAs forour nominally emmetropic subjects at high and low targetcontrasts, respectively. Note that in Figure 3, two subjects hadVAs less than 20/20. This was most likely due to the testluminance of 6.5 cd/rn (Sheedy, Bailey, and Raasch, 1984), whichis considerably lower than the 85 cd/r that is recommended forthe clinical measurement of VA (National Research Council, 1979).The luminance of 6.5 cd/r was selected because it matched theANVIS display luminance (Table 2).

The variability in VA among emmetropes is due, at leastpartially, to uncorrected refractive error, as shown in Figure 5(see also Table 1). Figure 5 demonstrates that unaided VA tohigh contrast letters is correlated with the amount ofastigmatism. Similarly, the ANVIS VA of emmetropes also can berelated to uncorrected refractive error. For example, Figure 6

9

1.0 200X R - 0.53

p = 0.030.9 .-

0 n=16 50

00o 0.8 c

C:0.8-- °3-

-o 0 Contrast= 12% -100<

Luminance = 6.5 cd/m 2 90

0.6 I I 800.00 0.25 0.50 0.75

Sphere (diopters)

Figure 6. The relationship between uncorrected NVG VA and theabsolute value of the spherical component of the re-fractive error for nominal emmetropes. The NVG eye-pieces were focused to infinity.

reveals that aided VA is correlated with the power of thespherical component of the refractive error when target contrastis low and the instrument eyepieces are focused to infinity.

Figure 7 demonstrates that there is also a correlationbetween aided and unaided VA for emmetropes when the eyepiecesare focused at infinity. This relationship is statisticallysignificant at both high (R = 0.73, p = 0.001) and low (R = 0.61,p = 0.01) contrast. However, the relationship ceases to besignificant when the focus is adjusted by the user for bestvision (R = 0.18, p = 0.5 at high contrast; R = 0.39, p = 0.13 atlow contrast).

Multivariate relationships

Multiple regression was used to predict aided VA by buildinga model which includes only those independent variables that addmarkedly to the strength of prediction. Tables 3 and 4 list thevariables that were tested as potential predictors of aided VAfor the fixed infinity focus condition, and Tables 5 and 6 listthe variables that were tested for the adjustable focus

10

Unaided visual acuity (Snellen denominator)10 20 30 40 50

1.0. 200

0.9 1 0 High contrast0 Low contrast 0

0.8- 0

o~ ~ .•• .S0.7- 100o a

So, 500 0.

S0.5- 0 .~C

00 6)6> 0.-- 050 r

0.2I I I32-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4

Unaided visual acuity (log MAR)

Figure 7. The relationship between uncorrected NVG VA and un-corrected unaided VA for nominal emmetropes. The NVGeyepieces were focused to infinity. Table 1 gives thevalues for high and low contrast for the aided and un-aided VA measurements.

condition. In these tables, partial correlation is equivalent toPearson's R in simple linear regression, and F-to-enter is tv'etest statistic for determining whether R is significant.

Fixed ifniyfocus condition

Aided visual acuity for high contrast targets

The list of candidate independent variables to predict aidedVA for high contrast targets is given in Table 3. The variablesthat were selected from this list are given by the equation be-low, in which AA represents aided acuity, Au represents unaidedacuity (high contrast), and NL represents the log of NVG flighthours (a log transform was performed because the distribution ofNVG flight hours was asymmetric).

AA - 0.50Au - O.O8NL + 0.58

The relative contribution of each independent variable to themodel can be inferred from the percent of variance explained.Unaided VA, the most predictive variable (highest F-to-entervalue in Table 3), alone explained 37 percent of the variance of

211

aided VA, i.e., R2 - 0.37. The combination of unaided VA and logNVG hours explained 60 percent of the variance of aided VA, i.e.,Re - 0.60. Thus, the addition of log NVG hours to the modelincreased the prediction of the dependent variable by 23 percent(60 - 37 - 23).

Table .List of candidate independent variables to predict aided

visual acuity (high contrast) when the eyepieceis focused at infinity.

Variable Partial F-to-enter

correlation

Unaided VA 0.61 6.50

Equivalent sphere 0.49 3.45

Sphere 0.39 1.99

Cylinder 0.41 2.16

Flight hours -0.32 1.25

Log flight hours -0.38 1.84

NVG hours -0.49 3.40

Log NVG hours -0.59 5.97

Table j.List of candidate independent variables to predict

aided visual acuity (low contrast) whenthe eyepiece is focused at infinity.


correlation



Sphere 0.42 2.14

Cylinder 0.53 4.30




Loa NVG hours -0.43 2.45

12

Table I.List of candidate independent variables

to predict aided visual acuity (high contrast)when the eyepiece is focused for best vision.


correlation



Sphere 0.18 0.37

Cylinder 0.21 0.53

Flight hours 0.09 0.09

Log flight hours 0.06 0.04


Log NVG hours -0.32 1.26

TableList of candidate independent variables to predict

aided visual acuity (low contrast) whenthe eyepiece is focused for best vision.

Variable Partial F-to-entercorrelation


Equivalent sphere -0.09 0.08

Sphere 0.22 0.58

Cylinder 0.17 0.33




SLog NVG hours -0.11 0.14

13

I I II

Aided visual acuity for low contrast targets

The list of candidate independent variables to predict aidedVA for low contrast targets is given in Table 4. The variablesthat were selected from this list are given by the equationbelow, in which AA represents aided acuity, Au represents unaidedacuity, and FL represents the common logarithm of total flighthours (a log transform was performed because the distribution oftotal flight hours was asymmetric).

AA = -0.0 4 FL + 0.34Au + 0.82

The most predictive variable was log flight hours (highest F-to-enter value in Table 4), whicP alone explained 36 percent of thevariance of aided VA, i.e., R - 0.36. The combination of logflight hours and una.ded VA explained 53 percent of the varianceof aided VA, i.e., R - 0.53. Thus, the addition of unaided VAto the model increased the prediction of the dependent variableby 17 percent (53 - 36 = 17).

adr stedfocus aided codt

When the NVG focus was adjusted by the user for best vision,aided VA was not predictable by any of the candidate independentvariables. This was true at both levels of contrast (Tables 5and 6).

We found that the between-subject variations in unaided VAof nominal emmetropes do manifest themselves as correspondingfluctuations in aided VA when the NVG eyepieces are focused atinfinity. This effect is robust with respect to changes intarget contrast. However, the effect diminishes significantlywhen the eyepieces are focused by the user for best vision. Thissuggests that the relationship between unaided and aided VA amongeumetropes is mainly due to an optical factor, e.g., clinicallyinsignificant refractive error.

Multiple regression revealed that, when the eyepieces werefocused at infinity, unaided VA and experience as a visualobserver (i.e., log NVG hours and log flight hours) wereimportant determinants of NVG VA. At high contrast, unaided VAexplained the greatest proportion of the variance of NVG VA. Atlow contrast, total flight hours explained the greatestproportion of the variance of NVG VA. This suggests thatexperience as a visual observer is more important under degraded

14

stimulus conditions than it is under optimal conditions.Refractive error was not selected for any of the multipleregression models although it is related to aided VA (Figure 6).This is because refractive error does not explain any of thevariability of NVG VA that is not already explained by unaidedVA.

Our data on the relationship between unaided and NVG VAamong emmetropes is consistent with data from other studies inwhich the subjects had reduced unaided VA either due toastigmatisa. (Kim, 1982) or to eye disease (Hoover, 1983). Thedata from all three studies are fit well by the same regressionline (R - 0.87) (Figure 8). Since there appears to be nosignificant difference between Kim's data from ametropes andHoover's data from visually impaired subjects, perhaps the sourceof reduced unaided VA is not important in predicting aided VA.

Unaided visual acuity (Snellen denominator)

8 10 20 501.1 I . m - 0

<tu1 200CP I0- m (1982) -- vuavgmata _,w

0 .

S0.9. E ovri8)vulkpu

Z 0.5 0

0 - 50 'D

4 "0.3

0.1 I I I 25

-0.40 -0.20 0.00 0.20 0.40 0.60

Unaided visual acuity (log MAR)

Figure 8. The relationship between NVG to unaided VAs, comparingobserved to predicted results. A simple mathematicalmodel seems to agree well with laboratory data fromthree independent studies.

Because Kim did not report unaided VA, we converted his measuredastigmatism data to unaided VA based on the known relationshipbetween the two (Peters, 1961). We controlled for between-studydifferences in NVG generation by comparing VAs from infinityfocus third generation NVGs to VAs from adjustable focus secondgeneration NVGs, because VAs have been shown to be similar underthese two conditions (Kotulak and Morse, 1994a). In addition, we

15

modified the exponents of the equation described in theintroduction (Farrell and Booth, 1984) to obtain a better fit ofthe data, i. e.,

1 1 1Rc1' Rj'' RL"''

As can be seen in Figure 8, the predictions based on the modifiedequation are in close agreement with the observed results. Thissuggests that the Farrell and Booth resolution model isapplicable to the eye-NVG system with only minor modification.Additional work needs to be done to determine the relationshipbetween NVG and unaided VA for subjects with unaided VAs beyondthe range of Figure 8.

The military significance of the present work lies withnight vision devices which are either not spectacle compatible orwhich have a fixed focus eyepiece. An example of the former isthe full faceplate AN/PVS-5 NVG that is used for ground troops,and an example of the latter is the helmet mounted display thatis under development for the Comanche helicopter. The AN/PVS-5has adjustable focus eyepieces, which when set properly, com-pensate for spherical refractive error (i.e., simple myopia orhyperopia) but not for astigmatism. The Comanche helmet mounteddisplay will be spectacle compatible, but will have eyepieces inwhich the focus is fixed at infinity. The results of this study,wnether considered alone or with the works of Kim (1982) andHoover (1983), suggest that for either type of device anydecrement in unaided VA produce3 an analogous loss in NVG VA.

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Donohue-Perry, M. M., Riegler, J. T., and Hausman, M. A. 1990.& compatibilitvy ofthesag2Liitegratedhood akw y=h ANVIS night uisin goggles. Wright-PattersonAir Force Base, OH: Armstrong Aerospace Medical ResearchLaboratory. Report No. AAMRL-TR-90-030.

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Jenkins, D., and Efkeman, A. 1980. Development of an aviator'snight vision imaging system (ANVIS). In Ontomechanicalsstemu desaign, 18-23. SPIE Vol. 250. Bellingham, WA.

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Kotulak, J. C., and Morse, S. E. 1994a. The effects of focusadjustment on visual acuity and oculomotor balance withaviator night vision displays. Aviai, apc andenvironmental medicj. 65:348-352.

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Kotulak, J. C., Morse, S. E., and Wiley, R. W. 1994. Theeffect of knowledge of object distance on accommodationduring instrument viewing. •Pjgjo. In press.

Kotulak, J. C., and Rash, C. E. 1992. Visual Acui N = secondAnd third genf2ratio night vison goggles. o b ined anfe methd 21 onIght ak simulatin2, across g wide rainge 21target . Fort Rucker, AL: U.S. Army AeromedicalResearch Laboratory. USAARL Report No. 92-9.

Levine, R. R., and Rash, C. E. 1989a. Visual Aut withALNI&S nig=yision goggles qnd simulatnd flashblindness2 lensne under varyinglevels 2f1 brigtnes andcna. Fort Rucker, AL: U.S. Army Aeromedical ResearchLaboratory. USAARL Report No. 89-16.

Levine, R. R., and Rash, C. E. 1989b. Atfltpn fig the luminouO t of the NLPvs-5A night vision goggj• and itsafts2ao visual ac y. Fort Rucker, AL: U.S. ArmyAeromedical Research Laboratory. USAARL Report No. 89-24.

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Miller, R. E., Woessner, W. M., Wooley, L. M., Dennis, R. J., andGreen, R. P. 1989. ComDatibilityo 1 nig•t vision gogglesa cfemical wrtr masks. Brooks Air Force Base, TX:U.S. Air Force School of Aerospace Medicine. Report No.USAFSAM-TR-89-3.

Miwa, T. 1992. Instrument myopia and the resting state ofaccommodation. Q~ ty Andn s ngyjjo . 69:55-59.

1s

National Research Council. 1979. Recommended standardcedures = Jr il measurement and specification ofisual acuity. Washington, DC: Committee on Vision,

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Riegler, J. T., Whiteley, J. D., Task, H. L., and Schueren, J.1991. Th effect f sianal-to-noise ratio 2n visualacuit • throuha niht vision gggales. Wright-PattersonAir Force Base, OH: Armstrong Laboratory. Report No. AL-TR-91-0011.

Sheedy, J. E., Bailey, I. L., and Raasch, T. W. 1984. Visualacuity and chart luminance. A Journal of optometryAnd Dhvsioloaical optics. 61:595-600.

Wiley, R. W. 1989. Visual acuity and stereo2sis with nightision agoggles. Fort Rucker, AL: U.S. Army Aeromedical

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Factors That Determine Visual Acuity Through Night ... - DTIC

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