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SSD Simulator Validation 

by 

Gemini Visual Acuity Experiment 

Barry Siegel 

12 June 1967 

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ABSTRACT 

A test to determine if proper contrast levels were being used in the SSD simulator 

was performed, based on the Gemini program visual acuity test. Due to the limited 

number of trials no statistically useful information was obtained and therefore 

validation of simulator contrast levels was not possible. 
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I. OBJECTIVE 

To compare visual acuity in space with that obtained on the SSD simulator 

for the purpose of validating that simulator contrast levels are appropriate. 

II. BACKGROUND 

The SSD simulator uses a flooding light to decrease the contrast of the 

photographic materials used. The intensity of flood illumination used has been 

determined through computations based on measurements of edge targets and the 

use of a model atmosphere. It has been requested that this approach be validated 

by a limited experimental activity. The Gemini viewing experiment, which was 

suggested by Dr. Duntley of the Visibility Laboratory, is an attempt to provide 

such a validation. 

III. EXPERIMENT CONCEPT 

The Gemini program had a visual acuity experiment whose primary objectives 

were to determine: 

1. the effect of the space environment on visual acuity 

2. visual acuity from space against ground objects 

Visual acuity was measured by testing the astronauts against a field of 

decreasing length rectangular bars displayed in various orientations. The astronauts were tested both on the ground and in space and it was determined that no 

change in visual acuity occurred as a result of being in the space environment. 

To evaluate the second objective the astronauts viewed a ground representation 

of the test field constructed near Larado, Texas (another field was constructed 

near Carnevon, Australia, but never used). The astronaut scores on this test 

indicated that there was no reason not to believe that visual acuity from space 

against ground objects could not be predicted through ground testing, if properly 

simulated viewing conditions were provided. It should therefore be possible to 

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determine if proper viewing conditions are being simulated at SSD by testing one 

of the astronauts against the bar target field as viewed through the SSD simulator. 

If his scores on this test are within his predicted performance envelope, there 

should be no reason for not believing that the SSD simulator is providing proper 

viewing and contrast conditions. 

IV. EXPERIMENTAL PROCEDURES AND EQUIPMENT 

A. Subject 

We would have wished to test Gemini astronaut Borman, who made 

the visual report on the Larado field in Gemini 7, but he was not available. We 

were able however, to use R. L. Stapleford of Visibility Laboratory for whom 

ground test data and therefore predicted performance curves were available. 

B. Material 

From aircraft photographs of the Larado site the Visibility Laboratory 

made eight slides, each with a field of eight bar targets. One set of four slides had 

a higher contrast than the other set. This document reports on test results using 

the low-contrast slides only. A diagram of a typical slide with the orientation and 

bar number convention used is shown in Figure 1. 

C. Simulator Setup 

Nominally, the SSD simulator has a magnification of 7. 5x which 

can be step changed to 15x. However, even at 7. 5x, the required slide dimensions 

to insure that the proper scene scale was provided at the eye were too small to 

permit slide manufacture. The simulator eyepiece was therefore changed to provide only 1. 5x and the test was run at this magnification. A biocular eyepiece 

was used throughout the test. 

D. Test Conditions 

1.) Two flood illumination conditions were run, namely 22 and 

78.5 foot-lamberts as measured through the left eyepiece with the table light 

off. The first level corresponds to the estimated setting needed to provide proper 

contrast levels and has been used throughout our previous tests. 

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2.) The table illumination was measured as 300 foot-lamberts 

through the left eyepiece and 140 foot-lamberts through the right eyepiece. 

3.) The estimated transmission of the slide background is 5%. 

This estimate was obtained by measuring the table illumination coming through 

the left eyepiece, with the slide in place and with the slide removed. 

4.) The subject was allowed a thirty-second response time for 

each slide. (For most slides, the subject required less than 15 seconds to 

respond.) 

E. Test Procedures 

All slides were run at a single floodlight setting, and then the flood 

setting was changed and the slides were run again. The set of low contrast 

slides were run first, and then the high contrast set. 

V DATA ANALYSIS AND REDUCTION 

A. Bar Contrast and Area 

Previous ground tests from which the subject's performance 

envelope was determined were run against bars whose length was four times 

their width. When the Larado field was constructed, it was determined that if 

4/1 bars were used, the contrast at the astronaut's eye in the spacecraft would 

be much greater than tested on the ground. Since the bar width was not resolvable from space, it was decided to decrease the width of the Larado bars and 

thereby decrease the apparent contrast of the bars as viewed by the astronaut 

from space. Table I compares the Larado bars with 4/1 bars. 

If densitometer measurements are made of the slides used in the 

SSD test, the total illuminance of each bar can be determined. Unfortunately, 

blow-ups of the slides show that the bars are not neat rectangles, but somewhat irregular ellipses. If it is assumed that the eye will see these ellipses 

as bars of the proper length, but unresolvable in width, then the total illuminance 

can be assumed to emanate from the actual rectangular Larado bar. Under this 

assumption, the average film contrast of the bars on the low contrast slide is 

7.16. When these bars are viewed in the simulator, the eye "spreads" the 

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bar width so that the bar area "looks" larger than it is, and in fact looks like 

a 4/1 bar at reduced contrast. The reduction in contrast is the ratio of the 

actual bar width to the width required to produce a 4/1 bar. Column 6 of Table 

I shows the contrast at the eye for each bar as viewed through the simulator 

with no flood illumination. 

The contrast at the eye with flood illumination can be computed 

for any bar as, 

C - B

o 

+ Bf 

where Q = Bar contrast with no flood illumination 

B

o 

= Background illumination (15 ft-lamberts for the left 

eyepiece) 

B

f = Flood illumination 

The contrast values for each bar for the two flood settings are 

shown in Columns 7 and 8 of Table I. 

B. SCORING 

To be compatible with the predicted performance data, we wish to 

determine from the raw data the bar at which, with high confidence, the probability of correctly determining bar orientation is .5. 

Table II shows the corrected scores for each bar. The correction, 

which deweights the extremes, is computed from 

Corrected Mean = X - 25 

100 - 25 

where X = the raw score mean 

If the data were Gaussian, a least squares straight line approximation should very nearly fit a cumulative probability curve based on the 

corrected data. It is apparent from Table II that a least squares straight line 

is a poor fit to the data, and therefore the scoring results are not Gaussian. 

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This result is undoubtedly due to the limited number of trials on each bar (4). 

A chi-square test was run which indicates the deviations of scores are not 

statistically significant--a result which is again due to the limited sample size. 

VI RESULTS AND CONCLUSIONS 

Figure 2 shows the contrast at the eye of the bars as a function of bar 

area for each of the floodlight settings used. In addition, the estimated bar 

contrast at the eye of Astronaut Borman during the Gemini overflight of the 

Larado site is also shown. It can be seen that the 22 foot-lambert flood setting, 

which has been used throughout our previous tests, happens to match the estimated bar contrast as viewed from space during the Gemini Test. If the slide 

and the material we have used previously were processed identically, we could 

conclude that this flood setting was not unreasonable for use. 

Statistically significant scores were not obtained, and therefore we 

cannot judge from this test the validity of the simulated contrast levels being 

used in the SSD simulator. 

DISTRIBUTION 

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