Optical and photometric properties of lunar surface and implications for human visual performanc

Taylor, J. H.

English

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UNIVERSITY OF CALIFORNIA, SAN DIEGO 
SCRIPPS INSTITUTION OF OCEANOGRAPHY 
VI  SI BI L I T Y  L A B 0  R A T 0  RY 
SAN DIEGO, CALIFORNIA 92152 
VISUAL PERFORMANCE ON THE MOON 
John H. Taylor 
SI0 Ref. 67-3 
January 1967 
Preprint of a paper given at the XVIIth Congress of the International 
Astronautical Federation, Madrid, Spain, 13 October 1966. Prepared 
under NASA Grant NGR-05-009-059 t e t h e  University of California. 
https://ntrs.nasa.gov/search.jsp?R=19670008756 2020-03-16T19:00:12+00:00Z
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Visual Performance on the Moon 
John H. Taylor 
Introduction 
This paper will deal with the properties of the lunar optical and visual environment a s  they 
may influence man’s ability to  function on the moon’s surface. A s  more and better quantitative 
data are gathered from the Luna, Ranger, and Surveyor programs w e  are increasingly able to es- 
timate illumination, brightness and contrast levels which will confront those lunar explorers and 
working scient is ts  whose visual performance will be of great importance to the success  of the 
venture. These new data a l so  enable u s  to design specific laboratory and field experiments to 
evaluate the effects of lunar environmental factors upon human vision, and may lead to  special  
operational techniques or optical aids to enhance the safety and productivity of the  mission. I 
will limit my remarks to phenomena related to the visible part of the electromagnetic spectrum 
(380-750 nanometers), assuming that adequate protection from stressful amounts of other wave- 
lengths is provided. We will begin with a review of the physical properties of the lunar visual 
environment as w e  now believe them to  be, and will show how human performance may be affect- 
ed. Some of the ongoing research which is addressed to specific problem areas w i l l  be described, 
together with some suggestions for other studies which need to  be done. Finally, there will be a 
few comments on the selection and training of personnel for the lunar missions and some possible 
experimental studies in human vision will be suggested which could be made in parallel with 
other behavioral investigations in the Lunar International Laboratory. 
The Lunar Visual Environment 
The natural illumination of the lunar surface comes from direct sunlight, reflected sunlight 
(primarily from Earth, but in some small degree from other planets in the solar system), and from 
starlight. During lunar day about 13,000 ft-C are incident on the surface, s o  that the brighter 
portions of the scene may have an apparent luminance in excess  of 1000 ft-L. Owing to the lack 
of atmospheric scattering, it may be expected that the deepest shadows will approach effectively 
zero luminance 
neighborhood of 10 ft-C for full Earth conditions, with a resulting maximum luminance of some- 
thing over 1 ft-L, but decreasing markedly with Earth phase angle. Starshine alone is estimated 
to produce about 3 x ft-C, and, unlike Earth and Sun, of a diffuse rather than uni-directional 
nature, so that a general average luminance might approach lo-’ ft-L, without the severe shadow 
casting effects j u s t  noted. These numbers are only approximations, and are intended merely to 
indicate the wide range of luminances and contrasts which inhere in the lunar scene. They give 
u s  an opportunity to estimate the adaptation of the observer, and hence to say something about 
his performance capabilities, at l ea s t  for simple discriminatory tasks .  The case  of elementary 
detection of luminance contrast has  been extensively studied by Blackwell (1946 and others, 
and Figure 1 shows the manner in which contrast and s ize  of objects are related a t  a wide range 
ft-L or less). Without sun, illumination levels are likely to  be in the 
1 
3.0 
2.5 
2.0 
1.5 
1.0 
0.5 
0 
- 0.5 
- 1.0 
-3.0 -2.5 - 2 . 0  -1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 
LOG LIMINAL CONTRAST 
Fig. 1.  Threshold contrast a s  a function of stimulus s ize  for various 
levels of adapting field luminance. (After Blackwell) 
of adapting luminances. The near coincidence of the curves a t  1000, 100, and 10-ft-L for objects 
subtending 10 minutes or more i s  important to  note, for it indicates that eye-protective filters 
which transmit 10 per cent or one per cent of visible light will not significantly impair contrast 
discrimination at the higher ambient levels of field luminance. Performance a t  1 ft-L and a t  
lop5  ft-L are a lso shown i n  this Figure. I t  i s  clear that  the expected luminance levels on the 
lunar surface are, in the main, well within the operational tolerances of ordinary seeing. There 
are,  however, certain special properties of the lunar visual environment which are unlike any 
naturally-occurring terrestrial ones,  and it i s  these uniquely lunar conditions which are of great- 
est interest to us.  
The lack of a lunar atmosphere has  several important consequences for the lunar explorer 
and scientist .  We are accustomed, in the terrestrial environment, to  natural illumination which 
results from direct sunlight plus a significant amount of diffuse light due to  molecular and par- 
ticulate scattering by our atmosphere. This  scattered light, combining with reflected light from 
nearby objects, serves to illuminate those parts of the scene which are shaded from the direct 
rays from the sun and therefore to  reduce the range of luminances in our field of view. The first 
consequence of a missing atmosphere, then, is that shadowed parts of the moon’s surface will be 
very black; the contrasts in the scene w i l l  be extremely high and it seems likely that details  of 
the shadowed areas can only be seen by use of some reflective device or auxiliary light source. 
Another property of our atmosphere, generally known as atmospheric haze,  w e  habitually use in 
t h e  estimation of distance and s ize  of features. Since this cue will not be available, and be- 
cause objects of familiar s ize  may not be available for direct visual comparison, it is believed 
that t h e  judgment of s ize  and distance will have to be aided by special  devices (theodolites, 
rangefinders) and by special observing techniques (motion parallax), a t  l ea s t  a t  distances where 
man’s accommodation and convergence cues are inoperative and stereopsis no longer helps. 
Since t h e  sky above the horizon w i l l  appear essentially black (unless the Sun or Earth is in the 
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field of view; see below), i t  may happen that the adaptive s ta te  of the man on the lunar surface 
will be highly variable, and his  visual performance under such conditions cannot confidently be 
predicted from existing data. Finally,  for the sunlit and earthli t  conditions, the highly direc- 
tional nature of the illumination will, especially at  low angles,  combine with the low average 
surface reflectance to  produce wide extremes in the appearance of the terrain. Some idea of 
this  can be gained from Figure 2 ,  in which w e  have photographed a small area of simulated 
moon surface (Average r =  0.15). The difference between the upper and lower photograph, which 
’ 
Fig .  2. Photograph of a lunar 
surface model of average reflectance 
0.15, showing differences in appear- 
ance with sun’s azimuth. Upper and 
lower exposures identical; camera 
and model positions same. 
were identically exposed and processed, are due solely to  a change in the sun’s azimuth, with 
elevation held constant. I t  is clear that  a man walking on the lunar surface is confronted by 
quite a different scene in the two instances.  Because visual cues  are essent ia l  to the a c t  of 
walking under conditions of 1/6 g. w e  must be sure that  he is able  to use  these cues  without 
errors of judgment. I t  may be that an operational strategy for getting about should be devised 
in order to optimize the man’s performance and reduce the possibility of a costly or hazardous 
accident. 
Up t o  this  point we have not considered one aspec t  of the lunar environment which may be 
of extreme importance to  man’s visual performance. This ,  of course,  is the presence of the sun 
i tself  in the otherwise dark sky of lunar day. With an apparent luminance of around 6.4 x l o8  
ft-L, and subtending a half degree, the sun constitutes a glare source of tremendous magnitude. 
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If the man on the lunar surface must operate with the solar d i sc  in his  visual  f ield,  it is impnra- 
tive that suitable protective devices be provided which will prevent discomfort or temporary or 
permanent visual disability. I t  has  been argued that  the man will always operate so a s  to  avoid 
looking into or near the sun, but w e  must recognize the probability that  accidental  exposure wi l l  
occur. Furthermore, the eye-protective devices which have been proposed are far from ideal  from 
the visual standpoint, tending t o  introduce contrast and acuity lo s ses  by reason of scattering 
or distortion. Other potentially serious glare sources may be introduced by man himself. The  
need for thermal regulation of lunar excursion vehicles and, eventually, fixed habitats,  has  dic- 
tated that  their surfaces be s o  treated a s  to be highly reflective. These  surfaces are likely, 
therefore, to be very much brighter than the surrounding scene ,  especial ly  if seen partly or 
wholly against the dark sky. A s  an extreme example, w e  may take the case of the highly pol- 
ished aluminum surface which has  been proposed for the Lunar Excursion Module. Figure 3 
Fig .  3. Photograph of LEM model with ascent  
stage of polished aluminum and descent  stage ano- 
dized. Sun at  14' elevation. 
Fig. 4.  Photograph of LEM model, a s  in Figure 
3, but rotated to show intense glare from reflected 
image of the sun. 
shows a model of t h e  LEM in which the surfaces of the ascent  s tage  are of mirror-like aluminum 
and those of t h e  descent s tage of somewhat more diffusely reflecting anodized aluminum. From 
the vantage point of the  photograph no serious visual problem may exis t ,  but if the camera is 
moved only a degree or two, a s  i s  shown in Figure 4,  an intensely bright and angularly large 
glare pattern i s  suddenly seen. Since, for a plane surface, the apparent luminance of th i s  re- 
flection is about 5.8 x109 tt-L (90 per cent  of that  of the sun)  and of the same angular extent  a s  
the sun i tself ,  it i s  clear that  the problem is severe,  especial ly  because of the element of sur- 
prise which makes any voluntary protective maneuver relatively ineffectual. The result  of ac- 
cidentally encountering such a beam of reflected light could be a temporary lo s s  of visual 
sensitivity in the extreme case ,  and impairment of performance in less extreme instances.  
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Some Problems for Research 
Although many facets of human visual performance have been systematically and quantita- 
tively studied over the past  century, these have largely been concerned with somewhat ab- 
stracted and highly artificial conditions. While w e  are able to make quite precise predictions 
of visual capabilities in simple situations and for simple sorts of discriminations, our predictive 
tools are frequeiitly inadeqtlate for the solution of operational problems. For this reason i t  i s  
often necessary to  resort to applied research methods, simulations, and field tests rather than to 
await the eventual appearance of appropriate data from comprehensive basic studies. The re- 
search program of the Visibility Laboratory includes both basic and applied studies of visual 
performance under sponsorship of the National Aeronautics and Space Administration. We are 
performing experiments which will provide, for the first time, precise and systematic data re- 
garding vision in the peripheral field a t  a wide range of adaptation levels,  exposure times, and 
stimulus s i zes ;  this is only one example of a long-term data acquisition program. A t  an inter- 
mediate level,  we are examining the effects on vision of a bright surround - n problem suggested 
by the problem of seeing into lunar shadows or into recesses  on the exterior of a space vehicle. 
And w e  have conducted studies during GEMINI flights 5 and 7 to discover whether prolonged ex- 
posure to the spacecraft environment would affect performance on a kind of visual acuity task.‘ 
Meanwhile, knowledge of the photometric properties of the moon are becoming better known. 
The recent resul ts  of the Surveyor program, especially from the analyses of luminances and 
color but a l so  of textural features of the surface, give us  some notion of the visual situation for 
one (rather dark) area. The Astrogeology Branch of the USGS and the Jet Propulsion Laboratory 
are, a t  this writing, engaged in reducing data from Surveyor 1’s photometric system. Some pre- 
liminary results by Rennilson3 which indicate the agreement between the data of Fedoretz and 
the Surveyor measurements are shown in Figure 5 .  
-180 -140 -100 -60 -20 0 20 60 100 140 180 
PHASE ANGLE [g] 
Fig. 5. Preliminary data (open circles) from Surveyor I compared 
with the lunar photometric model of Fedoretz. 
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There are many aspects of visual behavior which can be studied in terrestrial laboratories 
and the data applied to  the lunar problem. I t  is to  be hoped that scient is ts  of many nations will ' 
become interested in one or another phase of visual performance so that we will be able to  as- 
sess the effects a s  such things a s  highly intense glare, rapidly changing adaptation, special  gas  
mixtures, reduced gravity over long periods, distortions and higher-order phenomena resulting 
from confinement and the like. Only then can w e  take full advantage of the man's ability t o  u s e  
his  eyes  for scientific work on the lunar surface, while insuring his  visual safety and the value 
of his observations. 
Selection and Training of Personnel 
From a purely visual standpoint, it is desirable that due consideration be given t o  the ca- 
pacity of the individual for operating efficiently in the LIL environment. I t  is obvious that he  
should have very good visual acuity and be free from any ocular pathology, and he should be 
capable of better than average accommodation and convergence. Perhaps less obviously, he 
should be young, for the effects of glare due to  intra-ocular scattering become more severe with 
age. His  training should include sufficient time with simulators so that he w i l l  learn new skil ls  
in judging the features in his unfamiliar optical environment, and in optimizing his own safety 
and productivity during his mission. He must be taught to  use his  peripheral retina for opera- 
tion during periods of darkness,  and he should learn the best  techniques for search and 
scanning. Importantly, h e  must be made alert to  the dangers of debilitating amounts of glare 
and instructed how to avoid it. For the fine discrimination of hue differences, he  should have 
some knowledge of how best  to  make his observations. This  l i s t  could be greatly lengthened, 
but the point to be made is that selection should include tests not ordinarily performed by the 
practicing ophthalmologist, and that special training should be provided in advance of 
the mission. 
Vision Research on the Moon 
I believe that there are certain studies of human vision which are important to our overall 
space program, bct which are difficult or impossible to perform on the surface of the earth. 
For these,  it would be desirable to incorporate in the LIL schedule time for some visual ex- 
periments. With a relatively minor cost  in weight, t i m e ,  or funds, it appears feasible t o  
conduct these investigations as a part of the LIL program. 
One example is  the study of the effects of long term existence under reduced gravity. 
While we have already made a beginning on this problem in the GEMINI program, the data are 
limited to a single visual discrimination task,  very few observations, and an experimental 
population of only four individuals. On the moon, a wide range of visual discriminations could 
be examined over considerable periods of t i m e .  
Another unique research possibility concerns the study of highly intense glare effects,  to  
which I have already alluded. Earthbound data are relevant only to the case  of relatively low 
glare intensities and to a few kinds of visual tasks.  With an unvarying sun available, con- 
trolled experiments will be possible. 
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Finally,  in the absence of an atmosphere, certain studies will be possible during the lunar 
These include the detection and identification of dim- night which are impossible on Earth. 
light phenomena, vision in the peripheral field, and visual performance capability under these 
special  conditions. 
Conclusion 
Our experience in space thus far has generally affirmed the notion that human vision i s  a 
uniquely valuable a s s e t  in space operations. In order that w e  take full advantage of man's 
visual capabilities, w e  must study his  potential for operating under the unique environmental 
conditions anticipated for all  phases of space exploration. While w e  m u s t  safeguard his 
vision, w e  should, a t  the same time, provide ample opportunity for him to utilize his visual 
skil ls .  It i s  of the greatest importance that visual conditions be kep t  in the forefront of our 
attention a s  w e  plan future missions; that competent visual scient is ts  be consulted whenever 
an item of equipment or operational plan is considered. But it i s  a lso the responsibility of 
these scient is ts  to  provide their best  estimates of the capabilities and limitations of the human 
visual process. In this endeavor we must look for and encourage the contribution of ideas and 
scientific data from all  countries. 
References 
1. H. R. Blackwell, Contrast Thresholds of the Human Eye, J .  Opt. SOC. Am., 36, 624, (1946). 
2. S. Q. Duntley, et al., Visual Acuity Measurements during Prolonged Weightlessness, 
J.  Opt. SOC. Am., 56, 538(A), (1966). 
3.  J. J. Rennilson, Private Communication. 
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' .  0I)IGINATIN G ACTIVITY ICorponIe author) 
Visibility Laboratory 
University of California, San Diego, Calif. 
Scripps Institution of Oceanography . 
2. R E P O R T  I L C U R I T Y  c L A I I I F I C A t l O H  
- UNCLASSIFIED 
Z b  OROUP 
Text of a paper given a t  the XVIIth Congress of the I. DLSCRIPTIVC NOTES (rrp ol mrt md Inchalw &tea) 
International Astronautical Federation, Madrid, Spain, 13 October 1966 
I. AUTMONS) R n t  M., # a t  n a u ,  Inltld> 
Taylor, John H. 
I. R I P 0  RT 01 T I  
December 1966 
NASA Grant NGR 05409459 
1. CONTRACT OR S R A N T  NO. 
b CROJCCTNO. 
e. 
d. 
0. A VA IL  ABILlTY/LIYITATION NOTICLI 
Distribution of this document i s  unlimited 
11. SUPPLLYENTARY NOTCS 
- 
7.. TOTAL NO. OF I 7 b .  NO. oc nccm 
IS. SPOWIORIWO YILITARY ACTIVITY 
National Aeronautics and Space Administration 
Ames Research Center 
Moffett Field California 
13. ABSTRACT 
This paper discusses  the optical and photometric properties of the lunar 
amfacc envirmment and their implications for human visual performance. 
The need for specialized training of selenonauts i s  pointed out, and some 
suggested studies for the Lunar International Laboratory are given. 
UNCLASSIFIED 
Security Clarsificalon 
Security Class i f i ca t ion  
I i 
I E Y  Y O ” 0 S  
\ i s i o n  
Lunar environment 
Glare 
Lunar International Laboratory 
L I N U  A 
R O L E  -
L I N U  e -
R O L E  -
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C 


Visual Performance on the Moon

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