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APPENDI XES
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APPENDI X A: TESTING N IGHT VI ST Ob
THE MILITARY EXPERIENCE DURING AD FOLLOWING WO=D - R II
INTRODUCTI ON
Night vision tests have been proposed, produced, anct employed in
many places and under many c ircumstances. Their number and var. iety are
astonishing. A major source of experience in testing of night vision
occurred in the military during and just after World War IT. Since the
circumstances that were obtained in the 1940s are unlikely to recur, ana
large-scale night vision testing based upon the same goals and criter ia
may never again be essayed, the lessons of that t one are summarized in
this review.
Night vision testing aimed at evaluating large nurr.bers of inaivio-
uals and categorizing them according to levels of performance was a
development of wartime needs. highs blindness was of course a well-
known clinical syndrome, but there were no recognized standards that
could be applied to detect mild cases or to compare normal individuals.
The safety of a naval vessel could not be left in the trust of a look-
out whose night vision was even moderately impaired, and men selected
for night reconnaissance flights should certainly be ~r.en whose visual
senses were as keen as could be found. What was wanted was a test that
would give each man a score or rating that could be referred to when
duties called for night observations. Work was carried out in labora-
tories connected with various branches of the military service in a
number of the warring nations. Dozens of devices were developed,
hundreds of papers written, and a complete history would include
accounts of parallel attempts to determine the value of the different
devices. The knowledge gained, however, can be elucidated with a few
c hoice examples.
Th is appendix has been excerpted by Jo Ann K 1nney f ram the draf t of a
chapter for an unpublished book on night vision prepared by the late
Par ker Johnson.
321
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322
THE TYPES OF TE STS
Early emphasis in the military was on the development of ~one~shot,
single-score measures of general capabil ity or capacity, for use in
military selection or screening. Some envisioned only a rough test to
call out the small pe rcentage of personnel who were ser iously night-
blind. Others hoped for something more: for a test that would grade
men on some sort of scale . Appropr late night-vision standards could
then be set for var ious military duties. The task was approached with
the hope that night vision would be a stable, unitary trail-quickly
and reliably measurable, and valid as a predictor of any assignable
night vision duty. It was assumed that the test could be administered,
requiring only a modest expenditure of time and effort, somewhere in
the induction and training process. The score would then be inscribed
in the inductee's record, along with his height and weight, to be ready
as needed to predict night performance capabilities. As we shall see,
however, it never worked out that simply.
The tests that were developed may be divided, by and large, into
two categories: tests of light sensitivity and forms perception tests.
In addition, there were a number of field tests made; examples of each
of these will be g iven.
Tests of Light Sensitivity
The course of dark adaptation had been the object of study for some
time; the Gullstrand Photoptometer of 1905 and the bagel Adaptometer
(1907) were two early instruments. As research continued, showing the
two segments, rod and cone, of the dark adaptation curve, the word
"adaptometer" came to be assoc iatea with the measurement of dark-adapted
thresholds for light. Because adaptation slows and ceases after about
30 minutes in the dark, it seemed that the f inal, steady level might be
taken as representative of the indi~ridual's night vision capability.
In fact, the individual who totally lacks scotopic vision is readily
identif fed by such tests. He needs more than 100 times as much light
to see as does the normal, a degree of difference that is difficult to
overlook. The seriously night-blina may thus be screened f rom a popu-
lation with even the least sophisticated of devices. To discriminate
meaningfully among members of the large, n nor~r,al" population is much
harde r.
The Hecht-Shlaer Adaptometer
The instrument, originally used for research, appeared suitable
for screening service personnel and was offered to Britain's Flying
Personnel Research Committee in 1941. The apparatus u see a circular
test area with a diameter of 3 degrees, exposed periodically, at a
point 7 degrees above the fovea. A dim red light was provicea for
f ixation. The light was violet to ensure the least possible photopic
involvement. Wratten neutral density f liters and a wedge of f liters
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were used to effect changes in luminance. The luminance of the test
light was gradually reduced, the observer simply stated when he could
see the light, and the time noted.
The Hecht-Shlaer Adaptometer was widely used and numerous studies
prov ide norms and data on its reliability and validity (Berry, 19491.
Var ious modif ications were made to it, and in the end it became a
Family of devices" rather than a single instrument. Test-retest
correlations of various models, in reported studies that u sea 100 or
more men in the sample tested, ranged from .42 to .78 with a median
value of .64, levels not overly encouraging for mass testing.
Tests Using More Complex Forms
Some of the test developers felt that simple light sensitivity
might not be well correlated with successful night observation and
favored tests involving some deg ree of form perception. Thus the
A.R.L. Adaptometer of Britain's Royal Navy required location of the
position of a 45-degree, wedge-shaped, dark sector missing from an
otherwise circular lighted area; the test was given after a 30-minute
per iod of afar k adaptation.
The S.A A. (School of Aviation Medicine) N ight Vision Tester rem
quired an even greater degree of form perception since it presented a
Landolt ring. Other tests similarly employed a T in different posi-
tions. In each case, the figure remained the same but the or ientation
was different. The luminance level was systematically droppea through
a succession of fixed steps and the " score" determined by either the
lowest level with which an individual could cope or by the total number
of correct position identif ications for the whole series. Early ver-
sions of the Army N ight Vision Tester were similar in design.
Outdoor Courses
Another sort of test that had some appeal was the outdoor observa-
tion course, in which men were conducted over a prescribed course under
natural light conditions, locating and identifying as many as possible
f rom an assortment of real ob jects that lay along the way. But since
one cannot hold constant the deg ree of darkness desired, such testing
was not capable of being standardized for routine categor ization of
men. Such tests, however , had a g reat deal of ~ face-validity.: they
appeared to be sampling exactly the sort of skill required. Whenever
c ircumstances smi led on an outdoor range and a number of men were run
through the course under what seemed to be reasonably constant cond i-
tions of lighting, motivation, and experience, their scores were eager-
ly tabulated and compared with scores by the same men on the more pre-
cisely controlled indoor tests, tests that had them doing more artif i-
cial things like detecting dim lights, determining the orientation of
standard f arms, or reading letters.
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324
PROBLEMS OF TESTING NIGHT VI SION
A few examples will be given here to illustrate the problems encoun-
tered during attempts to test night vision during World War II. Both
indoor tests and the exper fences in an outdoor range are described.
The Livingstone Rotating Hexagon Night Vision Test
4
This test was used by the Royal Air Force, the Royal Australian Air
Force, and the Royal Canadian Air Force in the early 1940s. In an
effort to save time in the testing procedure, the apparatus was built
with six faces and could be administered to six men- at a time using one
machine and one technician. The task was one of recognition of letters
and of familiar forms such as styli zed sh ips and airplane silhouettes.
Several n night" luminance levels were employed; testing was started at
the higher luminance ~ .005 foot Lamberts) and progressed in steps down
to .0003 ft.L. The score was simply the number of correct recognitions
out of a possible 32. Instruction was given in the use of of f-center
vision and sub jects were allowed to scan f reely dur ing each exposure.
Setting aside questions of reliability and validity ~ in which the
Hexagon, adequately maintained and administered, revealed neither 9 reat
super for ity nor infe rior ity), a wide gamut of problems were encounterea
in its use which are inst ructive for night vision testing in general.
First, engineering to ensure adequate control of the scotopic lumi-
nance of the light stimulus is both essential and difficult. Varia-
tions in amount of light f rom one face of the Hexagon to another, f roar,
one instrument to another, and f ram one time to another occurred due to
aging, voltage fluctuations, homogeneity of lamp output, dirt, damage,
etc. Both the or iginal setting of the luminance and corrective ad just-
ments require considerable technical skill. Changes in the voltage, for
example, change the color temperature of the light which in turn can
produce marked deviations in the scotopic luminance. The wide range of
nighttime luminances requires adjustment over many log units of inten-
sity, again requiring considerable skill in the means of adjusting ano
maintaining such a range.
Second, not only must the instrument be standardized, but also the
testing procedures. There are several levels of concern here: (1) to
see that both the administrator of the test, who may be only a techni-
cal assistant, and the people being tested follow instructions to the
best of their ability; (2) to demand that these instructions be clear
and comprehensible to preclude lack of understanding as a reason for
individual variation; and (3) to ascertain that the auxiliary para-
phernalia, including the arrangements for communicating responses, not
require irrelevant skills.
The various forms of noncooperativeness--aisinterest, disdain,
malingering, cheating--cannot be ignored and the test must be designed
with them in mind. In the dark, men could and aid shorten the viewing
distance between them and the target to improve their chances. For res-
ponding, each man was provided with a clipboard and answer sheet aividea
into areas for response that had to be located, in the dark, by running
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one's finger over raised guides. His responses were to be written in
the proper area, but writing in the dark is a sadly neglected skill in
our society and those who had to score the sheets later found their
powers of judgment and ingenuity strained.
Another procedural problem is the amount of learning or practice
involved in taking the test. The Hexagon, with its unfamiliar scanning
procedure, showed signif icant learning over the f irst trials, with some
individuals catching on at a more rapid rate than others.
Third, a great deal can be learned by considering the scoring prom
cess used in the Hexagon and its meaning. The very decision to Reek a
single score for night vision performance reflects a belief either:
(1) that night vision is a unitary skill or ability, or (2) that,
though distinguishable components may exist and vary independently, the
test draws on a sufficient number of these to be representative of per-
formance in real-life tasks. The Hexagon, operating over a range of
luminances and employing a variety of targets, appeared to be based
upon the second assumption. However, it spread people on a scale whose
units were obscure; they indicated neither the variation in form per-
ception or acuity among observers at one, standard light level nor the
amount of light required to perform a standard task. Rather, an arbi-
trary composite of the two made up the individual scores. Moreover,
when using scores to grade man, there is always the implication that
the man who scores 24 is somehow better, even twice as god, as the
men who scores 12; in the ca se of the Hexagon, there was no reason to
believe this was true.
The scoring process may be summer ized as follows : ( 1) Scores that
are rationally related to specific performance variables are much more
meaningful than arbitrary scores reported on scales that reflect chiefly
the vagar ies of the testing procedure; and .~2) unless someone has a sped
c if ic reason for emphasizing some other aspect of performance, a score
that translates directly in "effective visual range" is probably best.
A Range Test for Validation
In order to determine whether the scores f roar, a specif ic test
related to on-the- job performance, there were many validation. stuaies
run in the 194 Os. Most proceeded by setting up an artif icial f ield
situation, outdoors, with natural nighttime illumination and employing
test targets for detection or recognition that were considered repre-
sentative of real military targets. For an adequate comparison among
subjects, conditions must be standardized; that is, each man should see
the same ob jects against the same backs round, under the same illun~ina-
tion. Lack of control of illumination level, cloud cover, and viewing
background; differences in the size and configuration of the targets
making them of unequal cliff iculty; obtaining data on enough men at once
to be meaningful; and controlling differing attention and motivation in
the sub jects are just a few of the innumerable cliff iculties in such an
endeavor.
A validation study run by the Navy in Long Island Sound illustrates
the difficulty. In setting up the validity trials, great care had been
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given to maintenance of realistic viewing conditions. The men whose
vision was being assessed were lined up along the rails of a large ves-
sel, all at the same height above the water. Their job was to watch a
specific section of the horizon for the approach of a smaller vessel
and to signal when they had it in view. The plan was simple. The
target ship would proceed to a position beyond the view of even the
best and then make a run toward the lookout ship. All men would be
alerted to start looking at the beginning of the run. The target ship
would continue toward the lookout ship until it had been seen by all
the ~lookouts.. Arrangements were made to record the time and distance
scores for each man as he called in his sighting. The target ship
would then retreat beyond view and come in again. It was hoped that
several such runs would provide performance measures for comparison
with scores generated from the same men on a dozen or so night vision
tests that had been administered back at the base.
It was not easy for a small group of night vision research people
to commandeer this much equipment and to get everything together for
such a trial. Everything had to go right. It didn't. The night
turned out to be one of the blackest of which the Sounc] was capable.
On its f irst run in, the target ship approached until its commander
stopped the run, fearing he would ram the lookout ship. lio one had
seen him at all. Being encouraged to try again, he circled around ano
approached, until a military ai rcraft, not associated with the enter-
prise, "checked things out" by turning on the beam of an airborne
searchlight. Instantly, every lookout being tested signaled n target
in view." Such hard luck stories were almost the predictable outcome
of trials attempting to incorporate high degrees of naturalness and
realism.
In examining the material on tests and validations from the World
War II literature, Berry (1949) summarized:
In short, it appears that the reliability of the tests was not
too impressive, varying for the most part from .60 to .80 with
occasional unreproduced examples of higher coefficients. They
correlated with each other either indifferently or badly . . .
the test results did not furnish any satisfactory basis for
prediction and there were far too many incongruities, published
and unpublished, of service men who n failed" the tests and yet
performed altogether adequately.
POST-WO=D WAR ~ I DEV~OP~NI S
The tests developed during the war gradually fell into disuse, for
a variety of reasons, including Berry's {1949) comprehensive assessment,
and in the decade that followed only two new tests appeared on the
scene (Kinney, 1962~. Both were grounded in the earlier experiences
and on additional new research. One of them, developed at the naval
Medical Research Laboratory (the NM*L Test) (Kinney et al., 1960),
sampled the sub ject's c~ark-adapted sensitivity at various locations
throughout his visual f ield. The rationale was that the previous poor
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correlations along tests were due in part to the fact that each tested
a different retinal area and that signif leant individual differences in
the pattern of retinal sensitivity existed (Sweeney et al., 1959~.
Moreover, leaving it up to the sub ject to f ind his own ~ best. area
simply added another source of variation. Because area and brightness
had previously been shown to be interchangeable (deGroot et al., 1952,
1953) in scotopic sensitivity, target size was varied, rather than tar-
get luminance, making control easier. This meant that the scores could
be related directly to minimum visual acuity and to visual range.
Scores were based upon overall performance and shown to be reliable and
normally distributed within the population of normal young Navy men.
The idea behind the second new test, the Army Night Seeing Tester
(ANST), a device developed within the Personnel Research Branch of the
Adjutant General's Office (Uhlaner and Zeidner, 19611, was that testing
carried out at higher mesopic levels, at which rod and cone vision were
both involved, would be as useful as purely scotopic for the purpose of
predicting performance at ~ low light levels.. It thus circumvented the
need for time-consuming dark adaptation and did not require specialized
dark rooms for testing. Resolution acuity was tested at a light level
of about .005 ft.L. Extensive f ield tests were done in which men were
scored on their ability to detect Enemies at night, against f told and
forest backgrounds (Martinek and Mellinger, 1959~. Though the correla-
tions between the indoor and the outdoor tests were not high, they were
sufficiently impressive, given the rather meager reliability of the
validation tests, to make clear that the ability being measured indoors
was significantly involved in the outdoor exercises.
At first blush, each of these tests looks like a continuation of
the traditional search for a one-shot test, but in rather important
ways, each represented a retreat from that goal. The justification
offered for the ANST was not that a mesopic test could predict scotopic
performance as well as a scotopic test; the argument was simply that
for many applications, a good scotopic test takes more time and trouble
to administer than it is worth. Admitting that a test carried out at
Topic levels was testing a somewhat different function, the juagment
was made that these higher, mesopic levels, besides being easier to
test, were also useful to know about.
Moreover, Kinney's (1968) work demonstrated the lack of correlation
among measures of photopic and mesopic acuities ana the NELL scotopic
sensitivity- test . The values ~ iven in Table A-1 were based on data
f rom 100 healthy young men ~ ithout pathology. Scotopic sensitivity
correlates poorly with either photopic or mesopic acuity, but it is
obvious that the closer together the light level of the tests, the
higher the correlations.
LESSONS LEARNED FRO}: THE MI LITARY EXPERIENCE
Sen s i t iv i ty and Form Pe rcept ion: One Func t ion or Two?
The assumption of one, single Quality of night vision gave way to a
m.ultiple-function hypothesis, involving at least sensitivity and form
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TABLE A-1 Correlations Among Photopic Acuity, Mesopic Acuity, and
Scotopic Sensitivity Measured on 100 Young Men
Test
Car relet ions
Photopic acuity _ i
~75* ~
At .0009 ft.L. ~I .17
Me Topic ecu ity
At .005 ft.L.
1 .37*
Scotopic sens it iv ity
*Significant at the .01 level
Source: Kinney (1968!
.15
_ ~ ~
~.29*.
perception, as soon as it was learned that adaptometer scores did not
correlate well with low-brightness form perception measures. It was
even argued that since sensitivity was presumed to be aided by maxi-
miz~ng spatial summation while acuity demanded a fine mosaic of func-
t toning receptor areas, the two functions or abilities might be con-
sidered fundamentally incompatible.
In 1955, Ogilvie et al. found that light thresholds were signif i-
cantly and positively related to acuity measures at the very lowest
scotopic luminance levels. This was conf irmed by Pirenne et al. (1957)
who dete rmined cor relet ions between absol ute threshold and scores
determined using various acuity criteria. Acuity was measured using
Landolt rings which range from 20/6, 000 (a Landolt ring with a gap size
of 295 minutes of arc) to 20/60 (3 min gap). ~ e corresponding light
levels ranged from close to absolute threshold to well up in the meso-
pic range. m ese correlations, determined for 22 subjects, are given
in Table A-2.
To be noted in these data are: {1) the remarkable high agreement
between sensitivity and acuity measures at the lowest light level ;
(2) the fact that the correlation was best at this light level, which
argues age inst any incompatibility of the functions being measured at
low scotopic levels; and (3 ~ the slow but ultimately precipitous de-
cline in cor relation with absolute threshold as the photopic level is
approached . These data, together with those of Kinney (1968), suggest
that a major determiner of the number of essential functions to be
tested is the overall light level, photopic and Scotopic vision being
essentially independent.
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TABLE A-2 Correlation Between Absolute Threshold and Acuity at
Va rious Levels
Gap size,
minu tes
Threshold log, Correlation
mmL Coef f ic tents
295 2.67 .927
191 2.78 .903
96 3.08 .846
24 4. 02 .8 41
4.74
5 95
6.59
Source: Pirenne et al. ( 1957~ .
Controlled Fixation Versus Free Movement?
The question of whether eye f ixation should be controlled or f ree
to roam is an interesting and complex one. The Hecht-Shlaer Adaptometer
was typical of most light-threshold devices in providing a fixation
point so that the test flash could always be presented to the same area
of the fovea. Because of known, gross variation in sensitivity in d if-
ferent parts of the retina, failure to provide such control would lead
to such var lability as to make the determination of a threshold almost
impossible. But having provided a f ixation, we must recognize the
arbitrary nature of the choice; the best area for one subject is not
necessarily the best for another. The NMRL test, by sampling a number
of the regions of the retina, sought to provide an average sensitivity
score for the eye as a whole (Kinney et al., 1960) .
However, it can still be asked whether this average score is what
is wanted; a person skilled in the use of his night vision will make
his crucial observations using that part of his retina that is best
under the prevailing conditions. And the best part. varies, even for
the same individual, depending upon his degree of dark adaptation, on
the amount of light available, and on the acuity requirements of the
task (Brown, 1954 ~ . Nor can a naive sub ject be trusted to select the
optimum peripheral angle for each condition.
Thus in choosing a procedure, the tester must look ahead to consi-
der the use to be made of the obtained information. Also, in any later
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interpretation of the results, it is important to look back and see
what procedure was used . Furthermore, before the tested individual may
be asked to use his own judgment, we must be sure he has had experience
that will qualify him to make proper use of this freedom.
The Role of Knowledge in Determining Performance
Testers necessarily try to keep irrelevant factors, as flagging
interest, shrewd guesses, series effects, cheating, etc., from influ-
encing the test scores. But even as we strive to keep knowledge of
the situation from influencing what is intended to be an assessment of
visual endowment, we also recognize that in any real situation where
night vision is exercised, where the job is to look, to see, and to
report or interpret correctly, something more than a good pair of eyes
is called for. Two additional factors are knowledge (pertinent to the
s i tua t ion) and intell igence .
During World War II, British Lancaster bombers regularly invaded
the skies over Germany at night. Me tail gunners peered from the rear
machine-gun tur ret, alert for closing German f ighter a ire rat t . It was
not, however, a situation in which the tail gunner could risk shooting
at any faint blur he saw, or thought he saw, since the air around him
was filled with other British bombers. If he could count the number of
engines in the approaching aircraft, he had what he needed to know; the
bombers were large, four-engine machines while the German fighters had
one or two engines. but when a plane came into view, approaching from
a distance, it was only a vaguely distinguishable blur; there was no
time to wait for engines to become distinguishable, since it it was a
fighter, a burst of gunfire was imminent. To enable the gunner to
reach the earliest possible decision, he was instructed not to look for
engines at all, but to concentrate on the general form of the blur; that
is, was it a compact spot (a single engine), an oval (twin engines), or
a dark horizontal line or bar (four engines)?
Not every nighttime task can be reduced to such a simple formula,
but in talking of Good vision,. we must always remember that practical
vision engages both the mind and the eye to an extent that varies with
the task. If we wish to measure capabil ities of the eye, independent
of such factors as motivation, experience, or intelligence, we fre-
quently f ind ourselves severely challenged. And to the extent that we
do succeed , we must recognize that we are ref. ining out factors that may
be important for success in other situations.
REFERENCES
Berry, W.
1949 Review of Wartime Studies of Dark Adaptation, Night Vision
Tests, and Related Topics. Armed Forces, National Research
Cou nc i 1 Vi s ion Commi ttee . Wash ing ton, D. C .: Nat tonal
Academy of Sc fence s.
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331
Brown, J . L.
1954 Effect of different preadapting luminances on the resolution
of visual detail dur ing dark adaptation. Journal of the
Optical Society of America 44:48-55.
deGroot, S .G., J.~. Dodge, and J.A. Smith
1952 Factors in night vision sensitivity: me effects of bright-
ness. Report No. 194. Groton, Canny: Nairal Medical Research
Laboratory, Sub Base.
deGroot , S .G ., J. M. Dodge , and J .A. Smith
1953 Factors in night vision sensitivity: The interrelation of
size, brightness, and location. Report No. 234. Groton,
Bonn.: Naval Medical Research Laboratory, Sub Base.
Fulton, am., D.G. Marquis, H.T. Perkins, and P.~. Hoff
1945 A Bibliography of Visual Literature 1939-1944, Supplement.
Unpublished reports on vision from United Nation as civilian
and military sources. Publication No. 11. New Haven,
Conn.: Historical Library, Yale Medical Library.
K inney, J .A. S. .
1962 Review of literature on night vision testing. Pp. 3-11 in
M.A . Wh itcomb, ea., Visual Problems of the Armed Forces.
Washington, D.C.: National Research Council.
Kinney, J.A.S.
1968 Clinical measurement of night vision. Pp. 139-152 in M.A.
Whitcomb, ea., me Measurement of Visual Function.
Washington, D.C.: National Research Council.
K inney , J .A. S ., E .J . Sweeney , and A . P . Ryan
1960 A net? test of scotopic sensitivity. American Journal of
Psychology 73 :4 61-46 7.
iSartinek, H., and J.J. Mellinger
1959 Field evaluation of ANST--fixed position observer. Memo No.
59-11. USA TAGO Personnel Res. Br. Res.
Mart~nek, H., R.N. Tanck, and J.J. Mellinger
1959 Field evaluation of ANST--armored patrol. Memo No. 59-12.
USA TAGO Per sonnet Res. Br. res.
Ogilvie, J.C., J.E.B. Ryan, R.F. Cowan, and E.I. Querengesser
1955 Interrelations and reproducibility of absolute light threshold
and scotopic acuity. Journal of Applied Physiology 7: 519-522.
Pirrene, M.H., F.H.C. Marriott, and E.F. O'Doherty
1957 Individual differences in night~ision sensitivity. Serial
No. 294. Medical Research Council Special Report. London:
Her Ma j esty' s Stat lone ry Of f ice .
Sweeney, E.J., J.A.S. Kinney, and A.P. Ryan
1959 Standardization of a scotopic sensitivity test. Report
No. 3 08. Groton, Conn.: Naval Medical Research Laboratory,
Sub Base.
Uhlaner, J.E., and J. Zeidner
1961 Abe Army Night Seeing Tester--development and use. HERB
Technical Research Report No. 1120. Human Factors Research
Branch, TAGO, DA.
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Representative terms from entire chapter:
light level
