A SURVEY REPORT ON HUMAN FACTORS IN UNDERSEA WARFARE

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SITPPLE31 ENT `'TO
A Survey Report on
HUMAN FACTORS
UND
}~;I'SEA WARFARE
Prepared in 1949 by the
PANEL ON PSYCHOLOGY AND PHYSIOLOGY .
COMMITTEE ON UNDERSEA WARFARE
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THE DESIGN AND USE OF OPTICAL INSTRUMENTS
IN, CONNECTION WITH UNDERSEA WARFARE
LORRIN A.. RIGGS
Brown University
INTRODUCTION
Optical instruments of importance in sub-
marine and anti-submarine warfare consist,
in the .main, of various forms of telescope.
Each instrument is designed for. one or more
specific functions, but they all have in com-
mon the general purpose of aiding the eye in
the detection and examination of relatively
distant objects. In the discussion which
follows, a survey will be made of factors
known to be of importance in visual de-
tection and examination. Consideration will
then be given to the limitations of optical
instruments, particularly in comparison with
the naked eye and with radar and sonar.
Special problems of optical instrument de-
sign will then be considered, and the human
factor will be considered in relation to these
design problems.
THE ADEQUACY OF TELESCOPIC
INSTRUMENTS FOR VARIOUS
PURPOSES
Increasing the Visual Range
One of the chief functions of telescopic
instruments is to extend the effective operat-
ing range of the eye by aiding the operator
(a) in the detection of targets too distant to
be detected by the unaided eye and (b) in
the examination or recognition of targets
too distant to be seen clearly by the unaided
eye.
Let us consider first the ideal situation of
viewing a black and white target in the ab-
sence of any absorption or scattering of
light, by the atmosphere. In this situation
the effect of range is simply to reduce the
size of the image, of the target on the retina of
the eye without altering the contrast, lumi-
nance, sharpness of contour, or temporal
stability of that image. A perfect telescope
is then able to compensate perfectly for
any increase in range by a corresponding in-
crease in the magnification of the instru-
ment, or, in the ideal case R - Mr, where R
is the effective telescope range; M, the magni-
fication; and r, the range of, unaided vision.
It is only in the idealized situation pic-
tured above that a telescope can be made to
extend indefinitely the range at which a
target may he observed. The remainder of
this paper may be regarded as a discussion
of how the value of telescopic magnification
is limited by other factors, such as atmos-
pheric absorption and diotortion, the 'in-
herent defects of telescopic instruments, and
the inevitable human factors pf Lde-
fects and errors of oo ration. It is obvi-
ously true, for example, that atmospheric
conditions set a limit beyond'which visual
observation cannot be effective, no matter
how good the telescopic instrument may be. ,
Daylight visual range is defined by interna-
tional agreement among meteorologists as the
greatest distance at which a large dark ob-
ject on the horizon is just recogniz-
able against a sky background. Another
unit, known as meteorological range, is also
in common use. This is the horizontal dis-
tance at which a large black target appears
to be 2% less bright than the sky back-
ground. This value is usually slightly greater
than the daylight visual range, for the reason
that a contrast of slightly greater than 2% is
commonly required for the recognition of
a large dark object on the horizon.
Whereas telescopic aids are of minimum
value in observing large objects at distances
approaching the meteorological limit, they
it
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are maximally effective in observing small
objects which are relatively close at hand.
The value of telescopic magnification is
chiefly in making small objects appear suffi-
ciently large, and A his can best be ac-
complished under good viewing conditions,
and at distances much smaller than that rep-
resented by the meteorological range.
Quantitative data to support this con-
clusion are given in the section on atmos-
pheric limitations appearing below.
To summarize the situation in respect to
extending the visual range by means of
telescopic instruments: At one extreme, the
meteorological range or daylight visual range,
either unit being defined in terms of very
large objects, cannot be extended. At the
other extreme, the effective range at which a
very small object may be viewed can be ex-
tended almost indefinitely up to the limiting
meteorological range by increasing the mag-
nification of the instrument, particularly
when atmospheric limitations are small. Be-
tween these two extremes, the detection
and examination of distant objects is facili=
tated by telescopic instruments.
Probably the most elaborate set of field
tests of visual range in which optical instru-
ments were employed is described in a report
entitled "Field Teats of Optical Instru-
ments" (35). This is a report of some ex-
periments under the active direction of W. S.
Verplanck at the U. S. Naval Submarine
Base at New London, Connecticut. For
the purposes of this section, we shall confine
our attention to the instrumental aspects
only of this study. The instruments used
in these experiments included hand-held and
mounted binoculars having magnifications of
from G to 25 power and objective diameters
from 33 to 120 mm. Monocular instru-
ments covering approximately the same
range were also used, hand-held and mounted,
and some obsrvations were made with the
naked eye. Fairly naturalistic targets were
employed in this study, and the instru-
ments were used on board ship by naval per-
sonnel. The targets ranged from a few inches
to many feet in width and were located on
the shore of a small island. They had var-
ious degrees'of contrast such as might be en-
countered under actual field conditions.
The results of the above experiments have
been analyzed for three different criteria of
visual performance: (1) when the observer
could just see the target for an-instant, only
to have it fade out of sight; (2) when he
could dimly see the target continuously; and
(3) when he could recognize the target by
its size, shape and position. The standard
7x60 binoculars were taken as the standard
of reference for the performance of the other
instruments.
The principal conclusions of this study are
as follows: (1) A binocular instrument is su-
perior to a monocular Tor night use by a
visual
To-Mr of at least 10 ercen in terms of
-rxffg-e-. ere is scarcely any erence e-
Teen monac ar instruments
o
for da ime (2) Magna cation up to
at east 10-power for a hand-held instrument
and 20-power for a mounted one increases
the effective range at night. Magnification
above 6-power does not improve daytime
performance for hand-held binoculars but
does increase the value of mounted instru-
ments up to at least 20-power. (3) The
best all-round hand-held binocular is the
10x50x7?. The best all-round mounted bin-
ocular is the 20x120x3?. (4) The range of
an instrument is extended by approximately
10 percent by the provision of suitable
mounts or rests for hand-held instruments.
(5) Differences among instruments are more
striking in the sighting and identification of
targets than in problems of search. . (6)
Further research is required relative to in-
dividual differences among observers under
various conditions of visibility in the use of
optical instruments of this character.
A British report (53) summarizes some ex-
periments in which binoculars have been em-
ployed for observing targets of high contrast
at low brightnesses in a perfectly clAar at-
mosphere. It is shown that even under op-
timal conditions binoculars do not extend
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the visual range by as great a factor as the
magnification.
Increasing the Probability of Detection
At any given range an object which is near
the threshold of detectability may or may
not be detected in the process of visual
search. Over a certain low range of con-
trast values of the target and its background,
there is a definite region of uncertainty.
Sometimes the target is seen, sometimes not.
In general, the probability of target detec-
tion is related to the luminance contrast,
I Lo - Leg where L,
L is the background lum-
inance and Lo, the luminance of the object
or target. This probability function, which
is sigmoid in shape, depends' on an apprecia-
tion of a minimal difference between the
numbers of nerve impulses delivered to the
brain per unit area and time from retinal
elements stimulated by target and back-
ground.
Hecht and his co-workers (15) have con-
cluded that the sigmoid function is based in
part upon statistical considerations within
the stimulus itself. For a source of constant
intensity and a series of flashes of equal du-
ration, there is a Poisson distribution of
number of quanta per flash of light. At in-
tensity values near the visual threshold, the
number of quanta per flash or unit of time
therefore varies considerably because of
variations in the stimulus itself. Thus, if a
certain minimum number of quanta is neces-
sary for seeing a flash, the probability that
it will be seen varies by reason of factors in
addition to physiological variations. It is
concluded that the absolute threshold under
the most favorable conditions of seeing is
characterized by the absorption of one
quantum of light by each of 5 to 14 retinal
rods. The sigmoid curve is, therefore, the
result of factors in the stimulus or in the ob-
serving individual, or both.
The military significance of increasing the
probability of detection lies, of course, in the
ability to oonduct a successful search for any
targets which may appear to 4e brighter or
less bright than their backgrounds. Field
studies, to which reference is made later in
this report, have been conducted to indicate
the degree of such success under given con-
ditions of illuminations, size of target, mag-
nification, and other particular factors which
may be present.
. With this statement of problems of thresh-
old detestability, let us turn to what is
known about the improvement of target de-
tection by the use of optical instruments.
Detection at Nighttime Levels of Illumination
Hartline and McDonald (36) have dis-
cussed the probability of detection or "fre-
quency of seeing" at low illuminations. The
laboratory studies of frequency of seeing
which are most pertinent to the present dis-
cussion are those conducted during World
War II at Dartmouth College, Brown Uni-
veristy, and the University of Pennsylvania
(46). In all of these studies, tests on indoor
observing ranges were made with binoculars
of various magnifications and exit pupils.
An evaluation was made of psychological and
physiological factors which govern the visual
assistance furnished by binoculars.
At Dartmouth college, the binoculars em-
ployed were 6x30, 7x35, 8x40 and 10x50.
The brightness levels were .000037, .00037,
.0037 and .037 footlamberts. The targets
were circular black dots against the dimly-
lit background. The observer was allowed
six seconds to look, at the end of which time
he signaled the position in which he saw the
target. Six different sizes of target were
used at each of four positions for each set of
brightness and magnification conditions.
Curves of frequency of seeing were deter-
mined as functions of the log of the diameter
of the target. The results of the Dartmouth
experiments may be summarized as follows:
the use of binoculars greatly increases the
range of detection of small targets at night.
This advantage is greater the higher the
magnification up to 10X and shows no sign
of falling off at this value. This conclusion
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holds true over the entire range of bright-
nesses.
The Brown University experiments (34,
46) were concerned with similar problems
but were considerably more extensive. In
one series of experiments, exit-pupil di-
ameters of 2, 4; 6, 8 and 10 mm. were em-
ployed with 5-power binoculars. It was
found that better performance resulted as
relevant factors were investigated.
mounting of the binoculars resulted En
average gain of about eight percent in terms
of visual range. Angular motions simulat-
ing those to be encountered on shipboard
produced no definite loss for periods of os-
cillation greater than 12 seconds; and for
shorter periods, the range was decreased by
a relatively small amount. The relative ad-
the exit-pupil diameters were increased up vantage of using two eyes over single-eye
to 6 mm. Beyond this point, however, in- o ervations range between 19 and 26 Der-
creasing diameter did not result in any im- cenfor the naked eyes. The corresponding
prevenient. This is somewhat surprising in
I view of the fact that the average diameter
of the natural pupil at that low level
of brightness is approximately 7 mm.
Another series of experiments compared the
efficiency of seeing with various magnifica-
tions in binoculars with 50-mm. objectives.
In these ex riments it was found that per-
e
. ground. For small targets at low levels of
illumination, retinal summation is complete
and it is'found that this threshold is propor-
tional to the product of the area of the target
by the target contrast. Since the area
varies inversely with the square of the range,
`the threshold range should vary in propor-
tion to the square root of the contrast. An
exception to this general rule was found for
contrasts below 50 percent where the ob-
served ranges were unaccountably lower that
the predicted ones.
In addition to these main findings of the
Brown University experiments, certain other
p
,
formance improved as the magnification was
increased up to 10X, then fell off for mag-
nifications greater than that. This result is
to be expected on the basis of the fact that
the exit-pupil diameter was inversely propor-
tional to the magnification in this series.
Other experiments, with 70-mm. objectives,
revealed that the visual range was improved
by only about 5 percent over that of the 50-
nun. instruments.
The results of the Brown University ex-
periments in general conform to the principle
that the threshold of visibility is determined
by the contrast between target and back-
gain with binoculars was a itt a ess t an 15
The general conclusion to be drawn from
the Brown University experiments is that
the design of binoculars affects performance
only to a minor extent within the usual range
of specification of these instruments.
Standard 7x50 binoculars, hand-held, are
only about 15 percent less efficient than
10x70 binoculars mounted on alidades
which yield optimal results under laboratory
conditions. The point may be made., how-
ever, that even the minor improvements
o e-d-are sificant wherever optimal per
formance at night is required. ^
The University of Pennsylvania experi-
ments (46) were designed to evaluate some
of the factors which served to detract from
the theoretical value of telescopic instru-
ments. Two such factors proved to be of
considerable importance. The first of these
was the misalignment of the exit pupils with
extent
Le
t o na ur pupils of the
o is misalignment was determined by
photographing the observer's eyes, imaged
in the objective aperture of the binocular.
It was concluded that there was a loss of
about 0.15 log units of range to be attributed
to the average amount of this misalignment.
Interestingly enough, the misalignment
seemed to result primarily from movements
of the eyes themselves rather than. move-
ments of the head.
The other factor which was analyzed in
these experiments was that of angular
tremor. Direct measurements of this angu-
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lar tremor were made in the laboratory by
a photographic technique and calculations
were made'of the probable extent to which
this tremor would detract from binocular
performance. This type of analysis ac-
counted fairly satisfactorily for the loss due
to tremor as calculated for the Brown Uni-
versity experiments. It was found that an-
gular tremor. was not greatly influenced by
the fit or balance of the binoculars, but
seemed rather to depend on external condi-
tions of vibration and wind. Accordingly,
it was concluded that provision should ordi-
narily be made. for vibration-free rests for
the elbows and for sheltering the, observer
from wind.
Detection at Daytime Levels of Illumination
Most of the data. on frequency of seeing
at high levels of illumination have been ac-
cumulated for the naked eye rather than for
telescopic observations There would seem
to be a definite need for research involving
optical instruments along the lines of the
naked-eye experiments recently reported by
Lamar, Hecht, Schlaer, and Hendley (22).
In submarine warfare, the daytime detec-
tion problem is particularly acute when
searching 'for targets by means of the peri-
scope. It is standard practice to raise the
periscope for such a short time that an ade-
quate check search is impossible. A great
deal of improvement on the use of the peri-
scope for this purpose may be achieved by
training the observer. Perhaps still more
promising is the improvement of photo-
graphic means of observation. A rapid se-
quence of photographs is taken for later anal-
ysis after the periscope is submerged.
Improving the Discrimination of Contour
The mere detection of a target is but the
first step in its observation by visual means.
In order to recognize and examine the finer
details of the target there must be a dis-
crimination of outline or contour.
One of the few experiments on the accu-
racy of contour discrimination during the
use of telescopic instruments was.done at the
Armored Medical Research Laboratory at
Fort Knox, Kentucky (29, 32). This study
was conducted under starlight conditions
with a Landolt ring as a target. . Standard
6x30 and 7x50 binoculars and also the 10x45
B. C. scope were used in this study. The
factors of magnification, exit pupil, and ob-
jective lens diameter were considered for
their effect on the distance at which the
acuity test object was clearly observed.
Some of the conclusions of this study in re-
gard to night visual acuity were the follow-
ing: (1) The Gx30 binoculars enabled an ob-
server to recognize a target at approximately
3.5 times the range at which the unaided eye.
can recognize it under the same star-
light conditions. (2) For the 7x.50 binocu-
lars, the corresponding figure is 4.75. (3)
Over the limited range of these experiments,
there is a direct relationship between mag-
nification and the distance at which targets
can be recognized. (4) The techniques of
efficient night seeing for the unaided eye
(dark adaptation, off-center vision, and scan-
ning) also apply when using binoculars.
Pointing and Tracking
There is a considerable literature on the'
subject of pointing and tracking with tele-
scopic instruments. such as gun sights,
mounted binoculars, rangefinders, etc. This
literature is listed in the classified bibliog-
raphy on visual research by Fulton, Hoff,
and Perkins (10). We mention here only
one set of experiments which bears directly
on the characteristics of the optical instru-
ments.
An article by Washer and Williams (28)
describes some experiments on the prevision
of telescope pointing for outdoor targets.
In these experiments, the telescope was
trained upon a target and the cross hairs
were set into apparent coincidence with the
image of the target formed by the objective
and seen through the ocular. The pointing
was accomplished by prisms external to the
telescope and located in front of its objec-
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tire. Rapid observations were taken by the
observer under various experimental condi-
tions. The probable error of a single point-
ing was computed on the basis of these ob-
servations and was found to have an average
value of 0.62 second of are. The cor-
responding value for an indoor target wa.g
0.24 second of arc. The relation of this
study to problems of atmospheric distortion
will be discussed below. It is sufficient here
to note the :author's belief that for magni-
fication in excess of 20 diameters, there is no
gain in the accuracy of outdoor pointing for
subjects at great distances.'
Range and Height Finding
Radar and submarine echo devices have
taken over the field of range-finding to such
an extent that optical devices have now come
to assume largely a stand-by function.
There is a telemeter device in the submarine
periscope which provides for measuring the
height above the water of the masthead of
a surface. target. The accuracy of this de-
vice for the purpose of measuring the range
is dependent upon a knowledge of the actual
height of the masthead, and a small error in
this figure may lead to a considerable error
in determining the course and speed of the
naval target. Further consideration of
these devices will also be postponed to a
section below.
Navigation
Problems of navigation are expected to
become more critical as submarines extend
their range of operations and as it becomes
increasingly necessary to remain submerged
for long periods of time. Attention should
accordingly be directed to the extent to
which the periscope may be used as an in-
strument for measuring the azimuth and ele-
vation of celestial objects. For tl}is purpose
' It might be mentioned here that the problems
of pointing and tracking are importantly related
to the typo of reticle pattern to be used in the gun
sight or telescope. See the discussion of this
topic that appears below.
it is essential to provide a gyroscopic stable
element for use in connection with the peri-
scope. No research on this problem is
known to the author at the present time,
but the development of such devices may be
in progress in connection with the current
process' of redesigning the periscope (37).
Concealment of the Observer
A submarine periscope is the chief repre-
sentative of a fairly large group of optical
instruments whose function, in part at least,
is to enable the observer to see without being
seen. Koenig (21) has listed and described
a number of other telescopic instruments
for this purpose. Some of these are to be
used on land for the purpose of elevating the
line of sight above the tree tops or other in-
tervening objects. Special periscopes for
use in aircraft have been developed and a
foxhole periscope is also ' in use (46)..
LIMITATIONS OF OPTICAL INSTRUMENTS
Atmospheric Conditions
It is obvious that even an ideal telescope
operated by a good observer cannot func-
tion under unfavorable atmospheric condi-
tions. This is one of the reasons, of course,
for the development of radar and other
means for the detection and identification of
targets. It need hardly be said, however,
that for purposes of detailed examination of
a target there is no substitute for visual ob-
servation.
Target Detection
The state of our knowledge concerning the
effect of atmospheric conditions upon, de-
tectability of targets is relatively highly de-
veloped. Much of this development oc-
curred in connection with OSRD projects
during World War II (54).
Hardy (13, 14) has outlined the manner
in which the principles of atmospheric at-
tenuation apply to the performance of tele-
scopes. It has long been known that the
apparent luminance of a target decreases
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exponentially with the target distance be- possible to enter these charts with data on
cause of'the atmospheric attenuation.
is in conformity with Koschmieder's
which may be written as
where Cs is the contrast of the target at
range R, Co is the contrast at range zero,
and 6 is a coefficient of attenuation. Cer-
tain instruments are available for the direct
determination of P. A value of approxi-
mately .02 represents the threshold condition
of Cie. On the basis of these facts, it is
possible to predict the effective range of
visual observation for values of ft correspond-
ing, for example, to very clear, hazy, or foggy
atmospheric conditions. As we have noted
above, it is possible under ideal atmospheric
conditions to extend the effective visual
range by the simple process of increasing
the magnification of the telescope. Hardy
has shown that, under ordinary atmospheric
conditions, the magnification must be great
enough to compensate for the reduction in
contrast of the target as well as the reduction
in size at great distances. Hardy's analysis
reveals that the increase in maximum range
is small unless the atmosphere is very clear
and that, for this reason, low power tele-
scopes ordinarily give as much increase in
range as telescopes of higher power. One
conclusion which emerges as a result of his
analysis is that it is futile to compare optical
aids for quality and precision by means of
field tests at maximum ranges. The optical
quality of instruments should be measured
indoors at relatively short ranges where the
state of the atmosphere is not a factor.
Duntley (7, 8) has summarised the prin-
cipal factors involved in the visibility of dis-
tant objects, making use of the Koschmieder
relationship and applying the results of the
Tiffany Foundation experiments on contrast
(1). Duntley has developed a series of no-
mographic charts for predicting the limiting
range at which a uniformly luminous object
can be detected by unaided vision. It is
This meteorological range, contrast, luminance,
law, and target area and thereby predict the
liminal target distance. By the use of cer-
tain corrections to the brightness. and' area
factors, it is possible to use Duntley's charts
to predict visibility along inclined paths of
sight. This use of the charts is of course
required for observations of aircraft from
submarines and vice versa.
The maximum distance at which a target
may be seen is predicted by the above ho-
mographic charts for excellent observers who
are forced to judge whether the object
is' present or absent. Consequently, the
ranges so predicted are to be regarded as
maximum values which are not always at-
tained in practice. Under some circum-
stances, it may be desirable to arrive at an
estimate of the range at which the average
observer is confident that he has indeed seen
the target. In this case, Duntley suggests
dividing the inherent contrast of the object
by two before entering the data on the chart.
This procedure is admittedly for expediency
only and has no quantitative justification.
Coleman (39) has extended the usefulness
of the nomographic visibility charts by mak-
ing provision for the variables a'sociated
with telescopic instruments. Coleman's
treatment enables one to predict the detec-
tion range for an object using a telescopic
instrument whose characteristics are known.
The particular characteristics so employed
are (1) magnification, (2) contrast rendition,
(3) light transmission, and (4) exit pupil size.
The factor of magnification, M, increases
the apparent magnitude of each dimension
linearly and hence increases the apparent
area of the target by a factor of Ms. There-
fore, the inherent target area given by the
chart is replaced by an apparent area which
is Ms times as large.
Contrast rendition is given by the relation,
contrast rendition (%)
contrast of the image
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In using the charts, this factor is used to
correct the inherent contrast of the target Target Resolution and Recognition
by the amount of the contrast rendition. The distortion of target images by. at-
LigU transmission is defined as the ratio mospheric factors is of importance in the
of the brightness of the telescopic image to detailed examination and recognition of ob,-
that of the naked eye image. This value, jects. It is well known that one of the most
determined photometrically, may be applied severe limitations of telescopic systems is to
as a correction for the value of luminance or be found in the extent of shimmer caused by
brightness with which the nomographic differential refraction of light over the optical
chart is entered. path from the target to the telescope. The
The wise of exit pupil is a determiner of extent to which shimmer causes deviations
the apparent brightness level. Since exit in the apparent position of points on the tar-
pupil.sizes are given in diameter units, it is get has been measured photographically by
necessary to square the ratio of the exit Riggs, Mueller, Graham, and Mote (26).
pupil size to the size of the natural pupil in This report shows that when a large amount
order to arrive at the appropriate correction of atmospheric shimmer is present, the aver-
to be ' used in the nomographic charts. age deviation in position of a point on a tele-
Average pupillary diameters for given values scopic image is of the order of 2.4 seconds of
of luminance are given by the experiments arc, and that the maximum deviation may
of Reeves (24, 25) and others. This factor, amount to 12 seconds or more. The result-
together with a minor correction for the in- ing distortions of the over-all target image
fluence of the Stiles-Crawford effect, is in- are shown by the instantaneous photographs
corporated into the charts. Numerical ex- upon which this report is based. Measure-
amples for the solution of problems on the ments were also made of the degree to which
liminal detection range are given. the images of two targets were similarly dis-
Coleman and Verplanck (5) have reported torted at the same instant of time. It was
field tests of the detection ranges of objects found that, for targets which were very close
viewed with telescopic systems from aboard together, the images suffered the same kind
ship. Approximately 80 telescopic systems of distortion, but that -the similarity became
of 18 different designs were used in these ex- less marked as the targets were separated
periments. A comparison was made be- laterally by a greater and greater distance.
'tween the computed performance of each in- These measurements suggest that, because
strument, as arrived at from the nomo- the value of telescopic magnification is
graphic charts devised by Coleman by the diminished by the fact that the apparent
method outlined above, and the field per- shimmer of a telescopic image is directly
formance as actually measured in these tests. porportional to the magnification, low mag-
A very satisfactory agreement was found be- nification is optimal under conditions of
tween the computed and predicted values marked shimmer. Analysis shows this to
under the conditions of this experiment. It apply to the case of stereoscopic rangefinder
is concluded by the authors that for them a operation as well as to simple problems of
e con of.o jests, it is posse a to predict target resolution.
fairly accurate y e range a w is they are
vno a nowmg on a sic measu Pointing and Ranging
menu necessary for the use the nomo- In the article by Washer. and Williams
- ? ----o----?,??