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SUGARY: FINDINGS AND ~CO~NDATIONS
em?
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SUMMARY: FINDINGS A2iD RECOMMENDATIONS
Night vision encompasses many different visual functions uncter a
variety of ambient lighting conditions. Since night operations are a
crucial part of around-the-clock combat readiness, the United States
Air Force has been interested in evaluating visual performance at night.
The Working Group on Night Vision addressed the topic of night vision
with four specific objectives in mind: (1) a definition of the relevant
parameters of night vision; (2 ~ an update of the literature pertaining
to night vision, especially new f indings, test procedures, and concepts
since 1950; (3 ~ the development of guidelines for establishing a compre-
hens~ve night vision laboratory; and (4) recommendations for the devel-
opment of night vision screening tests.
The first two objectives were addressed by convening a symposium at
Brooks Air Force Base in 1985 (see Appendix B. ~ The proceedings of the
symposium form the basis of this report. The findings and recommenda-
tions presented here are based on working group discussions following
that meeting. Reference is made throughout this section to the papers
in this volume that have some relevance to the recommendations under
· ~
discussion.
The recommendations address five broad topics: (1) the specifica-
tion of ambient illuminance levels; (2) task analysis and characteri-
zation of the work environment; (3) research areas of potential utility
to the development of night vision tests; (4) the development of night
vision screening tests; and (5) recommended equipment for a night vision
laboratory.
SPECIFICATION OF AMBIENT ILLU}lINANCE LEVELS
In order to predict performance under low-illumination conditions,
it is necessary to match information describing ambient illuminance
levels, task requirements, and psychophysical data relating the capa-
bilities and limitations of human performance to luminance levels. If
the task in question is to be carried out indoors, specification of
illuminance level is ~ .~1 at~vely simple procedure. Indeed, in most
artificially illuminated environments, it i s possible to ad just the
i Luminance levels to match the demands of the task.
3
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4
However, for tasks performed under natural lighting, specification
of illuminance is complicated by the continually varying levels of
natural illuminance from the sun and moon and their attenuation by
atmospheric conditions. Perhaps the most complex illumination condi-
tions are those associated with mixtures of natural and artificial
lighting, since there can be large variations in both the spatial and
temporal characteristics of ambient illumination.
Estimates of the ambient illumination during twilight and from the
moon can be calculated on the basis of the date, time, latitude, and
altitude. Less precise estimates can be made of the effects of atmos-
pheric conditions. We suggest that consideration be given to the devel-
opment of a hand calculator that would permit the rapid determination
of ambient illuminance levels between sunset and sunrise. These data
are particularly critical during twilight, when illum~nance is changing
rapidly (by a factor of 2 every 4-5 min at 40 degrees north latitude),
and during the darker portions of twilight and at night, when even the
low illuminance from moonlight can have implications for performance.
TASK ANALYST S AND CHARACTERI ZATION OF THE WORE: ENVIRONMENT
The design of visual screening tests for night vision must incor-
porate features that are based on the.nature of the tasks performed,
the properties of the work environment, and the typical illumination
and contrast levels that are present for night vision working opera-
tions. In a similar fashion, the definition of operational light
levels for various tasks requires an accurate assessment of the visual
skills used to perform certain tasks and the physical characteristics
of the work environment. Although this information is currently avail-
able for some tasks performed under low illumination, specif ications
are not yet available for the ma jor ity of night vision working opera-
tions.
In order to establish vision test procedures with h igh job rele-
vance and validity, it is essential that detailed information be made
available for task requirements and the work environment. We recom-
mend that this information be obtained as the first step in the analy-
sis of night vision. A quantitative description of the work environ-
ment should include the measurement of ambient lighting conditions,
contrast levels, the spatial and temporal frequency properties of the
environment, and the range of stimulus conditions under which certain
tasks are performed. Task analyses of various jobs should include a
comprehensive survey of the most frequent tasks performed, the most
critical task components, the types of visual skills necessary for
properly conducting the task, an evaluation of the frequency and con-
sequences of errors in task performance, and other related factors. It
is important to define the illumination levels under which tasks are
performed and whether they require visual detection, identification,
localization, or other skills. The visual requirements for individual
jobs performed at low Dominances (and the time, expense, and difficulty
of administering night vision tests) can vary widely, depending on the
specific characteristics of the task.
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We recommend that these evaluations be carried out by a team of in-
vestigators whose expertise includes visual psychophysics for measure-
ment of light and contrast levels, analysis of spatial and temporal fre-
quency properties of the work environment, and determination of visual
functions critical to task performance; industrial/organizational psy-
chology for conducting task analyses; human factors engineering for
ergonomic considerations of the work environment; and knowledge of the
operational concerns of the armed forces. The results of this analysis
will have a significant impact on defining operational light levels for
night vision tasks, defining the types of vision screening tests that
should be developed, and determining the priorities for a night vision
research laboratory.
RESEARCH AREAS OF POTENTIAL UTILITY TO NIGHT VISION
As indicated in the historical review (Appendix A), the majority of
night vision tests evaluated during and following World War II were
based on either scotopic detection thresholds or acuity-form perception
tasks performed at mesopic or scotopic luminance levels. Since that
time, the visual sciences community has produced many research findings
and methodologies that are relevant to night vision. We recommend that
the areas of research described below be considered for evaluation by a
comprehensive night vision laboratory facility.
..,
Validation Studies
Regardless of the specific visual functions and test procedures
that are examined by a night vision laboratory, we believe that it is
critical to correlate vision test results with task performance
measures f ram studies with simulators, "field" studies, performance
ratings of instructors, or other information sources. This information
will be useful in determining which test procedures are the best pre-
dictors of task performance and will thereby establish a basis for
designing appropriate night vision screening tests.
One of the goals of a military night vision laboratory should be to
develop a test or battery of tests that are able to predict individual
performance for various night vision tasks. As described In the his-
torical review (Appendix A), previous attempts to predict night vision
task performance on the basis of detection and acuity measures were not
very successful. Recent investigations of the contrast sensitivity
function, peripheral vision, accommodation and convergence, and other
visual functions suggest that they may have predictive value for task
performance. As the papers in this volume illustrate: (1) the accu-
racy of accommodation and convergence responses is reported to be
related to the visibility of objects during night driving (Owens);
(2) the status of the peripheral visual field is reported to be corre-
lated with driving performance (Johnson); (3) recognition of aircraft
appears to be related to an individuals contrast sensitivity function
(Harvey; Haber); and (4) the contrast sensitivity function and the
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peripheral visual field are reported to have predictive value for
orientation and mobility skills of patients with low vision (Bailey).
None of these visual functions has been correlated with task perfor-
mance under low-luminance conditions.
In addition to evaluating the basic visual functions described in
subsequent sections of this report, it is also important to correlate
these test results with performance of visual tasks under low-luminance
conditions. Emphasis should be placed on those tasks that are conducted
frequently and those that are critical (i.e., nonoptimal performance can
have serious consequences). There are a variety of procedures that may
be used for deriving evaluations of task performance, including (1) the
use of simulators of field studies, in which a task can be conducted
under controlled conditions that permit measures of performance accu-
racy and/or efficiency; (2) subjective performance ratings by instruc-
tors or supervisors; and (3) critical incident evaluations, in which
performance errors are reviewed. Performance evaluations can then be
compared with psychophysical measurements to identify tests that may be
good predictors of specific night vision tasks.
Longitudinal Studies of Night Vision Performance
We also believe it is important to establish a data base to track
individuals over a period of time and to establish a population sample
of reasonable size. Not only will this be helpful for present studies
but it will also be invaluable for examining issues that may arise in
the future. Many of today's questions might be answered simply if such
a data base were available from previous research.
We recommend that short-term (1-2 years) and long-term (7-20 years)
studies be performed for military personnel engaged in night vision
tasks. Both psychophysical tests under low-luminance conditions (e.g.,
contrast sensitivity, glare disability, peripheral visual function,
dynamic visual acuity, oculomotor function) and task performance mea-
sures should be obtained at periodic intervals. For both the short-term
and long-term studies, individual differences and their significance for
job-related night vision tasks should be evaluated. Short-term studies
(with sampling intervals of 1-4 weeks) will help identify the variabil-
ity of psychophysical and performance measures (both within and between
subjects) and will also provide an opport'un~ty to examine the effects
of practice and training. Long-term studies (with 1-year intervals of
time) will help to identify trends in night vision performance varia-
tions that may be related to aging, job experience, or environmental
influences. Nearly all studies of long-term visual changes have been
cross-sectional because of the logistic problems associated with lon-
g~tudinal follow-up of individuals in a mobile society. The military
environment is unique in that it is possible to conduct follow-up
testing of the same individuals over extended per iods of time. By mon-
itoring psychophysical and performance measures over many years, the
effects of aging, job experience, "nd environmental influences can be
readily examined. For example, changes in tonic accommodation and con-
vergence (or, alternatively, the resting or dark levels of accommodation
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and convergence) brought about by aging, greater amounts of course
work, or other influences may have significant long-term consequences
for cr itica1 night vision tasks. The data from long-term evaluations
will also be helpful in def ining norms for visual function and task
performance under low illumination conditions.
Oculomotor Effects and Spatial Orientation
Recent research has demonstrated that accor~odation and convergence
responses become inaccurate under degraded viewing conditions and dis-
sociate to different tonic or ~resting" levels in darkness (Owens).
Although the resting positions of accommodation and convergence are
located at intermediate distances (0.5-2 m) for most persons, large
individual differences are present. These individual differences in
oculomotor adjustment under low illumination may be an important compo-
nent of the large individual variability that is typically encountered
for night vision tasks such as target detection, recognition, and local-
ization. Distance perception can also be influenced by accommodation
and convergence responses. Tonic shifts in the oculomotor adjustments,
resulting from prolonged viewing of targets at fixed distances, can
also influence accommodation and convergence responses as well as pro-
duce shifts in distance perception (see, for example, Post and Beckman;
Ebenholtz). Reflexive and voluntary eye movements and their relation-
ship to se'f-motion sensation (vection), induced motion of objects in
the environment, spatial orientation, and postural stability are also
important topics for night vision research. Many of the night vision
tasks performed by Air Force personnel are conducted under "reduced
cue" situations, in which vection (illusory self-motion), induced
motion, spatial or ientation, and postural stability can signif icantly
Papa fir pert ormance .
The Spatial Contrast Sensitivity Function
The spatial contrast sensitivity function has received increasing
attention in the past 15 years ~ see, for example, Harvey) . I t has been
shown to be a sensitive indicator of early clinical visual abnormali-
ties, often in the absence of any deficits noted with standard clinical
tests of visual function. In addition, recent studies have demonstrated
that the contrast sensitivity function is a very good predictor of per-
formance for certain visual detection and identification tasks, visual
similarity judgments, letter identification and confusion, shape recog-
nition, and related visual tasks.
Many studies indicate that it is an important determinant of indi-
vidual differences in visual performance for populations with "normal"
vision. Although mesopic and scotopic contrast sensitivity functions
have been measured for various locations in the visual field, little is
presently known about the relationship between contrast sensitivity and
visual performance under low-luminance conditions. The relationships
between contrast sensitivity and visual performance that have been
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8
reported for photopic luminance levels indicate that this area of
research should be a high priority for investigation by a night vision
laboratory.
Peripheral Vision Function
The peripheral visual field has been shown to be an important factor
in tasks pertaining to orientation and mobility, vehicle guidance, and
other related functions (Johnson; Bedell; Bailey). At low luminances
the periphery becomes even more important, since performance with res-
pect to many visual functions (detection, contrast sensitivity, resolu-
tion, etc.) is better in peripheral vision than for the fovea under
these conditions. There are several aspects of peripheral vision
research that should be evaluated by a night vision laboratory.
First, optimum performance of visual functions under low-luminance
conditions occurs at different visual field eccentricities, depending
on the specific task (detection, identification, spatial localization,
etc.~. Observers are able to select and maintain a specific nonfoveal
locus for performing tasks under free-viewing conditions at low lumi-
nances, although it is not clear how this process is accomplished, or
what factors are involved.
A second area of peripheral vision research should be directed to
the effects of practice and training. It is generally believed that
the peripheral visual field is capable of significant improvement in
visual performance with practice and training. However, the duration
of these improvements, their generalization to nonlaboratory environ-
ments> and their significance for task performance are not well under-
stood. Third, there are many differences between central and peripheral
processing, including reduced velocity and/or disappearance of slowly
moving objects; changes in apparent brightness, size, and distance of
peripherally viewed targets; and the Troxler effect (fading of station-
ary peripheral stimuli in the absence of eye movements). The bases for
these differences and their relation to visual task performance are not
well understood. Finally, there is a paucity of information available
on flicker and motion sensitivity in the peripheral visual field at low
luminances, despite the fact that the perhiphery is particularly sensi-
tive to interrupted or moving stimuli.
Visual Search/Vigilance
Night vision often forces the observer to perform under conditions
in which imperfect visual information is available, and mental set and
search strategies would necessarily play an unusually signficant role.
Most laboratory studies of visual function involve the use of a static
display, maintenance of steady fixation by the observer, and a high
degree of certainty about the stimulus type, location, and time of pre-
sentation. However, visual performance tasks are typically conducted
in dynamic situations, with free viewing and a reasonable amount of
stimulus uncertainty.
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Target detection, identification, and localization are tasks that
are highly dependent on the properties of underlying neural mechanisms,
visual search behavior, vigilance, search strategies, mental set, and
other factors (see, for example, Copenhagen and Reuter; MacLeod and
Stockman; Makous; Leibowitz, Sheehy, and Gish; Sanders). Additional
research on these problems is needed, particularly with respect to
night vision tasks.
Prolonged and/or Stressful Conditions
Prolonged effort and/or stress, both of which are common in the
military environment, can produce significant alterations in the oculo-
motor system, functional visual fields, gaze stability mechanisms, and
cognitive processing functions (Leibowitz, Sheehy, and Gish). Very few
of the currently available tests of visual function are designed, how-
ever, to evaluate performance under those conditions. The majority of
current evaluation techniques and procedures were designed to reflect
performance for short-term tasks in a relatively unstressful environ-
ment. Many of the changes involved are critical for tasks performed
under low illuminance when the demands of the task frequently encroach
on human performance limitations.
One of the effects, for example, is visual field narrowing. Under
a wide variety of psychological and physiological stress conditions,
the functional visual field may constrict. As a consequence, visual
signals in the periphery may go unnoticed, reaction time may be signi-
ficantly lengthened, and a significant increase in stimulus energy may
be required for detection of signals. Since there is less overall
energy available under low illuminance conditions, it follows that the
consequences of visual field narrowing under stress may be more signi-
ficant at twilight and at night than under high illuminance conditions.
Evaluations of night vision performance should be performed for physio-
logical and psychological conditions that are known to produce visual
field narrowing.
Dynamic Visual Acuity
The majority of visual evaluation tests and procedures have been
designed for static observers viewing a static target. In most real-
life situations, however, the observer, the target, or both are moving.
Adequate tests for assessing functional capabilities under such dynamic
conditions are generally not available. At the present time, we have
little information pertaining to the functional properties of night
vision under static versus dynamic conditions, individual differences
in dynamic visual function, and the relationship between dynamic visual
properties and task performance capabilities. We recommend that dynamic
visual acuity and related tests be considered for evaluation by a night
vision laboratory. In addition to providing dynamic visual test cor.di-
t~ons, these procedures require the observer to coordinate oculomotor,
sensory, and cognitive functions, thereby creating a more demanding
visual task environment.
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Glare Sensitivity
The presence of a glare source can have a significant influence on
a variety of visual functions, especially under conditions of low ambi-
ent illumination (see, for example, Blackwell and Blackwell; Owsley;
Bailey). Most of the information that has been obtained for glare
disability has been derived from static displays. The influence of
glare for dynamic viewing conditions needs to be evaluated, especially
for task performance over extended periods of time.
Aging and Vision
It is important to evaluate the effects of normal aging on perfor-
mance under low illumination levels (Owsley). Some investigators
report that age-related changes in the ocular media produce an average
transmission loss of 50 percent (0.3 log unit) every 13 years, while
others have reported age-related transmission losses that are about
half as large (0.15 log units every 13 years). Under high illumination
these transmission losses may not be significant. During twilight and
at night, however, age-related transmission losses may be a critical
component of visual performance. We recommend that age-related changes
in night vision performance be investigated as part of a night vision
laboratory program.
Computer Modeling
The use of computer modeling and computational theories are playing
an increasingly important role in vision research (Watson). Empirical
data from optical, electrophysiological, psychophysical, perceptual,
and cognitive studies of the visual system can be used to design a
model, which can then be used to generate predictions about visual
response properties under a variety of situations.
Comparison of certain predictions of the model with experimental
results can then be evaluated to refine the model and improve its
applicability to specific problems. Computer modeling could be a
valuable e tool for the study of night vision. The design of such a
diodes requires specif ic, detailed information about the functional
properties, interrelationships, and constraints of all components of
the model. Thus, missing information, unspecified interactions among
components, unknown stimulus and/or response properties, and other key
aspects of the problem become readily apparent. This can serve as a
guide to directing research to critical areas of night vision that are
not well defined. In addition, the predictive capabilities of a com-
putational model can often provide insights for complex problems and
interactive, dynamic situations that might otherwise go unnoticed. In
view of the current rudimentary understanding of the critical factors
underlying the performance of night vision tasks, we recommend that
computer modeling be considered as one of the activities of a night
vision laboratory.
1
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DEVELOPMENT OF NIGHT III SION SCREENING TE STS
There are several purposes associated with screening night vision
capabilities of Air Force personnel: (1) to identify those individuals
with night vision problems stemming from ocular pathology; (2) to deter-
mine that an individual has sufficient night vision capabilities to per-
form a specific task; (3) to classify individuals on the basis of night
vision capabilities so that the most critical night vision tasks can be
carried out by personnel with the best night vision performance on
appropriate screening tests; and (4) to periodically monitor individuals
to ensure that adequate levels of night vision are being maintained.
The first objective is readily achieved with the use of existing
clinical evaluation methods (see Fishman; Berson), including a thorough
ophthalmologic exam and history, psychophysical measures (dark adapta-
tion, visual fields) and electrophysiologic studies (electroretinogram
[ERG], electro-oculogram [EOG]~. A more formidable challenge is to
design simple, rapid screening tests for night vision that will predict
task performance and that will permit the reliable classification of
individuals on the basis of their night vision capabilities.
Previous research indicates that it is not possible to accurately
predict visual performance at one background luminance level on the
basis of performance at other background luminances, especially If they
are separated by a large interval. Thus, it is not likely that a pho-
topic vision screening test will be able to provide information that is
predictive of night vision capabilities. In this view, the results of
a thorough task analysis of various night vision operations will play
an important role in the design of night vision screening tests.
Depending on whether specif ic tasks are typically performed at mesopic
or scotopic luminance levels, the time required for a night vision
screening procedure can vary considerably.
Presently, there are no standardized night vision screening tests
that have been shown to be related to task performance. The design of
night vision screening tests should ideally come from thorough labora-
tory studies that are correlated with actual performance measures of
night vision tasks. It will require considerable time and effort to
obtain this information. In the meantime, there is general agreement
that a performance-related night vision screening test should probably
incorporate elements of contrast sensitivity, target detection and
identification, and visual search in a free-viewing situation. A rapid
screening o~ oculomotor performance at low luminance s can also be per-
formed in a simple, straightforward manner.
RECOMMENDED EQUIPMENT FOR A NIGHT VI SI ON LABORATORY
A critical item for a night vision laboratory is a high-quality
photometer/radiometer to measure and calibrate luminance and contrast
levels accurately. The instrument should be capable of determining
luminance p illu.minance, color temperature, and other measurements for a
wide range of lighting conditions, object sizes, and temporal stimula-
t ion cond i t ion s .
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12
There are several commercial devices for clinical evaluation of
night vision that would also be appropriate for a night vision labora-
tory. A sophisticated dark adaptometer, a clinical electrophysiological
test system for measuring EGG and ERG responses, and an automated pro-
jection perimeter would be important equipment items for a night vision
laboratory. A number of clinical devices for the measurement of glare
disability have also become available. Many of these devices have not
undergone formal evaluations to determine their accuracy, reliability,
and validity. In addition, there are some glare devices that use a high
background luminance level that would not be appropriate for investiga-
tions of night vision. Although a device to measure glare disability is
highly recommended for a night vision laboratory, several issues must
be carefully considered prior to purchasing a specific glare tester. In
particular, the device should be applicable to the measurement of glare
disability under low luminance conditions, its performance characteris-
tics (sensitivity, accuracy, reliability) should be documented in pub-
lished studies, and normative values should be available for the general
population.
A key equipment item for a night vision laboratory is a general pur-
pose laboratory computer system. The computer system should be capable
of real-time control of stimulus displays and other laboratory devices,
as well as acquisition of psychophysical and electrophysiological res-
ponses. A full complement of analog and digital I/O interface boards
(and accompanying control software), peripheral devices (printer, plot-
ter, graphic terminal, etc.), and disk storage should be included in
the computer system. In addition, a high-quality spreadsheet/data base
management software package should be included to permit correlations
of laboratory findings with performance scores or ratings from night
vision tasks, longitudinal evaluations of individuals, determinations
of the efficacy of various training regimens, and other related issues.
The establishment of a comprehensive data base is an important aspect
of long-range planning for a night vision laboratory. In order to
accommodate the needs of computer modeling, there should be suf f icient
memory and mass storage available on the computer system to allow
computational models of night vision characteristics to be evaluated.
In addition to a computer system, other general psychophysical
laboratory equipment (high-resolution display oscilloscopes, function
generators, optical bench equipment, filters, etc.) should also be
available. For studies of per ipheral vision, a manual per imeter than
can measure a variety of v, sual functions (detection, resolution,
flicker sensitivity, dark adaptation, etc.) over a large range of back-
ground luminance s would be extremely useful. A large screen monitor or
CRT display would also be valuable for assessment of perhipheral visual
function.
Evaluations of oculomotor function can be carried out with several
devices. A laser or vernier optometer can provide steady-state measure-
ments of accommodation, and steady-state convergence measures can be
determined by the use of a device incorporating the nonius line tech-
nique. For measurements of the dynamic properties of accommodation, a
continuously recording infrared optometer is recommended. The dynamics
of conjugate and disconjugate eye movements should be evaluated with an
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infrared eye movement monitoring system. An infrared scleral reflec-
tion-type system mounted on a trial lens frame is a relatively inex-
pensive device that can be used for this purpose. For more comprehen-
sive evaluations of eye movements, an infrared eye-tracker that moni-
tors the relative positions of the Purkinje images would be a most
useful tool in a night vision laboratory.
Ideally, one would like to have night vision laboratory personnel
with expertise in the following areas: (1) visual psychophysics,
(2) human electrophysiology, (3) human factors, (4) clinical ophthalmic
sciences, (5) industrial psychology, and (6) illuminating engineering.
This multidisciplinary team would provide a comprehensive background
for addressing the variety of complex problems associated with night
· ~
vlslon.
,.(
OCR for page 14
Representative terms from entire chapter:
vision laboratory
