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OCR for page 141
SPATIAL AND TEMPORAL FACTORS AFFECTING NIGHT VISION
OCR for page 142
OCR for page 143
INTRODUCTI ON
Kenneth R. Alexander
The spatial characteristics of vision include visual acuity, which
is the ability to discriminate the fine detail of the visual environ-
ment, contrast sensitivity, which is the capacity to detect spatial
variations in light levels, and the perception of objects and their
relationships in space. Temporal processing refers to the discrimina-
tion of visual events that vary in time, including the detection of
brief flashes, motion, and flicker. In the visual tasks involved in
flying, such as reading cockpit instruments or judging terrain during
cross-country navigation, the ability of a pilot to assess the spatial
and temporal aspects of visual information accurately is critical for
optimum performance. However, the capacity of the visual system to
process spatial and temporal information varies considerably with
illumination level.
In a well-lit environment, vision at night may be cone-mediated, as
it is during the day. When the illumination level is reduced, both cone
and rod systems may govern visual sensitivity, while the roe system may
be the primary determinant of visual sensitivity at low light levels.
The cone and rod systems differ substantially in their response to spa-
tial and temporal visual information. Typically, the cone system pro-
vides much finer spatial and temporal resolution than the rod system.
Therefore, reliance on rod-mediated vision under dim illumination con-
ditions can result in a considerable decrement in visual performance.
Even if vision is cone-mediated, however, the level of illumination
can have a significant effect on visual tasks. For example, the visual
acuity for letters on an eye chart drops from 20/20 (normal) to 20/100
when the chart illumination is decreased by only a factor of 100 (the
range of illumination to which the visual system is sensitive is
10,000,000,000 to 10~. Results such as this imply that there is a
marked functional reorganization of the visual system that occurs
with reduced illumination levels. The papers in this section pro-
vide detailed information about the changes in spatial and temporal
processing that occur as a result of reduced illumination. The
following remarks briefly summarize the issues discussed in these
reports.
Under daytime illumination conditions, the fovea is the most sen-
sitive part of the retina and is used for fixation. However, with
reduced illumination, the fovea may no longer be the most sensitive
143
OCR for page 144
144
retinal region, and fixation may shift to a nonfoveal location. To
optimize visual performance, it is important to determine the factors
that govern fixation when light levels decrease. In the first report,
Harold Bedell discusses the sensitivity acuity and directionalization
properties of various parts of the visual field and their relationship
to eye fixation under conditions of reduced illumination. Also consi-
dered are the potential deleterious effects of optical blur, pupil size,
and accommodation on visual performance under reduced illumination con-
ditions.
The detection and identification of visual targets depend on more
than good visual acuity, as demonstrated by a number of recent studies.
Lewis Harvey presents a relatively new method for assessing visual per-
formance, in which targets are described in terms of their spatial fre-
quency characteristics. The detection and identification of these tar-
gets are derived from the contrast sensitivity function. As discussed
by Dr. Harvey, this method has proved to be more useful in predicting
the visual performance of pilots than the traditional measurement of
visual acuity. An extension of this technique to the rod system may
provide a successful way of predicting visual performance under condi-
tions of reduced illumination.
The ability of a pilot to perceive the layout of visual space and
its relationship to himself or herself is critical for safe navigation.
The visual cues that are available for accurate space perception change
dramatically with illumination. Ralph Haber details the visual task
demands required of pilots in the perception of visual space during
both day and night flying. He also discusses the role of individual
differences in night vision and the possibility of improved night vision
through training.
The ability to detect visual targets at night is strongly influ-
enced by the presence of extraneous sources of illumination, termed
disability glare. The Blackwells note in their paper that disability
glare degrades visual performance by reducing the contrast of the reti-
nal image of the target through light scatter. Dr. Blackwell provides
a quantitative evaluation of the factors that influence disability
glare, such as glare luminance, pupil size, and observer age. He also
discusses an instrument designed to measure disability glare objectively
and a device that measures the effect of disability glare on visual per-
formance.
The rod and cone systems in human vision do not function indepen-
dently. The response of one system can markedly influence the sensi-
tivity of the other, often to the detriment of visual performance.
Donald MacLeod and Andrew Stockman point out the ways in which rod and
cone signals can interfere with each other in such visual tasks as the
detection of flicker. He also presents evidence that the sensitivity
of the rod system to temporally varying information is substantially
greater than had been previously estimated.
Patients with "low vision" or severely impaired visual function can
exhibit many of the visual problems that normal observers experience
under conditions of reduced illumination. For example, reading is it
paired in normal observers if the illumination level is too low, just
as it is in patients with a loss of foveal function. Ian Bailey
OCR for page 145
145
discusses the problems of mobility exper fenced by low-vision patients
and evaluates the assistance offered by low-vision aids. Methods of
improving vision in such patients may be helpful as well in aiding
night vision in pilots.
OCR for page 146
ECCENTRIC REGARD, TASK, AND OPTICAL BLUR
AS FACTORS INFLUENCING VISUAL ACUITY AT LOW LUMINANCES
Harold E. Bedell
It is readily appreciated from casual experience and has been well
documented experimentally that visual acuity worsens as the level of
illumination is reduced (Koenig, 1897, cited in van Helmholtz, 1925a;
Schlaer, 1937~. This worsening of acuity is most pronounced at the
fovea, where under photopic illumination acuity is best and under sco-
topic illumination the eye is totally insensitive. One contribution,
then, to the reduction of acuity under scotopic illumination is the
necessity of viewing the target with a peripheral retinal locus that
is inherently less acute than the fovea.
The shift of the target from foveal to nonfoveal regard is not the
entire story, however, as can be seen in Figure 1, in which data from
Mandelbaum and Sloan (1947) are replottea on a logarithmic acuity axis.
A logarithmic scale for acuity is used because on such a scale response
variability remains constant, despite changes of the acuity threshold
due to the target's retinal eccentricity (Westheimer, 1979) , luminance
(Simpson et al., 1981), or blur (Prince and Fry, 1956; Homer et al.,
1985~. Figure 1 shows that between 4 and 8 degrees from the fovea, re-
duction of target luminance front about 3 to 0.01 cd/m2 results in a
worsening of visual acuity for Landolt ring targets of approximately
2.5-fold (0.4 log units). Further reduction of the target luminance by
a factor of 10 (to 0.001 cd/m2) produces about another twofold loss
of acuity at the same retinal eccentricities. These results, which are
confirmed by the data of other studies in which similar (Sloan, 1968)
and different (Aulhorn, 1964; Kerr, 1971; Koenderink, 1978) types of
acuity targets were used, indicate that, as in the fovea, acuity in the
near peripheral retina also depends substantially on the level of illu-
~r.inat ion.
COMPARI SON WI TH OTHER TASKS
In contrast to visual acuity, the precision with which targets can
be directionalized has been found to be much less dependent on the
Supported in part by research grants EY 05068 (to Harold E. Bedell) and
EY 03694 (to Merton C. Plot) from the National Eye Institute.
146
OCR for page 147
147
2.0
.8
.6
.4
1.2
al:
~1.0
o
J
.8
.6
.4
.2
o
o.oo 1 3 cd~m2
o.o 1 3 cd/m2
0 3.1 8 cd/m2
~ /N
~ -of- _
o
_¢
it,
~-
' ~
~,
.
__4~
-
_
-
~ _ -I
-
._
_ _-~
r I , , , , , , , , 1
o 2 4 6 8 10
ECCENTRICITY (DEG)
FIGURE 1 Extrafoveal visual acuity data (MAR = minimum angle of reso-
lution) for Landolt ring targets at different luminances are replotted
from the data of Mandelbaum and Sloan (1947) on a logarithmic acuity
axis.
level of illumination. About 30 years ago, Leibowitz and coworkers
(1955a,b) reported that observers could discriminate the meridian in
the field of a near peripheral (ca. 4 degrees from the fovea) target
with essentially equal precision whether the target was at its light
OCR for page 148
148
detection threshold or at much higher luminances. Recently, these
findings have been extended (Bedell et al., 1985) by showing that
observers' precision in specifying the direction of a target along a
meridian of the field (the horizontal) is similarly insensitive to
reductions of target luminance to near-threshold values. In our study
(Bedell et al., 1985) we assessed the precision of directionalization
with a variant of the classical monocular spatial partitioning task
(von Helmholtz, 1925; Ogle, 1932~. Observers successively partitioned
an 8.3-degree space in the right field into a series of perceptually
equal fractional spaces and then matched the direction of each parti-
tioning target in the opposite (left) half field. The targets were
luminous vertical lines, ranging in luminance from 40 to 0.04 cd/m2,
controlled by neutral density filters. For one subject, luminance of
the targets was further reduced with added Polaroid filtering to nearly
the detection threshold. Neither the precision of directionalization
(measured as the standard deviation of the psychometric functions fitted
to observers' partitioning judgments) nor the observers' small idiosyn-
cratic constant errors depended significantly on the luminance of the
targets. Together with the results of Leibowitz et al. (1955a,b),
results of our study (Bedell et al., 1985) indicate that the specifi-
cation of where a target is in the visual field is essentially inde-
pendent of its luminance. '
Figure 2 shows preliminary data (for the author's left eye) on the
effect of luminance on another directionalization task in which a
Vernier-type stimulus is viewed extrafoveally. The task (diagramed In
the inset to Figure 2) is to judge whether the small vertical line (3.S
by 30 min) is to the left or the right of an imaginary axis that con-
nect~ the tips of the two opposed triangles, which are separated by 1.5
degrees. The triangular reference targets are presented continuously,
and the line is flashed for 250 ms. Fixation is midway between two
vertical lines (each 1 degree long and separated by 1.5 degrees) to the
left of the triangles, a stimulus adequate to promote quite steady
fixation (Rattle, 1969; Sansbury et al., 1973~. The data plotted in
Figure 2 represent the horizontal distance, in log minutes of arc, that
the line must be displaced from its locus of subjective alignment (where
right and left responses occur equally often) to increase the probabil-
ity of a right response by 1 standard deviation (from 50 to 81 percent).
In the range of eccentricities examined, there was very little effect on
threshold of reducing the luminance of the display (achieved by inter-
posing neutral-density filters) from 40 to 0.4 cd/m2. When luminance
was further reduced to 0.01 cd/m2, thresholds' were elevates noticeably
at 1 and 2 degrees, but only slightly at greater eccentricities. In
fact, the substantially elevated line-displacement thresholds at 1 and
2 degrees when the display luminance was 0.01 cd/m2 are somewhat arti-
ficial in that the observer guessed the direction of the line's offset
on many presentations at 2 degrees and most of the presentations at 1
degree when the line was not detected at all. When the luminance of
the flashed line was adjusted to be near threshold (detected about 80
percent of the time) and the observer judged the liners direction with
respect to the triangles only when detection occurred, line-displacement
thresholds at eccentricities of 1 and 2 degrees (T's in Figure 2) were
OCR for page 149
149
2.0 ~
1.8
-
E 1.6
O 1-4 ~
:~
in
CC 1.2 _
:~
_
A_ 1.0 _
He
E.0.8_
a:
CL 0.6-
cn
LJ 0 4-
z
-
0.2
o
0.0
T
1 en'
An'
V
· 0.01 cd/m2
0.4 cd/m2
0 40 Cd/m2
/,~
W/~- _ ?~' ?
/,?
'/~
/;
A
2
6
ECCENTRICITY (BEG)
8
FIGURE 2 Thresholds for discriminating displacement of a small vertical
line with respect to the axis of a flanking reference target (see the
inset) are plotted for the author's left eye. Targets were presented
in the nasal visual field at the indicated luminances. Vertical bars
on data points represent l standard error. The T's show line displace-
ment thresholds when luminance of the l, ne was near detection threshold
at that eccentr ic ity in the f isle it deg ree, 0 . 06 cd/m2; 2 deg rees,
0.014 cd/m2; 3 degrees, 0. 005 cd/m ; ~and 8 degrees, 0.004 cd/m .
OCR for page 150
150
about 0.4 log units poorer than at the highest luminance. At greater
eccentricities, line-displacement threshholds were elevated by a
smaller amount--only about 0.2 log units--when the line's luminance was
at nearly the detection threshold. To compare these results with the
other data in Figure 2, it is useful to note that detection thresholds
for the line ranged from C.06 cd/m2 at 1 degree to 0.004 cd/m2 at 5 and
8 degrees.
Results at eccentricities beyond 2 or 3 degrees are reasonably com-
parable to those reported previously for judging a target's meridian
(Leibowitz et al., 1955a,b) or its direction along a meridian (Bedell
et al., 1985~. Reducing target luminance f ram moderate photopic levels
to nearly threshold results in only a small impairment (ca. O. 2 log
units, or 60 percent) of relative directionalization thresholds. Closer
to the fovea, Vernier thresholds increase about 2.5-fold (0.4 log units)
as the target's luminance is reduced to just detectability, a threshold
increase similar to those reported for Vernier acuity (Baker, 1949;
Berry et al., 1950; Leibowitz, 1955; Westheimer and McKee, 1977) and
spatial bisection (Klein and Levi, 1985) at the fovea. Whether or not
important differences exist between directionalization tasks is as yet
unclear; however, the picture that emerges is that reduction of target
luminance has a proportionately smaller effect on the precision of
directionalization outside the central few degrees than in the immediate
macular area. However, even in the macula, a reduction in target lumi-
nance has virtually no effect on directionalization until the luminance
falls to within 1 log unit or less of the detection threshold.
In contrast, recall that Landolt ring acuity at retinal eccentrici-
ties of 4 to 8 degrees decreases more substantially and regularly as
target luminance is reduced (Figure 1~. Incidentally, a comparison of
the data in F inures 1 and 2 reveals that the Vernier thresholds between
4 and 8 deg rees are similar in absolute magnitude to the acuity thresh-
olds for Landolt rings when luminance is 0.01 cd/m2. A likely reason
for the relatively poor Vernier thresholds in Figure 2 is that such
thresholds are highly dependent on the spatial separation between
targets (Sullivan et al., 1972; Enoch and Williams, 1983; Beck and
Halloran, 1985~. A related finding is that the separation between the
targets at which threshold is optimal increases regularly with eccen-
tricity in the field (westheimer, 1982; Yap et al., 19851. Because of
the fairly wide separation between targets, thresholds obtained with
the stimulus pictured in Figure 2 are not optimal r being on the order
of 1 min arc, even when this stimulus is presented at high luminances
near the fovea. It is clear that line-displacement thresholds would be
improved if the spacing between targets were smaller, and although as
yet untested, it is probable that comparable effects on these thresholds
would be obtained as luminance is reduced.
OCR for page 151
151
POS SI BLE INFLUENCE S ON THE ECCENT RI C
REGARD OF SCOTOPIC TARGETS
The limits of acuity and directionalization in the near peripheral
retina are of interest at low levels of illumination because at these
low light levels the fovea becomes insensitive and stimuli must be re-
garded extrafoveally. An assumption often made when measuring scotopic
visual functioning is that observers will use the most sensitive part
of the retina for the task at hand if targets are available for a suffi-
cient duration and if observers are instructed or trained beforehand to
use nonfoveal vision (Schlaer, 1937; Pirenne et al., 1957; Kinney,
1968~. However, a complicating factor is that the retinal locus with
the best sensitivity is likely to depend on the task. For example, it
is well established that the detection of scotopically illuminated
targets is best at an eccentricity of 15 to 20 degrees from the fovea,
where the density of scotopic receptors is greatest (Ten Doesschate,
1949; Sloan, 1950~. Scotopic visual acuity, on the other hand, peaks
closer to the fovea; Mandelbaum and Sloan (1947) report that, for a
range of scotopic luminances, the best acuity is found between 4 and 8
degrees from the fovea (Figure 11. Directionalization thresholds have
not yet been investigated systematically in the periphery at low mesopic
and scotopic luminance s. However, because directionalization falls off
faster with eccentricity than does resolution at photopic levels
(Westheimer, 1982; Levi et al., 1985) and, as discussed above, is less
affected by luminance, it would not be surprising if at scotopic lumi-
nance levels the best directi~onalization thresholds were found closer
to the fovea than the peak for resolution.
How observers identify and maintain the nonfoveal locus that they
use when freely viewing targets of low luminance has not been addressed
adequately. In Figure 3, two observers' monocular visual acuity for
scotopically illuminated targets is shown as a function of retinal
eccentricity. These data, collected in collaborat~on with Linda van
Schelt of the Southern California College of Optometry, are for illu-
minated Landolt rings (0.004 cd/m2) presented in a dark field. The
observer whose data are shown on the right is about a factor of two
more sensitive at each eccentricity, which can be ascribed to this
observer's much greater practice in making psychophysical aiscrimina-
tions in the peripheral field (Low, 1951; Johnson and Leibowitz, 1979~.
The arrows in Figure 3 mark the eccentricity to which each observer
adjusted a barely visible photopic fixation point to suc~ectively
maximize the discriminability of targets somewhat larger than those
corresponding to the best measured acuity. Targets were presented
continuously both for the measurement of acuity thresholds (50 per-
cent, corrected for guessing) and for adjustment of the fixation
target. Note that the experienced observer set the fixation mark such
that the targets were at an eccentricity slightly greater than that at
which the best acuity was found, although the difference in acuity
between the two loci is trivial. The second, less experienced observer
on two separate occasions placed the fixation mark so that targets were
nearer to the fovea than the locus of best acuity. Indeed, on one of
these occasions (the second) the observer adjusted the fixation point
OCR for page 224
224
. .
At.. ~,~i. it...
· _~.
_. ~
~.1
*
U.
· ~%
: ~
: ~
FIGURE 2 Bailey Contrast Sensitivity Test. me border contrast
sensitivity section of the Bailey Contrast Sensitivity Test.
it may be better than grating-based tests for the purpose of predicting
functional performance. The test is very easy to use and has surface
validity and apparent relevance to both the clinician and the patient.
The test should be sensitive enough to identify all but the rarest
abnormalities of contrast sensitivity that result from diseases to the
visual system.
Even though aging and night vision will be dealt with in greater
detail in another paper (C. Owsley, this volume), I should emphasize
that older people do have substantially more difficulty under dim light
conditions. Older individuals have less light reaching their retina,
partly because the ocular media absorbs more light and partly because
the pupil is smaller. If one were to consider retinal illuminance
alone, one could think of an old pair of eyes as being similar to a
young pair of eyes wearing sunglasses when the conditions are photopic.
But when the conditions are scotopic, it is as though the older eyes
are wearing a double layer of sunglasses or, perhaps, welding filters.
Figure 3 shows typical dark adaptation curves for a 40- and a 70-year-
old person. The curves are of similar shape, but they show that a 70
year old takes much longer to adapt to a dimmer luminance level. For
example, if one considers the level of sensitivity for detection at
which the 40-year-old person will be operating after 11 min of dark
adaptation, it will take the 70 year old 18 min to develop the same
degree of sensitivity at that same luminance level. In any considera-
tion of night vision, special attention must be paid to the strong
effects of age.
OCR for page 225
225
cat
J
. _
3 _
2
Log Ii \ \ \
;\ 1\ 1\
o
I \~\\\,: in yrs
1 1 \J~ \ 70-82
I ~ i\ \
~ i ; \~ 60-69
1 1 1 40-59
1 1 1
I'm ~ 1 1
10 20 30
Tlmc in minutes
FIGURE 3 Graph showing why older people take longer to reach a given
level of dark adaptation (after Birren and Shock, 1950~. Source:
Peale, 1963.
Under night vision conditions it should be expected that both
reading acuity and reading efficiency will be impaired. To consider
functional reading ability under dim light conditions, attention should
be paid not only to the size of the smallest type size that is legible
at all but also to the type size required for efficient reading per-
formance. Here I will describe some experiments that I have conducted
with Sam Berman and Robert Clear of the Lawrence Berkeley Laboratories.
We looked at reading efficiency and its relationship to type size,
luminance, and contrast. As our reading task, we used 40 Bailey-Lovie
Word Reading Charts (Bailey and Lovie, 1980) (Figure 4~. Each of these
charts contained samples of 17 type sizes. On the 11 lower rows, there
are 6 words per row (two 4-letter, two 7-letter, and two 10-letter
words). The print size ranged from 80 point (10 M) to 2 point (0.25
M). With the chart at 40 cm it is theoretically predicted that the
reading of the smallest row would require a visual acuity equivalent to
20/12.5 (0.40/0.25 M). The size progression on these charts is loga-
rithmic, with each row being about 0.8 times the size of the row above.
The charts were prepared as clear transparencies and viewed against a
retroilluminated opal screen. The luminance of the background screen
could be varied according to the number of incandescent bulbs engaged
and by the setting of a rheostat. An identical light box and opal
screen were positioned above the subject's head so that with reflectors
it could be optically superimposed on the light box with the test chart
A disk containing an open window and two apertures with partially re-
flecting mirrors was introduced close to the observer's eyes. The
OCR for page 226
226
~zs~' ~ ~
answers plnx
securities disease
luck collection
navy dynamic additional
incredible briefly gate
veteran encouraged lane
is historians gold carries membership bullets edge
me managed attempting stem fine remembered crawled
12 Stitch prooodu~ dew outdoor fail ~
FIGURE 4 Bailey-Lovie Word Reading Chart (Bailey and Lovie, 1980~.
Illustration shows 1 of a series of 40.
observer could view the test charts through the open window to achieve
maximum contrast (Michaelson's ratio of 97 percent) or through the two
partially silvered mirrors to achieve contrast levels of 64 and 17 per-
cent. For the experiment described here, the subjects read silently
while eye movements were monitored using an infrared reflection system.
Reading speed was measured for up to 11 different type sizes over a
luminance range that extended from 3 to 1,600 cd/m2. The measurements
were made on 17 normally sighted young adult subjects. In analyzing the
data, each subject's reading speed was normalized by first averaging
the reading speed with large (12- and 16-point) type, as this could be
read under every one of the various conditions tested. Then, for each
subject all other speeds were expressed as a ratio to his or her average
speed for the large type sizes. As expected, all subjects showed a
slowing of their reading speed as threshold type size was approached.
Under conditions of low contrast and low luminance, there was the
expected reduction in the ability to resolve the small print. Figures
5a and 5b show how, on average, the reading efficiency for 10-point
(magazine) and S-point (classified advertisement) type changed as a
function of luminance for the three contrast levels tested. For the
10-point type maximum speed could be achieved with either the high or
the medium contrasts over almost the entire luminance range. The
reading efficiency for the low-contrast material was only marginally
reduced, with the reduction being most pronounced when the luminance
was low. For the 5-point type, the reduction in contrast had an enor-
mous effect on reading efficiency. Even the medium contrast charts
produced a substantially slower reading speed for all luminances.
These results are not startling, but they do strongly illustrate that
reading efficiency is very dependent on type size and that size and
efficiency relationships can change dramatically when contrast or
luminance is reduced.
OCR for page 227
227
~ 2
e_
A)
·_
hi, 1 5
i_
~ 1
a
MEAINI RELATIVE READING TIME
VERSUS LUMINANCE
L = ~ ~ o/O c ontrast
M = 64 °/0 contrast
H = 97°/0 contrast
for 10 point print
3 10 22 46 100 250 630 1600 Cd/ ~
b
MEA N RELATIVE
READING TIME
VERSUS LUMINANCE
``Low
for 5 point print
M
10 22 46 100 250
LUMINANCE ( Cd/ M2 )
630 1 600
FIGURE 5 Reading time versus luminance functions for three contrast
levels and for two type sizes. Data represent averages of normalized
reading times per row for 17 normally sighted young adu It subjects
using a 40-cm viewing distance. ( a) Type size is 10 point ( 1.25 M) .
(b) Type size is 5 point (0. 63 M) .
OCR for page 228
228
The most interesting result to come from this study emerged when
each of the various reading efficiency versus type size curves was
plotted in a manner such that the type size was expressed as a ratio to
the threshold type size. This normalizes type size relative to thresh-
old for each condition. In Figure 6 we present curves for low-contrast/
low-luminance conditions, medium-contrast/medium-luminance conditions,
and high-contrast/high-luminance conditions. The three curves are vir-
tually identical in both shape and location. The very small lateral
displacement between the three curves is probably an artifact that can
be attributed to an underestimation of threshold type size under the
better viewing conditions because some subjects were able to read all
words on the charts, and consequently, their resolution limit about
which the curves were normalized was underestimated. Taking these
curves to be identical leads to the conclusion that people can perform
this particular reading task with maximum efficiency, provided that the
type size is about 2.5 to 3 times larger than the threshold size. This
is the case regardless of illumination and contrast. This suggests a
useful rule for illuminating engineering. For illumination to be ace-
quate for reading tasks, there should be at least a 2.S to 3 times
reserve above the threshold of resolution. In practice, a good test of
adequacy of illumination for a specific task might be to determine
whether an individual can perform the required tasks at a distance that
is 2.5 to 3 times the normal operational viewing distance. Printers
have long been aware that some reserve over resolution threshold is
required for efficient and acceptably comfortable reading performance.
Economic considerations encourage printers to use smaller type sizes,
but they are limited by the public's willingness to accept the smaller
type. It has evolved that 8-point type or thereabouts is used for most
newspaper material. This print is normally read at about 40 cm. The
distance at which newsprint is just legible to normally sighted indi-
viduals is just beyond 1 m. This fits in well with our findings that
suggest the desirability of a reserve of 2.5 to 3 times above the
threshold.
In summary, visual performance at mobility and reading tasks under
dim illumination have not been specifically addressed in the currently
available literature. Two limited studies directed at understanding
mobility skills of persons with low vision suggest that visually guided
mobility skills are likely to be somewhat reduced if maximum contrast
sensitivity is only in the 1~0- to 1.5-log-unit range. For contrast
sensitivities below 0.5 log units, visually guided mobility skills can
be expected to be minimal. It has been argued here that a test of
border contrast sensitivity under the given illumination conditions
might be a more relevant test of visual capacity for adequate mobility.
To determine whether an individual is able to perform reading tasks in
dim light, it is a simple practical matter to measure the threshold
type size for legibility under the illumination conditions of interest.
Based on photopic studies reported here, it might be expected that a
reserve of three times the threshold type size might be necessary to
achieve maximum efficiency under mesopic conditions.
OCR for page 229
229
, \
', \\
'\\
\ \
''\\
~ \ .
~ 2
.E
· _
hi;, 1 5
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-
~ 1
CHANGE IN READING TIME
AS THRESHOLD SIZE IS APPROACHED
~ L
·17 10
At\\
. \
·\ \
%\ \
"N \
a.
64 46
·9 7 250
1 2 4 ~
ratio- print size relative to threshold
FIGURE 6 Normalized reading times versus ratio of type size read to
threshold type size. Data from 17 young adult subjects reading
Bailey-Lovie Word Reading Charts under three conditions of illumination
(L).
REFERENCES
Bailey, I.L., and J.E. Lovie
1980 The design and use of a new near vision chart . Amer ican
Journal of Optometry and Physiological Optics 57: 378-3&7.
Birren, J.E., and N.W. Shock
1950 Age changes in rate and level of dark adaptation. Journal of
Applied Physiology 2: 407-411.
De Valois, R.L., M. Morgan, and D.~. Snodderly
1974 Psychophysical studies of monkey vision. III. Spatial
luminance contrast sensitivity tests of macaque and human
observers. Vision Research 14: 75-81.
Marron, J.A., and I.L. Bailey
1982 Visual factors in or ientation-mobility performance. Amer ican
Journal of Optometry and Physiological Ontics 59: 413-426.
Pelli D.G., and J.A. Serio
1984 The visual requirements for mobility. ARVO Abstract Investiga-
tions in Ophthalmology and Visual Science 25(Suppl.) :72.
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230
Weale R.A.
1963 The Aging Eye. London: Lewis.
Welsh R .L ., and B .B . B lasch
1980 Foundations of Orientation and Mobility. New York
Foundation for the Blind.
: Amer ican
Young, G.
1918 Threshold tests. British Journal of Ophthalmology 2 :384-392
~ .
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GENERAL DISCUSSION
-
ALEXANDER: I'd like to ask Dr. MacLeod if he'd care to speculate
on the mechanism that might underlie this two-system rod response. It
is in the photoreceptors, or is it further on downstream?
MACLEOD: That's obviously an important question, and unfortunately
I'm at a loss to give you any useful information as to the answer.
We've done our best to distinguish the two mechanisms spectrally or in
terms of the directional sensitivity, but we never find any distinction
between them, There are several hypotheses that are attractive. The
one that's most attractive, I think, is that there are fast cone path-
ways to the ganglion cell and slow rod pathways. I've tried to test
that by putting masking flickering stimuli in through the cones to try
to sort of block the fast cone pathway and upset the null between the
fast and slow rod mechanisms, but we haven't so far succeeded in up-
setting the null. So I just don't know. There is some relevant
receptor physiology. In the skate, D.G. Green and I. Siegel (Science
188:1120, 1975) found a two-branched CFF log intensity function, but so
far that has not been reported in the rods of other animals.
MAKOUS: I'd like to comment on Dr. Bedell's suggestion that the
Stiles-Crawford effect might account for some of the unexpectedly low
effects or small effects of defocus in photopic vision. One can apodize
the pupil so that transmission is reduced toward the edges. Of course,
if you defocus an image that passes through an aperture like the pupil
of the eye, naturally you get reduced optical transfer, but the inter-
esting thing is, an apodized pupil produces a uniformly higher transfer
function for a defocused image. So what the Stiles-Crawford effect
does is effectively apodize the human pupil and produce superior con-
trast sensitivity for defocused images. This applies even to images
that are defocused by a small amount corresponding to about a quarter
of a diopter.
PITTS: I appreciate Dr. Haber's paper very much, because I've
published in this area some years ago while I was in the Air Force.
But I'd like to suggest to him that in addition to losses of visual
input (which he emphasized), there's also increased confusion and
illusory effects as we get in dim illumination. Dim illumination is
not the reason for this increase in confusion and illusory effects,
because it is even worse under bad weather conditions.
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Regarding some of the examples that he gave, I would also like to
say that the stars are not fixed for a pilot at night, particularly for
a fighter pilot flying alone. If a fighter pilot does not look at his
instruments regularly, that star will move. We have many a pilot that's
locked onto a star, and you have one hell of a job trying to convince
him that it's not a light that he's looking at, that it's a star he's
flying toward. The other problem is that as you decrease light at
night, the patterns that we think are set up with the lighting of the
systems can be confused. There is a particular time when pilots con-
fuse lights in the sky, called stars, and lights on the ground, that
are streetlights. They have even landed aircraft low; Japan Airlines 2
or 3 years ago landed in San Francisco Bay just because of these sorts
of problems. And so they are real; they're there; and as you lower the
light levels, they get worse. As you decrease the weather conditions,
they get even worse.
HABER: I didn't touch on illusory perceptions at all, but those
are obviously very relevant. Your last example shows that as you
reduce illumination, and particularly when flying over boundaries
between water, cities, and sky, the pilot has to deal with three
horizons, the clear ones being the wrong ones to fly to. He is likely
to confuse the clear ones with the real one and impact the ground when
he didn't expect to.
In my comment about the visible stars not moving, I was being an
astrophysicist there rather than a perception psychologist. The stars
clearly move in that environment if you try to fixate or track them
with your eyes--the familiar autokinetic effect. Clearly I should add
those to the kind of presentation I gave.
WATSON: If I read your graphs correctly, Don [MacLeod], you only
showed that a reduction of a factor of about 2 from the peak temporal
sensitivity to the lowest chopper is going to reduce the amplitude of
your signal by a factor of 2. So it seems to me you'd be back to where
you started out. Indeed, the curve may go down much more than you
showed it at the low end. The second question has to do with your
description of phase lags and cancellation techniques. You treated
everything as though it were linear, and I wanted to know if you'd
thought through the consequences of very early nonlinearities and how
that would affect the ability to cancel things travelling through the
two pathways?
MACLEOD: On the first point--that there's not a sufficiently
severe loss of sensitivity at low temporal frequencies to make use of
the chopper worthwhile--it's true that at least some of the curves I
showed didn't have that much of a loss at low temporal frequencies.
But I omitted to tell you--and will apologetically tell you now--that
those were actually square-wave temporal modulations. That makes a
difference. For sine waves, you can get a factor of 10 between 1 Hz
and 10 Hz and in that case, for a sufficiently low spatial frequency
content, I think, you might expect a fivefold improvement (10 divided
by 2) from using the chopper. And with an image converter that inverts
contrast instead of chopping, the factor of 2 would not have to be lost.
There is a problem that with good edges in the stimulus, you have--
by virtue of fixation of eye movements--some sensitivity down to zero
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temporal frequencies, without any need for chopping. But when the
edges of the object are obscured by fog or haze or perhaps even retinal
resolution losses, then it might be helpful. I expect that the domain
of usefulness might be only below about 1 cycle/degree for rods, and a
bit above that for cones.
On the second point--about the role of nonlinearity in the pulling
of rod and cone signals--I did show a slide where a model that included
the assumption of linearity seemed to fit the data fairly well, although
not perfectly. So I think that shows that the consequences of nonline-
arity of the signals are not tremendously large. In principle, though,
you would expect that second ha``,~onic components in the rod and cone
signals would not cancel along with fundamental components. That would
lead to residual flicker in the conditions of the mesopic null. I think
there are some observations which do show that there is this sort of
distortion--one of them being that under some conditions there are two
phases that generate minimal flicker as you vary the phase between a
rod stimulus and a cone stimulus.
JOHNSON: I'd like to have Dr. Haber clarify a point with regard to
night visual approaches. The work that Conrad graft has done with
night visual approaches indicates that there are certain situations in
which pilots do not check their instruments and will come in too low on
their final approach because they think that they are higher than they
actually are. The common components of these situations include a body
of water or other dark expanse in front of the airport with a well-lit
city in the background. This creates a misperception that causes the
pilots to believe that they're actually higher than they are. What you
described, in terms of the problems with training and going from day-
light visual approaches to nighttime visual approaches, is exactly the
opposite. Pilots flare too high, the presumption there being that the
Pilots think they are actually lower than they are. I was wondering if
there was some kind of explanation that could account for these seem-
ingly opposite effects?
HABER: Yes, the two tasks are quite different. In the approach
over water--which were the examples by Conrad Kraft, who is the one who
has done the most work with the Boeing simulators on that--you have the
basic problem of identifying which horizon is the true horizon. And if
the pilot judges the horizon to be the water-land horizon, then he's
going to be aiming below that, because obviously the runway must be
below that horizon. So he comes in much too low. That' s not the only
characteristic that accounts for that kind of accident.
The general characteristic for night f lying, particularly for
beginning pilots--where you don't have a horizon at the near end of the
runway but where you've got city lights all around you--is simply
having trouble judging how high you are. So they will come in too high
over the numbers at the end of the runway, where they should be ready
to flare and touch. Instead, they may be a few feet too high (5, 10,
or even 50 feet) depending on the type of plane and type of approach,
and then they float down the runway. That's obviously a better outcome
than landing short, as long as the runway's long enough and they don' t
bounce too hard on it. Thus, the two tasks have quite different courses
and differ ent ou tcomes.
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M. O'NEAL: I'm at the Aerospace Medical Research Laboratory, at
Wright-Patterson AFB, Ohio. This is directed to Dr. Harvey. Contrast
sensitivity has been touted recently as a replacement for the acuity
chart. Recently, at a meeting at Wright-Patterson, it was suggested
that contract sensitivity testing could be used to select pilots for
specific missions. In your opinion, do you feel that contrast sensi-
tivity testing has progressed to the point where this is feasible and
proper? Also, what research is going on in contrast sensitivity per-
ception to correlate it with performance?
HARVEY: Let me say that the so-called contrast sensitivity func-
tion is only predicting a small number of operational things and is not
exactly correct. I mean, if you look in the vision literature, maybe
50 percent of the articles in vision research are devoted to some
aspects of the relationship between spatial frequency characterization
of stimuli and some sort of functional properties of the stimuli--some
sort of performance in terms of discrimination, detection, and whatnot.
So there's an incredible literature that relates contrast sensitivity
to basic psychophysical functions. In terms of actual real-world
tasks, I don't know right now what's going on in the Air Force. Are
there actual attempts to set up a proper evaluation of these contrast
sensitivity functions in relation to tasking?
There was a lot of evidence reviewed recently by the Committee on
Vision--that I've described--suggesting that the contrast sensitivity
function could become a better tool for selection if used in the proper
way. We recommended that research be carried out comparing individual
differences, not only on contrast sensitivity but on a number of basic
measures to look at the relationship between individual differences on
these tasks and individual differences on the performance of some tasks
which are of interest to the Air Force.
Representative terms from entire chapter:
visual acuity
