Night Vision Current Research and Future Directions, Symposium Proceedings (1987) / Chapter Skim
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Spatial and Temporal Factors Affecting Night Vision
Spatial and Temporal Factors Affecting Night Vision
Pages 141-234
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From page 143... ...
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)
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From page 144... ...
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 performance, in which targets are described in terms of their spatial frequency characteristics.
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From 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.
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From page 146... ...
, or blur (Prince and Fry, 1956; Homer et al., 1985~. Figure 1 shows that between 4 and 8 degrees from the fovea, reduction 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)
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From page 147... ...
__4~ _ ~ _ -I ._ _ _-~ r I , , , , , , , , 1 o 2 4 6 8 10 ECCENTRICITY (DEG) FIGURE 1 Extrafoveal visual acuity data (MAR = minimum angle of resolution)
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From page 148... ...
indicate that the specification of where a target is in the visual field is essentially independent of its luminance. ' Figure 2 shows preliminary data (for the author's left eye)
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From page 149... ...
are plotted for the author's left eye. Targets were presented in the nasal visual field at the indicated luminances.
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From page 150... ...
of relative directionalization thresholds. Closer to the fovea, Vernier thresholds increase about 2.5-fold (0.4 log units)
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From page 151... ...
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 illuminated Landolt rings (0.004 cd/m2)
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Targets were presented in the right visual field (to the right eye of the observer whose data are on the left and to the left eye of the observer whose data are on the right)
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From page 153... ...
. Their results show that small amounts of blur adversely influence visual acuity at all luminances examined and, surprisingly, that acuity is influenced by the greatest amount at the lowest illumination.
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From page 154... ...
LUMINANCE (cd/m2 ) FIGURE 4 Visual acuity (}4AR = minimum angle of resolution)
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From page 155... ...
If the effect of blur on contrast sensitivity is compared at spatial frequencies between 0.25 and 2 cycles/degree, it is seen that greater losses of contrast sensitivity occur when the luminance is low. Furthermore, the blurring lens has its first deleterious effect on contrast sensitivity for a coarser spatial frequency at low than at high luminance (about 0.75 cycles/degree at the lower luminance versus about 1.5 cycles/degree at the higher luminance)
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From page 156... ...
affected at the same spatial frequencies. The answer is not yet known, but there is at least one reasonable guess: even though pupil size was physically the same at both luminances, the much greater magnitude of the photopic than the scotopic Stiles-Crawford effect (van Loo and Enoch, 1975)
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From page 157... ...
Such information is necessary to develop viewing strategies for the optimal performance of particular visual tasks at dim light levels. The influence of optical degradation produced by papillary dilation and inappropriate accommodation on visual performance at dint luminance levels needs to be investigated more fully.
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From page 158... ...
Flom, M.C. 1966 New concepts on visual acuity.
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From page 159... ...
raction on visual acuity. Amer~can Journal of Optometry and Archives of Amer ican Acadernly Optometry 33 :353-373.
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Bedell 1981 The effect of optical blur on visual acuity for targets of different luminances. Opthalmic and Physiological Optics 6:279-281.
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Abplanalp 1986 Blindspot " fixation" in normal eyes: Implications for eccentr ic viewing in bilate ral macular disease . Amer ican Journal of Optometry and Physiological Optics 62 :259-264.
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From page 162... ...
The value of visual acuity measurements is well proven for correcting refractive errors. Yet, under some conditions, individual variation in standard visual acuity measurements often is not able to predict individual variation in performance on some visual tasks, such as target detection and identification (e.g., Ginsburg et al., 1983)
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From page 163... ...
Because it is measured in terms of the smallest identifiable, highcontrast target and because small sizes correspond to high spatial frequencies, visual acuity measures visual sensitivity largely in the higher frequency regions of the contrast sensitivity function. In brief, visual acuity is measured in terms of the size of the critical detail (stroke width of the Snellen letter, for example)
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From page 164... ...
Note the wide variations in low- and high-frequency sensitivity and that a low sensitivity at high frequencies does not necessarily imply a low sensitivity at low spatial frequencies. Visual acuity measurements, which are related primarily to high spatial frequency sensitivity, cannot predict contrast sensitivity to low spatial frequencies because threshold to spatial frequencies separated by more that about a factor of 2 (one octave)
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From page 165... ...
The contrast sensitivity function has the potential-of adding more information about the functioning of the visual system than that given by visual acuity, because it assesses sensitivity over a wide range of spatial frequencies, while visual acuity pr imar fly measures sensitivity at the high spatial frequencies. While visual acuity cannot predict the spatial contrast sensitivity function in people with abnormal vision, visual acuity also cannot predict contrast sensitivity in people with assumed normal vision.
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From page 166... ...
This approach allows both the visual stimulus and the visual system to be described with the same language: that of sinusoidal spatial frequencies. Basic Factors in Spatial Contrast Sensitivity Contrast The dependent variable which is the basis of the contrast sensitivity function is the contrast of the sinusoidal test grating.
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From page 167... ...
This change in the CSF is believed to reflect the unequal way in which the retinal visual field is projected onto the visual cortex (Daniel and Whitteridge, 1961; Schwartz, 1980~. When the contrast sensitivity function is measured at different retinal eccentricities with sinusoidal gratings, the size and spatial frequency of which have been adjusted to stimulate equal amounts of visual cortex, the CSF is approximately the same at all retinal loci (Virus and Rovamo, 1979; Rovamo and Virus, 19791.
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From page 168... ...
Psychophysical evidence suggests that the human visual system may also contain mechanisms with characteristics of Gabor functions (Daugman, 198Q; MacKay, 1981; Watson et al., 1987; Pollen et al., l984~. These developments may have consequences for the way in which the human contrast sensitivity function is measured, but it is too early to know with any certainty what they are.
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From page 169... ...
Another study showed that when the spatial frequency content of the stimulus patterns was combined with the infant's contrast sensitivity function, both the infant's looking preferences and looking times were better predicted than by the contour density measure (Gayl et al., 1983~. Finally, individual differences in contrast sensitivity functions may be the basis of ind ividual differences in performance on complex tasks.
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From page 170... ...
Visual acuity and contrast sensitivity functions were measured under high and low photopic levels of luminance. The correlation between the acuity measures and detection range was not statistically significant.
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Gervais, M.J. 1978 Spatial Frequency and Letter Confusions: A Test of Fourier Analysis as a Metric and a Model for Form Perception.
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Nachmias 1971 Detection of bating patterns containing two spatial frequencies: A comparison of single and multi-channel models. Vision Research 11:251-259.
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Stromeyer, C.F., III, and B Julesz 1972 Spatial frequency masking in vision: Critical bands and spread of masking.
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From page 174... ...
Section three examines the ways in which the visual system normally changes in its function when light levels decrease, especially with respect to flying performance tasks. I then distinguish between those changes in visual functioning that affect everyone in much the same way and those that are due to large individual differences that interfere with the normal visual functioning of some people during nighttime.
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From page 175... ...
PERFORMANCE TASKS REQUIRING VISION DURING FLIGHT Every flight manual contains a list of the principal tasks facing a pilot that require visual information and processing. What is missing from the flight manuals is a description of the visual information needed by the pilot for him to perform each of these tasks.
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From page 176... ...
In the latter cases, my viewing is not along the textured ground surface but through empty air. This makes it much harder to locate myself in the scene, and especially harder to perceive my distance to any particular object.
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From page 177... ...
Once on the final approach, the pilot must then perceive the slope of the ground and the attitude of the plane relative to the ground and the alignment of the flight path to the runway, corrected for wind conditions, and acquire sufficient visual information to be able to achieve and maintain proper aircraft attitude, glide slope, and speed over the course of the final approach, flare, and touchdown. There are three general kinds of visual tasks for the pilot contained in this list: perceiving the slope of the terrain and arrangements of the objects on the terrain, especially the runways in relation to surrounding terrain, buildings, and obstacles; perceiving the aircraft's location and attitude in relation to those arrangements; and maintaining visual control of the aircraft's movement through the scene.
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Optic flow arising from terrain texture; dynamic occlusion of terrain elevation variation; fine detail detection; familiar size of terrain features. See above.
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From page 179... ...
The cells of Table 1 have been filled in according to the visual tasks corresponding to each of the performance tasks and the sources of the visual information needed to carry out those visual tasks. Construction of a table like this one (and the ones to follow)
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Perceive and Contrast between figure and identify object background; familiar shape. Maintain ground track Perceive layout See Table 1 of space Maintain attitude Maintain spatial orientation Perceive horizon See above Perceive AGL change See Table 1 Perceive horizon See above Avoid obstacles and Perceive layout See Table 1 Perceive AGL See Table 1 Maintain formation Perceive Texture density; fine detail; distance; perspective; motion.
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From page 181... ...
Again, while this list can be found in any flight manual, the characteristics of the visual information and perceptual processing needed to carry out these tasks are only beginning to be understood. When the planned altitude is less than several thousand feet above ground level, the visual information needed to perceive ground clearance, heading, and ground track is the same as that discussed in the previous section on landing (see Miller, 1983~.
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From page 182... ...
Rather similar visual information is needed to accomplish midair refueling, except that the perceptual invariants now concern the rates of closure and joining, as well as relative position invariance. Navigation Many of the visual tasks required for cross-country flight are basically navigational performance tasks.
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From page 183... ...
The center columns note the visual tasks required to carry out each of the flying tasks. The righthand columns attempt to spell out the kinds of visual information potentially available to a pilot for each of those visual tasks.
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From page 184... ...
One concerns identification of fine detail, requiring high visual acuity of the kind assessed by a Snellen eye chart. Another concerns detection of targets against backgrounds, which usually requires good contrast sensitivity, especially in the middle- and low-spatial-frequency ranges (Ginsberg, 1981~.
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From page 185... ...
CHANGES FROM DAY TO NIGHT IN THE PROCESSING OF VISUAL INFORMATION The amount of available light affects virtually every function of the human visual system. In most cases functioning is so utterly transformed that the principles used to describe high light {photopic)
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From page 186... ...
Switch from su rf ace textur e to point sou rce illuminations from ground The most dramatic change concerns the ability to resolve fine detail. High-spatial-frequency visual acuity is monotonically dependent on luminance, with a lO-fold loss in acuity from the luminance available at high noon to room-level lighting.
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From page 187... ...
But point sources can be highly informative to a moving observer. A randomly arranged array of point sources attached to a contoured ground surface conveys information regarding the shape of the contours to a moving viewer.
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From page 188... ...
Another loss occurs with respect to visual search capabilities. Because cone functioning effectively ceases below twilight light levels, the fovea becomes a functional blind spot at night, so that directing one's gaze to the place where one might expect to find sDmething is the wrong strategy.
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From page 189... ...
(1984) showed substantial visual acuity losses at mesopic luminance ranges (presumably, primarily cone functions)
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From page 190... ...
showed that visual acuity and contrast sensitivity were substantially and monotonically lower for older as compared with younger subjects (ages ranged from 20 to 6G) under both photopic and scotopic conditions, but that the losses were larger when testing night viewing conditions.
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From page 191... ...
I have added a list of these variables to the lower half of Table 7. To find screening tests that predict these nighttime visual performance tasks, there must be a research program that develops both kinds of measures -- simple tests of visual functioning and realistic measures of flying performance at night.
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A number of critical questions can be answered from these kinds of data. Most importantly, how much variation is found in the night flying visual tasks among subjects equivalent to applicants to flight training school?
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From page 193... ...
Even so, there are several aspects of visual performance during night flight that might be amenable to practice and training. As far as I know, little research has been done on any of these.
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From page 194... ...
Hecht, S 1928 The relation between visual acuity and illumination.
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From page 195... ...
Osgood 1982 Effects of glare and background luminance on visual acuity and contrast sensitivity: Implications for driver night vision testing. Human Factors 24: 347-360.
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From page 196... ...
The evidence of Holladay and Stiles depended on the functional characteristics of the disability glare phenomenon, the loss in visual sensitivity being dependent on linearly additive effects from different localized glare sources, and the disability being describable in terms of the concept of an equivalent veiling luminance. The basic disability glare equation that emerged from the work of Holladay and Stiles may be rewritten for convenience in the following form: L K ~ s cos v ~2 (1)
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From page 197... ...
The numerical value of K is well established as approximately 10, at least for observers in the 20to 30-year age group and equilibrium adaptation to about 100 cd/m of luminance. K AS A FUNCTION OF LUMINANCE AND OBSERVER AGE The results of recent studies concerned with possible systematic effects of luminance, observer age, or both on the value of the disability glare constant can be found in a publication put out by the International Commission on Illumination (CIE, 1981~.
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From page 198... ...
FIGURE 2 Changes in Krel' the disability glare constant set to unity for 20- to 30-year-olds, as a function of observer age.
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From page 199... ...
It also sometimes occurs that the effect of disability glare is expressed relative to a situation of a perfectly uniform extended field which, without any glare sources as usually defined, produces 7.4 percent stray light. In this case, the effect of disability glare in reducing image contrast is expressed in terms of the following: CF = L/Le, where Le is Le = (L + LV)
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From page 200... ...
Comparison of the values of DGF and CF reveals the extent to which losses in target visibility due to reduction in image contrast are offset by increases in target visibility due to increased contrast sensitivity resulting from adaptation to a higher luminance. ASSESSMENT OF Lv IN NIGHT ENVIRONMENTS OF INTEREST Equation 1 implies that assessment of LO in a night environment of interest requires the measurement of luminance and angular size of each elementary source of disability glare, followed by weighted summations to obtain the total glare effect of all sources in the visual field.
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From page 201... ...
me agreement between the X's and the solid curve is considered remarkably good, considering that values of relative Lv varied over nearly 5 log units. It should be apparent that the optical attachment, often referred to as a glare lens, is well qualified to mimic the ocular stray light characteristics of the 20-year old eye when operating at relatively high luminances.
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202 Conformity of 1983 Glare Lenses to Formulation loo ~ 50 _ 10 5 S._ .
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From page 203... ...
DI SABI LI TY GLARE AS A MAJOR FACTOR IN NIGHT VISION AND TARGET VI SIBILITY ,. The loss in image contrast due to ocular light scatter in the presence of glare sources often dominates the assessment of night vision capability and target visibility.
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From page 204... ...
1 _ .005 .001 . 1 ~ \ \ \ \ \ \ \v~ 204 Conformity of 1983 Glare Lenses to Formulation my = Cos 9/~8x(~+l,51]
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Blackwell 1963 Design and calibration of a disability glare lens. Illuminating Engineering 58 :120-123.
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From page 206... ...
For instance, the rise in CFF with increasing intensity might occur because the visual signals persist for a longer time in dim light. But, alternatively, it could be explained -- and this was essentially how the model of Hecht and Shlaer (1936)
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From page 207... ...
With rod vision at a low scotopic light level, however, the freouency response looks very different, dropping sharply with increasing frequency until it reaches its limit at about 15 Hz. So rods, unlike cones, are extremely sluggish at low light levels.
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From page 208... ...
This rod saturation-related decline in CFF, from a peak close to 30 Hz, is illustrated in Figure 6 of Conner (1982~. So there is clear evidence for a duplicity within the rod mechanism itself that allows rods to pick up frequencies at mesopic light levels nearly twice as high as the scotopic limit of 15 Hz found by Hecht and Shlaer (1936~.
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From page 209... ...
No cone function whatsoever has been found in this subject, so if rhodopsin-filled cones do exist in humans, which now seems doubtful, they are clearly not the basis for the improved mesopic flicker detection in this true rod monochromat, or indeed under our conditions in the normal eye. Instead, the evidence shows that light-adapted rods can detect rapid
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From page 210... ...
That comparison is made in Figure 3. The result is that the cones still retain an advantage, but it is relatively slight: if rod and cone sensitivities are equated at low flicker frequencies, it is not until 20 Hz that rod sensitivity drops by a factor 2, or 0.3 log units, below that of the cones.
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211 .02 .05 o at_ ~ 1 3 o .' - I_ /\ O' o o ·\ o Cones 25 ph.td.
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From page 212... ...
1 212 \ FAINT TaRGET 6 Hz alternation me\ \ Uniform reference field is equated with target In space average luminance / ~\1 If\\ / A .. \\\ REFERENCE FIELD ( uniform diffuser)
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From page 213... ...
If the greater delay of the rod signal is compensated for by advancing the rod stimulus by an equal amount, the two signals wild be brought into opposite phase and will cancel to yield a constant sum, that is, a steady or minimally flicker ing light. The amount by which the rod stimulus must be advanced from the opposite phase to minimize flicker is an index of the rod signal's phase lag relative to that of the cones.
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From page 214... ...
By adjustment the subject found the phase difference at which the resultant flicker appeared to be null, or of least amplitude. Necessarily, the rod phase lags are relative to those of the cones; they are given by the amount that the rod stimulus must be advanced front opposite phase to yield a null.
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From page 215... ...
, and flicker visibility depends on the amplitude of the resultant or vector sum. If the rods have an ir,ternal duplicity, a scotopic null where cones are not involved, but where the fast and slow rod signals come out in opposite phase may be expected.
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From page 217... ...
Sharpe, MacLeod, and Stockman have measured the rod phase lag, relative to that of cones, at stimulus intensities just 0.4 log units apart, straddling the scotopic null. They were found to differ by nearly 180 degrees -- strong evidence that the null results from two separate rod signals that destructively interfere with each other.
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From page 218... ...
218 ·005 .01 ·02 c 05 .1 .2 S l\ o 0 Rods ~ beats)
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From page 219... ...
With this procedure, it was found (Figure 8) that rods can detect intensity modulations as small as 0.5 percent or 0.005 log units, corresponding to a Weber fraction of 1/200.
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From page 220... ...
Regarding reading skills, much is known about changes in visual acuity or resolution limits with illumination, but changes in efficiency of reading has been relatively ignored. In this paper some experiments will be described that might relate to the prediction of impairments of reading efficiency under dim illumination.
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From page 221... ...
The results showed that, within this sample population, visual acuity had very little predictive power for mobility performance. Both visual field size and contrast sensitivity, however, did show moderate predictive power.
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From page 222... ...
(1974~. Figure l shows five contrast sensitivity functions measured under different screen luminance levels ranging from 0.0017 to 17.0 cd/~.
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From page 223... ...
Theoretically, relating our test results to the contrast sensitivity function, the border contrast sensitivity score should approximate the peak of the contrast sensitivity function and the difference between the high- and low-acuity scores should provide a measure of the slope of the CSF as it approaches the high-spatial-frequency cutoff. The high-contrast visual acuity score should be an index of the cutoff spatial frequency.
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From page 224... ...
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 consideration of night vision, special attention must be paid to the strong effects of age.
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From page 225... ...
. 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)
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From page 226... ...
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.
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From page 227... ...
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.
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From page 228... ...
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.
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From page 229... ...
\ ·\ \ %\ \ "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.
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From page 230... ...
: Amer ican Young, G 1918 Threshold tests.
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From page 231... ...
, 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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From page 232... ...
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)
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From page 233... ...
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.
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From page 234... ...
HARVEY: Let me say that the so-called contrast sensitivity function 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.
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