Video Displays, Work, and Vision (1983) / Chapter Skim
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4. Display Characteristics
4. Display Characteristics
Pages 66-110
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From page 66... ...
For each of the pertinent display variables, we consider three categories of effects on human users: physiological effects, the effects of display variables on measurable and objective performance, and known relationships between display parameters and subjective estimates of display quality or related physical symptoms. Physiological effects are those in which the display parameter has a known, direct physiological effect on the human visual or other organic system.
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From page 67... ...
will have a background luminance of about 1 or 2 mL, and a character luminance of about 25 mL, with a character density of approximately 30 percent. This combination produces a display having an average (adapting)
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From page 68... ...
, the adaptation level is probably not a function of either background luminance or integrated luminance, but rather a function of the higher luminance of an irregular surface. Thus, Rupp suggests that the lighter of the two items, either the background or the character, will essentially control the adapting luminance level, thereby negating any effect on pupil size due to positive versus negative contrast.
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From page 69... ...
luminance (Hopkinson and Collins, 1970~. A secondary benefit of higher display luminance is the increase in visual depth of field Cased on a fixed diameter of the "blur circle")
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From page 70... ...
. This difference in pupil diameter corresponds to approximately a 30 percent difference in blur circle diameter (at the 50 percent intensity point)
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From page 71... ...
Effects on Flicker Threshold Another physiological effect on the visual system resulting from changes in display luminance relates to shifts in the flicker threshold. As illustrated in Figure 4.5, the temporal contrast sensitivity function becomes less sensitive with decreases in retinal illuminance.
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From page 72... ...
This recommendation appears to be reasonable and probably indicates the main reason that manufacturers have been reluctant to use negative-contrast displays in the past: standard television monitors cannot produce that repetition rate. Effects on Visual Task Performance The effect of display luminance on visual task performance has been investigated in a few studies.
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From page 73... ...
Research efforts to fill these data gaps are obviously needed. Contrast and Contrast Polarity As suggested in the preceding discussion, increases in contrast have been shown to produce significant increases in visual task performance.
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From page 74... ...
causes no elevation in reading time, which is otherwise the result of space between the dots. It must be recognized that most word processing and data processing displays today use either dot-matrix or raster-written characters, either of which can have visible spacing in the vertical dimension and, in the case of dot-matrix characters, also in
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From page 75... ...
- ~ FIGURE 4.6 Characters produced on a VDT screen from a raste' structure.
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From page 77... ...
Various means can be used to reduce the raster modulation, and one or more should always be used to eliminate visible raster or dot structure. Among the techniques to reduce raster loo is o ~ 80 o C' UJ CC 'A 60 o it CC UJ 40 J 20 o _ it' 2 4 /~L = Large Size Targets ( 13)
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From page 78... ...
and matching of the scanning spot size to the raster pitch. It is preferable to select a spot size in the dimension perpendicular to the raster such that the spot size and raster pitch are compatible to produce a visually flat field, that is, a field having no visible raster modulation.
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From page 79... ...
Anecdotal evidence also supports this contention. There are no known subjective preference effects for various character matrix sizes or numbers of raster lines with which individual characters are written.
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From page 80... ...
Although annoyance, headaches, and other negative subjective responses to flickering displays are often reported, it has yet to be demonstrated that a significant deterioration in visual task performance (except as a secondary effect of negative subjective responses) results from perceived flicker.
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From page 81... ...
have demonstrated that the red and blue portions of the spectrum are perceived to be brighter at equal luminance than either the green portions or achromatic displayed images. Whether this difference in perceived brightness is related to visual task performance has yet to be demonstrated.
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From page 82... ...
The specular reflection coefficients were determined by dividing the luminance of the reflected image FIGURE 4.10 Specular and diffuse reflections on a typical VDT screen.
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From page 83... ...
This unitless number indicates how susceptible the VDT screen is to specular reflections from ambient light sources. The smaller the number the lower the luminance of the specular reflection from any given source.
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From page 84... ...
under the identical illumination conditions in which a VDT will be used, it includes image degradation due to diffuse and specular reflections from ambient light sources. It is, however, possible to measure the MTF of the VDT in a dark room and accurately predict the hSTF that will result under any ambient condition if the diffuse and specular reflection coefficients of the display are known.
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From page 85... ...
An MTF greater than 0.95 is typically not technologically feasible in practical VDT applications. In general, lower MTFs result in a lower image quality, but this reduction in perceived quality is not linear with spatial frequency (Carlson and Cohen, 1978~.
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From page 86... ...
With the aid of a barium sulfate reflection, the type of photometer shown in Figure 4.12 can also be used to measure illuminance at the VDT screen. Barium sulfate reflects light in a highly diffuse manner and very nearly approximates a perfect diffusing surface.
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From page 87... ...
Sensitive Component measured (as shown in Figure 4.13 for the VDT screen) , the luminance of the barium sulfate can be measured and converted to the illuminance at the plane of the material.
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From page 88... ...
Measurement of Various Parameters Character Luminance The very term character luminance implies that there is a single luminance that is associated with a VDT character. As can be seen in Figure 4.14 the implication is not correct.
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From page 89... ...
Character Contrast To calculate the contrast of a character, it is first necessary to determine the character luminance and the background luminance. The previous discussion on what constitutes character luminance obviously applies to the uncertainty associated with determining contrast.
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From page 90... ...
Contrast Ratio Far from character 0.5 99 199 200 Near to character 2.0 96 49 50 NOTE: The assumed character luminance is 100 cd/m2. CRT screen typically carries a halo of light around characters due to several light-scattering and stray light sources, which results in a lower value of measured background luminance the farther from the character the measurement is made.
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From page 91... ...
An alternative technique is to measure the edge response of some selected character on a VDT. This technique does not c' a A J Photometer Scan Path VDT Character " I " \ X FIGURE 4.15 Luminance distribution across a single dot in the character I
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From page 92... ...
Specular reflection occurs from the front surface of the screen and satisfies the optical law that the angle of incidence of a light ray on a surface equals the angle of reflection. Diffuse reflection is a scattering in all directions of the incident light and occurs at the phosphor surface of the VDT.
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From page 93... ...
Determining the diffuse reflection coefficient is somewhat more involved. The phosphor surface acts as a fairly good diffusing surface, but it is by no means a perfect diffuser.
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From page 94... ...
Standardization As is apparent from the preceding discussion, there are currently no standard procedures for measuring the critical display quality characteristics of VDTs. Standard procedures should be developed so that accurate comparisons of image quality among VDTs can be made.
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From page 95... ...
A meaningful standard at the present time appears to be that offered by the proposed German TCA specification, which requires a minimum character size of 2.6 mm or 18 min of arc, whichever is greater. There is a large variability among observers and among tasks in viewing distance, and the character size must be compatible with the viewing distance.
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From page 96... ...
L' 12 CC o 111 8 UJ Z 4 LL oL I I I I I 0.50 0.75 1.00 1.25 1.50 1.75 ELEMENT Sl ZE (mm) FIGURE 4.19 Effect of element size on random search time SOURCE: Snyder and Maddox (1978~.
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From page 97... ...
. For example, it can be demonstrated that increasing the character size by increasing the pitch of the raster lines in a CRT display is detrimental because the visibility of raster lines significantly reduces performance even though the character size is increased.
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From page 98... ...
— LL en 2 A: u. m ~ Z LL _ ~ 1—c In 45 it 1 ~ _ o _ ~ Square Horizontally El ongated Vertically Elongated ELEMENT SHAPE FIGURE 4.21 Effect of element shape on reading time.
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From page 99... ...
It has been shown that reductions in object identification occur when the blur exceeds one-half of the width of the individual item, but these data apply to various cultural objects rather than to alphanumeric characters. Studies should be performed in which text reading and legibility, rather than object recognition, are the primary tasks.
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From page 100... ...
Contrast 7.5 7.5 7.5 3.2 20 UJ Cry CC UJ Z 10 CC o cr: 111 5 o 10 Decreased Co ntrast & Increased Reading Dist Condition D \ ,Condition D ,,L, If \~/ \ Condition C ~ Condition B ,< \ ·~ O ~ 20 30 40 50 60 70 80 MEASURED PERCENTAGE OF ACTIVE AREA FIGURE 4.23 Effect of percent active area on character recognition. SOURCE: Stein (1980)
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From page 101... ...
There are no useful guidelines from the literature to suggest optimum levels of display information density, and we strongly recommend research in this area. Dot-Matrix Display Quality Measures While image quality measures have been researched in some depth for CRT displays, very little attention has been given to suitable measures of image quality for flat-panel displays.
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From page 102... ...
summarized the best possible prediction of image quality and visual task performance from a variety of geometric and photometric variables that were measured from flat-panel displays. The pool of predictor variables is shown in Table 4.3.
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From page 103... ...
The major disadvantage of a flat-panel display is its extremely high cost relative to a CRT. At the present time, the few flatpanel displays that would meet the requirements of current data processing and word processing terminals cost in excess of $3,000 prohibitive compared with the cost of typical CRT displays.
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From page 104... ...
This type of filter reduces specular reflections in two ways: by reducing specular reflections from the filter itself through the use of the antireflection coating and by eliminating specular reflections from the underlying VDT screen through use of the circular polarizer. Diffuse reflections are reduced primarily by the light attenuation effects of the polarizer material, which allows only about 35 percent of the incident unpolarized light to
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From page 105... ...
And if those areas were kept somewhat dark, for example by an operator's wearing dark clothing, specular reflections should not be a problem. Diffuse reflections would be reduced as they are with any neutral density filter.
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From page 106... ...
As is the case with neutral density filters, control of specular reflections with this type of filter depends on the surface treatment of the filter. Directional Filters Directional filters use geometric or optical means to prevent ambient light from reaching the VDT or to prevent reflections from reaching the user.
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From page 107... ...
Third, matte-surface treatments are not very effective in dealing with specular reflections in terms of their effect on contrast, although they do reduce the sharpness of specular reflections. Unfortunately, matte finishes reduce the sharpness of the display characters as well, and this effect increases the farther from the VDT surface the filter is located.
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From page 108... ...
The diffuse reflection condition was achieved with a combination of normal room lights and a slide projector located off to one side to provide nonspecularly reflecting illumination (specular and diffuse reflections on a typical VDT screen are shown in Figure 4.10~. The illumination at the plane of the screen (which results in loss of contrast due to diffuse reflection)
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From page 109... ...
. The contrast and the luminance of the VDT characters were measured without a filter on two different types of VDT screens.
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From page 110... ...
, however, performed well under the diffuse reflection condition, but very poorly under the specular reflection condition. In general, filters are more effective in reducing diffuse reflections than in reducing specular reflections.
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