Effects on Visual Acuity

In general, increases in display luminance will cause decreases in pupil size, which in turn lead to increases in the optical depth of field and improvement in optical quality. Figure 4.1 illustrates reduction in pupil size as a function of retinal illuminance, assuming a uniformly illuminated retina.

Figure 4.1

Diameter of the pupil as a function of retinal illuminance. SOURCE: ten Doesschate and Alpern (1967).

This increase in retinal illuminance, which causes a decrease in pupil diameter, directly affects the visual acuity of the normal healthy eye, as shown in Figure 4.2. While the differences are not very great over the normal display operating range, an increase from approximately 1 or 2 milliLamberts (mL) to about 60 or 70 mL causes an increase in visual acuity of approximately 50 percent. Thus, displays having higher luminance permit an operator to see finer details on the display. The greatest proportional gain in acuity with increasing luminance takes place between approximately 1 and 10 mL.

Figure 4.2

Relationship between visual acuity and adapting luminance. SOURCE: Hecht (1934).

In general, a positive-contrast display (light characters on a dark background) 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) luminance of about 6 or 7 mL. By comparison, a negative-contrast display (dark characters on a light background) will have a background luminance on the order of 25 mL and a character luminance of about 1 mL, producing an average (adapting) luminance of about 17 mL. Accordingly, one might expect an increase in relative acuity from 1.4 to 1.6, or approximately 15 percent, for a change from positive to negative contrast.

This acuity increase, however, is probably neither important nor real. As suggested by Rupp (1981), 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. Whether this is actually the case has yet to be demonstrated experimentally for VDTs. There is cause for concern over such generalizations because of the lack of direct application of existing literature to VDTs. For example, there is overwhelming evidence that contrast sensitivity, as well as acuity, increases significantly with increases in overall retinal illuminance (see Figure 4.3); these and other data are based, however, on display fields in which the light and dark elements are approximately equal in area rather than on the unbalanced display typical of a VDT.

Figure 4.3

Effect of retinal illuminance on contrast threshold. SOURCE: van Nes and Bouman (1967).

It is known that people with poorer eyesight benefit more from increased levels of retinal illumination than do people with normal eyesight (Hopkinson and Collins, 1970). It is also known that maximum acuity is obtained when the surround (the area or surface around the display) is equal in luminance to the display (adapting) luminance (Hopkinson and Collins, 1970). A secondary benefit of higher display luminance is the increase in visual depth of field (based on a fixed diameter of the ''blur circle") as the pupil diameter decreases. Assuming that the luminance to which an observer adapts is in fact the space-average luminance of the display, a negative contrast display (higher space-average luminance) would typically yield a pupil diameter of about 4.5 mm while a positive contrast display (lower space-average luminance) would yield a pupil diameter of about 5.0 mm (see Figure 4.1, above). This difference in pupil diameter corresponds to approximately a 30 percent difference in blur circle diameter (at the 50 percent intensity point), as shown in Figure 4.4.

Figure 4.4

Blur circle diameter as a function of eye pupil diameter. SOURCE: Campbell and Gubisch (1966).

Again, however, application of these data to VDTs in the workplace should be experimentally verified. As with all lenses, aberrations in the eye are greatest in the periphery of the cornea and the lens. Thus, pupil constriction improves the quality of the image formed on the retina by excluding light that passes through the peripheral portions of the cornea and the lens (i.e., light rays beyond the border of the pupil at its adapted diameter). While pupil constriction is caused by increasing the amount of light in the adapting field, it also occurs synergistically with lens accommodation (focusing) for near objects. Thus, as the eye focuses on closer objects, such as a VDT at a working distance, the pupil will "automatically" constrict to obtain a somewhat sharper image. Thus, there is a significant interrelationship among display luminance, pupil diameter, blur circle, depth of field, and contrast sensitivity (or acuity). Generally, increases in display luminance will improve visual performance and tend to permit greater cancellation of spherical aberrations by the constricted pupil. On the other hand, positive contrast may tend to make the pupil larger, thereby reducing visual acuity (or contrast sensitivity), increasing the blur circle, and permitting greater spherical aberration.¹

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. Thus, as the average (adapting) luminance of a display increases, the eye is more likely to perceive flicker at any particular repetition rate. This effect has been reported in numerous experiments, including those that have included such variables as the wavelength of the light, the wave form of the stimulus, the size and shape of the stimulus, etc. A generalization from the research of de Lange (1958), which illustrates the relationship between the critical flicker frequency and the Fourier spectrum of the time varying stimulus, is shown in Figure 4.5. In general, de Lange found that the Fourier fundamental of the display could be used to predict the modulation at which flicker is perceived, as a function of repetition rate, irrespective of the wave form of the light.

Figure 4.5

Temporal contrast sensitivity function. SOURCE: de Lange (1958). Reprinted with permission of the Optical Society of America.

Unfortunately, large-area displays using negative contrast are perceived to flicker at much higher refresh rates than those using positive contrast in a typical VDT environment. Thus, a display with a 50 Hz refresh rate that is just at threshold for flicker at 10 cd/m² will flicker very noticeably if luminance is increased to 100 cd/m ². This effect is in conformance with the well-established Ferry-Porter Law, which suggests that the highest frequency at which flicker is perceived increases linearly with the logarithm of the adapting luminance, or by approximately 10 Hz for each tenfold increase in luminance. The data of Bauer and Cavonius (1980) clearly support this result. Bauer and Cavonius recommend a repetition rate of 100 Hz for VDTs with negative contrast. 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. Snyder and Taylor (1979) demonstrated that increases in character luminance caused significant increases in individual character legibility in several different viewing tasks. Unfortunately, in that particular experiment, the background luminance level was held constant, and therefore the character luminance was totally confounded with the contrast of the displayed image. Supporting evidence, however, for the effect of display luminance on performance is offered by Bauer and Cavonius (1980), who found that a higher-luminance negative-contrast display yielded both greater subjective preference and improved visual performance than a lower-luminance negative-contrast display. Further research on the subject of the effect of display luminance when separated from the influence of contrast and contrast polarity is needed, however, before this issue can be directly resolved.

Luminance Uniformity

There is very little research in the literature to provide information on the minimum requirements for the uniformity of visual displays. No studies are known to provide either thresholds of detection or tolerance limits for large-area nonuniformities. In general, we simply do not know how much large-area nonuniformity is a reasonable design goal.

The case for small-area nonuniformity is similar. Unless one applies basic sine-wave sensitivity data to a given form of small-area nonuniformity distribution and attempts to predict the detectability of nonuniformity, there is currently not even a suggested means for evaluation.

Except for an initial study by Riley and Barbato (1978), there is little knowledge of the effects of line errors (on or off) or of element errors (on or off) on display legibility and utility. Research efforts to fill these data gaps are obviously needed.

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. A character is typically made up of a series of dots and strokes, each of which has a distribution of luminance values across its extent. In general, the linear horizontal strokes (e.g., the top of the letter T and the top, middle, and bottom arms of the letter E) are higher in luminance than the short dots that make up the vertical elements of the characters. While a photometer with microscope objective lens can easily measure any aspect of this luminance distribution, the question is what should be measured and labeled as character luminance. There are several options but at present there is no accepted standard procedure. The three main options include: (1) a narrow vertical slit (width of the slit much less than the width of the vertical stroke of character) scan across a vertical character stroke (such as an I) with the vertical dimension of the slit covering several dots; (2) the same as above, except that the vertical height is limited to only one dot or a fraction of one dot; or (3) a small circular aperture with a diameter much smaller than the size of a dot. Each measurement will yield a luminance value, but the most appropriate measurement for determining character luminance is not apparent.

Figure 4.14

Light distribution that forms VDT characters.

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. The background luminance is also a problem because it differs depending on the ambient lighting conditions and the location on the screen at which the background reading is made relative to the location of the character or characters. A 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. Dark-room measurements of absolute contrast (calculated by the difference divided by the sum technique) will not be significantly affected by small absolute changes in background luminance due to the location of the measurement. Other values, however, such as differential contrast (difference divided by background) or the luminance ratio (also called contrast ratio, or character luminance divided by background), can change greatly with small changes in absolute background luminance readings.

To illustrate this effect, Table 4.2 shows a hypothetical set of data for character luminance and two different background luminances (one measured near the character and one measured farther away). Note that the modulation contrast is relatively insensitive to differences in background luminance compared with either of the other two calculations for contrast. It is apparent from the data in Table 4.2 that if a definition of contrast is used that is highly sensitive to background luminance measurement, the specific conditions under which background luminance is measured need to be specified and, preferably, standardized.

TABLE 4.2

Modulation Contrast, Differential Contrast, and Contrast Ratio for Two Measures of Background Luminance: Hypothetical Case.

Blur, Resolution, and MTF

The terms blur, resolution, and MTF all relate to the definition or appearance of sharpness of the VDT characters. No accepted standard procedure exists for measuring these parameters. Whatever procedure is used to measure them, it is still necessary to select and preferably standardize an appropriate photometer aperture.

Blur is usually based on a determination of how rapidly the luminance changes with distance at the edge of a character (Dainty and Shaw, 1974; Grandjean and Vigliani, 1980). This is not always easy to determine. Figure 4.15 shows the luminance distribution (measured with a vertical slit and a scanning microphotometer) across a single dot in the character I. It is difficult to define where the edge of either side of the I begins and ends in such a way that repeatable measures can be achieved.

Figure 4.15

Luminance distribution across a single dot in the character I.

The MTF can be measured in many ways (Schade, 1948; Snyder, 1974; Task and Verona, 1976; Gaskill, 1978). Probably the easiest method for VDT screens is by using the edge-response or line-response techniques described in Gaskill (1978). These methods are preferred because no special, separate signal generation devices are required; instead, selected VDT characters can serve as the input signals. For example, if the input signal is the narrowest vertical line that the VDT is capable of producing, then the luminance profile shown in Figure 4.15 is the line spread function (LSF) for the VDT. The MTF of the VDT can then be calculated by taking the normalized Fourier transform of the LSF, as described in Gaskill (1978).

An alternative technique is to measure the edge response of some selected character on a VDT. This technique does not require the assumption that the selected character represents the narrowest line possible, as does the LSF technique. It does, however, require an additional mathematical step. The edge response is simply the luminance distribution measured at the edge of a character. A typical edge response may look like that shown in Figure 4.16. The curve is first differentiated, which yields the line spread function, and then the normalized Fourier transform is taken to obtain the MTF (as described above).

Figure 4.16

Typical edge response (luminance distribution at the edge of a character).

Reflection Characteristics

As described in the section ''Filters for VDTs," there are two types of reflections that occur with VDTs: specular and diffuse. 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. There are no accepted, standardized procedures to measure either of these parameters.

The specular reflection coefficients for the data presented in Tables 4.1, 4.5, and 4.6 were obtained using the following procedure. A light box, measuring approximately 20 × 25 cm, was positioned 1.5 m from a VDT screen and tilted at an angle of about 17° from a straight line from the center of the VDT screen (see Figure 4.17). The photometer was located about 17° on the other side of the straight-line position and focused on the VDT screen. The luminance of the reflection was measured and divided by the luminance of the light box. It should be noted that much of the light from the light box was transmitted through the front surface of the VDT screen and fell on the diffuse phosphor surface. The measurement was not, therefore, a pure specular reflectance coefficient because of the diffusely scattered light in the area measured. It is extremely difficult to avoid this situation entirely. Since the effect depends on the size of the light box used and its distance from the VDT, it is necessary to specify these parameters in order to make meaningful comparisons between VDTs.

TABLE 4.5

Effect of Several Filters on Contrast and Luminance of VDT Characters for a Smooth-Finish VDT Screen.

TABLE 4.6

Effect of Several Filters on Contrast and Luminance of VDT Characters for a Matte-Finish VDT Screen.

Figure 4.17

Geometry for measuring specular reflection.

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. Consequently, the amount of light diffusely reflected depends both on the angle of the photometer with respect to the VDT surface and the angle of the light source with respect to the VDT surface. A geometry should be chosen that is representative of the viewing and illumination conditions typically encountered in VDT use. For the measurements shown in Tables 4.5 and 4.6, the photometer was located directly in front of the VDT screen, simulating an operator's view. The illuminating light source consisted of overhead room lights and a slide projector located about 45° from the straight-line position (see Figure 4.18). The resulting illumination at the VDT screen was approximately 400 lux, near the mid-range of the 300-500 lux recommended for office work areas. The illumination was measured using the barium sulfate technique described in the section "Measurement Techniques." The diffuse reflectance coefficient was calculated by dividing the resulting screen luminance (using a blank screen) by the incident illumination.

Figure 4.18

Geometry used for measuring diffuse reflectance coefficient.

Since individual characters are not measured in determining the reflection characteristics, the choice of photometer aperture is not critical.

Character Size Effects

In general, the size of the display should be adequate to present the necessary information for the task required of the operator. There are no standard requirements for screen size, but there are requirements for sizes of the information elements presented on the screen. Thus, as the task requires a greater amount of information, the screen generally increases in size to accommodate the larger amount of information, each element of which must be presented at a suitable size.

It has been shown that the proper character size varies with the nature of the task. For example, visual search for individual characters is improved as the size of the character gets larger. Snyder and Maddox (1978) showed that increases in individual dot sizes up to 1.50 mm improved random search time (Figure 4.19). They also showed that a dot size of 1.50 mm is greater than optimum for continuous reading of text (Figure 4.20). In essence, if the characters are too large, reading time is reduced because of the increase in necessary eye movements and numbers of visual fixations. On the other hand, larger characters are desirable in a search-type display, simply because peripheral characters can apparently be more easily located in peripheral vision. 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.

Figure 4.19

Effect of element size on random search time. SOURCE: Snyder and Maddox (1978).

Figure 4.20

Effect of element size on reading time. SOURCE: Snyder and Maddox (1978).

There is a large variability among observers and among tasks in viewing distance, and the character size must be compatible with the viewing distance. The above specification takes this into account. It generally assumes that the minimum viewing distance will be 50 cm, although longer viewing distances are feasible for some particular tasks and workplace geometries. If a longer viewing distance can be anticipated, then the greater character size necessary to meet the 18 minutes of arc requirement is justified.

Character sizes should not be obtained at the cost of a reduction in image quality (see below). 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.

Character Formation

In general, as for CRT displays, stroke-written characters are preferred to characters having visible dot or element structure (see discussion in the section "Raster Structure").

Where dot-matrix characters are used, there are known trade-offs among the design characteristics of the characters. In particular, the dot size and shape interact in a unique fashion. In all cases, vertical elongation of dots should be avoided; dots approaching a square aspect ratio are most desirable. This has been shown in several experiments, such as that summarized in Figure 4.21. When the shape of the dot is combined with what is known regarding spacing between elements (see discussion in the section "Raster Structure"), it is apparent that the best display design has square dots that are essentially adjacent to one another.

Figure 4.21

Effect of element shape on reading time. SOURCE: Snyder and Maddox (1978).

The size of the dot matrix or the number of video lines in a raster display used to form the individual characters is equally important. Figure 4.22 shows that the optimum matrix size depends on the character font used. As discussed above, a 7 × 9 character is generally more legible than a 5 × 7 character and a 9 × 11 character is more legible than a 7 × 9 one. Also, the more dots or elements available to form the character the better the individual legibility of the character, although diminishing returns appear to be reached beyond matrix sizes of 9 × 11. Similar results have been obtained with raster scan displays.

Figure 4.22

Interaction of font and character/matrix size in determining single character legibility. SOURCE: Snyder and Maddox (1978).

In recent years, there has been some interest in describing the intermittency of characters using a measure called percent active area, which is simply the proportion of the total display space which is illuminated by the dots. The percent active area is the square of the ratio of the dot diameter to the center-to-center dot spacing, multiplied by 100. As shown in Figure 4.23, increases in the percent active area lead to reductions in character recognition error rates. Under adverse reading conditions, active areas in excess of 50 percent are desirable, while under normal conditions active areas in excess of 30 percent appear to achieve asymptotic performance. In general, the percent active area measure is not critical if good design practice is followed by reducing space between character dots.

Figure 4.23

Effect of percent active area on character recognition. SOURCE: Stein (1980).

Contrast

Maximum legibility can be achieved for contextual (e.g., text) displays when the modulation is at least 75 percent.³ Very little gain is achieved beyond 75 percent. If the information presented on the display is noncontextual (e.g., isolated numerals or letters), a modulation of at least 90 percent is required to avoid reductions in legibility. It should be noted that these values of modulation are referenced to the display as viewed by the operator and therefore take into account the ambient illuminance and any reflections. Glare reduction, good workplace illumination design, and high intrinsic contrast of the display are all necessary to achieve an acceptable contrast level.

Characters with sharper edges, or less blur, are generally more legible than those with greater blur or reduced sharpness. Unfortunately, adequate measures of sharpness and their relationship to legibility are not well established. 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.

Font

The legibility of displayed alphanumeric information is greatly dependent on the character style or font. Legibility also interacts with the size of the matrix and the overall character size. As illustrated in Figure 4.22, the Huddleston font is the most legible of those studied for 5 × 7 characters, but the Huddleston and Lincoln/Mitre fonts are equally legible for either 7 × 9 or 9 × 11 matrix sizes. Since there is absolutely no standardization of fonts across existing systems, care should be taken by designers and users to select fonts that give optimum legibility rather than unique character designs. Generalizations from existing literature pertaining to stroke-written characters (e.g., printed text) appear reasonable and should be followed until more directly related data are generated.

Luminance Uniformity

Uniformity considerations are similar to those discussed previously in this chapter for CRT displays.

Information Density

Research relating the minimum, maximum, and optimum densities of information in the vertical and horizontal dimensions is urgently needed. Currently, word processing and data processing displays range from a few lines through a more typical 24 lines per display height to a full page of approximately 60 lines. The displays vary in physical size, and the characters also vary in size. It is clear that full-page displays are desirable for formatting purposes, but they are often very difficult to read because of the resulting small character size. Similarly, it is obvious that large character sizes on partial page displays produce legible characters but that formatting is a difficult and often tiring task. There are no useful guidelines from the literature to suggest optimum levels of display information density, and we strongly recommend research in this area.

Circular Polarizer with Antireflection Coating

A circular polarizer filter with antireflection coating can be used to reduce both specular and diffuse reflections. It is the most expensive filter available and probably one of the most effective.

The outside surface of this type of filter is coated with several layers of optically transparent materials to form what is called an antireflection coating. The effect of the coating is to significantly reduce specular reflections from the surface of the filter. The rest of the filter package consists of substrate material (typically glass) sandwiched around the more delicate components, a linear polarizer and a quarter-wave plate. The linear polarizer and the quarter-wave plate together form what is commonly known as a circular polarizer. The circular polarizer converts unpolarized incident light to circularly polarized light. The light is changed from right-handed circularly polarized light to left-handed circularly polarized light (or vice versa) on reflection from the VDT screen. Because of the optical physics of the circular polarizer, the light is blocked from getting back through the filter in much the same way that light is blocked by crossed linear polarizers.

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 pass through the filter to the phosphor surface of the VDT screen. The light is diffusely scattered by the phosphor surface, thus losing most of its polarization characteristics; and it is again reduced to about 35 percent as it passes back through the filter toward the user. This process results in an improvement of the display contrast since the ambient incident light (illumination) is attenuated twice by the filter (once as it arrives at the screen and again as it diffusely reflects through the filter toward the operator), while the VDT character luminance is attenuated only once as it passes through the filter to the operator.

Neutral Density Filters

A neutral density filter is probably the simplest of the contrast enhancement filters. It typically consists of a neutrally tinted plastic that allows the passage of some percentage (usually 15-25 percent) of the light that falls on it. This filter is most effective in reducing diffuse reflections. Light from ambient sources is attenuated twice as it passes through the filter to the VDT phosphor surface and is reflected from the phosphor surface through the filter toward the operator. Since the light from the VDT characters passes through the filter only once, the display contrast is improved.

Specular reflections may not be reduced by this type of filter unless the surface of the filter is treated with an antireflection coating (as discussed above) or with a matte finish coating that blurs the specular reflections.

A filter that is apparently not commonly available but that would appear to be both effective and inexpensive is a neutral density filter formed into a spherical concave shape. Because such a shape is opposite in direction to the curvature of the VDT screen, the edges of the filter would have to be located a short distance from the screen. If the radius of curvature of the screen were approximately equal to the operator's viewing distance, and the screen were tilted somewhat below the operator's eye level, reflection sources would be limited to the operator's chest and abdominal areas. 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.

A filter based on the physical curvature of the filter material is described in U.S. Patent 3,744,893 entitled "Viewing Device with Filter Means for Optimizing Image Quality" issued to Chandler (1973). As described, the filter was intended for use with a film viewing device but could be adapted to VDTs.

Notch or Color Filters

Notch or color filters are designed to allow transmission of a high percentage of incident light of some specified wavelengths (typically in the green portion of the spectrum) and a high absorption at other wavelengths. The principle of this type of filter is essentially the same as that of a neutral density filter, but in notch or color filters the bandpass (color) is tuned to the VDT screen color. A green filter placed over a VDT with a green phosphor will allow most of the display luminance to pass through the filter to the operator, while ambient illumination, which is usually broadband white, is largely absorbed by the filter (except for the green portion). This process reduces the ambient light that causes diffuse reflections on the VDT screen, thus improving contrast.

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. One type of directional filter is composed of a thin sheet of material with tiny, opaque, imbedded slats that are perpendicular to the surface of the sheet. The slats act as a miniature venetian blind, allowing light to travel only in certain directions. When the slats are oriented toward the operator, light from the VDT can pass to the operator but light from overhead cannot reach the VDT screen. This process reduces contrast loss due to diffuse reflections. Specular reflections would have to be reduced by surface treatment of the filter, as described above.

General Comments

Some general characteristics of filters should be noted. First, antireflection coatings tend to be somewhat delicate and will typically degrade with time, use, and cleaning. Second, plastics used in filters are softer than glass, and they also become scratched and degrade with time, thus reducing the effectiveness of the filter.

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. Some loss in character sharpness may be helpful, however, in reducing the dot structure of characters (see data on filters 1, 2, 3, and 4 in Tables 4.5 and 4.6).

Fourth, VDT screens are convex, curved surfaces and are therefore susceptible to specular reflections that are visible to the operator over a very wide range of angles (see Figure 4.24a). If a flat or concave filter is placed over the screen, the angles over which specular reflections may occur are drastically reduced and therefore more easily controlled (see Figure 4.24b), a subtle but significant advantage for such filters.

Figure 4.24

Specular reflection angles for a curved VDT screen (a) and for a flat VDT filter (b).

Effectiveness of Filters

Because the effectiveness of a particular filter depends on many variables and combinations of variables, it is not possible to fully discuss the issue of effectiveness in this report. For a limited comparison of the effectiveness of several filters and filter types, we measured the effects of seven filters on two different types of CRT screens:

1.

Amber filter with matte finish (curved)

2.

Gray filter with matte finish (curved)

3.

Green filter with matte finish (curved)

4.

Neutral filter with matte finish (curved)

5.

Circular polarizer with antireflection coating (flat)—manufacturer A

6.

Circular polarizer with antireflection coating (flat)—manufacturer B

7.

Green filter with smooth finish (flat)

The filters were measured under three conditions: in total darkness, in the presence of a specular source, and in the presence of a diffuse reflection source. The specular reflection source was a light box with a luminance of approximately 2,950 cd/m² positioned approximately 1.5 m from the VDT screen and approximately 17° off-axis (see Figure 4.25). Measurements under the specular reflection condition were taken with the room lights on, thus this condition was not one of a pure specular reflection. 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) was measured under each of the three conditions. The measurements are shown in Tables 4.5 and 4.6.

Figure 4.25

Top view of geometry used to test filter effectiveness against specular reflections.

The contrast and the luminance of the VDT characters were measured without a filter on two different types of VDT screens. Table 4.5 shows the measurements made on a VDT screen with a smooth surface; Table 4.6 shows the measurements on a screen with a matte surface.

There are several items worthy of special note in the data of Tables 4.5 and 4.6. Displays with both smooth and matte finishes have extremely high contrast in a dark room; it is the ambient environmental lighting that causes a loss of contrast. All filters reduce the luminance of the display characters. This means that when a filter is used, the VDT must be operated at a higher beam current to achieve the same character luminance as when no filter is used. The increased beam current causes the phosphor to age (become less efficient) more rapidly and reduces the lifetime of a CRT.

For the smooth-finish screen (Table 4.5), the circular polarizer filters improved contrast under the specular reflection condition; the improvement, however, was only moderate. For the matte-finish screen (Table 4.6), under the specular reflection condition, none of the filters resulted in a significant improvement over the no-filter condition. For both the smooth-finish and matte-finish screens, several filters improved the contrast under the diffuse reflection condition compared with the no-filter condition, but again the improvement was only moderate.

Filters 1, 2, and 3 not only did not improve contrast for either the smooth-finish or matte-finish screens, but resulted in poorer contrast under several conditions. Since the phosphor used in both VDTs was a P-4 white phosphor, these results indicate that color filters might not be expected to improve contrast unless the filter color is matched to that of the phosphor. Filter 7 (green, flat, smooth finish), 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. This is unfortunate because specular reflections cause the greater loss of contrast and probably contribute more to problems encountered in viewing VDTs.