VDT design factors
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Color
The perceived color of the display is the result of the eye integrating the
various wavelengths of light generated by the phosphor. Colors are not pure;
they are a mixture of a wide range of wavelengths. There are, however, dominant
wavelengths, the result of significantly more energy in one part of the spectrum
than in others.
There are standardized CIE Uniform Color Space diagrams available for making
estimates of display color contrast. Predicting human performance, based on
the analytic method has not been entirely successful. The reasons are:
- CIE mappings are based on 2° visual targets. The size of your fingernail
at arm's length is about 2°. Target size is well known to interact with color
perception.
- CIE data are based on averages of results across many observers obtained
from well controlled experimental conditions. An individual can not be expected
to map precisely to these averages and conditions.
Both the older US ANSI /HFS 100 -1988 Standard and the ISO 9241 - Part 8
Standard developed recommendations based on the 1976 CIE Uniform Chromaticity
Diagram and Uniform Color Space. Recommendations are given to ensure the discriminability
and identification of colors. One of the key issues addressed by both of these
standards is the use of color for coding. Modem displays are capable of addressing
a large number of points in color space, Gł. G is the number of gray
scale values. Typical value of gray levels are: 16, 32 and 64. These correspond
to 4,096; 32,768 and 262,144 color points respectively. Color coding, at maximum,
should be used at nine levels (see DeMars, 1975). It is important that application
designers refrain from using more than nine levels and that the color separation
between levels be large. A display with the addressability illustrated above
can permit tuning of four to nine colors to the individual user. And, of course,
realistic rendition of color images requires the use of many color points.
Photochrominance
Photochrominance criteria are an attempt to specify the equivalent contrast
when both color and luminance differences are present. Color difference criteria
are included in ANSI. The recommended minima, however, are not mandatory. They
apply only to colors that must be discriminated.
 The
1976 Uniform Color Space.
Figure 41 shows the approximately uniform 1976 color space.
There are two different situations in which color difference formulas apply.
Different formulas are used for each:
l. Color coding of characters for legibility where the color difference must
be clear but absolute identification of the color is not required:
ISO recommends Δ E(Yu'v')> 100 units from text to background.
2. Color differences where the user must be able to identify the color of
the character or graphic explicitly, without comparison to any other color
on the screen:
The older ANSI standard recommended Δ E*(L*u*v*)> 40 units.
This permits about 8 color points per application. ISO recommends that only
6 color points be used. For accurate identification, both the BSR/HFES 100
draft standard and ISO recommend a default color set of no more than 11 colors.
Readers interested in a fuller treatment of the use of color in computer
displays can find any number of sources on the Internet.
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