The level of brightness contrast required for letter recognition in daytime lighting conditions is a function of letter size, color contrast, and the observer's age. The average contrast required by older subjects was 2.13 times the contrast required by younger subjects. Eqs. (1-2) can be used in instrument panel design to meet or exceed the legibility criterion used here - that is, legible enough to read at least two english letters in one brief glance at a display.
Older individuals could not discriminate color differences as well as younger subjects. The same age effect has been found with the Farnsworth-Munsell 100 hue test (1-2).† Just-Noticeable-Distances (JNDs) in 1931 color space were larger than those found in previous experiments (3) – on average, 8.6 times larger for older subjects and 5.0 times larger for younger subjects. The JNDs reported here represent color discrimination with small, separated stimuli (like IP symbols), whereas previous work represents color discrimination with large, contiguous stimuli.
Older subjects were asked to rate a small set of CUT colors in terms of how desirable they would be for nighttime instrument panel lettering. Green and bluegreen were the most preferred colors; red and blue were the least preferred. Preferred luminance for such lettering differed greatly between colors, (ranging from 2.5 to 21.8 cd/m2), and differed in such a way as to suggest that subjects were adjusting the luminance to a constant subjective brightness (4-5).