Dominant wavelength and stimulus purity for VDT display color in terms of readability

This study was conducted to clarify complex relationships between VDT (Visual Display Terminal) display color subjective readability and dominant wavelength, and between the subjective readability and stimulus purity. In this study, readability was defined to be such that a human could read sentences on a VDT screen. Twelve men and eight women, who were all Japanese and had normal color vision, participated in three experiments. During all the experiments, the vertical illuminances on the screen were 700 Lux and 300 Lux. Contrasts were 3.1:1 and 4.2:1 under 700 Lux and 300 Lux, respectively. In the first experiment, 25 kinds of screen were used, there were 23 different colors and white shown on a black background screen (negative screen), and black shown on a white background screen (positive screen). The colors had six dominant wavelengths, ranging from 460 nm to 606 ran, whose stimulus purities involved three, four, or five levels for each wavelength. In the second experiment, three levels of stimulus purity for two dominant wavelengths, 480 nm and 520 nm, were added to the first experiment's colors. A total of 31 different colors were examined by a paired comparisons method. In the third experiment, subjects were required to complete a questionnaire about the impression of the colors.

The major result of the subjective evaluation was the identification of a most easily readable stimulus purity for each dominant wavelength, independently from different illuminance values. A difference between men and women was noted. Women's scores regarding higher stimulus purities than the optimal values were severely lower, while men's did not decrease notably. Middle dominant wavelengths, ranging from 503 nm to 566 nm, had similar evaluation values under the two illuminances. By applying a factor analysis to the questionnaire, a “Conspicuous factor” that mainly relates to conspicuousness and strength, and an “Uncomfortable factor” that mainly relates to tiredness and irritation, were extracted. VDT display color was found to be evaluated by these two factors.