A series of studies was conducted to investigate the effects of display background luminance, display contrast ratio and adaptation luminance on the legibility of selfluminous displays in aircraft cockpits. The studies were designed to avoid the typical confounding of these three variables, and employed luminance levels that are representative of operational conditions. The data show that with typical display symbols and luminances under low and moderate ambient illumination, contrast ratio is the major determinant of legibility; that increasing contrast ratios beyond as little as 1.4:1 may provide no significant improvements in legibility; and that, beyond their contribution to contrast ratio, the absolute luminances of the symbol and the background have no significant effect on legibility. Given the high ambient illumination typical of a sunny day, however, the pilot is adapted to luminances of up to 32 264 cd/m 2 (10 000 fL). Under these conditions, increases in contrast ratio of up to 30:1 are shown to improve legibility. When the adaptation luminance is more than 100 times that of the display background luminance, symbols on the display cannot be immediately identified, and the absolute luminances of the adapting field and the display background contribute to recovery time apart from their ratio relationship. Hyperbolic functions are applied to describe the multiplicative effects of these variables on legibility.