Legibility of Numbers as a Function of Contrast and Illumination

Abstract

While contrast is recognized as an important variable affecting legibility, scant information of what happens in the mid-ranges is available. Thus nine contrast conditions consisting of black or white lettering on white, black, or grey backgrounds were compared under three levels of illumination —0.06, 0.60, and 6.0 footcandles. 18 Ss were asked to search a stimulus array for a particular stimulus and then indicate its relative position among the other stimuli. Reaction time and errors were recorded. Illumination proved to be the single most important factor. Significant differences in performance were observed between the contrast conditions under poor illumination. It was concluded that for recognition tasks of short duration, varying contrast within wide limits has little effect on speed and accuracy of performance as long as illumination remains above 0.60 footcandles. Black lettering on a white background and white lettering on a black background did not differ significantly and were associated with the shortest reaction times and the least number of errors at all light levels.