Luminance and Contrast for Sign Legibility and Color Recognition

A laboratory study of sign legibility has shown that a contrast of between 30 and 50 percent is required to maintain 75th percentile legibility. Legibility distance increased gradually with greater contrast to about 80 percent; above a luminance ratio of 5 to 1, legibility did not increase greatly. As ambient levels increased, legibility distance increased linearly with the logarithm of either the letter or the sign luminance, whichever was greater. Five color combinations were measured. This report gives additional results on color recognition and applies the effects of luminance and contrast to legibility distance for 11 color combinations, after corrections were made for letter and stroke width. A method for estimating legibility was developed for black and white letter and sign combinations. The effects of luminance and contrast on color recognition at five ambient levels showed the need to increase luminance and contrast as ambient levels increase. Laboratory luminance data, confirmed by two sets of outdoor measurements, furnished a basis for determining the luminance ratios used in the legibility estimates. A basis for estimating glance legibility distance in relation to ordinary legibility (long viewing time) is suggested.