Effects of luminance and colour contrast on the search of information on display devices

Abstract

For black-and-white alphanumeric information, the speed of visual perception decreases with decreasing contrast. We investigated the effect of luminance contrast on the speed of visual search and reading when characters and background differed also with respect to colour. The luminance contrast between background and characters was varied, while colour contrast was held nearly constant. Stimuli with moderate (green/grey) or high colour contrast (green/red or yellow/blue), and three character sizes (0.17, 0.37, and 1.26 deg) were used. Eye movements were recorded during the visual search task. We found that the visual search times, number of eye fixations, and mean fixation durations increased strongly with decreasing luminance contrast despite the presence of colour contrast. The effects were largest for small characters (0.17 deg), but occurred also for medium (0.37 deg), and in some cases for large (1.26 deg) characters. Similarly, reading rates decreased with decreasing luminance contrast. Thus, moderate or even high colour contrast does not guarantee quick visual perception, if the luminance contrast between characters and background is small. This is probably due to the fact that visual acuity (the ability to see small details) is considerably lower for pure colour information than for luminance information. Therefore, in user interfaces, good visibility of alphanumeric information requires clear luminance (brightness) difference between foreground and background.