Effects of luminance contrast and character size on reading speed.

Both luminance contrast and character size are critical factors affecting reading performance. Previous studies reported on the effect of luminance contrast on the reading-speed function, that is, the relationship between reading speed and character size. In particular, when contrast was reduced, the critical print size (CPS) was found to shift to a larger character size even though the maximum reading speed and function shape did not change [Japanese Journal of Ophthalmology 52(1) (2008) 44-47]. In the present study, the effect of luminance contrast on the reading function was quantitatively examined. Japanese phrases with a luminance contrast of 0.03-0.99 were prepared as stimuli. Observers with normal vision were asked to read aloud phrases with several character sizes. Then, the reading functions were obtained for each luminance contrast. CPS was found to increase as the luminance contrast decreased. The relationship between contrast and CPS was linear in log-log coordinates, that is, log-CPS increased as the log-contrast of the characters decreased. It was found that the contrast of the stimulus systematically affects the location of the reading function.