Psychophysical Experiments on the Legibility of Letters

Abstract

The visual information available to us decreases selectively in hightemporal and highspatial frequency domains as viewing conditions change for the worse. Visual displays must therefore be legible in lowvisualacuity conditions and under conditions of short exposure. We ran three psychophysical experiments to evaluate legibility under these conditions, and have come up with several guidelines for display legibility.
We measured the minimum visual acuity needed to discriminate letters with the lowpass-filtering method and the multi frame detection task. The first experiment showed the crucial effect of letter size on legibility. The results of the second experiment suggested that the optimal line width may be determined by a critical band of spatial frequency describing the letter shape and the power of a low-spatial frequency. The third experiment showed that background luminance had a significant effect on the discrimination of low-contrast letters. In other words, letter contrast must be about 0.9 in order to avoid the effect of background luminance.