Visibility Index Functions for Positive Contrast Text

Abstract

In this paper, we propose Visibility Index Functions (VIFs), that enable evaluating and predicting the legibility of sentences quantitatively and conveniently. In light of nonlinearity attached to suprathreshold phenomena, we analyzed the data of the series of legibility evaluation experiments of Sato and Hara(1999), and constructed two formulations : VIFVL(VL, A) and VIFΔL(ΔL, Lb, A), as functions of Visibility Level VL, luminance difference ΔL[cd/m2] between a visual target and its background, background luminance Lb[cd/m2] and character size A[min]. We showed that both the VIFs can well describe both the experimental results of Sato and Hara and the legibility data derived by Inoue and Akizuki (1997) with no parameter change except low luminance conditions for background.