Visibility Level Analysis on the Relationship between Readability and Illuminance for Older People

Abstract

The recommended illuminance in the Japanese lighting standard (JIS Z 9110) was determined based on Indow and Kawai's experimental study concerning the relationship between illuminance and readability of printed text. As the observers in the experiment were young, the recommended illuminance derived from the experiment cannot be applied directly to older people. To estimate the recommended illuminance for older people, a set of numerical analyses wasconducted. We found that the readability scale in the above experimental study correlates with the Visibility Level (VL) defined by CIE publication No.19/21. The luminance difference threshold (ΔL_min) derived from the contrast threshold function (C_th) of the CIE publication was divided into two parts (ΔL_af for foveal adaptation and ΔL_eq for veiling luminance), assuming ΔL_min=ΔL_af+ΔL_eq. After considering age factors in each part, the luminance difference threshold function ΔL_min*was reconstructed. Applying the ΔL_min* function to the VL analysis, the recommended illuminance for older people was estimated. The result shows that at least about 700 lx of illuminance may be recommended for older people reading 3-mm high printed characters. The visibility level analysis is a useful method for assessing readability considering age-related effects.