A readability model of letters with various letter size, luminance contrast and adaptation luminance levels for seniors

In ageing societies, it is necessary to make the visual environment more appropriate for the elderly so that they can maintain visual performance as long as possible. To this end, we aimed at developing guidelines to determine appropriate adaptation luminance and luminance contrast for given letter sizes for the elderly. In this study, we first measured the contrast sensitivities of older participants. Second, we conducted an experiment in which the same participants evaluated the degrees of readability of Japanese sentences with various letter sizes, luminance contrasts and adaptation luminances. Based on the evaluations, we developed a readability model consisting of equal-readability-level contour lines that represent the average of older peoples’ readability scores. From the model, we found we could improve readability by increasing adaptation luminance while the adaptation luminance was lower than 200 cd/m2. However, an increase in luminance contrast became more helpful to improve readability than an increase in adaptation luminance when the adaptation luminance was higher than 200 cd/m2. We also found large individual deviations in readability evaluation among older participants depending on the contrast sensitivity. We divided the participants into three groups based on their contrast sensitivity levels and compared readability models among the three groups.