晴眼者を対象とした円形視標の視認閾値に関する基礎的検討

 It is vitally important to estimate the visibility of all visual targets in real environments in order to maintain a visual safety especially for low-vision people. As luminance images of real lit environments are relatively easily obtained, it is reasonable to attempt to establish a method to estimate visibility of objects using luminance images.<br> Authors have proposed C-A graph based on contrast profile method. In contrast profile method, luminance contrast is obtained by convolution of 9 by 9 matrix called n-filter into a logarithmic luminance image. This n-filter is a kind of the Mexican hat filter. A luminance contrast profile of the object is obtained by changing detection size of the filter. A logarithmic average of the object and the immediate background is obtained by the same size averaging filter as the n-filter in the logarithmic image of luminance. C-A graph presents luminance contrast and logarithmic average as vertical and horizontal axes for varying object sizes respectively. It expresses the three factors necessary for visibility estimation. By the author's past experiment using Landolt ring, that estimation of visibility using C-A graph is possible has been confirmed.<br> In this study, visibility evaluation experiments using circular objects as visual target was conducted as a fundamental study to develop estimation visibility for low-vision people. Circular objects with varying luminance contrast, background luminance and object size, were displayed on 27 inch digital display screen. Subjects observed circular targets displayed on uniform-luminance background with both eyes from 625 or 2500 mm distance. Subjects answered whether the circular target was visible or invisible. 10 subjects with median visual acuity 1.35 and 10 subjects with median visual acuity 1.70 participated in experiments.<br> Experimental results analyzed using contrast profile method and presented on C-A graph. By plotting visible and invisible contrast values for each target size and finding out contrast values that half of the subjects evaluated as visible, visibility threshold curve can be drawn between visible and invisible conditions on C-A graph for each circular object size. As object size became smaller, visibility threshold curve opened outward on C-A graph and both positive and negative visibility threshold contrast becomes greater. A visual threshold estimation model was established from threshold curves obtained for each object size. For difference of visual acuity of subjects, the variable obtained by multiplying object size and the visual acuity was used to concatenate the results. In this study, visibility threshold estimation model of circular objects adoptable to wider range of object size using C-A graph was established. The consistency between the established model and Blackwell's study was confirmed.<br> Whether these models are applicable to people with other visual ability, such as low-vision and average but weak vision, must further be examined.