Abstract
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.
Authors have proposed C-A graph based on contrast profile method. C-A graph presents C-value (luminance contrast) and A-value (adaptation luminance) as vertical and horizontal axes for varying object sizes respectively. It expresses the three factors necessary for visibility estimation. In the previous study, a visibility threshold estimation model of circular objects applicable for greater range of object size from 1.25 to 600 minutes was obtained.
In this study, two visibility evaluation experiments using circular objects as visual target was conducted to develop this visibility estimation method for low-vision people. One experiment was evaluated by sighted people with normal visual acuity, and the other experiment was evaluated by sighted people with lower visual acuity than normal. Circular objects with varying luminance contrast, background luminance and object size, were displayed on 27 inch digital display screen. Subjects observed circular objects displayed on uniform-luminance background with both eyes from 625 or 2500 mm distance. Subjects observed circular objects and evaluated visibility of circular objects using 5-step scale of ‘0’ to ‘4’. ‘0’ is invisible, ‘1’ is visible with considerable effort, ‘2’ is visible with effort, ‘3’ is visible effortlessly, and ‘4’ is easily visible. 14 subjects with normal visual acuity and 20 subjects with low visual acuity were participated in experiments.
Experimental results analyzed using contrast profile method and presented on C-A graph. When visibility threshold curve obtained from the previous study was shown in the same C-A graph, the visibility threshold curve fit between evaluation '0' and evaluation '1'. As the result, it was referring to the Blackwell's visibility level, and shown that visibility evaluation of sighted people with normal visual acuity can be estimated by the ratio of the visibility threshold luminance contrast value. As the size of the visual targets became smaller, the threshold ratio necessary for improvement for visibility became greater. In the case of sighted people with lower visual acuity, it was confirmed that visibility threshold contrast values can be estimated by the same method of sighted people with normal visual acuity. However, it was shown that higher threshold ratio than that of people with normal visual acuity is required to improve the evaluation of visibility for sighted people with low visual acuity.
In the future, it is necessary to develop into examination of the actual low vision people and to establish the estimation method for low vision people.
