弱視者を対象とした均一背景および不均一背景における円形視標の見やすさ評価に関する検討

－ 弱視者を想定した輝度コントラスト評価図を用いた視認性推定法に関する研究 (その4) －

抄録

 It is vitally important to estimate visibilities of all visual targets in real environments to assume and maintain visual safety. As luminance images in real lit environments are relatively easy to obtain, it is reasonable to attempt a method of estimating visibilities of objects using these images.

 The authors have proposed C-A graph that can estimate visibilities from luminance images using a contrast profile method. The C-A graph presents C-value (luminance contrast) and A-value (adaptation luminance) as vertical and horizontal axes for varying object sizes respectively. This graph expresses three factors necessary for the visibility estimation. In the previous study, the visibility threshold C-value estimation method of circular objects was established. Besides, it suggested that visibility evaluation of sighted people with normal visual acuity can be estimated from the ratio of the visibility threshold C-value.

 In addition, as preliminary steps of study for low-vision people, authors have conducted visibility evaluation experiments using circular objects with uniform or non-uniform (checkered) background for sighted people with low visual acuity. As the checker contrast and the checker size were increased, it became difficult to recognize the presence or the shapes of circular objects.

 In this study, same experiments were conducted for low-vision people with various disease types and disability grades. Twenty-seven low-vision people participated in the experiments. Circular objects and backgrounds with varying background luminance, luminance contrast and object size, were displayed on 27 inch digital display screen. When the background was non-uniform (checkered), luminance contrasts and sizes of the checkers were varied, too. The subjects observed circular objects displayed on uniform or non-uniform background with both eyes from a distance of 2500(625) mm. They evaluated visibility of circular objects using a 5-step scale of 0-4.0=cannot see any object, 1=can see some kind of object, 2=can see a circular object but not clearly, 3=can mostly see a circular object and 4=can clearly see a circular object. The experiment results were analyzed using a contrast profile method and were presented on the C-A graph.

 When the background was uniform, visibility threshold C-values of a part of low-vision people could be estimated by the estimation method established from experiment results of sighted people. However, it was suggested that required higher luminance contrasts in large circular objects depending on the visual field disturbance. Besides, it was suggested that visibilities of people with symptoms of photophobia or hemeralopia was decrease as the background luminance (A-value) increased.

 When the background was checkered pattern, as the checker contrast and the checker size were decreased, the visibility evaluation was similar to the uniform background evaluation. Additionally, as the checker contrast and the checker size were increased, it would become difficult to recognize the presence or the shapes of circular objects. There are the same as experiment results of sighted people with low visual acuity of the previous study. In order to improve the visibility, it is important to not increase the size and the luminance contrast of the background pattern, not to mention to increase the size and the luminance contrast of the visual object.

 Further development of the visibility estimation method is needed for applying to various conditions.