Legibility of printed small-sized Thai characters under different illumination by means of cataract experiencing goggles

Abstract

Most of the outside information is collected to us through the visual system. Since the ratio of population of elderlies to young people in Thailand is increasing rapidly, the proper infrastructure of the visual environment suitable for elderlies is one of the urgent tasks of the country. The printed labels representing visual environment have been found to be expressed by so small letters and are too difficult for elderlies to read. A serious problem of the visual performance of the elderly comes from the cloudy crystalline lens of the cataract that scatters the incoming light all over the retina and worsen the legibility of letters. The deterioration of the visual acuity investigated in the forgoing experiments should be because of the scattered light and cannot be avoided as far as the reading condition stays normal, that is the subjects read labels under illumination provided by ceiling light. To control scattered light from the environment, the two-room concept is introduced. A test stimulus is placed in one room and a subject stays in the connecting room with window on the separating wall. If the illumination of the subject room is lowered, while the luminance of the test stimulus is kept the same, the scattered light should be decreased and the visual performance should be improved. This research aims to investigate the visual acuity of printed small-sized letters under various illumination conditions by using cataract experiencing goggles. Results of one-room experiment showed that legibility of the eyes with goggles decreased highly with decreasing illuminance. Negative contrast gave slightly better legibility than positive contrast font. The three fonts tested gave no significant different in legibility. Low contrast stimulus causes high deterioration for eyes with goggles. Result from two-room experiment showed the advantage of two-room illumination system on preserving legibility for the eyes with goggles especially on the low contrast stimulus. The model for transferring letter height seen by young eyes to the letter height needed for the elderly for different backgrounds has been set up.