Optimal Optotype Structure for Monitoring Visual Acuity

To date, there are no generally accepted optotypes for monitoring visual acuity. All common optotypes are not completely suitable for some reasons. The tasks requiring visual monitoring - investigation of visual development, early diagnostics, assessment of treatment - impose heavy demands on the test stimuli. They must be: (1) suitable for patients of any age; (2) convenient for repeatable examinations; and (3) accurate enough for revealing the smallest physiologically significant changes of visual acuity. From theoretical consideration, one could conclude that the optotypes for monitoring visual acuity should be designed for measuring visual resolution but not recognition, unlike most popular optotypes. The best optotypes for visual resolution are gratinglike stimuli whose recognition could only be based on the high frequency part of the Fourier spectrum around the characteristic frequency (not on the low-frequency components). On the basis of theoretical analysis we elaborated modified 3-bar optotypes, which minimise the possibility of using low-frequency cues for stimulus recognition. In this paper we present the results of theoretical and experimental comparison of these optotypes with the two widely used ones: tumbling-E and standard 3-bar targets. According to the data obtained, our modified optotypes seem to be better than other investigated ones for monitoring visual acuity.