Legibility variations of Chinese characters and implications for visual acuity measurement in Chinese reading population

PURPOSE: Written Chinese contains an enormous number of characters with a wide range of spatial complexities. Consequently, the legibility of Chinese characters is expected to vary significantly, and this variability offers the challenge of deriving a simple visual function measurement for the Chinese reading population. The purpose of this study was to suggest a solution to the challenge through psychophysical studies of Chinese character legibility. METHODS: To illustrate legibility variations in Chinese characters, visual acuities for six groups of Chinese characters from low to high spatial complexities and one group of Sloan letters were determined in six normal-sighted Chinese observers. The relationship between legibility and optical defocus were then determined for the Landolt C, the Snellen E, and three groups of Chinese characters representing low, medium, and high spatial complexities in 26 normal-sighted Chinese readers. RESULTS: The acuity size of Chinese characters increased steadily with stimulus complexity, though at a slower rate than would be expected if visual acuity were based on the finest details of the stimuli. The acuity size versus optical defocus functions of three Chinese character groups and the Snellen E had similar slopes and differed only by a vertical shift, depending on the optotype spatial complexity. The function of the Landolt C was significantly steeper. CONCLUSIONS: The findings indicate that visual acuity assessment in Chinese readers is complicated by the spatial complexity of Chinese characters, but the fact that the Snellen E, which is the current national standard of acuity measurement in China, and Chinese characters showed similar dependence on optical defocus may indicate a potentially valid way to infer functional vision in Chinese readers with Snellen E acuity.