Legibility Variations of Chinese Characters and Implications for Visual Acuity Measurement in Chinese Reading Population Jun-Yun Zhang,1,2 Ting Zhang,1,2 Feng Xue,3 Lei Liu,*,4 and Cong Yu*,1,2 PURPOSE. Written Chinese contains an enormous number of for two related but different purposes. The first is to determine characters with a wide range of spatial complexities. Conse- the refractive error of the eye as well as the lens correction that quently, the legibility of Chinese characters is expected to vary makes the stimulus optically conjugate to the retina.1 For this significantly, and this variability offers the challenge of deriving purpose, visual acuity is usually defined as a measure of spatial a simple visual function measurement for the Chinese reading resolution of the visual system and is described by the finest population. The purpose of this study was to suggest a solution details (minimum angle of resolution, MAR) that can be re- to the challenge through psychophysical studies of Chinese solved. International2 and U.S.1 standards recommend the character legibility. eight- and four-orientation Landolt C as the primary visual METHODS. To illustrate legibility variations in Chinese charac- acuity test optotype. Both standards stipulate that the gap of ters, visual acuities for six groups of Chinese characters from the C target is the detail to be resolved and that the width of low to high spatial complexities and one group of Sloan letters the gap, which is one fifth of the target height, is the measure were determined in six normal-sighted Chinese observers. The of visual acuity. In China, the standard optotype is the Snellen relationship between legibility and optical defocus were then E,3 whose stroke width is one fifth of the optotype height. For determined for the Landolt C, the Snellen E, and three groups the purpose of refraction, visual acuity measurement should of Chinese characters representing low, medium, and high use a universal standard and should be free from the observers’ spatial complexities in 26 normal-sighted Chinese readers. visual environment and cultural background. RESULTS. The acuity size of Chinese characters increased However, there are situations in which the optical conjuga- steadily with stimulus complexity, though at a slower rate than tion between image plane and the retina is not the main would be expected if visual acuity were based on the finest concern.4 Even when refractive errors are optimally corrected, details of the stimuli. The acuity size versus optical defocus visual performance can be impaired by diseases and trauma of functions of three Chinese character groups and the Snellen E the eye or of the visual neural pathway or by demanding had similar slopes and differed only by a vertical shift, depend- operating conditions, such as low luminance, low contrast, and ing on the optotype spatial complexity. The function of the disabling glare. Therefore, for purposes such as diagnosis of Landolt C was significantly steeper. ocular diseases, visual rehabilitation, job qualification, and dis- CONCLUSIONS. The findings indicate that visual acuity assess- ability benefit, visual acuity is used as a measurement of func- ment in Chinese readers is complicated by the spatial complex- tional vision—that is, how well a person can perform vision- ity of Chinese characters, but the fact that the Snellen E, which related activities under certain conditions. For a visual acuity is the current national standard of acuity measurement in measurement to be functionally relevant, the stimuli should be China, and Chinese characters showed similar dependence on closely related to the observer’s visual tasks. For literate ob- optical defocus may indicate a potentially valid way to infer servers, the most important visual task is undoubtedly reading functional vision in Chinese readers with Snellen E acuity. text of the observers’ native language. (Invest Ophthalmol Vis Sci. 2007;48:2383–2390) DOI: Because of the importance of reading text in functional 10.1167/iovs.06-1195 vision, letter charts have become the dominant way of clinical evaluation of functional vision around the world. However, isual acuity is the most frequently used vision test in both letters are complex spatial patterns, and what constitutes the Vclinical practice and basic research. Though it is not al- “finest detail” in letters cannot be precisely defined. Therefore, ways explicitly stated, the fact is that visual acuity is measured uniting visual acuity for refraction and for functional vision has always been a challenge.1 Both international and U.S. standards stipulate that alternative optotypes, such as letters, should be From the 1State Key Laboratory of Cognitive Neuroscience and 2 equivalent to the Landolt C in test results. ISO 8597 (Interna- Learning, Beijing Normal University, Beijing, China; the Institute of 2 Neuroscience, Chinese Academy of Sciences, Shanghai, China; the tional Organization for Standardization) stipulates that a set of 3EENT Hospital, Fudan University, Shanghai, China; and the 4School of optotypes is equivalent to the Landolt C if they differ less than Optometry, University of Alabama at Birmingham, Birmingham, Ala- 0.05 log unit. In the U.S. standard, the allowed difference is bama. within 5%. If an alternative set of optotypes is not equivalent to Supported by Beijing Normal University and the Chinese Academy the Landolt C, a size conversion factor should be determined to of Sciences. scale the optotypes. In phonics-based languages, at least in Submitted for publication October 5, 2006; revised December 31, 2006 and January 17, 2007; accepted March 16, 2007. those using Roman alphabets, an agreement between refrac- Disclosure: J.-Y. Zhang, None; T. Zhang, None; F. Xue, None; L. tion oriented Landolt C acuity and functional vision oriented Liu, None; C. Yu, None letter acuity is relatively easy to achieve. Sloan et al.5 tested The publication costs of this article were defrayed in part by page acuity in 214 eyes with various refractive errors by using the charge payment. This article must therefore be marked “advertise- Landolt C and the uppercase English letters CDHKNORSVZ and ment” in accordance with 18 U.S.C. §1734 solely to indicate this fact. found that the two measurements correlated highly (Pearson *Each of the following is a corresponding author: Cong Yu, State ϭ Key Laboratory of Cognitive Neuroscience and Learning, Beijing Nor- r 0.90). When such agreement between primary and alter- mal University, Beijing, China; [email protected]. native optotypes can be established, the same unit for visual Lei Liu, School of Optometry, University of Alabama at Birmingham, resolution, MAR (minimum angle of resolution) or logMAR, can Birmingham, AL; [email protected]. be used for functional vision measurement, even though for Investigative Ophthalmology & Visual Science, May 2007, Vol. 48, No. 5 Copyright © Association for Research in Vision and Ophthalmology 2383 Downloaded from tvst.arvojournals.org on 09/25/2021 2384 Zhang et al. IOVS, May 2007, Vol. 48, No. 5 FIGURE 1. (a) Sloan letters and six groups of CCs used in the study. The number of strokes was 2 to 4, 5 to 6, 8 to 9, 11 to 12, 13 to 15, and 16 to 18 for the CC groups 1 through 6, respectively. (b) An illustration of stroke frequency calculation. letter stimuli, what is to be resolved may differ from letter to types on the same line have approximately the same legibility. letter. To date, none of the Chinese visual acuity charts have gained Because many phonics-based languages employ only a small popularity in clinical practice. The current Chinese national number of letters that often have simple and relatively uniform standard uses a logarithmic Snellen E chart.3 This chart, like the spatial complexities, a subset of 8 to 10 letters can adequately Landolt C, is preferable for refraction, but how its result can be represent the spatial complexity of the entire alphabet. The related to the legibility of Chinese characters has not been letters to be used as acuity optotypes can be determined by an systematically studied. exhaustive testing of the legibility of the entire alphabet, to In this article, we present a thorough investigation of the select letters with approximately identical legibility (The iden- legibility of written Chinese characters to lay the groundwork tical legibility requirement is essential to modern visual acuity for developing a more reliable way to evaluate functional vision chart design, in which multiple optotypes of nominally identi- in Chinese readers. We first measured the legibility of six cal legibility are arranged on the same line to increase testing groups of representative Chinese optotypes that covered the accuracy.) Once selected, the set of optotypes can be tested full range of spatial complexities of frequently used Chinese against the standard, such as the Landolt C, to determine acuity characters, by applying rigorous psychophysical procedures. equivalence and a conversion factor, if necessary. This method The results showed that acuity sizes of Chinese characters has been used in creating visual acuity charts in English, Ger- increased linearly with their spatial complexity. We then stud- man, Hindi and Gujarati, Thai, and Arabic.5–9 ied the effects of optical defocus on acuities of the Landolt C, This optotype selection method, however, may not be ap- the Snellen E, and three Chinese character groups representing plicable to nonalphabetic languages like Chinese. An enormous low, medium, and high spatial complexities, using a clinically number of characters are used in written Chinese. Primary accepted, forced-choice letter recognition method. We found school graduates are required to learn more than 2500 fre- that acuity-versus-defocus functions for the Snellen E and all quently used characters during their 6 years of schooling. The three Chinese character groups, but not for the Landolt C, criterion for literacy is the ability to recognize 2000 or more were parallel to each other.