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S. Ravikumar, A. Bradley, L. Thibos, X. Cheng; Letter Discrimination and Confusions within the Sloan Letter Set . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2783.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: A sub-set of ten letters originally identified by Sloan et al (1952) have become a standard for character recognition and visual acuity experiments, and are presumed to be similarly discriminable. We have examined the generality of this assumption by asking whether optical degradation affects the physical similarity of letters. Methods: Matlab was used to generate focused characters and characters optically degraded by defocus and/or higher order monochromatic aberrations. Physical similarity of the characters were quantified by normalized pairwise cross correlation. Psychophysical discriminability was assessed by quantifying letter identification as a function of letter size, from which letter acuity was determined. Confusion matrix analysis allowed for comparison between physical and psychophysical similarity. Results: Character similarity varies tremendously within the 26 letter alphabet (e.g. O and Q are very similar and A and U are dissimilar). Pairwise cross-correlation indicates that the Sloan letter set includes characters with similar alternatives, e.g. O and C (r=0.933), while some letter pairs are very different e.g. Z and V (r=0.521). Optical degradation generally increased physical character similarity and decreased letter discriminability. The relationship is complicated by optically induced phase shifts affected spatial structure of letters and their confusions. Psychophysical letter resolution in the presence of optical defocus indicates that acuity for individual Sloan letters differs by up to 0.15 logMAR (1 1/2 lines on a logMAR chart). Conclusions: The pairwise discriminability of Sloan letters is affected by optical aberrations. The spatial structure of aberrated images of letters suggests that performance on a visual acuity task will be influenced by the similarity of a degraded image to other degraded images and to the focused version of the character.
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