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Amy A. Kalia, Gordon E. Legge, Christopher S. Kallie; Effective Acuity for Low-Pass Filtering of Real World Images. Journal of Vision 2010;10(7):1249. doi: 10.1167/10.7.1249.
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Understanding of low-vision mobility problems would benefit from methods for predicting the visibility of environmental hazards such as steps. A useful tool for rehabilitation specialists and architectural designers would blur an image of a space according to a particular acuity level. Although many graphics programs have blurring functions, the relationship between blur filters, clinical measures of acuity, and the information transmitted in the resulting image is unclear. To examine this relationship, we tested the effective letter acuity associated with the bandwidth of two low-pass filters applied to a photograph of an eye chart. A high-resolution camera image was obtained of the Lighthouse Distance Visual Acuity chart at a standard viewing distance of 4 m. The image (with peak resolution of 80 pixels per degree) was filtered with Gaussian and 4th-order Butterworth filters with bandwidths (defined as the frequency at 50% of maximum) ranging from 1.33 to 35 cycles per degree. Five normally sighted subjects viewed the blurred images on a display screen at a distance that allowed easy identification of the 20/20 letters in the unblurred image. For the highest bandwidths, subjects were able to read letters smaller than the 20/20 line. For a lower range of bandwidths (1.33 to 8.78 cycles per degree) there was a linear relationship between filter bandwidth (expressed in degrees per cycle) and the smallest resolvable letter size (in degrees). Effective acuity was worse with the Butterworth filter compared to the Gaussian filter for equal bandwidths; the smallest resolvable letter size for a 1 degree per cycle bandwidth was approximately 1.1 degrees for the Butterworth filter and 0.63 degrees for the Gaussian filter. These results yield functions (effective acuity vs. filter bandwidth) for simulating the effects of reduced acuity on the information available in real-world scenes.
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