The Application of Digital Filters to Improve Visibility for People with Maculopathy
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Purpose: Previous studies have shown that some digital filters can enhance picture-image visibility for people with visual impairment. The ultimate purposes of this study are to determine the improvement of picture-image visibility for people with maculopathy using digital image enhancement, and to compare the enhancement effects of generic filters and custom-devised filters. The secondary interests are to investigate the effect of age and maculopathy on supra-threshold contrast matching and to investigate the spatial frequency characteristics of picture-images. Methods: In order to develop effective custom-devised filters, supra-threshold contrast matching and contrast thresholds for two age groups of subjects with normal vision (14 aged 20-50 years and 15 aged 51+ years) and three groups of people with maculopathy (13 with atrophic ARMD, 14 with exudative ARMD, and 8 with JMD) were measured. Amplitude spectrum at each spatial frequency and the slope of amplitude versus spatial frequency were measured to investigate the spatial frequency characteristics of single face and general scene images. To investigate the preference for filters, 7 generic filters and 4 custom-devised filters were applied to single faces and general scenes. The generic filters were high-pass/unsharp masking, contrast enhancement, Sobel edge enhancement, DoG convolution, DoG FFT, Peli’s adaptive enhancement, and a band-pass filter with equi-emphasis of spatial frequencies. The custom-devised filters were band-pass filters based on contrast sensitivity (CS) loss, contrast matching at 3.6% and 27.9%, and emphasis of the peak of the CS curve. Subjects with maculopathy were required to rate the visibility of each image with and without filtering. Nine subjects with maculopathy participated to assess the enhancement quantitatively during which the recognition of facial expression and details in general scenes was tested with and without filtering. Results: Contrast constancy was demonstrated in age-matched controls and people with maculopathy. Single faces were found to be of significantly lower average amplitude than the other groups of images. Eight filters were found to be effective in improving perceived visibility; contrast enhancement, Peli’s adaptive enhancement, DoG convolution, high-pass/unsharp masking, Sobel edge enhancement, band-pass based on 3.6% and 27.9% contrast matching and equi-emphasis band-pass filters. These filters specifically were found to be effective for one or more combinations of maculopathy type and image category. The most commonly preferred filters were the generic filters, contrast enhancement and Peli’s adaptive enhancement. The two highest rated filters for each subject significantly reduced the number of errors of facial expression and errors of recognition of detail within general scene images. Conclusions: The visual system adjusts to compensate for CS loss with aging and maculopathy. Single faces are unique in spatial frequency characteristics. Some generic and custom-devised filters are effective in enhancing image visibility. The custom-devised filters are not superior to the generic filters. Visibility enhancement can be assessed quantitatively.