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L. D. Hubbard, R. P. Danis, M. W. Neider, H. D. Wabers, D. W. Thayer, Age-Related Eye Disease Study 2 Research Group; Brightness, Contrast, and Color Balance of Digital vs. Film Retinal Images in the Age-Related Eye Disease Study 2. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2246. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To analyze brightness, contrast, and color balance of digital vs. film retinal images in a multi-center clinical trial, to propose a model image from exemplars, and to optimize both image types for evaluation of age-related macular degeneration (AMD).
The Age-Related Eye Disease Study 2 (AREDS2) is enrolling subjects from 90 clinics, ¾ using digital and ¼ using film cameras. Image brightness, contrast, and color balance (B/C/CB) were measured with 3-color luminance histograms. First, we analyzed exemplars (film and digital) from expert groups, and constructed an AMD-oriented model. Second, we analyzed impact of B/C/CB on appearance of typical AMD abnormalities. Third, we compared B/C/CB in AREDS2 images between film (156 eyes) and digital (605 eyes), and against the model. Fourth, we enhanced suboptimal images by adjusting B/C/CB to accord with our model parameters.
Exemplar images had similar brightness, contrast, and color balance, supporting an image model. Varying a specimen image through a wide range of B/C/CB revealed greatest contrast of drusen and pigment abnormalities against normal retinal pigment epithelium with the model parameters. AREDS2 digital images were more variable than film, with lower correspondence to our model. Ten percent of digital were too dim and 19% too bright (over-saturated), vs. 1% and 4% of film, respectively. On average, digital had lower green channel contrast (giving less retinal detail) than film. Overly red color balance (weaker green) was observed in 23% of digital vs. 8% of film. About half of digital (but fewer film) images required enhancement before AMD grading. After optimization of both image types, AREDS2 image quality was judged as good as that in AREDS (all film).
A histogram-based model, derived from exemplars, provides a pragmatic guide for image analysis and enhancement. In AREDS2, the best digital images matched the best film. Overall, however, digital provided lower contrast of retinal detail. Digital images taken with higher G/R ratio showed better brightness/contrast management. Optimization of images in our multi-center study helps standardize documentation of AMD.
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