The pallor of the optic disc. A quantitative photographic assessment by purple filter.

By employing purple colour filters with a large blue and a small red transmission, normal and atrophic optic disks were photographed on colour transparencies. Normal optic disks reflect red light, and atrophic optic disks reflect blue light. The degree of atrophy estimated by colour contrast is quantitated by microdensitometry in the blue (470 nm) and in the red (640 nm) region of the transparencies. The difference between the blue and red density is used as an expression of optic disc colour.