Digital measurement of pallor-disc ratio

A. R. Rosenthal, D. G. Falconer and P. Barrett

In an effort to quantitate optic nerve head pallor, a method has been devised to measure pallor-disc ratio using optic nerve head photographs. After electronically scanning photographs through various Wratten filters, it was observed that a red filter produces good contrast between the disc and the retinal surface, while a green filter produces good contrast between the pale area of the disc and the remaining normal-appearing nerve head. A computer program fitted an elliptical form to both the disc region of red image and the pale area of green image. Pallor-disc ratios were calculated on both area and radial bases. Reproducibility of 3.5% for the area ratios and 7% for the radial ratios were obtained.