Videographic quantification of optic disc pallor

Digitized images of the optic disc, acquired videographically (Rodenstock Analyzer) under green (540 nm) and red (640 nm) illumination, were used to quantify optic disc pallor. The pallor density of each pixel was defined as twice the reflectance under green illumination diveded by the sum of the reflectance under red and green illumination. Pallor densities can range from 0 (red) to 1 (white); typical median values were 0.25 for vessels, 0.40 for healthy disc rim, and 0.70 for the lamina cribrosa. The variability of pallor measurements using this technique was determined. Two uniform color fields (Farnsworth-Munsell 100-Hue Test color chips 1 and 18) were imaged five times each using a model eye. Significant (P<0.0001) drift of mean pallor densities occured between images of both sets. The optic discs of seven normal eyes and of seven glaucomatous eyes were imaged nine times each. The non-normal frequency distribution of pallor densities for each image was described by trimmed means and a measure of distribution width. Variability was defined as the standard of the measurements divided by the full scale pallor density. The variability of the trimmed means increased with pallor density (r=0.99, P<0.0001). The variability of distribution width was smaller than that of the mean values, and averaged 3.4% in normal and glaucomatous eyes. Videographic reflectometry may provide useful, quantitative measurements of optic disc pallor.