The optical density of erythrolabe determined by a new method.

1. A new method is described for the determination of optical density by retinal densitometry based on an analysis of stray light.2. The basic assumption used is that the stray light present in retinal densitometry is independent of wave-length. The justification for this assumption is considered.3. The stray light and hence the optical density was determined by a method of successive approximations using a Linc 8 computer.4. The optical density of erythrolabe for the direction of maximum Stiles-Crawford efficiency was determined for five subjects, two deuteranopes, two deuteranomalous and one red-rich normal subject. The mean values for three of the subjects were close to the grand mean value of 0.42, but the other two subjects had considerably higher and lower values respectively.5. The results are in satisfactory agreement with optical density values determined by the self-screening method using retinal densitometry, and also with determinations using microspectrophotometry and psychophysics.6. The relevance of dielectric wave-guide modes in the outer segments is considered.