Testing is done monocularly. Brightness comparison testing must be run before photostress testing. Subjective impressions of a difference in the quantity or brightness of a light perceived by a patient between their two eyes is a helpful method of differentiating an optic nerve and a retinal disease condition when ophthalmoscopy is inconclusive. Optic nerve lesions tend to cause a generalized drop in the sensitivity of light as though the light was turned down or dimmed. Subtle or even large macular lesions do not give the impression of less light entering the eye. This is assuming the media is equally clear in both eyes and there are no marked differences in the yellowing of the lens or a cataract present in one eye and not the other. (Marked pupil size differences can influence the results.)

The patient is presented with a light in front of their better seeing eye while the other eye is completely occluded and then repeated to the suspect eye. The patient is asked if they can tell a difference in the lights brightness between the two eyes. The best approach is to tell the patient if the brightness of the light as seen with their good eye is worth $1.00 how much would the suspect eyes perception of that same light be worth. It is important to point out that there must be at least a 20% difference before the test is truly of any diagnostic value. It should be noted that the brightness comparison test has been likened to or is the complement of the swing flash light tests for an afferent pupillary defect (APD).