Neuro-ophthalmic disorders may cause a subjective difference in the brightness perceived by each eye. We developed a unique clinical device to measure this difference. The device consists of a disk equipped with graded neutral-density filters that fits into a View-Master stereo viewer (View-Master International Group, Inc., Portland, Oregon). This device provides simultaneous viewing of a stimulus target for each eye, and is administered in a random, forced-choice format that allows assessment of patient reliability. We administered this test to 37 control subjects and 84 patients with a variety of disorders. Thirty-six of the 37 control subjects (97%) and 51 of the 84 patients (61%) had no measured relative brightness deficit and no errors. Of the 34 subjects who had abnormal results, 21 (62%) isolated a relative brightness deficit within 0.2-log unit steps without error, and 28 (82%) did so within 0.4-log unit steps. This device proved to be a simple, reliable instrument for measuring relative brightness perception.