The lancaster red-green test.

Walter Brackett Lancaster introduced his red-green test in 1939. He felt that any good test of heterophoria should assess the fixation of each eye, simultaneously, without the corrupting influence of a fusion stimulus. His basic concept was to utilize the strength of the cover test-namely, accuracy in measuring ocular deviations-while at the same time devising a way to "measure quickly and accurately the position of the covered eye when the other eye is fixating" in the diagnostic positions of gaze. The Lancaster Red-Green test is a convenient tool for assessing ocular misalignment in patients with paralytic or restrictive strabismus. It is a binocular, dissociative, subjective test that can quickly and accurately measure strabismus in the nine diagnostic positions of gaze. Comitancy, primary and secondary deviations, and torsional misalignment can all be detected and graphically portrayed.