In the fields of optometry and ophthalmology, the Lancaster red-green test is a binocular, dissociative, subjective cover test that measures strabismus in the nine diagnostic positions of gaze.
This largely eliminates the neurologically learned fusional vergence tone ("vergence adaptation") that is present in patients who are able to achieve fusion in a limited area of gaze, as is often the case for patients with incomitant strabismus.
[3] The Lancaster red-green test quantifies comitant and incomitant misalignments.
[citation needed] (Comitancy means that there is the same misalignment in all gaze directions.
[4] The test allows to determine and accurately quantify also latent forms of strabism heterophoria).