Binocular vision: The Mallett fixation disparity test

In the first of a two-part discussion concerning what influences measurements with the Mallett unit, Rajula Karania and Bruce Evans look at the theory behind fixation disparity and its measurement

The Mallett fixation disparity test is a widely used diagnostic tool within UK optometry. The purpose of this study was to evaluate for the first time the effect of test instructions on the results of this test in a clinical setting. Previous research indicates that the Mallett unit is a useful tool for detecting symptomatic heterophoria at near, having both good sensitivity and specificity. However, the significance of precise instruction technique has received little attention.