Trade name for a clinical instrument designed to measure fixation disparity at near. The target does not have a binocular fixation point and the fusion lock is parafoveal. The instrument fits on the near point rod of a standard phoropter. The Disparometer has two stimuli: one for vertical disparity measurement and the other for horizontal disparity measurement. The test consists of successive pairs of vernier lines of increasing angular separation within a structureless field, each line being viewed by one eye through polarizing filters. The edge of the field provides a peripheral fusion stimulus. Fixation disparity is measured when the vernier lines appear to be aligned and the amount is given by the angular separation (in minutes of arc) of the lines indicated on the back of the instrument. A fixation disparity curve can be obtained by determining the fixation disparity for various amounts of prism power placed in front of the eyes. See associated heterophoria; Mallett fixation disparity unit.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann