Fixation disparity at different viewing distances of a visual display unit

Fixation disparity was measured while subjects observed a realistic alphanumeric pattern on the cathode ray tube (CRT) of a visual display unit at viewing distances of 25-85 cm. The psychometric function of fixation disparity was determined by analysing the observers' responses to a series of short (100 ms) flashes of nonius targets that were superimposed on the CRT-image by means of a half-silvered mirror. Test-retest correlation of fixation disparity was high (r = 0.8) at viewing distances of 40 and 82 cm and lower (r = 0.55) at the shorter distance of 26.5 cm. Mean fixation disparity increased to a more exophoric condition as the display approached the eyes. However, this effect was observed only in some of the subjects, so the susceptibility of fixation disparity to viewing distance appears to be an individual characteristic. When subjects observed the VDU screen continuously for 30 min at viewing distances of 85, 47, 31 and 25 cm in different sessions, fixation disparity and accommodation remained unchanged over time.