Variability of fixation disparity and accommodation when viewing a CRT visual display unit

Fixation disparity and accommodation were measured while subjects viewed a target on a cathode ray tube at a 50 cm viewing distance. The psychophysical method of sequential testing was used. For measuring fixation disparity a series of brief dichoptic nonius targets was presented and the direction of perceived misalignment was reported by the subject. From these responses the psychometric function of fixation disparity can be derived which gives not only the mean fixation disparity but also the range of fluctuation during the period of measurement. During a 5 min period, this range was +/- 1.5 min arc averaged over subjects, when expressed as the range in which instantaneous fixation disparity fell with a probability of 68%. Repeated measurements on 5 days within 2 weeks gave a larger range of +/- 2 min arc. However, individual results can differ considerably from these average values. Repeated measurements and the analysis of the psychometric function can provide a more complete description of individual binocular status than is obtained with a single adjustment of continuously visible nonius targets. Laser optometry revealed that, compared to the stimulus distance of 50 cm, accommodation lagged behind in the range 1-2 D. The two display modes of bright characters on dark background and vice versa gave very similar results.