Precise placement of nonius lines on a personal computer screen for measuring fixation disparity

A method is described for measuring fixation disparity psychophysically using the conventional monitor of a personal computer. Both the nonius test stimuli and the fusion stimulus were presented on the same screen. To be able to adjust the offset of the nonius lines in very small steps, special electronic circuitry was developed that allows up to 16 steps within the pixel separation of the monitor. The method is flexible in that various geometrical dimensions of the visual stimuli and different psychometric procedures can be used.