The effect of illumination and foveal fusion lock on clinical fixation disparity measurements with the Sheedy Disparometer.

The Sheedy Disparometer is a clinical instrument designed to measure fixation disparity and hence determine forced vergence--fixation disparity curves. The target design includes a central 1.5 degrees fusion-free area. The effect of introducing a central fixation target to the instrument is investigated along with the influence of ambient illumination level on fixation disparity. All curve parameters, i.e. curve type, slope, X- and Y-intercepts were found to significantly alter with the introduction of a foveal lock. Illumination changes did not cause significant differences in any parameter.