A new method for determining squint angle in primary and all secondary positions using the first and fourth Purkinje images.

A new objective method is presented for measuring the squint angle in all gaze positions. While the subject fixates an object at a distance d, his eyes are illuminated one at a time from above or from the side with a collimated infrared beam. A high-sensitivity infrared camera observes the first and fourth Purkinje images from below. Camera and light source are mounted on the same holder, which can be turned around horizontal and vertical axes. The examiner turns the holder while observing the Purkinje images on a TV monitor until both images lie either on a vertical line (to measure a horizontal deviation) or on a horizontal line (to measure a vertical deviation). In either of these cases the direction of the incident light, which is determined by the position of the holder, and the direction of the optical axis of the eye coincide. Advantages of the method include: the angle between the optical axis and the fixation line can be easily determined; the subject does not have to fixate with both eyes; and the accuracy is +/- 1 degree.