Patent US 4,034,401

A head-up display system in a military aircraft provides projection of flight and weapon-aiming information into the pilot's line-of-sight through a partially-transparent reflector. The pilot operates a button firstly when he recognizes a target viewing through the display and then again when he fixes his eye on an aiming marker in the display. On each occasion of button operation a measure of his eye position is entered into the system to derive coordinates of the identified target related to the marker position in the display, for generation of the appropriate shifts to move the marker into register with the identified target. The measure of eye position is provided in each case by analysis of video-signal waveforms derived by a vidicon camera that is carried on the pilot's helmet and scans reflection in the helmet-visor of the pilot's eye as illuminated, also by reflection in the visor, from an infra-red source. The analysis involves determination from the video signals of successive line-scans of the eye-image, of the boundaries of the iris (or pupil) in relation to both the frame and line time-bases, and computation therefrom of coordinates of the centerpoint of the pupil. Compensation for head movement in the interval between the two measurements of eye position is provided in accordance with variation during that interval of the phasing (in the time-bases) of response to two arrays of point-sources mounted on said reflector.