Patent GB-1412707-A (GB)
IPC: F41G3/22
Filed: 1973-12-06
Abstract: 1412707 Detecting attitude of eyeball STANFORD RESEARCH INSTITUTE 6 Dec 1973 [11 Dec 1972] 56599/73 Heading H4D An eye tracker for continuously tracking the orientation of the optical axis 13 (Fig. 1) of an eye 11 which can translate and rotate in a horizontal and vertical direction with respect to its centre of rotation 13 is imaged by, e.g. a lens 12, in which image 11<1> is disposed a twodimensionally pivoted mirror 14, centred on the image centre 13<1>. A light source 16 produces a beam which is directed into the eye image whereby light reflected by the cornea forms a first Purkinje image and light reflected from the rear of the eye lens in the eye image forms a fourth Purkinje image, which images are detected by photo-detectors 21 and 19 respectively. The output from detector 21 controls a high speed servo system 23 which controls the rotational position of mirror 14 to centre the 1st Purkinje image, and further provides an output signal 27, indicating the eye's rotational position. The translational position of the eye as indicated by the 4th Purkinje image is provided by detector 19 which image is centred thereon by longer time constant servo system 26, which also outputs the translational position at 28. Detector 19 is physically driven by servo system 26, to track the translational position. Lens system 12 requires to be very powerful, i.e. have a low fnumber, to provide a sufficiently large signal from the dim 4th Purkinje image. In a further embodiment, Fig. 2, the eye 29 has an image 29<1> formed by a concave mirror 32. A small gimballed mirror 33 is pivoted in image centre 31<1>. Two light sources are provided. Light source 34 is vertically spaced and light source 36 horizontally displaced with respect to the eye axis, thereby each producing one pair of Purkinje images which are imaged on vertically separated split field photo-detectors 39, 41, and horizontally separated detectors 42, 43 respectively, which respectively measure horizontal rotational and translational displacement and vertical translational and rotational displacement of the eye. Split field photo-detectors 39, 41, 42, 43 detect translational and rotational changes in image 29<1>, in the vertical and horizontal axial directions. Each detector has its two outputs fed to a subtractor, e.g. 44, which provides an imbalance voltage which indicates the respective position change and drives a servo 46 to provide a restoring action. Servo 46 receives a signal corresponding to the horizontal translation of the eye and controls the horizontal positioning of photo-detectors 39 and 41. Similarly, servo 49 controls the horizontal rotation of mirror 33 in response to horizontal rotation of the eye detector by photo-detector 41. Photo-detectors 43 and 42 provide signals corresponding respectively to vertical translation of detectors 42 and 43 and vertical rotation of mirror 33. The high speed servo systems 49, 53 thereby continuously track the high speed rotational movements of the eye, of the order of 50-100 Hz. and translational movements of the order of a few Hz, are tracked by slow speed servos 46 and 57. In a further embodiment (Fig. 4, not shown) the translational movement of the eye is tracked by moving the complete concave mirror (62) and optical detection system by a servo system (69) so that the same portions of concave mirror (62) are used, independent of eye position.