A note on the remote oculometer technique for recording eye movements

The remote oculometer described by Merchant, Morrissette and Porterfield (1974) is an important development of cornea1 reflection methods for eye movement recording. It has the considerable advantage of requiring no attachments to the subject and can accommodate a small range of head movements whilst monitoring eye behaviour with an accuracy of some 1”. The measurement principle which was first detailed by Cowey (1963) employs recording the relative positions of two elements of the eye which move relatively for an eye movement but not for a head movement. This overcomes the sensitivity to translational head movements inherent in some cornea1 reflection systems. The two elements used are the centre of the pupil and the cornea1 reflection of a light source. In the remote oculometer the light source is located at optical infinity and is co-axially aligned with the recording camera. Incident infra-red light is commonly used which provides little subject awareness of the recording process. As a result of the co-axial alignment of the source and camera the normally black pupil of the subject appears bright due to light scattered back from the retina. This facilitates the detection of the pupil-iris boundary as the pupil appears as a bright disc with the cornea1 reflection of the oculometer source appearing as a much brighter spot. The pupil centre is then computed from measurements taken around the pupil edge. The displacement (D) between the cornea1 reflection and the pupil centre is shown in Fig. 1 and is given by the relationship: