The automatic eye alignment of an infrared optometer

Abstract

The ability of the human eye to change its overall refractive power so that people can focus on objects both far and near is termed accommodation. Research into how the eye automatically changes its accommodation, demands an instrument capable of tracking the accommodation with fine resolution and adequate corner frequency. An instrument capable of tracking the ocular accommodation is called an optometer. Reports of earlier optometers show that optometers using the older Scheiner principlecan have the required precision and dynamics required to track the micro fluctuations accommodation. However optometers using the Scheiner principle require precise alignment to the patient’s pupil to be maintained throughout the measurement time. Previous optometers have used the radiation reflected from the patient’s cornea (calledthe corneal reflection) to initially align the optical axis of the optometer to the centre of the patient’s pupil. Since the Scheiner principle optometer uses radiant energy reflected from the patient’s retina to make a refractive measurement, the idea of using this same radiant energy for patient alignment is investigated. Earlier optometers have blocked the corneal reflection from reaching the photodetectors for the retinal reflection using a small fixed light stop. Since it is not possible to use a fixed light stop if the retinal reflection is used for alignment, the feasibility of using crossed linear polarizers is experimentally evaluated. The results showed that about78% of the radiant energy reflected from the front lens of an artificial eye could be eliminated using crossed linear polarizers. Whether the Scheiner principle measurement of refraction of an artificial eye could be done with 78% of the front lens (corneal) reflection removed was investigated. The results were not conclusive. There was not a measureable indication of when the refraction of the experimental optometer matched that of the artificial eye. The experimental optometer system attempts to use a servo controlled mirror system to move the optical axis of the optometer so that it coincides with the optical axis of an artificial eye. The design, development and testing of the mirror system is described. The mirror system enables the optometer to perform a two dimensional scan over the pupil plane of the patient’s eye or an artificial eye. During the scanning, the total radiant power reflected can be measured. For the optometer to be aligned using radiation reflected from the retina, a scan of the pupil plane of should reveal the pupil boundaries. This was experimentally demonstrated to work. Unfortunately time limitations did not permit further development of an automatic eye alignment and tracking system.