Birefringence-based eye fixation monitor with no moving parts.

For the purpose of vision screening, we develop an eye fixation monitor that detects the fovea by its unique radial orientation of birefringent Henle fibers. Polarized near-infrared light is reflected from the foveal area in a bow-tie pattern of polarization states, similar to the Haidinger brush phenomenon. In contrast to previous devices that used scanning systems, this instrument uses no moving parts. It rather utilizes four spots of linearly polarized light-two aligned with the "bright" arms and two aligned with the "dark" arms-of the bow-tie pattern surrounding the fovea. The light reflected from the fundus is imaged onto a quadrant photodetector, whereby the circular polarization component of the polarization state of each reflected patch of light is measured. The signals from the four photodetectors are amplified, digitized, and analyzed. A normalized differential signal is computed to detect central fixation. The algorithm is tested on a computer model, and the apparatus is tested on human subjects. This work demonstrates the feasibility of a fixation monitor with no moving parts.