Abstract
Abstract: :
Purpose: We previously developed and reported 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 the previous device a point source of polarized near–infrared light was imaged onto the retina and was scanned in a 3° circle at frequency f. Reflections were analyzed by differential polarization detection. The detected signal was predominantly 2f during central fixation, and f during paracentral fixation. This method required a fast spinning motor which added noise and vibration and was generally of limited life. To avoid this problem, we developed an improved, no–moving–parts version of the instrument. Methods: The new instrument 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 four spots are produced by a modulated 785 nm, 100 mW laser diode and a multi–faceted prism. The light reflected from the fundus travels through a quarter–wave plate, a polarizer, and onto a quadrant photodetector, whereby the S3 component of the polarization state of each reflected patch of light is measured. The signals from the four photodetectors are amplified, filtered, and fed to a PC for further analysis. The normalized differential signal ND = ((A+C)–(B+D))/(A+B+C+D) discriminates between central fixation and the lack thereof, where A, B, C and D are the signals from the quadrant photodetector, counted in clockwise direction. Results: In 5 normal adults, properly consented, central fixation values for ND ranged from 0.17 to 0.35 typically, whereas paracentral fixation yielded ND < 0.6–0.7. Conclusions: The feasibility of a no–moving–parts fixation monitor is thus demonstrated. If proven in a larger number of test subjects, this design will be used in the next version of our Pediatric Vision Screener.
Keywords: screening for ambylopia and strabismus • strabismus: diagnosis and detection • eye movements: recording techniques