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Measuring Intraocular Scattering With Spatio-Temporally Modulated Light


Purpose: : Light scattering in the human eye is an important factor influencing retinal image quality and quality of vision. The purpose of this study was to develop a new optical instrument based on the double-pass principle but using flicker optical heterodyne detection to measure intraocular scatter.

Methods: : The instrument is a variation of a double-pass instrument using a specially designed source to be projected into the retina. The extended source was composed of four concentric annular diffusers back-illuminated by an array of 97 high-brightness green LEDs (530±10 nm). The LEDs corresponding to each annulus are temporally modulated at different frequencies ranging from 4 to 14 kHz. An annular diaphragm conjugated with the pupil plane allows light to enter the eye through an annulus with internal and external diameters of 4 and 5.5 mm respectively. A pupil camera controls the eye alignment. The retinal image of the source subtends a radius of 12 deg. Light reflected from the fundus is limited by a circular diaphragm conjugated with the pupil plane so that the incoming and outgoing pathways do not overlap. A pinhole conjugated with the retinal plane allows the light corresponding to the 15-arcmin central area of the retinal image to be detected by a photomultiplier tube. The Fourier transform of the photomultiplier signal reveals the contribution of each annulus and therefore the average intensity of scattered light between the corresponding angles. The system’s sensitivity was evaluated by means of an artificial eye and previously calibrated diffusers.

Results: : The system was capable of measuring the intensity of scattered light in three different regions (0.5-2, 3-5 and 6-12 deg of visual angle). The sensitivity was better that 0.1 log units of the equivalent straylight parameter -log(S)- for all angular regions. The Fourier method allowed the extraction of useful data for total exposure time equal to 300 msec.

Conclusions: : An instrument to measure scattering based in the double-pass method with improved heterodyne-type optical detection has been developed and validated in an artificial eye model. A high sensitivity was demonstrated by using diffusers that create comparable scatter to that expected in healthy young eyes. The system is compact and could be of use in future clinical applications.

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