Purpose: To develop a compact instrument to measure light scattering in the human eye for angles in the range between 3 and 9 degrees of visual angle.
Methods: The instrument is based on a previous laboratory setup using extended light sources in a double-pass (DP) configuration (Ginis et al., J of Vision, 2012). The light source is an array of green (528±10 nm) light emitting diodes (LEDs) spatially homogenized by light shaping diffusers. The field is separated in 2 zones: a central area (corresponding to visual angles from 0 to 3 degrees (in radius) and an annular area (3 to 9 degrees). In both zones LEDs are square-wave temporally modulated at 483 Hz and 769 Hz for the central and peripheral areas respectively. Two annular diaphragms conjugated with the cornea and the lens allow the projection of the source on to the retina while leaving the central part of the pupil free of back scattered light and reflections. Light reflected from the fundus is sensed through a circular diaphragm conjugated with the center of the pupil with no overlapping of the illumination and measurement paths. A pupil camera controls the alignment. The light reflected from the central retinal area (15-arcmin) is selected through a circular diaphragm and a pinhole by a photodiode. The Fourier transform of the signal reveals the contribution of each annulus and therefore the average intensity of scattered light for the corresponding angles. The total measurement time is 200 msec.
Results: Functionality, sensitivity and repeatability of the method were demonstrated with an artificial eye and two different previously characterized diffusers. The equivalent logarithm of the straylight parameter measured for the diffusers were 0.67 (SD=0.005) and 0.84 (SD=0.003), values not statistically significantly different than the anticipated. Pilot measurements in human eyes were also obtained. A careful analysis of the artifacts associated to alignment and refractive errors was also performed.
Conclusions: A new compact instrument suitable for routine or clinical measurements of light scattering in the eye was developed. It builds on previous experience with multi-wavelength, high-sensitivity, imaging double-pass system for the measurement the wide-angle point-spread function of the eye.