Determination of ocular defocus using the double-pass blur image of a point source

The double-pass blur circle from a point source can be used to determine the quality of focus of the human eye. An apparatus that can determine a threshold of defocus was devised. A circle/annulus “bulls-eye” photodiode that can distinguish focused from defocused light was used. The photodetector signal obtained was a function of the amount of defocus caused by the double-pass point spread reflected from the retina. The signal will thus be affected by refractive error, incomplete accommodation, media opacities, and abnormalities of retinal reflection. Such defocus detection can be combined with eye-fixation monitoring to be used as a screening tool to detect risk factors for amblyopia