Focus/defocus detection

Lack of proper focus is a risk factor for amblyopia. We have developed a simple method that helps us assess proper focus, or lack thereof.

Focus detection using a bull’s-eye photodetector

Focus detection is accomplished by using a bull’s-eye photodetector for differential detection of the goodness of focus of the retro-reflected light from the RBS scan. Briefly, fundus-reflected near-infrared light is focused by the optical system of the eye onto a bull’s eye photodetector, optically conjugate to the focusing target, and consisting of an annulus (A) and a center (C) of equal active areas. When the eye is in good focus, most returning light is imaged onto C, and C>>A. When the eye is partially or fully out of focus, the amount of light received by the annulus increases, with C≈A for complete defocus. The goodness of focus of each eye can be assessed by first computing the power spectrum of both the center (C) and annulus (A) signals, and then assessing the powers at the frequency of interest in the center and annulus signals via their ratio (C/A) or their normalized difference [(C-A)/C+A)]. The normalized signal (C-A)/(C+A) is in the range [0 … 1]. It accurately portrays the amount of focus achieved. The signal is independent of individual variations in reflectivity.

We also need to assess proper focus for both eyes simultaneously, as is done in the following binocular design:

You can also check out:

Nusz K.J., Congdon N.G., Tang Ho, Gramatikov B.I., Friedman D.S., Guyton D.L., Hunter D.G. Rapid, objective detection of cataract-induced blur using a bull's eye photodetector. J Cataract Refract Surg. 2005 Apr;31(4):763-70,

http://www.ncbi.nlm.nih.gov/pubmed/15899454