The directionality of photoreceptors in the human retina.

The directional sensitivity of photoreceptors is a result of their structure that make them act as optical fibers. Therefore measurement of photoreceptor directionality is a tool for testing the physical properties of photoreceptors in vivo. Clinical studies of photoreceptor directionality are limited by the fact that psycho-physical methods for measuring the Stiles-Crawford effect are time consuming and require excellent co-operation from the subject. Thus different reflectometric techniques have been developed recently. This paper describes such methods, that allow to characterize the optical properties of photoreceptors, i.e. their orientation and directionality. Mechanisms likely to explain the discrepancy between the directionality factor values given by these techniques are discussed. Finally the functional advantages of photoreceptor optics are considered.