Dielectric compound parabolic concentrators.

where θm a x is the angular acceptance (half-angle) and n is the index refraction of the collector relative to the sur­ rounding medium. This concentration is the maximum permissable by physical principles. The CPC is a nonimaging light funnel that derives its characteristic optical properties from the specific shape of the external wall, which is made specularly reflecting. The diameters of these structures are generally large relative to the wavelength of light. We have also investigated models of certain visual receptors, which strikingly resemble the compound parabolic shape and wherein the specularity was achieved by total internal reflection. In these examples, chosen to simulate the conditions found in nature, a frac­ tion of the light rays leaks out because they fail to satisfy the condition for total internal reflection. The novel suggestion in this communication is the obser­ vation that for certain values of parameters (of consider­ able practical importance), the interface between the CPC and the surround becomes a perfect total internal reflec­ tion mirror that permits no leakage of radiation. This re­ sult is unexpected because the condition for maximal con­ centration [Eqs. (1) and (2)], which determines the shape of the external wall, and the condition for total internal re­ flection (angle of incidence exceeds the critical angle) have no a priori connection. Incident rays that lie within the angular acceptance (θmax) of the trough CPC are refracted into an elliptic cone of semiminor angle θmg' and semimajor angle θc, where