NOVELTY - Optical device for concentrating photovoltaic system to convert incident solar radiation to electrical energy, including primary focusing element for collecting the radiation and directing it to surface of solar cell, comprises optical element (100) having entry aperture (102) for receiving light beams, exit aperture (103) for transmitting light beams to solar cell, and a region (105) made of two layers with different materials so that reflectivity of light beams off the region and transmitted to solar cell optimizes aggregate irradiance on the cell surface over incident solar spectrum.    USE - As an optical device for use in a concentrating photovoltaic system for converting incident solar radiation to electrical energy (claimed).    ADVANTAGE - The optical element reflects light at various wavelengths to a solar cell for the solar cell to operate efficiently. By optimizing the thickness of the first and second layers, the absolute reflectance of glancing angles can be improved at a variety of wavelengths. The improvement of the optical element improves III-V concentrator photovoltaic performance, especially during the winter months and early/late in the day when the spectrum is blue-poor. Moreover, by forming the primary focusing elements in an integral sheet production costs and assembly costs can be decreased because only one item i.e. the integral lens sheet needs to be aligned with the optical elements and solar cells.    DETAILED DESCRIPTION - An optical device for use in a concentrating photovoltaic system for converting incident solar radiation to electrical energy, including a primary focusing element for collecting the incident solar radiation and directing such radiation to the surface of a solar cell, comprises: an optical element (100) having an entry aperture (102) for receiving light beams from the primary focusing element, an exit aperture (103) for transmitting light beams to the solar cell, and a region (105) where at least some of the light beams reflect off the surface of the region and are transmitted to the solar cell. The region is composed of a layered structure with a first material layer having a first optical characteristic, and a second material layer disposed on the first material layer and having a second optical characteristic. The material composition and thickness of each layer is adapted so that the reflectivity of the light beams off such surface and transmitted to the solar cell optimizes the aggregate irradiance on the surface of the solar cell over the incident solar spectrum. An INDEPENDENT CLAIM is included for an optical element disposed in an optical path between a lens and a solar cell, where the optical element configured to concentrate incoming light onto the solar cell and comprising: a channel with an enlarged inlet that faces towards the lens and tapers to a reduced outlet that faces towards the solar cell, where the channel includes reflective inner walls; first and second layers positioned on the reflective inner walls; where the layered reflective inner walls configured to reflect the incoming light that enters through the inlet towards the outlet and onto the solar cell, where the layered reflective inner walls have a reflectivity at a glancing angle of the incoming light beam less than or equal to 30 degrees with respect to inner wall is 95% at 450 nm, about 93% at 400 nm, and about 82% at 375 nm.    DESCRIPTION OF DRAWING(S) - The figure shows a perspective view of an optical element.    Optical element (100)    Entry aperture (102)    Exit aperture (103)    Intermediate region (105)    Mounting tabs (120)