surfaces corresponding closely to the bragg planes an alternate embodiment of the invention illustrated in fig5 uses a concentrator 100 consisting of a fresnel lens made of spectrally dispersive material and configured to have an off axis focus ordinary glass and plastic materials are spectrally dispersive i e their index of refraction is wave length dependent the focus of such a concentrator is spectrally dispersed by an amount depending on the off axis angle and the coefficient of spectral dispersion of the fresnel lens material photocells 102 104 and 106 are placed in the focal region as in the preceding embodiments single element high efficiency spectrally dispersive solar concentractors such as those disclosed herein are inherently more efficient than prior art concentrators for photovoltaic power conversion whereas in the three element system such as described in u s pat no 4 021 267 can be expected to have at best an overall conversion effieiency of about 20 51 in the optical system and 40 in the photovoltaic system the present system is expected to have at best an overall conversion efficiency of about 32 80 in the optical system 40 in the photovoltaic system another significant advantage of the present invention is its simplicity of manufacture and construction either the spectrally dispersive off axis fresnel lens concentrator or the diffraction grating concentrators can be produced in large quantities and at very low cost no other optical elements are needed eliminating other optical elements further reduces cost by eliminating the cost of the elements and their supporting structures an additional advantage of the present system is its reduced weight which is important for use in spacecraft light energy converting means useful in this invention are not limited to photovoltaic cells other.