Flat Fresnel lenses are very popular in CPV. Since it is essentially an image-forming optics, it produces an image of the sun inside the solar cell, which must be large enough to allow for tolerances (acceptance angle). Moreover, due to its discontinuous nature, its magnification is nonconstant, so the image of the sun is blurred and the irradiance distribution is not stepped but bell-shaped. Furthermore, chromatic aberration of Fresnel lenses causes significant differences of the irradiance distributions for the different junction spectral bands, and it limits CAP. Nevertheless, due to its simplicity, Fresnel lenses without secondary elements are still a competitive solution at moderate concentrations (). The company Soitec, using the technology developed by Concentrix, utilizes this approach.25 With the concentrator system of together with the high-efficiency multijunction solar cells, their module efficiency reaches 31.8%.