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Transmission gratings that combine a large diffraction angle with ahigh diffraction efficiency and low angular and wavelength dispersion could be used to collect sunlight in a light guide. In this paperwe determine what characteristics a grating should have in order tobe useful for such a solar concentrator. To this end we compare thediffractive properties of polarization gratings and classical surface relief gratings. It is found that polarization gratings and classical surface relief gratings have qualitatively comparable diffractive properties as long as their thickness-parameters are within the same regime. The diffraction efficiency of these gratings can be closeto 100% for a broad range of incoming angles when the period is large compared to the wavelength of the incoming light. This no longerholds for small-period gratings. For solar concentrators the more easily producible surface relief gratings are preferred over polarization gratings.