We investigate the influence of surface texture on the effectiveness of reflectance by intermediate reflectors in thin-film tandem solar cells. Two distinct angular reflection regimes are identified. For large surfaces with large aspect ratios, frustrated total internal reflection or photon tunneling is found to be the dominating mechanism. We show that the parasitic reflection losses of commonly used randomly textured surfaces are explained by these distinct regimes and introduce a spectral reflectance parameter, which serves as a guideline for designing optimized spectrally selective intermediate reflectors.