Specularity of concentrator mirrors for CPV measured with VLABS

We investigate the specular reflectance properties of four different mirror samples suitable for massmanufacturing of CPV primary concentrators. The laboratory setup VLABS and its data evaluation procedure to calculate specular reflectance are introduced. We present measurement results of specular reflectance with a very high angular resolution around the specular direction making it possible to compare different mirror materials to a high degree of accuracy for arbitrary acceptance angles. It is shown that the specular reflectance can be significantly lower than hemispherical reflectance due to surface scattering. For a CPV system with 1° acceptance angle the loss of radiant flux due to surface scattering amounts to a maximum of 8% relative for the investigated samples.