The PACS optics employs a large number of mirrors in each instrument channel. Therefore, the loss per mirror is an important number for the overall transmission of the system. Losses occur by absorption in the mirror material, by scattering at the mirror surface, and by diffraction losses due to the finite mirror sizes. Without diffraction which can be treated separately, the combined scattering/absorption losses per mirror surface can be less than 1% at FIR wavelengths, but measurements also show that they vary with material and surface treatment, and we assume a value of 1%. From the number of reflections a loss of 26% for the spectrometer channels and of 15% for the photometer channels has been derived.