A method of determining materials of lenses contained in an optical system of a projection exposure apparatus is described. First, for each lens of a plurality of the lenses, a susceptibility factor KLT/LH is determined. This factor is a measure of the susceptibility of the respective lens to deteriorations caused by at least one of lifetime effects and lens heating effects. Then a birefringent fluoride crystal is selected as a material for each lens for which the susceptibility factor KLT/LH is above a predetermined threshold. Theses lenses are assigned to a first set of lenses. For these lenses, measures are determined for reducing adverse effects caused by birefringence inherent to the fluoride crystals.