By a unit for determining fractions of a substance in a gas or gas mixture, measurements are carried out on the gas or gas mixture for purging a lens in a projection apparatus for projecting patterns onto a substrate. The results of a first measurement on the gas fed to the lens are compared with the results of a measurement of the gas removed from the lens. If, the substance is a contaminating substance that leads to a deposit on the lens under the influence of high-energy radiation from an illumination source, the difference is used to infer photochemical reactions on the surface of the lens that lead disadvantageously to the deposition. A signal is generated as a consequence of the comparison and is used to take preventive measures against a degradation of the lens. Mass spectrometers, electric or optical sensors and other known methods for substance analysis are used as measurement units.