Abstract: The piezoelectric effect of an optical sensor (6) made from quartz is a function of temperature. Without correcting or compensating measures, this temperature dependence leads to a falsification of the measuring signal if the temperature of the sensor element is not held constant. In order to obtain accurate measurements for variable sensor temperatures T.sub.s, a temperature-corrected measuring signal M is provided in accordance with:M=U13.multidot.(1+.alpha..multidot.(T.sub.0 -T.sub.s)),U13 signifying a control signal, T.sub.0 a prescribable calibration temperature of the sensor (6), and .alpha. the temperature coefficient of the sensor (6). In this case, the sensor temperature (T.sub.s) is calculated in accordance with:T.sub.s =f(K,E(U.sub.Tr)),K being a contrast parameter which is calculated from light power signals (U1, U2), is a function of the sensor temperature (T.sub.s) and is proportional to the interference contrast, and U.sub.Tr signifying a signal receiver temperature signal and T.sub.