The invention relates to a method and a microelectronic sensor device for making optical examinations in an investigation region (3). An input light beam (L1) is emitted by a light source (20) into said investigation region (3), and an output light beam (L2) coming from the investigation region (3) is detected by a light detector (30) providing a measurement signal (X). An evaluation unit (40) provides a result signal (R) based on a characteristic parameter (e.g. the intensity) of the input light beam (L1) and the output light beam (L2). Preferably, the input light beam (L1) is modulated witha given frequency (omega) and monitored with a sensor unit (22) that provides a monitoring signal (M). The monitoring signal (M) and the measurement signal (X) can then be demodulated with respect tothe monitoring signal, and their ratio can be determined. This allows to obtain a result signal (R) that is largely independent of environmental influences and variations in the light source.