A microelectronic sensor device and a method for making optical examinations at a carrier for the detection of magnetic particles, for example, at a contact surface of the carrier by frustrated total internal reflection (FTIR), include a light source with a laser modulator for emitting an input light beam into the carrier. The input light beam is modulated such that optical interferences with reflections of the input light beam from the entrance window, or from other components of the carrier, are reduced or minimized. This can be achieved by a pulsed on/off modulation in which the first relaxation minimum of a currently emitted pulse (PN) coincides in the light source with the first relaxation maximum of a reflected pulse (PN-1′). By reducing the effect of interferences, the setup is less prone to disturbances from dimensional variations that are induced by thermal extension, for example.