SENSOR WITH CURVED REFLECTOR

Sensor device (21) comprising a retro-reflecting element (22) having a front boundary surface (23) configured for entry of measuring light (11) into the retro-reflecting element (22) and a back boundary surface (24) configured for reflecting a first part of the measuring light as reflected measuring light (12) and configured for transmitting a second part of the measuring light as transmitted measuring light (13), wherein the front boundary surface (23) and the back boundary surface (24) are arranged on opposite sides of the retro-reflecting element (22), and a sensor unit (40) configured and arranged so that the transmitted measuring light (13) is detectable by the sensor unit (40). The front boundary surface (23) and the back boundary surface (24) are of curved shape, the retro-reflecting element (22) is configured so that the measuring light (11) entering into the retro-reflecting element at the front boundary surface (23) is focussed at the back boundary surface (24), and an incidence angle of the measuring light (11) relative to the sensor device (21) is equal to an exit angle of the reflected measuring light (12) relative to the sensor device (21).