An optical sensor (10) for fitting to a washing machine or dishwasher is proposed, comprising: a housing (16), an arrangement, which is housed in the housing, of optical components, the optical components comprising a controllable light emitter (26) and a first and a second light receiver (28, 30), and an electronic analysis and control unit (36) which is connected to the light emitter (26) and the two light receivers (28, 30), the first light receiver (28) being arranged at the end of a first light measurement path (54) which starts at the light emitter (26) and runs on a portion outside the housing (16), and the second light receiver (30) being arranged at the end of a second light measurement path (56) which starts at the light emitter (26), and when the sensor is fitted to the machine as specified, the first light measurement path running on a portion through a washing space (14) of the machine, and is the second light measurement path running completely outside this washing space, the analysis and control unit (36) being set up to control the luminous flux of the light emitter (26), depending on an output signal of the second light receiver (30).