An optical sensor to measure the turbidity of wash water in a household washing machine or dishwasher, includes a housing having a housing interior containing a measurement module having a light-emitting element and a light-receiving element, the measurement module defining a measurement light path which extends from the light-emitting element to the light-receiving element and passes outside the housing over a part of its path length. The light-emitting element and the light-receiving element are arranged together in a first subspace of the housing interior, and the measurement light path extends over a part of its path length through at least one second subspace of the housing interior, which is sealed from the first subspace. All the electrical/electronic components of the measurement module are fitted in the first subspace, so that any ingress of wash water into the second subspace does not compromise the electrical functionality of the sensor.