A sensor includes an enclosure having a housing and a lower cassette that cooperate to provide a sealed volume in which an optical sensor assembly is enclosed and protected. The optical sensor assembly includes a circuit board with a light source and a light detector. The sensor assembly further includes a light pipe that guides light from the sensor onto a target and a lens guides reflected light from the target onto the light detector. Lower ends of the light pipe and the lens are supported by a recess in the lower cassette. Upper ends of the light pipe and the lens are supported by an upper cassette. The upper cassette is positively located and mounted to the circuit board and received in an internal receptacle in the lower cassette. Mounting the lower cassette to the housing encloses the optical sensor assembly in the proper alignment.