An optical sensor arrangement for time-of-flight comprises a first and a second cavity separated by an optical barrier and covered by a cover arrangement. An optical emitter is arranged in the first cavity, a measurement and a reference photodetector are arranged in the second cavity. The cover arrangement comprises a plate and layers of material arranged on an inner main surface thereof. The layers comprise an opaque coating with a first and second aperture above the first cavity, and with a third and fourth aperture above the second cavity. The measurement photodetector is configured to detect light entering the second cavity through the fourth aperture. The second and the third aperture establish a reference path for light from the emitter to the reference photodetector.