Calibration

A system, including an optical source configured to emit optical signals, and an optical receiver configured to receive optical signals; a housing having one or more optical windows; a user-detachable reflector attached to the housing, configured to enable emitted optical signals exiting an optical window to be reflected, so that the optical signals re-enter the housing via an optical window for reception by the optical receiver; and a calibrator configured to calibrate an output of the optical receiver using optical signals received by the optical receiver.