Embodiments described include a system comprising a position sensing device (PSD) and a light source. The light source is configured to, by passing one or more light beams through the PSD, cause one or more electrical currents to flow through the PSD. The system further comprises a processor, configured to (i) in response to the electrical currents, ascertain an amount of power that is delivered by the light source, and (ii) in response to the amount of power exceeding a threshold amount of power, inhibit the light source from further operation. Other embodiments are also described.