A wearable heads-up display includes a power source, laser sources, and a lightguide. A photodetector is positioned to detect an intensity of a test light emitted at a perimeter of the lightguide from an optical path within the lightguide. A laser safety circuit provides a control to reduce or shut off a supply of electrical power from the power source to the laser sources in response to an output signal from the photodetector indicating that the detected intensity is below a threshold.