Light source power tester

This invention relates to electromagnetic radiation output level testing, and more particularly to a device, removably attachable to the end of an optical fiber, allowing visual determination of whether light emitted by the fiber exceeds a threshold value. A device for testing the light power output of an optical system comprises a thermochromic element, a body element optionally providing collimation or other means of reproducible positioning, and optionally filters and attenuation. A preferred device fits onto the end of an optical power delivery system, and the thermochromic element changes color if the system output is above a defined threshold. An alternative device provides a flat target with a The device may be adapted to be sterilizable, and may be disposable. Optional means for ensuring that the light delivery system is operational during the measurement are provided.