Most power meters are suitable only for light beams with a quite limited beam radius, not for diffuse light, but there are e. special sensor heads with an integrating sphere, which can accept and precisely measure even highly divergent input beams, for example from. An optical power meter (OPM) is a device used to measure the power in an optical signal. The term "optical power meter" may sound generic, but in popular usage, it specifically implies a fiber optic power meter.