An Optical Power Meter (OPM) is used with a light source to measure signal loss in a fiber optic cable or channel. For light power measurements outside the field of. What are Optical Power Meters? An optical power meter (or laser powermeter) is an instrument for the measurement of the optical power (the delivered energy per unit time) in a light beam, for example a laser beam. This technical note explains how to measure and calculate the optical power of your light source. The source of light can be an LED (Light.