Results from a power meter are displayed in either decibels (dB), which indicate relative loss compared to a reference level, or decibel-milliwatts (dBm), which show the absolute power level relative to one milliwatt. Fiber Optic Measurement Units: "dB" and "dBm" Whenever tests are performed on fiber optic networks, the results are displayed on a power meter, OLTS or OTDR readout in units of “dB. 0 dBm is defined as 1 mW (milliWatt) of power into a power meter. It compares a power level to 1 milliwatt (mW). Think of it like a way to measure the brightness of a flashlight beam — but for invisible laser light.