Instruments measuring in dB can be optical power meters or optical loss test sets (OLTS), with optical power meters usually reading in dBm for power measurements or dB concerning a user-set reference value for loss. Loss (dB) = -10 log (Po/Pi) or 10 log (Pi/Po)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. It doesn't measure an absolute quantity; rather, it shows how one value compares to another. Thus, a source with a power level of 0 dBm corresponds to 1mW. In optical fiber networks, the units of optical power are often expressed in milliwatts (mw) and decibel milliwatts (dbm).