The basic process is straightforward: turn the meter on, set it to the correct wavelength, clean your connectors, plug in, and read the display. REF/dB key: Short press the dB to switch unit, click once nW/dBm/dB to enter the upper clear data, press and hold until REF is displayed on the screen, and set the current optical power as reference value, enter the relative. How to Use Optical Power Meter TR-504 | Optical Power Meter Working| Testing OPM, VFL, RJ45 | TRICOM. This document will serve as an overview of the major features and functions of the device and will offer tips for trouble shooting com on issues in optical networks. You measure optical power in dBm or insertion loss in dB. Consistent procedures ensure accuracy.