The relationship is: 1mw=0dbm, that is to say, 2mw=3dbm, 10*lgmw is the dbm value. In addition to measuring optical power, optical power meters can also be used with light sources to measure optical. Optical loss is measured in “dB” which is a relative measurement, while absolute optical power is measured in “dBm,” which is dB relative to 1mw optical power Loss is a negative number (like –3. 2 dB) while power measurements can be either positive (greater than the reference) or negative (less than. Since the logarithm for optical power ratio is base 10 and then multiplied by 10, each change of 10 in the ratio of the measured and reference power becomes a change of 10dB. +10 dB is a factor of 10 (10 times log10 10 which is 1), +20dB is a factor of 100 (10 times log10 100 which is 2). Engineers use the decibel-milliwatt (dBm) to quantify the absolute power level of the optical signal on a logarithmic scale, referencing it to one milliwatt (mW). Understanding the difference between them is crucial. It's very useful in many jobs, especially in communications, fiber optics, andelectronics. Using an optical power meter and light source or OLTS (Optical Loss Test Set), Tier 1 Certification can be performed against industry standard limits for cable and connectors.