(1) A decibel (dB) is a unit of measure of Light transmission. It is a logarithmic unit used to compare two power levels. (2) A unit to express differences of power level. Used to express power Gain in amplifiers or power Loss in passive circuits or cables. Also, a logarithmic comparison of two power levels, defined as ten times the base ten logarithm of the ratio of the two power levels.

A unit of measurement of Optical Power which indicates relative power on a logarithmic scale, sometimes called dBr. dB=10 log (power ratio)