where P 1 and P 2 are the actual powers. Power ratios may be expressed in terms of voltage and impedance, E and Z , or current and impedance, I and Z , since
Thus dB is also given by
If Z 1 = Z 2, these become
Note 2: The dB is used rather than arithmetic ratios or percentages because when circuits are connected in tandem, expressions of power level, in dB, may be arithmetically added and subtracted. For example, in an optical link, if a known amount of optical power, in dBm, is launched into a fiber, and the losses, in dB, of each component (e.g. , connectors, splices, and lengths of fiber) are known, the overall link loss may be quickly calculated with simple addition and subtraction.