In telecommunications, extinction ratio (r _e ) is the ratio of two optical power levels, of a digital signal generated by an optical source, e.g., a laser diode, where P ₁ is the optical power level generated when the light source is "on," and P ₂ is the power level generated when the light source is "off."

Note: The extinction ratio may be expressed as a fraction or in dB.

Source: from Federal Standard 1037C and from MIL-STD-2196

An example of an extinction ratio measurement taken on a eye diagram can be seen below: