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: