Noise Equivalent Power (NEP) is a basic indicator of detector performance or sensitivity. When a photo-detector does not get any input light, it still produces some noise output with a certain average power, which is proportional to the square of the r.m.s. voltage or current amplitude. Thus the Noise-equivalent Power (NEP) of the device is the light equivalent(light level) which produces an additional output power identical to that noise power(noise of the detector even when it does not get any light) for a given bandwidth that produces a signal-to-noise ratio of unity at the output of a given optical detector.