The Noise Equivalent Power (NEP) of a photodiode is a crucial parameter that characterizes the sensitivity of the photodiode to weak optical signals. NEP represents the minimum detectable optical power that can be distinguished from the inherent noise of the photodiode and its associated electronics. In other words, NEP quantifies the ability of a photodiode to detect very weak optical signals while accounting for the noise present in the system.

NEP is typically measured in units of watts per square root of hertz (W/√Hz) and is often denoted as NEP√Hz. It is related to both the photodiode's responsivity (detector sensitivity) and the noise characteristics of the photodiode and readout electronics. 

The formula for calculating NEP is given by:

Where K_B is Boltzmann's constant (approximately 1.38 x 10^-23 J/K), T is the temperature in Kelvin, R is the responsivity of the photodiode in A/W (amperes per watt), and R_L is the photodiode load resistance.