The noise spectrum of a p-i-n diode under a chopped light illumination is researched both theoretically and experimentally. The observed noise spectra can be presented in terms of a formula. The light-activated current of the device can be obtained from either a given light power or a given harmonic noise spectrum. Moreover, methods to measure the quantum efficiency and to estimate the amount of stored charge of a p-i-n diode (In_0.53Ga_0.47As) during the shaded period of chopped illumination are presented.