The amplitude modulated wave with any modulation factor is discussed in this paper. Its expression, spectrum and formula of the modulation factor, modulation efficiency, SNR gain are given.

The paper analyses the energy and signal noise ratio (SNR) with grating to measure passive laser wavelength in real time, provides the theoretical basis for measurement and error from the method.

The major factors which influence SNR of Direct Imaging Spectrometer ( DIS) are studied and the formulas of main noises from noise sources are given, finally the SNR formulas of DIS are deduced.