Transmission gratings have a simple optical path design. With applications such as spectrometer, a transmission grating has an obvious advantage. The incident beam can disperse at a fixed Angle on the back of the grating. Transmission gratings are insensitive to alignment error of some gratings. In order to obtain high diffraction efficiency, the groove density of transmission grating is not high, diffraction angle increases with the increase of the groove density. The refraction of the substrate limits the transmittance of long wavelength, which result in a degraded performance. The dispersion characteristics of gratings make it suitable for small detector array compact systems. This grating is also insensitive to polarization.