Longpass filter has the advantages of wide cut-off band, deep cut-off depth, high conversion steepness and high transmittance. It can be widely used in fluorescence excitation spectroscopy measurement, Raman spectroscopy application, spectral grouping application, and astronomical observation application, etc. It can also be widely used in imaging optical system, machine vision system application, used to eliminate the interference light beyond the available band, greatly improve the imaging clarity, as well as the sensitivity of the system.