The spectroscopic film is a film that divides the beam into two parts according to certain requirements and certain ways. The spectroscopic film mainly includes wavelength spectroscopic film, light intensity spectroscopic film and polarization spectroscopic film. 

Wavelength spectroscopic film is also called two-color spectroscopic film, as the name suggests, it is a film that divides the beam into two parts according to the wavelength region. This film can be a cutoff filter or a bandpass filter, the difference is that the wavelength spectrometer must not only consider the through light but also consider the reflected light, both of which require a certain shape of the spectral curve. Wavelength splitter is usually used at a certain incidence Angle, in this case, due to the influence of polarization, the spectral curve will be distorted, in order to overcome this effect, the depolarization of the film must be considered. 

Light intensity spectrometer is a film that divides the beam into two parts according to a certain light intensity ratio. This film sometimes only considers a certain wavelength, which is called monochromatic spectrometer. Sometimes you need to consider a spectral region called a broad band spectrometer; Broadband spectroscopic film for visible light, also called neutral spectroscopic film. This kind of film is also often applied under oblique incidence, due to the influence of polarization, the polarization state of the two beams of light can be very different, in some work, you can not consider this difference, but in other work (such as some interferometers), it is required that both beams of light are depolarized, which requires the design and preparation of depolarization film. 

The polarizing film is made by using the polarization effect of the film when the light is oblique incident. The polarizing membrane can be divided into two types: prism type and plate type. The prism-type polarization film utilizes the polarization effect of the interface at Brewster Angle incidence (see refraction and reflection of light at the interface). The plate type polarization film is mainly made by taking advantage of the difference of the reflection band bandwidth of the two polarization components of the dielectric reflection film at oblique incidence. The working wavelength range of prismatic polarization film is relatively wide, and the polarization degree can also be relatively high, but it is more troublesome to prepare, not easy to do large, and the anti-laser intensity is relatively low. The wavelength region of the plate polarizer is relatively narrow, but it can be made very large, and the laser resistance is also relatively high, so it is often used in strong laser systems.