The optical windows are used to separate the environment on both sides, such as separating the inside and outside of the instrument, so that the inside and outside of the instrument are isolated from each other, thereby protecting the internal devices. The window is the basic optical element, an optical flat plate that does not change the optical magnification and only affects the optical path in the optical path. The important parameters of the window are: light transmittance, surface accuracy, thickness, parallelism, substrate material and other attributes. The window with suitable parameters can be selected according to the specific application.