the function of the mirror: the shape of the mirror is a paraboloid of revolution, and the light bulb is located at its focus. The function of the reflector is to aggregate the light emitted by the bulb into parallel beams and increase the luminosity by hundreds of times. However, the beam emitted by the reflector is too narrow, the illumination range is not large, and the roadside illumination is not good, so it is necessary to increase the degree of astigmatism with a lens. Reflectors are used for beam turning, interferometry, imaging or illumination. Using glass or metal with polished surface as matrix, the light beam irradiates the surface to form specular reflection. In order to get as much reflected light as possible, a reflective film, including metal film and dielectric film, is often plated on the substrate. The commonly used materials of metal film are: aluminum, silver and gold. Aluminum has high reflectivity from ultraviolet region to infrared region, and a layer of alumina is formed on the surface of aluminum, so the film layer is firm and stable. The silver film has high reflectivity in the visible region and infrared region, and the polarization effect introduced when the incident angle changes is also the smallest. Gold film has high reflectivity in infrared region, and its strength and stability are better than silver film, so it is often used for infrared reflection. The metal film is soft and easy to be scratched and damaged, so a protective film is often plated outside the metal film. Metal film mirror is insensitive to incident angle, and can be used in a relatively wide wavelength range as long as it meets geometric reflection conditions. Generally speaking, metal film mirrors can meet the requirements of optical instruments. If higher reflectivity is required, multilayer dielectric reflective films can be used, which can achieve maximum reflectivity and minimum absorption loss. The incident angle and wavelength will affect the reflection efficiency of dielectric film, and generally the incident angle: or 45 and a specific working wavelength range will be marked.