The main optical thin film devices include reflection film, antireflection film, polarization film, interference filter and spectroscope, etc. They are widely used in national economy and national defense construction, and have been paid more and more attention by scientific and technical workers.

For example, the loss of light flux of complex optical lens can be reduced by ten times by using antireflection film. The output power of laser can be doubled by using a mirror with high reflective film ratio. The efficiency and stability of silicon cells can be improved by using optical films.

The simplest optical film model is a smooth, isotropic, homogeneous dielectric film. In this case, the optical properties of optical films can be studied by the interference theory of light. When a plane wave of monochromatic light incident on an optical film, multiple reflection and refraction occur on its two surfaces. The direction of reflected light and refraction light is given by the law of reflection and the law of refraction, and the amplitude of refraction light of reflected light is determined by Fresnel formula.

Optical film according to its use classification, characteristics and applications can be divided into: reflection film, anti-reflection film/anti-reflection film, filter, polarizing film/polarizing film, compensation film/phase difference plate, matching film, diffusion film/film, brightening film/prism film/concentrating film, shading film/black and white glue, etc.. Related derivative types have optical protective film, window film and so on.

The characteristics of optical films are: smooth surface, the interface between the film layer is geometrically segmented; The refractive index of the film can change at the interface, but it is continuous in the film. Can be transparent medium, can also be absorption medium; It could be normal homogeneous or it could be normal inhomogeneous.

Practical films are much more complex than ideal films. This is because: when preparing, the optical and physical properties of the film deviate from the bulk material, and the surface and interface are rough, resulting in the diffuse reflection of the beam; The interpenetration between the film layers forms a diffusion interface; Due to the growth, structure and stress of the film, various anisotropy of the film is formed. The film layer has complex time effect.

Reflective films can generally be divided into two categories, one is a metal reflective film, one is an all-dielectric reflective film. In addition, there is a metal dielectric reflection film combining the two, the function is to increase the reflectivity of the optical surface.

Metals generally have large extinction coefficients. When the light beam is incident to the metal surface by air, the amplitude of the light entering the metal decreases rapidly, so that the light energy entering the metal decreases correspondingly, while the reflected light energy increases. The larger the extinction coefficient, the faster the attenuation of light amplitude, the less light energy into the metal interior, the higher the reflectivity.

People always choose the metal with higher extinction coefficient and more stable optical properties as metal film materials. In the ultraviolet region commonly used metal thin material is aluminum, in the visible region commonly used aluminum and silver, in the infrared region commonly used gold, silver and copper, in addition, chromium and platinum are often used for some special films.

Because aluminum, silver, copper and other materials in the air is easy to oxidize and reduce the performance, so it must be protected by dielectric film. Commonly used protective film materials are silicon monoxide, magnesium fluoride, silicon dioxide, aluminum oxide and so on.

The advantages of metal reflective film are simple preparation process and wide wavelength range; Disadvantages are light loss, reflectivity can not be very high. In order to further improve the reflectivity of the metal reflective film, several layers of dielectric layer of a certain thickness can be added on the outside of the film to form the metal dielectric reflective film.

It should be pointed out that the metal dielectric film increases the reflectivity of a certain wavelength (or wave region), but destroys the neutral reflection characteristics of the metal film.