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High Reflective Coatings
Three: Classification of optical film characteristics
The main high precision optical thin film include reflective films, anti-reflection films, polarizing films, interference filters, beam splitters, and so on. They have been widely used in the national economy and national defense construction, and have gained science Increasing attention from technical workers. For example, the use of an anti-reflection film can reduce the luminous flux loss of a complex optical lens by a factor of ten; the use of a mirror with a high reflection film ratio can double the output power of the laser; the use of optical films can improve the efficiency of silicon batteries stability.
The simplest model of an optical film is a uniform dielectric film with a smooth and isotropic surface. In this case, the optical properties of optical films can be studied using the interference theory of light. When a monochromatic light plane wave is incident on an optical film, multiple reflections and refractions occur on its two surfaces. The directions of reflected light and refracted light are given by the law of reflection and the law of refraction. The size is determined by the Fresnel formula.
Optical films can be classified according to their uses, characteristics and applications: reflective films, antireflection films / reflection films, filters, polarizers / polarizing films, compensation films / phase difference plates, alignment films, diffusion films / sheets, Brightening film / prism sheet / condensing sheet, light-shielding film / black and white glue, etc. Related derivative types are optical-grade protective films, window films, and the like.
The characteristics of optical films are: smooth surface, and the interface between the film layers is geometrically divided; the refractive index of the film layer can change at the interface, but is continuous within the film layer; it can be a transparent medium or an absorption Medium; either normal or uneven. The actual film is much more complicated than the ideal film. This is because: during preparation, the optical and physical properties of the film deviate from the bulk material, and the surface and interface are rough, resulting in diffuse reflection of the light beam; the mutual penetration between the film layers forms the diffusion interface; due to the growth of the film layer , Structure, stress and other reasons, the formation of various anisotropy of the film; the film layer has a complex time effect.
Reflective films can generally be divided into two types, one is a metal reflective film, and the other is a all-dielectric reflective film. In addition, there is a metal dielectric reflection film that combines the two, which functions to increase the reflectivity of the optical surface.
Generally, metals have large extinction coefficients. When a light beam is incident on a metal surface by air, the amplitude of the light entering the metal is rapidly attenuated, so that the light energy entering the metal is correspondingly reduced, and the reflected light energy is increased. The larger the extinction coefficient, the faster the light amplitude decays, the less the light energy entering the metal, and the higher the reflectivity. People always choose metals with larger extinction coefficients and more stable optical properties as metal film materials. The thin metal materials commonly used in the ultraviolet region are aluminum, aluminum and silver are commonly used in the visible region, and gold, silver and copper are commonly used in the infrared region. In addition, chromium and platinum are also often used as film materials for some special films. Because aluminum, silver, copper and other materials are easily oxidized in the air to reduce performance, they must be protected with a dielectric film. Common protective film materials include silicon oxide, magnesium fluoride, silicon dioxide, aluminum oxide, and the like.
The advantage of metal reflective film is that the preparation process is simple and the working wavelength range is wide; the disadvantage is that the light loss is large and the reflectivity cannot be very high. In order to further improve the reflectivity of the metal reflective film, several dielectric layers with a certain thickness can be plated on the outside of the film to form a metal dielectric reflective film. It should be pointed out that the metal dielectric film increases the reflectivity of a certain wavelength (or a certain wave region), but destroys the characteristics of neutral reflection of the metal film.
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