What is an optical reflective film?
The basic principle of reflective film is to use the reflection and refraction of light between the medium,and reflect the light back to the original direction through a special reflective film material,so as to achieve accurate control and adjustment of the light.Among them,the thickness of the film layer and the choice of materials also determine the reflection and transmission effect of the reflective film on different wavelengths of light.
Reflective film can generally be divided into two categories,one is metal reflective film,one is all dielectric reflective film.In addition,there is a metal dielectric reflection film that combines the two.
General metals have a large extinction coefficient,when the beam is incident by air to the metal surface,the light amplitude into the metal rapidly decays,making the light energy into the metal correspondingly reduced,and the reflected light energy increased.The greater the extinction coefficient,the faster the attenuation of light amplitude,the less light energy entering the interior of the metal,and the higher the reflectivity.
People generally choose those metals with large optical coefficients and stable optical properties as metal film materials.Thin metal materials commonly used in the ultraviolet region are aluminum,aluminum and silver are commonly used in the visible region,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 and reduce their performance,they must be protected by electric dielectric films.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;The disadvantage is that the light loss is large,and the reflectivity cannot be high.In order to further improve the reflectivity of the metal reflective film,several layers of dielectric layer with a certain thickness can be plated on the outside of the film to form the metal dielectric reflective film.It should be pointed out that the metal dielectric radiation film increases the reflectivity of a certain wavelength (or a certain wave region),but destroys the characteristics of neutral reflection of the metal film.
All-dielectric reflective films are based on multi-beam interference.In contrast to the anti-reflection film,the reflectivity of the optical surface can be increased by plating a film with a higher refractive index than the base material.The simplest multilayer reflection is made by alternating evaporation of two materials of high and low refractive index,and the optical thickness of each film is one-fourth of a certain wavelength.Under this condition,the reflected light vector on the surfaces participating in the superposition vibrates in the same direction.The resultant amplitude increases with the increase of the number of film layers.
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