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The application principle of optical coating


From: owfeed optical network
With the development of science and technology and economic globalization, today human beings have entered the knowledge economy society and information society. Chinese mainland is also just unfolding and China's manufacturing industry is developing rapidly. China Optical manufacturing has begun to play an important role in the international economic stage, and China has ranked the first in optical glass production and optical parts production. Optical film is one or more layers of film which is coated on the surface of optical parts by changing the surface characteristics of optical parts. It can be a metal film, a dielectric film or a combination of these two types of films. Optical film is an indispensable part of various advanced photoelectric technology. It can not only improve the performance of the system, but also is a necessary means to meet the design objectives. The application field of optical film and all aspects of optical system, including laser system, optical communication, optical display, optical storage, etc., the main optical film devices include reflection film, antireflection film, polarization film, optical film, optical film, optical film, optical film, optical film, optical film, optical film, optical film, optical film, optical film, optical film, optical film, optical film, optical film Interference filters and spectroscopes, etc. They have been widely used in national economy and national defense construction, and they have been paid more and more attention by science and technology workers.
At present, more than 60 kinds of optical coating materials have been used, and their varieties and application functions are still being developed. In recent years, the metal film system has been developed. When the thickness of gold, silver, copper and aluminum is 7-20um, the transmittance of the film to visible light is 50%, while the infrared transmittance is less than 10%. The film has been successfully applied to the panel of Apollo spacecraft, which is used to pass through some visible light, and reflect almost all the infrared light for thermal control. The following paper mainly introduces the characteristics and classification of optical films.
1、 Definition of optical film
Optical thin film is composed of thin layered media, which propagates light beam through interface. The application of optical film began in 1930s. Optical thin film has been widely used in optical and optoelectronic technology fields, and various optical instruments have been manufactured. The preparation strip requires high and fine parts.
The definition of optical film is: it involves thin and uniform dielectric film attached to the surface of optical devices during the propagation path. Through the reflection, transparence and polarization characteristics of layered dielectric film, all the light in one or more band ranges can be transmitted or all the light reflection or polarization separation of light can be achieved.
Optical film is ubiquitous in our life, from precision and optical equipment, display equipment to optical film applications in daily life; For example, glasses, digital cameras, various household appliances, or anti-counterfeiting technology on banknotes can be called the extension of optical film technology application. Without optical film technology as the basis of development, modern photoelectric, communication or laser technology will not be able to make progress, which also shows the importance of optical film technology research and development.
Optical film refers to the coating or coating of one or more dielectric films or metal films on optical elements or independent substrates or the combination of these two films to change the transmission characteristics of light waves, including the transmission, reflection, absorption, scattering, polarization and phase change of light. Therefore, the transmittance and reflectivity of different band elements can be adjusted by proper design, and different polarization plane light can have different characteristics.
Generally speaking, the production methods of optical film are mainly divided into dry and wet processes. The so-called dry type is that no liquid appears in the whole process, for example, vacuum evaporation plating is in a vacuum environment, which uses electric energy to heat solid raw materials, and then attaches to the surface of a solid substrate after rising into gas, and finishes coating processing. The gold, silver or metal packaging film used in decoration is the product made by dry coating. But in the actual production consideration, the dry coating application scope is smaller than wet coating. Wet coating is generally used to mix the components with various functions into liquid coating, and then coating on the substrate in different processing ways, and then make the liquid coating dry and solidified to make the product.
2、 The principle of film interference
1. The fluctuation of light
In the 1960s, American physicist Maxwell developed the theory of electromagnetic, pointed out that light is an electromagnetic wave, which makes the theory of wave develop to a perfect level.
From the duality of the wave particle of light, light is the same as radio wave, X-ray, X-ray, and "X-ray"? Radiation is electromagnetic waves, but they have different frequencies. Wavelength of electromagnetic wave λ、 The relationship between frequency u and propagation rate V is as follows:
V= λ u
Because the electromagnetic waves of various frequencies are the same in vacuum, the wave wavelengths of different frequencies are different. The wavelength with high frequency is short and the wavelength with low frequency is long. For comparison, according to the wavelength (or frequency) of radio wave, infrared, visible light, ultraviolet, X-ray and gamma ray, they can be arranged into a spectrum in sequence, which is called electromagnetic spectrum.
In the electromagnetic spectrum, the longest wavelength is radio wave, which is divided into long wave, medium wave, short wave, ultra short wave and microwave due to the different wavelength. Next is infrared, visible and ultraviolet, which are called light radiation. Of all electromagnetic waves, only visible light can be seen by human eyes. The wavelength of visible light is between 0.76 μ m and 0.40 μ m, which only accounts for a small part of the electromagnetic spectrum. Again, X-rays. The shortest wave length is y-ray.
Since light is an electromagnetic wave, it is necessary to realize the characteristics of interference, diffraction, polarization and so on.
2. Thin film interference
The film can be a transparent solid, liquid, or a thin layer of gas sandwiched by two glass. The first beam of light is obtained after the incident light is reflected on the upper surface of the film. The refracted light is reflected on the lower surface of the film, and then the second beam is obtained after refraction on the upper surface. The two beams are separated from the same incident vibration on the same side of the film, which is coherent light, which belongs to the interference of partial amplitude. If the light source is an extended light source (surface light source), interference can only be observed in the specific overlapping area of two coherent beams, so it belongs to local interference. For the plane films with two parallel surfaces, the interference fringe is located at infinity, and is usually observed in the focal plane of the image by means of a convergent lens; For wedge-shaped films, the interference fringe is localized near the film.
Experiments and theories have proved that interference fringes can only be produced when two columns of light waves have certain relations, which are called coherent conditions. The coherent conditions of the film include three points: the frequency of two light waves is the same; The direction of the beam wave is the same; The phase difference of the two beams of light is constant.
The optical path difference formula of the two-phase dry light interference is as follows:
Δ= ntcos( α)±λ/ two
Where n is the refractive index of the film; T is the thickness of the film at the incident point; α It is the refractive angle in the film; λ/ 2. The additional optical path difference is caused by the reflection of two coherent light on two interfaces (one is from light sparse medium to light dense medium and the other is from light dense medium to light sparse medium). The principle of thin film interference is widely used in the inspection of optical surface, the precise measurement of small angle or linearity, the preparation of antireflection film and interference filter.
Light is radiated from the change of the motion state of atoms or molecules in the light source. Every time every atom or molecule emits light waves, there is only one short column, with a duration of about 1billion seconds. For two independent light sources, the three conditions for interference, especially the condition that the phase difference or the phase difference are the same, are not easy to meet, Therefore, two independent general light sources cannot form coherent light sources. Moreover, even the light emitted by different parts of the same light source will not interfere because they are emitted by different atoms or molecules.
3、 Classification of optical film characteristics
The main optical thin film devices include reflective film, antireflective 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 science and technology workers. For example, the loss of the optical flux of the complex lens can be reduced by ten times after the antireflection film is adopted; The output power of the laser can be doubled by using the mirror with high reflection film ratio; The efficiency and stability of silicon cells can be improved by using optical thin films.
The simplest optical film model is a homogeneous dielectric film with smooth surface and isotropy. In this case, the optical properties of optical thin films can be studied by the interference theory of light. When a single color light plane wave is incident on the optical film, it has multiple reflections and refractions on its two surfaces. The direction of the reflected light and refraction light is given by the law of reflection and the law of refraction. The amplitude of the reflected photorefractive light is determined by Fresnel formula.
According to its application classification, characteristics and application, optical thin films can be divided into: reflection film, antireflective film / antireflection film, filter, polarizer / polarizing film, compensating film / phase difference plate, directional film, diffusion film / sheet, brightening film / prism / condenser, light mask / black and white adhesive, etc. The related derivatives include optical protective film, window film, etc.
The optical film is characterized by smooth surface and geometric segmentation of the interface between the films; The refractive index of the film can jump at the interface, but it is continuous in the film; It can be transparent medium or absorption medium; It can be normal or normal. The practical application of the film is much more complex than the ideal film. This is because: when the optical and physical properties of the films deviate from the bulk materials, the surface and interface are rough, which leads to diffuse reflection of the beam; The diffusion interface is formed by the mutual permeation between the films; Due to the growth, structure and stress of the film, various directionalities of the film are formed; The film has complex time effect.
Generally, the reflective film can be divided into two types, one is metal reflection film and the other is full dielectric reflection film. In addition, there are metal dielectric reflection films which combine the two, which can increase the reflectivity of optical surface.
Generally, metals have a large extinction coefficient. When the beam is incident to the metal surface by air, the amplitude of light entering the metal decreases rapidly, which makes the energy of light entering the metal decrease correspondingly, while the reflected light energy increases. The larger the extinction coefficient, the faster the attenuation of the amplitude of light, the less light energy entering the metal, the higher the reflectivity. The metal with high extinction coefficient and stable optical properties is always chosen as the metal film material. The thin metal materials commonly used in the ultraviolet region are aluminum, aluminum and silver are commonly used in visible light area, gold, silver and copper are commonly used in infrared region. In addition, chromium and platinum are also used as membrane materials for some special films. Because aluminum, silver, copper and other materials are easy to oxidize in air and reduce their performance, they must be protected by dielectric film. The commonly used protective film materials include silicon oxide, magnesium fluoride, silica, aluminum oxide, etc.
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 is impossible to be high. In order to improve the reflectivity of metal reflective film, several layers of dielectric layer with certain thickness can be added to the outside of the film to form the metal dielectric reflection film. It is necessary to point out that the dielectric film increases the reflectivity of a certain wavelength (or a certain wave region), but destroys the characteristics of neutral reflection of metal film.

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