The application principle of optical coating technology is explained in detail

- Aug 19, 2019-

The application principle of optical coating technology is explained in detail


Optical film is ubiquitous in our life, from precision and optical equipment, display equipment to the application of optical film in daily life; For example, glasses, digital cameras, various household appliances, or anti-counterfeiting technology on banknotes can all be called extensions of optical thin film technology. Without optical thin film technology as the basis for development, modern photoelectric, communication or laser technology will not be able to make progress, which also shows the importance of optical thin film technology research and development. Today we bring you the application principle of optical coating.


One.Definition of optical thin film

Optical thin film is defined as: involved in the process of the propagation path of light, attached to the optical device thickness thin and uniform dielectric film on the surface of the layer, through the layered medium membrane layer reflection, through shot (passbook) and polarization characteristics, in order to achieve what we want in one or multiple wavelengths of light within the scope of all through all light reflection or polarization separation and so on various special form of light.


Optical film refers to the manufacture or coating of one or more dielectric films or metal films or a combination of the two kinds of films on optical elements or independent substrates to change the transmission characteristics of light waves, including the transmission, reflection, absorption, scattering, polarization and phase changes of light. Therefore, through proper design, the transmittance and reflectivity of the surface of the element with different wavebands can be adjusted, and the light with different polarization planes can have different characteristics.


In general, the optical film production methods are mainly divided into dry and wet production process. The so-called dry type means that no liquid appears in the whole processing process. For example, vacuum evaporation is to heat the solid raw material with electric energy in a vacuum environment. After sublimation into gas, it adheres to the surface of a solid substrate to complete coating processing. See the gold that adornment USES in daily life, silver or the packing film that has metallic qualitative feeling, it is the product that makes with dry type coating means. But in practical production considerations, dry coating application is less than wet coating. Wet coating the general practice is to have a variety of functional components mixed into liquid coating, coating in different processing methods on the substrate, and then make liquid coating dry curing products.


TWO.Membrane interference principle


1. The fluctuation of light

In the 1860s, the American physicist maxwell developed the theory of electromagnetism. He pointed out that light was a kind of electromagnetic wave, which made the theory of wave motion quite perfect.


From the wave-particle duality of light, light and radio waves, x-rays, are electromagnetic waves, but their frequency is different. The relationship between the wavelength of electromagnetic wave lambda, frequency u, and the propagation rate V is:



Since electromagnetic waves of different frequencies travel at the same speed in a vacuum, they have different wavelengths with different frequencies. High frequency wavelengths are short, low frequency wavelengths are long. For comparison purposes, radio waves, infrared, visible, ultraviolet, X-ray and gamma rays can be arranged into a spectrum, which is called electromagnetic spectrum.


In the electromagnetic spectrum, the longest wavelength is radio waves, which are divided into long wave, medium wave, short wave, ultrashort wave and microwave, etc. Next are infrared, visible, and ultraviolet, which are collectively known as light radiation. Of all electromagnetic waves, only visible light can be seen by the human eye. Visible light, with wavelengths ranging from 0.76 to 0.40 microns, makes up only a tiny fraction of the electromagnetic spectrum. Again, x-rays. The wave with the shortest wavelength is y ray.


Since light is a kind of electromagnetic wave, it should show its characteristics - interference, diffraction, polarization, etc.


2. Membrane interference


The film may be a transparent solid, liquid or a thin layer of gas sandwiched between two pieces of glass. The incident light is reflected by the upper surface of the film to get the first beam of light, the refracted light is reflected by the lower surface of the film, and the refracted light by the upper surface to get the second beam of light. These two beams are on the same side of the film and separated by the same incident vibration, which are coherent light and belong to fractional amplitude interference. If the light source is an extended light source (plane light source), the interference can only be observed in the specific overlapping region of the two coherent beams, so it is localized interference. For a planar thin film with two parallel surfaces, the interference fringes are localized at infinity and observed in the focal plane of the image by means of a convergent lens. For the wedge-shaped thin film, the interference fringes are localized near the thin film.


It has been proved by experiments and theories that only when two series of light waves have certain relations can interference fringes be produced, and these relations are called coherent conditions. The coherence conditions of thin films include three points: the frequency of two light waves is the same; Beam waves vibrate in the same direction; The phase difference between the two light waves remains constant.


The optical path difference formula of thin film interference with two coherent lights is:



Where n is the refractive index of the film; T is the thickness of the film at the incident point; Alpha is the refraction Angle in the film; Lambda /2 is an additional optical path difference caused by the reflection of two coherent beams of light at two different interfaces, one light dense to light dense, the other light dense to light dense. The principle of thin film interference is widely used in the inspection of optical surface, precise measurement of tiny Angle or linearity, preparation of anti-reflection film and interference filter, etc.


Light is light motion state of the atoms or molecules change radiation, each light from the atoms or molecules each time, only one short columns, duration of about 1 billion seconds for the light source of two independent, interfere the three conditions, in particular, the same phase or phase constant this condition, is not easy to meet, so two independent general light source cannot constitute a coherent light source. Moreover, even light from different parts of the same source, since they are from different atoms or molecules, does not usually interfere.


Three.Optical film characteristic classification


The main optical thin film devices include reflection film, anti-reflection film, polarization film, interference filter and spectroscope, etc. They have been 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 light flux loss of complex optical lens can be reduced ten times by using anti-reflection film. The output power of laser can be multiplied by the high reflector film ratio. The efficiency and stability of silicon cell can be improved by using optical film.


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


Optical films can be classified into reflective film, anti-reflection film/anti-reflection film, filter, polarizer/polarizer film, compensation film/phase difference plate, alignment film, diffusion film/film, brightening film/prism film/condenser film, shading film/black and white adhesive, etc. The related derivatives include optical grade protective film, window film, etc.

Optical films are characterized by smooth surface and geometric division of the interface between the film layers. The refractive index of the film can jump at the interface, but it is continuous in the film. It can be transparent medium or absorbing medium; It could be uniform in the normal direction, or it could be nonuniform in the normal direction. The practical application of the film is much more complex than the ideal film. This is because: during the preparation, the optical and physical properties of the film deviate from the bulk material, and the surface and interface are rough, which leads to the diffuse reflection of the beam; Diffusion interface is formed by mutual permeation between film layers. Due to the growth, structure and stress of the film, the anisotropy of the film is formed. The membrane has complex time effects.


Reflective film can be generally divided into two categories, one is metal reflective film, one is all dielectric reflective film. In addition, there are metal dielectric reflective films that combine the two to increase the reflectivity of optical surfaces.


Generally, metals have higher extinction coefficient. When the light beam is incident to the metal surface from the air, the light amplitude entering the metal rapidly decays, so that the light energy entering the metal interior decreases correspondingly, while the reflected light energy increases. The larger the extinction coefficient is, the faster the light amplitude decayed, and the less light energy entering into the metal, the higher the reflectivity. People always choose the metal with high extinction coefficient and stable optical property as the metal film material. 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 also often make some special film membrane material. As aluminum, silver, copper and other materials in the air easily oxidize and reduce the performance, so the dielectric film must be protected. Commonly used protective film materials are silicon oxide, magnesium fluoride, silicon dioxide, aluminum oxide, etc.


The advantages of metal reflective film are simple preparation process and wide wavelength range. The drawback is that the light loss is large, the reflectivity can not be very high. In order to further improve the reflectivity of the metal reflection film, we can add several layers of dielectric layer of a certain thickness on the outside of the film to form the metal dielectric reflection film. It needs to be pointed out that the emission film of metal dielectric increases the reflectivity of a certain wavelength (or a certain wave region), but destroys the characteristic of neutral reflection of metal film.


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