Optical film
A class of optical media consisting of thin layered media that propagate light beams through interfaces. The application of optical thin films began in the 1930s. Nowadays, optical thin films have been widely used in the field of Optics and optoelectronic technology to manufacture various optical instruments.
Chinese name
Optical film
Composition
Thin layered media
nature
Optical dielectric materials
Scope of application
Optical and Optoelectronic Technology
catalogue
1 Structure
2 characteristics
3 Classification
4 Application
5 common types
▪ Antireflection coating
▪ Optical film
▪ Reflective film
▪ Interference filter
▪ Spectroscopic film
The main optical thin film devices include reflective film, antireflective film, polarizing film, interference filter and spectrometer. They have been widely used in national economy and national defense construction, and have been paid more and more attention by scientists and technicians. For example, the use of antireflection film can reduce the luminous flux loss of complex optical lens by ten times; The output power of the laser can be doubled by using a high reflectance mirror; The efficiency and stability of silicon photocell can be improved by using optical thin film.
The simplest optical thin film model is a homogeneous thin layer with smooth and isotropic surface. In this case, we can use the interference theory to study the optical properties of optical films. When a monochromatic 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 refraction. The amplitude of reflected light and refracted light is determined by Fresnel formula (see refraction and reflection of light on the interface).
characteristic
The characteristics of optical thin films are as follows: the surface is smooth and the interface between the films is geometrically divided; The refractive index of the film can jump on the interface, but it is continuous in the film; It can be a transparent medium
Classification
According to the application, optical films are divided into reflective film, antireflective film, filter film
Optical protective film, polarizing film, spectroscopic film and phase film. The first four are commonly used. Optical reflective film is used to increase the mirror reflectivity, and is often used to make reflective, refractive and resonant cavity devices. Optical antireflection film is deposited on the surface of optical elements to reduce the surface reflection and increase the transmission of optical system, also known as antireflection film. Optical filter film is used for spectral or other optical segmentation, which has many kinds and complex structure. The optical protective film is deposited on the surface of metal or other soft erosive materials or films to increase their strength or stability and improve their optical properties. The most common is the protective film of metal mirror.
primary coverage
A kind of important optical elements. This field mainly includes the following aspects:
① The optical properties, mechanical properties and other related properties of the films were studied;
② The growth of thin films, the structure of thin films and their effects on the properties of thin films;
③ Design, fabrication and performance test of optical thin film elements.
application
Applications of optical film are everywhere, from glasses coating to mobile phone, computer, TV LCD display to LED lighting and so on. It is full of all aspects of our life and makes our life more colorful.
Optical thin film is defined as: it refers to the reflection, transmission (refraction) and polarization characteristics of the thin and uniform dielectric film attached to the surface of the optical device in the process of light propagation path, In order to achieve the special forms of light that we want, such as the total transmission of light or the total reflection of light or the polarization separation of light in one or more wavebands.
Optical films can be divided into "geometrical optics" and "physical optics". Geometrical optics is a geometric dielectric film formed on the surface of optical devices to adjust or redistribute the light beam by changing the light path; Physical optics is a kind of dielectric film formed by nano processing the unique optical material elements in nature to the required surface of optical devices. The characteristics of optical material elements through the dielectric film are enhanced to change the light polarization, transmission, reflection and other functions.
Generally, the preparation conditions of optical films are high and precise. The preparation of optical films can be divided into dry preparation method and wet preparation method. The dry preparation method (including vacuum coating: evaporation, magnetron sputtering, ion plating, etc.) is generally used for the preparation of physical optical films, and the wet preparation method (including coating method, tape casting method, thermoplastic method, etc.) is generally used for the preparation of geometric optical films.
Up to now (2013), the commonly used optical films are: high reflection film; Antireflection coating; Filter film; Color filter membrane; Antireflective film; Concentrating film; Diffusion film; Polarizing film and so on.
Common types
The liquid crystal display optical film of Veitch tech is an optical film formed by the principle of multiple refraction and focusing of light generated by microstructure. Its unique technology and process can reduce light absorption and ensure light penetration and higher brightness. In addition to improving the brightness benefits, it can also play the role of light diffusion and atomization through the refraction and scattering of light.
Brightness enhancement film
Brightening film (BEF) is an optical film on the transparent PET surface, which uses acrylic resin to precisely form a layer of uniformly distributed prism structure and light diffusion layer on the back. It is used to brighten the upper layer of liquid crystal display, so that the light can be recovered and concentrated through the brightening microstructure, resulting in brightening effect. It has high brightness design and diffusion function, Due to the basic principle of the diffusion layer, the wet out phenomenon is eliminated, and the light display is more uniform and softer.
Diffusion film
The diffusion sheet (DL series) is a very transparent PET surface, which uses acrylic resin to precisely coat a layer of randomly dispersed micron structure diffusion particles, and then precisely coat a layer of randomly dispersed micron structure antistatic particles on the opposite side of pet. It is used in the liquid crystal display to make light refract and diffract through the diffusion layer for many times, so as to play the role of light equalization, Make the light display more even and soft.
Reflective film
The reflector is made by tape casting method. HR polymer optical agent and plasticizer are doped in PET resin to achieve light shielding and high reflection effect. The reflection effect is reduced due to the absorption of light in the middle layer of the diaphragm. Therefore, a layer of HR dielectric film is added on the surface to achieve better reflection effect and anti ultraviolet yellowing function.
The simple model of optical thin film can be used to study its reflection, transmission, phase change and polarization. If we want to study the special properties of optical thin films, such as loss, damage and stability, a simple model can not do anything. At this time, we must consider the crystal structure, internal structure and surface state of the film, the anisotropy and inhomogeneity of the film, the chemical composition, surface contamination and interface diffusion of the film, etc. After considering these factors, we should not only consider its optical properties, but also study its physical properties
Chemical properties, mechanical properties and surface properties, as well as the penetration and influence between various properties. Therefore, the research of optical thin films has leaped out of the scope of Optics and become an interdisciplinary subject of physics, chemistry, solid and surface physics.
Although the optical phenomenon of thin films was noticed as early as the 17th century, the special research on optical thin films as a subject began after the 1930s. This is mainly because the development of vacuum technology provides a prerequisite for the preparation of various optical thin films. Up to now, optical thin films have been greatly developed, and the production of optical thin films has gradually moved towards serialization, programming and specialization. However, there are still many problems to be solved in the research of optical thin films, and the existing level of optical thin films can not meet the requirements in many work, and needs to be improved. In theory, not only the growth mechanism of the thin film needs to be clarified, but also the optical theory of the thin film, especially the optical theory applied to the very short wave band, needs to be further improved. In the process, there is still a lack of effective means to achieve accurate control of film deposition parameters
In this way, the growth of thin films has a certain degree of randomness, and the optical constants, thickness and properties of thin films also have a certain degree of instability and blindness, all of which limit the improvement of the quality of optical thin films. As for the optical film itself, in addition to the improvement of optical properties and the reduction of light losses such as absorption and scattering, its mechanical strength, chemical stability and physical properties need to be further improved. In laser system, the anti laser intensity of optical film is low, which is one of the most important problems in the research of optical film. The following describes several commonly used optical thin film elements.
Antireflection coating
Its main function is to reduce or eliminate the reflected light of lens, prism, plane mirror and other optical surfaces, so as to increase the light transmittance of these elements and reduce or eliminate the stray light of the system.
The simplest antireflective film is a single-layer film, which is plated on the optical surface of optical parts with low refractive index. When the refractive index of the film is lower than that of the substrate, the reflection coefficients R1 and R2 of the two interfaces have the same phase change. If the optical thickness of the film is one fourth of a certain wavelength, the optical path difference between two adjacent beams is just π, that is, the vibration direction is opposite. The superposition result makes the reflected light of the optical surface decrease. When the refractive index of the film is properly selected so that R1 and R2 are equal, the reflected light of the optical surface can be completely eliminated.
In general, it is difficult to achieve ideal antireflection effect by using a single antireflection film. In order to achieve zero reflection at a single wavelength or achieve good antireflection effect in a wide spectral region, antireflection films with double, three or even more layers are often used. A, B and C in Fig. 1 respectively plot the residual reflection curves of single, double and three antireflection coatings on the surface of kg glass.
Optical film
Antireflection film is one of the most widely used optical thin films with the largest output. Therefore, it is still an important research topic in optical thin film technology. The research focuses on finding new materials, designing new film systems, and improving the deposition process, so as to use the least number of layers, the simplest and most stable process to obtain the highest possible yield and achieve the most ideal effect. For laser thin film, antireflection film is the weak link of laser damage. How to improve its damage intensity is also one of the most concerned problems.
Reflective film
Its function is to increase the reflectivity of the optical surface. Reflective film can be generally divided into two categories, one is metal reflective film, the other is all dielectric reflective film. In addition, there is a metal dielectric reflective film which combines the two.
Generally, metals have large extinction coefficient. When the light beam is incident on the metal surface by air, the amplitude of the light entering the metal decreases rapidly, so that the light energy entering the metal decreases and the reflected light energy increases. The larger the extinction coefficient is, the faster the attenuation of light amplitude is, the less light energy enters into the metal, and the higher the reflectivity is. People always choose those metals with higher extinction coefficient and more stable optical properties as metal film materials. The commonly used metal film materials are aluminum in the ultraviolet region, aluminum and silver in the visible region, gold, silver and copper in the infrared region. In addition, chromium and platinum are also commonly used as film materials for some special films. Because aluminum, silver, copper and other materials are easy to oxidize in air and degrade their properties, they must be protected by dielectric membrane. 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 disadvantage is that the light loss is large and the reflectivity can not 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 the metal dielectric reflective film. It should be pointed out that the metal dielectric reflection film increases the reflectivity of a certain wavelength (or a certain wave region), but destroys the neutral reflection characteristics of the metal film.
All dielectric reflective film is based on multi beam interference. In contrast to antireflection coating, the reflectivity of optical surface can be increased by coating a film with refractive index higher than that of the substrate. The simplest multilayer reflective film is formed by alternating evaporation of high and low refractive index materials. The optical thickness of each layer is one fourth of a certain wavelength. Under this condition, the vibration direction of the reflected light vector on each interface is the same. The synthesis amplitude increases with the increase of film layers. Fig. 2 shows that the reflectivity of the reflective film varies with the number of layers.
In principle, the reflectivity of all dielectric reflective film can be infinitely close to 1, but the scattering and absorption loss limit the improvement of the reflectivity. Up to now, the reflectivity of high quality laser reflective film has exceeded 99.9%, but there are still some works to improve it. The reflective film used in high power laser system emphasizes its anti laser intensity more. The work on improving the anti laser intensity of this kind of film makes the research of this kind of film more in-depth.
Interference filter
It is a kind of optical thin film with the most kinds and complex structure. Its main function is to segment spectral bands. The most common interference filters are cut-off filter and band-pass filter. The cut-off filter can divide the considered spectral region into two parts. One part does not allow light to pass through (called cut-off region), the other part requires light to pass through fully (called band-pass region). According to the position of the passband in the spectral region, it can be divided into two kinds: long wave pass and short wave pass. The simplest structures of these two kinds of passbands are as follows: here h and l represent thick high and low refractive index layers respectively, and M is the number of periods. The membrane system with the above structure is called symmetric periodic membrane system. The structure of the film system will be more complicated if the spectral region considered is very wide or the ripple requirement of the passband transmittance is very high.
According to their different structures, they can be divided into Fabry Perot filters, multicavity filters and induced transmittance filters.
The structure of the Fabry Perot filter is the same as that of the Fabry Perot etalon (see Fabry Perot interferometer). Because the transmission spectrum obtained by the Fabry Perot filter is narrow, it is also called narrow band interference filter. The transmittance of this kind of filter is very sensitive to the loss of the film, so it is very difficult to prepare the filter with high transmittance and narrow half width. Multicavity filter is also called rectangular filter. It can be used as narrow band-pass filter and broadband band-pass filter. It is also difficult to prepare multicavity filter with wide wave area, high transmittance and small ripple.
Induced transmittance filter is a kind of filter which is matched with appropriate dielectric film on both sides of metal film to increase potential transmittance, reduce reflection and increase passband transmittance. Although its passband performance is not as good as that of all dielectric Fabry Perot filter, it has a wide cut-off characteristic, so it has great application value. Especially in the ultraviolet region, the general dielectric material absorption is relatively large, its superiority is more obvious. The typical curves of Fabry Perot filter, multicavity filter and induced transmittance filter are shown in Fig. 3.
Spectroscopic film
According to a certain requirement and a certain way to divide the beam into two parts of the film. Spectroscopic films mainly include wavelength spectroscopic films, intensity spectroscopic films and polarization spectroscopic films.
Wavelength splitter film is also called two-color splitter film. As the name suggests, it is a film that divides the beam into two parts according to the wavelength region. This kind of film can be a cut-off filter or a band-pass filter. The difference is that the wavelength splitter film should consider not only the transmitted light but also the reflected light. Both of them require a certain shape of spectral curve. The wavelength splitter film is usually used at a certain incident angle. In this case, the spectral curve will be distorted due to the influence of polarization. In order to overcome this influence, the depolarization of the film must be considered.
The light intensity splitter is a kind of thin film which divides the light beam into two parts according to a certain light intensity ratio. Sometimes this kind of thin film only considers a certain wavelength, which is called monochromatic splitter; Sometimes it is necessary to consider a spectral region called broadband spectroscopic film; The wide-band beam splitter used for visible light is also called neutral beam splitter. This kind of film is also often used in oblique incidence. Due to the influence of polarization, the polarization state of two beams can be quite different. In some work, this difference can not be considered, but in other work (such as some interferometers), both beams are required to be depolarized, which requires the design and preparation of depolarization film.
The polarizing spectroscopic film is made of the polarizing effect of the film when the light is obliquely incident. There are two kinds of polarizing spectroscopic films: prism type and flat type. The prism polarizing film utilizes the polarization effect of the interface when the Brewster angle is incident (see the refraction and reflection of light on the interface). When the beam is always incident on the interface of two materials at Brewster angle, no matter how many layers there are, the reflected light of horizontal vibration is always zero, while the reflected light of vertical vibration increases with the increase of the number of layers. As long as the number of layers is enough, the transmitted light is basically parallel vibration, while the reflected light is basically vertical vibration, Since the Brewster angle on the interface between the two films can not be reached by air, the film must be plated on the prism, and the incident medium is not air but glass. The flat polarizing film is mainly made of the difference of the reflection band bandwidth of the two polarization components of the dielectric reflection film at oblique incidence. With the increase of the incident angle, the reflection bandwidth of the vertical component increases, while that of the parallel component decreases. When the high reflection region of vertical component and the high transmission region of parallel component are selected as the working region, the polarizing film reflecting vertical component through parallel component can be formed. The incident angle of the polarizing film is generally selected near the Brewster angle of the substrate. The working wavelength range of prism polarizing film is wide, and the degree of polarization can be high, but its preparation is troublesome, not easy to be large, and the laser resistance is low. Flat plate polarizer works in a narrow wavelength region, but it can be made very large and has high laser resistance, so it is often used in high-power laser systems.
Fig. 4 and Fig. 5 show the reflection spectrum curves of neutral light intensity splitter and flat polarization splitter respectively.
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Diffusion film The diffusion film is translucent and has the function of light diffusion. It is a kind of film material that can promote the uniformity of light intensity. It is mainly used in LCD backlight module, plane lighting and other fields. Product features When light passes through the diffusion layer with pet as the substrate, it will pass through the medium with different refractive index, causing a lot of refraction, reflection and scattering of light. The light can be modified into a uniform surface light source to achieve the effect of optical diffusion. In the backlight structure, it can correct the diffusion angle and cover the LGP defects, which will increase the radiation area. After the light source passes through the diffusion film material, it can become a secondary light source with larger area, better uniformity and more stable chromaticity. Product advantages It has excellent covering effect, can be used to replace the diffusion plate in the direct backlight, and has the advantages of excellent optical performance and low cost. Brightening film Brightness enhancement film (BEF) is a new type of high-performance optical film. Because of its micro prism array structure, it is also widely known as prism film. Brightening film is widely used in LCD backlight of TV, display, flat panel, mobile phone and other electronic devices to improve the overall brightness of LCD, so as to achieve the effect of energy saving. Product features The brightening film usually uses UV curable glue to micro copy the prism structure on the surface of pet. Through the refraction and reflection of light, the prism structure is used to correct the direction of light, so as to make the light front concentrated, and the unused light outside the viewing angle is refracted, reflected, recycled and utilized, so as to improve the brightness of the backlight. Product advantages Composite brightening film In the TFT-LCD backlight module, the thin film or sheet which can improve the luminous efficiency of the whole backlight system can make the light in all directions concentrate to the central viewing angle by using the special prism structure of the brightening film and the optical principles of refraction, total reflection and light accumulation, so as to improve the brightness of the LCD panel and control the viewing angle. Product features The backlight module of traditional LCD is mainly composed of light source, light guide plate, lower diffusion film, lower enrichment film, upper enrichment film and upper diffusion film. However, the backlight module with the above structure has many optical diaphragms, high cost, complex assembly process and large thickness after assembly. In order to solve the above problems, Jizhi technology has assembled a variety of different optical films (2 or 3 kinds) into a single multifunctional composite brightening film through advanced composite technology and resin formulation technology. According to the different types of optical films in the composite brightening films, they can be divided into diffusion prism composite film (DOP), prism prism composite film (POP), microlens prism composite film (MOP), prism microlens composite film (POM), etc. Product advantages Compared with the traditional structure of backlight module, the advantages of composite brightening film are as follows: 1. Avoid the repeated use of the module substrate, reduce the total thickness of the module and the total cost of the product; 2. Reduce the assembly time and improve the assembly efficiency; 3. The energy loss of the light source is avoided and the display brightness is improved; 4. The sharp top of the prism is covered inside by composite, which avoids the scratch of the prism; 5. The composite membrane has better stiffness and warpage resistance Backlight module and reflective polarizing brightening film In addition to the above composite brightening films, the company has developed a variety of new composite films based on reflective polarizing brightening film (hereinafter referred to as "core layer"), such as diffusion core prism composite film (COP), diffusion core prism prism composite film (copp) and copp plus composite film bonded with optical transparent adhesive (OCA). The core layer is the multilayer film of polyethylene terephthalate and its copolymer. Its special optical structure can convert s light into P light, so as to improve the light energy utilization and cause the brightness gain in a large viewing angle. Taking cop as an example, the traditional diffusion coating is coated on the core layer, and the core layer is bonded with the traditional prism by the composite process to form a single multifunctional composite film. Using this kind of multi-functional composite film, a single film can not only achieve the effect of brightening and diffusion, but also effectively reduce the thickness of the whole film, simplify the difficulty of module assembly, and provide excellent backlight module products for the subsequent development of ultra-thin models. Quantum dot film Quantum dot film is a kind of special optical film with wide color gamut, which is made of quantum dot, barrier resin and optical grade water oxygen barrier film. Product features Quantum dots, also known as nanocrystals, are nanoparticles composed of II-VI or III-V elements. The particle size of quantum dots is generally between 1 nm and 10 nm. Due to the quantum confinement of electrons and holes, the continuous energy band structure becomes a discrete energy level structure with molecular characteristics. When a certain electric field or light pressure is applied to this kind of nano semiconductor material, they will emit light of a specific frequency. The frequency of the light will change with the size of the semiconductor, so the color of the light can be controlled by adjusting the size of the nano semiconductor, and the pure monochromatic light with high quality can be obtained. Different from the traditional liquid crystal display, quantum dot display uses blue LED as the light source. Quantum dot film will excite pure green light and red light under the blue light excitation, and then mix the blue light to form high-quality white light. This special nanotechnology realizes the high color gamut coverage of the display and restores the color. Product advantages The advantages of QD films can be summarized as "high purity and stability". "High" means high gamut. Under NTSC standard, the gamut of ordinary LED TV is only 72%, that of the first generation high gamut TV is only 82%, that of OLED display can reach 100%, and that of quantum dot TV is as high as 110%; "Pure" means pure color. The color purity is more than 50% higher than that of ordinary led, which accurately presents natural color; "Stable" means stable performance, long life of the film, stable quantum dots of inorganic nano materials can ensure the color will not fade for a long time. Composite silver reflective film Composite silver reflector (LSR) is a kind of backlight unit (BLU) reflector used in mobile phone terminals. It can reflect most of the light emitted from Blu downward, so as to improve the luminous flux emitted upward, reduce the backlight leakage, and improve the brightness of the whole machine. Product features LSR uses precision coating and composite technology to pass 1-1.5 mil thick reflective layer (silver plated PET) and 1.5-2 mil thick protective layer (transparent, white, black and silver PET) through 5-7 layers μ Finally, the total thickness is about 80 μ The reflector can be covered with 40 ~ 50 mm on one side μ The protective film with a thickness of 1.5 m is used for dust and scratch protection. The average reflectivity of LSR in the range of 460-700 nm is more than 95%. Because the silver coating has smooth surface, high finish and high reflectivity, it can fully carry out specular reflection, so as to improve the upward luminous flux. Product advantages The LSR products developed by our company are divided into two kinds of design structures: plain bright and bright bright, and have four types of products: Silver transparent (also known as silver gray in the industry), silver white, silver black and double silver. It has the advantages of high reliability, matte sense, high shadowing and fine taste, and can be applied to a variety of backlight types of mobile terminals. Naked eye 3D grating film Grating film, or cylindrical lens film, is a kind of special optical film which can be applied to large, medium and small full-size display terminals, and enable the viewer to view 3D display effect naked without the help of 3D glasses. Product features The cylindrical lens structure made of acrylic resin was transferred onto the front of pet substrate with a total thickness of 60-300 mm by using precision engraving and UV micro replication technology μ M, and covered with 40 ~ 50 μ The protective film with a thickness of 1.5 m is used for dust and scratch protection. According to the principle of geometrical optics, two parts of 2D image information with parallax between left and right eyes are independently entered into the left eye (only left eye information) and the right eye (only right eye information) by using the refraction and light splitting function of lens structure. After processing, the viewer's brain obtains the stereoscopic scene depth. Product advantages The naked eye 3D grating film developed by our company is divided into convex grating, concave grating and composite grating (invisible grating). It has been widely used in the fields of dual view and multi view 3D advertising machine, 3D picture frame, 3D TV, 3D game display, especially 3D tablet and 3D mobile phone, which can make customers easily enjoy 3D vision at any time! Protective film of educational machine The protective film of educational machine is applied to large black (white) board and large educational machine panel in schools, conference rooms and office buildings. Product features The market education machine mainly uses Ag treated glass as the protection screen. Although it has high wear resistance, the writing performance of Ag glass is poor, and the cost is high. The explosion-proof writing film of education machine replaces the glare prevention of Ag glass, which significantly reduces the cost, increases the blue light resistance, and takes into account the explosion-proof and writing performance. In the field of traditional education whiteboard, the explosion-proof writing film of educational machine is used to replace the paint baking panel, which makes the writing smooth, easy to write, easy to wipe, easy to use, and greatly reduces the cost. Product advantages 1. Excellent writing ability Suitable for chalk, water-based pen and pencil writing The writing performance is easier and clearer than traditional glass 2. Erasure resistance Easy to wipe, resistant to alcohol and other conventional solvents 3. Light and convenient Compared with the traditional Ag glass, the film material has the characteristics of light weight 4. Anti glare, anti blue light: Replace Ag glass for anti glare, reduce cost, and increase anti blue light function. Customizable 5. Replace the traditional blackboard, The length of the product is unlimited, and the width can reach 2m, which can completely cover the whole traditional black (white) board 6. Low maintenance cost. In the process of long-term use, only the diaphragm attached to the surface of the educational machine / blackboard needs to be replaced, so the maintenance cost is extremely low |