Optical film is ubiquitous in our life, from precision optical equipment, display equipment to optical film application 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. In this paper, we only discuss the optical film industry of wet coating technology.
  optical film can be divided into: reflection film, antireflection film / antireflection film, filter, polarizer / polarizing film, compensating film / phase difference plate, directional film, diffusion film / film, brightening film / prism / condenser, shading film / black and white adhesive, etc. The related derivatives include optical protective film, window film, etc.
  1. Reflection film
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.
  the general metal has 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.
The reflection film of all dielectric is based on the interference of multiple beams. In contrast to the antireflection film, the reflectivity of the optical surface can be increased by coating a film with a refractive index higher than that of the substrate. The simplest multilayer reflection is made of two materials with high and low refractive index. The optical thickness of each layer is one quarter of a wavelength. Under this condition, the reflection light vector on each interface participating in superposition has the same vibration direction. The amplitude of the synthesis increases with the increase of the number of layers.
  aluminum foil reflective film dike aluminum foil insulation coil, also known as diaphragm, insulation film, heat insulation foil, pull-out film, reflective film, etc. The aluminum foil is made by pressing the aluminum foil surface + polyethylene film + fiber woven material + metal film through the hot melt adhesive layer. The aluminum foil coil has the functions of heat insulation, water proofing and moisture-proof. The solar absorption rate (solar radiation absorption coefficient) of aluminum foil insulation coil is very low (0.07), which has excellent heat insulation and insulation performance, which can reflect more than 93% of radiation heat, and is widely used in the insulation and insulation of building roof and exterior wall.
The main effect is to improve the diffraction of light, so that people can watch the text and graphics for a long time. This requires a surface smooth reflection less anti reflective film.
  2. Antireflection / antireflection film
The main function of the antireflection film is to reduce or eliminate the reflection light on the lens, prism, plane mirror and other academic surfaces, so as to increase the light transmission of these elements and reduce or eliminate the stray light of the system.
The antireflection film is based on the fluctuation and interference of light. The amplitude of the two waves is enhanced when the two waves with the same amplitude and the same wavelength are superimposed; If the two waves are originally the same, the wave history is different, if the two light waves are superimposed, they are offset. The antireflection film is to use this principle to coat the surface of the lens with Ar-coating, which makes the reflection light produced on the front and rear surfaces interfere with each other, thus offsetting the reflection light and achieving the effect of reducing reflection. The simplest antireflection membrane is a single layer membrane. In general, it is difficult to achieve the ideal effect by using single layer antireflection film. In order to achieve zero reflection at single wavelength or better penetration enhancement in a wider spectral region, it is often used double layer, three layer or more layers of antireflection film.
The practical application of the antireflection film is very extensive, the most common is the lens and solar cell - the power watt value of photovoltaic module can be improved by preparing the antireflection film. At present, the antireflective film material used in crystalline silicon photovoltaic cells is silicon nitride. Plasma enhanced chemical vapor deposition technology is used to separate ammonia and silane and deposit on the surface of silicon wafer, which has high refractive index and can play a better effect of reducing reflection. In the early stage, silicon dioxide and titanium dioxide were used as antireflective layer.
  3. Filter
  the filter is made of plastic or glass and special dyes. Red filter can only let red light pass through, and so on. The refractive index of glass sheet is similar to that of air. All color light can pass through, so it is transparent. However, after dye is dyed, the molecular structure changes and refractive index changes. The passing of some color light changes. For example, a white light passes through a blue filter, and it emits a blue light, while green light and red light are very few, most of which are absorbed by the filter.
The filter products are classified by spectral band, spectral characteristics, film materials, application characteristics and so on.
  spectral band: UV filter, visible filter, infrared filter;
Spectral characteristics: band pass filter, cutoff filter, spectroscopic filter, neutral density filter, reflection filter;
Film materials: soft film filter, dural filter. The dural filter is not only the hardness of the film, but also the laser damage threshold, so it is widely used in laser system. The soft film filter is mainly used in biochemical analyzer.
Band pass type: the light passing through the selected band and the light cut off outside the pass band.
  short wave pass (also called low wave pass): the passage of light shorter than the selected wavelength, and the optical cutoff longer than that wavelength. For example, infrared cut-off filter, ibg-650.
  long wave pass (also known as high wave pass): the light passing through longer than the selected wavelength, and the light cut-off shorter than that wavelength, such as infrared transmission filter, ipg-800.
  color filter is an important part of TFT LCD backlight module.
  4. Polarizer
The full name of polarizingfilm should be polarizing film. The imaging of LCD must rely on polarized light. The main function of polarizer is to make the non polarized natural light become polarized and transformed into polarizing light. In addition, the liquid crystal molecular torsion characteristics can control the passing of light, thus improving the transmittance and the range of view angle, and forming anti dazzle function.
The polarizer can be widely used in modern LCD products: LCD TV, laptop, mobile phone, PDA, electronic dictionary, MP3, instrument, projector, etc., and also used in fashionable polarizing glasses. Among them, LCD application is the main force to promote the development of polarizer industry.
  5. Compensating film / phase difference plate
The compensation principle of compensation film is to correct the phase difference of liquid crystal in various display modes (tn/stn/tft (va/ips/ocb). In short, it is to compensate the symmetry of birefringence property of liquid crystal molecules. If we want to distinguish from its functional purposes, we can divide the phase difference film, the color difference compensation film and the angle of view expansion film. The compensation film can reduce the light leakage in the dark state of LCD, and can greatly improve the contrast, chromaticity and overcome some gray order inversion problems in a certain angle of view.
  6. Directional film
The   coordination film is a film with straight streaks, which guides the arrangement direction of liquid crystal molecules (Fig. 1.1). On the glass base plate with evaporated transparent conductive film (ITO), a parallel groove is printed on ITO film by PI coating and roller. Then, the liquid crystal can lie in the groove in the direction of this groove, so as to make the liquid crystal arrange in the same direction. The film with one direction is the directional film.
The reason why liquid crystal can be applied to the screen is that the induced electric rate of liquid crystal is different from that of vertical molecular direction. Therefore, it can be driven by electric field. On the other hand, because the liquid crystal also has refractive index (i.e. birefringence) which varies according to the molecular direction, the polarizing direction of polarized light can be changed, Finally, because of the strong force on the interface between liquid crystal and the film, the liquid crystal will return to the original arrangement by the elastic coefficient (recovery force) after the electric field is closed. Therefore, it can be seen that the liquid crystal can not work without the presence of the film. But in the application of liquid crystal screen, the liquid crystal molecules and the surface of the film are inclined at an angle (i.e. pretiltangle), so as to achieve the uniform alignment effect.
The coating method of the film is non roll wet coating, which includes traditional directional brush grinding, UV light matching, electron plasma and ion beam.
  7. Diffusion film
  diffusion film is a key component of TFT LCD backlight module, which can provide a uniform surface light source for LCD. Generally, traditional diffusion film mainly includes chemical particles in the diffusion film substrate, which is used as scattering particles, while the particles of the existing diffusion plate are scattered among resin layers, so when light passes through the diffusion layer, It will continue to pass through two medium with different refractive index, so many phenomena of refraction, reflection and scattering will occur in light, which will cause the effect of optical diffusion. See Chapter II for details.
  8. Brightening film / prism / condenser
The "brightening film" is also called prism sheet, which is often referred to as bef (brightnesenhancementfilm), which is the key component of TFT LCD backlight module. It is mainly used to modify the direction of light by the refraction and reflection principle of light, and to make the light focus on the front, and to recover and utilize the unused light outside the viewing angle, and improve the overall brightness and uniformity, Achieve the effect of brightening, also known as the focus film. The composite optical film mainly integrates the function of original focus film and diffusion function, so that the use of one diffusion sheet can be reduced, which is conducive to the downstream manufacturers to simplify backlight design, save process and reduce cost, and improve brightness efficiency. For optical film manufacturers, although composite brightening film will replace traditional spotlight (brightening film), the unit price and profit are better.
  9. Light mask / black and white adhesive
The black and white light mask film is mainly used on the back light source, which plays a fixed and light blocking role (covering the edge light and light position light), also called the light mask and black-white film, or black-and-white adhesive (can be said to be a double-sided tape). Compared with the backlight used by TFT LCD, the requirements of backlight are higher, so most black and white glue is applied to the back light source of TFT LCD. Besides black and white glue, there are black black glue (black on both sides), and the main function is still fixed and light shading; Black silver glue (one side black, one side silver), except for shading, silver surface has reflection effect. The relative black and white glue is the mainstream product of LCD market. Compared with the viscosity of black surface and white surface, the white surface needs to be larger. Because the white surface is connected with rubber frame, while the black surface is connected with glass, the adhesion of the black surface to the rubber is worse than that of the glass. Therefore, the white surface is needed to be more viscous to ensure the stability of the whole module.