What is Optical Thin Film
Optical thin film refers to the coating or coating of one or more layers of dielectric or metal films or a combination of these two types of films on optical components or independent substrates to change the transmission characteristics of light waves, including light projection, reflection, absorption, scattering, polarization, and phase change.
Therefore, by appropriately involving, the transmittance and reflectivity of surfaces in different wavelength bands can be adjusted, and light with different polarization planes can also have different characteristics.
All liquid crystal display devices can be considered to be composed of two substrates with transparent conductive electrodes, a liquid crystal layer clamped, packaged into a polarizing box, and then polarizing films attached to the outer surface and backlight modules assembled to provide light sources.
The related article: How Does the LCD Work?
The optical thin films applied in TFT-LCD can be mainly divided into two types: polarizers and backlight module optical thin films.
Polarizer
LCD imaging must rely on polarized light, and the LCD module must contain two polarizers.
There are two polarizers in the LCD module attached to both sides of the glass substrate.
The lower polarizer is used to convert the light beam generated by the backlight into polarized light, and the upper polarizer is used to analyze the polarized light electrically modulated by the liquid crystal to produce a contrast between light and dark, thereby producing a display screen. Without any polarizer, the LCD module cannot display images.
Polarizers are composed of multi-layer films, and their raw material costs account for 80% of the total production costs.
The raw materials mainly include TAC (tricellulose acetate) film, PVA (polyvinyl alcohol) film, pressure-sensitive adhesive, protection film, and release film. TAC accounts for about 50% of the cost, PVA accounts for 12%, pressure-sensitive adhesive 5- 10%, protective film and release film 15%, chemical materials 5%, and other costs account for 10%.
The core technology of polarizers is the preparation of TAC (tricellulose acetate) film and PVA (polyvinyl alcohol) film. Due to the high technical threshold, the market is mainly monopolized by several major manufacturers such as LG Chemical, Sumitomo Chemical, and Nitto Denko.
However, as emerging markets continue to accumulate technology, the market concentration of Japanese and Korean manufacturers has decreased. Manufacturers in Taiwan and mainland China have gradually increased their production capacity, and their market share has begun to gradually increase.
Global polarizer manufacturers are mainly concentrated in Japan, South Korea, Taiwan, and China, and the proportion of mainland China’s production capacity in global production capacity is increasing year by year.
Basic Performances
The basic performance indicators of polarizers mainly include optical performance, durability, adhesive properties, appearance performance, and other special properties.
Optical Performance
It includes three main performance indicators: polarization, light transmittance, and coloring. Others also include ultraviolet protection performance and the light transmittance, total reflection, and diffuse reflectance indicators of the semi-transmissive polarizer semi-transmissive film.
Durability
The technical indicators include four items: resistance to high temperature, heat and humidity, low temperature, and hot and cold shock. The most important one is the resistance to heat and humidity.
Adhesive Performance
Technical indicators mainly refer to the various characteristics of the polarizer pressure-sensitive adhesive, generally including the peeling force between the pressure-sensitive adhesive and the glass substrate, the pressure-sensitive adhesive and the peeling film, the polarizer protective film, and the polarizer, and bonding durability of pressure-sensitive adhesives.
Appearance Performance
Technical indicators mainly refer to the surface flatness of polarizer products and the number of imperfections in appearance. These technical indicators mainly affect the utilization rate of polarizer products during patching.
Additional Functions
Additional functions of the polarizer include a wide viewing angle function, improved resolution, enhanced brightness, and energy saving.
The wide viewing angle function requires that the LCD monitor can observe images with basically unchanged image quality within a wider viewing angle range. An optical compensation film can be attached to the polarizer to correct the phase difference produced by the liquid crystal at each viewing angle, thereby improving the Picture fidelity. Improving the display resolution mainly involves targeted surface treatment of the TAC film, such as anti-glare treatment, anti-scratch treatment, anti-reflective/low-reflective treatment, anti-fouling treatment, etc.
Pasting a brightness-enhancing film on the polarizer is currently the main method to increase the brightness of the panel and save energy consumption.
Backlight Unit (BLU) Optical Film
The Backlight Unit (BLU) is a key component that provides the backlight source for LCDs.
The LCD panel does not have luminous properties, so a light source must be added to the LCD panel to achieve the display effect.
LCD backlight modules are mainly composed of backlight sources, optical films, adhesive products, insulation products, plastic frames, etc.
Among them, various types of optical films are key components of backlight modules, which can be mainly divided into six types according to their functions: reflection film, diffusion film, ordinary prism film, multifunctional prism film, microlens film, reflection polarization enhancement film, etc.
A rough breakdown of the cost of the LCD panel shows that the material cost accounts for more than 70% of the total LCD manufacturing cost. Among the material costs, the backlight module accounts for 18.2%, the color filter accounts for 14.7%, the polarizer 9.5% and Glass substrates 8.9%.
Blacklight enhancement film(BEF) and diffusion film are important components of the backlight. The cost proportions of BEF, diffusion film and reflective film in the backlight module are 32%, 7%, and 2% respectively, accounting for 41% of the total.
Brightness Enhancement Film (BEF)
Brightness Enhancement Film (BEF), also called PrismSheet, is used to improve the luminous efficiency of the entire backlight system.
When the light emitted from the backlight passes through the prism structure, only the light that is incident within a certain angle range can be refracted and emitted. The remaining light is reflected to the light source by the edge of the prism due to not meeting the refraction conditions and then re-emitted by the reflection plate at the bottom of the light source.
The light in the backlight is continuously recycled under the action of the prism structure. The light that originally diverged in all directions was controlled within 70 degrees of the normal after passing through the prism sheet, thereby achieving the effect of axial brightness enhancement.
The core technology of prism sheets lies in the technology of engraving prism patterns on the roller.
In lens extrusion production, lens extrusion roller processing technology directly affects the prismatic lens molding quality of the product. The common practice in the world is to coat the processed steel roller with a layer of metal with lower hardness, such as copper, and use diamond tools to process the required fine prismatic surface structure.
The surface of this layer cannot reach the state of extrusion production, and a layer of harder nickel or chromium must be plated on the surface of the processed microstructure to meet the process requirements of diamond tool carving prismatic lenses.
Diffusion Film
The main function is to provide a uniform surface light source for the display. The core lies in precision coating technology. It has the function of diffusing light, that is, the light will scatter on its surface, spreading the light softly and evenly.
The basic structure of most diffusion films is to coat optical astigmatism particles on both sides of a transparent substrate such as PET.
When light passes through the diffusion layer with PET as the base material, it will pass through media with different refractive indexes, causing the light to undergo many refraction, reflection, and scattering phenomena. The light can be modified into a uniform surface light source to achieve the effect of optical diffusion. In the backlight structure, it mainly plays the role of correcting the diffusion angle, which increases the light radiation area but reduces the light intensity per unit area, that is, reduces the brightness. After the luminous light source is diffused by the diffusion material, it can become a secondary light source with a larger area, better uniformity, and stable chromaticity.
The Global Supply Trend of Optical Film
The R&D and production of optical film materials require the accumulation and matching of technology, talents, equipment, and raw materials.
Currently, Japanese manufacturers occupy a leading position in the polarizer field, American, Japanese, and Korean companies dominate the backlight optical film field, and the diffusion film market is occupied by Japanese and Korean companies.
Around 2005, affected by the transfer of international industries to mainland China, the first batch of optical film manufacturers began to appear. At present, the local optical film manufacturers with the largest market share in mainland China mainly include Shuangxing New Materials, Ningbo Exciton Technology, KDX, Sichuan EM Technology Co., Ltd and other companies.
For different applications, we need to choose a suitable combination of optical films to achieve the desired display effects and requirements, like high brightness, haze surface to reduce the reflection, wide operation temperature tolerance, etc.
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