Optical coatings are ubiquitous in our lives, from precision optical devices and display equipment to everyday applications of optical films; for example, the glasses we wear, digital cameras, various household appliances, or the anti-counterfeiting technology on banknotes can all be considered extensions of optical film technology applications. Without the foundational development of optical film technology, modern optoelectronics, communications, or laser technology would not have made progress, highlighting the importance of research and development in optical film technology.

Optical films refer to one or more layers of dielectric films or metal films, or a combination of both, deposited or coated on optical components or independent substrates to alter the transmission characteristics of light waves, including transmission, reflection, absorption, scattering, polarization, and phase change. Therefore, through appropriate design, the transmittance and reflectance of the surface of different band components can be adjusted, and light with different polarization planes can exhibit different characteristics.

Generally speaking, the production methods of optical films are mainly divided into dry and wet production processes. The so-called dry process does not involve any liquid throughout the processing, for example, vacuum deposition occurs in a vacuum environment, where solid raw materials are heated using electrical energy, sublimated into gas, and then adhered to the surface of a solid substrate to complete the coating process. The decorative gold, silver, or metallic textured packaging films seen in daily life are products made using the dry coating method. However, considering actual mass production, the application range of dry coating is smaller than that of wet coating. The general practice of wet coating is to mix various functional components into a liquid coating, apply it to the substrate using different processing methods, and then dry and cure the liquid coating to make products. This article will only discuss the optical film industry using wet coating technology.

Optical films can be classified according to their uses, characteristics, and applications into: reflective films, anti-reflective films, filters, polarizers, compensating films, alignment films, diffusion films, brightness-enhancing films, prism films, light-concentrating films, and shading films. Related derivative types include optical-grade protective films, window films, etc.

The most common types of coatings in optical processing:

1. Reflective Films

Reflective films can generally be divided into two categories: metal reflective films and all-dielectric reflective films. In addition, there are metal-dielectric reflective films that combine both, which function to increase the reflectivity of optical surfaces.

Generally, metals have a large extinction coefficient. When a light beam enters a metal surface from air, the amplitude of the light entering the metal rapidly decreases, resulting in a corresponding reduction in the light energy entering the metal, while the reflected light energy increases. The larger the extinction coefficient, the more rapidly the light amplitude decreases, the less light energy enters the metal, and the higher the reflectivity. People always choose metals with a larger extinction coefficient and more stable optical properties as materials for metal films. Commonly used metal thin materials in the ultraviolet range are aluminum, in the visible range are aluminum and silver, and in the infrared range are gold, silver, and copper. Additionally, chromium and platinum are also often used as materials for some special thin films. Since materials like aluminum, silver, and copper easily oxidize in air and degrade performance, they must be protected with dielectric films. Common protective film materials include silicon monoxide, magnesium fluoride, silicon dioxide, and aluminum oxide.

The advantages of metal reflective films are that the preparation process is simple and the working wavelength range is wide; the disadvantage is that there is significant light loss, and the reflectivity cannot be very high. To further increase the reflectivity of metal reflective films, several layers of dielectric layers of a certain thickness can be added to the outer side of the film, forming a metal-dielectric reflective film. It should be noted that metal-dielectric reflective films increase the reflectivity at a certain wavelength (or wavelength range) but destroy the characteristic of neutral reflection in metal films.

All-dielectric reflective films are based on multi-beam interference. In contrast to anti-reflective films, depositing a layer of film with a refractive index higher than that of the substrate material on the optical surface can increase the reflectivity of the optical surface. The simplest multilayer reflection is formed by alternating the deposition of two materials with high and low refractive indices, with each layer's optical thickness being a quarter of a certain wavelength. Under these conditions, the reflected light vectors at the participating interfaces vibrate in the same direction. The synthetic amplitude increases with the number of film layers.

Aluminum foil reflective film, also known as barrier film, thermal insulation film, thermal foil, heat-reflecting film, reflective film, etc., is made of aluminum foil facing + polyethylene film + fiber woven fabric + metal coating, laminated through a hot melt adhesive layer. Aluminum foil roll materials have functions such as thermal insulation, waterproofing, and moisture-proofing. The solar absorption rate (solar radiation absorption coefficient) of aluminum foil roll materials is extremely low (0.07), providing excellent thermal insulation performance, capable of reflecting more than 93% of radiant heat, and is widely used in building roofs and external wall thermal insulation.

Correspondingly, there is an anti-reflective film, whose main function is to enhance the diffraction of light, allowing people to view text and graphics for extended periods. This requires a smooth surface with minimal reflection from the anti-reflective film.

2. Anti-Reflective Films / Transmittance-Increasing Films

Anti-reflective films, also known as transmittance-increasing films, primarily function to reduce or eliminate the reflected light from the surfaces of lenses, prisms, plane mirrors, etc., thereby increasing the light transmission of these components and reducing or eliminating stray light in the system.

Anti-reflective films are based on the wave nature of light and interference phenomena. When two light waves with the same amplitude and wavelength overlap, the amplitude of the light wave is enhanced; if the two light waves are originally the same but differ in path length, they will cancel each other out when they overlap. The anti-reflective film utilizes this principle by depositing an anti-reflective coating (AR-coating) on the surface of the lens, causing the reflected light from the front and back surfaces of the film layer to interfere with each other, thereby canceling out the reflected light and achieving the effect of reducing reflection. The simplest anti-reflective film is a single-layer film. Generally, using a single-layer anti-reflective film is difficult to achieve the ideal anti-reflective effect. To achieve zero reflection at a single wavelength or good anti-reflective effects over a wider spectral range, double, triple, or even more layers of anti-reflective films are often used.

The practical applications of anti-reflective films are very extensive, with the most common being lenses and solar cells—improving the power output of photovoltaic modules by preparing anti-reflective films. Currently, the anti-reflective film material used in crystalline silicon photovoltaic cells is silicon nitride, which is deposited on the surface of silicon wafers using plasma-enhanced chemical vapor deposition technology, ionizing ammonia and silane, resulting in a high refractive index that provides a good anti-reflective effect. Earlier photovoltaic cells used silicon dioxide and titanium dioxide films as anti-reflective layers.

3. Filter

Filters are made of plastic or glass with special dyes added. A red filter only allows red light to pass through, and so on. The refractive index of the glass is originally similar to that of air, allowing all colors of light to pass through, making it transparent. However, after being dyed, the molecular structure changes, and the refractive index also changes, affecting the passage of certain colors of light. For example, when a beam of white light passes through a blue filter, it emits a beam of blue light, while green and red light are minimal, with most being absorbed by the filter.

Filter products are mainly classified by spectral band, spectral characteristics, coating materials, application features, etc.

Spectral bands: ultraviolet filters, visible filters, infrared filters;

Spectral characteristics: bandpass filters, cutoff filters, dichroic filters, neutral density filters, reflection filters;

Coating materials: soft film filters, hard film filters. Hard film filters not only refer to the hardness of the film but more importantly, their laser damage threshold, so they are widely used in laser systems. Soft film filters are mainly used in biochemical analyzers.

Bandpass type: allows light of a selected band to pass through, while light outside the passband is cut off.

Shortwave pass type (also called low pass): allows light shorter than the selected wavelength to pass through, while light longer than that wavelength is cut off. For example, infrared cutoff filter, IBG-650.

Longwave pass type (also called high pass): allows light longer than the selected wavelength to pass through, while light shorter than that wavelength is cut off. For example, infrared transmitting filter, IPG-800.

Color filters are an important component of TFT-LCD backlight modules.

A few less commonly used film systems:

4. Polarizer

The full name of a polarizer (Polarizing Film) should be a polarized light film. The imaging of liquid crystal displays relies on polarized light. The main function of a polarizer is to convert unpolarized natural light into polarized light, combined with the twisting characteristics of liquid crystal molecules, to control the passage of light, thereby improving light transmission and viewing angle range, forming anti-glare functions.

Polarizers can be widely used in modern liquid crystal display products: LCD TVs, laptops, mobile phones, PDAs, electronic dictionaries, MP3 players, instruments, projectors, etc., and can also be used in fashionable polarized sunglasses. Among these, the application in LCDs is the main driving force for the development of the polarizer industry.

5. Compensation Film/Phase Difference Plate

The compensation principle of the compensation film is to correct the phase difference generated by liquid crystals in various display modes (TN/STN/TFT(VA/IPS/OCB)) at different viewing angles. In simple terms, it compensates for the birefringent properties of liquid crystal molecules to achieve symmetry. If we distinguish by functional purpose, we can roughly categorize them into phase difference films that simply change the phase, color difference compensation films, and viewing angle expansion films. Compensation films can reduce the amount of light leakage in the dark state of liquid crystal displays and significantly improve image contrast, color, and overcome some gray scale inversion issues within a certain viewing angle.

6. Alignment Film

Alignment film is a thin film with straight scratches, which serves to guide the arrangement direction of liquid crystal molecules (see Figure 1.1). On a glass substrate that has been coated with a transparent conductive film (ITO), a PI coating liquid and a roller are used to print parallel grooves on the ITO film, allowing the liquid crystals to lie horizontally within the grooves, achieving the goal of aligning the liquid crystals in the same direction. This film with directional grooves is the alignment film.

The reason liquid crystals can be used on screens is that their dielectric constants differ in the parallel and perpendicular molecular directions, allowing them to be driven by an electric field. On the other hand, since liquid crystals also have a refractive index that changes with the molecular direction (i.e., they exhibit birefringence), they can change the polarization direction of polarized light. Finally, due to the strong anchoring strength at the interface between the liquid crystal and the alignment film, when the electric field is turned off, the liquid crystal returns to its original arrangement due to its elastic coefficient (restorative force). Thus, it can be seen that without the presence of the alignment film, liquid crystals cannot function. However, in the application of liquid crystal screens, the liquid crystal molecules are inclined at a certain angle to the surface of the alignment film (i.e., the pretilt angle), which is necessary to achieve uniform alignment.

The coating method for alignment films involves non-roll wet coating, with traditional directional brushing methods and modern UV alignment methods, electronic paste alignment, and ion beam alignment.

7. Diffusion Film

Diffusion film is a key component in TFT-LCD backlight modules, providing a uniform surface light source for liquid crystal displays. Generally, traditional diffusion films mainly incorporate chemical particles as scattering particles within the diffusion film substrate. The existing diffusion plates have their microparticles dispersed within the resin layer, so when light passes through the diffusion layer, it continuously traverses two media with different refractive indices, resulting in numerous refractions, reflections, and scattering phenomena, thus creating an optical diffusion effect. See Chapter 2 for details.

8. Brightness Enhancement Film/Prism Sheet/Concentration Film

Brightness enhancement film, also known as prism sheet (Prism Sheet), commonly abbreviated as BEF (Brightness Enhancement Film), is a key component in TFT-LCD backlight modules. It mainly utilizes the principles of light refraction and reflection to correct the direction of light, concentrating light directly in front and recycling and utilizing light outside the viewing angle, while enhancing overall brightness and uniformity, achieving a brightness enhancement effect, also known as concentration film. Composite optical films mainly integrate the original functions of concentration films and diffusion functions, thus reducing the need for an additional diffusion sheet, benefiting downstream manufacturers by simplifying backlight design, saving processes, and reducing costs, while also improving brightness efficiency. For optical film manufacturers, although composite brightness enhancement films will replace traditional concentration films (brightness enhancement films), they offer better unit prices and profits.

9. Light Shielding Film/Black and White Glue

Black and white light-blocking adhesive | Light-blocking film is mainly used on backlight sources, serving to fix and block light (blocking side light and lamp position light). It is also called light-blocking sheet, black and white film, and is abbreviated as black and white adhesive (which can be considered a type of double-sided tape). The light-blocking requirements for backlight sources used in TFT-LCDs are relatively high, so most black and white adhesives are applied to the backlight sources of TFT-LCDs. In addition to black and white adhesive, there is also black adhesive (both sides are black), which mainly serves to fix and block light; and black and silver adhesive (one side black, one side silver), which, in addition to blocking light, has a reflective effect on the silver side. Compared to black and white adhesive, black and silver adhesive is a mainstream product in the LCD market. The stickiness comparison between the black side and the white side shows that the white side needs to be stickier because the white side connects to the rubber frame, while the black side connects to the glass. The adhesion of the adhesive to glass is relatively better than to rubber frames, so the stickiness of the white side needs to be greater to ensure the stability of the entire module.