Summary of Nine Types of Optical Thin Films

Optical thin films are essential materials used in the field of die cutting. These films possess unique properties that make them ideal for various applications in optics, electronics, and telecommunications.

With precise die cutting techniques, thin films can be accurately cut into desired shapes and sizes, ensuring a perfect fit for specific optical devices. The die-cutting process allows for the creation of intricate patterns and designs, enabling the production of custom optical components.

Optical thin films used in die cutting are known for their superior light transmission properties, making them crucial for optical applications. These films are designed to minimize reflection and maximize light transmission, ensuring high optical efficiency and clarity.

Furthermore, these materials provide excellent protection against external factors such as moisture, dust, and scratches. Die-cut optical thin films can be laminated onto optical surfaces, acting as a protective barrier while maintaining the integrity of the optical system.

In addition to their optical properties, these films offer versatility in terms of material selection and thickness. Die-cutting allows for the utilization of a wide range of materials, including polymers, plastics, and specialized coatings, ensuring compatibility with different optical devices and applications.

Optical thin films are ubiquitous in our lives, from precision and optical equipment, display devices to everyday applications such as eyeglasses, digital cameras, various household appliances, and even anti-counterfeiting technologies on banknotes. Without the foundation of optical thin film technology, the development of modern optoelectronics, communications, and laser technologies would not have progressed. This highlights the importance of research and development in optical thin film technology.

Optical thin films refer to the deposition or coating of one or more layers of dielectric or metallic films, or a combination of both, on optical components or independent substrates to alter the transmission properties of light waves. These properties include transmission, reflection, absorption, scattering, polarization, and phase change of light. Through appropriate designs, the transmittance and reflectance of the surface of different wavelength components can be adjusted, and different polarization planes of light can exhibit different characteristics.

Generally, the production of optical thin films can be divided into two main processes: dry and wet methods. Dry methods involve processes without the presence of liquids, such as vacuum deposition, where solid raw materials are heated by electrical energy in a vacuum environment to be sublimated into gas and adhere to the surface of a solid substrate. Decorative packaging films with gold, silver, or metallic appearance seen in daily life are products made using dry coating methods. However, in practical mass production, the application range of dry coating is smaller than that of wet coating. Wet methods usually involve mixing various functional components into liquid coatings, which are then applied to substrates using different processing techniques and dried to solidify into products. In this article, only wet coating technology in the optical thin film industry will be discussed.

Optical thin films can be categorized based on their applications, characteristics, and uses. These categories include reflective films, anti-reflection films, filters, polarizers, compensators/phase difference plates, orientation films, diffusion films/sheets, brightness enhancement films/prismatic films/focusing sheets, light-blocking films/black-and-white adhesives. Related derivatives include optical-grade protective films and window films.

1. Reflective Films

Reflective films can generally be divided into two types: metallic reflective films and all-dielectric reflective films. There are also metal-dielectric composite reflective films that combine both types, increasing the reflectivity of optical surfaces.

Metals typically have large extinction coefficients. When a light beam enters a metallic surface from the air, the amplitude of the light oscillation rapidly decays upon entering the metal, reducing the light energy entering the metal and increasing the reflected energy. The larger the extinction coefficient, the faster the decay of the light amplitude and the less light energy entering the metal, resulting in higher reflectance. Metals with larger extinction coefficients and stable optical properties are usually selected as metal film materials. Aluminum is commonly used for ultraviolet applications, while aluminum and silver are used for visible light applications. Gold, silver, and copper are commonly used for infrared applications. Chromium and platinum are also used in some special films. Since aluminum, silver, copper, and other materials easily oxidize in air, leading to reduced performance, they must be protected with dielectric films. Common protective film materials include silicon dioxide, magnesium fluoride, silicon dioxide, and aluminum oxide.

The advantage of metallic reflective films is their simple preparation process and wide working wavelength range. However, they have significant optical losses, making it impossible to achieve high reflectance. To further increase the reflectance of metallic reflective films, several layers of dielectric films with a certain thickness can be deposited on the outer side of the film to form metal-dielectric reflective films. It should be noted that metal-dielectric reflective films increase the reflectance of a particular wavelength (or a specific wave range) but sacrifice the characteristic of neutral reflection in metal films.

All-dielectric reflective films are based on multi-beam interference. By depositing a thin film with a refractive index higher than that of the substrate material on the optical surface, the reflectance of the optical surface can be increased. The simplest multi-layer reflection is composed of two materials with high and low refractive indices alternately deposited. The optical thickness of each layer is one-quarter of a certain wavelength. Under these conditions, the reflected light vectors on different interfaces that participate in superposition vibrate in the same direction, and the synthesized amplitude increases with the number of layers.

Aluminum foil reflective film, also known as barrier film, heat insulation film, heat insulation foil, heat removing film, and reflective film, is made by laminating aluminum foil face + polyethylene film + fiber weave + metal coating with hot-melt adhesive. Aluminum foil roll has functions such as heat insulation, waterproofing, and moisture resistance. The solar radiation absorption rate (absorption coefficient of solar radiation) of aluminum foil roll is very low (0.07), and it has excellent heat insulation performance. It can reflect more than 93% of radiant heat and is widely used in heat insulation for building roofs and exterior walls.

Conversely, anti-reflective films are mainly used to improve diffraction of light, allowing people to view text and graphics for extended periods. This requires anti-reflective films to have smooth surfaces with minimal reflection.

2. Anti-reflection Films

Anti-reflection films, also known as anti-reflective films, primarily reduce or eliminate reflected light from surfaces of lenses, prisms, mirrors, etc., thus increasing the transmittance of these elements and reducing or eliminating stray light in systems.

Anti-reflection films are based on the wave nature of light and interference phenomena. When two light waves with the same amplitude and wavelength superpose, the amplitude of the light waves increases. If the two light waves are originally in phase but have a phase difference, when they superpose, they cancel each other out. Anti-reflection films utilize this principle by depositing anti-reflection films (AR coating) on the surface of lenses, effectively interfering with the reflected light between the front and back surfaces of the film to cancel out the reflected light, achieving anti-reflection effects. The simplest form of an anti-reflection film is a single-layer film. However, in most cases, achieving ideal anti-reflection effects with a single-layer film is challenging. To achieve zero reflection at a single wavelength or good anti-reflection effects over a broader spectrum, double-layer, triple-layer, or even more layers of anti-reflection films

3, the filter

Filters are made of plastic or glass with special dyes, red filters can only let red light through, and so on. The refractive index of the glass sheet was originally about the same as air, and all colors of light can pass through, so it is transparent, but after dyeing the dye, the molecular structure changes, the refractive index also changes, and the passage of some colors of light changes. For example, a beam of white light through the blue filter, emitted is a beam of blue light, and green light, red light is very little, most of the filter is absorbed.

Filter products are mainly classified by spectral band, spectral characteristics, film material, application characteristics and so on.

Spectral band: UV filter, visible filter, infrared filter;

Spectral characteristics: band pass filter, cut-off filter, spectroscopic filter, neutral density filter, reflection filter;

Film material: soft film filter, hard film filter. Hard film filter not only refers to the hardness of the film, but more importantly, its laser damage threshold, so it is widely used in laser systems. Soft membrane filters are mainly used in biochemical analyzers.

Band-pass type: The selected band of light through, the light outside the passband cut-off.

Short-wave pass type (also called low-wave pass) : the light passing shorter than the selected wavelength, and the light cutoff longer than the wavelength. Such as the infrared cut-off filter, IBG-650.

Long wave pass type (also called high wave pass) : longer than the selected wavelength of light through, shorter than the wavelength of light cutoff, such as infrared filter light, IPG-800.

Color filter is an important part of TFT-LCD backlight module.

4. Polarizing Film

The full name of a polarizing film is polarized light film. The imaging of liquid crystal displays relies on polarized light. The main function of a polarizing film is to convert unpolarized natural light into polarized light, which, when combined with the twisting characteristics of liquid crystal molecules, allows for control over the passage of light. This, in turn, improves transmittance, viewing angle range, and provides anti-glare functionality.

Polarizing films are widely used in modern LCD display products such as LCD TVs, laptops, smartphones, PDAs, electronic dictionaries, MP3 players, instruments, projectors, and even fashionable polarized sunglasses. Among these, the application in LCDs is the main driving force behind the development of the polarizing film industry.

5. Compensation Film/Phase Difference Plate

The compensation film corrects the phase difference in the liquid crystal generated at different viewing angles in various display modes (TN/STN/TFT(VA/IPS/OCB)). In simple terms, it compensates for the birefringence properties of liquid crystal molecules to achieve symmetry. The different types of compensation films include those that primarily change the phase, color compensation films, and wide viewing angle films. The compensation film can reduce light leakage in dark states of the liquid crystal display and significantly improve contrast and color fidelity within a certain viewing angle while overcoming partial gray-scale inversion issues.

6. Alignment Film

The alignment film is a thin film with parallel striped scratches. Its purpose is to guide the alignment of liquid crystal molecules (Figure 1.1). On a glass substrate with a transparent conductive film (ITO) coated, a PI solution is applied and a roller is used to imprint parallel grooves on the ITO film. Later, the liquid crystal can align horizontally along these grooves to achieve a uniform alignment. The film with a pattern of parallel directions is called an alignment film.

The reason liquid crystals can be applied to screens is that their dielectric constants differ depending on the alignment of the molecules, which allows them to be driven by an electric field. Moreover, since liquid crystals have a refractive index that changes with molecular orientation (birefringence), they can alter the polarization direction of polarized light. Finally, strong anchoring forces between liquid crystals and alignment films allow the liquid crystals to return to their original alignment after the electric field is turned off, thanks to the elasticity of the liquid crystals. From this, we can understand that liquid crystals cannot function without alignment films. However, for the application of liquid crystal displays, the liquid crystal molecules inclined at a certain angle (pretilt angle) relative to the alignment film surface are required to achieve a uniform alignment.

Alignment films involve non-roll-to-roll wet coating methods, including traditional orienting brushing and grinding methods, and modern UV exposure, electron beam, and ion beam methods.

7. Diffusion Film

The diffusion film is a critical component of TFT-LCD backlight modules that provides a uniform surface light source for liquid crystal displays. Traditional diffusion films mainly include chemical particles dispersed in the diffusion film substrate, acting as scattering particles. On the other hand, existing diffusion films disperse micro-particles within the resin layers. When light passes through the diffusion layer, it repeatedly traverses between media with different refractive indices, causing refraction, reflection, and scattering phenomena, resulting in optical diffusion. Please refer to Chapter 2 for more details.

8. Brightness Enhancement Film/Prism Sheet/Focusing Film

The brightness enhancement film, also known as a prism sheet (Prism Sheet) or simply BEF (Brightness Enhancement Film), is a key component of TFT-LCD backlight modules. It corrects the direction of light through the principles of refraction and reflection. By focusing the light and collecting the unused light outside the viewing angle, it enhances overall brightness and uniformity, which is why it is also called a focusing film. Composite optical films integrate the functions of focusing films and diffusion films, reducing the need for an additional diffusion film in backlight designs. This simplifies the backlight design process, reduces manufacturing steps, lowers costs, and improves brightness efficiency. For optical film manufacturers, although composite brightness enhancement films replace traditional focusing films, the unit price and profit margin are generally better.

9. Light-Shielding Film/Black and White Adhesive

The light-shielding film, also known as a light-shielding sheet or black and white film, is mainly used in backlight sources to provide fixation and light-shielding functions (blocking edge and lamp light). It is commonly referred to as a black and white adhesive (similar to double-sided adhesive tape). Due to the higher light-shielding requirements for backlight sources used in TFT-LCDs, most light-shielding films are applied on top of the backlight sources in TFT-LCD displays. In addition to the black and white adhesive, there is also a black black adhesive (both sides black) primarily used for fixation and light-shielding, and a black silver adhesive (one side black, one side silver) that provides light-shielding and reflection. The black and white adhesive is the mainstream product in the LCD market. The adhesiveness of the black and white sides differs, with the white side requiring greater adhesiveness. This is because the white side is connected to the rubber frame, whereas the black side is connected to the glass. In terms of adhesion to the glass, the rubber frame is relatively less adhesive, so a higher adhesiveness on the white side is necessary to ensure the stability of the entire module.