Introduction and Classification of Optical Coatings

In the clear display of smartphone screens, in the images of distant galaxies captured by astronomical telescopes, and in the precise operation rooms of laser surgeries, an "invisible technology" is playing a core role - this is optical coating. It is an ultra-thin material layer deposited on the surface of optical substrates through a special process, usually only at the nanometer level in thickness, yet it can precisely control the reflection, transmission, absorption and other properties of light, enabling optical devices to break through performance bottlenecks. Optical coating applications are ubiquitous, ranging from daily consumer goods to cutting-edge technology fields. This article will systematically interpret the essence of optical coating and focus on three core products: AR anti-reflection film, HR high reflection film and filter, revealing their technical mysteries and application values.

Optical coating is not a single technology but a general term for a category of processes that form thin films on the surface of optical substrates such as glass, plastic, and metal through physical or chemical methods. The core principle is based on the interference phenomenon of light - when two beams of light with the same frequency and a constant phase difference meet, they will produce an effect of mutual reinforcement (constructive interference) or weakening (destructive interference). By precisely designing the material, thickness and number of layers of the film, engineers can utilize this principle to achieve directional control of light.

Common coating processes include physical vapor deposition (PVD) and chemical vapor deposition (CVD). Among them, the PVD process is the most widely used, covering vacuum evaporation, magnetron sputtering and other methods. It can atomize the coating material and uniformly deposit it on the surface of the substrate in a high vacuum environment, ensuring the purity and uniformity of the film. In contrast, the CVD process generates films through chemical reactions and is more suitable for preparing coatings with special functions. No matter which process is adopted, there are extremely high requirements for environmental cleanliness, temperature control and deposition rate. Even the slightest deviation may lead to the failure of coating performance.

Optical devices that have not undergone coating treatment often have obvious defects. For instance, the surface of ordinary glass reflects approximately 4% to 5% of visible light. For a camera composed of 10 lenses, the loss of reflection alone can prevent over 40% of the light from reaching the sensor, resulting in a dim image accompanied by severe glare. The emergence of optical coating is precisely to solve such problems. It is like putting on a "performance armor" for optical devices, making the propagation and utilization of light more in line with actual needs.

AR Anti-Reflective Coating is the optical coating closest to daily life. Its core function is to reduce the reflected light on the optical surface while maximizing the light transmittance. The myopia glasses we wear, the screens of our mobile phones and the lenses of our cameras almost all rely on its support.

The working principle of AR anti-reflection coating is a typical application of cancellation interference. It is usually composed of single or multi-layer dielectric materials, the most common of which are silicon dioxide, magnesium fluoride, etc. Engineers will precisely control the thickness of the film layer at one quarter of the target light wavelength. When light shines on the surface of the film layer, part of the light will be reflected from the upper surface of the film layer, while the other part will penetrate the film layer and be reflected from the interface between the film layer and the substrate. The optical path difference between these two reflected light beams is exactly half a wavelength, and their phases are completely opposite. When they meet, they will cancel each other out, thereby significantly reducing the reflectivity.

Early AR anti-reflection coatings were mostly single-layer structures and could only achieve anti-reflection effects at specific wavelengths, with limited application ranges. Modern AR anti-reflection films have developed into multi-layer composite structures. By superimposing film layers of different materials and thicknesses, the reflectivity can be reduced to less than 1% and the light transmittance can be increased to over 95% across the entire visible light spectrum (400-700nm). Some high-end AR films also add hydrophobic and oleophobic layers, which can not only reduce reflection but also prevent fingerprints and stains, becoming a "standard" technology for smartphone screens.

The value of AR anti-reflection films is fully demonstrated in various fields. In the photovoltaic industry, the AR film on the surface of solar panels can increase the light transmittance by 5% to 10%, directly translating into an improvement in power generation efficiency, which is of great significance for the utilization of new energy. In the aerospace field, the AR film on aircraft windshields can reduce the interference of sunlight reflection on pilots' vision and enhance flight safety. In medical equipment, the AR film on the endoscope lens enables doctors to obtain clearer internal images, providing a guarantee for precise diagnosis.

Contrary to the "anti-reflection" function of AR anti-reflection Coating, the core role of HR High-Reflective coating is to maximize the reflectivity of optical surfaces, even achieving a reflection effect of over 99%, far exceeding the reflection capacity of ordinary metal mirrors. Therefore, it is widely used in scenarios that require precise light reflection.

The working principle of HR high reflection film is based on reciprocal interference, and its structure is usually an alternating superposition of "high refractive index material + low refractive index material". When light shines on the film layer system, the reflected light from each layer will strengthen each other due to the consistent phase, thus forming an extremely strong reflection effect. The number of film layers directly determines the reflection performance - a film system with about 10 layers can achieve a reflection rate of over 95%, while a high-precision film system with more than 30 layers can exceed a reflection rate of 99.9%. Compared with traditional metal reflective layers such as aluminum film and silver film, HR high reflectivity film not only has a higher reflectivity but also avoids the defects of metal materials such as easy oxidation and absorption of light energy. It performs particularly well in the infrared and ultraviolet bands.

Laser technology is the core application scenario of HR high-reflectivity film. The resonant cavity of the laser requires a pair of high-reflector mirrors to achieve reciprocating reflection and amplification of light. One of the mirrors uses HR high-reflector film to achieve nearly 100% reflection, while the other uses partial reflector film to output the laser beam. Without the precise control of HR high-reflectivity films, lasers cannot form stable energy output, and technologies such as industrial cutting, medical lasers, and lidar would all be out of the question.

In the field of astronomical observation, HR high-reflective films are also indispensable. The primary mirror of the Hubble Space Telescope adopts a multi-layer HR film system, which can efficiently reflect the faint light of distant celestial bodies and help humans capture images of galaxies billions of light years away. In the field of lighting, after the reflector cups of LED lamps are treated with HR coating, they can concentrate the light and emit it in a specific direction, significantly improving lighting efficiency and reducing energy waste. In addition, in equipment such as projectors and stage lighting, HR high-reflectivity films also play a crucial role in guiding light.

If AR films and HR films are the "comprehensive regulation" of light, then filters are the "precise filters" of light - they can selectively allow light of specific wavelengths to pass through while blocking light of other wavelengths, thereby meeting the needs of light signal extraction in different scenarios. According to different screening methods, filters can be classified into absorption type, interference type and polarization type, etc. Among them, interference type filters have become the mainstream in industrial applications due to their high precision and stable performance.

The working principle of interference filters is similar to that of AR and HR films, both based on the interference phenomenon of light, but their structures are more complex. It precisely controls the constructive and destructive interference of light of different wavelengths by superimposing dozens or even hundreds of dielectric film layers, thereby achieving the "release" of target wavelengths and the "interception" of interfering wavelengths. For instance, a common narrowband filter can only allow a specific wavelength (such as 650nm red light) to pass through, with a bandwidth controlled within a few nanometers, which is equivalent to installing "monochromatic glasses" on the optical system.

In the field of digital imaging, filters are at the core of color imaging. The image sensors of smart phones and cameras themselves cannot distinguish different colors of light. This must be achieved through a color filter array (CFA) covering the surface of the sensor - this array is composed of a large number of red, green, and blue filter units, which respectively screen the light of corresponding wavelengths and then synthesize color images through algorithms. In addition, the UV filter commonly used in camera lenses can block ultraviolet rays and prevent the image from having a foggy appearance. Infrared cut-off filters can filter infrared light to ensure the accuracy of color reproduction.

In the field of medical diagnosis, the precise screening ability of filters plays a crucial role. The blood glucose detector can identify the light signal produced by the reaction between glucose in the blood and the test reagent through a filter of a specific wavelength, thereby achieving rapid measurement of blood glucose levels. Fluorescence microscopes use filters to separate excitation light from fluorescence signals, allowing researchers to clearly observe fluorescently labeled substances within cells. In environmental monitoring, gas detection instruments can accurately detect the concentration of pollutants in the air by filtering out the characteristic absorption wavelengths of the target gas through filters. In the field of security, infrared filters can be used in conjunction with night vision cameras to capture clear infrared images in dark environments.

With the development of technology, optical coating technology is moving towards the direction of being "thinner, smarter and more all-round". In the field of flexible electronics, ultra-thin flexible coating technology has achieved a breakthrough and can be applied to the flexible glass of foldable screen mobile phones. It not only maintains anti-transmittance and anti-scratch performance but also can adapt to repeated bending. In the field of intelligent regulation, new products such as electrochromic coating and thermochromic coating have emerged. They can dynamically adjust the light transmittance or reflectance according to external signals and can be applied to scenarios such as smart car Windows and adaptive glasses in the future.

Meanwhile, in extreme environmental applications, the performance of optical coatings is also constantly upgrading. In response to the demands of space exploration, special coatings that are resistant to radiation and high temperatures have been applied to the optical equipment of Mars probes. For deep-sea exploration, the high-pressure resistant and corrosion-resistant coating ensures the stable operation of underwater cameras. In addition, with the development of nanotechnology, new coatings based on graphene and two-dimensional materials are becoming a research hotspot, and are expected to achieve better optical performance and a wider range of application scenarios.

From daily necessities to cutting-edge technology, optical coating, with its precise light regulation ability, has become the core support of modern optical technology. The AR anti-reflection film makes our field of vision clearer, the HR high-reflection film makes the utilization of light more efficient, and the filter makes the extraction of light signals more accurate. With the continuous advancement of technology, these "invisible films" are bound to create value in more fields and provide more powerful tools for humanity to explore the world of light.