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What is optical coating?

Optical coating refers to the process of depositing one or more thin films—either metallic or dielectric—onto the surface of optical components. The purpose of coating the surfaces of optical components is to achieve specific optical effects, such as reducing or enhancing light reflection, beam splitting, color separation, filtering, and polarization. Commonly used coating techniques include vacuum coating (a type of physical coating) and chemical coating. By employing physical or chemical methods, a transparent dielectric film or a metallic film is deposited onto the surface of a material, with the aim of altering the material's reflection and transmission characteristics.

The main optical thin-film devices include reflective coatings, anti-reflection coatings, polarizing films, interference filters, and beam splitters, among others. These devices have found extensive applications in the national economy and national defense construction, and are receiving increasing attention from scientific and technological researchers. For example, the use of anti-reflection coatings can reduce light loss in complex optical lenses by as much as a factor of ten; high-reflectivity mirrors can increase the output power of lasers several times over; and optical thin films can enhance both the efficiency and stability of silicon photovoltaic cells.

I. Structure

The simplest model of an optical thin film is a uniform, isotropic medium with a smooth surface. In this case, the optical properties of the thin film can be studied using the theory of light interference. When a monochromatic plane wave is incident on an optical thin film, multiple reflections and refractions occur at its two surfaces. The directions of the reflected and refracted light are determined by the laws of reflection and refraction, while the amplitudes of the reflected and refracted light are specified by Fresnel’s formulas (see Refraction and Reflection of Light at Interfaces).

II. Characteristics

Optical thin films are characterized by their smooth surfaces, geometrically defined interfaces between film layers, and refractive indices that can undergo abrupt changes at these interfaces yet remain continuous within each individual layer. These films can be either transparent media or optical coatings.

Absorbing media can be either normally homogeneous or normally inhomogeneous. In practical applications, thin films are far more complex than idealized ones. This is because, during fabrication, the optical and physical properties of thin films deviate from those of the bulk material; their surfaces and interfaces are rough, leading to diffuse scattering of light beams. Moreover, mutual penetration between film layers gives rise to diffusion interfaces. Due to factors such as film growth, structural characteristics, and stress, thin films exhibit anisotropy, and their layers display complex time-dependent effects.

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Champion Optics is a high-tech enterprise integrating the research and development of optical thin films, the development of new spectral instruments, and spectral image processing and application.

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