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.

First, let’s clarify one point: sapphire cannot properly be referred to as glass. Glass refers to a transparent, molten substance based on silicon dioxide that solidifies upon cooling. To meet various requirements, different elements are added internally, but the primary component remains silicon dioxide. Sapphire, on the other hand, is a crystalline material—a single-crystal structure composed of aluminum oxide. Because sapphire becomes transparent after polishing, some people also refer to it as sapphire glass.

Sapphire glass—contrary to our initial understanding—is not a naturally occurring sapphire. Rather, it’s a synthetic material produced in laboratories that boasts the same chemical composition and physical properties as natural sapphire. As a result, its price has dropped significantly, no longer commanding the exorbitant costs once associated with rare collectibles. Today, sapphire glass is increasingly being incorporated into a wide array of applications. It exhibits excellent thermal properties, outstanding electrical and dielectric characteristics, and exceptional resistance to chemical corrosion. Moreover, it withstands high temperatures, conducts heat efficiently, possesses high hardness, is transparent to infrared light, and demonstrates remarkable chemical stability. For these reasons, sapphire glass is frequently used as a substitute for other optical materials in the fabrication of optical components and infrared-transparent optical windows. It finds extensive use in military equipment operating in the infrared and far-infrared spectrums—for instance, in night-vision infrared and far-infrared sights, night-vision cameras, and other instruments, as well as in satellite and space technology instrumentation. Additionally, it serves as window material for high-power lasers, various optical prisms, optical windows, UV and IR windows and lenses, and observation ports for low-temperature experiments. It is also widely employed in instrumentation for marine, aerospace, and aviation applications. Furthermore, this material is commonly found in the screens of familiar mobile phones and in the watch faces of the timepieces we wear.

Quartz glass is a special industrial technical glass made by melting various pure natural quartz materials (such as crystal and quartz sand). It is composed solely of silicon dioxide. This type of glass has a hardness that can reach up to Mohs’ seven, and it exhibits excellent properties including high-temperature resistance, a low coefficient of thermal expansion, resistance to thermal shock, chemical stability, and electrical insulation. Moreover, it can transmit both ultraviolet and infrared light. With the exception of hydrofluoric acid and hot phosphoric acid, it demonstrates good resistance to most common acids. Based on transparency, quartz glass is categorized into two main types: transparent and opaque. According to purity, it is further divided into three grades: high-purity, standard, and doped. It is produced from raw materials such as crystal, silica, and silicates through high-temperature melting or chemical vapor deposition. Melting methods include electric melting and gas refining. Quartz glass has an extremely low linear thermal expansion coefficient—only about one-tenth to one-twentieth of that of ordinary glass—giving it outstanding resistance to thermal shock. It boasts exceptional heat resistance, with typical operating temperatures ranging from 1100°C to 1200°C, and can withstand short-term exposure up to 1400°C.

Quartz glass is primarily used in laboratory equipment and in refining devices for special high-purity products. Due to its high spectral transmittance and its resistance to damage from radiation—while other glasses darken when exposed to radiation—quartz glass is also an ideal material for spacecraft, wind-tunnel windows, and the optical systems of spectrophotometers.

K9 glass is a type of glass made from K9 material, used in fields such as optical coating. K9 material is classified as optical glass. Due to its crystal-clear transparency, many factories have emerged that specialize in processing K9 material. The products they manufacture are commonly referred to as crystal glass items in the market.

The composition of K9 is as follows:

SiO2 = 69.13% B2O3 = 10.75% BaO = 3.07% Na2O = 10.40% K2O = 6.29% As2O3 = 0.36%

Its optical constants are: refractive index = 1.51630, dispersion = 0.00806, Abbe number = 64.06.

The national standard for optical glass classifies glasses according to their Abbe number. Glasses with an Abbe number ≥50 are designated as crown glasses and represented by the letter “K”; glasses with an Abbe number <50 are designated as flint glasses and represented by the letter “F”. Under these two main categories, further subdivisions are made using light “Q”, heavy “Z”, extra-heavy “T”, and chemical element symbols preceded by a prefix and followed by a suffix. In total, there are 18 major categories and 141 specific grades.

For example: BaK11 (barium crown) K9 (crown)

Generally speaking, crown glass belongs to the alkali-silicate system, while the vast majority of flint glass belongs to the lead-silicate system.

According to the spectral characteristics of filters, filters can be divided into six categories:

Bandpass filters, cutoff filters, spectroscopic filters, neutral-density filters, reflective filters, and notch filters (negative filters);

Bandpass type: Light within the selected wavelength band passes through, while light outside the passband is blocked. Its optical specifications primarily include the center wavelength (CWL) and full-width at half-maximum (FWHM). It is categorized into narrowband and broadband types.

Shortwave-pass (also known as low-wave-pass): Light with wavelengths shorter than the selected wavelength passes through, while light with wavelengths longer than that wavelength is blocked.

Long-pass filter (also known as high-pass filter): Allows light with wavelengths longer than a selected wavelength to pass through, while blocking light with wavelengths shorter than that selected wavelength.

Biochemical analysis is one of the frequently used diagnostic methods in clinical practice. It involves analyzing blood or other body fluids to measure various biochemical indicators, such as transaminases, hemoglobin, cholesterol, creatinine, and glucose. By integrating these results with other clinical data for comprehensive analysis, it can help diagnose diseases, evaluate organ function, and establish benchmarks for future treatment decisions.

Our company has developed a series of biochemical analyzers and specialized filters for enzyme-linked immunosorbent assay (ELISA) readers. These filters are primarily used in biochemical analyzers, ELISA readers, and other biochemical analysis applications. The biochemical filters feature a hard-coated film with stable spectral performance and no drift, and their products comply with national standards.

The main filters include: 340 nm narrowband filter, 405 nm narrowband filter, 420 nm narrowband filter, 450 nm narrowband filter, 492 nm narrowband filter, 505 nm narrowband filter, 510 nm narrowband filter, 546 nm narrowband filter, 578 nm narrowband filter, 610 nm narrowband filter, 630 nm narrowband filter, and 650 nm narrowband filter.

Filters are classified according to the length of the spectrum (i.e., the spectral region in which they operate) as follows: ultraviolet filters, visible-light filters, near-infrared filters, mid-infrared filters, and far-infrared filters.

Spectral wavelength range:

UV filter 1–380 nm

Visible light filter, 380–780 nm

Near-infrared filter, 780–1500 nm

Infrared filter, 1500 nm – over 10 μm