Sunday, March 4, 2012

Antireflection Coating

Antireflection coatings are optical coatings that are typically used to decrease an object’s reflectivity. Depending on the exact composition of a given antireflection coating, the extent to which reflectivity can be reduced varies. No matter how antireflective a coating is each antireflective coating is made the same way: alternating layers of materials with opposing refractive index are layered to block certain wavelengths of light and transmit the desired spectrum. Antireflective coatings are typically designed to affect infrared, visible or ultraviolet light, depending on the application, with some coatings blocking a greater range of light than others.

Product Variants: Types of Antireflection Coatings

Typically, antireflection coatings are classified based on the number of layers: single- or multi-layer. A third kind of antireflection coating, called absorbing antireflection coating, is also available.

Key Terms

When considering types of antireflection coating and optical coatings in general, it’s helpful to understand a coating’s relationship to wavelengths of light, as well as various key terms.

Thick-film If a coating is referred to as thick-film, the film is thicker than the wavelength of light hitting the coating. Of course, the effect a coating has on reflectivity will depend not only on the light’s wavelength, but the angle at which light hits the coating’s surface. Typically, thick-film coatings increase reflectivity.

Thin-Film A thin-film antireflection coating is one that features a thickness that is a quarter of the thickness of the wavelength of light. As a result of this ratio, thin-film antireflection coatings reduce reflectivity.

Thin-Film Interference Thin-film interference can occur when the upper and lower layers of an antireflective coating reflect incident light waves. These light waves then interfere with each other and merge to form another light wave. Thin-film interference can sometimes indicate the coating’s thickness or refractive index.

Other Considerations

Other terms, such as broadband, narrowband and dual band, are often used to describe the kind of spectrum of light an antireflective coating is designed to block. For example, some high-efficiency coatings block infrared light. Broadband coatings, on the other hand, reduce reflectivity over a wide range of wavelengths, which enables more light to be transmitted and enhances contrast in some optical applications. Depending on the exact spectrum of light that one is trying to transmit or block, one of these coating options may be appropriate.

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