Optical coatings are composed of thin film layers which produce interference effects to change transmission or reflection properties of optical systems. The performance of optical coatings depends on the number of layers, the thickness of each layer and the refractive index between different layers.
The common types of coatings in precision optics are antireflection film, the high reflection film, spectroscopic film, and filter film. The antireflection film is suitable for most refractive optical components and can increase the optical flux and reduce unnecessary reflection; the high reflection film can provide the maximum reflection in a single wavelength or in a certain wavelength range, which is mostly used in speculums; the spectroscopic film is used to divide incident light into transmitted light and reflected light output; filter plate coatings are suitable for a wide range of life sciences and medical applications, which can transmit, reflect, absorb or attenuate light at a specific wavelength.
Optical coatings are usually applied to specific incident angles and specific polarized light, such as S polarization, P polarization, or random polarization. If the light Angle of the coating is different from that of the design, the performance will be significantly reduced; if the deviation between the incident Angle and the designed incident Angle is very large, the coating function may be completely lost. Similarly, using polarized light that is different from designing polarized light can produce wrong results.
Optical coatings are made from deposited dielectric and metallic materials such as tantalum pentoxide (Ta2O5) or alumina (Al2O3) in alternating film layers. Usually, coatings have the one-fourth wavelength optical thickness (QWOT) or half-wave optical thickness (HWOT) in order to maximize or minimize interference in applications. These films are composed of the high refractive index and low refractive index layer alternately to induce interference effect.
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