An optical coating is composed of a combination of thin film layers that create interference effects used to enhance transmission or reflection properties within an optical system. The performance of an optical coating is dependent upon the number of layers, the thickness of the individual layers and the refractive index difference at the layer interfaces. The most common types of coatings used on precision optics are: Anti-Reflection (AR) Coatings, High Reflective (Mirror) Coatings, Beamsplitter coatings and Filter coatings. Anti-reflection coatings are included on most refractive optics and are used to maximize throughput and reduce ghosting. High Reflective coatings are designed to maximize reflectance at either a single wavelength or across a broad range of wavelengths. Beamsplitter coatings are used to divide incident light into known transmitted and reflected light outputs. Filters are found in a large number of industry applications and are used to transmit, reflect, absorb, or attenuate light at specific wavelengths. Edmund Optics can also offer a variety of Custom Coatings to meet any application need.
Figure 1: Sample Three Layer BBAR Coating Design [View Larger Image]
Optical Coatings are designed for a specific incident angle of light and for a specific polarization of light such as S-polarized, P-polarized, or random polarization. If a coating is designed for light at a 0° angle of incidence, but is used with light at a 45° angle of incidence, the coating will not perform at the stated transmission/reflection specifications. Similarly, coatings are generally designed for randomly polarized light so using S-polarized or P-polarized light with a coating designed for randomly polarized light will again fail to produce the stated specifications.
Optical Coatings are created by depositing dielectric and metallic materials such Ta2O5 and/or Al2O3 in thin layers that are typically quarter-wave optical thickness (QWOT) or halfwave optical thickness (HWOT) the wavelength of the light used in the application. These thin films alternate between layers of high index of refraction, and low index of refraction, thereby inducing the interference effects needed. Refer to Figure 1 for a sample illustration of a broadband anti-reflection coating design.
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