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Chapter 8:
Coatings That Contain Absorbing Layers
Every effort is made to minimize, if not eliminate, absorption in many of the multilayers described previously. § demonstrates, for example, that the performance of the multilayer reflector is degraded by absorption. There are numerous instances in which the absorption of metals and other layers is a welcomed effect. Quite often the absorption of a bandpass filter is not undesirable. For example, if the bandpass filter rejects radiation by reflecting it, the reflected flux is liable to scatter from baffles in the optical system and thence leak through the bandpass filter, especially at high incidence angles. An absorbing coating (such as the type mentioned in §8.5) is coated on each baffle that reduces its reflectances to small values. The performance of the coating is usually superior to the black lacquer often applied to visual optics. The IR reflectance of such coatings is quite small. Another advantage is that they do not outgas in a vacuum, as does the organic lacquer. Selective absorbers are used extensively in photothermal collection systems.
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