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Thin coatings are widely used to improve the transmittance of a window by reducing reflection and to reject unwanted optical or radio frequency wavelengths. Protective coatings endure rain and particle impacts and provide scratch resistance. Occasionally a thin coating can enhance mechanical strength. This chapter describes the use of coatings to reduce reflection and to reject radio and microwave frequencies. Coatings to protect against rain and sand erosion are discussed in the next chapter.
Whenever light encounters an interface at which the refractive index changes, partial reflection occurs. In Table 1.3 we saw that approximately 4% reflection occurs at the interface between air and glass. Reflection from a zinc sulfide surface is 14%, while that from germanium is 36%. Since light entering almost any optical system encounters a series of interfaces, it is imperative that reflection be reduced at each interface; otherwise, the light will be attenuated to almost nothing.
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