Optical coatings sometimes consist of one layer, but in most cases consist of multiple thin layers of material deposited on an optical element to change the manner in which it reflects, transmits, or absorbs light.
Without optical coatings, applications would be severely hindered. Mirrors must have high reflectance, lenses require high transmittance, and filters and beam dividers require controlled transmission and reflection over specific spectral bands. Some applications require high absorption.
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