Film stack

Design

Optical thin film interference coatings rely on the difference in refractive index of two or more materials to produce interference effects within a multilayer structure to modify the transmission and reflection spectra of optical components.

The most common example optical thin films are antireflection coatings designed to reduce reflection from the surface of glass. These antireflection coatings are now common place for spectacles, but the earliest antireflection coatings were to be found in optical equipment such as binoculars, microscopes and telescopes, often consisting of single layers of low index material, cryolite or magnesium fluoride. Today, most AR coatings consist of multiple layers of different materials whose interference effects create much lower reflection over a greater bandwidth.

Other common thin film coatings rely on an alternate structure of high (H) and low (L) index materials to produce coloured filters, reflectors, narrow bandpass filters to isolate narrow portion of the spectrum. In all of these, the spectral characteristics depend on the number of layers, the refractive indices and the layer sequence.

The design of thin films requires many complex calculations, and many thin film design programmes have been written to facilitate this, although a thorough understanding of the principles involved is a key factor. As important is the ability to manufacture filters that have been designed, so an intimate knowledge of the optical and mechanical properties of manufactured films from any given system is mandatory.


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