How Optical Coatings Work
Whenever light passes from one medium into another medium with different optical properties, part of the light is reflected and part transmitted. The intensity ratio of the reflected and refracted components is a function mainly of the difference in refractive index among the materials, the polarization of the incident light, and the angle of incidence. Optical coatings are used to alter the reflectance, transmittance, absorption, or polarizing properties of optical components. The optic being coated is called the "substrate". The coating is deposited in a high vacuum, using a process of evaporation on the substrate. Coating materials include metals (Au, Al, Ag, Ni-Cr, Cr, etc.), dieletrics (Oxides, Fluorides and Sulfides) and semiconductors (Si, Ge). In application, it requires the reflection ratio of incident light to approach 0% for transmitting optics such as lenses and 100% for reflective optics such as mirrors, or some different percentage for partial reflectors such as beamsplitters. For certain applications there is a need to accurately control the reflection/transmission ratio within specific spectral ranges; this is done with antireflective, high-reflective and partial reflective coatings using multilayer dieletric coatings. This is how nebula filters work.
Ref: web site of Casix Corp. (an optical coatings firm)