where p is the ratio of the refractive indices of the two materials
(n1/n2). Intensity is the square of this expression.
The amount of reflected light is therefore larger when the disparity between the two refractive indices is greater. For an
air/glass interface with the glass having a refractive index of 1.5, the intensity of the reflected light will be 4% of the
incident light. For an optical system containing ten such surfaces (e.g., five optical elements), the transmitted beam will be
attenuated by 34% from reflection losses alone. To reduce the reflections to an acceptable level, antireflection (AR) coatings
can be applied to the interface surfaces. The theoretical basis for these coatings can be found in the subchapter
The Reflection of Light.
There are three basic types of AR coatings in general use: single-layer coatings, multilayer broadband dielectric coatings, and
multilayer narrow-band (V) coatings. Magnesium fluoride (MgF2) is the most common material used for single-layer AR coatings. A
properly designed coating can reduce reflections to between 1% to 2% per surface for normal incidence. Broadband multilayer
dielectric coatings, of which the Melles Griot HEBBARTM coatings are excellent examples, reduce average
reflectivity to approximately 0.5% per surface. Narrow-band V coatings provide the best performance, typically less than 0.25%,
but only at a specific design wavelength.
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