Reflection, transmission and coatings

When light encounters a surface, a fraction of the beam is reflected, another fraction is refracted (transmitted) and the rest is absorbed by the material. In lens design, we must achieve the best transmission while minimizing reflection and absorption. While absorption is usually negligible, reflection can be a real problem: the beam is in fact not only reflected when entering the lens (air-glass boundary) but also when it exits the lens (glass-air). Let’s suppose that each surface reflects 3% of incoming light: in this case, a two lenses system has an overall loss of 3*3*3*3 % = 81%. Optical coatings – one or more thin layers of material deposited on the lens surface – are the typical solution: a few microns of material can dramatically improve image quality, lowering reflection and improving transmission.

Transmission depends considerably on the light wavelength: different kind of glasses and coatings helps to improve performance in particular spectral regions, e.g. UV or IR. Generally, good transmission in the UV region is more difficult to achieve.

Percent transmittence of different  kind of glasses.
Percent transmittence of different kind of glasses.

Anti-reflective (AR) coatings are thin films applied to surfaces to reduce their reflectivity through optical interference. An AR coating typically consists of a carefully constructed stack of thin layers with different refractive indices. The internal reflections of these layers interfere with each other so that a wave peak and a wave trough come together and extinction occurs, leading to an overall reflectance that is lower than that of the bare substrate surface.

Anti-reflection coatings are included on most refractive optics and are used to maximize throughput and reduce ghosting. Perhaps the simplest, most common anti-reflective coating consists of a single layer of Magnesium Fluoride (MgF2), which has a very low refractive index (approx. 1.38 at 550 nm).

Hard carbon anti-reflective HCAR coating: HCAR is an optical coating commonly applied to Silicon and Germanium designed to meet the needs of those applications where optical elements are exposed to harsh environments, such as military vehicles and outdoor thermal cameras.

This coating offers highly protective properties coupled with good anti-reflective performance, protecting the outer optical surfaces from high velocity airborne particles, seawater, engine fuel and oils, high humidity, improper handling, etc.. It offers great resistance to abrasion, salts, acids, alkalis, and oil.

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