Whenever working with lenses and mirrors, adjusting their properties in terms of transmitting and reflecting light is something we often need. Easy to assume, we can achieve this result by applying different types of optical coatings on the filtering environments. Inside this article, we will discuss about all these options, in general, and dielectric coatings, in particular.
As already suggested, the coating process is meant to diminish the reflections from any given surface. Before we get to explain how we do it, we should, however, take some time and understand the reflection process. Considering that the light itself comes as a wave, whenever it hits a surface, part of the wave is sent back from where it came, on a different angle, and part of it will surface the new environment. The reflection phenomenon, from the glass or lens’s surface, manifests because of the difference of refractive indices in between the two environments: the one through which the light has traveled and the one that the light must now go through.
Even people with little knowledge on optics have heard about antireflection properties on eyeglasses or photo camera lenses. Even so, the applicability of this process is incredibly wider and specialists can achieve it through several different methods. In a few words, optical coatings consist of metals, dielectric materials and antireflection materials. Thin layers from one of these are applied to the lenses and, in consequence, whenever the light hits the lens, an opposite wave created by the thin layer will block the wave of light that tries to bounce back. And this is how they prevent reflection.
Aluminum, silver and gold are some of the most accessible options in terms of metal coating, only that the first one is also the cheapest. Their degree of antireflection yield varies from 88 – 92 % for aluminum, 95 – 99% for silver and 98 – 99% for gold. Different densities and thicknesses of the metal layer allow us to play with the degrees of transmission and reflectivity. If you are familiar with the term of a one way mirror, you would like to know that this is how they get it!
Back to one of the most popular types of coating, dielectric coating consists from layers of metal oxides or fluorides. We can use more than one layer of substance, yet the most important aspect is to choose something with a different index of refraction than the substrate has. The AR coating resulted from here can be so efficient that it cuts off the process under 0.2% of the coming light!
Needless to insist on the benefits that derive from here, we can state that this strategy is among the most popular ones for its versatility. Optical microscopes and lasers, interferometers and refracting telescopes, photographic lenses, spectacles, binoculars and many more take advantage of the tiny dielectric layers.
The next stage after these two would be a combination of dielectrics and metals, which will result into coatings that are much more professional. If you really want to understand the way that these processes work, discussing with a specialist will sort things out.