Optical Coating Design & Development
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An optical coating is one or more thin layers of material deposited on an optical component or substrate such as a lens or mirror, which alters the way in which the optic reflects and transmits light. A popular type of optical coating is an antireflection (AR) coating, which reduces unwanted reflections from surfaces, and is commonly used on spectacles, sunglasses and photographic lenses. Another type is the high-reflector coating which can be used to produce mirrors reflecting more than 99.99% of the light falling on them. Yet, more complex optical coatings exhibit high reflection over some wavelength range, and anti-reflection over another range, which can be used in the production of dichroic thin-film optical filters.
The simplest optical coatings are thin layers of metals, such as aluminium, which are deposited on glass substrates to make mirror surfaces. The metal used determines the reflection characteristics of the mirror; aluminium is cheapest and most common coating, and yields a reflectivity of around 88%-92% over the visible spectrum. More expensive is silver, which has a reflectivity of 95%-99% even into the far infrared, but suffers from decreasing reflectivity (<90%) in the blue and ultraviolet spectral regions. Most expensive is gold, which gives excellent (98%-99%) reflectivity throughout the infrared, but limited reflectivity at wavelengths shorter than 550 nm, resulting in the typical gold colour.
By controlling the thickness and density of metal coatings, it is possible to decrease the reflectivity and increase the transmission of the optical surface, resulting in a half-silvered mirror. These are sometimes used as "one-way mirrors".
The other major type of optical coating is the dielectric coating (i.e. using materials with a different refractive index to the substrate). These are constructed from thin layers of materials such as magnesium fluoride, calcium fluoride, and various metal oxides, which are deposited onto the optical substrate. By careful choice of the exact composition, thickness, and number of these layers, it is possible to tailor the reflectivity and transmissivity of the coating to produce almost any desired characteristic. Reflection coefficients of surfaces less than 0.2% can be achieved, producing an antireflection (AR) coating. Conversely, the reflectivity can be increased to greater than 99.99%, producing a high-reflector (HR) coating. The level of reflectivity can also be tuned to any particular value, for instance to produce a mirror that reflects 80% and transmits 90% of the light that falls on it, over some range of wavelengths. Such mirrors can be called beamsplitters, and used as output couplers in lasers. Alternatively, the coating can be designed in such a way that the mirror reflects light only in a narrow band of wavelengths, producing an optical filter.
The versatility of dielectric coatings leads to their use in many scientific and industrial optical instruments (such as lasers, optical microscopes, refracting telescopes, and interferometers) as well as consumer devices such as binoculars, spectacles, and photographic lenses.
Dielectric layers are frequently applied on top of metal films, either to provide a protective layer (as in silicon dioxide over aluminium), or to enhance the reflectivity of the metal film. Metal and dielectric combinations are also used to make advanced coatings that cannot be made any other way. One example is the so-called "perfect mirror", which exhibits high (but not perfect) reflection, with unusually low sensitivity to wavelength, angle, and polarization.
Designing of optical coatings requires specialized expertise and experience. There are a number of software programs being used by our optical coating designers. For any projects involving the design, testing, troubleshooting or research & development of coatings, contact us and our World class optical coating designers will help you.