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Mirror, Mirror on the Wall…..Optical Coatings

September 10, 2020

It all started in the 1800’s. (Optical coatings have been around for a while.) Scientists of that century developed chemical processes for depositing silver on either the front or back surface of a piece of glass. “Silvering” the back of the glass improved the quality of household mirrors. Silvering the front of the glass allowed for vast improvements in telescopes and other optical equipment.

These techniques seem primitive compared to the performance enhancing optical coating technologies available today, a broad selection of coatings that can reflect, absorb, enhance transmission, split or filter light. Modern coating techniques utilize thermal evaporators, electron beam gun evaporators, magnetron sputtering, DLC or ion beam sputtering. Modern real-time, insitu process monitoring of thin film deposition is far more precise and affords greater control of the thickness and uniformity of the coating.

Anti-Reflective Coatings

An anti-reflective (AR) coating reduces reflection at each coated surface, increasing the amount of light that passes through an optical substrate. In corrective lenses, the anti-reflective coating reduces reflection and glare from glasses making it easier to see details such as facial features through the glasses. This particular coating is also important in any light-starved application that relies on light to create an image. Because less light is reflected off each surface and therefore more light travels through the optic, those covered with an anti-reflective coating optimize transmission and produce sharper images in low-light settings. Anti-reflective coatings also improve the ability of telescopes to capture images of distant galaxies.

While magnesium fluoride and zinc selenide are two of the most common anti-reflective coatings, there are dozens of more exotic elements that are used in AR coatings. These compounds form a dielectric coating that has a refractive index that is different from the glass substrate. Using Snells law, coatings can be customized by selecting coating materials based on their refractive indices and then alternating indices and layer thicknesses.  By adjusting the evaporated materials and layer thickness, technicians can reduce the amount of reflection to close to 0.002% per surface in ideal situations.

High-Reflective Coatings

When an application requires light to bounce off a surface (reflection), a high-reflective coating comes into play. In the visible light spectrum, this would be a mirror. However, the thickness and type of coating will determine how well the surface reflects the light. Technicians can manufacture dielectric mirrors by coating a substrate with layers of substances with different refractive indices. Some optics labs prefer dielectric mirrors because compounds such as magnesium fluoride and silicon dioxide are much less expensive than their metal counterparts. Advanced coating techniques still allow for excellent reflection.

However, there are some applications where traditional metals are the only appropriate coating. The three most common metals for this use are aluminum, silver and gold. As we discussed, the world has appreciated silver’s reflective properties for centuries, as one of the first materials used for coating mirrored glass. Gold, one of the most expensive materials for this use, is the metal of choice when reflecting light in the infrared spectrum. Aluminum is popular due to its lower price. However, aluminum has a slightly lower reflection rate than silver.

Complex Coatings

While many applications require a coating that is as reflective or anti-reflective as possible, there are times when it is necessary for a coating with both properties. A one-way mirror is a familiar example from television crime shows. This type of mirror uses a thin coating of aluminum that reflects some light while the rest passes through. In a typical scenario, the lights are on in one room and off in the other. People in the darkened room can see through the glass. Those sitting in the other room see a mirrored surface as the light is reflected back at them.

A one-way mirror is a simple example of a complex science. Using a variety of coating materials, mirrors can reflect different wavelengths of light while letting others pass through. In laser technology, a beam splitter is a coated glass plate that can both reflect and transmit light depending on the angle and wavelength of the beam. There is often a second anti-reflective coating to prevent unwanted interference.

Optical filters are devices that allow a limited spectrum of wavelengths to pass while reflecting the rest. They are fabricated using a multi-layered coating process. For applications such as spectroscopy and chemical analysis, this is critical technology that requires advanced coating techniques.

Optical Coating Techniques

For use in advanced optical devices, obtaining the necessary coating thickness is a challenge. Most manufacturers create precision coatings using some form of physical vapor deposition (PVD). Every PVD technique can provide a thin coat. However, some techniques are more precise than others.

In PVD, the coating material begins in a solid state. It is then forced into a vapor form using high-voltage electricity, high heat or another high-power source. The vapor then coats the substrate where it returns to a solid state. Such PVD techniques need special equipment and facilities. For example, several PVD techniques require a vacuum chamber.

Ion beam sputtering is a PVD technique that provides a high level of precision. It works well with both metal and dielectric coatings. The target and substrate sit close to one another in the sputtering device. An ion beam shoots at the target, and the sputtered material deposits on the substrate.

The great advantage of this technique is the amount of control it allows. Other PVD techniques can have imperfections that lower the quality of the coating. In a sputtering device, the technician can control the sputtering rate and ion energy to provide a uniform coating of predetermined thickness. This technique lends itself to the manufacturing of high-precision optical devices.

A supplementary technique creates an even more precise film. Ion-assisted deposition enhances ion beam sputtering with a second ion beam that points directly at the substrate. The secondary beam knocks out incorrectly placed molecules. Using this technique will produce thin-film optical coatings of an even higher quality.

Choosing the Right Thin-Film Optical Coating

With current deposition technology, there are many options available for thin-film optical coatings. Technicians can design coatings that reflect or transmit light of almost any wavelength. Before placing an order, the customer must consider the necessary level of precision for their coating. For instance, if a surface does not require the highest level of reflectivity, a less-expensive aluminum coating may be preferable to silver or gold. In the same way, it is less expensive to filter a broad spectrum of light than a single wavelength in dielectric coatings.

IRD’s approach is to provide customers with unlimited access to our technical people, “engineer to engineer”, to produce a design that not only meets performance requirements but does so at the lowest lifetime cost and most consistent quality.

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