Anti-Reflective Coatings: Invention Through Current Industry Applications

While materials like glass have the wonderful property of transparency, they also reflect a huge amount of light. This reduces their see-through characteristic. Anti-reflective (AR) coatings, as their name suggests, are thin smooth transparent layers applied on a surface to remove the glare being reflected.

The possibility of anti-reflection is easy to assume. Because, why can’t some substance exist that is completely transparent and absolutely non-reflective? In short, a material that is like a one-way door for light. But the actualization of anti-reflection is far trickier than we might expect. In fact there aren’t any such ‘special’ materials. Anti-reflection is a consequence of light’s wavelike behavior and the geometry of the mediums it passes through.

There are a variety of materials used as AR coating, each one suited to best block reflected light in specific ranges of the electromagnetic spectrum. The shorter the incident wavelengths get, the thinner the layer needs to be to block these reflections. UV-AR is a popular layering material for blocking UV rays, which requires thinner layering at about 5-10 nm.

Below is a diagram showing the different materials and the range over which they work for blocking reflected light.

AR coating materials by their respective wavelength range. Courtesy of Edmund Optics

AR in Cutting Edge Optics

Research with lasers demands a high level of precision. Modern laser experiments work with complicated set-ups, using multiple lenses, beam splitters and mirrors. Sometimes a single beam deviates multiple times before it reaches the detector. All such optical devices in its path must have specialized AR coating to be resistant to temperature and pressure differences. On the other hand, lasers typically operate around a single wavelength, which does simplify the coating process a little.

Current astronomy pushes the limits of detecting faint radiation. Vital information is lost if radiation is reflected. Additionally, it also produces unwanted noise due to scattering. Reflected rays from telescope lenses produce unwanted stray light as they bounce around in the interior. This lowers the quality of the observation. Some convex lenses have multiple AR layers that can bring reflection down to 0.01%. Accordingly, the process is complex and expensive. However there is the additional benefit of anti-reflection at angles outside normal incidence.

Additionally, more and more astronomy is focused around individual wavelengths instead of a range of them. As in the case with lasers, AR coating process simplifies when targeting limited wavelengths.

AR in Consumer Products

In our era of online social networks, popularity in photography has greatly increased. And camera companies are bringing in all sorts of tech for their digital and smartphone products. All camera lenses have AR coatings, making cameras and smartphones the biggest market for the AR tech. That is, any picture involving incoming light such as during sunrises, sunsets, or street lights at night, would contain unwanted effects. AR coated camera lenses can produce far better contrast ratios by removing such noise captured by the sensor.

Opticians no longer view AR coating in corrective lenses as a luxury. In some countries like Japan, there is an 80% market penetration. Indeed, it is definitely a major draw to its customers. Reducing the glint and making the eyes more visible to others is a must. Especially in our times of selfies and video chats. But the viewers themselves also benefit as they perceive less peripheral glare. Moreover, this greatly reduces strain. And viewers are less likely to remove their specs.

With ever-improving processes in manufacturing, AR coating is finding its way into any product that has some transparent sheet. Indeed, most modern cars have AR coating on their headlights. Switched on lights no longer produce halos around them. Watches, interior windows, glass cabinets, projector lamps, all have AR coatings as an option for those who can afford.