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Anti-reflective Technology: Features and Benefits Explained

January 01, 2018

This short article explains AR technology and the features and benefits of these coatings on watch crystals. We hope this short write-up is helpful and can help in your understanding towards its technology and functions.

What are AR coatings anyway?
AR (Anti-Reflective) coatings are nanometer (billionth of a meter) thick layers of metal oxide materials that are applied, generally, to both sides of the crystal substrate. The materials include titanium or zirconium oxides. The outside layer is silicon dioxide. Each surface, front and back, of the substrate will have at minimum three metal oxide layers applied. 

How are the coatings applied?
The coatings are most often applied using a vacuum deposition system. Targets of the metal oxide are vaporized in a vacuum chamber. Since the crystal has an opposite electrical charge from the target material, they attract each other and bond to the crystal surface in a very homogeneous and uniform thickness. Each metal oxide layer is applied separately. Each layer has a different index of refraction and hence each bends the light differently creating the desired optical interference. One wavelength of light comes along and cancels out another. 

With AR coatings, reflection is reduced to <1% and light transmission is increased to +98% of available light. If there is no AR treatment, 92% of available ambient light is transmitted through the crystal, 8% having been reflected. Loss of light transmission reduces the ability to see with clarity the dial’s colors and textures. 

What are AR coatings designed to do?
AR coatings are designed to reduce light reflection (increase transmission) by optical interference. The principal is the same as in noise canceling headphones. As stated, the AR coatings bend incoming light wavelengths and eventually one wavelength cancels out another. Also, reflection is reduced to < .5% per side and transmission is increased to over + 98% depending on the substrate and its thickness. 

What is the major benefit of AR coated crystals? 
The major benefit of AR coating on a watch crystal is increased light transmission. With more ambient light striking the dial, more color and texture is visible. 

Why do I sometimes see more reflection from the dial? 
AR coatings are angle dependent. The maximum reduction in reflected light is at a 90-degree viewing angle. At an acute viewing angle, the % R increases.

Why coat both sides of the crystal?
Coating only one side of the crystal substrate is half as effective as coating both sides. If you only coated the backside of the crystal next to the dial, you would get ~ 4% reflection off the front side plus ~ .5% on the backside. The total reflection would be high at +4.5% vs. the goal of < 1% total reflection. With increased %R, the %T is reduced and you lose the real benefit of clarity in colors and textures. Two sided coatings are, therefore, optimal for minimal reflection and to obtain maximum light transmission onto the watch dial. The small negative is that the AR coatings are softer than the crystal itself and can scratch. With normal care, scratching can be negligible. It is also good to note that while sapphire crystal is scratch-resistant, it is by no means scratch-proof. 

P.S. All Tyche timepieces are applied with 3 layers of coating, and not just only to the inside surface of the crystal, as with most manufacturers. This allows our customers to be able to look good telling the time even under a very glaring sun. Living in sunny Singapore, this has always been a big problem for us, which is why we decided to have a good anti-reflective coating on all our watches. 

Hopefully this brief article has helped you understand more on the subject.

Cheers,
Darren






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