Here are some considerations for AR Coating:
Feel free to add to the list.
As far as the explanation, I found this very informative write-up online:
AR Coatings on Watch Crystals
An Explanation and FAQ’s
What are AR coatings?
AR 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. Some manufactures apply the coating only to the inside surface of the crystal.
How are the coatings applied?
The coatings most often are 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. The side benefit is reduced reflection that, if not reduced, is annoying. In a sense the term AR, anti-reflective, is the opposite of the real benefit of increased transmission. Perhaps the technology should be called IT, increased transmission. You get both benefits, but higher transmission, in my opinion, is the greater good.
Why is there a bluish color of reflection with AR coated crystals?
The color of reflection can be controlled in the optical system. Blue light is the color of reflected light chosen by the optical engineers to be less garish than, let’s say, red. The thickness of the layers determines the color of reflection. Optical engineers can design the optical system to reflect any color, but blue is often chosen as being less intense and its complimentary color of transmission is a light orange /yellow that is less noticeable and intrusive. That is, the coatings on watch crystals transmit a light yellow (kind of like incandescent light) because it’s complementary blue color is being reflected / taken out of the visible color. You could have a violet / red color of reflection and a slight greenish color of transmission, but that would be a not be a great system for a watch crystal.
Why do I sometime see a different color of reflection from the crystal? Why is the transmitted color striking the dial sometimes different?
The color of reflection is angle dependent and at an angle the color and intensity of the reflected light shifts slightly to violet and the color of transmission changes with it. In addition, the color of the ambient light affects what you see. Daylight has a much different Kelvin temp than incandescent or fluorescent light; that too affects what is reflected from and transmitted to the dial.
Why do I sometime see more reflection from the dial?
AR coatings are angle dependent as noted. 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 cheaper to replace the whole crystal than to have an existing one re-coated.
- Do you like how it makes the watch look?
- Do you want it to match the gen?
- Am I worried about scratches?
Feel free to add to the list.
As far as the explanation, I found this very informative write-up online:
AR Coatings on Watch Crystals
An Explanation and FAQ’s
What are AR coatings?
AR 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. Some manufactures apply the coating only to the inside surface of the crystal.
How are the coatings applied?
The coatings most often are 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. The side benefit is reduced reflection that, if not reduced, is annoying. In a sense the term AR, anti-reflective, is the opposite of the real benefit of increased transmission. Perhaps the technology should be called IT, increased transmission. You get both benefits, but higher transmission, in my opinion, is the greater good.
Why is there a bluish color of reflection with AR coated crystals?
The color of reflection can be controlled in the optical system. Blue light is the color of reflected light chosen by the optical engineers to be less garish than, let’s say, red. The thickness of the layers determines the color of reflection. Optical engineers can design the optical system to reflect any color, but blue is often chosen as being less intense and its complimentary color of transmission is a light orange /yellow that is less noticeable and intrusive. That is, the coatings on watch crystals transmit a light yellow (kind of like incandescent light) because it’s complementary blue color is being reflected / taken out of the visible color. You could have a violet / red color of reflection and a slight greenish color of transmission, but that would be a not be a great system for a watch crystal.
Why do I sometime see a different color of reflection from the crystal? Why is the transmitted color striking the dial sometimes different?
The color of reflection is angle dependent and at an angle the color and intensity of the reflected light shifts slightly to violet and the color of transmission changes with it. In addition, the color of the ambient light affects what you see. Daylight has a much different Kelvin temp than incandescent or fluorescent light; that too affects what is reflected from and transmitted to the dial.
Why do I sometime see more reflection from the dial?
AR coatings are angle dependent as noted. 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 cheaper to replace the whole crystal than to have an existing one re-coated.
