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ArcticMyst Security by Avery

How do AR coatings work?

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If we can produce optics with a 1/4 wave layer of AR "whatever it is" material on the surface, can we make different materials transmit microwaves, radiowaves, or for instance.. radar gun wavelength waves better than previously?

How would one "AR coat" your house, so you could get better FM radio signals?

Or AR coat a TV satellite dish antennae?

Or a Satellite radio antennae?

All waves are reflected to some amount, how do you reduce that amount?
 





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Glass is semi transparent to light waves, and I believe the AR coating allows more of the wave to pass into the surface plane of the lens thus allowing more output. other materials might be similarly transparent to a different energy wavelength. As for a house there are too many materials involved to do a AR coating or equivalent treatment to be more transparent to other wavelengths. someone correct me if i'm wrong.
 
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surface plane?

So if you're not using bk7 glass, you could put a thin slice of bk7 in front of a slightly worse piece of glass, and the bk7 would improve the light transmission through that "other" type of glass? Is that what you mean? Or am I being a bad interpreter?
 
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Not exactly what I was trying to say. The bad glass would still have transmittance issues because of it's poor optical qualities. Think of the surface tension of water, and if you place a needle very carefully on the surface it will sit on top of the water and not sink..The surface tension is supporting the needle..If you then place a drop of detergent into the water the needle will instantly sink as the surface tension is broken..same principle for an AR coating only instead of detergent a film of chemicals is applied.. hence the light aka needle passes through easier...hope this helps some.
 
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Interesting analogy for sure.. I'm just the type that visualizes things happening in my head, so I'm working on applying detergent... so the light waves can go through....

Still working on it!
 

HIMNL9

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Some different types of AR coatings, but, mainly, 2 principals, exists.

The more diffused is to form a layer (or multiple layers, in some cases), with a substance with a certain refraction index (normally is used magnesium fluoride), and with a certain thickness, that depend from the wavelenght and must usually be 1/4 of the wavelenght of the center wavelenght that you want to be passed more, so the reflection of the glass and the reflection of the layer are rotated 180 degrees in phase, and virtually "cancelled" for interference ..... the other way is to produce a reticular grid of "bumps" on the surface, that have the highness and spaces smaller than the visible wavelenght, so the light see it as a "virtual continuous gradient of refractivity" between the air and the object ..... this works better than quarter wave layers, but is much more difficult to obtain and high cost process.
 
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If you consider the two mediums that light travels through to be transmission lines, I think you can consider the AR coating as a quarter-wave transformer matching the impedance (as index of refraction) of the two mediums.
 
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[wrong]To fix one confusion here: an AR film often uses a half-wavelength film thickness.[/wrong]A quarter-wavelength thickness can also result in an HR coating. A DBR, which is how high-reflectivity mirrors are made on laser diode facets, is a stack of alternating films, where each is a thickness of a quarter-wavelength.

So the simplest AR coating uses [wrong]a half-wavelength thickness[/wrong], and you basically use a material that has an index of refraction that is inbetween the indexes of the two original materials. So for instance, the simplest case of a glass element in air: the index of air is 1, the index of glass is ~1.5, so the simplest would be to put a 1/2-wavelength film of a material that has an index of refraction of 1.25 on the surface of the glass.

It can get more complicated though, but a simple [wrong]half-wavelength[/wrong] of an intermediate-index material on a planar surface is a prime example of a simple AR coating. Broadband AR coatings get more complicated, of course. But the simple case helps show the explanation: destructive interference of waves that reflect off of surfaces.


And of course, the optical thickness of a film and the index of refraction both must take into account the index of refraction and the wavelength of the light you're interested in.
 
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Yeah, I'm an idiot, I forget the phase flip is only when going from low-to-high index. I work with DBRs typically, in which the 1/4-wave thickness repetition results in an HR coating, so I get that confused.
 
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Yeah, I'm an idiot, I forget the phase flip is only when going from low-to-high index. I work with DBRs typically, in which the 1/4-wave thickness repetition results in an HR coating, so I get that confused.
The phase flip is indeed from low to high, this happens at both reflections, the first from air to the low index material, the second from the low index material to the higher index glass. DBR's have alternating low to high and high to low. But I had to look up the details of that phase shift so I'm the same type of idiot :)
 




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