Post by jeanmichel on Dec 13, 2005 5:05:12 GMT -6
All you wanted to know about anti-reflection coating without having to ask
I would like to explain briefly to the forum members how work an anti-reflection coating (AR coating), I’ll try to be as simple as possible to make a physics lesson a little fun.
You have seen that on many modern watches your sapphire crystal is equipped with an AR coating, limiting to a very low level the reflection on the crystal, bringing the glare almost to zero except for a little purple glare.
The phenomenon of reflection on a sapphire crystal is explained in drawing n°1, (sapphire is more reflective than the glass due to its higher index of refraction : 1,8 compared to glass = 1,47)
The basic principle of an AR coating is depositing one or more layers of material (generally oxides) with a reflective index somewhere in between air and sapphire : the incident light will then undergo reflections both in the air/AR boundary and the AR/sapphire boundary. If the thickness of the AR layer is properly chosen a destructive interference between the two reflected waves will occur which will decrease the reflected wave
For more complex AR coating such as the coatings of our watches, multiple layers are deposited in order to create a multiple destructive interference between the waves, bringing the reflected light to almost a zero level.
You see in the drawing N°2 the principle of a sapphire crystal AR coated on the top.
The problem of an AR layer is that it is only valid for ONE wavelength (one colour), but sunlight consist of a large variety of visible colours from violet to red, therefore an AR coating could not offer the minimum reflection for ALL colours and that is why you see purple reflection : the AR coating chosen by most watch manufacturers reflect a little amount of the purple (on camera lens you will commonly see a green glare, because the AR only reflect this part of the visible spectrum)
Some sapphire crystals are coated on BOTH faces, this costly choice offer the lowest reflection on the crystal. Most watch brands only coat the INTERNAL face of their crystal : this of course cut by half the amount of reflection , but still the glare is close to 5%, with a double face AR coating Breitling offers a glare under 1%
See drawing N°3
Cost of the AR process is multiplied by two (because you need to coat the two faces separately) but it offers to double coated crystal an incomparable advantage
The drawback of this double side AR coating is the scratch resistance - the oxides deposited on the sapphire DO NOT offer the same scratch resistance compared to bare sapphire - that is why you CAN scratch a double coated sapphire : you do not scratch the sapphire by itself (it can only be scratched by sapphire or diamond) but you simply scratch its coating
Please when you compare the scratch resistance of double coated crystals to simple coated ones take this into account! ... you can always remove the AR coating on the face of the crystal ... but you will loose the benefit of it.
Bear in mind that the scratches to the AR are almost invisible (but I have looked to my AR coatings under microscopes and of course they are micro-scratched)
FAQ on AR:
Is it ?scratch resistant??
YES but ... not as much as sapphire, something in between glass and sapphire : for sure better than plastic but the hardness of an AR coating will never be as resistant as a bare sapphire
Is it possible to re-coat my scratched sapphire?
YES but ... it will cost you a fortune: an AR batch cost from 1.000 to 2.000 dollars, but the manufacturer put hundreds of crystals in the evaporator ! (I personally have had, the experience and yes it is possible, I have done it in our company’s facility)
Is there different quality of AR?
YES! All AR manufacturers keep their secret, most companies do not coat their sapphire : they surely sub-contract this very specialized operation the different quality came from, scratch resistance , quality of reflection .... one of the leader of thoses crystals and coating is Comadur a company belonging to Swatch group
Is there any different quality on Sapphire crystal?
NO , all sapphire crystal are synthetic sapphires same chemical and mechanical properties, here again this operation is subcontracted to specialized companies, the only difference came from the level of quality inspection and on shapes : domed shapes are far harder to obtain compared to flat crystals because they must be polished.
I hope you like this little article
Cheers
JM
I would like to explain briefly to the forum members how work an anti-reflection coating (AR coating), I’ll try to be as simple as possible to make a physics lesson a little fun.
You have seen that on many modern watches your sapphire crystal is equipped with an AR coating, limiting to a very low level the reflection on the crystal, bringing the glare almost to zero except for a little purple glare.
The phenomenon of reflection on a sapphire crystal is explained in drawing n°1, (sapphire is more reflective than the glass due to its higher index of refraction : 1,8 compared to glass = 1,47)
The basic principle of an AR coating is depositing one or more layers of material (generally oxides) with a reflective index somewhere in between air and sapphire : the incident light will then undergo reflections both in the air/AR boundary and the AR/sapphire boundary. If the thickness of the AR layer is properly chosen a destructive interference between the two reflected waves will occur which will decrease the reflected wave
For more complex AR coating such as the coatings of our watches, multiple layers are deposited in order to create a multiple destructive interference between the waves, bringing the reflected light to almost a zero level.
You see in the drawing N°2 the principle of a sapphire crystal AR coated on the top.
The problem of an AR layer is that it is only valid for ONE wavelength (one colour), but sunlight consist of a large variety of visible colours from violet to red, therefore an AR coating could not offer the minimum reflection for ALL colours and that is why you see purple reflection : the AR coating chosen by most watch manufacturers reflect a little amount of the purple (on camera lens you will commonly see a green glare, because the AR only reflect this part of the visible spectrum)
Some sapphire crystals are coated on BOTH faces, this costly choice offer the lowest reflection on the crystal. Most watch brands only coat the INTERNAL face of their crystal : this of course cut by half the amount of reflection , but still the glare is close to 5%, with a double face AR coating Breitling offers a glare under 1%
See drawing N°3
Cost of the AR process is multiplied by two (because you need to coat the two faces separately) but it offers to double coated crystal an incomparable advantage
The drawback of this double side AR coating is the scratch resistance - the oxides deposited on the sapphire DO NOT offer the same scratch resistance compared to bare sapphire - that is why you CAN scratch a double coated sapphire : you do not scratch the sapphire by itself (it can only be scratched by sapphire or diamond) but you simply scratch its coating
Please when you compare the scratch resistance of double coated crystals to simple coated ones take this into account! ... you can always remove the AR coating on the face of the crystal ... but you will loose the benefit of it.
Bear in mind that the scratches to the AR are almost invisible (but I have looked to my AR coatings under microscopes and of course they are micro-scratched)
FAQ on AR:
Is it ?scratch resistant??
YES but ... not as much as sapphire, something in between glass and sapphire : for sure better than plastic but the hardness of an AR coating will never be as resistant as a bare sapphire
Is it possible to re-coat my scratched sapphire?
YES but ... it will cost you a fortune: an AR batch cost from 1.000 to 2.000 dollars, but the manufacturer put hundreds of crystals in the evaporator ! (I personally have had, the experience and yes it is possible, I have done it in our company’s facility)
Is there different quality of AR?
YES! All AR manufacturers keep their secret, most companies do not coat their sapphire : they surely sub-contract this very specialized operation the different quality came from, scratch resistance , quality of reflection .... one of the leader of thoses crystals and coating is Comadur a company belonging to Swatch group
Is there any different quality on Sapphire crystal?
NO , all sapphire crystal are synthetic sapphires same chemical and mechanical properties, here again this operation is subcontracted to specialized companies, the only difference came from the level of quality inspection and on shapes : domed shapes are far harder to obtain compared to flat crystals because they must be polished.
I hope you like this little article
Cheers
JM
