Reason(s) for single-sided AR coatings?

[AvidHiker]

Am I correct in assuming that Don's choice to use single-sided AR sapphire is a durability-related design decision? Does anyone know how it would compare to a double coated window in terms of transmission?

 

[Kiessling]

I guess double-sided AR means more light coming through but also a vulnerable coating outside that would defeat some of th eusefulness of the sapphire being used.

IIRC the losses for the sapphire lenses used are roughly 12%, but I could be wrong abnd quoting old figures.

bernie

 

[easilyled]

I'm not sure how an outer coating would handle rain, but my guess
is not very well.

 

[McGizmo]

Am I correct in assuming that Don's choice to use single-sided AR sapphire is a durability-related design decision? Does anyone know how it would compare to a double coated window in terms of transmission?

As I understand it and I could be way off, the good and durable A/R coatings we enjoy on some of the glass windows is a coating that is added to the glas while it is still a large plate and prior to the plate being cut up and then ground into the round windows.

The sapphire windows are not made in a large plate but individually "grown" if I am not mistaken. The company I get my windows from only offer one type of A/R coating and it is quite fragile and easily disturbed. I don't know how effective it is either.

Newbie once posted about all types of great and efficient A/R coatings but in contacting sources for the sapphire windows, these people had never heard of the types of coatings Newbie had mentioned.

I think it would take a good amount of research to find a viable solution and I also suspect that there would need to be quantities well in excess of my activities to approach some cost effective solution if it could be found.

In my opinion, the durability and scratch resistance of the sapphire is worth the loss in light transmission it entails relative to a very clear window that is much more prone to breakage and scratching. I prefer a known and factored in loss to one of a potentially catastrophic nature. I know there are those who disagree with my priorities here and I don't say they are wrong. It's a judgment call.

 

[AvidHiker]

For sure, I definitely share your priorities Don - very interesting, just what I was looking for. Thanks to all for the responses!

I know that even Surefire uses (or has used) only internal coatings on some of their borosilicate glass lenses (like my L2 from a few years back), so I wonder if even the best AR coatings are still somewhat fragile. Anyway, if only another 3-4% of transmission is to be gained from having the exterior coated, then I can certainly do without it.

 

[paulr]

I think they use AR on sapphire watch crystals. They'd have the same scratching issues, I guess.