I am needing to use Schott UG11 filters (and other coloured glass filters, of which UG11 is the most sensitive) that degrade on a UAV setup. The degradation is a cloudy film that forms from the material the lens is made of reacting with humidity and/or oxygen in the atmosphere that then needs to be polished off. This seems to be a common trait of any filter whose passband dips into the ultraviolet region.
This is an integrated assembly with photodiodes and not something like a camera where the filters can be easily removed for maintenance and storage after use, or installed each time before use.
Do any coatings or treatments exist for situations like this? From where I stand it's almost as if Schott UG11 and similar filters can't be used anywhere outside of a lab or camera kit where they are constantly babied.
UPDATE: So I've been calculating and found that once you stack one of the more common NIR cutoff filters (BG39, BG42) with UG1, you get essentially the same response as UG11 for my purposes. But UG1 is cheaper and less humidity sensitive.
Surprisingly (to me, anyways) the real problem is turning out to be the NIR cutoff filters. Between BG39 and BG42, both have excellent NIR suppression but BG42 has only only 60% the UV transmission of BG39 but less humidity sensitive.
By "less humidity sensitive" for UG1 and BG42 I mean no particular warnings about unusual humidity sensitivity in their datasheets (no single or double thunderstorm cloud icons).
However, I think I have see photographer reports of BG39 (and probably UG1) still clouding over in long term storage and needing periodic polishing and such. But it's not clear since they never seem to be clear about exactly what filter glass it is they have.
I have samples of UG11 and BG39 which I managed to get on clearance and is the ideal combination as far as performance goes.
Presumably sandwiching them between two plates of fused silica or fused quartz and sealing the edges with copper foil tape should go a long way to protecting them. The problem is that I can't get ahold of fused quartz and fused silica is really expensive and overblown for my needs since its transmission dips way lower into the UV than I need it to.
But that makes really expensive, but inexpensively acquired test pieces that cannot be re-obtained again at the same price.