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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.

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  • What do you mean by degrade? Does the glass somehow melt into goo? Become semi-opaque (to wavelengths it is supposed to allow through)? Something else?
    – Michael C
    Jun 2 at 4:52
  • @MichaelC Apparently the stuff the lens is made inherently reacts with moisture in the air and clouds over forming a film. I've also heard it referred to as oxidation which doesn't sound quite right but scholarly papers use that term. In any case, it chemically reacts with the atmosphere and clouds over, which also isn't conducive transmission and needs to be polished or scraped off.
    – DKNguyen
    Jun 2 at 4:53
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    But... we are not simply talking about "condensation" (moisture collecting on cold surfaces) and "contamination" (soot, smoke, grunge, gunk, and other highly scientific chemicals collecting on the glass), right? Jun 2 at 8:12
  • "This is an integrated assembly" - Would you provide a photo, link, model numbers, etc of the assembly you're using?
    – xiota
    Jun 2 at 8:20
  • @xiota It's one I'm building so it doesn't exist yet. Basically a printed circuit board in an enclosure with holes cutout for windows and some method of mounting the glass filters into it.
    – DKNguyen
    Jun 2 at 13:17
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From the Schott catalog (emphasis added):

"For the optical filter glass types BG42, UG5, UG11, BG39, S8612, S8022 and S8023 a change in the glass surface is possible after a few months of normal storage. For this reason, applying a protective coating or lamination is recommended for durable optical filter glass from Group 1 (SCHOTT can provide both)"

I would suspect the option of laminating with non-ionic glass would be the most durable (and costly); but that's only a guess.

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  • Ugh...that sounds...expensive...and the filters are already godawful expensive on top on top of needing to stack filters of the photodiodes already being godawful expensive. Optics are expensive.
    – DKNguyen
    Jun 2 at 14:45
  • I suppose you could laminate your filter yourself; between any glass of suitable quality (just seal the perimeter). But that would probably cost just as much (or more) in the end. Jun 2 at 14:53
  • That might actually hold promise because I can search for optical window surplus stick. Depending on the available methods to seal. Whether it's soldering or a film or adhesive or something. I don't necessarily need a mechanically rugged seal since the mounting can take care of that.
    – DKNguyen
    Jun 2 at 18:48
  • Might help to also store everything in a temperature/humidity controlled environment when not in use.
    – xiota
    Jun 2 at 20:11
  • @StevenKersting, Re, "any glass of suitable quality" That would have to be some type of glass that is mostly transparent at the same wavelengths that are passed by the UG11 filter. I think soda-lime glass (a.k.a., ordinary window glass) falls off steeply somewhere mighty close to the UG11 pass band. Might need something more exotic. Best option might be some other, broad-band Schott filter or optical window with which the UG11 could be stacked. Jun 3 at 22:12

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