Optical filter that blocks infrared and allows UV?

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Hello. I want to build camera for ultraviolet photography and reflectography. I plan to use debayer'ed backlit CMOS camera with or without Wood's ZWB3 glass UV filter. But I also need to find infrared cut filter because abudant IR radiation will ruin photos.

Problem is that there are plenty of hot mirrors which block not only infrared but also UV. I managed to find only one hot mirror that passes 365nm LED radiation that I scavenged from JVC gz mc100e older but high quality camcorder and it is small. I want to use same camera for infrared photography so putting it inside is not good option besides I'm not sure how far filter passes UV radiation.

I know there are specialized only UV pass filters such as U-Venus Baader but they are expensive, rare and not so versatile. I want camera to see into ultraviolet as much as possible although since UV-C is blocked by air alone I'll be good with filter that allows fully UV-A and preferably UV-B spectrum.

I'm curious what kind of materials are used to make hot mirrors and in particular those that pass UV radiation?
 
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  • #2
Andy Resnick
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I know there are specialized only UV pass filters such as U-Venus Baader but they are expensive, rare and not so versatile. I want camera to see into ultraviolet as much as possible although since UV-C is blocked by air alone I'll be good with filter that allows fully UV-A and preferably UV-B spectrum.

I'm curious what kind of materials are used to make hot mirrors and in particular those that pass UV radiation?
You didn't specify the diameter of the filter you need- that can drastically impact the cost. In any case, someone took the time to measure and post transmission spectra of a variety of glass filters that you may be interested in, I agree the ZWB3 filter is not what you are looking for:

http://www.ultravioletphotography.com/content/index.php/topic/1313-filter-transmission-charts/page__st__20

As an alternative, you may be able to find some excitation filters used for fluorescence microscopy: Hoechst or DAPI excitation filters would work well for you, I think.
 
  • #3
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Forgot to mention. Filter needs to be 27mm or larger diameter. Well I seen small filter also costing a lot, I think their exotic nature determines price. Thanks for link, from quick look I see there are shown most popular UV filters with low infrared leak that are expensive, excitation filters also have hefty price. I might try looking for used one although I read somewhere UV filters have tendency to age and transmit less UV so I want to be careful when investing into them.
 
  • #5
hilbert2
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I think any material will absorb UV at least in some wavelength interval. The ratio of the charge and mass of the electron is such that it leads to practically any atom or molecule to have some electronic transitions with an energy in the UV range. It's just about finding something that is transparent in the UV region you're interested in. The finding of compounds that are "infrared neutral" is much easier.
 
  • #6
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Yes, lots materials absorb UV and little allow only it, that is why UV spectrum photography requires expensive equipment. So far best infrared cut UV pass filter I was able to find on Ebay is this and it is QB21 37mm diameter - more than enough. Filter actually blocks completely IR and allows UV-A with visible light. Only downside is that filter does not pass UV-B and UV-C:
$_57.jpg
 

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Have you considered a very thin silver film? Silver has significant transmission in the UV. You might not be able to afford the loss in sensitivity depending on how bright your source is.
 
  • #8
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No I didn't consider silver thin film filter. Can you share spectrum of it? There is better slightly infrared cut filter BG40 but these are expensive. For UV photography great filter is 330WB80 although it's 25mm diameter.
 
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My experience is that silver becomes as reflective as its going to get at about 200 nm of thickness. I suggest a thickness of 100 nm as a test. Perhaps you could find data on the reflectivity of silver as a function of thickness. Then you could make a better estimate of the thickness you want.
 
  • #12
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Forgot to mention. Have the silver deposited on fused quartz, not glass which would absorb a lot of the UV.
 
  • #13
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I was wrong about camera lens UV-B transmission. Modern camera lens use optical cement with a lot of lens, use various glass and AR coating which may cut significantly ultraviolet transmission. Although BK7 glass alone can pass well even UV-B. Camera sensor besides CFA has also silicon nitride coating which makes deep UV vision impossible. So it's better to focus on near UV-A, but 350nm should work with right equipment.
UV grade fused silica lens would be best but I have fixed lens camera. But I do plan getting interchangeable lens camera with quartz lens.
Folks in ultraviolet photography forum recommend using at least BG39 or BG40 IR-cut filters but they didn't mention silver film filters. It does pass 350nm well enough although there is some infrared leakage. I can't find such filters on Ebay, where they could be found?
 
  • #14
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What looks to be leakage in the IR if you look solely at the absorption spectra isn't an increase in transmission, but an increase in reflectivity. Similarly much of the decrease in reflectivity around 400-500 nm is really an increase in absorbance.
As to getting such a filter, I would be surprised if one was available. You will probably have to have one custom sputtered or vacuum deposited for you.
I learned about the UV transmission of silver in the 1980's while I was developing reflective coatings for CT x-ray detector scintillators. You might be able to find a company that does low cost silver deposition for decoration of glass or plastic. That way you wouldn't have to spend more at an optical company.
 
  • #15
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Another option is to silver the substrate yourself by reducing silver from a solution of silver nitrate. If you use a glass tray with a light underneath you can monitor the glass getting more opaque. Your substrate in the tray will be getting coating. By pulling the substrate and rinsing at different times, you get different thicknesses. There are instructions on-line. Keep in mind if a silver solution gets on your skin, the silver reduce in your skin turning it black-brown. Normally people just wait a week or two for it to go away, but if you mix dilute ammonia water and dilute hydrogen peroxide, it will remove the stain and then you can just the same rinse with water. You still have to deal with silver tarnishing. Maybe epoxy another piece of quartz or glass over the film.
 
  • #16
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Seems like Schott S8612 and BG40 are best IR-cut UV-A pass filters money can buy. Although I will have opt for QB21 which has lower UV transmission but is significantly cheaper.
 

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