IR attenuation film - does it exist?

[Guest]

Hi everyone,

 

I have a UV source I like to use, which is simply a bank of 4 x 18" fluorescent BLB bulbs, filtered through a pane of UV-pass glass (FL-20 type). This filter glass does pass a lot of IR, but also attenuates IR slightly; which helps (every little bit helps!). The BLB bulbs also have relatively low IR emission, once they are warmed up. With the combination of the slight IR attenuation from the FL-02 glass, and the low IR emission of the (warmed) bulbs I think I am close to an acceptable IR "free" UV source. I just need to attenuate the IR a little bit more - this because it is of course not perfect... there is still some getting through, albeit not much.

 

So, I was wondering if there is a thin film of some sort which could absorb the remainder of this IR, while still passing UV?

 

I have a booklet of Rosco filter gel samples, and in hopes of finding one that might serve my purpose I picked a few candidates and put them to a test. I photographed the set against a foil background, to help show how clear each is under each waveband.

 

Here is a summary of the five conditions I tested:

 

Unfortunately, as you can see, while a couple of the films ('gels') looked promising in that they didn't attenuate UV too much, at the same time they didn't have any observable effect on IR. Not even the #375, which has a posted transmission profile indicating it could have been very good at absorbing IR.

 

So this test was a bust. It would be great to find such a film - something affordable, available in 'large' sheets, easy to cover various light sources, etc.

 

Does anyone know if such a film exists?

[Andrea B.]

I don't know if such a film exists.

 

I think I'm surprised that the #375 film shows a chart which promises total IR blocking between 640-740 nm but then this apparently does not happen. So what's up with that? The chart indicates that Tungsten/Daylight lighting was used and that does contain IR. But maybe the composition of their Tungsten source had very low IR -- unlike your IR source or bare flash?

[rfcurry]

Ah, the Holy Grail of filters!

In lieu of finding a large filter that passes UV well and blocks NIR thoroughly, have you thought of using a small aquarium in front of your lamp? Throw some CuSO4 in the water and you will be all set. :)

[Cadmium]

This seems convoluted. Why filter the light if you have a UV-only filter that transmits UV only?

If you want to shoot UV-only, then it is best (easiest) to just use a UV filter that transmits UV-only.

If you want to shoot UVIVF, then it is best to use a light source that is 365nm (which don't have Red/IR), and filter the 365nm light to below 400nm/420nm,

or use a stack on the light that transmits UV-only, such as some use on flashes (U-340 2mm + S8612 2mm).

[Guest]

Actually, I have thought of liquid copper sulfate - and I made a homemade filter with some old filter rings, etc. And it worked very well. Unfortunately, being very corrosive to metals, it didn't last long (not even one day) before the rings were corroded. And working with a toxic liquid like that just isn't fun or safe. I've been wondering lately if it is possible/easy to melt CuSO4 into glass - maybe a glass blower could do this for me? Just an idea at the moment.

 

As for the approach being convoluted, yes I agree with your point. But, to the point, I want to use a source with a broader range of UV - for example, that which is emitted from a fluorescent BL-B, as compared to the very narrow UV emitted from LED's. This would help ensure I could image something which has more or less, or both/mixed, absorbance/reflectance to longer or shorter UV wavelengths (i.e., maybe a given subject absorbs strongly at/around 350 nm, but not at 380 nm, etc.). It may be a long shot, but I like the idea of keeping my options open.

 

Also, I would use this for CyIIF (cyan induced infrared fluorescence) imaging - which would definitely require removing IR (albeit from a different source).

[Bill De Jager]

This seems convoluted. Why filter the light if you have a UV-only filter that transmits UV only?

If you want to shoot UV-only, then it is best (easiest) to just use a UV filter that transmits UV-only.

If you want to shoot UVIVF, then it is best to use a light source that is 365nm (which don't have Red/IR), and filter the 365nm light to below 400nm/420nm,

or use a stack on the light that transmits UV-only, such as some use on flashes (U-340 2mm + S8612 2mm).

 

One possibly very good reason could be when using UV flash on cut flowers indoors. Almost all of the energy is still visible and IR. My understanding from posts on this site is that many repeated flashes (such as for focus stacking) end up wilting and damaging the flowers. Some of that may be from the UV itself but a lot could be due to heating from the much more abundant visible and IR. So attenuating the IR (and a good chunk of the visible energy if that can be achieved as well) before it hits the subject could be useful. This is not an issue I've personally run into yet but I expect I will at a later date.

[nfoto]

Bill, you make very valid points. I certainly wish there had been IR-reducing, UV-transparent filtration available that simultaneously could protect delicate biological specimens for overheating due to the heat output from the flash(es). The Corning filters might stand the heat, but they are not easily available and/or have unspecified UV behaviour. Copper sulphate solutions certainly help, but their deployment is messy and not easy to integrate in the ordinary flash set ups, plus there is a danger of the container breaking or cracking in use. My studio has seen its fair amount of copper sulphate leakage over the years ....

 

So far, I have managed the worst damage by placing specimens on a heat sink (either a massive aluminium block or a thinner one combined with a low-voltage fan underneath it).

[aphalo]

I will try to get #375 measured this week or next. The spectrum from Rosco ends at 750 nm and transmittance could be higher at longer wavelengths. For the copper sulphate solution one could use a tray or tank made of UV-transparent acrylic Plexiglas from Evonik in Europe or Acrylite from Evonik in USA, type 2458 (https://www.acrylite-shop.com/US/us/search.go?fromtemplate=%2Fsearch.htm&q=2458&sendsearch=Do+search). The only thing about copper sulphate/water is that it warms up if there is enough IR, so there must be room for expansion. Copper sulphate is toxic and corrosive, so not too easy to handle, but it can be used with care. (Plexiglas in USA is a TM of a different company...)

 

Another thought is where the IR could be emitted from? Have you tried blocking a few centimeters near the ends of the tube? It could be that the IR comes from the electrodes/filaments on the inside of the end caps of the lamp. If this is the case wrapping the lamp ends could help. The start-up current is a lot higher, so this could maybe explain the initial burst in IR emission. Easy to test anyway... although it is a wild guess.

[nfoto]

Electronic flash emits a lot of IR. Not surprising since they use xenon-filled tubes and have an output spectrum pretty similar to sun light.

[rfcurry]

You should think about sheet plastic filters such as http://www.astraprod...cut-filters.asp the Clarex IR Cut filter. However, as you can see, the cut filter is made of polycarbonate which restricts the UV transmission. So, you would need to ask Astra to put an IR-blocking coating on acrylic instead (PMMA). Here is the transmission spectrum at present:

http://www.astraproducts.com/images/ir-cut-filter-01.jpg

 

PMMA would get the UV below 300nm.

 

Just some thoughts.

[rfcurry]

Also, you might want to buy tanning bed plastic sheets, e.g., http://www.tanningbedacrylics.com/index.php?route=product/product&manufacturer_id=14&product_id=54 as that is acrylic and passes UV well. Then coat against NIR.