nfoto, on 30 November 2018 - 23:19, said:
As you describe the setup, using the LED will enhance UV, whilst the speedlight certainly delivers a huge amount of IR (besides visible light).
Reflective UV will pick up surface detail and IR can sometimes show non-visible inks or lettering. I used IR (830 nm filters) for an earlier documentation of Dead Sea scrolls.
Thank you Birna, so maybe the changes in the inks I saw was due to IR light from the speedlight, leaking in through the Baader U filter on my lens?
Andrea B., on 01 December 2018 - 02:02, said:
Hi Lieah -
That is a very nice reflected UV kit you have put together. And that
Polilight 400 is amazing with all those different output peaks possible! (And my goodness, that thing is
very expensive. Woo!)
What is not clear to me from your post is what kind of Speedlight UV filter you are referring to? On the typical photo website the phrase "UV filter" indicates a UV-blocking filter which cuts haze. Here a "UV filter" usually refers to a UV-pass filter used to capture reflected UV.
Usually speedlight flashes have UV-blocking or UV/IR-blocking filters. So if your Speedlight is blocking UV, then it is output-ing lots of Visible and perhaps some IR light. Thus I would not be surprised that the inks look much different under mixed Vis/IR than under UV illumination. But as mentioned, it's not clear to me which "UV filter" is in use. Perhaps you can clarify?
If you look under our search tag
UV Lighting then you will find most of our posts about UV illumination. UV-Led torches are discussed a lot. We have found that many of them emit some visible violet light. So we filter them with a good UV-pass filter. We've also found that some UV-Led torches do not meet the stated 365nm (or other) peak because of poor quality LED chips. Nichia chipped torches are usually the best. But even within Nichia, the chip bin lots can vary. So check the type and quality of the LED chip in any torch you use for forensics work. UV-LED torches are somewhat narrowband. But I don't know what you need for forensics? Broadband UV or narrower band UV? The current favorite UV-Led torch is the Convoy. Here is just one post about it:
http://www.ultraviol...-uv-flashlight/
The search tag will bring up others.
Most of us doing documentary work (such as UV botanical signatures) use modified Xenon tube flashes for a broader and stronger UV illumination than can be offered by a UV-Led. Such flashes have had any tube coatings removed and also have UV-pass filters in place to supress most of the Vis/IR output. A few folks use Xenon lamps like the Blak-Ray, but they are large and thus a bit difficult to filter. Our founding member, Bjørn Birna, uses an enormous Broncolor UV studio flash which is
quite nice and probably has the best broadband UV output of anything I've seen so far. [Added: The output is peaked I think around 350?]
(Birna, please return and offer a few details about that Broncolor because I have totally forgotten what model it is and how strong its output is.)
Do you have any reflected UV standards that can be used to confirm that your gear is working properly? Well, that is not quite what I mean..... We know from your gear list that it is all going to work properly!!! But when setting up any documentary kind of photo shoot, one usually makes an initial photograph of some known UV-reflecting and UV-absorbing subjects as "proof" that the gear and the illumination is all OK at the time of the shoot. I've been using
Spectralon standards such as those made by Labsphere.
Targets can also be used. And being a botanical photographer, I always try to find a sunflower to shoot when testing gear under UV illumination.
Sunflowers have a known UV-dark central bullseye and UV-bright petal tips. And you can usually find them even in winter at the florist or grocery stores.
[UVP has no affiliation with any vendor. Any vendor links are provided only for your information.]
Thank you so much Andrea for your response, it has really helped. And I agree, the Polilight is pretty great! We’re very lucky to have a healthy government budget to work with
Sorry for the confusion about the speedlight… what I mean is that the speedlight has not been modified (so I’m assuming the UV blocking filter that comes with it would still be on there). So I figured the speedlight must be outputting a lot of visible and IR light, but since I’m using a Baader U filter on the lens, I figured the IR and visible light was being blocked, and so what I was seeing was a very little UV output that must be escaping from the speedlight… but I guess it could also be that the Baader U is leaking IR/visible light? Or I’m just not understanding the process?
I did have a search through the UV lighting section, and I did see some very good UV LED torch discussions, but I was still unsure of why I was getting the results I got, so I decided to ask my specific question. But I will continue to read through the discussions - there’s definitely a lot of good info in there! And I did read that many LED torches emit violet light, but I figured I was ok on that front, because I’m using the Baader U on my lens. But maybe I have that wrong?
And I too considered the possibility that the LED torch I’m using is perhaps not actually 365mn, so I wrote to the manufacturer, and they said that they “produce their forensic lights specific to the nm”, and that this is achieved by the LED supplier. They went on to say they have “very little spectral bleed due the the high tolerances they meet.” So their concerns were that the LED could be malfunctioning, or that there might have been a mishap when the dealer provided the light, thinking it was a 365nm but actually wasn’t. So they suggested I contact the Canadian distributor, so they can work with them to resolve any issues. So before I got into that process, I decided to reach out to you knowledgeable folk to get your thoughts first… but I still plan to start this process if I can’t figure out what’s going on. But it’s sounding like the targets and standards you gave me links for will tell me whether the LED torch is working as it should… I will be getting some of those ASAP, thank you!
And I will definitely check out the Convoy, thank you for the link. And I really like the sounds of those light sources… Birna’s studio flash sounds awesome!
So I think I will start with getting standards / targets to test the LED torch, as well as try out some stronger light sources, such as modified Xenon tube flashes or lamps.
Thank you so much, this has been so helpful and informative.
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