using low-E film as a reflective filter

[GlennE]

So this is in the realm of asking whether an idea is complete idiocy. Reading in this board I found someone mentioning that Paul C. Buff Einsteins have a removable UV exclusion dome - I have one, and yep, there's the dome, and it looks like it's designed to come off when the tube needs replacing.

 

My interest is in UV flourescence (black light photography). Clearly the issue is blocking visual light and getting UV-A (trying to avoid B and C). In wracking my brain on whether there's a way to use the Einstein, I realized there's the stuff they sell you to put on your windows to keep your house cool. Their goal in making the film is that it let through visible light, so you're not just using a glorified curtain, while "blocking" other forms of radiation which would heat your house. Some significant amount of the "blocking" is in the form of reflection.

 

So: is it possible to use such a film not strictly as a filter, but as a reflector?

 

I've tried to attach a schematic jpg of the idea.

 

Assuming that it isn't just goofy: has anyone tried anything similar? I don't expect there to be zero visible reflection; how good would it have to be to get something where the flourescent substance was strongly present in the image? Making non-painted surfaces completely black doesn't have to be the goal. Is there likely to be frequency shifting by power levels, and if so, is one "direction" - strong pops, for example - likely to shift the spectrum more in favour of UV-A?

 

the diagram of my Rube Goldberg device:

 

I

[OlDoinyo]

If I understand your post and diagram, you are trying to eliminate UV contamination from visible fluorescence images. This presupposes that is a problem in the first place, and I am not aware that it usually is at the exposures generally used--the camera is many times more sensitive to visible than UV anyway, especially if one is using a non-converted camera and/or non-UV-friendly lenses. Or do I misunderstand?

[Cadmium]

Boil it down for me, ask a simple question.

I have read it three times, and it seems to jump around, and I don't have a clue what you are trying to do.

 

Sorry...

ask a one line question.

[GlennE]

I want UV flourescence, but of course any visible light dulls the effect in the photo. So given that I have a strobe that produces both visible and UV, how do I get the UV to the subject (which will flouresce) while removing as much visible light as possible from that same subject? there isn't, as far as I can tell, a "filter" that removes the visible spectrum for a strobe this size without it being like, a thousand dollars.

 

So the idea is to let the visible light go out the back, absorbed into a black cloth or something, while reflecting the UV towards the subject.

 

Have I made it clearer?

[Andy Perrin]

Why does this have to be done with that strobe? Why not just light paint with a UV torch or a few of them? They are like $20 each.

[GlennE]

Of course anyone "can" do it with continuous lights, but I find the question odd on this site. This whole discussion board is filled with people discussing how to remove UV filters from outdated speedlites so they can use them as UV sources, do you ask each of them the same question?

 

Possible answers to the question are: the short duration of the strobe (as with an equivalent speedlight) allows you to virtually eliminate ambient light - you don't have to do it in a darkened room, because the room becomes dark at 1/200th of a second. The short duration of the flash also eliminates questions of camera shake, so you don't have to try to work out whether you can handhold, as you might if your $20 blacklight requires you to go to 1/15th of a second at a high ISO.

 

I haven't been able to track down how much UVA I might expect out of the strobe at full power, but it's possible that with a 640Ws source, there might be enough that I can carve away a few stops of noise.

 

Again, given the number of people discussing how easy it is to modify an out-of-date Canon 199a to be a UV source, I find it strange to ask why I might want to explore using a strobe I already have.

[Andy Perrin]

Of course anyone "can" do it with continuous lights, but I find the question odd on this site. This whole discussion board is filled with people discussing how to remove UV filters from outdated speedlites so they can use them as UV sources, do you ask each of them the same question?

No, only people who tell me that filters big enough to work are too expensive. (I'm not sure if that's true, by the way. I'm also not sure how big a piece of filter glass you need for that flash.)

 

Anyway, I'll let other people try to answer your questions.

[JMC]

Glenn. If you were to fit a filter over the light source, how big a diameter filter do you need? I'm certainly not afraid to try whacky ideas, and please go ahead and try it (I'm sure we'd all be intrigued to know if it works), however a filter would be a simpler option.

 

Of course cost is a factor, depending on diameter. But I've seen some research fluorescence systems using fairly big flash lights which used squares of Kopp glass which had good visible blocking combined with letting the UV through, and it was a fairly cost effect approach.

[Cadmium]

Glenn, if you want your light to emit only UV, then you would absolutely not block it with a material that filters out UV,

which if I understand you correctly here, the window filter material does, it blocks UV, and passes visual.

 

What you do, is get one of the reflectors made for that flash, they make several, and remove the plastic dome,

them put a large round U-340 2mm and S8612 2mm on the front of the reflector,

those two filters when stacked together will filter out visual and IR, blocking everything except UV down to below OD5.

Then your flash will only be emitting UV below 400nm.

[Cadmium]

If you want to use the window material as a UV reflector, then I have no advise.

[Cadmium]

Also, filter glass is available in 165mm x 165mm sheets (6 inches square). So if you want to use filter glass then the reflector you choose should be that diameter or smaller.

You can even use the square as it is, without cutting it into a circle if that works well enough. Cutting it into a circle is slightly more expensive, not much for one circle though.

Then glue or tape the glass onto the front of the reflector.

[Alaun]

Welcome here Glenn.

 

What you describe is a dichroic filter or better a dichroic mirror.

You might have a look here: https://en.wikipedia.org/wiki/Dichroic_filter

[Andy Perrin]

A dichroic really would be too expensive (and overkill) for filtering a huge flash though.

[ulf]

A large round U-340 2mm and S8612 2mm would not be cheap either!!

[Andy Perrin]

Yup. I think there would still be a big multiple in between those two expensive solutions however.

[ulf]

One thing that must be considered first, do the flash-tube for this big flash emit enough UV in the first place.

Many studio flashes have UV-filtering tubes. High power is not enough.

Even if there is different ad-on UV-filters that do not mean that the tube is unfiltered.

The standard tubes for Bowens, Godox etc are UV-filtered.

[Andy Perrin]

Good call, Ulf:

[GlennE]

ah, ok, I'd been given to understand through other posts on this site that the einstein had its' UV protection in the dome, which is removable. Someone had even posted a UV photo which they said was from an Einstein.

[Cadmium]

Glenn, Can you post links to those topics, people here?

I was under the same impression, thought I had read that before, although I don't have one of those, and the info Andy shows differently.

Perhaps some people are using an alternative tube?

Can you post your links so we can look.

Thanks.

[Guest]

Just FYI - Paul C Buff does offer non-UV-coated flash tubes... they're just not offered online (you need to contact them otherwise to get the info).