• Ultraviolet Photography

Secondary illumination in UVIVF

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#1 Mark


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Posted 14 May 2017 - 12:17

Aside from the ongoing battle with ever-present lint fluorescing in my UVIVF images there is another challenge I've been contemplating recently. That is, illumination of my UVIVF scenes/subjects by the fluorescent light emitted by the subject itself. In many scenarios it seems like a practical impossibility to photograph VIS fluorescence without the fluorescence itself lighting up the scene; especially when that fluorescence is strong.

For example, in the following image the left panel shows the subject illuminated with a VIS flash; the panel on the right is the UVIVF image illuminated only with 2 UV LED torches. What concerns me is how much of the normal VIS color is seen in the UVIVF image. This seems wrong. In the very least I would expect the leaves not to look green in the UVIVF image. They must be getting illuminated by VIS somehow. Maybe they are being VIS illuminated secondarily by the fluorescent light the subject is emitting? Maybe my UV source is bleeding VIS?
Attached Image: 05-13-2017_20-39-58.jpg

To rule out my UV source (2 x MTE-U303 torches) as the problem I tested it by comparing them filtered (U340) and unfiltered to a normal VIS shot, and also compared a different UV source for good measure. The same camera, lens filter, and exposure settings were used for all shots. The 'subject' here is a pile of stainless steel ball bearings (a mix of two different sizes), on a sheet of aluminum foil (dull side up).
Attached Image: 05-14-2017_00-19-51.jpg

As expected, the unfiltered LED torches produced a strong blue cast. And the fluorescent BL-B bulb looks blue & purple (looked almost entirely purple to my eyes (which were behind clear UV safety glasses)). What I was happy to see is that the filtered LED torches yield practically no illumination past my lens filters (save for the fluorescing drop of water off to the right; which dripped from a plant clipping from an earlier shoot). In that last shot there is some of what looks like ambient light contamination reflecting off the foil in the background. But at 15 seconds exposure time I'm not surprised, since it wasn't exactly completely dark in the room. Its rather difficult to get a room 100% dark, as in this case, even the power lights from my PC on the other side of the room throw some light.

Since it looks like my UV source is not throwing VIS into my UVIVF images, would it be correct to surmise that images of my UVIVF subjects are prone to secondary illumination contamination from their own fluorescence? And if so, is it necessarily impossible to avoid this?

- Nikon D750 [broadband] + 50 mm Nikon series E lens + 20 mm extension tube
Lens filters

425 nm longpass + GG435 longpass + Baader UV/IR [stack]
- VIS: Sigma EF-500 DG Super flash

- UVIVF: 2 x [MTE-U303 +/- U340]; 6W fluorescent BL-B
- ISO320 / f16 / 15s

#2 Andrea B.

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Posted 14 May 2017 - 19:18

I think the thing to do is to eliminate as much stray light as possible. Even from your small PC lights across the room. I'm pretty sure you would pick up any stray ambient light in a 15 second exposure.

1) Shooting tent.
Put a blanket over the dining room or kitchen table and shoot UVIVF under there.
(Pretend you're 5. Make a hideout!!)

2) Or, closet. With rolled towels stuffed along the bottom edge if light leaky.
(Humans sitting in closets tends to make pets a little crazed. I get paws sneaking under the door.)

3) Or, BIG cardboard box. Slot for lens. Slot for illuminator. Towels or whatever around slots.

4) There are formal "dark boxes" you can buy for such shooting. But that's no fun. Invent your own, yes?

After trying one of these solutions, you might be able to better determine whether your photo above is picking up stray ambient light or if it is showing some secondary illumination from the fluorescence.

In many cases, fluorescence has only one colour. Like red from chlorophyll. So I was thinking that in such cases secondary illumination would/should have a colour cast?? Try a chlorophyll fluor shot, put a PTFE disk in the shot, look to see whether it reflects red light if close? If further away?

Also, contrary to typical laboratory fluorescence practices, we are using rather broadband filtration. If you were using both a targeted narrowband excitation on the illumination and narrowband capture on the taking lens, then that might eliminate some of the 2ndary illumination?? If indeed 2ndary illum is what you've got instead of stray ambient.

Just some thoughts. You are a good experimenter, so I'll be looking forward to the results.
Andrea G. Blum
Often found hanging out with flowers & bees.

#3 Andrea B.

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Posted 14 May 2017 - 19:20

P.S. Forgot to say -- LOVE the foil + ball bearing shots. A good example of Shane's Shiny Metal Test.
Andrea G. Blum
Often found hanging out with flowers & bees.

#4 Alaun


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Posted 15 May 2017 - 17:07

Just a side note to the ball bearings on aluminum:

As Al is a quite good/better reflector for UV compared to other metals I would have
expected brighter surfaces of the foil compared to the balls. As the foil is rather dark
too (picture 2xMTE+U340 filter) maybe there is little amount of light in that spectrum
range coming out of the MTEs? But still: the balls seem to be a bit darker.