IR luminescence - is my theory correct?

[Adrian]

I am still struggling with IR luminescence.The following images are:

 

Visible,

UV fluorescence

UV reflected (Baader U)

UV with Schott UG5 + S8612 stack (I have not attempted to white balance the UV images)

"IR luminescence" induced with UV. Lit with two electronic flashes modified to UV with plastic windows removed and replaced with old Kodak 18 A filters, The transmission curve for the 18A shows there is no transmission past 880nm, so I have used an IR 950nm IR filter on a 100mm Series E Nikon lens

 

Is the final image IR luminescence or reflected IR ?

Thank you for looking!

[Cadmium]

Hi Adrian, I think with the last image, the UV induced 950 IR luminescence shot, it looks like it has a lot of reflected IR, so I would guess your flash filter is not blocking IR.

I will go shoot a sunflower with MTE and 850nm.

 

Here is a comparison between 850 IR reflected and MTE 365nm induced 850nm IR LUM.

[Adrian]

Thank you for this. I will try with my MTE 303 source. It's very possible also I suppose that my 950nm filter is not very narrow cut (it's a relatively cheap Chinese one!)

[Cadmium]

Hi Adrian, by narrow cut, do you mean sharp cut-off? As longpass filters get higher in nm they tend to have less sharp cut-off slopes (for example below graph).

However I would not expect that would make a difference here. I would expect your 950 IR filter is only seeing IR.

So this makes me wonder if your light filter is really blocking IR from the flash well enough.

Expect some long exposure time with the MTE, and moving the light around (painting the scene).

 

[nfoto]

In comment to the first post, my view is that the final sunflower image is practically pure IR.

[Cadmium]

Hi Adrian, I think the flash filter is transmitting IR, so try stacking it with some kind of BG filter, S8612 2mm would be best, but if you have something else use that.

[Adrian]

Many thanks for all the replies to my query. I have re-done the IR luminance shots, using my MTE U303 source, and various IR filters over the lens. I also tried putting a Baader U filter over the MTE, but that made very little difference. I now have interesting IR luminescence shots I think"! The torch was at a distance of around 6 feet, and moved throughout the surprisingly short exposure (the 720nm filter was 10 seconds @ f/8 @ 200 ISO with a full spectrum modified Nikon D300).

 

I will now try to find a blue-green light source for the excitation.

 

The images are: visible, 720nm filter, 950nm filter. The colours are straight from the camera. I would assume there is no point in trying to "white balance " them?

[Andrea B.]

I just ran through a few exercises in shooting IR fluorescence and it reminded me of the following which I'll share FWIW:

• It is important to make sure you have absolutely no ambient IR through windows or doors or from room lights when attempting to shoot IR fluorescence, so shoot completely in the dark (after setting up the shot in the light, of course). Even a little bit of ambient IR will wash out the desired results.
  

• Any light source must be filtered to prevent leakage. For example, every UV torch we've tested so far does leak some visible light. Test for those leaks before proceeding.
  

• And of course filter your lens with a filter thick enough to pass the desired wavelengths and block the others. Test for leakage here also.
  

• Make sure none of your filters are fluorescent under the excitation wavelength. For example, most all longpass filters are fluorescent under UV until you make it all the way up to about 695n nm.
  

• Make reference shots in all of Visible, UV and IR so you will know what the subject looks like normally in Vis, UV and IR. There are some foliage and flowers subjects which exhibit some IR absorption in some areas.
  

• Use enough excitation illumination so that you don't cross the line between recording fluorescence and forcing unwanted light through a filter. Like try to get the fluorescence shot in a 1 - 10 second exposure. Boost ISO if need be. Hold the illumination closer to the subject. And so forth.
  

• If you have indeed induced fluorescence in the subject, it should be very bright compared to the non-fluorescent areas. EDIT: I will back off on this one. This may not always be true.
  

[Cadmium]

Adrian, Nice to see your new tests. I don't know why our results look so different. Your shots don't have any 'pattern'. It sounds like we are doing it the same way.

[Adrian]

Again, very many thanks for the replies. I am away now for two weeks, probably without internet access, but will do so,me more tests on my return.

One things that has got me wondering: why would IR (reflected or luminescence) show any hidden patterns that are revealed by UV? Surely the flower is trying to attract insects which have UV, B, G vision, so IR probably would not reveal the same patterns that UV does??

[Cadmium]

I do find the 'pattern' interesting in fluorescence and luminescence, regardless of what bugs see. I don't know how fluorescence or luminescence would happen naturally for bugs. The lighting situation we use is not natural.

My thinking is that not everything happens because of bugs. ;)

Also notice how the light/dark pattern we normally associate with UV-only is reversed with 365nm UV induced 850nm IR LUM when shooting a sunflower, yet it is not reversed when shooting a rudbeckia.

Again interesting.

[Andrea B.]

Infrared luminescence is a byproduct of the plant's photosynthesis. Any light absorbed by the plant which is not used during the energy production process has to be dissipated as heat or light (conservation of energy). So certain plants and their pigments evolved to support this by producing a small amount of fluorescence under some lighting conditions.

 

The age or condition of the plant can affect the amount of fluorescence you see. Also whether the plant material has been dark-adapted or not. Could make for some interesting experiments. :)