How does ultraviolet photography (fluorescence) work?

[Graeme M]

I was reading my latest Reader's Digest in which appears an article about ultraviolet photography. The article explains that this kind of photography utilises a technique known as ultraviolet induced visible fluorescence. The article says that the technique allows flowers to reveal spectacular colours "you'd expect on another planet". I thought that rather unlikely as presumably if we can see them they are simply different arrangements of colours we see everyday.

So I did a little digging and found that the colours are a result of fluorescence, something I've not ever really thought about before. The situation is that these flowers absorb the UV wavelengths and then emit light in longer (and therefore visible) wavelengths. So far so good.

But here's where I need a little clarification. I assume that these emissions are always occurring in typical sunlight - that is fluorescent materials must be both reflecting and emitting visible wavelengths (as everyday sunlight contains light across the spectrum from UV to IR). I assume that we do not usually "see" emitted wavelengths as they are presumably of less intensity than the reflected wavelengths and hence our cones are "swamped" by the reflected wavelengths?

Similarly, does it also follow that fluorescence doesn't depend on a particular set of wavelengths? That is, presumably fluorescent materials can absorb visible wavelengths and emit visible longer wavelengths (or even IR wavelengths)? Perhaps it is quite commonplace in nature? Does that mean then that the everyday world of colour is really a mix of both reflected and emitted wavelengths in some or many cases?

 

[phinds]

I assume that we do not usually "see" emitted wavelengths as they are presumably of less intensity than the reflected wavelengths and hence our cones are "swamped" by the reflected wavelengths?

I believe that's correct. I do fluorescent photography of wood sometimes and while I can see just a little of the emitted visible light when I shine UV on a flourescent piece in the daytime, that's only inside on a rainy day. In sunlight it would be TOTALLY swamped by the reflected light.

Similarly, does it also follow that fluorescence doesn't depend on a particular set of wavelengths? That is, presumably fluorescent materials can absorb visible wavelengths and emit visible longer wavelengths (or even IR wavelengths)?

No, this is not correct. There is a very limited band of UV light that causes fluorescence in wood and I believe that's also true of flowers. It centers around 365nm. Here's one of my educational slides:

 

[Graeme M]

Thanks phinds. So while fluorescence is by definition the absorption by objects of light at some wavelength(s) and consequent emission of light at longer wavelengths, it is a property only of material absorbing UV wavelengths? That is, no objects absorb say visible wavelengths, or IR wavelengths, and then emit (not reflect) longer wavelengths? If so, what is the peculiar property of UV wavelengths (or objects I guess) that causes that?

 

[phinds]

Wikipedia is your friend. Best I can do.

 

[Cutter Ketch]

Some materials can and do fluoresce in the infrared. For example fluorescence is the principle mechanism of essentially all optically pumped lasers, and the most common lasers are pumped and emit in the near IR. However fluorescence tends to happen at higher energies (shorter wavelengths). This is because the absorption and emission of light is governed by transitions in energy states of electrons in the atom, molecule, or material. The energy spacing between electronic states is usually large enough to absorb and emit NIR, visible, and UV light. Even higher energies are available because the electrons can transition further than just adjacent states. However lower energies are tough because the electrons can’t transition less than one state. Low energy longer wavelength transitions require lower energy transitions and so involve collective motions like vibrations or rotations of a molecule. Still, even then there are plenty of mechanisms that allow for absorption of one wavelength and reemmision at a lower wavelength.

 

[Graeme M]

Thanks Cutter Ketch. Thinking about it a little earlier and I suspected it was to do with the higher energies at shorter wavelengths. I think that I more or less understand how the whole fluorescence and UV photography hangs together now.