Infrared fluorescence from UV excitation

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Lou Jost
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Infrared fluorescence from UV excitation

Post by Lou Jost »

Edit: Google tells me that near-infrared fluorescence is actually a standard and well-known tool in some branches of biology, though they usually use visual light to excite the fluorescence.

There is no reason to think that fluorescence emissions miraculously end at the wavelength sensitivity limits of the human eye. I recently got my hands on a full spectrum converted A7R camera, so I decided to look for infrared fluorescence excited by UV light. It turns out that infrared fluorescence is even more common in nature than visible-light fluorescence, when 365nm UV light is used for excitation.

Here are three leaves from two species of plants in my yard, photographed with the full-spectrum camera fitted with a filter that only passes IR light (R72 Hoya, 720nm cut). The leaves are being illuminated by a 365nm UV flashlight fitted with a visible-light-blocking filter. There is a big Erythrina leaflet at left, a "ventral"-side-up leaf from a Malvaceae weed, and a "dorsal"-side-up leaf of the same weed. The middle leaf fluoresces strongly under UV light, the right leaf has a weak fluorescence in a distinct fishbone pattern, and the left leaf hardly fluoresces.
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Some UV filters could let IR light pass through, so this could be IR reflection rather than fluorescence, though it doesn't look like it (among other things, there is no trace of glare on the middle leaf, and violation of Snell's Law is evidence for the light's fluorescent origin). To prove it is fluorescence, I illuminated the same leaves with a normal flashlight to see what IR reflection looked like. The camera is still fitted with the same IR-bandpass filter. The three leaves are more or less equally bright, and in the rightmost leaf, the fishbone pattern seen in the fluorescence photo is gone. Also note that the IR illuminator caused reflections in the teflon-coated frying pan holding the leaves; there are no reflections in the picture showing fluorescence. So I conclude that I am really seeing infrared fluorescence.
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The glowing underside of that middle leaf is one of the most fluorescent leaves in the visible spectrum as well. Here you see a frying pan of different leaves exposed to 365nm UV light; the bright red and yellow leaf is the same species as the one that fluoresced in the infrared. The leaves to the left of it are "dorsal"-side-up leaves of the same species; note that the same fishbone pattern shows up in the visible-light fluorescence as in the IR fluorescence. In visible light this leaf is uniformly plain green with no pattern.

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