Many substances fluoresce under UV light. Are there any
    which do so under IR or are the energy levels all wrong ?

The energy levels are wrong.


    I think short wavelength UV is converted to longer wave-length visible
    light by the fluorescee and IR is longer than visible.

Exactly.  For fluorescence, you absorb a photon at one wavelength/energy,
and emit at a different (lower!) energy level.  It's tough to go from a
lower energy photo to a higher energy one.  (altough - I think radio shack
(of all places) used to sell an "IR detecting phosphor thing" that could be
activated by exposing to UV, and would then emit in the visible region when
stimulated with IR...)


    Just wondering if there's some quirk of nature/physics that would make
    it possible to use invisible flood- lighting

Well, modern semiconductor image sensors (CCDs and CMOS) are pretty
sensitive to near-IR, and there is no shortage of vendors selling "see in
the dark" video system consisting of IR-sensitive cameras and IR LEDs as a
light source.  Likewise, most "image intensifiers" work in the IR range.
If your "floodlighting" is for use by a camera, you're all set.

BillW

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