In article ,
Pat wrote:

Back to the original question, most of the newer cameras have an IR
filter to keep daytime IR light from fuzzing up the picture. If the
camera has nighttime IR illumination, then that filter is turned off
at night so your IR scheme would work at night.

If that's the case, then I suspect that a bunch of high-output narrow-
dispersion IR LEDs, aimed in the direction of the camera, and driven
with periodic high-current pulses, might be the way to go. Think "IR
flash". You can get significantly higher peak intensity from LEDs by
pulsing them - their peak-current capacity is higher than their
continuous-current capacity.

Camera sensors tend to have some "memory", and so if "blinded" by a
bright flash they'll take a fraction of a second (or more) to recover,
just as human eyes do.

Strobing a bank of IR LEDs several times a second might "give 'em
fits".

If you really want to get cute, build a sizable panel of IR LEDs in a
rectangular layout with individual drivers (e.g. one transistor per
LED, or row-and-column drivers), hook it up to a PC or single-board
computer through a suitable interface, and write some software which
"strobes" a message across it.

"STOP SPYING ON US!"

It'd be invisible to the eye, but visible to the camera.