Harry Bloomfield wrote:
Ian Stirling submitted this idea :
Just in case you do not already know, IR will need some light to be
able to see. Ordinary lights will work perfectly well, IR lights will
illuminate an area invisibly.

Generally not completely invisibly.

Well not quite invisibly if we are talking about IR filament lamps, but
you would need to be on the look out for IR lights, to notice them. The
alternative to filament lamps is IR diodes (as used in TV remotes), the
output of which is completely invisible to the eye. These work well to
illuminate at shorter ranges. Even the glow from a cigarette is enough
to illuminate someones face.

Not completely invisible.
Much less visible.

Try looking into the LED of a remote in a dark room, with dark adapted eyes.
Many will glow noticably when activated.

The source is often visible as a crimson glow.
Pity thermal IR cameras are not yet cheap.