Further to my last post, I've just discovered Ultra Magenta! It's not really ultra magenta of course - that's just my name for it. I experimented further with my digital camera and discovered that it could also see ultraviolet, which the blue and red sensors render as a kind of visible magenta!
Add my ultra magenta to my infra green (see previous post) and you've got invisible white light!
Think about it - a flash unit with an infra red and an ultraviolet source, both invisible to the human eye but the camera sees it as white light! The ultimate unobtrusive flash.
No - this is too good. It's probably been done...
I need to do tests, tests, yes, tests (mutter mutter...)