Looking at an IR-transmitter, but have no clue weather it is on or off?
Turn on your camera, video or still, as long as it is digital.
This blue spot does not show for the human eye - but digitize it, and you can see :)
Sometimes you just get a little black piece of plastic, have nu clue, and nothing happens.. Camera!!! Blue spot!!!
(You can also use it to test if your TV's remote needs new batteries, or something is wrong with the TV:)
