blackout

Found this strange object in my Dad's loft the other day. Apparently it was developed during the second world war and is called a blackout bulb. It's remarkable property is that when turned on, it emits only darkness.

You can imagine how important this device could be, useful for concealing virtually anything in darkness.

The first picture below shows it when it's turned off.

The second picture was taken by a high-speed camera in the split-second before it is fully deluminated.

The third picture is a photograph of the bulb when it is working.





Thanks to Robert for the information.