In the days before 24-hour TV, channels would often shut down for several hours a day. But this caused a problem for TV shops and engineers, who needed a signal to test their equipment.
Originally, this was literally a camera pointing at an image on a card, but the static image could burn itself into the camera. The invention of the monoscope camera allowed the test-card image to be built into a camera and broadcast without any problems.
The image is test card C, but the most famous is test card F - the little girl and the clown playing noughts and crosses - and a version of that was still being used in 2007 by Sky's HD service.
Comments
Dear Sirs
Do you by any chance have a spare monoscope camera? I have a brand new CRT (came from the ITA transmitter at Crystal Palace) and I'd really like to get this working.
Yours sincerely
Malcolm Beeson (TrueBlueFrog)