Photographic Display Unit

The entire developing process took one minute, after which the frame was pulled into a large film projector that shone upwards into the bottom of the plotting table.

The plotting table was normally covered with a large sheet of semi-transparent paper that contained a National Grid map and allowed the operators to make notes directly on its surface.

When the film or developer system ran low or there was any sort of problem, the operator could switch to the second unit simply by moving a mirror under the projector.

The US SAGE system used the PDU hardware to project an even larger display onto a movie screen, which was used for overall battle control in the "subsector command post", more widely known as the "blue room".

In these systems the original CRT was replaced with a charactron driven by the FSQ-7 computer, outputting both graphics and character data onto the film.