Photographic Display Unit
The Photographic Display Unit, or PDU, was a large-format display system used by the Royal Air Force to present radar images for interpretation by a number of operators and commanders. Made by Kelvin Hughes, it projected a 6 feet (1.8 m) diameter image that could optionally be overlaid with a map and range rings.
Source: Wikipedia — Photographic Display Unit (CC BY-SA 4.0)