Plan Position Indicator (PPI)
The classic radar screen: a rotating line sweeps from centre to edge like a clock hand, brightening wherever an echo returns. The antenna rotates in sync. After one sweep the screen shows a map of everything around the station. Persistence phosphor leaves the blips visible for a few seconds, so the operator sees a complete picture even though the beam only passes each target once per rotation.
Phosphor chemistry
Short-persistence phosphors (P1, P31) for fast-updating tactical displays. Long-persistence phosphors (P7, P14) for search radars, where the operator needs the target to stay visible for several seconds after the sweep passes. Colour phosphors (rare in radar, common in TV) never caught on because radar didn't need colour — it needed contrast and speed.
A-scope and B-scope
Before PPI, there was the A-scope: a horizontal line with vertical blips — distance on the x-axis, signal strength on the y-axis. Still used inside modern radars as the raw signal view. The B-scope added azimuth on the x-axis and range on the y-axis, giving a rectangular map slice. Both survived as secondary displays long after PPI took over.
Decline and legacy
Solid-state displays replaced CRTs in the 1990s. But the visual language — sweep, blip, fade — is still how we imagine radar. Every movie radar screen, every video game UI, every control-tower display mimics the phosphor glow. The CRT invented radar's visual grammar.