Vectorscope
A circular display showing an image's colour information as hue by angle and saturation by distance from the centre.
In depth
Its most practical feature is the skin tone line, a diagonal along which correctly rendered human skin falls regardless of complexion. Pulling a trace onto that line is the fastest reliable way to correct a colour cast on a face, and the display makes any imbalance visible instantly.
Examples in practice
A trace sitting to the green side of the skin line confirms a fluorescent cast before any eye judgement is made.