A Vectorscope is, very simply, an X-Y plot of the U and V signals, the U signal vertically and the V signal horizontally, resulting in a display that's effectively a colour wheel of hues with red at or near the top. Rotating a hue control in a grading application will cause the vectorscope display to rotate. More saturated colours provoke greater deviation of the U and V signals and will be displayed toward the edges of the circular graticule of the vectorscope. Completely unsaturated colours are displayed as a single spot in the centre. Most Vectorscopes provide targets into which the dots representing the blocks of colour in a colour bar test signal should fall, which was their original purpose – to allow VTRs to be set up to display accurate colour. Digital signalling means that colour pictures are generally either guaranteed perfect or entirely absent, although there are a number of subtleties, based on the exact mathematics used to go between RGB and YUV, which could be caught with a Vectorscope and colour bars.
A vectorscope displaying colour bars. Notice that the highly saturated colour blocks each appear as a dot near the outside of the display, which has targets into which the test colours should fall
Apart from broadcast television, it's uncommon that a Vectorscope would be used on set by a cinematographer, but they are very relevant to colourists who may use them to accurately match colours shot to shot, and to ensure technical standards are followed. Given that the U and V signals are displayed at 90 degrees to one another, it's clear that any position within a square region can be plotted on the Vectorscope, whereas most will actually display only the central circular area. This is a reference to the earliest days of colour television and the use of a signalling technique called quadrature amplitude modulation to graft colour onto black-and-white television while maintaining backward compatibility; the colour gamut was limited, broadly speaking, to the central circle of hues. Anything falling outside the circle – and often, things falling near its edge – may be colours that tape decks, transmitters and televisions can't handle.