Chrominance refers to a measurement of the quality of the light obtained by comparing the hue, saturation, and luminance of a given color to a reference color source. In video systems, chrominance refers to the signal used to convey the color information of the picture, separately from the accompanying luma signal (or Y for short).
Chrominance is usually represented as two color-difference components: U = B′ − Y′ (blue − luma) and V = R′ − Y′ (red − luma). Each of these different components may have scale factors and offsets applied to it, as specified by the applicable video standard.
In composite video signals, the U and V signals modulate a color subcarrier signal, and the result is referred to as the chrominance signal; the phase and amplitude of this modulated chrominance signal correspond approximately to the hue and saturation of the color. In digital-video and still-image color spaces such as Y′CbCr, the luma and chrominance components are digital sample values.
Separating RGB color signals into luma and chrominance allows the bandwidth of each to be determined separately. Typically, the chrominance bandwidth is reduced in an analog composite video by reducing the bandwidth of a modulated color subcarrier, and in digital systems by chroma subsampling.
Digital video and digital still photography systems sometimes use a luma/chroma decomposition for improved compression. For example, when an ordinary RGB digital image is compressed via the JPEG standard, the RGB colorspace is first converted (by a rotation matrix) to a YCbCr colorspace, because the three components in that space have less correlation redundancy and because the chrominance components can then be subsampled by a factor of 2 or 4 to further compress the image. On decompression, the Y′CbCr space is rotated back to RGB.