Luma is the brightness, or luminance, channel of a video signal, the Y component in the YCbCr color representation, separated from the two color channels Cb and Cr. It carries the light-and-dark detail of the picture. The reason it matters is a fact of human vision: the eye is far more sensitive to brightness detail than to color detail, so luma carries most of the information a viewer actually perceives as sharpness. This is why codecs preserve luma at full resolution while subsampling chroma, and why structural metrics such as SSIM compute their core comparison on the luma channel and why quality work weights it heavily. The catch is that focusing on luma alone can miss genuine color artifacts: a metric that scores only brightness will not see color bleeding on saturated edges or text. Luma is the counterpart to chroma, and the asymmetry between them, the eye favoring luma, is the basis for chroma subsampling schemes like 4:2:0.
