luma

Luma, in the context of digital imaging and video, refers to the component that represents image brightness or luminance. It is distinct from chrominance, which encodes color information. In most video systems, the image is stored or transmitted as a combination of luma and chroma signals, allowing color information to be compressed more efficiently without greatly affecting perceived brightness.

In digital video, luma is denoted as Y' in color spaces such as Y'CbCr or Y'PbPr. The value of Y' is derived from the scene's RGB values through a weighted sum that varies by standard. For example, sequences based on ITU-R BT.601 (standard-definition television) use roughly Y' = 0.299 R' + 0.587 G' + 0.114 B', while ITU-R BT.709 (high-definition television) uses Y' = 0.2126 R' + 0.7152 G' + 0.0722 B'. In practice, R', G', B' may already be gamma-corrected, so Y' reflects perceptual brightness rather than linear light.

Luma enables several important properties. It allows chroma subsampling (for example 4:2:0), which reduces color resolution while preserving perceived luminance, enabling efficient compression. It also supports compatibility with black-and-white displays, since a pure grayscale image can be produced from the luma channel alone. In image processing, luma is commonly used for tasks such as grayscale conversion, edge detection, and luminance-based adjustments.

Beyond technical uses, the term luma may appear as a brand name or proper noun in various contexts. In those cases, its meaning is separate from luminance and should be interpreted within the relevant domain.