BT709

BT.709, or ITU-R BT.709, is the ITU-R recommendation that defines the color space and related parameters for High-Definition television (HDTV). It specifies the primaries, white point, transfer function, and the Y'CbCr encoding used to represent color in HD video. The standard establishes a coherent framework for color reproduction across broadcast, consumer displays, and production workflows.

The color primaries in BT.709 are defined for a chromaticity triangle with red, green, and blue at approximately red (x=0.64, y=0.33), green (x=0.30, y=0.60), and blue (x=0.15, y=0.06), with the white point at D65. These primaries define a color gamut that is similar to but not identical with sRGB. Y' represents luminance, computed with luma coefficients close to Y' = 0.2126 R' + 0.7152 G' + 0.0722 B'. The chroma components Cb and Cr carry color information, and the standard specifies how RGB is transformed to and from Y'CbCr for encoding and decoding.

BT.709 also defines the transfer characteristics, i.e., the nonlinear EOTF that maps encoded values to linear light. In practice, this is a gamma-like curve with a nominal gamma around 2.4 and a small linear segment at the dark end to accommodate display response. The standard also encompasses recommended sampling formats used in HDTV, such as 4:2:2 and 4:4:4, and is widely adopted in broadcast, Blu-ray, and streaming.

In modern workflows, BT.709 remains the baseline SDR color space for 1080p and 720p content, contrasted with newer HDR and wider-gamut standards such as BT.2100 and DCI-P3 where applicable.