Luma (video, $Y^{\prime }$)

Luma is a video signal formed by combining nonlinear RGB component signals. It is conventionally written $Y^{\prime }$, with the prime warning that the components have already undergone a nonlinear encoding. It differs from physical luminance, which describes light with a photometric scale.

Under Recommendation ITU-R BT.709-6,The coefficients sum to one, so a neutral signal $R^{\prime }=G^{\prime }=B^{\prime }=t$ has luma $Y^{\prime }=t$. The same recommendation specifies a nonlinear conversion $f$ from a normalized linear primary signal $E$:The primed components are formed separately before combining them.^[ref]

An original calculation shows why the order matters. For a linear red primary $(R,G,B)=(1,0,0)$, the primed triple is also $(1,0,0)$ and luma is 0.2126. Combining the linear channels first gives normalized linear luminance $Y=0.2126$; applying $f$ to that scalar gives approximately 0.4485. ThusNonlinear encoding does not commute with this channel combination.

In YCbCr, luma is stored alongside colour-difference information. Chroma subsampling can preserve the luma sampling grid while reducing the other components' sampling. A luma image can therefore retain fine spatial variation without carrying every colour distinction of the original image; it is not a complete colour description.