into XYZ space, but may be negative for transforming back to RGB. The green coefficient is normally the largest and blue normally smallest, and normally form the middle row of the RGB-to-XYZ color transformation matrix. For nonlinear gamma-compressed R′G′B′ color spaces as typically used for computer images, a linearization of the R′G′B′ components to RGB is needed before the linear combination. Relative luminance should not be confused with luma (Y prime), which is a weighted sum of nonlinear (gamma encoded) R′G′B′ components, where in some implementations the weighting coefficients are applied to the gamma encoded signal. Some colorspaces that use luma include Y′UV, Y′IQ, and Y′CbCr. To determine relative luminance, The must be used with the subcomponents to create the gamma encoded R′G′B′ components, which are then linearized to RGB by inverting the gamma correction. These linearized RGB channels can then have the appropriate linear coefficients applied (based on the primary chromaticities) and summed to relative luminance . Relative luminance and perceptual spaces is linear to light, but human perception has a non-linear response to lightness/darkness/brightness. For L*a*b* and L*u*v* space, the component is perceptual lightness (also known as "Lstar" and not to be confused with luminance). is intended to be linear to human perception of lightness/darkness, and since human perception of light is non-linear, is a nonlinear function of relative luminance . See also Luminance CIE 1931 color space Chromaticity References Color Photometry Film and video technology