Luma vs lightness
Luma is calculated from (gamma-encoded) sRGB coordinates by the formula Luma = 0.299 * R + 0.587 * G + 0.114 * B
Lightness is the L* component of the L*a*b* color space. The coordinates of the form (L*, 0, 0) have been mapped to the sRGB color space for image storage.
