International Commission on Illumination
A Lab color space is a color-opponent space with dimension L for lightness and b for the color-opponent dimensions, based on nonlinearly CIE XYZ color space coordinates.
CIE Lab Colorspace:
L is always pasitive and represents brightness
a>0 represents red conponent, a< 0 green conponent
b>0 represents yellow conponent,and b <0 blue conponent