Basic Color Spaces (Part 2)

L*a*b  Color  Space

          L*a*b  Color  Space  L*a*b  Color  Space The  L*a*b*  Color  space  (also referred to as CIELAB)  is  presently  one  of the  most popular  color  spaces  for  measuring  object  color and  is  widely  used  in virtually  all  fields.  It  is  one of  the  uniforn color spaces defined by  CIE  in  1976 in order  to  reduce  one  of  the major problems of the original  Yxy  color  space : that equal distances on the x,y chromaticity  diagram  did  not correspond  to  equal  perceived   color  differences.  In this color space,   L* indicates lightness and a*  and b* are the chromaticity  coordinates.   Figure 6 shows the a*, b* chromaticity  diagram.  In  this  diagram, the a*  and b* indicate  color directions : +a*  is the red direction,  -a*  is  the green  direction, +b*  is  the yellow direction, and –b* is the blue direction.  The center is achromatic; as the a* and b*  values increase  and  the point moves out from the center,  the saturation of the color increase. Figure 8  is a representation of  the  color solid  for the L*a*b*  color  space;  Figure 1 is a view of this  solid cut horizontally  at  a constant L* value.

Figure 1 : L*a*b* color space chromaticity          Figure 2 : Portion of a*,b* chromaticity diagram of Figure 1

L*C*h* Color space

The L*c*h color space uses the same diagram as the L*a*b* color space , but uses cylindrical coordinates instead of rectangular coordinates. In this color space, L* of indicates lightness and is the same as the L* of the L*a*b* color space, c* is chroma, and h is the hue angle. The value of chroma C* is 0 at the center and increases according to the distance from the center. Hue angle h is defined as starting at the +a* axis and is expressed in degrees; 0 would be –a* (green), and 270 would –b* (blue). If we measure the apple using the L*C*h color space, we get the resuts shown below.

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