TIFF 6.0 Specification
Final—June 3, 1992
Color Difference Measurements in CIELAB
The differences between two colors in
L*, a*,
and
b*
are denoted by DL*, Da*,
and Db*, respectively, with the total (3-dimensional) color difference represented
as:
∆E*
ab
=
[
(∆E*)
2
+(∆a*)
2
+(∆b*)
2
]
1/2
.
This color difference can also be expressed in terms of
L*, C*,
and a measure of
hue. In this case,
h
ab
is
not
used because it is an angular measure and cannot be
combined with
L*
and
C*
directly. A linear-distance form of hue is used instead:
CIE
1976
a,b hue-difference,
∆H*
ab
∆H*
ab
=
[
(∆E*)
2
-(∆L*)
2
-(∆C*)
2
]
1/2
.
where DC* is the chroma difference between the two colors. The total color dif-
ference expression using this hue-difference is:
∆E*
ab
=
[
(∆L*)
2
+(∆H*)
2
+(∆b*)
2
]
1/2
.
It is important to remember that color difference is 3-dimensional: much more can
be learned from a DL*a*b* triplet than from a single DE value. The DL*C*H*
form is often the most useful since it gives the error information in a form that has
more familiar perception correlates. Caution is in order, however, when using
DH* for large hue differences since it is a straight-line approximation of a curved
hue distance.
The Merits of CIELAB
Colorimetric.
First and foremost, CIELAB is colorimetric. It is traceable to the internationally-
recognized standard CIE 1931 Standard Observer. This insures that it encodes
color in a manner that is accurately modeled after the human vision system. Col-
ors seen as matching are encoded identically, and colors seen as not matching are
encoded differently. CIELAB provides an unambiguous definition of color with-
out the necessity of additional information such as with RGB (primary
chromaticities, white point, and gamma curves).
Device Independent.
Unlike RGB spaces which associate closely with physical phosphor colors,
CIELAB contains no device association. CIELAB is not tailored for one device or
device type at the expense of all others.
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