TIFF 6.0 Specification
Final—June 3, 1992
Section 23: CIE L*a*b* Images
What is CIE L*a*b*?
CIE La*b* is a color space that is colorimetric, has separate lightness and chroma
channels, and is approximately perceptually uniform. It has excellent applicability
for device-independent manipulation of continuous tone images. These attributes
make it an excellent choice for many image editing functions.
1976 CIEL*a*b* is represented as a Euclidean space with the following three
quantities plotted along axes at right angles:
L*
representing lightness,
a*
repre-
senting the red/green axis, and
b*
representing the yellow/blue axis. The formulas
for 1976 CIE
L*a*b*
follow:
L*=116(Y/Y
n
)
1/3
-16
L*=903.3(Y/Y
n
)
for
Y/Y
n
> 0.008856
for
Y/Y
n
<= 0.008856
*see note below.
a*=500
[
(X/X
n
)
1/3
-(Y/Y
n
)
1/3
]
b*=200
[
(Y/Y
n
)
1/3
-(Z/Z
n
)
1/3
]
.
where
X
n
,Y
n
, and
Z
n
are the CIE
X, Y,
and
Z
tristimulus values of an
appropriate
reference white. Also, if any of the ratios
X/X
n
,
Y/Y
n
, or
Z/Z
n
is equal to or less than
0.008856, it is replaced in the formulas with
7.787F + 16/116,
where
F
is
X/X
n
,
Y/Y
n
, or
Z/Z
n
, as appropriate (note: these low-light conditions are
of no relevance for most document-imaging applications). Tiff is defined such
that each quantity be encoded with 8 bits. This provides 256 levels of
L*
lightness;
256 levels (+/- 127) of
a*,
and 256 levels (+/- 127) of
b*.
Dividing the 0-100
range of
L*
into 256 levels provides lightness steps that are less than half the size
of a “just noticeable difference”. This eliminates banding, even under conditions
of substantial tonal manipulation. Limiting the theoretically unbounded
a*
and
b*
ranges to +/- 127 allows encoding in 8 bits without eliminating any but the most
saturated self-luminous colors. It is anticipated that the rare specialized applica-
tions requiring support of these extreme cases would be unlikely to use CIELAB
anyway. All object colors, in fact all colors within the theoretical MacAdam lim-
its, fall within the +/- 127
a*/b*
range.
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