Standard color space with color-opponent values
The
CIELAB color space
, also referred to as
L*a*b*
, is a
color space
defined by the
International Commission on Illumination
(abbreviated CIE) in 1976.
[a]
It expresses color as three values:
L*
for perceptual lightness and
a*
and
b*
for the four
unique colors
of human vision: red, green, blue and yellow. CIELAB was intended as a
perceptually uniform
space, where a given numerical change corresponds to a similar perceived change in color. While the LAB space is not truly perceptually uniform, it nevertheless is useful in industry for detecting small differences in color.
Like the
CIEXYZ
space it derives from, CIELAB color space is a device-independent, "standard observer" model. The colors it defines are not relative to any particular device such as a computer monitor or a printer, but instead relate to the
CIE standard observer
which is an averaging of the results of color matching experiments under laboratory conditions.
Coordinates
[
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]
The CIELAB space is three-dimensional and covers the entire
gamut
(range) of human color perception. It is based on the
opponent model
of human vision, where red and green form an opponent pair and blue and yellow form an opponent pair. The lightness value,
L*
(pronounced "L star"), defines black at 0 and white at 100. The
a*
axis is relative to the green?red opponent colors, with negative values toward green and positive values toward red. The
b*
axis represents the blue?yellow opponents, with negative numbers toward blue and positive toward yellow.
The
a*
and
b*
axes are unbounded and depending on the reference white they can easily exceed ±150 to cover the human gamut. Nevertheless, software implementations often clamp these values for practical reasons. For instance, if integer math is being used it is common to clamp
a*
and
b*
in the range of ?128 to 127.
CIELAB is calculated relative to a
reference white
, for which the CIE recommends the use of CIE
Standard illuminant
D65
.
[1]
D65 is used in the vast majority of industries and applications, with the notable exception being the
printing industry
which uses D50. The
International Color Consortium
largely supports the printing industry and uses
D50
with either CIEXYZ or CIELAB in the Profile Connection Space, for v2 and v4
ICC profiles
.
[2]
While the intention behind CIELAB was to create a space that was more perceptually uniform than CIEXYZ using only a simple formula,
[3]
CIELAB is known to lack
perceptual uniformity
, particularly in the area of blue hues.
[4]
The lightness value,
L*
in CIELAB is calculated using the cube root of the
relative luminance
with an offset near black. This results in an
effective
power curve with an exponent of approximately 0.43 which represents the human eye's response to light under daylight (
photopic
) conditions.
The
sRGB
gamut (
left
) and visible gamut under D65 illumination (
right
) plotted within the CIELAB color space.
a
and
b
are the horizontal axes;
L
is the vertical axis.
The three coordinates of CIELAB represent the lightness of the color (
L*
= 0 yields black and
L*
= 100 indicates diffuse white; specular white may be higher), its position between red and green (
a*
, where negative values indicate green and positive values indicate red) and its position between yellow and blue (
b*
, where negative values indicate blue and positive values indicate yellow). The asterisks (*) after
L*
,
a*,
and
b*
are pronounced
star
and are part of the full name to distinguish
L
*
a
*
b
* from Hunter's
Lab
, described below.
Since the
L*a*b*
model has three axes, it requires a three-dimensional space to be represented completely.
[5]
Also, because each axis is non-linear, it is not possible to create a two-dimensional chromaticity diagram. Additionally, it is important to understand that the visual representations shown in the plots of the full CIELAB gamut on this page are an approximation, as it is impossible for a monitor to display the full gamut of LAB colors.
The green-red and blue-yellow opponent channels relate to the human vision system's opponent color process. This makes CIELAB a
Hering
opponent color space. The nature of the transformations also characterizes it as an
chromatic value
color space.
Perceptual differences
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The nonlinear relations for
L*
,
a*
and
b*
are intended to mimic the nonlinear response of the visual system. Furthermore, uniform changes of components in the
L*a*b*
color space aim to correspond to uniform changes in perceived color, so the relative perceptual differences between any two colors in
L*a*b*
can be approximated by treating each color as a point in a three-dimensional space (with three components:
L*
,
a*
,
b*
) and taking the
Euclidean distance
between them.
[6]
RGB and CMYK conversions
[
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]
In order to convert
RGB
or
CMYK
values to or from
L*a*b*
, the RGB or CMYK data must be linearized relative to light. The reference illuminant of the RGB or CMYK data must be known, as well as the RGB primary coordinates or the CMYK printer's reference data in the form of a color lookup table (CLUT).
In color managed systems,
ICC profiles
contains these needed data, which are then used to perform the conversions.
Range of coordinates
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]
As mentioned previously, the
L
* coordinate nominally ranges from 0 to 100. The range of
a
* and
b
* coordinates is technically unbounded, though it is commonly clamped to the range of ?128 to 127 for use with integer code values, though this results in potentially clipping some colors depending on the size of the source color space. The gamut's large size and inefficient utilization of the coordinate space means the best practice is to use floating-point values for all three coordinates.
Advantages
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Unlike the
RGB
and
CMYK
color models, CIELAB is designed to approximate human vision. The
L*
component closely matches human perception of lightness, though it does not take the
Helmholtz?Kohlrausch effect
into account. CIELAB is less uniform in the color axes, but is useful for predicting small differences in color.
The CIELAB coordinate space represents the entire
gamut
of human photopic (daylight) vision and far exceeds the gamut for sRGB or CMYK. In an integer implementation such as TIFF, ICC or Photoshop, the large coordinate space results in substantial data inefficiency due to unused code values. Only about 35% of the available coordinate code values are inside the CIELAB gamut with an integer format.
[7]
Using CIELAB in an 8-bit per channel integer format typically results in significant quantization errors. Even 16-bit per channel can result in clipping, as the full gamut extends past the bounding coordinate space. Ideally, CIELAB should be used with floating-point data to minimize obvious quantization errors.
CIE standards and documents are copyright by the CIE and must be purchased; however, the formulas for CIELAB are available on the CIE website.
[8]
Converting between CIELAB and CIEXYZ coordinates
[
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From CIEXYZ to CIELAB
[
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]
where
t
is
or
:
X
,
Y
, and
Z
describe the color stimulus considered and
X
n
,
Y
n
,
Z
n
describe a specified white achromatic reference illuminant. for the CIE 1931 (2°) standard colorimetric observer and assuming normalization where the reference white has
Y
= 100
, the values are:
For
Standard Illuminant D65
:
For
illuminant D50
, which is used in the printing industry:
The division of the domain of the
f
function into two parts was done to prevent an infinite slope at
t
= 0
. The function
f
was assumed to be linear below some
t
=
t
0
and was assumed to match the
part of the function at
t
0
in both value and slope. In other words:
The intercept
f
(0) =
c
was chosen so that
L
*
would be 0 for
Y
= 0
:
c
=
16
/
116
=
4
/
29
. The above two equations can be solved for
m
and
t
0
:
where
δ
=
6
/
29
.
[9]
[10]
From CIELAB to CIEXYZ
[
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]
The reverse transformation is most easily expressed using the inverse of the function
f
above:
where
and where
δ
=
6
/
29
.
Cylindrical model
[
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]
The
sRGB
gamut (
left
) and visible gamut under D65 illumination (
right
) plotted within the CIELCHab color space.
L
is the vertical axis;
C
is the cylinder radius;
h
is the angle around the circumference.
The "CIELCh" or "CIEHLC" space is a color space based on CIELAB, which uses the
polar coordinates
C
* (
chroma
, relative saturation) and
h
° (hue angle, angle of the hue in the CIELAB color wheel) instead of the
Cartesian coordinates
a
* and
b
*. The CIELAB lightness L* remains unchanged.
The conversion of
a
* and
b
* to
C
* and
h
° is performed as follows:
Conversely, given the
polar coordinates
, conversion to Cartesian coordinates is achieved with:
The LCh (or HLC) color space is not the same as the HSV, HSL or HSB color models, although their values can also be interpreted as a base color, saturation and lightness of a color. The HSL values are a polar coordinate transformation of what is technically defined RGB cube color space. LCh is still
perceptually uniform
.
L
is the vertical axis;
C
is the cylinder radius;
h
is the angle around the circumference.
Further,
H
and
h
are not identical, because HSL space uses as primary colors the three additive primary colors red, green and blue (
H
= 0, 120, 240°). Instead, the LCh system uses the four colors red, yellow, green and blue (
h
= 0, 90, 180, 270°). Regardless the angle
h
,
C
= 0 means the achromatic colors (non saturated), that is, the gray axis.
The simplified spellings LCh, LCh(ab), LCH, LCH(ab) and HLC are common, but the letter presents a different order.
HCL color space
(Hue-Chroma-Luminance) on the other hand is a commonly used alternative name for the
L*C*h(uv)
color space, also known as the
cylindrical representation
or
polar
CIELUV
. This name is commonly used by
information visualization
practitioners who want to present data without the bias implicit in using varying
saturation
.
[11]
[12]
The name Lch(ab) is sometimes used to differentiate from L*C*h(uv).
Other related color spaces
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]
A related color space, the CIE 1976
L
*
u
*
v
* color space (a.k.a.
CIELUV
), preserves the same
L*
as
L*a*b*
but has a different representation of the chromaticity components. CIELAB and CIELUV can also be expressed in cylindrical form (CIELCh
ab
[13]
and
CIELCh
uv
, respectively), with the chromaticity components replaced by correlates of
chroma
and
hue
.
Since the work on CIELAB and CIELUV, the CIE has been incorporating an increasing number of
color appearance phenomena
into their models and difference equations to better predict human color perception. These
color appearance models
, of which CIELAB is a simple example,
[14]
culminated with
CIECAM02
.
Usage
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Some systems and software applications that support CIELAB include:
- CIELAB is used by
Datacolor
spectrophotometers
, including the related
color difference
calculations.
- CIELAB is used by the PantoneLive library.
- CIELAB is used extensively by
XRite
as a color space with their hardware and software color measuring systems.
- CIELAB D50 is available in
Adobe Photoshop
, where it is called "Lab mode".
[15]
[16]
- CIELAB is available in
Affinity Photo
by changing the document's Colour Format to "Lab (16 bit)". The
white point
, which defaults to
D50
, can be changed by
ICC profile
.
- CIELAB D50 is available in
ICC profiles
as a
profile connection space
named "Lab color space".
[2]
- CIELAB (any white point) is a supported color space in
TIFF
image files.
[17]
- CIELAB (any white point) is available in
PDF
documents, where it is called the "Lab color space".
[18]
[19]
- CIELAB is an option in Digital Color Meter on
macOS
described as "L*a*b*".
- CIELAB is available in the
RawTherapee
photo editor, where it is called the "Lab color space".
[20]
- CIELAB is used by
GIMP
for the hue-chroma adjustment filter, fuzzy-select and paint-bucket. There is also a LCh(ab) color picker.
[21]
- Web browser support for CIELAB was introduced as part of
CSS
Color Module Level 4, and is supported in all major browsers.
[22]
[23]
Hunter Lab
[
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]
See also
[
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]
Notes
[
edit
]
- ^
Referring to CIELAB as "Lab" without asterisks should be avoided to prevent confusion with
Hunter Lab
.
References
[
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]
- ^
CIE Colorimetry 15
(Third ed.). CIE. 2004.
ISBN
3-901-906-33-9
.
- ^
a
b
International Color Consortium,
Specification ICC.1:2004-10 (Profile version 4.2.0.0)
Image technology color management ? Architecture, profile format and data structure,
(2006).
- ^
MacEvoy, Bruce.
"Explanation of this history"
.
- ^
A discussion and proposed improvement
, Bruce Lindbloom
- ^
3D representations of the
L*a*b*
gamut
, Bruce Lindbloom.
- ^
Jain, Anil K. (1989).
Fundamentals of Digital Image Processing
. New Jersey, United States of America:
Prentice Hall
. pp.
68
, 71, 73.
ISBN
0-13-336165-9
.
- ^
"LAB Integer Gamut ?Bruce Lindbloom"
.
brucelindbloom.com
. Retrieved
2020-12-12
.
- ^
"CIELAB Formula"
.
CIE Terms List
. Retrieved
2024-05-10
.
- ^
Janos Schanda (2007).
Colorimetry
. Wiley-Interscience. p. 61.
ISBN
978-0-470-04904-4
.
- ^
"CIE 1976 L*a*b* colour space | eilv"
.
eilv.cie.co.at
. Archived from
the original
on 2019-12-28
. Retrieved
2020-12-12
.
- ^
Zeileis, Achim; Hornik, Kurt; Murrell, Paul (2009).
"Escaping RGBland: Selecting Colors for Statistical Graphics"
(PDF)
.
Computational Statistics & Data Analysis
.
53
(9): 3259?3270.
doi
:
10.1016/j.csda.2008.11.033
.
- ^
Stauffer, Reto; Mayr, Georg J.; Dabernig, Markus; Zeileis, Achim (2015).
"Somewhere over the Rainbow: How to Make Effective Use of Colors in Meteorological Visualizations"
(PDF)
.
Bulletin of the American Meteorological Society
.
96
(2): 203?216.
Bibcode
:
2015BAMS...96..203S
.
doi
:
10.1175/BAMS-D-13-00155.1
.
hdl
:
10419/101098
.
- ^
CIE-L*C*h Color Scale
- ^
Fairchild, Mark D. (2005).
"Color and Image Appearance Models"
.
Color Appearance Models
. John Wiley and Sons. p. 340.
ISBN
0-470-01216-1
.
- ^
Margulis, Dan (2006).
Photoshop Lab Color: The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace
. Berkeley, Calif. : London: Peachpit; Pearson Education.
ISBN
0-321-35678-0
.
- ^
The Lab Color Mode in Photoshop
, Adobe TechNote 310838
- ^
TIFF: Revision 6.0
Archived
2000-08-15 at the
Wayback Machine
Adobe Developers Association, 1992
- ^
Color Consistency and Adobe Creative Suite
Archived
2008-07-25 at the
Wayback Machine
- ^
Adobe Acrobat Reader 4.0 User Guide
"The color model Acrobat Reader uses is called CIELAB…"
- ^
"Lab Adjustments - RawPedia"
.
rawpedia.rawtherapee.com
. Retrieved
2018-05-08
.
- ^
"2.6. More use for CIE LAB and CIE LCH"
.
docs.gimp.org
.
- ^
"Color Module Level 4"
.
w3.org
. Retrieved
2023-10-06
.
- ^
"lab() - CSS: Cascading Style Sheets MDN"
.
developer.mozilla.org
. Retrieved
2023-10-06
.
- ^
Hunter, Richard Sewall (July 1948).
"Photoelectric Color-Difference Meter"
.
JOSA
.
38
(7): 661.
(Proceedings of the Winter Meeting of the Optical Society of America)
- ^
Hunter, Richard Sewall (December 1948).
"Accuracy, Precision and Stability of New Photo-electric Color-Difference Meter"
.
JOSA
.
38
(12): 1094.
(Proceedings of the Thirty-Third Annual Meeting of the Optical Society of America)
- ^
Hunter, Richard Sewall (July 1948).
"Photoelectric Color-Difference Meter"
.
JOSA
.
38
(7): 661.
(Proceedings of the Winter Meeting of the Optical Society of America)
- ^
Hunter, Richard Sewall (December 1948).
"Accuracy, Precision, and Stability of New Photo-electric Color-Difference Meter"
.
JOSA
.
38
(12): 1094.
(Proceedings of the Thirty-Third Annual Meeting of the Optical Society of America)
External links
[
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]