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a
b
) and the other is Hunter Lab (strictly, Hunter L, a, b). Lab is an informal abbreviation, and without further checking should not be assumed to be one or the other. The color spaces are related in intention and purpose, but are different.

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In Adobe Photoshop, image editing using "Lab" is CIELAB.

In ICC Profiles, the Lab color space used as a profile connection space is CIELAB.

In TIFF files, the Lab color space is CIELAB.

In PDF documents, the Lab color space is CIELAB.

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a
b
(CIELAB) is the most complete color model used conventionally to describe all the colors visible to the human eye. It was developed for this specific purpose by the International Commission on Illumination (Commission Internationale d'Eclairage, hence its CIE initialism). The
after L, a and b are part of the full name, since they represent L
, a
and b
, derived from L, a and b. CIELAB is an Adams Chromatic Value Space.

, L
=0 yields black and L
=100 indicates white), its position between magenta and green (a
, negative values indicate green while positive values indicate magenta) and its position between yellow and blue (b
, negative values indicate blue and positive values indicate yellow).

a
b
is based directly on the CIE 1931 XYZ color space as an attempt to linearize the perceptibility of color differences, using the color difference metric described by the MacAdam ellipse. The non-linear relations for L
, a
, and b
are intended to mimic the logarithmic response of the eye. Coloring information is referred to the color of the white point of the system, subscript n.

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a
b
, not understanding that RGB and CMYK are not absolute color spaces and so have no precise relation to an absolute color space such as L
a
b
. To convert between RGB and L
a
b
, for example, it is necessary to determine or assume an absolute color space for the RGB data, such as sRGB or Adobe RGB. For each of these absolute spaces, there are standard techniques for converting to and from the XYZ absolute color space (see for example SRGB color space#Specification of the transformation) which can be combined with the following transformations to convert them to L
a
b
.

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$L^$

= 116,f(Y/Y_n) - 16

$a^$

= 500,f(X/X_n) - f(Y/Y_n)

$b^$

= 200,f(Y/Y_n) - f(Z/Z_n)

$f(t)\; =\; t^,$ for $t\; >\; 0.008856,$

$f(t)\; =\; 7.787,t\; +\; 16/116$ otherwise

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define $f\_yequiv\; (L^$
+16)/116

define $f\_xequiv\; f\_y+a^$
/500

define $f\_zequiv\; f\_y-b^$
/200

if $f\_y\; >\; delta,$ then $Y=Y\_nf\_y^3,$   else $Y=(f\_y-16/116)3delta^2Y\_n,$

if $f\_x\; >\; delta,$ then $X=X\_nf\_x^3,$   else $X=(f\_x-16/116)3delta^2X\_n,$

if $f\_z\; >\; delta,$ then $Z=Z\_nf\_z^3,$   else $Z=(f\_z-16/116)3delta^2Z\_n,$

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u
v
(CIELUV) is based directly on CIE XYZ and is another attempt to linearize the perceptibility of color differences. The non-linear relations for L
, u
, and v
are given below:

$L^$

= 116 (Y/Y_n)^ - 16,

$u^$

= 13L^
( u' - u_n' ),

$v^$

= 13L^
( v' - v_n' ),

$u\text{'}\; =\; 4X\; /\; (X\; +\; 15Y\; +\; 3Z)\; =\; 4x\; /\; (\; -2x\; +\; 12y\; +\; 3\; ),$

$v\text{'}\; =\; 9Y\; /\; (X\; +\; 15Y\; +\; 3Z)\; =\; 9y\; /\; (\; -2x\; +\; 12y\; +\; 3\; ),$.

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$x\; =\; 27u\text{'}\; /\; (\; 18u\text{'}\; -\; 48v\text{'}\; +\; 36\; ),$

$y\; =\; 12v\text{'}\; /\; (\; 18u\text{'}\; -\; 48v\text{'}\; +\; 36\; ),$.

$u\text{'}\; =\; u^$

/ ( 13L^
) + u_n,

$v\text{'}\; =\; v^$

/ ( 13L^
) + v_n,

$Y\; =\; Y\_n((\; L^$

+ 16 ) / 116 )^3,

$X\; =\; -\; 9Yu\text{'}\; /\; ((\; u\text{'}\; -\; 4\; )\; v\text{'}\; -\; u\text{'}v\text{'}\; ),$

$Z\; =\; (\; 9Y\; -\; 15v\text{'}Y\; -\; v\text{'}X\; )\; /\; 3v\text{'},$

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