CIE Lab

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20 Sep 2026 - Nicola Asuni

Lightness on one axis and two opponent chroma axes, scaled so that equal numeric steps are approximately equal perceptual steps. Lab coordinates describe a colour independently of the device rendering it.

Two parts of the library use that: perceptual distance on Nearest Colour, and the Lab alternate colour space of a spot colour in a PDF.

Model

Item	Value
Type string	`LAB`
Enum case	`ColorModelType::Lab`
Class	`\Com\Tecnick\Color\Model\Lab`
PDF colour space	DeviceRGB, by conversion
Notations	`lab()`

Components

Component	Key	Range	Out of range
L* (lightness)	`lstar`	0.0 to 100.0	clamped
a* (green to red)	`astar`	-128.0 to 127.0	clamped
b* (blue to yellow)	`bstar`	-128.0 to 127.0	clamped
Alpha	`alpha`	0.0 to 1.0, default 1.0	clamped

These are the only components in the library that are not fractions in [0..1], and the only ones that go negative.

Construction

<?php

require_once __DIR__ . '/vendor/autoload.php';

$lab = new \Com\Tecnick\Color\Model\Lab([
    'lstar' => 42.008144,
    'astar' => -0.151700,
    'bstar' => -32.846040,
]);

echo $lab->getCssColor(), "\n";
echo $lab->getRgbHexColor(), "\n";
echo $lab->getPdfColor();

lab(42.0081% -0.1517 -32.846)
#336699
0.200000 0.400000 0.600000 rg

Output

Every accessor, against #336699:

Call	Result
`getCssColor()`	`lab(42.0081% -0.1517 -32.846)`
`getRgbHexColor()`	`#336699`
`getRgbaHexColor()`	`#336699ff`
`getComponentsString()`	`42.008144 -0.151700 -32.846040`
`getPdfColor()`	`0.200000 0.400000 0.600000 rg`
`getPdfColor(true)`	`0.200000 0.400000 0.600000 RG`
`getJsPdfColor()`	`["RGB",0.200000,0.400000,0.600000]`
`getArray()`	`['L' => 42.0081436628616, 'a' => -0.15169986265223256, 'b' => -32.84603952194887, 'A' => 1.0]`
`getNormalizedArray(255)`	`['L' => 42.0, 'a' => -0.0, 'b' => -33.0, 'A' => 1.0]`
`getPDFacArray()`	`[0.20000005882999666, 0.3999999766783507, 0.6000000097846151]`

Two results from that table. getNormalizedArray(255) ignores its argument and rounds to integers, so the -0.1517 the CSS form keeps arrives as -0.0; getArray() returns the unrounded components. getPdfColor() emits DeviceRGB, since PDF has no Lab colour space for a plain fill. A Lab value reaches a PDF as the alternate space of a spot colour, not as a colour operator.

The White Point

D65, fixed, as the row sums of the sRGB to XYZ matrix the conversion uses. A Lab value computed elsewhere under D50, which is the illuminant most print workflows use, will not match one computed here.

addSpotLabColor() takes a white point option, but it only sets the metadata written into the PDF Lab colour space. The CMYK equivalent it stores alongside is computed under D65 regardless.

Clamping

$lab = new \Com\Tecnick\Color\Model\Lab([
    'lstar' => 120.0,   // clamped to 100
    'astar' => 200.0,   // clamped to 127
    'bstar' => -200.0,  // clamped to -128
]);

echo $lab->getCssColor(), "\n";
print_r($lab->getNormalizedArray(255));

lab(100% 127 -128)
Array
(
    [L] => 100
    [a] => 127
    [b] => -128
    [A] => 1
)

The a* and b* axes are not bounded in theory. The [-128..127] interval is what this model represents, and addSpotLabColor() clamps its range option to the same interval, so a spot colour cannot declare a range wider than the components it stores.