Introduction to Colour Spaces
CIE Lab & Lch
This is a very basic introduction to two related colour models which are becoming increasingly important in the world of colour reproduction. A colour model is merely a way of describing colour. These are among the tristimulus (three-dimensional) colour models ('spaces') developed by the CIE.
What is the CIE?
is short for 'Commission
l'Eclairage', which in English is the 'International
A professional scientific organisation founded some
90 years ago to exchange
information on 'all matters relating to the science and art of
The standards for colour spaces representing the visible
were established in 1931, but have been revised more recently.
For those of us involved in creating colour which will be reproduced on a printed page, it is easy to forget that there are other industries which need to accurately describe colour! RGB or CMYK descriptions won't be of any use to paint or textile manufacturers! Terms such as 'maroon' or 'navy blue' won't be precise enough.
There are many CIE colour spaces, more correctly known as models, which serve different purposes. They are all device independent, unlike RGB or CMYK colour spaces which are related to a specific device (camera, scanner, or press, etc.) and/or material type (paper, ink set, film emulsion or lighting, etc.). These RGB and CMYK spaces usually do not cover the entire visible colour spectrum or gamut. The CIE also specify lighting conditions.
This is possibly a little easier to comprehend than the Lab colour space, with which it shares several features. It is more correctly known as L*c*h*. Essentially it is in the form of a sphere. There are three axes; L* , c* and h°.
The L* axis represents Lightness. This is vertical; from 0, which has no lightness (i.e. absolute black), at the bottom; through 50 in the middle, to 100 which is maximum lightness (i.e. absolute white) at the top.
The c* axis represents Chroma or 'saturation'. This ranges from 0 at the centre of the circle, which is completely unsaturated (i.e. a neutral grey, black or white) to 100 or more at the edge of the circle for very high Chroma (saturation) or 'colour purity'.
The h* axis represents Hue. If we take a horizontal slice through the centre, cutting the 'sphere' ('apple') in half, we see a coloured circle. Around the edge of the circle we see every possible saturated colour, or Hue. This circular axis is known as h° for Hue. The units are in the form of degrees° (or angles), ranging from 0° (red) through 90° (yellow), 180° (green), 270° (blue) and back to 0°.
The Lch colour model is very useful for retouching images in a colour managed workflow, using high-end editing applications. Lch is device-independent.
A similar colour model is HSB or HSL, for Hue, Saturation and Brightness (Lightness), which can be used in Adobe Photoshop CS, CC, Lightroom, Camera Raw and other applications. Technically this is 'device-dependent', however it is particularly useful for editing RGB images. For example to edit a green: Adjust the Hue angle by increasing it to make it 'bluish' or by reducing it to make it 'yellowish'; Increase the Saturation (Chroma) to make it 'cleaner'; increase the Brightness or Lightness to make it lighter. Go on give it a try!
correctly known as CIE L*a*b*.
Just as in Lch, the vertical L* axis represents Lightness, ranging from 0-100. The other (horizontal) axes are now represented by a* and b*. These are at right angles to each other and cross each other in the centre, which is neutral (grey, black or white). They are based on the principal that a colour cannot be both red and green, or blue and yellow.
The a* axis is green at one extremity (represented by -a), and red at the other (+a).
The b* axis has blue at one end (-b), and yellow (+b) at the other.
The centre of each axis is 0. A value of 0, or very low numbers of both a* and b* will describe a neutral or near neutral. In the case of paper, the whitepoint in terms of a* and b* is usually carried through to the black, being gradually reduced towards '0'.
In theory there are no maximum values of a* and b*, but in practice they are usually numbered from -128 to +127 (256 levels).
The CIE Lab colour model encompasses the entire spectrum, including colours outside of human vision.
CIE Lab is extensively used in many
industries apart from printing and photography. It's uses
include providing exact colour specifications for paint
(including automotive, household, etc.), dyes (including textiles, plastics, etc.),
printing ink and paper. Nowadays it is becoming of increasing importance in
specifying printing standards such as in ISO-12647,
where it is usually used instead of densitometry.
For example Paper Type 1 (115gsm gloss coated white, wood-free) has 'Paper Shade' described as 'L* 95, a* 0, b* -2'. So the L*95 is very light, the a*0 neutral, and the b*-2 very slightly 'blueish'.
Paper Type 5 (115gsm uncoated yellowish offset) is described as 'L* 90, a* 0, b* 9'. So it is a darker, more 'yellow' paper. If you compare the different Lab values for Type 1 & 5 you will understand the descriptions.
can be used to control printing, typically by monitoring a 3-colour
neutral grey mid-tone patch. It is also very useful for specifying
a spot colour, perhaps an important
'house' or 'corporate' colour such
as 'Coca-Cola Red'. The same colour definition could be used for
vehicles, clothing, buildings, and of course tin cans.
To obtain CIE Lab measurements from an RGB image in Photoshop etc.,
you will need to have assigned the correct ICC profile to that image.
In ICC Colour Management CIE Lab is often used as the Profile Connection Space (PCS) where it provides a link between two colour profiles, such as Input RGB (scanner or camera) and Output (CMYK or RGB press or inkjet). All ICC profiles contain a PCS. In an input profile the tables will convert the scanner's or camera's RGB space to the PCS (Lab). An output profile will convert the PCS (Lab) to the digital printer or printing press colour space (CMYK). The other PCS colour space is CIE XYZ, which is often also used by spectrophotometers to report colour, see the next article.
The difference between two colour samples is often expressed as Delta E, also called DE, or ΔE. 'Δ' is the Greek letter for 'D'. This can be used in quality control to show whether a printed sample, such as a colour swatch or proof, is in tolerance with a reference sample or industry standard. The difference between the L*, a* and b* values of the reference and sample will be shown as Delta E (ΔE). The resulting Delta E number will show how far apart visually the two samples are in the colour 'sphere'.
Customers may specify that their
contract proofs must have tolerances
within ΔE 2.0 for example. Different tolerances may be specified for
primary colours. A value of less than 2 is
common for greys, and less than 5 for the primary
CMYK and overprints. This is somewhat contentious however.
RIPs sometimes have verification software to
check a proof against a standard scale, such as a Ugra/Fogra Media Wedge,
using a spectrophotometer. Various software applications are
available to check colour swatches and spot colours, proofs, and
The Wikipedia page on CIE colour differences has a more in-depth explanation of the complex mathematics involved. http://en.wikipedia.org/wiki/Color_difference
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Introduction to the CIE Lab Colour Values - Basic theory of the CIE Lab Tristimulus Colour co-ordinates...