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 C.I.E.

What is the CIE? C.I.E. is short for ‘Commission Internationale de l’Eclairage’, which in English is the ‘International Commission on Illumination’.  A professional scientific organisation founded over 90 years ago to exchange information on ‘all matters relating to the science and art of lighting’. The standards for colour spaces representing the visible spectrum 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.

The CIE LCH Colour Space or Colour Model.

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’.

If we take a horizontal slice through the centre, 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 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!

The CIE Lab Colour Space or Colour Model

CIE Lab colour space used in ICC Colour ManagementThis is more correctly known as  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 theory there are no maximum values of a* and b*, but in practice they are usually numbered from -128 to +127 (256 levels).

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.

Lab measurements 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 printed matter, 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 nextarticle.

Delta E Differences and Tolerances.

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 greys and primary colours. A value of less than 2 is common for greys, and less than 5 for primary CMYK and overprints. This is somewhat contentious however. Proofing 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 printed sheets.

Delta E displays the difference as a single value for colour and lightness. ΔE values of  4  and over will normally be visible to the average person, while those of 2 and over may be visible to an experienced observer. Values for neutrals are lower. Note that there are several subtly different variations of Delta E: CIE 1976, 1994, 2000, cmc delta e. When no particular ‘flavour’ of DE is mentioned, it is usually DE76 (as in 1976).  The later DE2000 is more perceptually uniform, and becoming more common.

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