Introduction to Colour Models ('Spaces')
CIE Lab & Lch
This article was first written several years ago in response to our
customers who found the concept of 'Lab colour' difficult to
understand. It's written for photographers, designers and printing
industry people, in other words, laymen rather than colour scientists!
So you won't find the maths, but just the basics.
It is an introduction to two related colour models
which have become very
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? CIE 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 models 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, photo or a computer screen, 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 either.
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, such as a camera, monitor or inkjet printer, etc. in the case of RGB or, a Printing Industry Standard in the case of CMYK, and/or associated material type or illumination (paper, ink set, lighting, etc.). These RGB and CMYK spaces usually do not cover the entire visible colour spectrum or gamut. The CIE also specify lighting conditions for their colour models.
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'.
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.
HSB and HSL Colour Models
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!
The CIE Lab Colour Space or Colour Model
This
is more
correctly known as CIE L*a*b*.
Just as in Lch, the vertical L* axis represents Lightness, ranging from 0-100. 0 being black (no light); and 100 white (maximum illumination).
The other values are now a* and b*.
These are now horizontal axes. They appear to travel horizontally through the colour circle used in L*c*h*.
Indeed they are often represented by a diagram similar to that used to that for L*c*h*., as above.
These axes 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 (as opposed to a neutral)
cannot be both red and green at the same time,
or blue and
yellow at the same time.
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.
You could think of the '+' values (red and yellow) as 'warm' colours, while those with a ' -' (green and blue) are 'cold' colours.
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) had '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 (offset litho, etc.) 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 next article.
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'.
Standards Organisations specify Delta E tolerances. Printing Industry
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 ΔE 2.0 is
common for greys, and less than ΔE 5.0 for the 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.
This article is intended to give photographers, designers, etc. an introduction to colour theory, and is not intended for colour scientists!
The Wikipedia page on CIE colour differences has a more in-depth explanation of the complex mathematics (to non mathematicians) involved. http://en.wikipedia.org/wiki/Color_difference
If you like this
article, please link to it from your site. However please do not copy
the text or hot-link the images.
Introduction
to
the CIE Lab Colour Values - Basic
theory of the CIE Lab Tristimulus Colour co-ordinates...
Our Remote Custom Printer Profiling Service
This page updated April 18, 2020