As mentioned in the Introduction to Fine Art Printing, the printer, the ink and paper play an important role in the printing process but in order to achieve the desired output in a constant and predictable manner, one has to ensure all the devices and media (paper, fabric…) can communicate in full symbiosis. This is where Colour Management comes into play and in this episode, we are going to learn about its foundations.
Colour ModelsAt the core of any colour space is a colour model. A colour model is a method/formula used to describe a colour. While I know five types of colour models: RGB, CMY(K), LAB, HSV & HSL, we only need to be concerned by RGB, CMY(K), LAB and HSL for photography (The latter being found as one developing tool in Adobe Lightroom and Adobe Photoshop). With both RGB and CMY(K) models, the colours are created by the combination of three primary colours which are neither the same nor do they behave identically. While the RGB colour model represents the colours that is emitted by the light, CMY(K) represents the colours after the absorption of the light by a surface (like paper). Hence the RGB colour model is used primarily for cameras, scanners and displays ( each creates the colours directly from a light beam) while the CMY(K) is used in printing (where the colour is the result of the printer, ink and paper combination).
The RGB colour modelRBG stands for Red, Green and Blue and each colour has a value going from 0 to 255. From those three colours, we can get most of the colours visible to the human eye which avoid the need to use other colours. RBG is called an additive colour model because the colours are achieved by the mixing of different light colours. While there is no such thing as “white” light, with the RGB colour model, the greater the value of a primary colour the brighter it becomes.
- Black is represented like this in the RGB model: Red: 0, Green: 0, Blue: 0
- Green is represented like this in the RGB model: Red: 0, Green: 255, Blue: 0
- White is represented like this in the RGB model: Red: 255, Green: 255, Blue: 255
The CMY(K) colour modelCMY(K) stands for Cyan, Magenta, Yellow ( & Black) and each colour has a value expressed in percentage. This colour model is called subtractive because as mentioned earlier it is used for printing and when you add colour ink onto a white surface, you subtract (reduce) the paper’s brightness from it. Subsequently, the greater the value of a CMYK primary colour, the darker the resulting colour becomes.
- White is represented like this in the CMYK model: Cyan: 0%, Magenta: 0%, Yellow: 0%, Black: 0%
- Green is represented like this in the CMYK model: Cyan: 63%, Magenta: 0%, Yellow: 100%, Black: 0%
Why the K?Just like with paint, when you mix 100% of Cyan, Magenta and Yellow you do not really get a real black. It is closer to a dark muddy brown. In order to achieve the perfect black, the fourth colour (Black) was added to the mix. There are also three other reasons why the black colour was added:
- In order to get the darkest colour when using 100% of each CMY colour, we end up using 300% of paint. Which means it cost 3 times more to obtain black than any other colour.
- Using a 300% mix of paint also means a greater drying time
- In order to have the perfect mix to achieve the darkest tone, the three colours need to be fully aligned which is not always easy to get with offset printers.
Why K and not B if it means black?K stands for the colour black but does not come from the word black per se. The letter K comes from the key plate (black colour) which is used in the press print process as a base to align all the other plates. In press printing, each CMYK colour has its own plate.
Magenta only exists in our brainTo explain this phenomenon we first need to explain how we perceive colours. At the back of a human eye there are three cells ( called “cone cells“) and each is sensitive to a specific light colour: Red, Green and Blue. Does this remind you of anything? Here is the colour spectrum as seen in a rainbow: Mixing light colours together produces some interesting and rather unexpected results:
- Mix Green and Red to get Yellow
- Mix Blue and Green to get Cyan
LAB colour modelThe LAB colour model is a three axis model that enables us to represent all colours, exceeding those represented by the RGB and CMYK colour model. The axes are:
- L: Luminance value. The value 0 represents the darkest black while 100 brightest white.
- A: The value 0 represents neutral grey on the red/green axis with green in the negative values and red in the positive.
- B: The value 0 represents neutral grey on the yellow/blue axis with blue in the negative values and yellow in the positive.
Colour spacesColour spaces define the range of tones available according to a colour model . That range of tone is called a gamut. There are basically three RGB colour spaces and here they are ordered by greater gamut:
- ProPhoto RGB (90% all perceivable colours and some invisible one as well) created by Kodak
- Adobe RGB (1998)
- sRGB created by Hp & MS in 1997 and often nicknamed “stupid RGB”
Free application to view your colour spacesWhether or not you own a Mac computer, you can still view and compare the colour spaces and as we will see later your ICC profiles. Here are two links to download ColorThink:
- ColorThink 2.3.1 for Mac OSX: 10.5 – 10.11 (15.5865 MB)
- ColorThink 2.3.2 for Windows: XP SP2+, Win 7, Win 8 (7.99239 MB)
ICC colour managementIn order to convert/translate the colour values across all devices (i.e. from the camera to the printer), we need an ICC profile. ICC stands for International Colour Consortium which was formed in 1993 by eight vendors (Adobe, Apple, Agfa, Kodak, Microsoft, Silicon Graphics, Sun Microsystems and Taligent) to create an open, vendor-neutral colour management system which would function transparently across all operating systems and software packages.
Let’s look at a common eco-system for us photographersEach device has its own ICC profile which is a conversion table from either RGB to LAB or CMYK to LAB. The computer is at the core of the eco-system and on it runs a colour engine (i.e. Camera RAW for Adobe Lightroom & Photoshop) which uses the LAB colour model to link up all the different ICC profile from all the devices of the ecosystem.
Setting the Colour Space in your cameraYou may remember that in your camera menu you can define the colour space. Either sRGB or Adobe RGB. Note this only matters when you shoot JPEG. When shooting RAW, the resulting RAW file (i.e. CR2 for Canon, NEF for Nikon) does not have a colour space since it is not an image. It is the raw data recorded by your sensor. Each camera has its own colour gamut, but in general, we can say that modern high-end DSLRs have almost the same colour gamut, as the Adobe RGB colour space.
A well-calibrated displayComputer displays are not all equal, and their price tags go from £100 to several thousands of pounds. The reason comes down to their gamut, their ability to provide a rather constant colour rendition and their size. That being said, all monitors do require to be regularly calibrated to ensure their colour rendition is accurate.
Colour Space in Adobe LightroomAdobe Lightroom uses two RGB colour spaces depending on what you are doing:
- Adobe RGB:
- Preview in Library, Map, Book, Slideshow, Print and Web modules
- When printing in Draft mode
- In exported PDF slideshows and uploaded web galleries
- When sending a book to Blurb.com (If you export books as PDF or JPEG from the Book module, however, you can choose sRGB or a different colour profile.)
- For photos uploaded to Facebook and other photo-sharing sites using the Publish Services panel
- ProPhoto RGB (Adobe’s own version): Preview in the Develop module
Colour Space in Adobe PhotoshopPhotoshop has no restriction when it comes to colour space. While by default, its working space is set to use sRGB, you can replace it by any colour space or ICC profile you have installed on your computer (Edit->Color Settings). We will learn about adding colour spaces and ICC profiles on your computer when we will cover soft-proofing.
ConclusionThere is a lot of information in this article, and I hope you find them insightful and easy to absorb. In summary, colours are represented differently depending on how they are produced or perceived. Their representation is done according to a colour model which is a mathematical formula. Each device and media (i.e. paper) will have a specific colour gamut which is represented by a colour space. For all the devices and media to work together and produce the result you expect, they need to rely on an ICC Profile which is a file that translates the Colour Space of a device to a generic one, which is call LAB. As a rule of thumb, I would follow these simple rules when you wish to print your photographs:
- Camera: use the biggest colour space available if you shoot JPEG
- Display: use a display that can render most (if not all) of the Adobe RGB colour space
- Editing tool: use the correct colour space when editing your work (when not using Adobe Lightroom)
- Printer/ink/paper: use the correct ICC profile for the printer, ink and paper combination. More to come in the future episode about paper