Introduction to Colour Management - Part 2

The Mechanics of Colour Management

The nitty gritty of Colour Management is performed by ICC profiles (International Colour Consortium) and CMMs (Colour Management Modules). The great thing is you don't need to know their internal workings, all you need is to have a high quality ICC profile made for your particular colour device(s).

An ICC profile is a piece of software that contains data describing the precise behaviour of your input/output device in terms of the colours it actually records/produces versus the colour control signals it was sent. ICC profiles can stand alone or be embedded into images.

ICC profiles are processed by the Colour Management Modules (sometimes referred to as Colour Management Engines) which are incorporated into all Colour Managed software such as Photoshop, printer drivers and RIPS.

A CMM uses an input device profile to translate the colours in an image (captured by the device) into 'real world' colour**. It uses an output device profile to adjust the colour signals sent to an output device to ensure that colours are rendered accurately.

** to be more precise it's a mathematical model of real world colour (CIE XYZ or CIE LAB).

Colour Management Examples

Example 1 : A colour managed scanner is used to capture an image which is then displayed on a colour managed monitor.


Explanation: An accurate ICC input profile (scanner profile) is embedded into the image data at the time of scanning. The Image is later opened in Photoshop and viewed with a monitor that also has an accurate ICC profile.

Behind the scenes the CMM in Photoshop reads the 2 ICC profiles and first translates all the colours of the image to 'real world' colour and then to the output signals the monitor needs to render those colours accurately.  The result is that the image displayed on the monitor will be a good visual match to the original scanned photo.

It should be noted that the data in the scanned image is not changed by the profiles. Only the temporary data sent to the monitor is adjusted.

If the scanner had not been accurately profiled the colours in the image would not be a faithful reproduction of the scanned photo. If the monitor had not been profiled it would not correctly render the colours in the image even though they had been scanned correctly.  

Example 2 : A colour managed scanner is used to capture an image which is then printed with a colour managed printer.



Explanation: An accurate ICC profile (scanner profile) is embedded into the image data at the time of scanning. The image is then sent to print on a printer that has an accurate ICC profile (printer profile). Incidentally a printer profile is more accurately described as a printer/media profile because colour output will vary depending on the media used.

During the printing process the CMM reads both the input profile embedded in the image and the printers output profile. First the image colours are translated into their 'real world' colours and then the CMM modifies the colour signals sent to the printer in order to ensure that the colours in the image are printed correctly. The result is that the print will be a good visual match to the original photo. For the process to be successfull both devices in the workflow must have accurate ICC profiles.

Stand alone ICC Profiles

As mentioned above. ICC profiles do not have to be embedded into images. The only difference is that the user must manually tell the CMM what ICC profile relates to the source image that is being viewed or output. This is known as assigning a profile to an image.

Colour Management is not just colour matching.

The biggest miss-conception about Color Management is that it is designed solely to ensure the rendering of colour will be exactly the same on every device. If 2 devices of matching make and model were calibrated and profiled then an exact colour match between the 2 devices can be created with little difficulty.  It is not necessarily the case for devices that are fundamentally different in their characteristics.  Ultimately we are bound by the laws of physics and some devices can produce more colours than others.  Colour Management is designed to make the available colours of a device accurate rather than limit every device to a some kind of standardised palette. 

Additionally it is more important to manage the relationship between the colours of a device than it is to make every colour absolutely accurate. This is because humans perceive a colour relative to any surrounding colours. For example if I were to print a photograph on plain paper using ICC profiles and asked the observer, "Do the colours of this image appear accurate?" I would expect them to say yes because instinctively they are judging the relationship between the colours in the image. If I re-printed the image on photographic quality fine art paper and asked the observer the same question, I would expect them to agree again. It is only when we compare the prints side by side that one appears to have far more saturated colours than the other. Colour Management allows for the fact that a perpetual match is often more important than an absolute match.

Perceptual colour matching is one of the great strengths of Colour Management. The world contains billions of colours and contrast ratios of sometimes 100,000:1 and yet we can effectively simulate real world colour with a printer that has a fraction of that palette and contrast ratios on paper of just 100:1. 

Happy image making!

© Phil Tanner
Owner of Finer Image