Image Colour Management Using Profiles

The idea of image colour management is to obtain a good match across different colour devices. For example, you would expect your digital camera file image to appear the same colour on your computer monitor, television screen or on a digital fine art print. Colour management helps to achieve the same appearance on all of these devices, provided the devices and media are capable of delivering the needed colour intensities.

In digital imaging systems, colour management is the controlled conversion between the colour representations of various independent devices, such as image scanners, digital cameras, monitors, television screens, digital printers, offset presses, and corresponding media.

International Colour Consortium

The ICC - International Colour Consortium - was set up by a consortium of major manufacturers with the aim of creating a means for predictable colour. The idea was to create colour profiles, the function of which, would be the mapping of colour values between different independent devices. They achieved this by defining an open standard for a Colour Matching Module (CMM) at the operating system level, and colour profiles for the devices and working space (colour space the user edits in Photoshop or other image editor).

Adobe Photoshop users will recognise colour working spaces, such as sRGB or Adobe RGB, as colour spaces that facilitate good results while editing. For instance, pixels with equal values of Red, Green, Blue should appear neutral.

Colour Space

To map the colour values between different independent devices, a colour space was devised to accommodate all the colours visible to the human eye, through which this mapping might be achieved. This colour space was conceived as a device independent reference space. The idea of having the largest theoretical, device independent space as a reference, is that colour values can be re-mapped between devices, such as, digital cameras, scanners, monitors, film recorders, printing machines etc. In fact, between any device that is capable of being measured or calibrated and a profile created for.

This colour space is now commonly known as CIE LAB (often just LAB space), a combined space, where CIE refers to the aforementioned ICC commission and Lab refers to the Hunter Lab 1948 space. The Hunter colour space co-ordinates are L, a and b, where L stands for lightness or luminance and a and b refer to colour opponent axes. The term colour opponent, refers to a theory of how the human eye interprets or processes the information it receives to create the illusion of colour. This colour space is theoretical, as it is larger than the gamut of any computer controlled device, such as a monitor or printer, as well as the human eye.

The visible spectrum that the human eye can detect is measured in Nanometers, between approximately 380nm (violet) - 750nm (red). The human eye is most sensitive to the green wavelength at around 550nm. Some colours are theoretical, as although they cannot be seen or indeed printed, their wavelengths can be measured. The idea is that the colour engines in proprietary imaging software will remap invisible colours to the nearest reproduceable colour.

Gamut refers to the range of colours that can be rendered by any device. Thus CIE LAB is a reference or translation colour space, as colours are mapped through this space from source to destination.

ICC Profiles

ICC profiles may be created for any device that is capable of being measured or calibrated and profiled. A hard ware device, such as a colourimeter, is often used for monitors, whereas a more sophisticated device, such as a spectrophotometer is normally used to profile a printer.

When calibrating a monitor, a target, consisting of a number of colour patches, is displayed on the screen and the patches are read by the hardware device. When calibrating a printer, the target is printed with all colour management disabled and then read. In both cases the measured results are compared with aim values and a profile is then built by the specialised software.

The display of or printing of a target is a characterisation process and the regularisation of the gathered data is the profiling process. Profile creation not only involves complex mathematical calculations by the software used, but profiles nearly always require editing and in this process subjective assessments are made. These subjective assessments and subsequent editing require a high degree of skill and experience, which is reflected by the, often encountered, "Black Art" references to profiling.

How to Achieve Predicatable Colour in Your Prints

Thus it may be seen that the correct use of colour profiles in your workflow will go a long way to achieving predictable colour. The use of image files without an embedded ICC profile, whether displayed on a screen or sent to a printer, will result in generic settings being used. The parameters for these generic settings are much wider and may therefore result in less accurate colour reproduction on your screen or in your print.

If you are frustrated with achieveing predictable results in your colour printing, then a quick call to our resident colour expert, Tim Sale, will help you understand the steps required for a first class result every time.

For more information or advice on Giclee Prtinting call Tim Sale or Adrian Chambers on 0117 952 0105 or use the contact form to tell us your requirements, we can suggest the most appropriate solution for your fine art printing needs.