The magazine of the Melbourne PC User Group True Colours Colour Profiles and how they can have a dramatic effect on your work... Calvin Jones |
Have you ever looked at a vibrant, colour-rich image on your computer monitor only to be disappointed
with the accuracy of the colours when you see it in print? Does scanning a colour image on your flatbed
or film scanner result in a screen image whose colours vary significantly from the scanned original? Do
the photographs from your digital camera look different when viewed on your monitor and different again
when printed on your colour inkjet?
For most casual users these subtle colour shifts are perfectly acceptable, but if you want to achieve
truly accurate end-to-end colour then you need colour management.
The Problem
Colour digital devices; like scanners, digital cameras, monitors and printers; all handle colour
differently. Each individual device has its own unique range or gamut of colours. For example, when
an image is digitised using a scanner or digital camera the values stored to represent the colours in
that image are based on the particular colour gamut of the device in question. If that image is then
viewed on a computer monitor the original colour values are represented using a different colour gamut -
often resulting in a perceptible colour shift. The same applies when printing an image from your computer
to your colour printer.
The Colour Management Solution
The International Color Consortium (ICC) http://www.color.org
was formed in 1993 to create, promote and encourage a standard, vendor-neutral, cross platform colour
management system. The result is ICC Colour Profiles, which effectively provide a standard way to describe
how the colour characteristics of a particular device differ from a standard, device independent colour
space.
How Does It Work?
The Colour Management Module (CMM)
At the heart of the ICC Colour Management System is a piece of software called the Colour Management
Module (CMM), sometimes referred to as the Colour Engine. There is a CMM incorporated into your computer's
operating system, and in some software applications (eg. Adobe Photoshop). The CMM uses a standard, device
independent colour space with a range broad enough to encompass all possible colour gamuts for input,
display and output devices.
ICC Colour Profiles
In order for the CMM to accurately compensate for the unique colour characteristics of each device it needs
some very specific information. This information is provided in ICC colour profiles.
Manufacturers' Profiles
Most digital cameras, scanners, monitors and printers ship with a manufacturer's default profile that
defines the colour characteristics of the device at factory specifications under ideal conditions. Real
world performance tends to vary considerably from this factory benchmark. While manufacturers' profiles
are a good place to start, to obtain truly accurate colour across all devices you will need to create
custom ICC profiles.
Custom Profiles
Creating custom ICC profiles involves explicitly measuring the colour characteristics of a particular
device and how they differ from a known standard. Special profiling software (and sometimes hardware)
will analyse the colour characteristics of a device and generate an ICC profile.
Input Device Profiles
Creating profiles for digital cameras and scanners involves using a standard colour target - a print,
slide or transparency that contains a number of standard colour and greyscale swatches - and special
profiling software. Essentially you photograph or scan the colour target, and use profiling software to
analyse the resulting image file and generate a profile.
Display/Monitor Profiles
Profiling a monitor involves first calibrating it to a known standard, and then measuring its particular
colour characteristics and recording them in an ICC Profile. Software tools, like Adobe Gamma (included
with Photoshop), allow you to calibrate and profile your monitor visually. However, accurate monitor
profiles require a piece of hardware that physically measures the colour emanating from the screen. One
produced by ColorVision is named the Spyder
http://www.colorvision.com/.
Using these measurements the profiling software can generate a custom profile for a monitor.
Output/Printer Profiles
Printer profiles are obtained by printing a known colour target file (usually provided with profiling
software). More expensive (and accurate) profiling methods involve using a spectrophotometer to explicitly
measure the colours on the printed output. Profiling software then uses the data to generate an ICC colour
profile. Less expensive profiling options employ flatbed scanners to compare the printed output to the
original target and generate an ICC profile in that way. An important point to remember with printer
profiles is that you need a different ICC colour profile for each combination of ink and media you use.
Summary
The fundamental principles of colour management are straightforward enough: a colour management system
uses device specific profiles to ensure accurate, consistent colour across all devices in your system.
Having said that, it is an extremely complex subject, and this article offers only the briefest of
introductions. Profiling your entire system can be both time-consuming and costly, but if you're looking
for consistent, professional and predictable results it can make all the difference.
About the Author
Calvin Jones, calvin@nivlac.net took the plunge in 2002 and left
the IT industry after 10 long years to pursue his passion for writing and photography. He picked up his
first camera at the age of ten, got his hands on his first SLR by the time he was twelve and was
immediately hooked. An avid photographer ever since, he has embraced digital photography and now rarely
ever shoots film. To view samples of Calvin's work visit his Web site on
http://www.nivlac.net.
Reprinted from the March 2003 issue of PC Update, the magazine of Melbourne PC User Group, Australia
