Tom Arah explains why achieving accurate colour isn't simple but shows how it can be done.
Colour Spaces: Understanding the Problem
In these days of full-colour displays, budget colour lasers and photo-quality inkjets it might seem that computer-based colour handling is hardly an issue. It’s certainly easy to specify any colour you want (see HSV boxout) but when you need accurate and consistent colour it’s another matter entirely.
To understand why, it’s necessary to go back to first principles. The eye perceives colour as a response to different wavelengths of light hitting the retina and stimulating its photoreceptor cells or “cones”. Humans have three types of cone each primarily receptive to a different wavelength – long (L), medium (M) and short (S) - and different wavelengths produce different combinations of response which are perceived as different colours within the full visible spectrum between ultra-violet and infra-red.
This underlying framework provides the key to how we can go about mechanically reproducing the perception of colour. By combining different mixes of Red (R), Green (G) and Blue (B) light broadly corresponding to the L, M and S cones, it’s possible to reproduce a particular colour so that a combination of 100% red and 100% green produces the impression of yellow. By using 8-bits to specify 256 varying intensities of light that each of the red, green and blue phosphors in a computer display emit, it’s possible to reproduce no less than 16-million colours (256 x 256 x 256).
This “additive” RGB mixing model is ideally suited for computer displays, and for scanners and digital cameras, but print works very differently. Rather than emitting light, inks (and toner and dyes) absorb certain wavelengths and reflect others. However, by mixing the “subtractive” colour primaries Cyan (C), Magenta (M) and Yellow (Y) you can control the wavelengths that eventually hit the eye with cyan ink, for example, absorbing red and reflecting blue and green. Throw in a separate BlacK (K) ink for handling solid black (particularly useful for text) and you have the traditional CMYK “process print” system that underpins all commercial print.
This might all sound relatively straightforward. By using the RGB additive primaries and the CMY subtractive primaries, and mapping between them, presumably we can accurately and consistently display and print the full visible spectrum. Absolutely not. To begin with, the wavelengths emitted by the RGB phosphors in a computer display only roughly approximate to the responsivity of the LMS cones so that the full range or “gamut” of colours that the RGB model is capable of describing is only a subset of the full visible “colour space” – many visible colours can’t be reproduced onscreen.
This mismatch is fundamental but in practical terms pales in comparison to the mismatch between the RGB and CMYK colour spaces – not surprising really when you consider that CMYK is trying to precisely control the combination of reflected light through the very imperfect medium of ink. While the majority of colours are reproducible and convertible between models there are whole areas (nearer the respective primaries where colours are most saturated) where this just isn’t possible – whole swathes of colours can be displayed but not printed and, to a lesser extent, vice versa. This explains why you’ll often be disappointed when you compare the flat, matt colours in a commercial print run to the vivid onscreen hues. It also explains the benefits and attractions of printing with additional Pantone inks and of additional inkjet cartridges which extend the printable colour space and so offer much better RGB reproduction (that’s why modern inkjets are effectively treated as RGB devices).
These core differences between colour spaces are only the beginning of our troubles. To produce reliable and accurate colour on screen and in print we need to be able to reliably translate between colour spaces and here we hit another fundamental problem: there isn’t one display colour space and one print space to work with - instead there are any number! The reason is simple. Each colour space is “device-dependent” so that the gamut of colours that a particular monitor or printer can produce depends entirely on its particular combination of phosphors or inks and how they interact with each other. This is why when you walk into a TV showroom you’ll see that the colour on each device is radically different.
In fact it’s even worse than this as changes to brightness, contrast and colour settings, or a different batch of ink or paper, mean that exactly the same hardware device can and will output the same colours differently. And to top it all whether you view the end colours in natural or artificial light will also effect the final perception of colour. Ultimately then the apparent precision of a numerical colour reading such as R:38 G:66 B:194 is very misleading: yes the colour will be a dark blue but the particular shade perceived by the end viewer will vary enormously – unless you take action.
RGB screen and CMYK print colour spaces are entirely different (visualized here in Microsoft Color Control Panel Applet for Windows XP).
Colour Management: Understanding The Solution
With each device effectively handling colour as it sees fit there’s no hope of ensuring accurate colour. Clearly we need a way to impose some order and get each device singing off the same hymn sheet. That’s exactly what the CIE (Commission Internationale de l'Eclairage) international standards body provided with its CIE XYZ (1931) model and its later refinement CIELAB (1976) based on three channels L* (indicating lightness), a* (indicating red-greenness), and b* (indicating yellow-blueness). What makes these models different is that unlike RGB and CMYK they don’t define a given colour in terms of colourant but rather in terms of perceived colour – in other words they are fixed, unambiguous, device-independent and cover the entire human visible spectrum.
Crucially these device-independent colour standards can be used like the Rosetta stone to help translate accurately between device-dependent colour spaces. The key for this is to have ICC (International Color Consortium) profiles for each device, which describe their colour behaviour in relation to either the CIE XYZ or CIELAB standard. Using these, a Colour Management System ( CMS) based on a CMM (Colour Matching Method) engine can adjust the numerical values that get sent to and received from different devices so that the perceived colour remains consistent.
The success of such colour management depends entirely on the quality of the ICC profiles. These days most devices ship with an ICC profile but a single generic profile is only a starting point. For desktop printers you can often find libraries of customized profiles for different paper types via the manufacturer’s site or the wider web. For commercial CMYK presses, it’s relatively straightforward to use a densitometer to compare the actual printed ink density to that specified by the application (that’s what the off-page calibration bars are for) and so to produce custom profiles for different papers and inks. If your commercial printer doesn’t provide this, you can use the default coated, uncoated and newsprint profiles provided by Adobe (see page ).
An accurate profile for your monitor is also essential – after all the display acts as your window onto the colour workflow and it’s onscreen that you’ll make most colour decisions. To ensure an accurate colour picture you first need to “calibrate” your monitor to bring it into line with a known standard in terms of black and white luminance (the latter is usually referred to as colour temperature) and gamma (which controls midtones). Secondly you need to “characterize” your monitor to produce an ICC profile describing its calibrated colour behaviour in relation to the device-independent CIE XYZ or CIELAB standards.
By far the best way to calibrate/characterize is to use a colorimeter which works like the commercial printer’s densitometer to objectively compare actual and specified RGB intensities. Alternatively you can use onscreen, software-only approaches such as that offered by the Adobe Gamma control panel utility which comes bundled with Adobe Photoshop or the QuickGamma and QuickMonitorProfile freeware utilities (in both cases use online gamma tests to fine-tune). What you can’t do is just to assume/hope that your monitor is somehow showing you accurate colours.
With accurate ICC profiles of calibrated devices you’re now ready to put colour management into action.
Calibrating and characterizing your monitor as an ICC profileis crucial.
Microsoft Colour Management
Rather than leaving colour management to individual applications or suites of applications, it clearly makes sense to build it into the operating system. However, this is an area that Microsoft has largely steered clear of, limiting itself to providing the Image Colour Management ( ICM) 2.0 engine and an API that allows applications to take advantage of it.
However Microsoft is beginning to take the issue more seriously. Using the new central Microsoft Color Control Panel Applet for Windows XP (free download from microsoft.com), for example, you can install and uninstall ICC profiles and even view and compare 3D representations of their colour spaces so that you can see which colours are out of gamut. Most practically useful is the Devices tab, a central area where you can associate particular profiles with your scanners, printers and monitors (including dual-display set-ups) and select which should be currently applied – handy for example for swapping between printer profiles for matt and photographic paper.
However the limitations are equally striking. In particular while the applet provides a Default Windows Colour Space setting there’s no option to change this from the screen-oriented sRGB and there’s no CMYK-oriented setting to enable soft proofing for commercial colour-separated print and no controls for managing how embedded profiles should be handled or for setting rendering intent. The biggest limitation is the lack of any Adobe Gamma-style onscreen calibration and characterization – instead there’s a rather cheeky option to set weekly calibration reminders!
For the moment at least Windows in-built capabilities are a useful if partial solution for digital photographers producing desktop prints, but inadequate for professional use.
The free Microsoft Color Control Panel Applet for Windows XP is a step towards OS-level colour management.
Adobe Colour Management
Clearly accurate colour handling is most important in a commercial print workflow and here Adobe dominates with its Creative Suite applications – Photoshop, Illustrator, InDesign and Acrobat Professional – each of which offers shared colour handling. As colour accuracy is most essential at the colour-editing coal face, the best place to see how colour management principles are put into practice is in Adobe Photoshop’s Edit > Colour Settings dialog.
The good news is that, in its default state, the dialog is divided into just two sections, of which the first is concerned with Working Spaces. The obvious question is: so what is a “working space”? As you might expect by now it’s another ICC profile, this time describing the colour space within which the current image can be edited. You can set the RGB working space to your Adobe Gamma calibrated monitor profile, for example, to limit your working gamut to viewable colours.
However, rather than tying your RGB working space to an individual physical device, it makes more sense to use an abstract and device-independent common space - especially as, since Photoshop 6, all onscreen colours are automatically colour managed based on the monitor profile specified in the Adobe Gamma control panel utility whichever working space you set. Click on the dropdown list and you will see four standards to choose from. Apple RGB and ColorMatch RGB are only provided for backwards compatibility so it boils down to a choice between the two remaining options. sRGB (standard RGB) was developed by HP and Microsoft and is designed to reflect the characteristics of the average PC monitor and so is suitable for web and general purpose work. However sRBG’s gamut is limited and cuts out whole areas of CMYK printable colour so for prepress work the best choice is Adobe RGB (1998).
That’s the RGB working space taken care of, now on to the CMYK. Here the default options are based on the behaviour of coated, uncoated and newspaper paper stocks and you’ll find separate generic profiles for European, Japanese and American users based on typical ink and press set-ups. If your printers have provided you with a customized profile for particular press conditions this is where you should load it or, if you know exactly what you’re doing, you can select the Custom CMYK option to create your own.
The CMYK working space profile that you choose is then used to match colours when you permanently convert images from RGB to CMYK with the Convert to Profile command. Even better, having a CMYK working space also lets you non-destructively “soft proof” your image, matching your display to your printer rather than the other way around. Using the View > Proof Colours command you can see how your current RGB image will look printed in CMYK on coated or uncoated paper for example, while the View > Gamut Warning command highlights non-printable colours.
After editing an image in Photoshop the relevant RGB or CMYK working space profile is automatically embedded in the image which enables accurate colour management throughout the publishing workflow right through to the press-based Acrobat PDF digital master. How these embedded profiles are handled is managed in the second, Colour Management Policies, section of the dialog.
For prepress work you might think that the ideal would be to choose the Convert to Working Space options to automatically convert profiled colours, but having numeric colour values changing behind the scenes is a recipe for confusion. Instead it’s generally better to opt to Preserve Embedded Profiles, while making sure that you are prompted about Profile Mismatches and Missing Profiles on opening. Then you can simply edit in the image’s native working space or use the Assign Profile and Convert to Profile commands as desired.
That’s it for the default settings but, for a full picture of ICC-based colour management, there’s one other important option which is only accessible in the Color Setting dialog’s advanced mode. The Intent setting determines how colours are mapped between spaces and in particular how out-of-gamut colours are handled. The default Relative Colorimetric setting first scales the white of the source space to the target space, then renders all colours accordingly, clipping out-of-gamut colours to the nearest hue by sacrificing saturation and brightness as necessary. By comparison: Absolute Colorimetric rendering doesn’t scale the white space first; Perceptual rendering compresses the entire gamut of the source space into the destination space so preserving the relationship between colours though not the actual colours; and Saturation rendering chooses to sacrifice hue and lightness when mapping out-of-gamut colours in favour of saturation. For photographic images Relative Colorimetric is usually the best choice but, for other work, you might want to select another rendering intent.
With just a few settings to manage, Adobe’s colour management takes you right the way through from monitor calibration to final render ensuring the best possible colour range, accuracy and consistency on screen, on proof and in print. However it’s important to remember that when you’re dealing with different colour spaces even the best colour management system can never guarantee the same colours. Ultimately when it comes to colour you’re looking for the best match not the perfect match.
For further information on colour management check out Bruce Fraser’s Out of Gamut articles at www.creativepro.com
The Colour Settings dialog is the key to Adobe’s professional colour management.
So what about HSV?
RGB and CMYK are the two main models for colour reproduction, but when specifying colours in your desktop application you might well come across a third, HSV. This is a colour model that defines a colour based on its Hue, Saturation and Value (also referred to as Brightness and the model as HSB). The big advantage of HSV over RGB and CMYK is that colour specifying is very intuitive – you can quickly choose a colour based on its Hue – red, green, whatever - then vary its Saturation to make the colour stronger or more neutral and its Brightness to make it lighter or darker. It’s important to realize however that the model only acts as a convenient middleman, you can’t save a photo to HSV mode and the colour you see onscreen or on paper is actually determined by its RGB or CMYK equivalent.
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