Why do your digital photos look different on every screen and in every print? This article explains how digital colour is defined and how color management tools link the digital numbers to human vision so that you can see, edit and print digital color images reliably and accurately.
Digital technology offers a combination of creative flexibility and productivity that photographers of previous generations could only dream of. By allowing duplication without loss of quality, instant viewing and endless experimentation at minimal cost, digital imaging beats its analogue predecessors hands-down.
But digital imaging, and more specifically, digital color, bring its own problems. Instead of the master transparency that was the definitive version of an image, you now have an endless range of interpretations, with each screen or print showing you something different – what you saw on your camera isn’t quite the same as what’s on your computer screen and neither of them match the print. And possibly none of them are right.
Color management is the technology that solves this, allowing you to see your images reproduced faithfully on screen, providing the means to make editing decisions based on reliable display, as well as previewing and making printed output in a predictable way. Display calibration and profiling is one of the most important parts of color management and is one of the easiest and cheapest to do.
It is also especially easy to keep your screen up to spec now that relatively stable LCD flat panel displays have largely replaced the older and more variable CRT types, which required calibration every seven days or so and sometimes daily or even more often in critical applications. However, if you do still have a CRT monitor you certainly can calibrate and profile it and get excellent results, but you may have to carry out the process more frequently.
Whether you use Windows (XP onwards) or Mac OSX, you can use custom-made display profiles to ensure accurate display with image editing and Raw processing applications such as Adobe Photoshop, Adobe Lightroom, Phase One’s Capture One, Aperture from Apple and DxO Optics Pro.
So why do displays and prints look different? Digital image files are normally made up of pixels with numerical values which by themselves have no reference to the ‘real’ colors that we see. What’s more, digital devices are all different: each output device (display or printer/press) will need different numerical values to display or print the same final color, and image capture devices such as digital cameras or scanners yield different numbers in response to the same original color.
Without a mechanism to give unequivocal meaning to these pixel values, any display or printing of an image will rely upon assumptions in software as to the actual colors intended. These default settings often aren’t too bad but if you’re reading this, you’ve probably already found out that they’re not good enough.
To define an unambiguous color and tonal value for each image pixel, the numbers in your files need to be referenced against ‘real’ color values that are related to human vision rather than to specific items of hardware. The L*a*b* colour model provides this ‘absolute’ description of color and tonal values, based on experiments into human vision.
It describes colors using three components: L (luminance, roughly the same as brightness, from deepest black to brightest white), a (a green-to-red scale) and b (yellow-to-blue). The * (‘star’) denotes a mathematical transformation of these values but is frequently omitted in non-scientific discussions and the term ‘Lab’ color is used to mean the same thing.
An ICC profile provides the link between the numbers in your files to the ‘real’ L*a*b* colors that people see and it is used in the ICC (International Color Consortium) profile specification. Once referenced against L*a*b* via the profile, the pixels’ colors can then be calculated for accurate display on a calibrated display. ICC profiles can also describe the color behaviour of digital cameras or scanners, working color spaces such as sRGB or Adobe RGB (1998), or the characteristics of a press or an inkjet printer.
Display profiles are created by dedicated color management tools that comprise a measuring instrument (usually a colorimeter, but in display and printer profiling offerings, a spectrophotometer) and supporting software. There are choices at a wide range of price points, with accuracy and detailed control and functionality increasing with price.
Color management software built into the Windows and Macintosh operating systems is used by professional quality image editing software to read these profiles and use the information within them to convert color for editing, display and print.
Together, these will give you a solid basis for achieving and maintaining accurate color. You will be able properly to see what you’ve shot, exactly how your edits will affect it and what it will look like when it’s printed. This brings the promise of digital imaging to full fruition.
This was a guest article from Michael Walker.
This article is based on Practical Colour Management for Photographers and Digital Image Makers, an e-book that’s full of detailed practical tips on how to ‘do’ color management on real computer displays to support high quality image editing. It includes tips on choosing and using colour-critical equipment and the optimum system software settings and working environment for reliable results.