When viewing one of your digital images in Adobe Photoshop, it's often difficult to see the forest for the trees. It's hard to realize that all you're viewing is a huge collection of colored pixels, each one represented by a set of numbers.
Colored pixels defined by numeric values is the only way a computer can produce what appears to be a photograph. Your digital darkroom is nothing more than an expensive adding machine, and all Photoshop can really do is modify pixel values.
When viewing an image on-screen that appears too dark, you wouldn't crank up the brightness on your display to "fix" this condition. If your prints appeared too cyan, you could, in theory, add a red filter to the light source, but would you? No—you'd edit your images by changing the pixel values until you produced the desired color appearance.
Similarly, you expect the color and tone you see on your display and the color and tone you output on your printers to match reasonably well. Do the numbers in your document define an image correctly, or does the image look too dark because the display is incorrectly producing a dark preview? The only way to remove such ambiguity is to use color management, which is nothing more than the correct management of numbers!
Color management is necessary when handling digital images because numbers alone don't fully describe the appearance of color; they are only a partial ingredient. Photoshop treats a document filled with a single solid color and a beautiful photographic image the same.
Colors You Expect
Imagine a single solid color displayed in Photoshop. Let's use R240/G78/B98 as an example. What color is this, and how should it appear? You can surmise this is a shade of red because the R value is so much higher than the G and B values. But how red is this red, and will this shade and tone of red reproduce the way you expect?
Two key components are necessary to ensure that the numbers you manipulate produce a desired color appearance on-screen and when output. First, you need an ICC device profile for your display to describe to a color management system how it reproduces color. Second, you need a profile to define the color space of your documents—the scale of these numbers as they relate to human vision.
Having a set of RGB values associated with a color space allows for the exact reproduction of that color. R240/G78/B98 in two different color spaces—for example, sRGB and ProPhoto RGB—don't produce the same color, even though both share identical numbers. Photoshop expects each document to have an embedded profile so it can display the numbers as they should appear, defined by their color space. Color management provides a way for Photoshop to properly display, and ultimately output, the numbers as the colors you expect.
To output an image to a particular printer, you need an ICC profile that defines how that output device behaves. Photoshop does its mathematical magic by altering every pixel value to produce a new set of numbers optimized for your printer. Just as the same numbers in sRGB and ProPhoto RGB produce a different color, the same set of numbers sent to two different printers will produce two different colors.
That's not what you want. When Photoshop knows the conditions of your output device based upon an ICC profile, it can produce the correct color values and simulate on-screen how these colors should appear. This on-screen simulation, known as a soft proof, is an incredibly powerful feature. You get to see how your image will appear printed before you spend the time and money actually making a print. This is true if you are working with RGB, CMYK, or grayscale images.
We need ICC profiles for all the devices we plan to use—displays, scanners, printers, and so on. ICC profiles define the images we will edit, but they know nothing about images, only the device behavior they define. Therefore, human intervention is usually necessary. Just as ICC profiles (and Photoshop) only understand color one pixel at a time, it is up to you to view the pixels in context and decide how you want your images to appear.
To handle these images properly, calibrate and profile your display on a regular basis. Displays behave inconsistently, even from similar models, and alter their behavior over time. When you profile devices on a monthly basis using a colorimeter, the same set of numeric values should produce the same color appearance every time you view them.
If you send files to a lab for output, ask for an output profile so you can properly view your images as they will appear when output. Refrain from altering your calibrated display to match a reference print or file from a lab—this will ruin your ability to use Photoshop to produce an accurate soft proof. When properly configured, Photoshop will correctly display and output your numbers— mean images.
Color management—nothing more than the correct management of numbers.
This article is based on Andrew Rodney's book Color Management for Photographers: Hands-On Techniques for Photoshop Users (www.focalpress.com). To order a copy, call 800.545.2522