The material in these guides is recommended for advanced users – such as creative professionals – who require the highest possible precision in rendering color. For most users, an understanding of the advanced concepts of color management is not necessary to produce a great-looking book with Blurb.
For more information, visit our Color Management Resource Center.
Color management is a technology that if followed correctly ensures color fidelity throughout a digital workflow process. To get the best results, you need to understand the basic principles that follow.
In today’s digital workflow, we use a wide variety of image capturing devices and an equally large number of output devices to create prints of this data. Due to a variety of factors, a digital file cannot perfectly correspond to a printed sheet. The most glaring mismatch is because we use Red, Green, and Blue (RGB) to describe digital image colors, but we print with Cyan, Magenta, Yellow, and Black ink (CMYK). Color management helps to minimize these factors so that a consistent and fairly accurate color match is possible.
Most digital images come from digital cameras and scanners. Regardless of their source, all digital images exist as RGB type color files. Each pixel in a digital image is described by the amount of Red, Green, and Blue light that makes up its color. This is known as additive color since the total amount of RGB equals white (see Figure 1).
RGB is often thought of as a definitive color number. However, this is a misconception and there are actually many varieties of RGB available in the digital world. Although all RGB varieties use the same numbering system (0-255) each variety uses the numbers differently, which means the same set of numbers can mean different colors.
Each individual RGB variety is known as a Color Space and the range of color it contains is its color – Gamut.sRGB and Adobe RGB are two popular color spaces. sRGB is the more common color space and is usually the default for most consumer and prosumer digital cameras. Adobe RGB is often used in commercial photography and is the preferred space for photographers who use the RAW file format and process their images into Adobe RGB files. Adobe RGB is the larger of the two color spaces and has more richly saturated colors. Figure 2 shows a 2-D representation of a color gamut. The larger the shape, the more colors it contains.Figure 2. RGB Color Gamuts
A third RGB color space is the monitor that we view our images on. Regardless of whether we use a CRT or LCD monitor, the pixels are composed of RGB elements. Some monitors have a large color gamut, while others may be much reduced. To properly evaluate digital images, we must have a stable monitor with a large color gamut that has been properly calibrated. (More info regarding calibrating your monitor can be found at X-Rite or Datacolor.) Some monitors can cover most of sRGB, but very few can display the entire range of Adobe RGB.
The gamut projection image in Figure 2 shows a typical Monitor RGB gamut in relation to the larger sRGB and Adobe RGB gamuts.
Most output devices are printers that use CMYK to mix colors. Since the total combination of CMYK produces black, CMYK is subtractive color and the opposite of RGB. Printing is a subtractive color space because the color we see is reflected light, not the direct light seen when looking at a digital image on a monitor. Just as in the case of RGB, there are a variety of CMYK color spaces and gamuts, each describing the particular color capabilities of a particular printer (see Figure 3).
When a digital image is printed, its RGB numbers are converted to CMYK numbers for the printer. This conversion will produce unexpected color if not done in a controlled and predictable manner. Overlay RGB and CMYK color spaces (see Figure 4), and you’ll see that colors in RGB that do not have an exact equivalent in CMYK. What this means is that there are often colors that we see on our monitors that cannot be reproduced perfectly on the printed sheet. Color management is a process by which we pick the best possible CMYK color to match a given RGB color.
With all of these conversions from one color space to another, from one device to another, the color of an image would naturally appear differently on each. What a color profile does is describe what the colors will look like on a particular device whether it be a computer monitor, an ink-jet printer, or an HP Indigo printer. The color profile contains a Look-Up Table it uses when fed the data that describes a certain color on your monitor and converts it to the same color on a digital press.
The Blurb ICC Profile is based on the GRACoL2009 reference used in high-end commercial printing. Our entire print network adheres to this standard on all of their print devices for the most consistent results possible with print on demand. By using this color profile, you may soft proof your images while in RGB to see how they will look when printed, or use the profile to actually convert your images to the CMYK color space of the print device to eliminate the press-side conversion. This gives you more control over the images and how they will eventually print.
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