Color spaces: which one?
Color space is a catchall term we use to describe the range of colors a digital image file can contain, as well as the range of colors an output device is capable of reproducing. Every digital image file we work with and every device to which we output files resides in a color space. Each color space is described by a profile. There are three main categories of color spaces in the RGB-to-CMYK workflow:
- Reference space: Lab is the absolute and unambiguous definition of color used as the hub of the color management system. Lab is the only reference color space.
- Working spaces: These are color spaces in which we create, edit, and save files.
- Output spaces: These color spaces describe the color behavior of our output devices such as printers, monitors, and offset presses.
Reference Space: Lab
Lab is the reference space used by the CMS to translate from one color space to another during the conversion process. Because the color it defines is based on human vision and not any particular output device, it is called device independent. In fact, it is the only device-independent color space we encounter in our normal RGB-to-CMYK workflow. Even though we use it multiple times every day, our actual involvement with Lab is only tangential because it operates behind the scenes in the basic Source-to-Lab-to-Destination color management move.
Lab can also be used as a working space in which to edit images; however, it is beyond the scope of this book to address that subject. For the definitive source on using Lab as a working space, refer to Dan Margulis's Photoshop LAB Color (Peachpit Press, 2005).
RGB Working Spaces
RGB working spaces, as their name implies, are color spaces in which we create and edit digital image files. As is the case with all color spaces, each working space is described by a profile.
All color spaces except Lab are considered device dependent because they describe the color output of a particular output device, and as such, render their RGB triplets (or quads, in the case of CMYK) dependent on the color behavior of that device. RGB working spaces are a unique subset of device-dependent output spaces; they don't describe actual physical output devices. Rather, they are color spaces that describe idealized synthetic output devices and were created to fulfill specific editing needs.
RGB working spaces share one important feature: they are RGB neutral. This means that when the RGB triplet is made up of equal values, the working space renders that triplet as a neutral color—in technical terms, zero saturation. In a working space, an RGB triplet of R128/G128/B128 yields a perfectly neutral middle gray (see Figure 4.10). Likewise, R10/G10/B10 yields a neutral black, and R250/G250/B250 yields a neutral white. This is of great value when color correcting images. (We'll cover this in Chapter 6.)
Figure 4.10 An ideal workspace is RGB neutral, meaning that when a triplet is R=G=B, the working space renders that RGB triplet as a neutral value with zero saturation. This concept is critical to using the working space for editing color. All four RGB working spaces that are included in the Adobe Creative Suite are RGB neutral.
There are four main RGB working spaces; they differ primarily in the size of their gamut. The general rule of thumb when choosing a working space is that its gamut should totally encompass the gamut of the ultimate output space. In the case of our RGB-to-CMYK workflow, the ultimate output space describes the gamut and color reproduction characteristics of the offset press.
Here are the four RGB working spaces:
- sRGB: This is the smallest of the four, primarily suited for images destined for Internet and multimedia use. Its gamut does not encompass the gamut of an average CMYK press, leaving out significant chunks of the blue and cyan spectrum, and as such, sRGB is not an ideal working space for editing image files headed for CMYK.
- Colormatch RGB: This is a working space that is larger than sRGB, but still not large enough to encompass current CMYK press spaces. It's rarely used.
- Adobe RGB (1998): The ideal space for the RGB-to-CMYK workflow. Of all the color spaces that ship with the Adobe Creative Suite, this is the one best suited for CMYK output because its gamut is large enough to describe virtually all the colors of which an average offset press is capable. Adobe RGB (1998) is by far the most popular, and a good choice.
- ProPhoto RGB: ProPhoto RGB is a huge color space. It is used primarily by photographers to edit and archive digital camera files, and it allows them to retain as much color range as possible in the image file. Because it contains colors that cannot be reproduced by any output device, ProPhoto RGB must be used with care and caution. Conversions from ProPhoto RGB to CMYK require careful attention to gamut issues.
Among the devices to which we output digital image files are monitors, projectors, inkjet printers, dot proofers, photographic laser printers, film recorders, and offset printing presses. Each one resides in its own unique color space and is described by its own profile. Most devices are shipped with a standard, or "canned," profile. The canned profile is usually very good. However, due to unit-to-unit variances in the manufacturing process, for accurate results in a professional environment custom profiles should be built for each device in the workflow.
Monitor color spaces are a subset of output spaces. The color behavior of your monitor is described by your monitor profile. Due to the variation between individual monitors—even two that are the same model and brand—you must create a custom profile for your specific monitor in order to view color accurately. We reviewed this procedure in Chapter 3, The Tools.
Your monitor profile should be used only to view images on your monitor. Never use it as a working space for creating or editing files because it describes a physical output device (the monitor) and is not necessarily R=G=B neutral. To be a collaborative partner in the RGB-to-CMYK workflow, we each must work in RGB working spaces, not monitor spaces.
Although you will be actively color managing your working spaces and your output spaces, your monitor space will be static. Use only the custom-built profile you create for your monitor and nothing else.