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will recognize this and know how to correctly interpret the color data. The same thing applies to profiled CMYK files. Photoshop uses the computer display profile information to render a color correct preview on the computer display. It helps to understand here that in an ICC color managed workflow in Photoshop, what you see on the display is always a color corrected preview and you are not viewing the actual file data. So when the RGB image you are editing is in an RGB workspace, such as Adobe RGB, and color management is switched on, what you see on the display is an RGB preview that has been converted from Adobe RGB to your profiled display RGB via the PCS (see Figure 12.6). The same thing happens when Photoshop previews CMYK data on the display. The Photoshop color management system calculates a conversion from the file CMYK space to the display space. Photoshop therefore carries out all its color calculations in a virtual color space. So in a sense, it does not really matter which RGB workspace you edit with. It does not have to be exactly the same as the workspace set on another user's Photoshop system. If you are both viewing the same file, and your displays are correctly calibrated and profiled, a color image should look near enough the same on both.

Figure 12.6 Photoshop can read or make use of the profile information of an incoming RGB file and translate the data via the Profile Connecting Space and make an RGB to RGB conversion to the current RGB workspace in Photoshop. As you work in RGB mode, the image data is converted via the PCS and uses the display profile to send a profile-corrected signal to the computer display. When you make a print, the image data is then converted from the workspace RGB to the printer RGB via the PCS.

The ideal RGB working space

If you select an RGB workspace that is the same size as the display space, you are not using Photoshop to its full potential and more importantly you are probably clipping parts of the CMYK gamut (see Figure 12.7). For many years I would have advised you to choose Adobe RGB as your workspace, because it appeared to offer the best compromise between encompassing most of the CMYK gamut but without being so large as to be unwieldy. However, a few years ago I was in conversation with the late Bruce Fraser and he convinced me that cautionary warnings against ProPhoto RGB were perhaps a little overstated (even if you are going to end up converting a file from 16-bit to 8-bit RGB). So following Bruce's advice I mostly now use ProPhoto RGB as my principal RGB workspace, although I would still strongly advise making the big tone edits in 16-bit before converting to 8-bit. The only other thing I would caution you about is to never supply clients with ProPhoto RGB master files. If I am sending a file to someone who I believe is ICC color management savvy, I'll send them a profiled Adobe RGB version. If I am sending a file by email or to someone who may not understand color management, I always play safe and send them an sRGB version instead.

Figure 12.7 A CMYK color space is mostly smaller than the computer display RGB color space and not all CMYK colors can be displayed accurately due to the physical display limitations of the average computer display. This screen shot shows a continuous spectrum going through shades of cyan, magenta and yellow. The image was then deliberately posterized in Photoshop and then screen captured from the computer display. Notice how the posterized steps grow wider in the yellow and cyan portions of the spectrum. This simple exercise helps pin-point the areas of the CMYK spectrum which fall outside the gamut of a typical display. Most LCD displays have a color gamut that is not much bigger than sRGB, while the more advanced displays I mentioned in Chapter 2 are capable of handling an Adobe RGB color gamut.

Choosing an ideal RGB workspace

Although I highly recommended that you should switch on the color management settings in Photoshop, you cannot assume that everyone else will be doing the same. There are many other