Section 2.4. Rendering Intents

2.3. Color Space

A color space model shows the color gamut of a given device or image. Viewing the graphs allows you to compare profiles and images. It is relatively easy to see which colors are within the gamut of a color space and which colors are outside the gamut.

Simply put, the gamut is the range of colors that a specific device can see, display, or output. The larger the gamut, the more colors.

A gamut can be displayed as a two- or three-dimensional object. A gamut represented as a 2-D figure is easy to read and is a good way to get an idea of its size. However, color is actually a three-dimensional space and is usually displayed as LAB values, with a light/dark axis (L), a red/green axis (A), and a yellow/blue axis (B).

The LAB color space consists of the range of colors that the human eye can see and has a very large gamut. There are three axes. The L axis represents the brightness, with pure white on the top and pure black on the bottom. The A axis ranges from -A (green) and +A (red). The B axis ranges from -B (blue) to +B (yellow). The resulting 3-D graph shows the available gamut. However, the devices that we use for digital imaging and photography are much more limited in their color gamut, and each device's gamut is unique.

The illustrations on this page show the visualization of a variety of gamuts of both color spaces and output color devices.

A representation of Adobe RGB 98 color space, which is considerably larger than the sRGB

A 3-D representation of the sRGB color space

A 2-D model of the smaller gamut of sRGB inside the larger Adobe 98

A 3-D model of the smaller gamut of sRGB inside the larger Adobe 98

2.3.1. Device-Dependent Color Spaces

Both RGB and CMYK are device-specific color modes: for a given set of CMYK or RGB numbers, the colors you get will vary from device to device. If, for example, you have two identical printers and you send the exact same file to each, the printed images will not match each other exactly, even though the same RGB or CMYK values were sent to both devices. If the printers are from different manufacturers, these differences will be even greater. With an ICC profile, you can make these different devices match each other in terms of final product, but the actual RGB or CMYK values will be different.

2.3.2. Device-Independent Color Spaces

Independent color spaces are defined regardless of device. For instance, CIE XYZ was developed by the CIE (Commission Internationale de I'Eclairage) as a mathematical model to describe how the average person sees color. Since that time, other variations on this color space have been developed, the most common being CIE L*a*b* (usually abbreviated as LAB).

If you wish, images can be edited in this color space with programs such as Photoshop. However, LAB is a very unintuitive color space to work with and edit images in. For the most part, it is used as a connection space for translating colorsfor instance, when converting from one space to another, such as from RGB to CMYK. LAB understands the relationship between itself and each dependent color space. Because LAB is close to a linear and uniform color space, it does a very good job as a translator.

A 3-D model of the standard CMYK profile in Photoshop, which is a SWOP printing press and is a smaller gamut than that of an ink-jet printer

The orange is the Abobe RGB 98 color space while the blue is an ink-jet printer; Adobe space encompasses most of the printer space

The blue represents the gamut of an ink-jet printer compared to the gamut of sRGB color space; notice that there are quite a few colors in sRGB that are outside the gamut of the printer

A 2-D model of the standard CMYK profile in Photoshop, which is a SWOP printing press that is a smaller gamut than that of an ink-jet printer

2.3.3. Intermediate Color Space (Working Space)

In addition to input, monitor, and output profiles, we also have working color space profiles. Until Photoshop 5 came along, we really didn't have to deal with color spaces very much. (In previous versions of Photoshop, the working space was usually the monitor profile.)

With Photoshop 5, Adobe introduced working spaces, which were selectable in the color preferences. The two main advantages to this are:

• The image resides in its own color space and is not limited to the profile of the monitor.

• The image won't change from workstation to workstation as long as the color settings are set up to use the same working space.

A wide variety of color spaces are available, but the two most common RGB working spaces that photographers use are sRGB and Adobe RGB 98. Of the two, sRGB has a smaller gamut, while Adobe RGB has a considerably larger gamut. (There's an even larger color space that you can use: Wide Gamut RGB.)

You might think that using the widest color space is best. However, as we'll see in the next section, using a wide color space can cause problems later on when you are trying to convert the image for output.

The bright green in this image represents the colors of a printing press that are out of gamut from the working space of Adobe RGB 98

The bright green in this image represents the colors of sRGB that are out of gamut from the working space of the image, which is Adobe RGB 98