Color Channels; Measuring Color Values In The Info Palette; Adjusting The Monitor Display - Adobe PHOTOSHOP 5.0 User Manual

Therefore, some colors, such as pure cyan or pure
yellow, can't be displayed accurately on a monitor.
Note: The Wide Gamut RGB mode extends this color
range beyond what can be previewed on a monitor,
creating a larger RGB color range in which to work.
For information on editing images in this mode, see
"Entering RGB setup information" on page 84.
The CMYK gamut is smaller, consisting only of
colors that can be printed using process-color
inks. When colors that cannot be printed are
displayed on-screen, they are referred to as out-of-
gamut colors—that is, outside the CMYK gamut.
(See "Identifying out-of-gamut colors" on
page 110.)
For a color illustration of the Lab, RGB,
and CMYK color gamuts, see figure 4-5
on page 223.

Color channels

Every Adobe Photoshop image has one or more
channels, each storing information about color
elements in the image. The number of default
color channels in an image depends on its color
mode. For example, a CMYK image has at least
four channels, one each for cyan, magenta, yellow
and black information. Think of a channel as
analogous to a plate in the printing process, with a
separate plate applying each layer of color.
In addition to these default color channels, extra
channels, called alpha channels, can be added to an
image for storing and editing selections as masks;
and spot color channels can be added, to add spot
color plates for printing. (See Chapter 10, Using
Channels and Masks.)
An image can have up to 24 channels. By default,
Bitmap-mode, grayscale, duotone, and indexed-
color images have one channel; RGB and Lab
images have three; and CMYK images have four.
You can add channels to all image types except
Bitmap-mode images. For information on these
image types and other distinctions, see the sidebar
"Color modes" on page 229.
Measuring color values in the
Info palette
Depending on how your Info palette is set up, you
can select any tool, position the pointer over any
part of an image, and determine the color value
under the pointer. You can customize the Info
palette and color samplers to express color values
using the HSB, RGB, CMYK, Lab, or Grayscale
modes without changing the mode of the image
itself. For more information on measuring color
with the Info palette and color samplers, see
"Previewing color values" on page 107 and the
procedure to change Info palette options in "Using
the Info palette" on page 30.

Adjusting the monitor display

Although the RGB color model used by computer
monitors is capable of displaying much of the
visible spectrum, the video system sending data to
a given monitor often limits how many colors can
be displayed at once. By understanding how color
data is measured in digital files and on-screen, you
can better adjust Photoshop's preferences to offset
the limitations of your video system.
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User Guide