Adobe PHOTOSHOP 5.0, User Manual, Page: 73 / 400

65 ADOBE PHOTOSHOP 5.0
User Guide
L*a*b model
The L*a*b color model is based on the model
proposed by the Commission Internationale
d’Eclairage (CIE) in 1931 as an international
standard for color measurement. In 1976, this
model was refined and named CIE L*a*b.
L*a*b color is designed to be device independent;
creating consistent color whatever the device (such
as monitor, printer, computer, or scanner) used to
create or output the image.
L*a*b color consists of a luminance or lightness
component (L) and two chromatic components:
the a component (from green to red) and the
b component (from blue to yellow).
For a color illustration of the luminance
model, see figure 4-4 on page 223.
Lab mode
In Photoshop’s Lab mode (the asterisks are
dropped from the name), the lightness component
(L) can range from 0 to 100. The a component
(green-red axis) and the b component (blue-
yellow axis) can range from +120 to –120.
You can use Lab mode to work with Photo CD
images, edit the luminance and the color values in
an image independently, move images between
systems, and print to PostScript
®
Level 2 and Level
3 printers. To print Lab images to other color
PostScript devices, convert to CMYK first.
Lab color is the internal color model Photoshop
uses when converting from one color mode
to another.
Specialized color modes
Photoshop also provides specialized color modes.
Bitmap mode Uses one of two color values (black
or white) to represent the pixels in an image.
Images in Bitmap mode are called bitmapped, or
1-bit, images because they have a bit depth of 1.
(See “About bit depth” on page 68.)
Grayscale mode Uses up to 256 shades of gray.
Every pixel of a grayscale image has a brightness
value ranging from 0 (black) to 255 (white).
Grayscale values can also be measured as
percentages of black ink coverage (0% is equal to
white, 100% to black). Images produced using
black-and-white or grayscale scanners typically are
displayed in Grayscale mode. You can convert both
Bitmap-mode and color images to grayscale.
To convert a color image to a high-quality black-
and-white image, Photoshop discards all color
information in the original image. The gray levels
(shades) of the converted pixels represent the
luminosity of the original pixels.
You can mix information from the color
channels to create a custom grayscale
channel by using the Channel Mixer command.
When converting from grayscale to RGB, the color
values for a pixel are based on its previous gray
value. A grayscale image can also be converted to a
CMYK image (for creating process-color
quadtones without converting to Duotone mode)
or to a Lab color image.