Hardware Reference
In-Depth Information
built up from the linear superposition of the three additive primary colors, which
are red, green, and blue.
In contrast, reflected-light images, such as color photographs and pictures in
glossy magazines, absorb certain wavelengths of light and reflect the rest. These
are built up from a linear superposition of the three subtractive primary colors,
cyan (all red absorbed), magenta (all green absorbed), and yellow (all blue
absorbed). In theory, every color can be produced by mixing cyan, yellow, and
magenta ink. In practice it is difficult to get the inks pure enough to absorb all
light and produce a true black. For this reason, nearly all color printing systems
use four inks: cyan, magenta, yellow, and black. These systems are called
CMYK
printers
. The K is sometimes attributed to blacK but it really stands for the Key
plate with which the color plates are aligned in conventional four-color printing
presses. Monitors, in contrast, use transmitted light and the RGB system for pro-
ducing colors.
The complete set of colors that a display or printer can produce is called its
gamut
. No device has a gamut that matches the real world, since typically each
color comes in 256 intensities, giving only 16,777,216 discrete colors. Imperfec-
tions in the technology reduce the total more, and the remaining ones are not al-
ways uniformly spaced over the color spectrum. Furthermore, color perception has
a lot to do with how the rods and cones in the human retina work, and not just the
physics of light.
As a consequence of the above observations, converting a color image that
looks fine on the screen to an identical printed one is far from trivial. Among the
problems are
1. Color monitors use transmitted light; color printers use reflected light.
2. Monitors have 256 intensities per color; color printers must halftone.
3. Monitors have a dark background; paper has a light background.
4. The RGB gamut of a monitor and the CMYK gamut of a printer are
different.
Getting printed color images to match real life (or even to match screen images) re-
quires hardware device calibration, sophisticated software for building and using
International Color Consortium profiles, and considerable expertise on the part of
the user.
Inkjet Printers
For low-cost home printing,
inkjet printers
are a favorite. The movable print
head, which holds the ink cartridges, is swept horizontally across the paper by a
belt while ink is sprayed from tiny nozzles. The ink droplets have a volume of
about 1 picoliter, so 100 million of them fit in a single drop of water.
