CHAPTER 12 HOW COMPUTERS AND PRINTERS CREATE PHOTOGRAPHS^191
Finally, the software writes a file that
contains the monitor’s color profile.
This is simply a short record of the
color space values for a few key col-
ors the monitor can produce. This
information can be used by scanners,
printers, cameras, and other devices
to adjust their colors to match those of
the monitor. Because the color values
are different for different devices,
color matching is not perfect.(^9) Color Calibration on the Cheap
Color calibration systems that include a colorimeter cost
at least $100. You can take a software-only approach
for free. PPTools provides a free printable color chart
and software to calibrate a monitor (http://www.pptfaq.
com/FAQ00448.htm). Monitor Wizard 1.0 is freeware
that also does the job (http://www.hex2bit.com/). How
well do they work? If they were perfect, they wouldn’t be
free. They all require your perceptions, which are subjec-
tive. In my opinion, the best software-only tool for moni-
tor measurement and maintenance is Display Mate
(http://www.displaymate.com/). It costs about $70, but
in addition to calibrating, it lets you test every aspect of
your display.
On the side of the colorimeter facing
the screen are three to seven photon
counters, similar to light meters. The
counters are set into walled cells to
prevent them measuring ambient light
from the sides.
(^4) If the colorimeter is being used with an LCD screen, light
from the screen first passes through a filter that removes
ultraviolet light that the photon counters would otherwise
have added into their count hits, inflating their readings.
On a cathode ray tube (CRT) monitor, the thick glass at
the front of the display absorbs the ultraviolet.
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After the light is evenly spread out by passing
through adiffusermade of a material that could
be mistaken for a plastic milk jug, it passes through
thecolor filters mounted above each photo
counter. The seven filters roughly correspond to the
main divisions of the visible spectrum: red,
orange, yellow, green, blue, indigo, and violet.
The software switches the color on the screen
among red, blue, and green to give the
meter a chance to gauge each color.
A microprocessor in the colorime-
ter converts the counters’ readings
for red, green, and blue into digi-
tal values on the scale of the color
space the meter uses. A color
space lets any color be defined by
a set of three numbers called the
tristimulus values.
The software uses this information to
adjust the monitor’s settings so it has
the widest possible ranges for the
three colors (RGB). If the monitor can-
not be controlled by the software, the
program turns to the video card,
tweaking the controls that send color
signals to the monitor.
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