Graphics Programs Reference
In-Depth Information
what unstable and change on a regular basis, calibration is necessary. The
process of calibration requires that we have a target color space for the
display. A display target includes the specification of white point, dynamic
range (black level), and the tonal response (TRC). The TRC often is
described using a value for the gamma formula. A typical target color
space for a display might be D65, a TRC of gamma 2.2, and a dynamic
range of 500 : 1. The dynamic range is simply a ratio of luminance
between black and white. The process of calibration and profiling a
display is similar to the process of calibration and profiling other devices
in the imaging pipeline.
To effect calibration of the monitor, the software will display colors
with standard reference values. An instrument will be used to measure
the results. The software will then use the results of this measurement
to adjust the display toward the specified target color space. After the
target has been reached the software again will display a set of standard
colors. The data gathered from these color patches will be used to create
the ICC profile for the device. Hardware devices such as a Colorimeter
or Spectroradiometer are examples of instruments that may be used in
this process.
Often, the line between calibration and profiling is unclear in several
respects. Many software packages perform both in a single operation. It
may also be unclear when purchasing a package whether the package
allows physical calibration of the display. With some products, the extent
of the calibration options is limited.
When calibrating a display, the goal is to reach a target color space.
The components of a target color space are as follows.
Tonal Response Curve (TRC)
Often described using gamma (see Chapter 1). Over the years, most users
have come to believe that the “right” TRC for a Macintosh user is gamma
1.8, and for the PC user, gamma 2.2. This is primarily due to the TRC
assumption made by the operating system. When Apple created the
Macintosh, they chose a system TRC of gamma 1.8, which is very similar
to the response of printing technology. There was no color management
at the time. The operating system and the output devices were all
Grayscale. By using a TRC similar to the output device, images on the
screen tended to match output closely. Today the use of gamma 1.8
within the Macintosh OS is simply a legacy of these early days. If a user
calibrates the display to something other than a TRC of gamma 1.8, the
appearance of color outside ICC-aware applications will be incorrect. It
is preferable to use a target TRC of gamma 2.2. As this is much closer to
the natural TRC of the display, aliasing artifacts are reduced.
The ICC profile provides the TRC of the display to Photoshop and
other ICC-aware applications. These applications use the ICC profile to
provide color correct previews. Outside these applications, on the
