Graphics Programs Reference
In-Depth Information
in aliasing artifacts. Each part of calibration that can be conducted in the display electronics
reduces the amount of these artifacts. For example, a display that provides three presets for
white point will allow us to get closer to our final target in hardware. The reason a display
system like the Barco or Artisan can produce such smooth gradients is they are able to conduct
virtually all the calibration in the display and leave the CLUT untouched. If you see banding on
one of these displays, it's in the image.
Many LCDs have only a single physical control (backlight luminance). All the other adjust-
ments have to be produced in the CLUT. New LCDs will provide more capabilities in the future.
The bright side (no pun intended) is the user has a lot less to deal with when calibrating an
LCD. The bad news is the degree of banding that can present itself on the display. A unique
solution is the Eizo ColorEdge LCD. That product uses a 10-bit LUT inside the display to adjust
color temperature and gray tracking. This greatly reduces the aliasing artifacts crated by LUT-
based calibration. It should be noted that this solution still does not provide control over black
level, thus dynamic range remains fixed. See Chapter 9, Tutorial #10: “Testing Your Display
Profile” for information on evaluating your display.
Spectroradiometers and Colorimeters
There are two primarily instrument technologies available for calibration
and profiling displays.
Colorimeters
are instruments that use a set of color
filters to mimic the response of human vision.
Spectroradiometers
break the
visible spectrum into many bands providing individual intensity values
for each. You may be more familiar with the term Spectrophotometer. A
Spectrophotometer is a device that shines an illuminant onto a sample
and then measures the spectra (see Fig. 3-1). When measuring an emis-
sive surface no illuminant is used; instead we are measuring the energy
radiating from the source (hence the term radiometer). Some spectrora-
diometers can also be used as Spectrophotometers. This allows a single
instrument to perform both jobs, although not optimally.
Colorimeters may be less expensive to manufacture, but this does not
mean they are inferior. When measuring displays, dark colors and black
are very important, as we have seen. If you divide the light into more
than three components your ability to differentiate darker colors is greatly
reduced. A colorimeter optimized for your display device always will
provide the best results. Colorimeters are not necessarily the best solu-
tion for measuring printed output.
Some items to consider when calibrating your display are as follows.
It is important that ambient light be kept to a minimum when calibrat-
ing your display. Stray light striking the surface of the display can travel
along the glass under the sensor. This will affect calibration. It is impor-
tant to ensure a tight fit and no movement of the sensor during the cal-
ibration process. Most instruments for LCDs are placed on the surface and
hang on for dear life. They use a soft seal around the sensor and a coun-
terweight (see Fig. 3-2). Try tilting the LCD back a bit so gravity assists
