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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<title>Calibration vs. Characterization</title>
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<h2 style="text-decoration: underline; font-weight: bold;">Calibration
vs.
Characterization<br>
</h2>
Some of the terminology can be confusing. Many people are initially
confused about the difference between <span style="font-weight:
bold;">Calibration</span> and <span style="font-weight: bold;">Characterization/Profiling</span>.<br>
<h3>What is Calibration ?</h3>
"Calibration" is a short hand Graphic Arts term for adjusting a
devices behavior to meet calibration targets.<br>
Calibration is the process of modifying the color behavior of a
device. This is typically done using two mechanisms:<br>
1) Changing controls or internal settings that it
has.<br>
2) Applying curves to its color channels.<br>
<br>
The idea of calibration is to put a device in a defined state with
regard to its color response. Often this is used as a day to day
means of maintaining reproducible behavior. Calibration is often the
most practical way of setting parameters such as white point and
brightness of displays. Typically calibration will be stored in
device or systems specific file formats that record the device
settings and/or per channel calibration curves.<br>
<br>
For some specific applications (such as Video playback), calibration
may be used to allow a video signal with a specific encoding (i.e.
colorspace) to display correctly on a display without color
management software or hardware being needed to transform between
color spaces. For each source colorspace, a specific calibration
needs to be made for the display, and the display can only show one
source correctly at a time. If you have N source color spaces, and M
displays, then N x M calibrations would need to be created to
display any source on any display.
<h3>What is Characterization/Profiling ?</h3>
Characterization (or <span style="font-weight: bold;">profiling</span>)
is <span style="text-decoration: underline;">recording</span> the
way a device reproduces or responds to color. Typically the result
is stored in a device <span style="font-weight: bold;">ICC</span>
profile. Such a profile does not in itself modify color in any way.
What it does is allow a system such as a CMM (Color Management
Module) or color aware application to modify color when combined
with another device profile. Only by knowing the characteristics of
two devices or colorspaces, can a way of transferring color from one
device representation to another be achieved.<br>
<br>
Given a source profile and a destination (i.e. display) profile, a
direct transform can be computed between the two. In ICC terminology
this is a <b>Device Link</b>.<br>
The benefit of managing color this way is that such transforms can
be created on the fly between any source colorspace and the display,
and all the results rendered into the same output. Many sources with
different color spaces, or color spaces that are unknown at the time
of profiling the display can be rendered correctly. If you have N
source colorspaces and M displays, then only N + M profiles need to
be created to display any source on any display.<br>
<br>
Note that a characterization (profile) will only be valid for a
device if it is in the same state of calibration as it was when it
was characterized/profiled.<br>
<h3>What about display calibration and profiles ?</h3>
In the case of display profiles there is some additional confusion
because often the <span style="font-weight: bold;">calibration</span>
information is stored in the <span style="font-weight: bold;">profile</span>
for convenience. By convention it is stored in a tag called the
'vcgt' tag. Although it is stored in the profile, none of the normal
<span style="font-weight: bold;">ICC</span> based tools or
applications are aware of it, or do anything with it, it is just
"along for the ride". Similarly, typical display calibration tools
and applications will not be aware of, or do anything with the ICC
characterization (profile) information.<br>
<br>
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