Calibration vs. Characterization

Some of the terminology can be confusing. Many people are initially confused about the difference between Calibration and Characterization/Profiling.

What is Calibration ?

"Calibration" is a short hand Graphic Arts term for adjusting a devices behavior to meet calibration targets.
Calibration is the process of modifying the color behavior of a device. This is typically done using two mechanisms:
    1) Changing controls or internal settings that it has.
    2) Applying curves to its color channels.

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.

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.

What is Characterization/Profiling ?

Characterization (or profiling) is recording the way a device reproduces or responds to color. Typically the result is stored in a device ICC 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.

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 Device Link.
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.

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.

What about display calibration and profiles ?

In the case of display profiles there is some additional confusion because often the calibration information is stored in the profile 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 ICC 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.