What's the difference between calibration and characterization?
Many people are initially confused about the difference between calibration and characterization. Calibration is the process of modifying the color behavior of a device. This is typically done using two mechanisms:
Changing controls or internal settings that it has
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. Typically calibration will be stored in device or systems specific file formats that record the device settings or per-channel calibration curves.
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. It allows a system such as a CMM (Color Management Module) or a color aware application to modify color when combined with another device profile. Only by knowing the characteristics of two devices, can a way of transferring color from one device representation to another be achieved.
Note that a characterization (profile) will only be valid for a device if it's in the same state of calibration as it was when it was characterized.
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. Similarly, typical display calibration tools and applications will not be aware of, or do anything with the ICC characterization (profile) information.