Monitor Calibration and Characterization

 

What is Monitor Calibration ?

Calibration is the process of modifying the color behavior of a device to achieve a known defined state.

Usually this is done by changing the controls or internal settings of the monitor or by adjusting the 1D Look Up tables [1DLuts] in the graphics board by ICC profiles or other OS specific programs.

The goal of calibration is to put a display device into a defined state with regard to its color response. Different devices have their own intrinsic stability and it is not unusual to perform a calibration on a day to day basis to maintain reproducible behavior.

A calibration is stored in device or systems specific files that archive the device properties [ White Point, Color Primaries and per channel calibration curves].

Usually the calibration information is stored in the ICC [ICM] profile for convenience. It is stored in the 'vcgt' tag.

Although the calibration information is stored in the profile, none of the normal ICC based tools or applications are aware of it.

Also, in a typical OS color management, the system uses the calibration curves stored in the 'vcgt' tag, but nothing else. This leads to color problems with monitors that have expanded color gamut space.

The role of the ICC [ICM] profile and the OS system color management program is to provide a set of 1DLuts for the graphics board that achieve the desired white point in luminance and color [as defined by X Y Z values] as well as a neutral tone-scale gamma aim position.

If the monitor has hardware setting for a gamma of 2.2 [usually for sRGB], and the user wants to calibrate to a gamma of 2.4 [usually the gamma of a Rec-709 specified monitor], then the 1DLuts have the gamma change in the curves along with the RGB scaling to achieve the desired white point from black to white.

A wide gamut monitor calibrated to a specific position, i.e., Rec-709 with a gamma of 2.4 and a white point of 6500K will achieve that specified neutral position. However, the COLOR position of the monitor will NOT adhere to the Rec-709 colorimetric standard if the color RGB primaries are not identical to the Rec-709 standard. The usual result would be enhanced saturation of colors vs. the standard.

This is problematic in facilities where there are many different monitors with widely different RGB primaries leading to a potpourri of color position even though they are all "calibrated" to a common neutral position.

 

What is Monitor Characterization ?

Characterization, sometimes referred to as profiling, is a representation of the way a device produces color based on the digital code value input. Typically, a large set of code values is sent to the device and the colorimetry of the output on the device is measured with a colorimeter or spectrophotometer. The resultant data is a mapping of display code values and the corresponding X Y Z colorimetric values.

The characterization result is stored in a device ICC profile or a data file which is used to create specific 3DLuts for programs that are not friendly to ICC color management.

When a profile is associated with a device in the system OS, it as stated before, only uses the 1DLuts for the graphics board, even though it may have an embedded 3DLut derived from the characterization process. At this point a wide-gamut monitor still has the problems stated above.

What a profile [ICC,ICM] does is allow a system such as a CMM (Color Management Module) or color aware application [such as Photoshop] to achieve the proper color when combined with another profile.

In the Motion Picture industry, most color correction software for Digital Intermediates or VFX programs, both 2D and 3D are not color aware and the characterization is introduced via 3DLuts [the format of what the VFX software will accept varies].

These characterization (profile) entities are only be valid for a device if the state of calibration is the same as it was when characterized.

 

MJB Consulting can provide advice and assistance in calibrating monitors and characterizing monitors. The characterization can be folded into a 3DLut or combined with other existing or newly created 3DLuts for specific DI and VFX software.