Digital Projector Calibration and Characterization

 

What is Digital Projector Calibration ?

Calibration is the process of modifying the color behavior of a device to achieve a known defined state. [ see: Digital Projector Standards Setup]

Usually this is done by changing the controls or internal settings of the digital projector.

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.

Digital projectors are fairly stable, but they should be checked on a daily basis for color accuracy and consistency. A calibration should always be done after replacing a bulb. Also, besides the problem of bulb age, it is not uncommon in a large facility to have multiple calibrations to various standards on a single device, so a check before using to assure the device is in the desired color space can avoid problems.

A digital projector calibration is stored in the device or in proprietary files used with manufacturer supplied software.

Typically, the digital projector is calibrated to either P3 4:4:4 RGB color space or Rec-709 4:4:4RGB color space. If Digital Cinema files are to be displayed, then the calibration position is 4:4:4 X Y Z color space. Other standards to be considered are any of the above formats in 4:2:2 which is Y Cb Cr. Proper conversion matrices are required for 4:2:2.

The digital projectors today have a good sequential contrast ratio, usually around 2000:1 . The term sequential contrast ratio refers to the ratio of the luminance of a full on white full screen patch to the luminance of a full off black full screen patch. The effects of optics and surround do not usually change this ratio.

A stressful contrast ratio is the in-frame contrast ratio. A single frame with alternating white and black patches in a checker board pattern [usually a 3 X 4 array or a 4 X 6 array] is used for this in-frame contrast ratio determination. All of the patches are read with a luminance meter [i.e., Konika Minolta LS-100] and the white patches are averaged and the black patches are averaged and the in-frame contrast ratio is the ratio of the white average over the black average. This metric is very sensitive to surround light flare [bouncing back to the screen from objects in the room] and optical flare [lens and dichroic beam splitting module etc.]. A "good to excellent" in-frame contrast ratio is above 150, a"typical" in-frame contrast ratio is about 100 to 150. Typically, the lower in-frame contrast ratio projectors show poor low-key or dark scene reproduction.

We can measure a projector sequential contrast ratio and in-frame contrast ratio to determine the effect on image quality.

The 3D calibration of a digital projector will depend on the 3D system employed.

 

What is Digital Projector 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 data file which is used to create specific 3DLuts for film emulation in a Digital Intermediate venue.

In the Motion Picture industry, these 3DLuts can be transformed to different device color spaces for use with other color correction software for Digital Intermediates or VFX programs, both 2D and 3D. Most all VFX programs are not color aware and the characterization and emulation is introduced via 3DLuts [the format of what the VFX software will accept varies].

The characterization 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 Digital Projectors and characterizing Digital Projectors. We can create film emulation 3DLuts for many uses. The characterization can be folded into a 3DLut or combined with other existing or newly created 3DLuts for specific DI and VFX software.