Color calibration

This article is about the color calibration on output devices such as displays and printers.

The aim of Color calibration is to adjust the colors of one output device to match that of another. The device that is to be calibrated is commonly known as calibration source the device that serves as a comparison standard is commonly known as calibration target. Both target and source can be a Color space such as Adobe RGB or CMYK color space, a test print, color chart or material sample.

Information flow and output distortion

A computer program that sends a signal to the computer's graphic card in the form RGB (Red,Green,Blue)255,0,0, signals only a device instruction, not a color itself. The instruction then causes the connected display to show Red to the maximum achievable brightness, while the Green and Blue components of the display remain dark. What resulting color is being displayed however is dependent on two main factors:

• The phosphors or crystals actually produce a light that falls inside the red spectrum and
• the overall brightness of the color results in the desired color perception. (An extremely bright light source will always been seen as white, irrespective of spectral composition.)
  

Hence every output device will have its unique color signature, displaying a certain color according to manufacturing tolerances and material deterioration through use and age. If the output device is a printer, additional distorting factors are the qualities of a particular batch of paper and ink.

The conductive qualities and standards compliance of connecting cables, circuitry and equipment can alter the electrical signal before at any stage in the signal flow. (A partially inserted VGA connector can result in a monochrome display, as some pins are not connected.)

Color Perception

Color perception is subject to ambient light levels, and the ambient white point. (A red object looks black in blue light.) It is therefore not possible to achieve calibration that will be perceived evenly in different lighting conditions.The computer display and calibration target will have to be considered in controlled predefined lighting conditions. Controlled lighting conditions such as D65 help to suppress the effect of metameric colors which would further complicate the issue.

Techniques and procedures

The most common form of calibration aims at adjusting monitors and printers for photographic reproduction. The aim is that a printed copy of a photograph appears to be identical in saturation and dynamic range to the source file on a computer display. This means that two independent calibrations need to be performed:

• The computer display needs to represent the colors of the image color space.
• The printer needs to match the computer display.

In the first stage an external calibration device is attached flat to the displays surface, shielded from all ambient light. The calibration software sends a series of color signals to the display and compares the values that were actually sent against the readings of the calibration device. This establishes the current offsets in color display. Depending on the calibration software and type of monitor used, the software either creates a correction matrix (i.e. an ICC Profile) for color values before being sent to the display, or gives instructions for altering the display's brightness/contrast and RGB values through the OSD. This tunes the display to reproduce parts of a desired color space (It is technically and procedurally not possible to reproduce an exact color space on a computer display). The calibration target for this kind of calibration is that of print stock paper D65 at 120 cd/m².

In the second stage the software sends a test print to the printer, and compares the print result with the original file with the use of an external calibration device, similar to the display calibration. A calibration profile per printer/paper combination is necessary.

Calibration Devices

Several devices available for calibrating display monitors are listed here:

• Huey, from Pantone is a low-cost, easy to use unit.
• Xrite Eye One Display
• ColorVision Spyder2 Suite Color Calibration System
• Pantone Eye-One Display 2 Colorimeter
• ColorVision Spyder2Express professional but easy to use and affordable

External links

• Free online tools for calibrating monitor brightness, contrast, color depth and resolution.
• Online monitor calibration tool
• A discussion of some color calibration tools