Color calibration

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The aim of color calibration is to measure or adjust the color response of a device (input or output) to establish a known relationship to a standard color space.^[1] The device that is to be calibrated is sometimes known as calibration source; the color space that serves as a standard is sometimes known as calibration target.^{[citation needed]}

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. This 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. The resultant color being displayed, however, depends on two main factors:

• The phosphors or crystals actually producing a light that falls inside the red spectrum and
• the overall brightness of the color resulting in the desired color perception. (An extremely bright light source will always be 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 also alter the electrical signal at any stage in the signal flow. (A partially inserted VGA connector can result in a monochrome display, for example, as some pins are not connected.)

Color perception

Color perception is subject to ambient light levels, and the ambient white point; for example, a red object looks black in blue light. It is therefore not possible to achieve calibration that will make a device look correct and consistent in all capture or viewing conditions. The computer display and calibration target will have to be considered in controlled, predefined lighting conditions.

Calibration techniques and procedures

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

• The scanner needs a device specific calibration to output the originals colors.
• The computer display needs to represent the colors of the image color space.
• The printer needs to match the computer display.

Scanner

An an IT8.7 Target by LaserSoft Imaging

For creating a scanner profile it needs an IT8-target, an original with many small color fields, which was measured by the developer with a photometer. The scanner reads this original and compares the scanned color values with the target's reference values. Taking the differences of these values into account an ICC profile is created, which relates the device specific color space (RGB color space) to a device independent color space (L*a*b color space). Thus, the scanner is able to output with color fidelity to what it reads.

Display

For calibrating the monitor a colorimeter is attached flat to the display's 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 from 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 fairly accurately the in-gamut part of a desired color space. The calibration target for this kind of calibration is that of print stock paper illuminated by D65 light at 120 cd/m².

Printer

The ICC profile for a printer is created by comparing a test print result using a photometer with the original reference file. The testchart contains known CMYK colors, whose offsets to their actual L*a*b colors scanned by the photometer are resulting in an ICC profile. Another possibility to ICC profile a printer is to use a calibrated scanner as the measuring device for the printed CMYK testchart instead of a photometer. A calibration profile is necessary for each printer/paper/ink combination.

See also

• color chart
• Color management
• IT8

References

1. ^ Hsien-Che Lee (2005). Introduction to color imaging science. Cambridge University Press. http://books.google.com/books?id=CzAbJrLin_AC&pg=PA388&dq=%22color+calibration%22+camera+monitor&lr=&as_brr=3&ei=v7QTSL3-OI2qtgPI54CgCA&sig=5_7RELfTow9Pm98V6TXQ__KPlKo#PPA388,M1. 

External links

• DisplayCalibration.com Free online tools for calibrating monitor brightness, contrast & color depth.
• monitorsetup.com Free website for checking the monitor calibration and the color management capabilities of web browsers
• nla.gov.au/preserve/dohm/sipc.html SIPC - an open source IT8.7 scanner calibration software by Andrew Stawowczyk Long