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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
Color space

A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components ....
. 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.

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.






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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
Color space

A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components ....
. 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.

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
    Cathode ray tube

    The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen, with internal or external means to accelerate and deflect the electron beam, used to create images in the form of light emitted from the fluorescent screen....
     or crystals
    Liquid crystal display

    A liquid crystal display is an Electro-optic modulator shaped into a thin, flat panel made up of any number of color or monochrome pixels filled with liquid crystals and arrayed in front of a Light#Light sources or reflector....
     actually producing a light that falls inside the red spectrum and
  • the overall brightness of the color resulting in the desired color perception
    Color vision

    Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths of the light they reflect or emit. The nervous system derives color by comparing the responses to light from the several types of Cone cell in the eye....
    . (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
VGA connector

A VGA connector as it is commonly known is a three-row 15 pin D-subminiature. There are four versions: and pinouts, the far older and less flexible DE-9 connector, and a Mini-VGA used for laptops....
 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
White point

A white point is a set of tristimulus or chromaticity coordinates that serve to define the color "white" in image capture, encoding, or reproduction....
; 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
Computer display

A visual display unit, often called simply a monitor or display, is a piece of electrical equipment which displays images generated from the video output of devices such as computers, without producing a permanent record....
 and calibration target will have to be considered in controlled, predefined lighting conditions.

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 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 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
ICC profile

In color management, an ICC profile is a set of data that characterizes a color input or output device, or a color space, according to standards promulgated by the International Color Consortium ....
) 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
On-screen display

An on-screen display is an image superimposed on a screen picture, commonly used by modern televisions, video cassette recorders, and DVD players to display information such as volume, channel, and time....
. This tunes the display to reproduce fairly accurately the in-gamut
Gamut

In color reproduction, including computer graphics and photography, the gamut, or color gamut , is a certain complete subset of colors....
 part of a desired color space. The calibration target for this kind of calibration is that of print stock paper illuminated by D65
D65

CIE Standard Illuminant D65 is a commonly-used standard illuminant defined by the International Commission on Illumination . It is part of the D series of illuminants that try to portray standard illumination conditions at open-air in different parts of the world....
 light 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 is necessary for each printer/paper combination.

Limited calibration can be done visually using a color chart
Color chart

In color-related fields, a color chart is a physical arrangement of standardized color samples, used for color comparisons and measurements such as in checking the color reproduction of an imaging system such as a camera....
. This is not a substitute for hardware calibration.

See also


  • Color management
    Color management

    In digital imaging systems, color management is the controlled conversion between the color representations of various devices, such as s, digital cameras, monitors, TV screens, film printers, computer printers, offset presses, and corresponding media....


External links

  • Free online tools for calibrating monitor brightness, contrast & color depth.
  • Free website for checking the monitor calibration and the color management capabilities of web browsers