Glossary
  • Accuracy: How close measurements are to those made with a standard measurement device (for color this would be a spectrophotometer). The closer the agreement the better the accuracy.
  • Color temperature: Temperature in K of a black body of the same color. Used for illuminants and white points. When the spectrum of such an illuminant is very different from that of a black body radiatior (Planck's law) one speaks of 'correlated color temperature'.
  • Gamma correction: A non-linear transformation of tristimulus values meant to compensate for the non-linear response of cathode-ray tube type display devices. By pure coincidence this makes the resulting values more perceptually uniform so that it lowers their bandwidth requirements.
  • Gamut: The colors a device or a color space can represent. E.g. the sRGB color space has a rather limited gamut compared the human visual system, and the cyan color of the MacBeth Color Chart cannot be represented in it.
  • Illuminant: this is more than a light source, more like a well-defined spectrum. The CIE has defined several standard illuminants. Related to the white point.
  • Chromatic adaptation: adjustment of the color balance of the human visual system so that the brightest object in view looks more or less 'white'. This adaptation is usually achieved within 2 minutes, but is quite complex and is still the subject of continued research. Mimicked by white-balancing in cameras.
  • CIE: 'Commision International de l'Eclairage' (Internation Commission on Illumination), a standardizing body in the field of lighting and colorimetry.
  • CIE A: A standard illuminant at 2856 K. It is used to represent incandescent lighting and is quite yellow.
  • CIE F: A series of standard illuminants used to represent different types of fluorescent lighting. F2 is cool white, F7 and F8 are daylight white.
  • CIE D65: A standard illuminant with a color temperature of 6500K used to represent daylight at noon.
  • CIE XYZ: A standardized color space spanning all visable colors, derived from CIE RGB. Colors are defined by positive amounts of the three primaries called X,Y and Z. Note that these primaries are virtual and do not exists in the real world.
  • CIE RGB: A standardized color space spanning all visable colors. Colors are defined as combinations of three monochromatic primaries (red, green and blue). Some colors require negative amounts of red, which lead to the developpement of CIE XYZ. Based on color matching experiments made in the beginning of the 20th century.
  • CIE L*a*b*: A perceptually uniform color space, i.e. a color space in which the Euclidean norm between 2 colors is proportional to the perceived color difference between those 2 colors. Derived from CIE XYZ.
  • Primaries: base colors of a color space.
  • Reproducibility: The amount of spread of different measurements of the same quantity around the average of those measurements. The lower the spread, the better the reproducibility.
  • RGB: A generic color space based on red, green and blue primaries. Without exact knowledge of these primaries and white point, data defined in such a color space is almost useless for any quantitative work.
  • Saturation: occurs in cameras when there is either too much or too little light for the camera sensor(s) under the current settings. Usually this results in a sensor output either at the minimum or maximum level possible, e.g. 0 or 255 for an 8-bit digitized signal. When this happens data is irrevocably lost. Saturated data should be recognized and discarded because they are wrong and can induce errors, e.g. when used for calibration.
  • sRGB: A standardized RGB color space with a D65 white point. Many monitors and printers use this color space which allows realistic viewing and printing of sRGB images. Also said to be the color space 'of the web'. Has limited gamut. Known relationship with the CIE color spaces.
  • Thrichromatic principle: 'Almost all colors can be reconstructed using a combination of 3 suitable base colors (primaries)'.
  • Tristimulus values: 3 color coordinates in a color space representing the strengths of the 3 primaries of that color space needed to reconstruct a certain color.
  • White balancing: adjustment of the color balance so that the brightest object in view looks more or less 'white'. Similar to what human vision does (chromatic adaptation).
  • White point: The color obtained when the full strenght of the 3 primaries of a color space are combined. Also the tristimulus values of an illuminant.