Color Theory Glossary
Imaging - Imaging
References Definition
Additive mixing Method of color mixing for which two or more primary colors are combined in specific amounts to create a new color. The newly created color was not present in any of the primaries but is created through the interaction of the primaries. Therefore this method is said to be subjective.
Subtractive mixing Method of color mixing which filters the incoming colored light, letting through only a particular color. This method is objective: the desired color must be part of the incoming light
Colorimetric system A (mathematical) way of rigorously classifying and identifying colors by specific attributes
Color space The sum of all the colors that can be represented starting from a set of primaries and a blending (color mixing) rule.

In another definition, which is interchangeable with 'colorimetric system', a color space is a mathematical description of how colors can be uniquely identified by a suite of values called color components.

Gamut The 'volume' of a color space, the collection of all the colors that belong to a color space. A 'wider' gamut implies that the space contains more colors
White point The reference white point in CIEXYZ is the point for which x and y equal 1/3. In a color space, the white point is the purest white that can be obtained using the primaries of that color space.
Color temperature The associated temperature of a color is the temperature at which heated, a black body would emit light of that color. Colors become bluer as the color temperature increases and more red as it decreases.
Planckian locus The geometric locus formed within the chromaticity diagram by all the colors that the radiation (light) coming from a black body can possibly have when heated to different temperatures. Colors with equal color temperature but varying tint are situated on the iso temperature lines drawn across the Planckian locus.
Tint The deviation of the white balance of the image from the Plankian locus. This can be a green tint or a magenta tint.
Color depth The number of discrete colors in a digital representation of a particular color space. For obvious reasons this is a power of 2 and the common representation is the actual number of bits needed to describe the colors. A large bit depth is 48 bits. A regular one is 24 bits. Because each color has three color coordinates, an additional notation only takes into account the bits per color coordinate, making 48 bits equivalent to 16 and 24 to 8.
Color management Ensemble of techniques employed for maintaining consistent color representation across devices with different input or output characteristics
CMM Color Matching Module (or Engine) - routines in the image manipulation software responsible for the conversion from the source color space to the destination color space according to the desired rendering intent. CMM's from different manufacturers have different names, for example:

Microsoft: ICM 2.0 - Image Color Management

Adobe: ACE - Adobe Color Engine

Apple: ColorSync

Rendering intent Set of instructions for the Color Matching Module describing the way translation between the source and the destination color spaces needs to be made. The rendering intent is chosen by the user and depends on the source image, source and destination color spaces and the ... rendering intent.

Four rendering intents exist:

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  • Saturation or Graphic is for business graphics
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  • Absolute colorimetric or Match clips hues outside the destination space's gamut and does not preserve the white point graphics
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  • Relative colorimetric or Proof clips hues outside the destination space's gamut and preserves the white point
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  • Perceptual or Picture stretches or squeezes the input space's gamut to fit exactly in the output space's. Colors are usually modified but no color information is lost in the process.
    Usually, Perceptual and Relative Colorimetric are used in the digital processing of photography.
    Chromaticity diagram, Horseshoe diagram, CIEXYZ The three-dimensional color space of the colors visible to the 'average human eye'. Also, the colorimetric system used to describe colors by their parameters x, y and z. The z axis is perpendicular to the chromaticity diagram. The chromaticity diagram is a 2D representation of all the visible colors at maximum brightness. It is bordered by the pure (saturated) colors - those found in the visible spectrum, and the purple line. Colors toward the center are less saturated.
    Hue Intrinsic property of a color which differentiates it fundamentally from other colors. (A green is certainly not red no matter what kind of green it is). Hue is measured on a circle, in degrees, with red being at 0 deg.
    Saturation, Purity A color a is the more saturated, the closer is to the 'edge' of the chromaticity diagram, considering the white point as the 'center'. Pure or completely saturated colors are on the periphery of the chromaticity diagram. These are the colors in the visible spectrum and in the purple line
    Lightness, Value, Brightness, Luminance Intrinsic property of a color which allows classification by comparing it to a neutral gray scale.
    CIELAB, Lab, L*a*b* A geometrical transformation of the CIEXYZ three-dimensional color space (other description of the same entity). It allows a more uniform and intuitive representation of visible colors, having as axes: L for luminance, a for green-magenta and b for blue-yellow. Correct synonym for CIELAB is 'L*a*b*' but 'Lab' liberally used. If white point is specified, CIELAB is an absolute (device independent) color space
    ICC profile For devices: file that denotes the capabilities of an input or output device in terms of gamut and other rendering characteristics. For color spaces: file that represents the actual color space by its gamut.

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