Glossary of Terms :

Here are the most commonly used terms and acronyms in alphabetical order. To find a particular term, use your browser's Find (Ctrl+F) command.


- A

  • Absolute white - an ideal material that totally reflects all the light at every wavelength in the visible region. In practice, a white material that reflects most of the incident light equally across the visible region. When calibrating a spectrophotometer, often a white ceramic plaque is measured and used as the absolute white reference.
  • Absorption - when molecules of a material are exposed to light having an energy that matches a possible electronic transition within the molecule, some of the light energy will be absorbed as the electrons are promoted to a higher energy levels. The absorption of light makes an object dark or opaque. When light of certain wavelength is absorbed, the remaining (transmitted) light will then assume the complementary color to the wavelengths absorbed.
  • Adapted white - color stimulus that an observer who is adapted to the viewing environment would judge to be perfectly achromatic (perfect white diffuser). The adapted white may vary within a scene - see color temperature.
  • Adopted white - spectral radiance distribution as seen by a measurement device and converted to color signals that are considered to be perfectly achromatic. Such conversion is arbitrary and thus adopted white is result of our definition (e.g., D50 illuminant).

- B

  • Black - an ideal material that totally absorbs the incident light; the absence of any reflection. In practice, any color of very low saturation and very low luminance.
  • Brightness - the attribute of visual perception in accordance with which an area appears to emit or reflect more or less light regardless of hue or saturation.

- C

  • Calibration - the process of adjusting the performance of input, display, and output devices to a known state.
  • Characterization - the process of describing and defining the unique reproduction capabilities of a device.
  • Chromaticity coordinates (CIE) - The ratios of each of the three tristimulus values X, Y and Z in relation to the sum of the three - designated as x, y and z respectively.
  • Chromaticity diagram (CIE) - a two-dimensional graph of the chromaticity coordinates (x as the abscissa and y as the ordinate), which shows the spectrum locus of the visible light (chromaticity coordinates of monochromatic light, 380-770nm).
  • CIE - French name Commission internationale de l'éclairage is the international authority on light, color, and color spaces.
  • CIELAB (or CIE L*a*b*, CIE Lab) - color space in which values L*, a*, and b* are plotted orthogonally to one another to form a three-dimensional coordinate system. Equal distances in the space approximately represent equal color differences. Value L* represents Lightness; value a* represents the Magenta/Green axis; and value b* represents the yellow/blue axis. CIELAB system was recommended for common use by CIE in 1976.
  • CIE Standard Illuminants - known spectral data established by the CIE for different types of light sources. When using tristimulus data to describe a color, the illuminant must also be defined. These standard illuminants are used in place of actual measurements of the light source.
  • CIE Standard Observer - a hypothetical average observer matching the tristimulus color-mixture data recommended in 1931 by the CIE for a 2� viewing angle. A supplementary observer for a larger angle of 10� was adopted in 1964. If not specified, the 2� Standard Observer should be assumed. If the field of view is larger than 4�, the 10� Standard Observer should be used.
  • CIE Tristimulus Values - amounts of the three components necessary in a three-color additive mixture model to match any color (designated as X, Y, and Z).
  • Color difference - a color difference space is a 3-D space with approximately uniform visual spacing in terms of color difference judgments. In color tolerancing, the symbol ΔE (DE) is used to express the difference in between two colors. The letter "E" does not refer to an "Error", but its meaning comes from the German word empfindung (difference in sensation). There are several equations leading to different flavors of for color differences. In general, the color difference calculated as the square root of the combined squares of the lightness, chroma, and hue. A difference of 1 ΔE is used as a threshold of what is perceptible to the human eye (a just-noticeable difference).
  • Color Matching Functions - Tables of values determining how much of each of the R,G,B primaries are needed to match colors of a reference spectrum. These results in form of graphs are called the color-matching functions. The interpretation of these graphs is that they show how much of each primary (the tristimulus value) is needed to match unit of intensity of a single wavelength of light. The term is generally used to refer to the CIE Standard Observer color matching functions designated.
  • Color Space - a mathematical model with a three-dimensional geometric representation of the colors that can be seen and/or generated using a certain color model. Colorimetric color spaces have an exact and simple relationship to CIE colorimetric values. Colorimetric color spaces include those defined by CIE (e.g., CIE XYZ, CIELAB, CIELUV, etc.), as well as color spaces that are simple transformations of those color spaces (e.g., additive RGB color spaces).
  • Color Management System (CMS) - a software system that ensures color consistency and repeatability across all devices in a production workflow.
  • Colorimeter - an optical measurement instrument that responds to color in a manner similar to the human eye-by filtering reflected or emitted light into its dominant regions of red, green, and blue.
  • Color temperature - chromaticity of the light emitted over a range of temperatures perceived as a "white" (bluish, yellowish or reddish). Perceived "white" color is commonly referred to as the color temperature of the black body.

- D

  • D50 -the CIE Standard Illuminant that represents a color temperature of 5000K. This is the color temperature that is most widely used in graphic arts industry viewing booths.
  • D65 - The CIE standard illuminant that represents a color temperature of 6504K.
  • Daylight illuminants (CIE) - series of illuminant spectral power distribution curves based on measurements of natural daylight and recommended by the CIE in 1965. Values are defined for the wavelength region 300 to 830 nm. They are described in terms of the correlated color temperature.
  • Device-dependent - describes a color space that can be defined only by using information on the color-rendering capabilities of a specific device. For example, the RGB color space must be generated by a monitor, scanner, and printer - devices which have specific capabilities and limitations for achieving its gamut of colors.
  • Device-Independent - describes a color space that can be defined using the full gamut of human vision, as defined by a standard observer. It is independent of the color-rendering capabilities of any specific device.
  • Device Profile - device-specific color information that characterizes a device's color rendering capabilities. Monitor profiles, scanner profiles, and printer profiles are utilized in a color management system to help the devices communicate color information with each other. Profiles are created by process of profiling.
  • Dynamic Range - the range of tones a device is capable of measuring, sensing, or rendering from the lowest amount it can detect to the highest amount it can handle.

- G

  • Gamma - a number representing the power coefficient in the general formula: Y = inputgamma + constant. It relates the luminance output to the input values. A gamma of 1.0 represents a linear device. Gamma is associated with contrast because increasing the gamma increases the contrast in shadows and midtones.
  • Gamma curve - a plotted curve representing the relationship between input and output tonal values for a device. Gamma correction (described by the gamma curve) is normally used in the context of displays, and is performed in part to correct for the nonlinear light-output versus signal input characteristic inherent to CRT displays. The relationship between the light input level and the output signal level, called the optoelectronic transfer function (OETF), provides the correction curve for an image capture device. The gamma correction is usually an algorithm, lookup table, or circuit which operates separately on each color component of an image.
  • Gamut - the range of colors that can be interpreted by a color model or generated by a specific device. Color gamuts are device-specific.
  • Gamut Mapping - converting the coordinates of two or more color spaces into a common color space. Often results in tonal range compression.
  • Gray balance - the balance between primary colors required to produce neutral grays with no color bias.
  • Grayscale - a tonal scale that represents the lightest white to the blackest black, with intermediate shades of gray in between. A grayscale is used as a guide to ensure proper calibration and unbiased color balance settings.

- H

  • Hue - the attribute of color by which we distinguish red from green, blue from yellow, etc. Munsell defined five principal hues (red, yellow, green, blue and purple) and five intermediate hues (yellow red, green-yellow, blue-green, purple-blue and red-purple. These 10 hues (represented by their corresponding initials R, YR, Y, GY, G, BG, B, PB, P and RP) are equally spaced around a circle divided into 100 equal visual steps, with the zero point located at the beginning of the red sector. Adjacent colors in this circle may be mixed to obtain continuous variation from one hue to another.

- I

  • ICC profile - set of colorimetric values and transforms from one color encoding to another, e.g. from device color coordinates to profile connection space, prepared in accordance with ICC standards.
  • Illuminant - incident luminous energy specified by its spectral distribution.

- J

  • Just-Noticeable Difference (JND) - attribute JND is a measure of detectability of appearance changes, whereas the quality JND is a measure of significance or importance of stimulus differences in terms of their impact on quality. An attribute JND is a useful unit for predicting how observers would react to an images carefully matched in all respects but one, and drawing the attention of the observer to the attribute varying.

- L

  • Lightness - the attribute of visual perception in accordance with which an area appears to emit or reflect more or less light. Also refers to the perception by which white objects are distinguished from gray objects and light-from dark-colored objects. L* is a lightness metric defined in both the CIELAB and CIELUV color spaces. The lightness is calculated using the cube root of the relative luminance.
  • Linear device - a device in which input values produce equivalent output values across the entire tonal range. Most devices are non-linear and require some correction to make them linear.
  • Luminance - refers to the measurable aspect of perceived brightness. Typically expressed as Y that is proportional to absolute brightness. Unit is cd/m2 (older unit nit). The CIEXYZ color system was designed so that Y component was a measure of the brightness or luminance. An increase in luminance is usually perceived as an increase in brightness. The relationship, however, is not linear. Doubling luminance will not necessarily double perceived brightness.

- P

  • Phosphors - typically inorganic materials that emit light when irradiated by certain wavelengths of light radiation.
  • Profile - a file containing data which describes how a device interprets colors.

- R

  • Rendering intent - a way of mapping color values from one device to another. ICC defines four rendering intents (absolute colorimetric, relative colorimetric, perceptual and saturation).
  • Reflectance - the percentage of light that is reflected from an object.

- S

  • Saturation - the attribute of color perception that expresses the amount of departure from the neutral gray of the same lightness. Also referred to as chroma, vividness or purity of a color.
  • Spectral Data - The most complete description of the color of an object. Spectral data can be visually represented as a spectral curve.
  • Spectrophotometer - an instrument that measures the characteristics of light reflected from or transmitted through an object, which is interpreted as spectral data.

- T

  • Tonal range - the range of visible tones in an image.
  • Tristimulus values (CIE) - percentages of the components in a three-color additive mixture necessary to match a color; in the CIE system, they are designated as X, Y and Z. The illuminant and standard observer color-matching functions used must be designated; if they are not, the assumption is made that the values are for the 1931 observer (2� field) and illuminant C. The values obtained depend on the method of integration used, the relationship of the nature of the sample and the instrument design used to measure the reflectance or transmittance. Tristimulus values are not, therefore, absolute values characteristic of a sample, but relative values dependent on the method used to obtain them. Approximations of CIE tristimulus values may be obtained from measurements made on a tristimulus colorimeter that gives measurements generally normalized to 100. These must then be normalized to equivalent CIE values.

- W

  • White balance - the balance between R,G,B primaries required to produce neutral gray with no color bias.



This section is reserved for comments and tips from other users.

Last update : February 16, 2008