256 colors |
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When a computer?s graphics system uses 8 bits of information to store each pixel in an image, the most effective way of gaining the widest range of colors is to use a Color Look Up Table (CLUT). This is called indexed color. Each pixel can be one of 256 colors (2^8) and the colors are chosen from a palette of 16 million. If the palette is chosen carefully and the image doesn?t have too wide a range of color, you can get a reasonably pleasing result. The choice of pallete, and what your operating system does to it, however can really mess up your images. Both the Mac OS and Windows have standard 256-color palettes (not that same colors of course). They are called the system palettes and were chosen to have the widest application possible. Photoshop can create custom palettes for your images but beware that the system displaying your image may not have access to the custom palette.
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Chroma |
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Chroma is the C component of the LCH color space. Chroma represents how far out from the center of the color space (radially) a color lies. The farther out the more saturated the color. The "colorfulness" of a sample judged proportional to the brightness of a white reference sample in the same medium and under the same illumination.
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Chromaticity Diagram |
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The Chromaticity Diagram is a horseshoe-shaped graph of the CIELab color space where the L component has been removed and the a and b components are then graphed in 2 dimensions. It is a very useful diagram for getting a quick feeling for not only the range of colors humans can perceive but the ranges of colors (gamuts) that different devices can (or cannot) display.
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CIE - Commission Internationale De l'Eclairage |
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Link: http://www.cie.co.at/cie/
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CLUT |
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A CLUT is a Color Look Up Table. This is used with indexed color schemes when a lower number of bits are used to store each pixel's color so a look up table is used to increase the number of colors available. See Indexed Color for a more complete description.
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CMM - Color Matching Method |
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The CMM is the "engine" that actually does the work of color management. There are many different CMM's available from vendors such as Linocolor, Kodak, Adobe (within Photoshop), and others. The CMM contains the actual executable code that uses the ICC Profile information supplied by the user to transform the colors of a document. While the debate may rage on about which CMM is best to use (and that probably differs the most depending on your workflow), one thing most color scientists would agree on is you should choose one CMM and use it throughout your workflow. At this time, in our opinion, this removes the Adobe CMM from most workflows as it is only available within Photoshop (and presumably more Adobe products as time goes on). Unless you do all your color work in one application, you should choose a CMM that is available to all applications.
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CMYK Cyan, Magenta, Yellow, Black |
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The four colors commonly used in process color printing. Black is added to CMY to enhance the density of dark areas and solve gray balance problems encountered when trying to make neutral grays with CMY alone. The letters stand for: C: Cyan M: Magenta Y: Yellow K: Key Color (which is usually black)
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CMY Cyan, Magenta, Yellow |
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The three "subtractive" primary colors used as the basis of all dye or pigment - based printing systems and color photography. A full range of colors including neutral grays and blacks can be produced with good CMY colorants (e.g. color film dyes) but typical printing inks are less than perfect.
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Color Engine |
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A software utility running inside a RIP or application that applies links to images to create a mode change, e.g. from RGB to CMYK. See CMM as an example
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Color Gamut |
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The specific range of colors seen by or reproduced by a device.
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Color Management System |
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See CMS
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Color Model |
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A mathematical way of predicting the behavior of human vision or color devices (see Color Space)
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Color Space |
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A description of the range of visible colors in standardized numerical terms. As human vision is based on three types of light sensor ("red", "green" and "blue" cones) color spaces are usually "3-dimensional".
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Colorimetric |
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Measured or expressed in tristimulus terms, such as XYZ.
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Colortron |
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A spectrophotometer from Light Source Technologies capable of measuring reflection and transmission samples as well as color monitors. (Lightsource is owned by X-Rite)
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CRD Color Rendering Dictionary |
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A table that can be downloaded to a PostScript Level II RIP to control color transformations handled by PostScript. CRD?s gan often be generated by the same software that produces ICC Profiles.
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Delta E (dE) |
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A measurement of "color error", or "color difference" based on L*a*b* coordinates. A dE of 1.0 is a just- perceptible difference. A dE of 4 to 7 is the typical limit of acceptable error in commercial reproduction, but the limit varies in different color regions and applications.
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Density |
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A measure of darkness or amount of color used widely in photography and printing. The logarithm to base 10 of the reciprocal of fractional transmittance or reflection.
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Device-Dependent Color |
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Measuring and "profiling" input and output devices in terms specific to each device. A scanner's RGB values have no simple relationship to a printer's CMYK values for the same color because both values are device-dependent.
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Device-Independent Color |
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(1) The concept of automatically good color reproduction of an image from any source on any destination device. (2) Measuring and "profiling" input and output devices in terms that are independent of their native color spaces. (3) An intermediate color space to which scanners and printers are calibrated and through which images are translated from one device to another.
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Digital Swatchbook |
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A spectrophotometer from X-Rite Corporation. It is a hand-held device which is good for taking readings of anything you may have at hand. If you are reading many patches, as when creating a printer profile, you may want a more automated solution like the DTP41 strip-reading spectrophotometer or the GretagMacbeth Spectrolino / Spectroscan combination which combines a spectrophotometer with an automated x-y reading table.
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Gamma |
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A non-standardized measure of contrast for color scanners, video monitors and photographic materials. A gamma of 1.0 usually means normal or no change in contrast, except in Macintosh monitors where gamma 1.8 is standard. Higher gamma numbers usually mean more contrast with darker mid-tones. Lower gamma numbers usually mean lower contrast and lighter mid-tones. Gamma pivots on (does not change) the white and black point of an image. Gamma is typically applied by raising fractional L*, fractional RGB or fractional XYZ to the power of gamma.
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Gamut Compression |
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A form of gamut mapping in which the range of color values produced by an input device (e.g. a color scanner) is reduced to fit within the smaller available gamut of the output device (e.g. a CMYK press). Gamut compression can be crucial to good color reproduction but colors that have been gamut compressed will seldom match the original.
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Gamut Mapping |
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The redistribution of color values from an input device (e.g. color scanner or printer) to fit the smaller or larger gamut of the output device. If the input gamut is larger than the output gamut, gamut mapping is the same as Gamut Compression. If the input gamut is smaller than the output gamut, colors and contrast can be intensified through Gamut Expansion, or colors can be mapped to their exact equivalents, e.g. for digital proofing.
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GCR Gray Component Replacement |
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The replacement of some or all of the CMY inks in a CMYK pixel with enough black ink to restore the pixel to the same visual appearance. (see UCA, UCR).
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Gradation Control |
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A means of adjusting tonal contrast of specific tonal regions, either equally for all colorants or imaging channels, or separately for independent channels.
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Gradation |
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The relationship of reproduced lightness values to original lightness values in an imaging process, usually expressed as a "tone curve".
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Hi-Fi Color |
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Any process that increases the color gamut of an output imaging device (printer). Usually refers to adding extra inks and plates to the traditional CMYK set to improve the color gamut of offset lithography. The three main Hi-Fi methods are; the Küppers approach (CMYK+RGB), Pantone(r) Hexachrome? (CMYK + Orange & Green) and MaxCMY (CMYK + extra CMY).
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ICC International Color Consortium |
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An international group that has proposed open, cross-platform standards for the description and handling of device-independent color.
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IT8.7/1 |
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An ISO standard transmissive color target available from all major color film manufacturers. CIE data provided with each copy of the target enable scanning devices to be characterized or "profiled" in CIE terms.
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IT8.7/2 |
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A reflective version of the IT8.7/1 provided with its own CIE data.
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Kelvin |
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An absolute temperature scale used in scientific circles. The "color temperature" of a light source, monitor or image area, defined in "degrees Kelvin" (°K), is the temperature a perfectly black radiating object would be if it glowed that shade of white. Average daylight is standardized at 6500°K (Europe) and 5000°K (North America). Higher temperatures are bluer, lower temperatures are redder.
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Lightness |
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The brightness of an area expressed relative to the whitest area in the viewer's field of vision or the brightest white the device is capable of seeing or producing.
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Linearization |
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The process of measuring and correcting for a device's inability to see or reproduce a straight line of tones from black to white. Most commonly used to ensure an image setter reproduces the same halftone dot values predicted by the imaging software. Linearization
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Link |
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Combining an input profile with an output profile produces a mathematical look-up-table (LUT) that translates colors from an input device into the best available matching colors on the output device. Changes or edits can be combined in the link to alter or improve the image.
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Metamerism |
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The phenomenon by which two materials that match under one circumstance appear different to different viewers or under different lighting. Metameric mismatch occurs when tristimulus values are the same but spectral characteristics are not.
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Mode Change |
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Transformation of an image from one mode to another, e.g. RGB to CMYK.
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Output Target |
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A series of color patches sampling the full range of output device colorant combinations. When printed and measured the resulting CIE values are tabulated with the colorant combinations that produced them to create the output profile.
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Patch |
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A single color sample in an input or output target.
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Plug-ins |
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A third-party filter or sub-program that works within a host application such as Adobe Photoshop(r).
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Postscript |
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The most common page description language, invented by Adobe and used almost universally in desktop publishing. Postscript code tells the RIP how to construct page elements such as text or linework and indicates the scale, rotation and cropping of continuous tone images.
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PPD Postscript Printer Description |
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A file located in the System Folder and accessible to any printing application, describing the features and capabilities of a printer.
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Printing Technology |
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A type of printing method, e.g. offset lithography, ink jet, dye-sublimation, etc.
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Reference |
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A standard against which others are measured.
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Relative Colorimetry |
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Colorimetric measurements expressed relative to the white point of the device or image, rather than as "absolute" CIE values. In relative colorimetry, the white point would be 100 X, 100 Y, 100 Z.
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Relative Densitometry |
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Densities expressed relative to the white point of the image or device. Relative reflection densities have the white paper density subtracted. Relative transmission densities have the clear film base subtracted.
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RIP Raster Image Processor |
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A software or hardware mechanism that converts page description data such as Postscript code into lines (rasters) of micro-image units (pixels or microdots) that are arranged for high-speed sequential output to the imagesetter.
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SCID Images Standard Color Image Data |
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Eight standard images representing difficult reproduction challenges.
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Serial Port |
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A connection for computer peripherals. The Macintosh has two serial ports, one marked with a modem (phone) symbol and one with a printer symbol. Serial devices like colorimeters can usually be connected to either port.
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Transformation |
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The process of transforming an image from one color space or color mode to another, usually through a Link or Color Table.
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