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Glossary of Terms


Aliasing - Visibly jagged steps along angled lines or object edges, due to sharp tonal contrasts between pixels.

Batch Scanning - Sequential scanning of multiple originals using previously defined, unique settings for each.

Bilevel - A type of image containing only black and white pixels.

Bit Depth - The number of bits used to represent each pixel in an image, determining its color or tonal range.

Bitmap - A digitized, or scanned image. The image is mapped into a grid of pixels. The color of each pixel is defined by a specific number of bits.

Black Point - A movable reference point that defines the darkest area in an image, causing all other areas to be adjusted accordingly.

CCD - Charge-coupled device. An integrated, micro-electronic light-sensing device built into some image-capturing devices.

Clipping - The conversion of all tones lighter than a specified grey level to white, or darker than a specified grey level to black, causing loss of detail. This also applies to individual channels in a color image.

CMS - Color management system. This ensures color uniformity across input and output devices so that final printed results match originals. The characteristics or profiles of devices are normally established by reference to standard IT8 color targets.

Colorimeter - A light sensitive device for measuring colors by filtering their red, green, and blue components, as in the human eye.

Contone (CT) - An abbreviation for continuous tone. A color or greyscale image format capable of illustrating continuously varying tonal ranges, as opposed to line art.

Densitometer - A measuring instrument that registers the density of transparent or reflective materials. Colors are read as tonal information.

Density - The degree of opacity of a light absorbing filter, pigment, or exposed photographic emulsion.

Descreening - Removal of halftone dot patterns during or after scanning printed matter by defocusing the image.

Dichroic Mirror - A special type of interference filter, which reflects a specific part of the color spectrum. Used in scanners to split a beam of light into RGB components.

Dmax/Dmin - The point of maximum (or minimum) density in an image.

Down-sampling - The reduction in resolution of an image.

DPI - Dots per inch. A measurement of output device resolution.

Dye Sublimation - A printing process using small heating elements to evaporate pigments from a carrier film, depositing these smoothly onto a substrate.

Gamma Correction - The correction of tonal ranges in an image, normally by the adjustment of tone curves.

Gamut - The limited range of colors provided by a specific input device, output device, or pigment set.

Grey Balance - The balance between CMY colors required to produce neutral greys.

Greyscale - A continuous tone image comprising of black, white, and grey data.

Halftone - A simulation of continuous tones by the use of black or overlapping process color dots of varying size or position.

Halo - A light line around object edges in an image, produced by the unsharp masking (USM) technique.

Highlight - The lightest tones in an image. A specular highlight is a bright, reflected light source.

Histogram - A chart displaying the tonal ranges present in an image as a series of vertical bars.

Imagesetter - A device used to record digital data onto monochrome film or offset litho printing plates by means of a single or multiple light beams.

Interpolation - In the image manipulation context, this is the increase of image resolution by the addition of new pixels throughout the image, the colors of which are based on neighboring pixels.

IT8 - Industry standard color reference target used to calibrate input and output devices.

Lossy - Image compression that functions by removing minor tonal and/or color variations, causing a slight loss of detail.

LPI - Lines per inch. Units of measurement for screen ruling.

Midtone - The middle range of tones in an image.

Monochrome - Single colored. An image displaying only black and white or greyscale information. Greyscale information displayed in one color is also monochrome.

Mottling - A texture similar to orange peel, sometimes caused by sharpening. It is particularly visible in flat areas such as sky or skin.

Noise - In the scanning context, this refers to random, incorrectly read pixel values, normally due to electrical interference or device instability.

Optical Resolution - In the scanning context, this refers to the number of truly separate readings taken within a given distance (such as dots per inch, or dpi).

Pixel - Picture element. Digital images are composed of touching pixels, each having a specific color. The eye merges differently colored pixels into continuous tones.

Posterization - The conversion of continuous tone data into a series of visible tonal steps or bands.

PPI - Pixels per inch. Units of measurement for scanned images.

Quarter Tones - Tones between shadow and midtones are known as 3/4 tones and those between highlight and midtones are known as 1/4 tones.

Raster - A synonym for grid. Sometimes used to refer to the grid of addressable positions in an output device.

Resampling - An increase or reduction in the number of pixels in an image, required to change its resolution without altering its size.

Sampling - The process of converting analog data into digital data by taking a series of samples or readings at equal time intervals.

Saturation - The extent to which one or two of the three RGB primaries predominate in a color. As quantities of RGB equalize, color becomes desaturated towards grey or white.

Speckling - Isolated light pixels in predominantly dark image areas, sometimes caused by incorrect readings or noise in the scanning device.

Specular Highlight - A bright reflection from a light source having little or no detail.

Spectrophotometer - An extremely accurate color measurement device using a diffraction grating to split light into its component wavelengths, which are then measured by numerous light sensors.

Tone Curves - Also known as gamma curves. These are used to smoothly adjust the overall tonal range of an image, or the individual tonal ranges of each color channel.

White Point - A movable reference point that defines the lightest area in an image, causing all other areas to be adjusted accordingly.

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