Digital Imaging Terminology

Archive image: A lossless high-quality master image used for reproduction and long-term storage. The image is free of compression artifacts. Requires reprocessing as image standards change. Formats: TIF, PNG, PhotoCD.
Bit Depth: 24-bit color, 8-bit color, 8-bit grayscale, 1-bit bitonal
The number of bits used to define each image pixel. The greater the bit depth, the greater the number of tones (grayscale or color) that can be represented. Digital images may be produced in black and white (bitonal, lineart), grayscale, or color. Some new color display systems offer a 32-bit color mode. The extra byte, called the alpha channel, is used for control and special effects information.
Bitonal: A bitonal, a.k.a. black & white or lineart, image consists of pixels that can represent one of two tones.
Black & White: See bitonal.
Collaborative image: An image with limited file size, < 200 KB, for efficient image transfer. Formats: GIF, PNG, JPG.
Compression: A process used to reduce image file size. Image-compression algorithms may be either information-preserving, a.k.a. lossless, or non-information-preserving, referred to as lossy.
Compression artifact: The visible distortion in a compressed image.
Dot Gain: The tendency of ink dots to increase in size when they are printed. Porous paper soaks up ink and the halftone dot swells causing the image to become darker. Dot gain can be reduced by using coated paper.
dpi (dots per inch): A measure of printer resolution. It refers to the dots of ink or toner used by a laser or inkjet printer to print text and graphics. For example, many color ink jet printers have a resolution of 720 dpi.
EXIF (Exchangeable Image File): A metadata standard that supports the storage of extended camera information within the header of an image file. Nearly all digital cameras use this format to record such information as the time and date the image was taken, exposure information (shutter speed, aperture) and other extended (and often custom) camera details. The specification uses existing file formats such as JPEG, and TIFF Rev. 6.0, with the addition of specific metadata tags. It is not supported in JPEG 2000 or PNG.
Gamma correction: A method for controlling how midtones are rendered in a scanned image.
GIF (Graphics Interchange Format): A lossless image file format that is limited to 256 colors. Suitable for images with few distinctive colors (e.g., documents and online graphics). GIF files use the ".gif" extension.
Grayscale: A grayscale image is composed of pixels represented by multiple bits of information. The most common representation uses eight bits per pixel resulting in 256 tones.
Halftoning: The process of converting a continuous tone image into an image that can be printed with only one color ink. Halftoning uses dots of varying size or density to give the impression of intermediate tones.
Highkey: An image where the important areas are light.
Highlights: The lightest image tones, one of the three tonal ranges of an image.
Historgam: A plot of the number of pixels for each possible grayscale value.
IPTC (International Press Telecommunications Council: An information standard developed by the Newspaper Association of America and the IPTC to identify transmitted text and images.
JPEG (Joint Photographic Expert Group): A lossy image file format best suited for onscreen photos. Takes advantage of limitations in the human vision system to achieve high rates of compression that can be controlled by the user. JPEG files use ".jpeg" or ".jpg" extensions.
Lineart: See bitonal.
Lowkey: An image where the important areas are dark.
Lossless/Lossy compression: lossless, lossy
Lossless compression methods are methods for which the original, uncompressed data can be recovered exactly; lossy compression sacrifices information and the original data cannot be recovered exactly. For most images lossy compression achieves substantially higher compression ratios than does lossless compression, often without sacrificing much in fidelity to the original.
lpi (lines per inch): A measure of the halftone resolution, also referred to as the line or screen frequency. Printers simulate continuous tone images by printing lines of halftone cells. To optimally reproduce an image at 100 lpi, the digital file should have a ppi resolution of 1.5 to 2 times the screen frequency (i.e., 150-200 ppi).
Midtones: Neutral gray tones, one of the three tonal ranges of an image.
OCR (optical character recognition): A process that converts a scanned text image into a machine-readable text file, e.g. a word file.
PhotoCD: A proprietary image file format developed by Eastman Kodak to store digitized photographic images on CD-ROM disks. The default file extension is “.PCD”.
Pixel: The smallest unit of visual information that can be used to build an image. Short for picture element, pixels are the little squares that can be seen by enlarging an image.
PNG (Portable Network Graphics): A lossless file format for bitmapped graphic images approved as a replacement for GIF files. PNG is patent and license-free. The default file extension is “.PNG”.
ppi (pixels per inch): A measure of digital image file resolution.
Resampling: Resampling occurs in two forms: down sampling and up sampling. Scanning often occurs at a higher resolution than is necessary and the required resolution is obtained by resampling the image. Aliasing can occur when the image data are downsized. To minimize its effects low-pass filtering can be applied to the image before it is downsized. Up sampling should be avoided because no additional image information can be created.
Resolution: A measure of image detail. Spatial resolution is measured in pixels per inch (ppi), samples per inch (spi), lines per inch (lpi), and dots per inch (dpi). Brightness resolution is measured in bits per pixel.
RGB color (red, green, blue): Refers to a system for representing the colors to be used on a computer display. Red, green, and blue can be combined in various proportions to obtain any color in the visible spectrum. Levels of red, green, and blue can each range from 0 to 100 percent of full intensity. The figure on the left shows RGB values at 100% intensity, the figure on the right shows RGB values at 50% intensity.
Service image: A good-quality image intended for short-term use. May be compressed for efficient storage and transfer. Formats: GIF, PNG, JPG, TIF.
Shadows: The darkest image tones, one of the three tonal ranges of an image.
Sharpening: See unsharp masking.
spi (samples per inch): A measure of scanner resolution. Also referred to as scanning samples per inch (sspi).
Thumbnail: A small image, e.g. 100 pixels x 150 pixels, used for content preview.
TIFF (Tagged Image File Format): A lossless image file format that has become the standard archiving format for library use. TIFF files use ".tiff" or ".tif" extensions.
True color a.k.a. 24-bit color: The specification of the color of a pixel on a display screen using a 24-bit value. Each RGB level is represented by the range of decimal numbers from 0 to 255. The total number of available colors is 256 x 256 x 256, or 16,777,216 possible colors.
TWAIN: An open standard interface that makes it possible to capture image data from a scanner while using a graphics application. a.k.a. Technology Without An Interesting Name.
Unsharp masking: An artificial transformation that can make images appear more in focus by exaggerating transitions that have been dulled by scanning.
XMP (Extensible Metadata Platform): An image metadata framework introduced by Adobe Systems.

Digital Imaging Terminology

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