Glossary of terms

Digital images created from another digital image through some kind of automated process, usually involving a loss of information. Techniques used to create derived images include sampling to a lower resolution, using lossy compression techniques, or altering an image using image processing techniques.

A document that has been scanned or digitized and converted into a computer saved file and is made up of a set of picture elements (pixels). Digital files for the Portal are saved as TIFF file types.

An electronic photograph, made up of a set of picture elements ("pixels"). Each pixel is assigned a tonal value (black, white, a shade of gray, or color) and is represented digitally in binary code (zeros and ones). The term "image" does not imply solely visual materials as source material; rather, a digital image is simply a representation of whatever is being captured, whether it be manuscripts, text, photographs, maps, drawings, blueprints, halftones, musical scores, 3-D objects, etc.

The process of converting analog information into digital format.

Any object that is not yet digital. For example, a photograph, a book, a newspaper, a yearbook, a page from a diary, an entire diary, etc. that has not yet been scanned

(Also see DPI) A measure of resolution used for printed text or images and monitor display. 

(Also see Dots Per Inch)

A measure of resolution used for printed text or images and monitor display.

(Also See Bit Depth)

The number of colors or shades of gray that can be represented by a pixel. The smallest unit of data stored in a computer is called a bit. Dynamic range is a measurement of the number of bits used to represent each pixel in a digital image. 1 bit or bitonal means that a pixel can either be black or white. Bitonal imaging is good for black and Montana History Portal white images, such as line drawings and text. However, scanning in grayscale rather than bitonal may produce a better looking image. 8 bit color or 8 bit grayscale means that each pixel can be one of 256 shades of color or one of 256 shades of gray. 24 bit color means that each pixel can be one of 16.8 million colors. When moving to 48 bit color the available colors in the display will number 2800 times one trillion. This is an enormous number of colors. Plus a higher bit resolution can display more shades of gray. With 30 bit color depth, four times more gray can be represented in the display. Eight times more gray, or even higher, can be represented by a 36 or 48 color bit-depth display.