Digitizing Images: Terminology

Pixel: A pixel is a single dot on a computer monitor. Depending on the bit depth of a computer monitor, each pixel can be displayed as anywhere from two to millions of colors. Each pixel is assigned a tonal value (black, white, shades of gray or color), which is represented in binary code (zeros and ones).

Resolution: A measure of the quality of a digital image; as the resolution increases, the quality of the image increases. Resolution is measured in terms of dots per inch (dpi) or pixels per inch (ppi). Monitors measure in ppi; flatbed scanners, drum scanners, and printers measure in dpi.

Screen Resolution: Resolution measured in pixels per inch (PPI). It helps to think of the image as a grid. As the resolution increases, the size of the grid cells get smaller, in effect increasing the number of cells (pixels) per inch.

Bit Depth: Bit depth concerns the number of bits used to convey tonality for each pixel; that is, bitonal (black and white), grayscale, or color. In general, the more bits per pixel, the larger the file size.

Tonality: Pixel depth or bit depth.

Halftone Frequency (or) Lines Per Inch (LPI): The number of lines per inch stored by the digital file. Used for printing screens.

Bitonal Image: An image consisting only of a foreground colour and a background colour.

Monochrome: Literally "one color". Usually used for a black and white (or sometimes green or orange) monitor as distinct from a color monitor.

Line art: Perfect for use either in print or on the web, these images can be emailed to you on whatever platform and graphics suite is currently used.

DPI: Number of pixels per inch stored by the digital file. Used for: printing screens and laser printers.

PPI: Number of pixels per inch stored by the digital file. Used for: monitor display resolution.

SPI: Number of samples per inch stored by the digital file. Used for: scanners; optical resolution vs. interpolated resolution.

LPI: Number of lines per inch stored by the digital file. Used for: printing screens.

Archival or Master image: An uncompressed, unedited rendition of data which serves as a long term storage form. Archival data storage is high quality and requires a large file size. Also called preservation format.

Presentation or Access image: A compressed, easily accessible rendition of data which serves as a general access working form primarily for the web. Presentation data storage is of medium quality and constitutes a reasonable file size for fast download time. Also called access format and display format.

Thumbnail image: A reduced-size version of an image, used to present a faster loading, albeit lower quality, alternative to larger image formats such as a GIF or a JPEG. This format is not recommended for archival purposes, but can be useful when a large number of images must be displayed on the same webpage.




The content of this page was developed following the recommendations of the E-MELD working groups and the Electronic Text Center.

User Contributed Notes
Terminology for Digitizing Images
+ Add a comment
  + View comments

Back to top Credits | Glossary | Help | Navigation | Site Map | Site Search