A halftone, or halftone image, is an image comprised of discrete dots rather than continuous tones. When viewed from a distance, the dots blur together, creating the illusion of continuous lines and shapes. By halftoning an image (converting it from a bitmap to a halftone), it can be printed using less ink. Therefore, many newspapers and magazines use halftoning to print pages more efficiently.
Originally, halftoning was performed mechanically by printers that printed images through a screen with a grid of holes. During the printing process, ink passed through the holes in the screen, creating dots on the paper. For monochrome images, only one pass was needed to create an image. For multicolor images, several passes or "screens" were required.
Today's printers are more advanced and typically do not contain physical screens. Instead, the halftone images are generated by a computer and the resulting image is printed onto the paper. By using a process called dithering, modern printers can randomize the dot patterns, creating a more natural appearance. This produces realistic images using far less ink than fully saturated ones.
Like a standard bitmap, the quality of a halftone image depends largely on the its resolution. A halftone with a high resolution (measured in LPI), will have greater detail than a halftone with a low resolution. While the goal of halftoning is typically to create a realistic image, sometimes low resolutions are used for an artistic effect.
Updated: September 2, 2014