Dots per inch

Dots per inch (DPI) is a measure of printing resolution, in particular the number of individual dots of ink a printer or toner can produce within a linear one-inch space. The term is also commonly used to describe a variety of unrelated measurements, including pixels per inch on a computer display, or samples per inch taken by a scanner; usage of the DPI measurement in these cases is considered by some to be inaccurate and misleading, though the intended meaning is usually clear based on context.

Generally, printers with higher DPI produce clearer and more detailed output. The DPI measurement of a printer is dependent upon several factors, including the method by which ink is applied, the quality of the printer components, and the quality of the ink and paper used. A dot matrix printer, for example, applies ink via tiny rods striking an ink ribbon, and has a relatively low resolution, typically in the range of 60 to 90 DPI. An ink jet printer sprays ink through tiny nozzles, and is typically capable of 300 DPI. A laser printer applies toner through a controlled electrostatic charge, and may be in the range of 600 to 1200 DPI.

The DPI measurement of a printer often needs to be considerably higher than the PPI measurement of a video display in order to produce similar-quality output. This is due to the limited range of colors typically available on a printer: most color printers use only four colors of ink, while a video monitor can often produce several million colors. Each dot on a printer can be one of only four colors, while each pixel on a video monitor can be one of several million colors; printers must produce additional colors through a halftone or dithering process.

The printing process could require a region from four to six dots on each side in order to faithfully reproduce the color contained in a single pixel. An image that is 100 pixels wide may need to be 400 to 600 dots in width in the printed output; if a 100x100-pixel image is to be printed inside a one-inch square, the printer must be capable of 400 to 600 dots per inch in order to accurately reproduce the image.

There are some ongoing efforts to abandon the dpi in favor of the dot size given in micrometre (µm). This is however hindered by leading companies located in the USA, the only remaining country to not use the metric system exclusively. A resolution of 72 dpi for example equals a dot size of about 352.78 µm, 96 dpi => 264.58 µm, 300 dpi => 84.67 µm, 4000 dpi => 6.35 µm.