Dots per inch
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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.
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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 inkjet 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 pixels per inch (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 100×100-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 micrometres (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 350 µm, 96 dpi → 265 µm, 160 dpi → 160 µm, 300 dpi → 85 µm, 4000 dpi → 6.4 µm. Going the other way, 1 µm → 25400 dpi, 30 µm → 8500 dpi, 200 µm → 127 dpi. Note that 25400 = 1 dpi·µm, so dividing 25400 by a measurement in one of these units gives the measurement in the other unit.
Some have also proposed using dpcm.
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Misuses of DPI measurement
Owing in part to its conceptual similarity with other measurements of graphical resolution, the DPI measurement is frequently misused. For instance, it is common for an image scanner's sampling resolution to be specified in terms of DPI, though a more accurate measurement would be samples per inch. The number of pixels per inch in a computer display is sometimes specified in this way as well. 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.
Another misuse results from an incomplete understanding of what the DPI measurement means. In order for any "per inch" measurement to be meaningful, some number of inches must be specified. A digital image captured by a scanner or digital camera has no inherent "DPI" resolution until it comes time to print the image (and even then, a more accurate measurement of image resolution is pixels per inch); for example, a 1000×1000-pixel image could be printed at 4×4 inches and 250 pixels per inch, or at 10×10 inches and 100 pixels per inch. Digital images contain some number of pixels; the size at which they are printed is relatively arbitrary. When someone asks for a "300 DPI image", they may be expecting an image with 300 pixels per inch of printed output; unless the size of the printed output is known beforehand, the measurement is meaningless. A more complete specification would include the desired print size in addition to the number of desired pixels per inch. A yet more complete specification would also include the DPI capability of the printer that will be used to print the image; if the printer is only capable of faithfully reproducing 100 pixels per inch, there is no reason to use a higher-resolution image.