DPI Measurement in Printing
DPI is not really used to describe the resolution number of dots per inch in a digital print and the printing resolution of a hard copy print dot gain; the increase in the size of the halftone dots during printing. This is caused by the spreading of ink on the surface of the media.
Up to a point, printers with higher DPI produce clearer and more detailed output. A printer does not necessarily have a single DPI measurement; it is dependent on print mode, which is usually influenced by driver settings. The range of DPI supported by a printer is most dependent on the print head technology it uses. 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 300-600 DPI. A laser printer applies toner through a controlled electrostatic charge, and may be in the range of 600 to 1,800 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 colours for each dot typically available on a printer. At each dot position, the simplest type of colour printer can either print no dot, or print a dot consisting of a fixed volume of ink in each of four colour channels (typically CMYK with cyan, magenta, yellow and black ink) or 24 = 16 colours on laser, wax and most inkjet printers.
Higher-end inkjet printers can offer 5, 6 or 7 ink colours giving 32, 64 or 128 possible tones per dot location. Contrast this to a standard sRGB monitor where each pixel produces 256 intensities of light in each of three channels (RGB).
While some colour printers can produce variable drop volumes at each dot position, and may use additional ink-colour channels, the number of colours is still typically less than on a monitor. Most printers must therefore produce additional colours through a halftone or dithering process. The exception to this rule is a dye-sublimation printer that utilizes a printing method more akin to pixels per inch.
The printing process could require a region of four to six dots (measured across each side) in order to faithfully reproduce the colour 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.
Read more about this topic: Dots Per Inch
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