Dots Per Inch (DPI)
You have probably heard this term many times before especially in the context of printing your digital photos. DPI is sometimes used as a measurement of digital photo printing quality while the truth is that DPI is an indicator of the printer quality.
Dots per inch (DPI) is a measure of spatial printing or video dot density, in particular the number of individual dots that can be placed within the span of one linear inch (2.54 cm.) The DPI value tends to correlate with image resolution, but is related only indirectly.
DPI is a physical characteristic of a printer. Every printer prints dots that when put next to each other comprise a photo. Each dot has a physical size. DPI is also known as the maximum resolution that a printer is capable of. Low-end printers have lower DPI while high-end printers have higher DPI.
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 1800 DP.
if a printer supports 1200 DPI it means that the printer can print 1200 dots per inch (on both X or Y axis). When printing it is important to make sure that the DPI is higher or equal to the number of pixels per inch in the source photo. If the DPI is lower the printer will not be able to fully display the high resolution photo. When the DPI is higher the printer will use multiple dots to represent one photo pixel. DPI is not relative to the page size. DPI is a fixed number for a given printer.