DPI in digital imaging and digital printing is a base measure of the resolution of an image to be printed or the scanning ability of a machine. The higher the DPI, the greater the dot density, and it usually equates to more detail, smoother gradation, and fewer visible dot patterns. DPI is, however, commonly confused with PPI (pixels per inch), which is used to measure the digital image resolution on screens. Good DPI testing will ensure that the hardware equipment (printers, scanners, and monitors) is of quality, the prints are printed as intended, and that the interplay between the resolution of the digital file and physical output is properly controlled to produce a professional print.
Fundamental Principles
DPI represents a resolution output. The essence of this is that printed images consist of a grid of small dots that are microscopic in size. The size of an image of 1200 x 1800 pixels printed with 300 PPI will result in a 4 x 6-inch print. The DPI of the printer should be at least as great as the PPI of the file itself to capture all that data. True DPI testing is a measure of the accuracy of dot placement and ink droplet consistency, as well as the absence of banding or streaking, in addition to the maximum claimed number of dots.
Industry Standards & Calibration
Calibration is a process by which spectrophotometers and densitometers are used to establish ICC profiles, which make the printer output match the desired standard (e.g., SWOP print in North America, JOGRA print in Europe). It is necessary to have regular calibration, as the performance of the DPI can vary with the wear of the printheads or ambient conditions.
Advanced Applications
Microprinting and Security Printing: Requires a completely high DPI (e.g., 2000 dpi) to print very small, difficult to replicate text or designs onto currency, tests, and IDs. Fine Art Reproduction (Giclée): Forms the reproductions of original works of art that demand using excessive-DPI printers with pigment inks and managed colour control to breed authentic artwork correctly. Textile printing DPI requirements vary with the weave of the material; direct-to-garment printers may be decreasing DPI because of ink flow.
Future Trends
Increased Accuracy in Print Heads: Developments in MEMS technology allow for increased nozzles and more accurate and smaller-sized droplets. On-Printhead ensures in-built sensors for real-time dot inspection and automatic correction of print runs.
Nanographic and Digital Offset Printing: Emerging technologies that confuse high-LPI offset printing technologies with digital DPI printing technologies. The integration of DPI into a multicurrency, automated system of quality assurance of print production.
