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 in a line within the span of 1 inch (2.54 cm).
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.A close-up of the dots produced by an inkjet printer at draft quality. Actual size is approximately 0.25 inch by 0.25 inch (0.403 cm2). Individual coloured droplets of ink are visible; this sample is about 150 DPI.
People often ask me how big a digital image needs to be for printing. This is a very relative question, based on variables such a print quality and viewing distance. But I’ll try and answer it first on the basic level of resolution.
First off, I want to make it clear that we are not talking about file byte size. We cannot judge an image based off the file byte size, since most image files are often compressed. But we can judge an image based on the total number of pixels that make up an image, which can be determined by the height multiplied by the width of the image in pixels. These pixel dimensions are usually measured in megapixels.
So how do we measure height and width? When you hear people talking about photo “resolution” they often use terms like DPI (Dots Per Inch) and PPI (Pixel Per Inch). DPI is a physical measurement for photographs, because dots have an actual size. You can see the size of the dots if you look carefully at photo printed out, or an image generated on a computer screen. PPI is a virtual measurement used for digital images. This is because pixels are points in space we define mathematically, adding informational characteristics to such as color and contrast. They have no actual size or physical characteristics until converted to something real, such as a dot of ink or colored light on a computer screen.
Now that we have made this clear, it’s easy to understand why most people do not talk about digital images in terms of PPI. They usually only refer to pixel dimensions. This is because you can’t mathematically assume there will be a specific number of pixels in an inch. You simply can’t. They are relative points in space! The concept of “per inch” can really only apply to a physical entity, such as a dot. Thus DPI resolution.
At 300 DPI the eye at an average small photo viewing distance stops distinguishing between dots and creates a resulting shape. Computer monitors were found to work well at a minimum of 72 DPI, Newspaper text at 125 DPI, and more recently 600 DPI has become more of a photography printing standard. But for over a decade we have thought of 300 DPI to be a visual minimum for printing a standard photo.