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The Basics: DPI (Dots Per Inch)

DPI (Dots Per Inch) in digital imaging means how many pixels per square inch make up the image. For example, if I scan a 1 inch by 1 inch piece of paper in 300 DPI, then there are 300 pixels in the image that I scanned. If I looked at the digital image and realized that it did not show enough detail of the image, then I would scan it at 600 DPI. Now, the digital image has 600 pixels that represent the image. You can keep going up and up, but there reaches a point where adding more pixels does not gain you any more detail. Why? Because at some point you start picking up detail about the paper (or medium) that the items is on. You start to see the fibers in a piece of paper, or the dimples on it’s surface. Once you start picking up that level of detail, you are actually creating more “noise” in the image than is necessary and you need to back off on the DPI.

Most printed text, for example, is printed in 80-90 DPI on a page. However, this does not mean you can scan the image at 80-90 DPI, because your dots and the dots on the page might not match up. You will want to raise the DPI of the scan 2 to 3 times (say 200-300 DPI) to get a decent image of the printed text.

For micro-text (like on Microfiche, microfilm, or micro-opaque cards), this gets much more complicated. For micro-opaque cards, for example, you may need to scan them at 1200 DPI to get a clear image. This is because the text is so small.
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