What Does DPI Mean in Scanners?
DPI stands for "dots per inch," but this is just the surface of what DPI really means for getting a good scan of your photos, artwork or documents. You should know about DPI values as well as the real meaning of "interpolated" DPI.
In "dots per inch," a "dot" is a single point of color. Unlike film or a digital camera, the scan head is just one row of sensors, with each sensor capturing one point. By capturing row after row, you eventually get a complete image.
DPI and File Size
A higher DPI value usually means that more sensors along the scan head are used and that the sensors are capturing more times with every inch scanned. All of this adds up to more information, which in turn makes a larger file. If you know the DPI of where scanned image will go, try to match that value. For example, a typical LCD monitor will be around 100 DPI, so even a wallpaper image at full size (1280x1024) and in full color (48 bit) should stay under 8MB.
DPI as Two Values
Some scanners describe DPI as two numbers; for example, "1200x2400." Both numbers represent scan quality, but in different ways. The first---usually smaller---number is the number of sensors-per-inch on the scan head. The second number is the calibration of the scan-head motor; meaning how many "steps" per inch the scan-head can be moved. This means the DPI values are expressed like "X by Y." The smaller number, in this case 1200, would be the "actual" DPI for the scanner.
When something is "interpolated," it usually means the same thing as "an educated guess." For image scanning, this means that the scanner software is doing part of the work; filling-in between the dots actually captured by sensors, similar to enlarging a digital photo. The result is usually fuzzy or blurry when compared to the original.
Screen vs. Print
Before scanning, it's important to think about how the image will be used. A typical monitor can be from 72 to 100 DPI, where a typical inkjet printer prints at 300 or 600 DPI. If the image is going to be put on the Web or on a digital picture-frame, it can use a lower DPI and resolution. When a scanned image is going to be included in a project for printing, a higher value should be used.
DPI and OCR
Scanning a printed document---like a report or pages from a book---uses a technology known as OCR. This stands for "optical character recognition," meaning special software that can "read" the scanned document. The program generates text that can be copied into other programs, like a word processor or spreadsheet. OCR is more accurate with higher DPI values, and since only text is saved, the size of the scanned image is not an issue.
Some scanner software uses the term "resolution" along with "DPI" in settings. DPI would still mean the actual quality of the scan. The "resolution" determines the total number of pixels that will become an image file. With most scanner programs, both the resolution and DPI settings together will determine the size and shape of the area that will be scanned.