Know the difference between DPI, PPI, and LPI
Simply put, DPI stands for dots per inch, PPI stands for pixels per inch, and LPI stands for lines per inch. Although all three abbreviations are used in a general sense to describe properties regarding digital imaging, each term has a very distinct and very different definition. In addition, because DPI and LPI are also commercial printing terms that have been applied somewhat differently to digital printing technology, quite a bit of misunderstanding has resulted. Let’s take a very brief step back in time to sort out all the confusion.
Dots per inch
In the late 1800’s, printers perfected the halftone process. This enabled them to reproduce photographs and drawings in publications such as books and newspapers. To do this, they photocopied originals by placing a piece of glass containing many evenly spaced etched lines in front of a copy negative. The result was a halftone negative comprised of hundreds of evenly spaced dots of varying sizes. The more etched lines per inch the glass contained, the more dots per inch the halftone contained—and the better the image quality of the resulting image.
So the terminology remained until the digital era, when digital printer manufacturers, using the same varying dot size principle to reproduce images, applied the same terminology. Today, the term dots per inch, or DPI, refers to how many dots of printer toner or ink a printer prints per inch. The greater the number is, the greater the quality is of the reproduced image. But while DPI refers effectively to the same thing for both commercial and digital printing, LPI doesn’t.
Lines per inch
Because LPI originally referred to the etched ruled glass screen used to produce halftone negatives, an LPI of 100 always produced a halftone negative of 100 DPI. In turn, this produced a printed image containing 100 evenly spaced dots per every linear inch of image. Said another way, one line produced one printed dot.
Today, though commercial printers generally use image setters rather than screens to produce halftone negatives, they still use the term LPI to refer to a printed image. But, because of digital technology, each line may not necessarily contain only one dot. Instead, it may contain several dots. For example, an image of 300 DPI may be commercially printed at 100 LPI. This means that that there are three dots used per line rather than one. DPI and LPI are terms that refer to a printed image. But before a printed image exists, it’s an electronic image file—which PPI is used to describe.
Pixels per inch
Digital images—whether created by a digital camera, a digital scanner, or a software application—are composed of hundreds of picture elements, commonly referred to as pixels. Image pixels, like halftone image dots are expressed in terms of units per inch meaning pixels per inch, or PPI. Like DPI, the higher the number is, the greater the image quality is. Equipped with this knowledge, you’ll better understand such things as digital camera specifications and digital printing options.
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