(Adapted from the Adobe FAQ Community Forum)

In graphics terminology, image resolution is generally stated in one of three ways: as dots per inch (dpi) for imaging device output, pixels per inch (ppi) for actual image resolution, or lines per inch (lpi) for commercial (generally offset) printing with halftone screens. Although each of the terms refers to the smallest element of an image, they don’t mean the same thing.

And if that sounds confusing, you are not alone. From printer and scanner manufacturers to designers and prepress departments, the terms are sometimes used interchangeably, or without a clear understanding how or when they should be used. Nonetheless, it is important to understand and use the terms correctly to prevent confusion in communication, or worse, expensive, time-consuming mistakes.

PPI

Let’s look at the term you will most often be concerned with, ppi, or your Image’s true resolution from pixel-based applications, such as Photoshop. This is specified in pixels per inch (ppi). Magnify an image several hundred percent on your monitor, and each square in the checkerboard grid you see is a single pixel (from the term, “picture element”). Image detail is formed from the color values of the individual pixels and how they are arranged in relationship to other pixels. The more original pixels, the more detail. And you generally get to choose from a palette of nearly 16.8 million colors.

There is no additional or hidden detail in any one pixel you can conjure up from a low-resolution image. Resampling the pixels upward in a low-resolution image simply gives you a larger low-resolution image. This is a common problem when attempting to use a standard low-resolution (72 ppi) Web-based image in high-resolution (300 ppi) print reproduction at anywhere near its original dimensions. However, if you reduce the dimensions of a Web image to about 25% of its original size without resampling, you will now have an image at about 288 ppi resolution which will appear sharp in commercial offset printing.

Devices that have output specified in ppi include flatbed and film scanners, and computer monitors.

LPI

For commercial offset printing, images have to converted electronically or by traditional litho film and halftone screens to a pattern of dots, with a resolution measured as so many lines per inch. This step forms a grid of equidistant dots of varying sizes on the printing plates, each dot attracting ink and simulating a specific percentage of solid color. The resulting printed images are called halftones. Before computer technology, halftones were created using finely scored glass plate screens, so you will also see the term “halftone screen” or “line screen”. Higher quality printing uses finer screen values. Newsprint may use 85-110 lpi; web offset 133 lpi (printed from rolls of paper, like consumer magazines); standard sheetfed offset 150 lpi (printed from “lifts” of trimmed sheets of paper); fine quality (art book, etc.) 175-200 lpi. Finer dot patterns can be used under ideal conditions. Otherwise they will tend to “plug” (block up) or skip. Silkscreening and rubberplate printing (for printing directly on uncoated corrugated boxes) may use very coarse values with very visible dot patterns. In addition, there are stochastic screens that use a random pattern to create a less visible dot with offset pinting.

For offset printing, the standard image resolution is 266-300 ppi at 100% image size, for printing at 133-150 lpi; providing a ppi/lpi ratio of 2:1. This allows for a bit of “wiggle room” if you have to make a small or last-minute increase in image size without scanning again. If you remove the safety margin, you can use a ppi/lpi ratio of as little as 1.55/1 without image degradation (according to Agfa, a leading manufacturer of imagesetting equipment). You can certainly use images that are sampled at somewhat higher rates. But there is no benefit. It won’t give you more detail in your printed image due to the limiting resolution of the halftone screen.

Vector-based images, such as those created in Illustrator, or most editable fonts, are based upon mathematical curves, not pixels. As such, they are infinitely scalable, and will be absolutely sharp at almost any size. But when they are finally output, these vectors are still first converted to to the finite elements of the desktop printer or commercial press, just as pixel-based images are. Exceptions include “solid” ink color (unscreened, maximum ink density) in offset printing, such as black type or linework, which have no dot pattern.

DPI 

This is the usual specification for resolution of output devices, such as desktop printers, film recorders, RIPs (imagesetters), and (again) computer monitors. For computer monitors and film recorders, there is a 1:1 ratio between the ppi of the digital image file and the dpi of the output device. In most other cases, especially with today’s typical desktop inkjet or laser printers, there is a vast difference between dpi and ppi. At an inkjet printer’s, say, 2880 dpi resolution, many micro dots of ink are used to build one image pixel. Let’s do the math: say that we have an image that is being reproduced at 300 ppi on our 2880 x 2800 dpi inkjet. Dividing, we see that 9.6 dots of ink horizontally and vertically (about 92 dots altogether) are needed to “build” one pixel. Even so, you often cannot tell the difference in print quality betweeen a 2880 dpi print and one made at 1440 dpi.

But, rescanning or recreating your pixel-based reflective art image at 2880 ppi is not a practical option for enhancing detail. First, you will end up with an unwieldy file, over 90 times larger than necessary. Secondly, the high native dpi of your inkjet is to give you smoother, more photo-like tonal transitions and perhaps crisper type and fine rules with your 300 ppi images. It won’t give you any usable increase in image detail for higher resolution images.

Note that transparencies and slides are generally scanned at very high resolution. But that is only because they are usually enlarged considerably. For example, a 35 mm slide (24 mm x 36 mm image area) scanned at, say, 4000 ppi will yield an image with an effective resolution of 300 ppi when it is enlarged to 12.6″ x 18.9″. If you have a good scanner of this capability and a 13″ x 19″ inkjet printer, you can print a high-quality images filling the sheet from slides.

Service bureau or printshop RIPs that produce film, repros (remember them?) and plates use proprietary, high-density dot technology to translate pixels into halftones, and to create crisp rules and type from the original files, often running at 2540 dpi or greater, so that the dots themselves are invisible to the naked eye.