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IMAGE RESOLUTION

An often confusing aspect of desktop publishing and digital imaging is resolution and how it’s measured—SPI (samples per inch), PPI (pixels per inch), DPI (dots per inch), and LPI (lines per inch).

The terms are often interchanged. The most well-know, DPI, is often used in place of SPI and PPI although they really aren’t the same. And that creates a lot of confusion.

Short definitions for each term

SPI (samples per inch)

This is scanner resolution. In scanning an image a scanner takes a sampling of portions of the image. The more samples per inch it can take, the closer the scan is to the original image. The higher the resolution, the higher the SPI.

PPI (pixels per inch)

This is the number of pixels displayed. Digital images are composed of samples that your monitor/screen displays in pixels. PPI is the display device’s resolution not the image resolution.

DPI (dots per inch)

This is a measure of the resolution of a printing device. It refers to the dots of ink or toner an imagesetter, laser printer, or other printing device uses to print text and graphics. As a rule, the more dots, the better and sharper the image. DPI is printer resolution.

LPI (lines per inch)

This refers to the way printing devices reproduce images. They simulate continuous tone images by printing lines of halftone spots.

The number of lines per inch is the LPI. This is also called line frequency. You can think of LPI as the halftone resolution.

How SPI, PPI, DPI, and LPI relate

Digital images are composed of samples—information about how an image is to be displayed. In scanning an image, the SPI is its scanning resolution (the amount and type of information stored for that image). Through resizing and resampling the stored information can be changed so that the image resolution is different from the original scanning resolution. In either case, both are expressed as SPI.

Many scanner manufacturers use DPI instead of SPI in describing their scanners resolution capabilities. However, there are no dots in an image until it is printed.

SPI is not as common as DPI or even PPI. In general, the terms are used as if they were the same.

SPI and Scanning

Before you begin scanning a photographic image, you need to know the final output method. This is important so that you scan at an appropriate resolution (SPI). Too low a resolution and printed image quality will suffer. Too high a resolution results in wasted information and unnecessarily large files.

SPI and Digital Images

Images created through digital photography, from the Web, or from CD collections need the right amount of resolution for the final output method.

When manipulating an image in your photo-editing software, it’s good to start with an image at a higher resolution (SPI) than what you will eventually need. It’s better to have a file with more image data than needed. You’ll have more margin for error and experimentation. Once image editing is complete, downsample the image to the required SPI.

SPI and Printing

You need to know the final output method when printing photographic images. This way you can insure your digital images have enough resolution (SPI) for printing or for screen display. The LPI formulas below can help you determine if a digital image has enough resolution for printing.

SPI and the Web

Monitors typically display images at 72 or 96 PPI. A lot less sampling (SPI) is needed for on-screen display. Any information in images over 96 SPI is wasted, doesn’t improve the display, and only increases file size and download time. Scan images at a higher SPI so you have "pixels to play with". But you’ll need to downsample the image before using it on the Web.

SPI and PPI are used interchangeably. DPI is the one that’s frequently used in place of one or both terms. Even if you call it DPI, each dot or "unit of measure" will behave differently depending on whether it is a scanner (or scanned image), a monitor (or on-screen image), or a printer (or printed image).

LPI Charts and Formulas

LPI is important when it comes to the way printers reproduce photographic images. LPI is dependent on the output device and the type of paper.

The charts in the sidebar will help you get an idea of the typical LPI needed based on the printing method and paper type.

How to determine the photo pixel size you need:

You publish a newsletter and want to include a photo to illustrate an article. You determine the size of the photo, as it will be printed, should be 5" wide by 3" tall.

Your publication is offset printed on uncoated paper at an LPI of 133. What resolution must your image file be to be properly reproduced?

Using the formula, LPI x 2:
133 x 2 = 266 SPI (or PPI).

For the image size on the printed page, you’ll need an image file that’s at least 1,330 pixels wide (266 PPI x 5") and 798 pixels tall (266 PPI x 3"). That means a photo file a little larger than 2 megapixels.

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