Playing Fast and Loose with Scanner Specs

By M. David Stone  |  Posted 2008-02-08

Playing Fast and Loose with Scanner Specs

Specsmanship is a time-honored game that marketing departments like to play. But there are times when they take the game so far that the specs don't mean very much. 

Consider, for example, these common scanner specs that are widely misused and widely misunderstood. 

DPI or PPI? 

References to scanner resolution are almost always in dots per inch (dpi). The better metric is pixels per inch (ppi). It's not that dpi is wrong, but that it's too easy to confuse with printer resolution. Saying you scanned at 600 ppi and printed at 1,200 dpi is clear. Saying you scanned at 600 dpi and printed at 1,200 dpi could mean you changed the image resolution.

Resolving Detail: The Missing Spec

Scanner resolution should tell you how well the scanner resolves detail. Unfortunately, it only tells you how many pixels are in the scanned image. If the optical system of a 600-ppi scanner is limited to resolving, say, 400 ppi, the real resolution is only 400 ppi.

Dynamic Range

Dynamic range, a measure of how many different shades a scanner can see between black and white, is related to color depth, and as with color depth, it's almost always based on theoretical capabilities rather than a measure of the actual dynamic range. Here again, though, almost any scanner will have sufficient dynamic range for anything but transparencies.

Showing Resolve


Optical and Mechanical Resolution

Optical resolution tells you how many pixels the scan element can see at once across the width of a page. Mechanical resolution tells you how many steps the scan element takes going down the page (or how many the page takes going past the scan element). A spec like "600-by1,200-dpi optical" really means "600-ppi optical by 1,200-ppi mechanical."

N by M Optical Resolution. Not.

Even ignoring the distinction between optical and mechanical resolution, a spec such as "600-by-1,200 optical resolution" is misleading. The scanner can't pass a 600-by-1,200-ppi image to your computer, so the top resolution you'll get without interpolation is 600-by-600 ppi.

Interpolated Resolution

Scanner manufacturers like big numbers, such as "19,200-ppi interpolated resolution." Whatever the interpolated resolution, this is a good spec to ignore. You can get results that are just as good or better (particularly for photos) by doing the interpolation in separate steps in Photoshop or other software.

High Optical Resolutions

Even high optical resolutions usually don't matter. For typical office tasks such as copying, faxing and scanning to PDF files, and even for scanning photos to print at the same size, a 600-ppi scanner is almost always all you need. Higher resolutions are rarely useful unless you're scanning, say, slides or otherwise need to resize the image.

Meaningful Measures


Measurements: What About the Trays?

Some manufacturers' measurements include the input and output trays for their scanners, but many don't. In the real world, that extra few inches for a tray can make the difference between a scanner fitting or not fitting where you want it.

Duplex Scanning

"Duplexes" as a spec can mean either a duplexing scanner that scans both sides of the page at once or a duplexing ADF (automatic document feeder) that scans one side, turns the page over and scans the other side. The more meaningful specs to look for are pages per minute (ppm) and images per minute (ipm). For duplexing scanners, the ipm is double the ppm.

Pages Per Minute: At What Settings?

The ppm ratings for document scanners can vary greatly depending on the resolution and color mode settings (black and white, grayscale, or color). Scanner manufacturers are usually good about stating the settings, but different manufacturers base their ratings on different settings.

When Pages Per Minute Doesn't Matter

Most document scanners come close to their rated speed when scanning to image files. But some slow down a lot more than others if they're also recognizing text and saving the scan as, say, a searchable PDF file. If recognizing text is part of what you need from a scan, the faster scanner will sometimes be the one with the slower rating.

Legal Size Paper

Being able to scan legal size paper doesn't necessarily mean a scanner has a legal size flatbed. With most scanners that have an ADF, you can scan legal size paper even if the flatbed is letter size. (Having an ADF doesn't necessarily mean the scanner can scan legal size paper, however.)

PDFs and Bits

Scans to PDF: What Format?

If you want to scan to searchable PDFs for document management, don't assume that "scans to PDF format" means "scans to searchable PDF format." Some scanners come with software that scans directly to PDF image format only.

Scans to Searchable PDF

Even scanners with spec sheets that say they scan to searchable PDF format don't all scan directly to PDF files in a single step. Make sure that "scans to searchable PDF" means the scanner (or, more precisely, the software it comes with) scans, recognizes text and saves the file, all in one step.

30 Bits (and Up)

Claimed color depths over 24 bits are almost always the number of bits the scanner uses to describe color, not the number of different colors the scan element can detect. For most scanners, some of those bits are just noise. Fortunately, almost any scanner offers at least a true 24 bits, which is all you need for anything but transparencies, such as slides or film.

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