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Im ages intended to be view ed on a com puter screenare best m easured directly in pixels, because the outputdevices, m onitors and projectors are generally thoughtof in term s of the num ber of pixels they display, 600x 800 for exam ple.

D eterm ining the resolution of im ages intended for printm ust take into account several additional criteria. Firstof all, an im age on paper is m ore com m only thoughtof by it’s physical size.

A lso the output devices, printers, are m easured in tw ow ays. The m ore com m on m easure is the size of thesm allest dot of a single color that the printer can m ake,generally described by how m any can fit in an inch,hence dots per inch or dpi.

This describes how sharp the printer can m ake a singlecolor im age in the color inks available. This is appropriatefor im ages referred to as line copy w here there are nointerm ediate tones, only the w hite of the paper and

the solid application of one of the colors of the inkavailable, generally cyan, m agenta, yellow and black.

Im ages of this type should be scanned at the fullresolution of the printer. In actual practice, it is difficultfor the hum an eye to tell the difference of anythingover 400 dots per inch, so no m atter w hat the resolutionof the printer, scanning line copy over 400 dots perinch yields dim inishing returns.

In order to sim ulate grey tones, printers use a schem eknow n as halftoning, w here clusters of the dots theprinter is capable of are used to sim ulate shades ofgrey. In color printing the im age is divided into it’s cyan,m agenta, yellow and black com ponent and each is

printed in a halftone pattern, w hich the eye and brainreconstruction into the full spectrum of colors.

These halftone patterns are m easured in lines per inch.

It is this halftone frequency that determ ines how sharpan im age a given printer can produce.

It is quite rare for this m easure to be published inprinter m anuals, and m odern color printers especiallyuse variants of the schem e for w hich this halftonefrequency is not as obvious as w ith a traditional printedhalftone.

The best schem e is to m ake a assum ption of w hat thehalftone frequency is based on the full resolution ofthe printer.

N ew laser printers w ill have a halftone frequency about85

C olor inkjet printers printing at 720 dpi have a halftonefrequency about 90 lines per inch and at 1440 dpi,about 130 lines per inch.

The general recom m endation for the correctresolution for an im age is one and a half to tw otim es the halftone frequency, depending on howcritical you are.

So for a black and w hite laser printer, you w ouldhave to have betw een 120 and 170 dots per inch.

A s with any application, you w ant to have enoughdata to get a sharp im age, but no m ore.

The im ages on the accom panying page dem onstratesthe practical im plications of this.

The top row of im ages range from the top end ofthe range, 180 dpi, a little m ore than tw ice thehalftone frequency, to 105 dpi, only about 125% ofthe halftone frequency.

It’s rather difficult to tell the difference between theseim ages. O n close inspection the higher resolutionim ages seem sharper, but it’s hard to identify specific

details that contribute to that difference, and w ithoutthe 180 dpi im age to com pare it to, it w ould be easyto assum e the 105 dpi im age w as as good as it couldbe.

The 80 dpi im age at first glance looks pretty good,but on careful inspection, the curves of the lam pbase and the outer edge of the cat’s eye reveal thestair step pattern of the square pixels that m ake upthe im age. The points of the tufts of fur on her chestare not as sharp and w ell defined.

In the 60 dpi im age the pixels can be seen throughoutthe im age to a careful view er, and at 40 dpi, theydom inate the im age.

Image resolution for print

800 dpi 400 dpi

200 dpi 100 dpi

Line copy examples

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180 dpi 140 dpi 105 dpi

80dpi 60dpi 40dpi