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DPI vs LPI: What's the difference? - SOS Print + Media Group

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To understand the difference between dots per inch (dpi) and lines per inch (lpi), look at the way laser printers and printing presses create grey by using ... Skiptocontent Tounderstandthedifferencebetweendotsperinch(dpi)andlinesperinch(lpi),lookatthewaylaserprintersandprintingpressescreategreybyusingonlyblackinkortoner. Toproduceshadesofblack,anypicturethathastobeprintedmustfirstbebrokenupintoaraster.Throughthisrastertheblacktoner(orink)canthedistributedonthepaperasdotsofavaryingsize.Thesizeoftheblack dotandthewhitepaperarounditcreatetheimpressionofgreys.Lightareashavesmalldotsanddarkerareasorshadowshavelargerdots.Theimagebrokenupbytherasteriscalledahalftone,therasterisahalftonegrid,dividedintohalftonecells. Thehalftonegridisdependentonthefinenessofdotstheoutputdevicecanproduce.Inotherwords,ifyouhavea300dpilaserprinter,yourgridcanonlybeproducedusing300dotsperinch,whichmeansthatyouarelimitedinthevarianceofyourhalftonedot,muchmorethanwitha3600dpiimagesetter. Thislimitationmeansthatthedistributionofyourhalftonecellsdependsonyourgrid,ifyourcellsaretooclosetogether,thegridwillnotallowtheprintertovarythesizeofyourhalftonedotandyoucannotproduceenoughshadesofgrey,theimagebecomesblotchy.Theclosenessoftherowsofhalftonecellsismeasuredinlinesperinchorthelinescreen. Inprintingthenumberoflinesperinchuseddependonthedpioftheoutputdeviceandalsoonthetypeofpressusedandthepaper.Asmallinstantprintpresscannotprintdotsasfineasabig,commercialprintingpress.Jobsprintedonuncoatedpaperornewspapertypicallyrequireacoarselinescreen(85lpi)becausethesepapersabsorbal otofinkandthedotsspreadout(dotgain).Highqualitycommercialprintingisusuallydoneonhighqualitycoatedpaper,onwhichamuchsharperdotcanbeproduced(150-200lpi).Thedotsincoarselinescreenscaneasilybeseenbythenakedeyewhereasfinelinescreenslookalmostlikeacontinuoustone. Whiletheaboverelatedtooutput,dpiandlpiarealreadyveryimportantwhenscanningyourimages.Toreproducewithclarityatacertainlinescreen,ascanneedstohaveacertainresolution.Aruleofthumbistoscaninatapprox.twicethefinallinescreen,e.g.apictureprintedata150lpineedstobeataresolutionof300dots(orpixelsonscreen)perinch. Theimagewillbebrokenupintoarasterdeterminedbythelinescreen.Thereneedstobeenoughinformationinthepicturetosupportthelinescreen,superfluousinformation,e.g.morethan300dpiat150lpi,willbediscarded. Anotherwayofproducingshadesofblackoracolourisbyvaryingthenumberofverysmalldotsonapage,asopposedtothesizeofthedot.Thistechniqueisusedinmostinkjetprintersandproducesverygood,photolikeresults.Itiscalledstochasticscreening,orfrequencymodulation.Ithastheadvantageofproducingrelativelygoodresults,evenwhentheimageresolutionisbelow300dpi. Searchthissite Searchfor: QuickLinks RecentNews WhereGoingGreenIstheOnlyChoice WhenBiggerisALWAYSBetter PrintOnDemand:TheEfficientPrintingSolutionThatSavesYouTime,MoneyandExcess  

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