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Moire, causes and curescan’t pick out any detail. Your customerwill have specified an lpi that’s significantlyhigher than the resolution of their customers’eyes at the normal viewing distance,hence there is a reasonable margin of resolutionto protect against moiré.The following series of images gives youan idea of the phenomenon. Each imagehas a strong moiré, but the frequency isincreasing.It’s debatable where the cutoff is betweenugly moiré and pleasant half-tone pattern,but 4 is a good approximation.Note that the example with a MRN=2 isthe pattern between two colours at 30°.MRN=10 MRN=6 MRN=4MRN=3MRN=2106
This Rule of 4 has some important exceptionswhich I’ll discuss later.2 - AngleThe angle of a moiré has some small effecton the visibility of the moiré. The humaneye is more sensitive to vertical and horizontalmoirés. But in general, the angle is auseful ally for the printer. It offers a clue asto which parts of the system are producingthe moiré. As a simple rule, the angle ofthe moiré is 90° + the angle between thetwo patterns. So a screen at 15° and oneat 30° will have a moiré at 90+22.5°=112.5°.If you have a simple dot screen then youwill also see a moiré at right angles to this(i.e. 22.5°) but if you have an elliptical dot,one of the angles will be stronger than theother. If you use a geometric screen thenyour moiré will be a line rather than a grid.Using this simple rule, you can often pinpointwhich things are interacting. Forexample, a 3.7° (or 93.7°) moiré is a symptomof a mesh (0°) clashing with your 7.5°colour.3 - AmplitudeWe’ll soon be discussing one type of moiréthat’s always with us, but which we seldomsee. Its frequency is close to our Rule of4 cutoff so it’s a marginal call whether wesee it or not. If you look closely at any print(or if you overlay the original film positives)you’ll be able to pick it out. Yet most of thetime it’s not a problem. However, this samemoiré can sometimes leap out and hit youin the face. Why? Because some other effectsare increasing the amplitude of themoiré.The word amplitude is the scientificallycorrect word for discussing these opticaleffects, but it also provides a familiaranalogy with sound. A quiet sound hasa low amplitude, a loud sound has a highamplitude. A low amplitude optical effectis hardly visible (subtle changes in tint), ahigh amplitude optical effect (black andwhite stripes) is highly visible. In generalwe hear loud sounds but don’t hear quietsounds. Yet if everything else is very quiet,then a quiet sound can appear loud (“Youcould hear a pin drop”). So a moiré of lowamplitude is generally not visible; but ifit’s in an area (e.g. a light, even tint) of lowgeneral amplitude (i.e. absence of strongimage contrasts) then it can become visible.Similarly a reasonably loud soundgets drowned in a noisy room, so a reasonablyhigh amplitude moiré can disappearin ‘busy’ areas of a print.If there were a simple way of predictingamplitude, we’d have resolved all moiréissues a long time ago. But it turns outthat you need a sophisticated theoreticalmodel to calculate the amplitude of mostpractical moirés and that’s not somethingyou’d have next to your press! The Mac-Dermid <strong>Autotype</strong> Moiré Modeller provideshelpful insights (and the images) for thisChapter. One thing that emerges fromsuch a model is that moirés with higher107
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HowHow to be agreat ScreenprinterEd
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About this eBookI’m passionate ab
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06Table ofContents07 The Theory Bit
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The wrong way: a strong-deep flood.
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why a light flood is recommended so
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This is the single most important f
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When I first pointed this out to my
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The Theory Bit20Why is the 1-2-3 so
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The mesh does the meteringThe 1-2-3
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This gives us the ‘real’ TIV fo
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simple, in-control process.Years of
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The Optional Theory28At the shear r
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I’ve come across examples in all
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The Optional TheoryOur 50µm line b
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oughness can slow slumping; it prob
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The Optional TheoryThe exact part o
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atches of the ‘same’ mesh. My f
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The Optional Theory40The industry r
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the screen frame itself to compensa
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Print Faults - Making and fixing th
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Print FaultsThey include tape marks
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Wrong ink - Too much viscous drag (
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DustPrint FaultsWe threw some dust
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What’s the best order to print yo
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Colour shifts54high-frequency (non-
Page 56 and 57: We formed our own (subjective) opin
Page 58 and 59: And the winner is...Colour shifts58
Page 60 and 61: Getting the right exposureAnna Harr
Page 62 and 63: Getting the right exposureThe light
Page 64 and 65: 3/4 full exposureEFFECTS: Emulsion
Page 66 and 67: Getting the right exposureThere is
Page 68 and 69: Undercutting from a rough stencilSo
Page 70 and 71: Getting the right exposure(or numbe
Page 72 and 73: Solving problems in the real worldA
Page 74 and 75: Solving problemsThe simple act of r
Page 76 and 77: Emulsion coatingThe advice about cl
Page 78 and 79: Solving problemsA quick tip to help
Page 80 and 81: stencil causes a thick edge needed
Page 82 and 83: Solving problemsThis screen-shot fr
Page 84 and 85: A modest Rz gives amodest absolute
Page 86 and 87: Solving problemsThe root cause of n
Page 88 and 89: Now the ink fully fills the extra s
Page 90 and 91: Solving problems90Stencil X gives b
Page 92 and 93: Moiré, causes and curesSteven Abbo
Page 94 and 95: Moire, causes and cures94fluential
Page 96 and 97: Figure ii. The overlayedpattern fro
Page 98 and 99: Moire, causes and cures98Causes and
Page 100 and 101: 2 15° moiréIt’s an unfortunate
Page 102 and 103: Moire, causes and cureson top of an
Page 104 and 105: Ideal. Find a RIP with an option fo
Page 108 and 109: frequency tend to have a lower ampl
Page 110 and 111: Moire, causes and curesis a half-tr
Page 112 and 113: Incidentally, note that the ratio o
Page 114 and 115: Moire, causes and cures114So, surpr
Page 116 and 117: grid) in a conventional set that wa
Page 118 and 119: Moire, causes and cures118what this
Page 120 and 121: Arguably the skipping takes place p
Page 122 and 123: Moire, causes and curesSummaryAs Ma
Page 124 and 125: MoiréDescriptionAn unexpected patt
Page 126 and 127: Problem solving GuideCause1. The mo
Page 128 and 129: Dark spots / HickiesDescriptionDark
Page 130 and 131: Problem solving GuideTechnical refe
Page 132 and 133: 6. Substrate unstable.7. Changes in
Page 134 and 135: Glossary of terms used in this eBoo
Page 136 and 137: Glossary136Moiré amplitudeThe ampl
Page 138: A Macdermid Autotype HowTo eBookEdi
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