Click on the advert above to visit the company web site

Product category: Screen Printing Systems and Materials
News Release from: MacDermid Autotype | Subject: Screenprinting
Edited by the Printingtalk Editorial Team on 30 March 2005

Screenprinting Moire - Its Main
Characteristics

In the final part of the series on moire effects, Professor Stephen Abbott of Autotype International looks at moire characteristics.

In the final part of the series on moire effects, Professor Stephen Abbott of Autotype International looks at moire characteristics "Previously, we have discussed how moire effects are created in screen printing, what causes them to occur and some proven recipes for success, as well as discussing moire look-alikes

In this final instalment we will consider the three main factors than characterise any moire effect - frequency, angle and amplitude (intensity).

"Although most of the confusion surrounding moire effects arises from insufficient knowledge relating to these three characteristics, there is also a fourth factor that is often forgotten and that is the human eye; it is the eye that dictates the relative importance of these three moire characteristics.

"Frequency is extremely important and is simply the number of times something repeats itself in a regular pattern, so a moire frequency can be described by the number of repeating moire spots or lines that can be found in each inch or centimetre.

We are all used to specifying frequencies in screen printing.

When we talk about an 80 lines per inch (lpi), or 32tpc halftone, we are effectively describing its frequency.

Similarly, when we talk about moire effects it is normal to say that they repeat, for example every 0.5" (1.3cm).

That half inch is what is called the period and is the inverse of the frequency.

So the frequency of a 0.5" moire is 1/0.5=2lpi (1/1.3=0.8 lpc).

"Although it is easy to prove theoretically that a moire effect will be present in a screen print, the more important question to ask is, 'will the effect actually be visible in the final print?' For example, a moire effect may be present in a large poster but actually only be seen under a loupe or microscope and will certainly not be noticeable under normal viewing conditions.

To make life easier, there is a simple calculation that can be used - the Moire Ratio Number (MRN).

Simply divide the frequency of the print (the halftone ruling) by the frequency of the moire effect.

The higher the MRN the higher the visibility of the moire.

In simple terms, a MRN of four or more indicates a likely problem.

As an example, on a 80lpi (32tpc) print, a moire frequency of less than 20lpi (8tpc) should not be a problem.

"We recently had a startling reminder of the validity of this rule having been shown some prints with a very strong moire effect that showed at close inspection.

The prints had, however, a very low lpi (32lpi, 12tpc) and were designed for viewing from a reasonable distance.

Sure enough, when you put them at their correct viewing distance, not only did the moire disappear, but the prints also looked extremely good.

The designer and printer obviously had a good understanding of moire effects and had the knowledge to ignore the strong moire that was visible at close distances.

This particular moire had a frequency of 8.4lpi (3.3tpc) so its MRN was 32/8.4=3.8, just below the rule of foour limit, and great for the viewing distance of a 32lpi image; by comparison, an 8.4lpi moire effect on an 80lpi print (MRN=9.5) would have been a catastrophe! "The human eye has an impact on the MRN rule as it only has the ability to resolve effects to a certain definition, below which it will be unable to detect any detail.

Generally, customers will specify an lpi figure that is significantly higher than the resolution of their eyes at normal viewing distance, so there is usually a reasonable margin of resolution to protect against moire effects.

"The angle of a moire effect has a small impact on the actual visibility as the human eye is more sensitive to vertical and horizontal moire alignments.

In general, however, angle is a useful tool for a printer as it offers evidence confirming which part of the system is producing the moire.

As a simple rule, the angle of the moire effect is 90 degrees plus the angle between the two patterns; so a screen at 15 degrees and one at 30 degrees will have a moire effect at 90+22.5 degrees = 112.5 degrees.

"Note that if there is a simple dot screen then a moire effect will also be seen at right angles to this (i.e 22.5 degrees).

If there is an elliptical dot, however, one of the angles will be stronger than the other, while if a geometric screen is used then the moire effect will appear as a line rather than a grid.

Using this simple rule, it is often possible to pinpoint which components are interacting.

For example, a 3.7 degrees moire is a symptom of a mesh (0 degrees) clashing with the 7.5 degrees colour.

"The amplitude of sound is essentially a term to describe its loudness; sound with a high amplitude can be easily heard, whereas sound at low amplitude is soft and difficult to hear.

Similarly, the amplitude of a moire effect is a measure of its visibility; the higher the amplitude the more visible the moire effect becomes.

For example, a low amplitude optical effect is hardly visible, due to the subtle changes in tint, while a high amplitude optical effect consisting of black and white stripes is highly visible.

Although a moire effect with a low amplitude is generally invisible, it should be noted that if the effect appears in an area where there is a light, even tint and an absence of strong image contrasts then it can become visible.

Similarly, a reasonably high visibility moire effect can almost disappear in busy areas of a print.

"Although there is not a simple way of predicting amplitude, there is a sophisticated theoretical model that can be used - the Autotype Moire Modeller - that will help you calculate the amplitude of most practical moire effects.

Interestingly, one factor that does quickly become apparent from the use of such a model is that moire effects with higher frequencies tend to have lower amplitudes.

This theory supports the rule of four as previously discussed.

At an MRN of four, the moire effect is likely to be of a lower amplitude than one with an MRN of eight.

Although a moire effect at the rule of four frequency should be visible at a reasonable viewing distance, if the amplitude is low then the effect is unlikely to appear.

This amplitude effect is what makes moire such as enigma, as you might print a moire-free job today, with a careful set of press parameters, then get a bad moire effect tomorrow with the same parameters.

This is because the amplitude of some marginal moire effect happens to have increased above the visibility limit.

"Even with such as sophisticated model as the Autotype Moire Modeller, there are some moire effects that can't be predicted.

This is due to the fact that the human eye is incredibly sensitive to patches of white within a dark area and even a tiny change in the number or size of white areas is sufficient to produce a visible moire effect that can be seen.

"In summary, in this recent series of articles, we've now identified that there are actually just three types of moire effect that can cause problems in screen printing and that other phenomenon, although that may look like a moire effect, generally arise from different causes.

"In addition, we have looked at methods of recognising true moire effects and at a variety of ways of understanding why they occur and how to control them. Request a free brochure from MacDermid Autotype ...

I hope that this knowledge will enable you to produce even better quality screen images and prints and to use the phenomenon of moire effects to your advantage.".

• MacDermid Autotype: contact details and other news
• Email this article to a colleague
• Register for the free Printingtalk email newsletter
• Printingtalk Home Page

Search the Pro-Talk network of sites