This article is part of the FLEXO FLAW FIXING series

What is the moiré effect in flexo printing?

The moiré effect in flexo printing occurs when certain images are viewed on a screen or printed. The term comes from the French word for “maze” and refers to the appearance of fine, repeating patterns within a picture. The patterns can be repeated or randomly distributed, resembling jagged lines, swirls, or latticework.

When did the moiré effect appear?

In the early 1800s, the moiré effect was discovered. Prints with higher resolution than what was available at the time were initially noticed when attempting to produce them. The images resembled having layers of fabric or lace embedded within them.

The moiré effect was first observed in 1827 by an English scientist named Thomas Young, who correctly theorised that it is caused by interference patterns generated by two wavelengths of light passing through the same medium.

Why does the moiré effect happen?

The cause of the moiré effect is usually due to the way images are captured and displayed on screens. Each pixel of an image is arranged in a grid when captured. However, when the image is displayed on a screen, the pixels are not evenly spaced. Due to this uneven spacing, pixels overlap one another, creating the moiré effect.

As a visual distortion, moiré appears as jagged, monochromatic lines. It is caused by the interference of lightwaves at different frequencies. The interference creates an image that does not resemble a smooth curve or surface.

The most common source of the moiré effect is spinning disks in images. During the image creation process, the jagged lines are created as a result of interference waves caused by uneven, spindly disk pieces.

Various Types of Moiré Effect

There are various types of the moiré effect in flexographic printing, but they all share some common features.

Chevreul’s Illusion

The first type of the moiré effect is known as “Chevreul’s Illusion”. This occurs when a pattern of dots or lines moves across the screen and gradually fades away. The dots or lines create the illusion of a continuous background pattern.

Sine-Gordon Moiré

The second type is referred to as Sine-Gordon Moiré, which occurs when two closely spaced patterns overlap one another and create an interference pattern. The interference causes the patterns to appear as though they are moving together, giving rise to the moiré effect.

Poynting-Robertson Moiré

As for Poynting-Robertson Moiré, the third type of the moiré effect, it occurs when two patterns with different colours overlap and create a rainbow-like pattern. The colours blend together to form new colours, resulting in the moiré effect.

Hanns Euler Moiré

The last type of the moiré effect is an interference pattern that occurs in some optical systems. It is named after the Swiss mathematician and physicist Hanns Euler, who first described it in 1746.

Hanns Euler Moiré patterns are often observed in fibre-optic systems, where they can pose issues with data transmission. In computer graphics, they can occur when two surfaces are in close proximity, causing them to merge.

How can you eliminate the moiré effect in flexo printing?

If you encounter the moiré effect in your photos, there are steps you can take to resolve it.

Initially, ensure your photo is in focus. This implies that everything in the photo, from the background to the objects in the foreground, is in focus. If your photo is blurry, rectifying the moiré effect becomes more challenging.

Next, consider using a higher resolution photo where feasible. This will provide more detail and reduce the number of pixels to address.

Lastly, employ a filter that aids in minimising glare.