I don’t know if you have ever had such an experience. You see a fun picture on your computer or TV and want to take a picture and send it to your friends. However, no matter how you take the picture, some strange light and dark stripes will appear on the picture. But when you remove the camera and look directly at the screen, you will find that these stripes do not exist at all. What are these inexplicable stripes? Is there any way to eliminate them?

Today we will talk about these "mysterious stripes".

What is Moiré?

These strange stripes are "Moiré", which is a transliteration of the word, and some people also call it "Moiré". This fringe is caused by interference phenomena.

To put it simply, two waves with similar frequencies are superimposed on each other in space to produce a new wave. This is rather abstract, let’s take the simplest example:

When two sets of parallel vertical lines overlap each other, special light and dark stripes will appear between the lines, which are moiré patterns.


Moiré pattern, image source: Wikipedia

These two sets of parallel lines can be regarded as two sets of waves. The bright lines and dark lines in the parallel lines can be regarded as the peaks and troughs of these waves respectively.

If the two sets of parallel lines move, the new waves (moire patterns) generated will also move and change, making them look more obvious.


Moving moiré pattern, image source: Wikipedia

Moreover, moiré patterns will not only appear on parallel lines, but the angle is slightly inclined, or other regular graphics, such as a pattern composed of two sets of concentric circles, will also form moiré patterns.


Moiré pattern formed by two sets of concentric circles, picture source: Wikipedia

Of course, not only simple lines, but also many regular and transparent objects in life can produce moiré patterns. For example, by pressing two layers of materials such as silk together, you can create clothing with a moiré pattern (the word moiré in French originally referred to textiles with this pattern).


Clothing with moiré pattern, picture source: Wikipedia

In addition, if we look at the scene on the opposite side through two layers of screens or mesh structures, we will also find moiré patterns on the screens.


Two layers of light-transmitting mesh produce moiré patterns. Image source: Wikipedia

In addition, a similar phenomenon will appear in some animals. For example, in birds, moiré patterns will appear between two layers of feathers that are arranged parallel and slightly staggered.


Moiré patterns on bird feathers, image source: Wikipedia

Then the next question arises. The pattern on the screen of a computer or mobile phone is obviously not a regularly arranged line, and there is no moiré pattern when viewed with the eyes. Why does moiré pattern appear when taking a photo with a camera?


The polar bear obviously does not have regular stripes, but also has moiré patterns. Picture source: Taken by myself

Why

Will the screen also have moiré patterns?

In fact, no matter what kind of pattern is displayed on your screen, the electronic screen used to present the picture itself is composed of neatly arranged luminous points, but these luminous points are extremely small and almost indistinguishable with the naked eye.

Nowadays, the mobile phones and digital cameras we use to take photos use charge-coupled devices, which are often called CCDs.

The CCD is densely covered with rows of small photosensitive units (usually capacitors). After receiving light, each capacitor can convert the light intensity into a corresponding electrical signal, thus recording the outside world.

So you see, the rows of light spots on the electronic screen can be regarded as a column of waves, and the neatly arranged photosensitive units on the CCD used for taking pictures can be regarded as a column of waves. When the frequencies of these two waves are similar, new regular light and dark stripes will appear on the photos taken - that is, moiré patterns.

How to eliminate moiré?

Knowing the principle of moiré pattern generation, it is easy to eliminate moiré pattern.

For example, if the shooting equipment you use is not a digital camera, but a film camera, there will be no moiré pattern in the photo. Because the photosensitive part of a film camera is a whole piece of film, there are no rows of small photosensitive elements on it. Of course, this method is not easy to implement. After all, few people still have film machines at home.

Another simpler operation method is to change the angle of the mobile phone camera so that the "bright spot lines" in the electronic screen and the lines of the photosensitive elements in the CCD are no longer parallel. Such operation tutorials have been circulated on the Internet before, and they are indeed feasible.


Picture taken from the Internet

Of course, in actual shooting, this angle rotation cannot be too small, otherwise you may get other shapes of moiré patterns. The two pictures below were taken by rotating the phone at an angle. It can be seen that the moiré pattern is very obvious in the first photo, but it is almost invisible in the second photo.


The picture on the left, taken at angle 1, has obvious moiré patterns. In the picture on the right, when the phone is rotated at an angle, the moiré pattern is significantly reduced. Picture source: Taken by myself

Therefore, if moiré appears when you shoot with a mobile phone camera, the simplest thing is to rotate the mobile phone camera at an angle. But if this trick doesn't work, then you have to resort to image processing software.

Generally, professional image processing software has the function of automatically removing moiré, and there are also some online websites that can directly remove it with one click. If your requirements are not high and you can use it by slightly weakening the moiré pattern, you can also leave it to AI.


Use bean bag AI to remove moiré, but the removal is not very clean

What is the use of moiré pattern?

In fact, in addition to "adding trouble" to taking pictures, moiré patterns have some special applications.

Needless to say, use them to generate cloud patterns and other types of patterns on textiles or artworks. There is also an area you may not have thought of - anti-counterfeiting.

Many currencies also have very densely arranged stripe patterns. These patterns are very clear to the naked eye, but if you want to use a scanner to scan them, the sensor array on the scanner will interfere with these pattern arrays to produce moiré patterns. This is one of the anti-counterfeiting methods on banknotes.

In the movie "Wu Shuang", there is also a plot like this. The gang that makes counterfeit money directly hires a painter to draw the template for the counterfeit money, instead of scanning it with a scanner. The anti-counterfeiting principle of moiré pattern is involved behind this.

In addition to banknotes, moiré patterns can also be used on some special items to prevent counterfeiting. For example, anti-counterfeiting marks are printed on product labels. These marks are nothing special when viewed directly with the naked eye. At most, you can see very fine stripes on them.

But when they overlap with the decoding piece (another set of cards with fine stripes), the Mori text produced by their interference will show special patterns, and the authenticity can be verified through these patterns. And if the logo is fake, there is no way to restore such a pattern. This design is also very clever.

Therefore, as long as we make good use of it, moiré pattern, a phenomenon that originally caused us trouble when taking photos, can also bring unexpected applications.

Planning and production

Author丨Scientific scraps, popular science creator

Review｜Li Ming Researcher, Institute of High Energy Physics, Chinese Academy of Sciences

Planning丨Yang Yaping

Editor丨Yang Yaping

Reviewer｜Xu Lai Zhang Linlin