Is there something wrong with my eyes? Why do these weird stripes appear when I take photos of a TV screen with my phone?

When we pick up our phones or cameras and shoot at the screen, we sometimes encounter a mysterious "invisible magic" - strange stripes appear on the photos. Don't worry, this is not a problem with your phone or camera, but an optical phenomenon called "moiré".

What is moiré

Imagine you stack two combs together and rotate one of them. You will find that light and dark stripes appear between the teeth of the comb. This is the moiré pattern. In screen shooting, similar stripes are produced when the pixels or lines of the screen interfere with the pixels or sensors of the camera lens.

The word moiré comes from the wavy stripes on silk imported from China by the ancient French. In addition to the stripes produced by screen shooting and overlapping combs, the wavy stripes produced by reflections on mosquito nets or curtains on sunny days, and the rainbow stripes that appear when taking photos after wearing striped clothes are also moiré patterns. Moiré patterns are actually a manifestation of the beat frequency phenomenon.

The so-called beat frequency phenomenon, in simple terms, is that when two equal-amplitude sine waves with similar frequencies meet, they will synthesize a new signal, and the amplitude of this signal will change with the difference between the two frequencies.

Here is a simple metaphor to help us understand the beat frequency phenomenon. Imagine you are in a room with two tuning forks, whose frequencies are very close but not exactly the same. When you strike the two tuning forks at the same time, you will hear a special "buzzing" sound.

This "buzzing" sound is actually a manifestation of the beat frequency phenomenon. Since the frequencies of the two tuning forks are different, the sound waves they generate will produce periodic amplitude changes when they are superimposed. This amplitude change is like a "breathing" rhythm, sometimes strong and sometimes weak, creating a rhythmic feeling similar to heartbeat or breathing.

Specifically, when the two tuning forks vibrate in phase, their amplitudes add together to produce a larger amplitude, while when they are in opposite phases, their amplitudes subtract from each other to produce a smaller amplitude. Because the frequencies of the two tuning forks are very close, this phase change occurs periodically, causing the amplitude to also change periodically.

The "buzzing" sound in this metaphor is like an audible representation of the beat frequency phenomenon.

After we understand the beat frequency phenomenon, we can extend its definition from time to space: when stripes with slightly different spatial frequencies are superimposed, the overlapping positions will gradually shift due to the difference in stripe intervals, and moiré patterns will be formed.

The picture below shows two groups of stripes with slightly different spacing between them, and the overlapping position gradually shifts to the right.

We aligned the left sides of the two sets of stripes and overlapped them, and the overlapping area began to show periodic changes from white lines to complete black, which is the moiré pattern.

The interval difference between the two groups of stripes was increased, and the periodic change from white lines to completely black appeared 4 times.

The two sets of stripes overlap completely. If you are nearsighted, you can choose to take off your glasses to observe; if you have normal vision, you can choose to step back a few steps and squint your glasses to observe. At this time, you can observe the typical moiré pattern

What are the characteristics of moiré?

Periodicity: As you can see from the previous demonstration , moiré patterns repeat themselves at regular intervals, just like the hands of a clock.

Angle dependence: Just like your shadow will be different lengths at different times, the sharpness of moiré patterns also depends on the shooting angle.

Color changes : Moiré patterns on the screen can also change color like a chameleon.

Mobility: If you move the camera or screen, they will "dance" with it.

Tips: How to reduce moiré

In order to take perfect photos of the screen, we can reduce the generation of moiré through the following measures according to its generation principle and characteristics:

Adjust the camera angle: Since moiré is caused by the angle of the camera to the subject, changing the camera angle slightly by rotating the camera can eliminate or change any moiré that is present. Additionally, changing the angle relationship by moving left or right or up or down can also reduce moiré.

Adjust the focal length of the lens: By using different lenses or focal length settings, you can reduce the impact of moiré by making the pixel distribution of the screen to be photographed closer to that of the shooting device.

Use professional software for processing: There are many professional image processing software that can help us eliminate or reduce moiré fringes.

Moiré is not some scary "invisible magic". As long as we master its characteristics and how to crack it, we can easily cope with the challenges in screen shooting and take clearer and more beautiful photos!

Moiré is not limited to screen photography, but is an optical phenomenon that is widely present in our daily lives. Understanding the principle of moiré can help us better understand and apply related science and technology. Do you know where moiré is used?