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Why the LED screen photos taken with the mobile phone are not displayed clearly?

by:Rika Sensors     2021-11-04
Why is the LED screen photo taken with a mobile phone not displayed clearly?
In recent years, LED screens have become more and more widely used. Whether in the studio, monitoring center or even environmental monitoring equipment, you can see the figure of the LED screen. Compared with other display screens, LED screens have greater advantages in brightness and size.

However, in the long-term use process, we will find that when the mobile phone or other shooting equipment is aimed at the LED screen, occasionally inexplicable water wave-like stripes and strange colors, as well as strange faults, will appear. The same is true for the photos of, many people think that the current is unstable, but it is not caused by this reason. Today, I will talk to you in detail why the LED screen photos taken with the mobile phone are not displayed clearly.

Stripes like water waves are called 'water ripplesThere will be inexplicable water wave-like streaks. Simply put, the moiré pattern is a manifestation of the beat principle. Mathematically speaking, two equal-amplitude sine waves with close frequencies are superimposed, and the amplitude of the synthesized signal will change according to the difference between the two frequencies.

The reason for the proliferation of moiré is that the refresh rate of the LED screen used is insufficient. Some lines on the LED screen are not lit at the moment of taking pictures and imaging, and a series of moiré phenomena occur. The refresh rate is often referred to as the hertz (hz) of the screen.

Most of the LCD monitors currently in use default to 60hz. Because the screen is not always luminous, but refreshed every certain period of time. For example, 60hz means refreshing 60 times per second, but the human eye has a visual delay, so the human eye is completely unable to detect any flashes on the screen. moveable.

When the mobile phone shoots the screen, most of it will have a relatively fast shutter speed. When the shutter speed is faster than the refresh rate, the screen refreshing process that we can't see with the naked eye can be captured, which will produce 'streaks'.

Take a 1/4 scan screen equipped with a common cross-current driver IC and a refresh rate of 1000Hz as an example. Set the camera's exposure time to 1/1000 second, 1/500 second, 1/800 second, 1/2000 second, The following situations will occur.

Only at a shutter speed of 1/500 second, all lines are lit twice, and the brightness is consistent. At the shutter speeds of 1/800 sec and 1/1000 sec, part of the lines are lit up once more, resulting in different degrees of moiré. At a shutter speed of 1/2000 sec, half of the lines are not lit. You can imagine the number of moiré patterns.

Only when the refresh rate is greater than 2 times the camera shutter speed, the LED screen will not appear moiré.

Under the camera's general shutter speed of 1/2000 seconds, the nearly 1000 refresh rate of the ordinary cross-current driver IC and the 1920Hz refresh rate of the dual-latch driver IC are not enough to avoid moiré. Only a high-definition high-end PWM driver IC with a refresh rate above 3840Hz can better ensure that the LED screen does not appear moiré when taking pictures.

So are there any shooting techniques that can reduce or eliminate this kind of interference?

(1) Change the camera angle of the phone. Since the angle between the mobile phone and the LED screen will cause moiré, slightly changing the angle of the mobile phone camera (by rotating the phone) can reduce or eliminate the moiré.

(2) Change the location of the phone camera. By moving left and right or up and down to change the angle relationship, the moiré can be reduced.

(3) Change the focus. Overly sharp focus and details on fine patterns may cause moiré. Changing the focus slightly can change the sharpness and help eliminate moiré.

(4) Change the focal length of the lens. Use different lens or focal length settings to change or eliminate moiré.

(5) Use software to process, use Photoshop plug-in, etc. to eliminate any moiré on the final image.

(6) Use the front filter to install directly in front of the CCD to make the exposure conditions meet the spatial frequency, thoroughly filter the high spatial frequency part of the image, reduce the chance of moiré occurrence, but this will also reduce the image simultaneously Sharpness.



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