LED video screen, refresh rate and human eye

Primarily, refresh rate affects image perception. We usually perceive an image on a screen as smooth and do not notice a flickering effect because the frequency of flickering is rather high. Our visual perception is a psychological as well as physical in nature. The individual flashes of light are summed up into a “smooth” image by our brain. According to the Bloch’s Law, this summing up lasts approximately 10 ms and depends on brightness of light flashes. If light flickers with sufficient frequency (so called threshold CFF – Critical Flicker Frequency), human eye does not notice pulsation according to the Talbot-Plateau Law. The threshold CFF depends on many factors such as the spectrum of the light source, positioning of the light source in relation to the eye, brightness level. However, in normal conditions this frequency never exceeds 100 Hz.

Thus, a human eye will not distinguish any differences in LED video screen images formed with a PWM or modified PWM methods with refresh rates varying from 100 Hz to 1 kHz.

LED screen, refresh rate and a video camera

However, a human eye is not the only instrument that may perceive images. Sometimes we use video cameras to record LED video screens, and video equipment is based on principles drastically different from the one employed by the human brain. This is especially important for all LED video screen installations in the sports stadiums, trade shows or concert halls where events are recorded with cameras. Exposure time or shutter speed in modern video cameras may vary from seconds to a millisecond.

Let us say that we look at a LED screen where image is formed using traditional PWM method with 100 Hz refresh rate. The video screen displays a static image. If we try to record this image with a video camera using a 1/8 second shutter speed (i.e. exposure time of 125 milliseconds) the photo sensor will record light from the screen image produced by 12.5 refresh periods. The LED screen and our video camera are not synchronized and each frame recorded by the camera will correspond to different time related to the beginning and the end of the refresh cycle. But with this high shutter speed there will be no conflict and the camera will record a smooth image of the LED video screen.

If we reduce shutter speed to 1/250 seconds when exposure time equals 4 ms, one camera frame will be 2.5 times shorter than refresh period on the LED video screen. This time the discrepancy between the beginning of the camera frame and beginning of the PWM cycle will be significant. Some frames will correspond to the beginning of the PWM cycle, others to the middle, and others still to the end of the cycle. Each frame will record different light flow and gradually the error accumulates. When we view the recorded video the brightness of frames will be noticeably different. Typically, all objects recorded with short exposure time appear less bright. The camera will record “flickering” effect on the LED video screen. If the exposure time reduced even further we will definitely see some black frames (when the beginning of the camera frame corresponds to the short PWM period when LEDs are switched off) and the recorded video will flicker even more.

Thus, if we use a video camera to record a LED screen with traditional PWM function, the refresh rate should be compatible with or exceed the camera exposure.

On LED video screens with modified PWM function the same logic applies. Since in high brightness mode the turning on time of LEDs is “spread” over the PWM cycle, the recorded image will be more stable compared to traditional PWM function. But at low brightness the situation will remain the same: the recorded image will either lose brightness or will flicker.

As you see without proper synchronization any video recording of a LED screen will result in distortions in recorded image. We can compare this to recording the analogous TV with an analogous camera: the differences in scanning modes of both devices will lead to an effect of diagonal black lines separating TV frames.

Another important issue is the synchronization of LED video screen controllers. Large LED screens are made of blocks (LED modules and/or cabinets) that display imaging generated by different controllers. If these controllers do not synchronize the beginning of the PWM cycle (i.e. the beginning of the cycle on different parts of the screen) we may encounter the following problem: refresh cycle on some parts of the LED screen will correspond to camera frames and on other parts of the screen it will not. If the exposure is compatible with refresh cycle, part of the video screen will seem brighter, another darker. The whole image will consist of dark and bright rectangles and will be uncomfortable to watch.

How to enhance viewers experience with high refresh rate LED video wall?

The relationship between LED video wall, refresh rate

How LED video wall can enhance stage events experience?

3 main benefits to use LED video wall with high refresh rate in your stage events

It is not a fresh news that event planners will use LED video wall to enhance activity atmosphere in all kinds of stage events. As a fact, the LED wall promote exactly the engagement and excitement of the audience in activities. At the same time, for audience, except watch the performance by eyes, they also prefer to use camera to record the splendid moment. But most of the time, the camera will record “flickering” effect on the LED video screen. That will greatly influence their experience. Not to mention the whole event recorders.
To solve this problem, LED wall with high refresh rate will be better for audience to take pictures. And let us take a look what benefits will have if use high refresh rate LED video screen. 

1. Reduce visual fatigue 

The refresh rate will affect the image perception. Normally, when we watch a LED wall with regular ICs, we do not notice a flickering effect on the LED screen and perceive the image is smooth. That just because the frequency of the flickering is quite high. So the human eye cannot distinguish any differences. However, it will last for several hours in an event, and a human eye will easily feel fatigue if the screen is in a low refresh rate. If improving the fresh rate, all will be different. The higher of refresh rate, the screen picture will be more stable. And a human eye will feel less fatigue.
That is one of the reasons why the operators should invest in a high refresh rate stage rental LED wall. 

2. Eliminate the scanning line when took by video camera

Event recorders would like to use camera to record the events. And LED video wall, as an important part of the stage events, it is no doubt that it will be took in video. However, if you use a video camera to record a LED screen with low or regular refresh rate, there will be scanning lines and shadow in the video. 

You may wonder why you see the LED screen video is normal but it appears this scanning lines on camera videos. Although a human eye can not notice, the video camera will. Exposure time or shutter speed in modern video camera may vary from seconds to a millisecond. If the LED video wall with lower refresh rate, the refresh rate of LED video wall and the shutter speed of video camera are not synchronized. In this way, when we use a video camera to record the image of the screen, it will record each refresh cycle of the screen, that is the scanning line we see in the video.

Using high refresh rate LED video wall, this problem can be solved easily. It will enable the LED wall to show a better picture quality and won’t have any scanning line or shadow in the camera video.

3. Enhance events atmosphere and audience experience

Audience is also an important part in a stage event. And LED video wall is vital to set an atmosphere and creating a dramatic impression. A LED wall with perfect performance will create a dynamic event and its various colors will give an astonishing visual impact. Thus, it definitely attracts audience attention to fit into the event. The communication between audience and events will positively promote the live atmosphere of the event. On the other hand, it will be a fantastic experience for audience.

Setting an atmosphere and creating a dramatic first impression is why mood lighting is so vital to any event. Get the lighting (and sound) right and you are well on your way to creating a dynamic event. In our opinion you need to bring in professionals to manage all of these needs. Lighting professionals can transform a venue space, creating the atmosphere you desire using varying colours and intensities of light to give an astonishing visual impact.

Among the lighting techniques that can be used are LED uplighters, which can enhance the venue's internal architecture, colour washes to give impact to the stage, ceilings and walls, and gobos, which can liven up dead spaces by shining patterns or corporate logos onto them.

At award ceremonies your lighting engineer and event designer from the production team will create an exciting lighting design in accordance with key elements from lighting to tie in with the music such as winner announcements to guest speakers and the host going on stage.

a human eye will not distinguish any differences

However, a human eye is not the only instrument that may perceive images. Sometimes we use video cameras to record LED video screens, and video equipment is based on principles drastically different from the one employed by the human brain. This is especially important for all LED video screen installations in the sports stadiums, trade shows or concert halls where events are recorded with cameras. Exposure time or shutter speed in modern video cameras may vary from seconds to a millisecond.