What is flicker?
Flicker is a major problem for the lighting industry. A flicker means the fluctuation of light occurred for any reasons. It may be occurred by the design of lighting products, mostly caused by some external factors.
Harm of flicker
Flicker causes the light fluctuates repeatedly and it will be harmful to health. By the research, it is known that the flicker may bring out such as brain cell injury, poor vision, migraine headache. And it may also affect the efficiency for some jobs, especially for the workspace that people exposed to light for a few hours, even all day, flicker become a serious problem.
For the film industry, flicker is also severe trouble because it may affect the instruments such as a camera. When a camera working for video shooting, it has a frame rate and shutter speed. If each frame contains equal light, the picture or video captured will be looked nonweird. On the other hand, while flicker occurred in light source, the camera is difficult to catch same light for each frame (light source getting bright and dark alternatively), then we, unfortunately, get a video with flash. Despite some of the flicker cannot be sensed by human eyes, since the camera works frame by frame, it detects even slight flicker in light source with a high frame rate.
External factors cause flicker
Even though luminaries’ manufacturers try to design and produce no flicker light applications, when people use those high-quality products, there are still flicker in light caused by their usages.
One of the common factors is the dimming of the light. As for many lighting industries, we always have to adjust our lightness of the lamps or LED in order to tackle with different occasions. And the most-used dimming method is Pulse Width Modulation (PWM). PWM control the brightness of LED via voltage change applied on the LED. For changing the voltage, we import a concept which is duty, to adjust the applied voltage on the LED.
Duty means the ratio of the switch-on time and total time (switch on + switch off). The longer the switch-on time is, the more duty we can get. When the switch on time equals the total time, duty is 100%, and the voltage is maximum on this occasion. By changing the switch on time, we can adjust the duty. However, duty changing means a variation of voltage, this causes a flicker occurred.
Commonly people eliminate flicker by increasing the carrier frequency of PWM. Such a method makes the time between switch on and off shorter, then the flicker is decreased. Here we should notice, for human eyes, a higher frequency could make people feel no flashing in the light. However, for film industries, because they record the scenes for each frame, even a slight flicker will be detected by devices. So, we have to know film lights request a quite high carrier frequency of PWM.
Another factor causes flicker is our power supply. During a filming process, dysprosium lamp is often used as an exterior light due to its color temperature is close to 5600 K (similar to daylight). Since the dysprosium lamp is a strobe light, the flashing frequency is synchronized with the city power supply. For example, the frequency of power supply is 50Hz, that is to say, the emitted light from this type of lamp change 50 times between bright and shade in 1 second. As we said above, our eyes are not so sensitive so we don’t see any flicker. The camera is different, and each frame it takes is an instant. If the illumination brightness of each frame is the same, there will be no flicker. If the brightness is inconsistent, there will naturally be a change in brightness when you playing the video, and flashing will occur. To ensure that the brightness of each frame is the same, the same amount of exposure is obtained for each frame. Therefore, the time required for the shutter (leaf board) to open is the same as the frequency of the strobe light, so that the blinking period of the “capture” of each frame is the same, and the obtained exposure value is also uniform.
Due to the above, we could know that if you want to shoot a picture with no flicker, the shutter should be set at 1/50 second (50Hz), which is consistent with the strobe light frequency. At a frame rate of 24 fps, the 1/50 second fender opening angle is 172.8 degrees. Strobe is caused by the difference between the shutter frequency and the strobe light frequency, which makes the brightness of each frame different. Therefore, digital cameras will also encounter such problems. When lighting with incandescent lamps, tungsten lamps, etc., there is no such stroboscopic problem.
We discussed some concepts of flicker above, but what are the specific parameters of flicker? If we don’t know quantification of flicker, we can’t judge how the flicker affects a light source.
The flicker factor represents the flicker content in the light source. It computed by the maximum-to-minimum ratio of the illuminance. A flicker factor is 0% shows the light source is an absolutely constant flicker-free output. On the other hand, if the flicker factor is 100%, that means the light extinguishes entirely at its minimum.
Where E represents light power.
Also known as flicker percent, is a relative measure of the cyclic variation in the amplitude of light. The lower the percent flicker, the less substantial the flicker.
A measure of the cyclic variation taking into account the shape of the waveform, whose range is 0~1. The lower the flicker index, the less substantial the flicker.
Stroboscopic Effect Visibility Measure. Switch a light waveform from the time domain to the frequency domain by Fourier Transform. SVM shows the summation of weighted Fourier components.
SVM < 1: Flicker is not visible
SVM = 1: Flicker is just visible
SVM >1: Flicker is visible
See here (https://www.uprtek.com/en/faq/What-is-Flicker-Metrics-Percent-Flicker-Flicker-Index-SVM-/FAQ-Knowledge-015.html) for more information.
How to eliminate flicker
As for the PWM dimmer, since we have known the reason cause flicker, thus we can figure out a method to avoid flicker occurring.
The main two parameters of PWM is duty and carrier frequency. Duty determines the value of emitted light, frequency determines the times of brightness and darkness happen in a period.
See the figure below, it shows a PWM wave with duty = 80% and frequency = 1Hz. What does that mean? For dimming a LED, it represents the LED emits 80% of its maximum luminance, and for each second, bright interval and dark interval only emerge one time respectively. As we see, since the dark interval causes flashing, and this dark interval at 1Hz is quite wide, thus we may get a light with severe flicker.
But what if we arise the frequency to 10Hz? As the figure below shows, we could see that in the same 1 second period, bright interval and dark interval emerge 10 times. The total time of the bright and dark have not changed, but for each time of bright or dark emerge, it is shorter than the 1Hz case. You could image that arising frequency just like you cut the bright or dark interval in many small pieces. This will make the flash happening in a shorter time interval, thus the flicker is eliminated.
Actually, 10Hz is still not enough for flicker-free LED dimming, here we only want to show you the concept of eliminating the flicker. Besides, we should notice that here we only show you the case for PWM period is 1 second. For the actual situation, we should adjust the frequency based on the frame time of the camera or other implements.