Before starting the wave, first make a statement. Our "LCD Teaching" and "Anti OLED Alliance" can be collectively referred to as "Anti low-frequency PWM Alliance". We The objection is low frequency PWM dimming , rather than other display technologies.
We are not aiming at anyone. Whether it is LCD, OLED, or lamp, any display and lighting equipment that uses low-frequency PWM dimming is the object of our condemnation.
DC dimming and high-frequency PWM equipment are our allies. However, Samsung AMOLED, low-end notebooks and desktop displays, as well as a few but also flashing LG p-OLED screens (Pixel 2 XL: black question mark?), All of them are objects of our "anti low frequency PWM alliance". Love to play computer games Outline of the PWM science popularization:
The first half, mainly about the harm of flicker, the definition of PWM dimming and some pitfalls in the history of displays;
The second half is mainly about how to assess the harm degree of low-frequency PWM, personal detection methods of PWM and the survival guide for eye protection in the era of A-screen.
Seriously, if you usually play with your mobile phone and watch the monitor continuously for less than 1 hour, and you are still in a non sensitive constitution, then low-frequency PWM dimming will not have a great impact on you. However, the use of mobile phones and computers is too long. Users spend several hours every day, even more than 10 hours. For heavy users, it is necessary to consider the dimming mode of the screen.
PS： Personal works will inevitably have omissions. If you have any comments or suggestions, please go to the comment area for advice.
All evil stroboscopic
This time, the starting point of the story is higher, and the story starts directly before the display appears. The core of this popular science article is "stroboscopic". "The history of artificial lighting is the history of flashing" This is really not exaggerated. The moment when Tesla's AC power defeated Edison's DC power, it seemed that it decided the flicker history of human lighting. Because all light sources working under AC power supply will flash
50Hz AC
Since 1891, people began to use polyphase alternators on a large scale, and finally 50Hz (the frequency being used in China) and 60Hz AC power occupied the market. The frequency of AC determines that the flash frequency of most household fluorescent lamps (fluorescent lamps) from antique tungsten filament lamps to the present is 100 Hz, that is, 100 flashes per second (50 Hz sinusoidal AC has 50 peaks and troughs per second, and passes through zero point 100 times).
Strobe of different LED products
The LED light source that was once expected to have a very complex stroboscopic form because of different manufacturers' drive circuits. If the light up function is added, the stroboscopic types will be more and more complex. Of course, good products can not flash or flash very slightly. However, after the price of LED products plummeted, many manufacturers used simple driving circuits, which led to the deterioration of stroboscopic conditions.
Various low-frequency flickering light sources indoor and outdoor
Now I understand why the slow motion video shot indoors flashes. Because it's really flashing In addition to being seen flickering intentionally or unintentionally by humans, even the scanning gun of the supermarket will be affected by the flickering light and the recognition rate.
As defined in the technical report CIE TN006-2016 of the International Illumination Association, Scintillation can be divided into "flicker, flicker effect and phantom effect", The latter two refer to the physical effects of observing objects and moving observers respectively. In order to simplify the description, it is uniformly called flicker and stroboscopic.
Sensitivity curve of human eye to flicker frequency
The sensitivity of some different people to stroboscopic is very different, and even there is a saying of "flicker sensitive constitution". Human's most sensitive frequency is 8.8 Hz, and then it keeps dropping. Most people After 80Hz, no flicker can be seen, but even so, some people will feel tired and painful eyes. The 2015 IEEE Std 1789-2015 document of the Institute of Electrical and Electronics Engineers points out that lighting flicker can have potential adverse effects:
Photosensitive epilepsy or flicker induced seizures (0.1% of the population)
Migraine or severe headache, often accompanied by nausea and visual disturbance
Increase the repeated behaviors of people with autism
Visual impairment includes eye fatigue, fatigue and blurred vision
Baokemeng Impact Event
Now looking back at the most famous "stroboscopic accident" in modern times, there is an air of urban legend, and who would have thought that the protagonist was Baokemeng's Pok é mon... On December 16, 1997, the Japanese television station broadcast Baokemeng's 38th remark, "Computer Warrior Multilateral Beast (formerly translated as 3D Dragon)", in order to play up the battle in the computer world, A large number of 12Hz red and blue flashes were used to display the explosion scene, which directly led to 700 cases of epilepsy (650 children) in Japan. This low-frequency flicker, no matter what the display looks at, may lead to related symptoms (never felt that human beings are so fragile, so easy to roll over).
This event is called "Baokemeng Impact" in the history. It made Baokemeng, a cartoon that seems harmless to humans and animals and is loved by the world, enjoy the treatment of being ordered to stop broadcasting. It did not continue broadcasting from the 39th speech until April 16 of the next year. The 38 words were permanently banned on TV (and on the Internet). And the poor multilateral beast, the whole family has never appeared in the TV version again. Coincidentally, the London 2012 promotional trailer in 2007 and the Twilight: Breaking Dawn in 2011 have all had incidents of "audience epilepsy due to flickering color patches".
If you are interested, you can go to the video website to watch the 38th sentence of Baoke Dream, station B , the airborne position is 18min 51s. Although after digitization, The fineness, color saturation and irritation of the video are not as good as they were in those days, but we suggest you pay attention to safety before starting , put on the seat belt before driving (plus a scary nonsense: we didn't know we were sick before we got sick).
What is PWM?
Back to our main character PWM dimming. PWM， Its full name is Pulse Width Modulation, which translates to pulse width modulation. In the final analysis, it is a technology to modulate analog signals into pulse waves. It is already a brightness control scheme for widely used displays/light sources. In addition, we will mention DC dimming later (CCR constant current dimming in LED field, for convenience, this series of science popularization is uniformly called DC dimming).
Before we get to the point, we need to popularize the following two categories of signals:
Analog signal can have a series of values between 0-100% (close to stepless speed change).
Digital signal, characterized by "0 and 1". These two are Geek figures. After all, the entire human IT system is based on the control of logic circuits "0 and 1".
Although the cost of digital signal control is low, the most embarrassing aspect of digital signal control is that it only corresponds to "0" and "1" states, whether it is marked by high or low levels, or the power off and power on of the light source. For the problem of dimming mentioned this time, the control of digital signal can only generate two states: "on" and "off". The brightness is either 0 or 100%. It is doomed to fail to achieve the stepless brightness adjustment of analog signals.
Then why can our screen adjust the brightness steplessly? Because intelligent people find that the naked eye reaction speed is limited, and has brain tonic effect. As long as the switching speed of the light source is fast enough, the naked eye cannot detect it. PWM dimming is just like this The "carrier" is formed by flashing, and then the brightness can be controlled by controlling the time ratio (duty cycle) of "bright and bright":
In more than N flashes per second, if the required brightness is low, then increase the proportion of extinction time. For example, if you want to adjust the brightness to 10%, just let the proportion of light time reach 10%. Although it sounds like a chicken, PWM dimming has many advantages, such as simple structure, high precision, control of "brightness change" will not cause obvious color deviation, but also saves power and low heat. The eyes of human beings and even most of the creatures on the earth have evolved under continuous natural light. If the brightness is adjusted by flickering, which is a visual deception, will the car roll over?
Although it is mentioned above that most people cannot detect flicker above 80Hz, the residual light part can actually detect flicker at a higher frequency, The nervous system and cerebral cortex can detect 160Hz stimulation, and the retina is more sensitive to 200Hz flicker These have been proven to cause headaches, migraines and fatigue.
Review the definition of "stroboscopic phenomenon" given by the national standard (according to IEEE, this is actually the definition of "stroboscopic phenomenon"). Stroboscopic refers to the phenomenon that "when observing a continuous moving object under a flickering light source, the image that should be continuous appears discrete". When we read or move our eyes from the screen, because The flicker of hundreds of hertz can be detected by the naked eye due to the stroboscopic effect.
outside reading: Similarly, the most famous example of using the physiological characteristics of the human eye to "cheat" vision must be 24 frames per second movies. It can also be extended here. Why is 24 frames for a movie enough, but 60 frames for a game to start? If you are interested, you can poke our old science popularization 。
Human display history, various dark histories
In history, every change of display equipment has gone out of the pit. If you are old enough, you will surely remember those big and heavy CRT televisions and monitors in the past. In addition to the flicker of some pictures caused by the scanning speed of the cathode ray tube, are they particularly transparent in the impression? Especially gorgeous?
The LCD monitors that appeared later, except for their volume, were all products that were rubbed on the ground by CRT in all directions. The early desktop LCD has poor backlight, resulting in poor color gamut coverage, and the visual angle is a big hole. What's more, LCD was more expensive at that time, but it was small and handsome (the attitude of people at that time was like that of mobile phones with full screen. Handsome, you can really do whatever you want).
It took five or six years for the backlight of mainstream LCD to catch up, and the large-scale use of IPS panels, and the image quality performance barely caught up. During this period, the desktop LCD screen experienced image scaling and resolution growth. The picture scale ranges from 5:4, 4:3 to 16:10, and the final setting is 16:9. The resolution ranges from 1280 * 1024 to 1440 * 900, and the final resolution is 1920 * 1080.
In 2010, the backlight of LCD monitors experienced a transition from CCFL (cold cathode fluorescent lamp) to LED. CCFL was banned from production in many countries and regions in the following two or three years due to environmental protection, power consumption and other factors. then, For the first time, human beings felt the horror of being ruled by "LED+low-frequency PWM dimming". A large number of users found that new displays were more eyesore and tiring to look at, and even there was a problem of hoarding CCFL displays to live.
Fan stroboscopic method
If you are old enough, you should also remember that the test method of "waving, pencil and fan" was once popular in China. The CCFL backlight displays that have fallen into the history trash can, their PWM dimming frequency is almost 175Hz, but they have an obvious afterglow effect (they are still glowing when the PWM control is off), which greatly reduces the stroboscopic effect.
Later LED backlight displays continued to use low-frequency PWM dimming, and the frequency is usually between 180-420Hz. But because LED light source has a fast response speed, the afterglow effect is close to 0, and its stroboscopic effect is serious enough to produce "stroboscopic effect" (That is, the picture of moving objects can be fixed like high-speed photography).
Some display manufacturers seized the opportunity to use "non flashing screen" as a means of promotion, and began to use DC dimming or DC+high-frequency PWM dimming on a large scale. Up to now, a large part of monitors still use non flashing screens as propaganda.
The laptop field is sandwiched between the desktop and the mobile phone, which has always been sprayed without a good panel, and the backlight is also a pit. There are a large number of notebook markets Products with low frequency PWM dimming Some are PWM only at low brightness, and some are PWM all the time. A large number of new products of notebook manufacturers, even high-end products, are still using low-frequency PWM dimming (Jilao expressed strong condemnation).
Fortunately, The LCD screen of mobile phones has hardly been troubled by low-frequency PWM dimming. Even though the brightness of some mobile phone LCD screens may fluctuate, it is not at the same level as the low-frequency flicker of OLEDs (the next section of science popularization will explain the difference in detail). However, just like the large-scale use of PWM dimming for desktop displays in those days, Samsung and LG The low-frequency PWM dimming used by OLED manufacturers reminds people of the horror of being dominated by PWM
If it is a progress from 16:9 screen to 18:9 full screen, it is a regression from full screen to Liu Haiping. From the moment when OLED screen uses low-frequency PWM dimming, it has regressed to the pit of historical cycle.
In the second half, we will conduct science popularization on "how to evaluate the harm degree of low-frequency PWM, the method of personal PWM detection and the guide for eye protection and survival in the era of A-screen". Please wait.
reference material: IEEEstd 1789-2015、 International Illumination Society Report CIE TN006:2016, IEC TR 61547-1, GB/T 9473-2017 Performance Requirements for Reading and Writing Table Lamps, Taiwan Lighting Output Industry Association notebookcheck、TFT CENTRAL
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