It's complicated - multiple factors may affect human health
There are many potential sources of flicker on LED screens, including:
flicker of the screen backlight or OLEDs (may include pulse with modification, PWM)
flicker of colors introduced to expand the color palette by application software, the operating system, or the graphics processing unit (temporal dithering)
flicker of colors introduced by some monitors to simulate rendering of a larger color palette (frame rate control, a type of temporal dithering)
screen-wide color flicker due to LCD pixel inversion
forms of full-screen flicker deliberately introduced to reduce motion blur effects (strobe backlight flash or black frame insertion)
flicker in videos or animations
flicker associated with the screen refresh rate
flicker due to electrical interference
In addition to flicker, sensitive individuals may also experience adverse health effects when using screens due to:
People with concussion, dry eye, migraine, or other conditions with photophobia experience pain and/or a worsening of symptoms due to ordinary levels of light. Some of these patients are sensitive to light brightness, while some may be particularly sensitive to only artificial lights. Migraine patients, who tend to be extremely sensitive to light brightness during a migraine, also tend to be somewhat more sensitive to very bright lights between migraines than the general population, suggesting that some individuals might be sensitive to screen brightness.
When a sensitive individual is bothered by a screen, it's difficult to figure out which of these potential triggers are present and which are actually triggering adverse health effects. The difficulty arises because:
There are many potential triggers, making it difficult to know which of them are triggering health effects, either individually or in combination.
Some of the effects aren't obviously visible. Detecting them may require a sophisticated high-speed camera and/or microscope.
Some of these effects may be encoded by proprietary software, so there isn't publicly-available documentation acknowledging that they exist,
There may not be a way to turn an effect off, so there's no way to test whether eliminating it may solve the problem.
I am not a computer expert, but have assembled some of the sources I've found that explain these issues below. I refer readers to LEDStrain.org, a forum where sensitive individuals are seeking solutions and many contributors have significant tech savvy for discussions of specific devices.
Flicker in, flicker out
When images, videos, or animations are displayed on screens, the original content may be first processed by application software, system software, and the graphics processing unit before it is passed on to the monitor where it will be processed further. Flicker may be introduced at any point along this path. Once the flicker is incorporated, there isn't a way to remove it and it will be displayed on the screen.
LCD screen backlights may flicker
How do LCD screens create images?
In color liquid crystal display (LCD) screens, white backlights shine through red, green, and blue filters in the subpixels of each pixel. A combination of polarizing light filters and the application of voltage to control the shape of liquid crystals controls how much light is able to pass through each subpixel. There are several varieties of LCD monitors that work using this general principle, although they vary in terms of the nature of the light source or the strategies for combining polarizing filters and liquid crystals to control light transmission.
This video uses a fantastic cardboard model to demonstrate the roles of polarizing filters and the untwisting of liquid crystals as voltage is applied to subpixels in an LCD monitor.
Backlight flicker: Pulse width modification (PWM)
In the past, LCD monitors and televisions were backlit by cold cathode fluorescent lamps (CCFLs), but LCD screens backlit by light-emitting diodes (LEDs) had become common in the marketplace by about 2010 (TFT Central: LED Backlighting, 2010). While CCFL monitors tend to have very little backlight flicker at full brightness, when dimmed they generally flicker the backlight off about 175 times per second (at 175 Hz) in order to create the illusion that the backlight is dimmed. To create a dimmer screen, the length of time that the backlight is off is lengthened, a dimming strategy called pulsed-width modification (PWM; see description in Background: LED Lights). This works because the nervous system tends to average the light brightness or darkness that it senses when there is rapid flicker (see discussion of the Talbot-Plateau Law in Background: LED Lights). Therefore, PWM is a dimming strategy that works by creating flicker.
LED LCD monitors also often use PWM. In addition to PWM being a dimming strategy, for LED monitors, PWM is also a strategy implemented at full brightness to avoid overheating that occurs when constant current is applied to LEDs. The PWM flicker of LEDs also tends to be more bothersome to sensitive people than the PWM flicker of CCFLs because there is a sharper, more distinct difference in the on and off states for LED PWM flicker than for CCFL PWM flicker. This happens because LEDs turn off immediately when the current supplied to them is stopped. However, white fluorescent lights work by generating high energy light that strikes a phosphorescent coating on the glass that then fluoresces red, green, and blue, creating white light. Fluorescent lights have an afterglow because the phosphorescence takes a little time to fade after the current goes off. The result is a gentler slope in the waveform for CCFL PWM flicker than for LED PWM flicker (TFT Central: Pulse Width Modification, 2015).
Interestingly, for CCFL backlights the timing of the flicker of the blue channel is out of sync with the flicker of the red and green channels, creating color flicker, as well as further smoothing brightness flicker. However for LED backlights, the flicker of the three color channels are the same as each other, so the color temperature doesn't flicker (TFT Central: Pulse Width Modification, 2015). It would be interesting to know what, if any, impact the asynchrony of the flicker of the CCFL color channels has on the nervous system's sensing of the flicker. Note that the differential color flicker from CCFL screen backlights is consistent with earlier observations that a specific type of blue-blocking lens, FL-41, may reduce the perception of flicker specifically from fluorescent lights because fluorescent lights have more blue light flicker than the flicker of other colors (Wilkins & Wilkinson, 1991).
This article provides very good illustrations and a clear, thorough comparison of PWM backlight flicker for CCFL LCD monitors and LED LCD monitors, including data on the flicker of the separate red, green, and blue light channels and data for different brightness levels of sample monitors.
NotebookCheck includes PWM measurements in their evaluations of laptops, tablets, and phones.
Flicker as a strategy to reduce motion blur on LCD screens
Screen motion blur refers to a phenomenon where an image moving across a screen appears to be blurry when tracked by the human eye (or a specialized pursuit camera) because the screen refresh rate is too slow for the eye (or camera) to perceive the motion as smooth. The blur partly results from a lag time for liquid crystal molecules to change shape in response to a change in voltage, but this is less significant than a slow refresh rate. Screen motion blur is considered to be a problem for gaming, virtual reality, and augmented reality because it is hypothesized to trigger nausea, vomiting, headaches, and eyestrain (see Background: Health Effects: More Screen Sensitivity Literature and Anecdotal Reports of LED Sensitivity). A strategy for reducing the motion blur is to strobe the backlight on very briefly each frame after the monitor has finished refreshing the pixels. Alternatively, black frames can be interspersed between the normal image frames, creating a similar effect. Either the use of a strobing backlight or black frame insertion creates flicker. BlurBusters.com mentions that some of their readers are more bothered by motion blur, some are more bothered by the flicker created with motion blur reduction strategies, and some are bothered by both.
OLED screens do not have backlights. Instead, they work because the red, green, and blue subpixels are LED lights. To create different colors, the brightness of each subpixel is controlled using pulse width modification (PWM) - see an explanation of PWM above and in Background: LED Lights.
Explains how and why OLEDs flicker using PWM. Also provides some examples of flicker frequency for a few OLED phones.
NotebookCheck includes PWM measurements in their evaluations of laptops, tablet, and phones.
How does a monitor create colors?
The color that we perceive from each pixel on a screen is a combination of the amount of the red, green, and blue light produced in its subpixels. The RGB system for creating colors on screens works by specifying a value between 0 and 255 for each color, red, green, and blue. This means that there are 256, or 2 to the 8th power, possible values for each color and it therefore takes 8 bits of computer memory to encode the value of each color. This is called "8-bit" color or sometimes "24-bit" to reference the sum of all 3 color channels. In some circumstances, a fourth "alpha" channel may be encoded in addition to the RGB values and may then be referred to as "32-bit" color.
The 8-bit color system can produce 16,777,216 different colors because 256 red values x 256 green values x 256 blue values = 16,777,216 possible RGB combinations.
Temporal dithering: Flickering colors to expand the color palette and smooth gradients
If two different colors alternately flicker rapidly enough, we think we see the single blended color. This phenomenon was first demonstrated by Joseph Plateau in 1829 using sectors painted different primary colors on a spinning disk to create the appearance of blended, intermediate secondary colors (Plateau, 1829). Similarly, computer software, either in applications, in system software, or in the graphics processing unit, may cause pixels to flicker between two different colors in order to create the appearance of the blended, intermediate color. This "temporal dithering" may be done in order to create the appearance of smooth color gradients in photos and graphics. It may also create the appearance of smooth edges. Temporal dithering might be employed to create a highlight color overlay rather than calculating alternate colors. If isn't clear whether various kinds of software that shift the color temperature throughout a screen use temporal dithering to create the new colors. Because software is usually proprietary and temporal dithering flicker tends to not be obviously visible, it isn't obvious when temporal dithering is used. However, without temporal dithering, color gradients tend to show more discrete color banding.
Frame rate control
As described above, an 8-bit monitor can produce 16,777,216 different colors. However, not all monitors are capable of actually creating 8-bit color and instead use a system of 6-bit color and then simulate the remaining colors by alternately flickering between other colors each frame to create the appearance of the intermediate colors. This is a form of temporal dithering that is introduced by the monitor, rather than by the computer operating system or applications, and is called frame rate control (FRC). The vast majority of the colors will flicker because a 6-bit system can only display 262,144 colors without using FRC flicker. A monitor that is marketed as having 8-bit color might either have "true" 8 bit color that doesn't use FRC flicker or might actually be a "6-bit + FRC" monitor that uses flicker.
Recently, monitors with "10-bit" color that allows for 1 billion colors have become available. These have the advantage of having smoother color gradients compared to 8-bit monitors. Depending on the monitor, FRC may or may not be utilized to create part of the advertised color pallet. Monitors with true 10-bit color are more expensive than 8-bit + FRC monitors.
Frame rate control creates flicker at half the refresh rate of the monitor. So if the monitor refresh rate is 60 Hz, the FRC flicker will be at 30 Hz, and therefore definitely in what is considered the obviously visible range, even though most people may be able to ignore it.
Wikipedia: Frame Rate Control (Warning - this site includes a visibly flickering graphic, which may bother sensitive individuals)
Plateau, J. Dissertation sur quelques propriétés des impressions produites par la lumière sur l'organe de la vue. Université de Liège (1829) http://hdl.handle.net/2268/501
LCD pixel inversion
As described above, in LCD screens the amount of light passing through a subpixel is controlled by changing the shape of liquid crystal molecules by applying a voltage. To prevent damage to the pixel that could result from long-term application of voltage, the polarity of the voltage switches between positive and negative each frame. There may be slight differences in the pixel appearance in these two states, resulting in flicker. A monitor with a 60 Hz refresh rate would have 30 Hz flicker from pixel inversion, which is in the visible range. The amount of flicker from LCD pixel inversion varies among monitors.
The Lagom LCD Monitor Test Pages: Inversion (pixel-walk) Warning: some of the animations may bother individuals sensitive to flicker.
Flicker in videos or animations
Filming videos at a frame rate incompatible with ambient light flicker
Videos recorded in flickering ambient light should be recorded at a frame rate that is a factor of the ambient light flicker frequency so that the flicker doesn't become visible in the video. For example, in North America, a smartphone video filmed at 30 frames per second (fps) will not show the flicker of ambient light that has a frequency of 120 Hz. This is because the ambient light flicker is in sync with the video frame rate and each video frame would be equally illuminated (120 divided by 30 is a whole number - each video frame receives exactly 4 cycles of light flicker). However, a video at 25 fps would have visible flicker because 120 Hz light flicker would be out of sync with the video frame rate and some frames would be brighter than others (120 divided by 25 is not a whole number). Such a video would have visible flicker.
Such asynchrony between ambient light flicker and video frame rate is often apparent as visibly flashing ambient light in Zoom video feeds, at least in the U.S. (personal observation; also supported by a report that Zoom video uses 25 fps that expresses concern that visible flicker may be present in Zoom videos in countries that use 60 Hz power, as in North America). Rotating ceiling fan blades also visibly flicker in Zoom video feeds (personal observation) due to the interaction of the fan's rotational speed and video frame rate. Such flicker from fans is visible even if there is not noticeable ambient light flicker.
The flicker of ambient light is also visible in some of the videos of some YouTube vloggers and sometimes even in professional videos. I have personally observed this in some scenes of television programs filmed in the past few years where a lamp or other indoor lighting obviously flickers on the order of several times a second.
Explains frame rate and light flicker compatibility in videos.
Low light webcam video
When ambient light is relatively low, a webcam video feed tends to appear grainy and the colors of those grains in the video visibly flicker. This can occur in Zoom video feeds. This flicker can still be present when there is more ambient light, although it becomes less obvious. Such grainy video flicker was sometimes present in remote video feeds on professional news programs, particularly early in the COVID-19 pandemic when videos of remote guests were obtained with less than optimal equipment.
There are multiple examples of ways that visible flicker may be used in animations:
Computer-created special effects in movies sometimes introduce flashing lights or flashing graphics
Animated cartoons may have visibly flickering images that are deliberately introduced as visual effects.
Computer, tablet, and phone software might animate part of the screen in flashing waves to indicate loading content. Some images on the edge of a window may repeatedly flash during scrolling. There are other ways that devices use flashing images as well.
Flashing video frames
I have personally noticed that some, but not most, videos created for television have rapid flicker throughout the entire video. I first noticed this in some, but not all, animated cartoons, but have since noticed it in a live-action video as well. These cartoons (or other videos) have relatively few distinct individual images in the animation, but have intermediate frames where the next image is partially produced as a transparent overlay on top of the previous image, which I assume is a strategy to smooth an otherwise choppy video. This creates flicker of the colors in the changing area of the screen - from the original color, briefly to an intermediate color, and then to the final color. This pattern of color flicker can repeat many times as an object moves (see Self-tests).
Screen refresh rate creates flicker
The higher the monitor refresh rate, the more times the image is updated per second. The flicker of the screen refreshing is more obvious when the refresh rate is lower. For example, a 30 Hz refresh rate will result in obvious flicker. 60 Hz is a common screen refresh rate, although higher refresh rates are becoming increasingly common. Additional flickering effects may be created if the computer's refresh rate setting is incompatible with the monitor.
Screen flicker from electrical interference
Especially if there is visible screen flicker on a television, projector, or on an external computer monitor, there may be a loose cable, faulty connection within the computer, faulty port, or electrical interference from other nearby devices. Ferrite cores included on some VGA cables are designed to mitigate interference to an analog signal.