What flicker means on a screen
Flicker is a rapid change in brightness over time. If the changes are slow enough, you see them directly: a flashing light. If they are fast enough, the eye blends them into a steady glow, and you do not consciously notice. The trouble is that “fast enough” differs between people, between viewing conditions and between kinds of flicker. Flicker that is invisible to many people can still, for some, be linked with discomfort. The research on this is not settled, so claims about harm should be treated cautiously.
How PWM dimming works
To dim a light, you can reduce the current flowing through it, which lowers brightness smoothly. Another approach is pulse-width modulation, usually shortened to PWM. Instead of lowering the level, the light is switched fully on and fully off many times a second. The fraction of each cycle spent on, called the duty cycle, sets the average brightness. A light that is on for half of each cycle looks roughly half as bright as one that is steady, because the eye averages the pulses.
Switching is attractive to designers for practical reasons. It keeps the light’s color more consistent across brightness levels, since some light sources shift color when their current is lowered. It is also simple and inexpensive to control with digital circuits.
The downside is that the light is not actually steady. At full brightness, a PWM-driven light may be on all the time and show no flicker at all. As brightness is turned down, the on-time per cycle shrinks, and the effect becomes stronger. The switching rate, called the PWM frequency, matters too. A higher rate is generally easier on the eye, and a lower one more likely to be noticed. Makers choose a rate based on cost, design and other factors, and there is no single standard value.
Which screens use it
The answer depends on the design and brightness setting, and is not simply a matter of technology type.
LCDs. The backlight is the part that is dimmed. Some LCD backlights are dimmed with PWM and some with steady current control. The role of LCD backlights explains why the light source is separate from the pixels. Where PWM is used, the pixels themselves still change on their own schedule, so the flicker comes from the backlight.
OLEDs. Since each emitter produces its own light, dimming can be done by driving the emitters at a lower level, by switching them, or by a combination of both. Many OLED screens are reported to use switching at lower brightness levels, though it varies by product and settings.
Phones, laptops and monitors. Different models use different approaches, and some offer a setting that changes how dimming is done. Specifications seldom describe it clearly.
Mini LED and local dimming. Backlights divided into many zones can dim each zone separately, and their drive method may also involve switching. See mini LED for how zones are used.
Why people perceive it differently
Several things influence what you notice:
- Frequency. Very low rates can be seen as an obvious flicker. Higher rates may not be consciously visible but can still produce effects when your eyes move.
- Eye movement. When you move your eyes quickly, or look at a scene with fast motion, a pulsed light can leave a trail of separate images instead of a smooth blur. This is sometimes called the stroboscopic or phantom array effect. Some people see it, and some do not.
- Peripheral vision. The edges of vision tend to be more sensitive to flicker than the center.
- Brightness and surrounding light. Flicker is often more noticeable in a dark room, where the screen is the only light source, and at low brightness settings.
- Individual sensitivity. Some people report headaches, eye strain or fatigue around flickering screens; others report nothing at all. Because feelings of strain can come from many causes, including reading distance, posture, dry eyes and long hours, as eye comfort discusses, it is hard to know how much any one cause contributes. People with conditions such as migraine or photosensitive epilepsy may be advised by medical professionals to take extra care. Anyone with medical concerns should ask a qualified health professional rather than rely on general articles.
Other sources of flicker
Not all flicker is PWM. A flickering picture can also come from:
- A faulty or loose cable or connection, as the account of display connections suggests.
- Refresh rate mismatches between a source and screen, or problems with adaptive sync. See refresh rate and screen tearing for the surrounding ideas.
- Failing backlight or power components, which may cause erratic flashing and dimming.
- Artificial room lighting, which can flicker on its own and combine with the screen.
- Software settings such as changing power modes or auto brightness. The behavior of ambient light sensors can make brightness change in steps, which is not flicker but can look like it.
Before assuming PWM is the culprit, it is sensible to check these.
How people check
A few general approaches, none perfect:
- Phone camera test. Many cameras record rows of the image at slightly different times, so a pulsed screen may show dark bands or stripes in a photo or video. The pattern depends on the camera, so the presence of bands suggests flicker, but their absence does not prove there is none.
- Waving a pen or finger in front of the screen in a dark room. A trail of separate images instead of a smooth blur can hint at pulsed light. It is a rough test.
- Specialized measurement equipment can record light over time, but results depend on the tools and methods, and they are outside typical home use.
- Checking at several brightness levels. Because flicker tends to be worse when dimmed, testing at the brightness you actually use is more meaningful than at maximum.
What can be done
Some displays offer a setting that changes the dimming method, or advertise reduced flicker. Descriptions are not standardized, so claims are worth reading carefully. Other options are working at a higher brightness and dimming the room lights instead, using a dimmer screen setting in software rather than hardware, or choosing a different model. If you suspect it is causing discomfort, trying a different device for a while is a simple way to find out, and a conversation with an eye-care professional is sensible if problems continue.
Flicker is a real and measurable property, but how much it matters differs widely from person to person, and for many people it is not a concern at all.