PWM – Is your phone making you sick? (smartphone guide)

19/12/2024
By Ray Shaw
In Featured Guides

PWM (Pulse Width Modulation) is a low-cost way to dim AMOLED or OLED phone screens. It results in low-frequency, imperceptible flickering that can cause nausea and headaches.

PWM is the smartphone industry’s dirty little secret. It severely affects over 10% of the global population, and several times that number suffers some effects. Why the industry has been allowed to get away with it for so long is a testament to the power that Apple and Samsung yield—two of the worst offenders.

What is PWM?

This is because smartphones can only issue ‘digital’ commands—on/off—to the screen. Digital is popular because it is cheap and reliable, which means better battery life.

Most AMOLED and OLED screens (there are some exceptions) use it to dim the screen. For example, if you want 30% brightness, it turns off the screen for 70% of the time. At 100% brightness, it will leave the screen on.

The opposite of Digital is Analogue, where you can continuously vary the screen backlight—there is no flicker. Compare it to a light bulb dimmer, where you vary the voltage to change screen brightness. Analogue is not really practical in a phone.

It is not generally an issue with LED screens where the backlight is DC dimmed.

It is a problem with cheap OLED TVs, including from some big brand names. PWM can also occur in 8-bit/16.7 million colour screens that use 8-bit+2FRC (Frame Rate Control) to perceive 10-bit/1.07 billion colours.

PWM effects

It depends on the flicker frequency (how many times a second), modulation depth (difference between maximum and minimum luminance), and display brightness. This academic research shows:

Visible Low-frequency PWM (3-80Hz) is almost visible as a strobing effect and can induce epileptic attacks and trigger neurological problems like autistic behaviour, ADHC (Attention-deficit/hyperactivity disorder) and the emergence of mental health issues.
Imperceptible PWM (80-300Hz) and the typical PWM of OLED smartphone screens can induce headache, eye strain, blurred vision, and impaired visual performance.
Mid-range (300-2000Hz) can result in less severe symptoms for PWM-sensitive people, but limit screen time.
High-range (2000-3000Hz) are more person-specific. It may be that they cannot look at the screen for a long time, get migraines, suffer anxiety, or general malaise.
At even higher frequencies, there is no perceptible flicker.

Many smartphone companies are conning you with the terms Flicker-free and DC-dimming

Many companies claim TUV certification for flicker-free, but the Catch22 is at 100% brightness. As brightness reduces, flicker increases.

Some claim DC Dimming, but the reality is that it decreases sharpness and colour accuracy at less than 60% brightness and can increase the propensity for burnt-in images. DC dimming may reduce flicker but not eliminate it.

What can you do?

PWM generally does not occur at full brightness, but who wants to wear sunglasses to read the screen?
PWM can reduce with the screen set to maximum refresh rate but at the expense of battery life.
PWM is more impactful at closer distances. Holding the phone over 30cm can reduce the effects.
PWM effects worsen with exposure time, so limit screen time on suspect devices.
Vibrant colours can exacerbate the risk. Dark Themes can reduce the risk if properly implemented.
Avoid screens that are 8-bit+2FRC
Use a smartphone video camera set to ISO 6400 and 1/8000 or higher shutter speed to look for flicker. It is not perfect, but it can help identify them.
Look at your environment. Ceiling lights, cheap TVs, computer monitors, and laptop screens can increase the PWM load.

Better still, do not buy the product. IPS LED screens are generally flicker-free

CyberShack’s view: PWM is here to stay as AMOLED/OLED takes over smartphone screens

PWM is a real issue, and for some, it is intolerable. Readers have long asked for PWM reports on the phones we review, and where we can, we will advise accordingly.

Our PWM testing was a Light Master 3, but this and the next version have been discontinued, and the software has been removed from Google Play. We are looking for an alternative, but these all seem oscilloscope-based and way out of our pay grade. We will continue to report on PWM on a yes/no basis using other methods to identify such.

Our beef is with the industry for not being upfront on PWM frequencies that would help PWM-sensitive users select other screen types. Apple iPhone, Google Pixel, and Samsung are the biggest offenders, all now using Samsung AMOLED, which, according to PWM-sensitive readers, is unusable.

You can’t see this but this is what an S24U Utra screen looks like at 25% brightnes.

PWM — You can’t see this but this is what an S24U Utra screen looks like at 25% brightnes.

Other companies, like OPPO, are starting to use 2160Hz PWM dimming (below 70 nits and DC dimming above that) and 10-bit OLED screens in premium devices that are much safer and more expensive.