Flicker and PWM Test

The Flicker and PWM Test can't detect PWM flicker directly — no browser can read your monitor's backlight-dimming hardware — so instead it walks you through the two methods that actually work: the pencil test and the phone-camera test. Click Start Test to fill your screen with a mid-grey field, lower your monitor's brightness below where you'd normally run it, then wave a pencil in front of the field or record it with your phone's camera and watch for banding or flicker in what you see. PWM usually only shows up below a certain brightness threshold, so if nothing turns up at 50% brightness, keep dropping it before you rule your display out. Run the free ghosting test for a few seconds under normal room lighting for the most reliable result.

Click Start Test to fill the screen with a mid-grey field, then try both methods below at a few different brightness levels.

Pencil test: hold a pencil a few inches in front of the grey field under normal room light and wave it side to side quickly. A smooth, single blur means no visible PWM at this brightness. A series of separated, stroboscopic "multiple pencil" images means the backlight is pulsing fast enough for your eye to catch.

Phone-camera test: open your phone's camera, point it at the grey field, and slowly drag your finger across the screen while recording. Horizontal dark bands scrolling through the video are a strong sign of PWM — this works because most phone camera sensors scan line by line faster than the backlight is pulsing.

Brightness threshold: many displays only use PWM dimming below roughly 50% brightness, switching to a steadier method (DC dimming) above that. Test at 100%, 50%, and 20% brightness and note where flicker first appears — that's the threshold to avoid if you're sensitive to it.

Ever finished a lengthy session feeling like your eyes have been sandpapered from the inside? The culprit is often invisible — a rapid on-off pulsing of your panel's illumination source that your brain registers as fatigue long before your eyes consciously notice the flicker. This Flicker and PWM Test gives you clear, actionable evidence of whether your display is flickering and at what intensity, so you can make an informed decision about your monitor, laptop, or phone before eye health deteriorates further.

What Is PWM Dimming and Why Does the Flicker and PWM Test Reveal Eye Strain?

Pulse Width Modulation (PWM) is the brightness-reduction method used by a large proportion of LCD and OLED units on the market. Rather than reducing the actual electrical current flowing through the light source — which is what DC dimming does — a PWM-based unit achieves lower brightness levels by switching the illumination on and off at a fixed frequency, often hundreds of times per second. At 100% brightness most panels use continuous DC dimming, so no flicker occurs. Drop the brightness to 50–75% brightness or below, and the PWM circuit kicks in, cycling the light source at the panel's designated pulse rate. Use the ips glow test to confirm whether your monitor shows this issue before you adjust settings or return it.

The human visual system cannot consciously track individual flashes when the flicker rate rises above roughly 60–100 Hz, but the brain still processes the alternating light levels subconsciously. This covert processing is the root cause of eye strain, head pain, blurry text, and eye fatigue during extended use. People with a history of migraines, photosensitivity, or visual stress disorders are disproportionately affected and can experience symptoms even at PWM frequencies well above what most users notice. Visual fatigue accumulates gradually, which is why many sufferers don't immediately connect their discomfort to their panel settings.

How PWM Frequency Affects Your Health and Comfort

Research within the calibration and ergonomics community generally treats frequencies below 1000Hz as the threshold where sensitivity to pulsed illumination becomes a genuine concern. At frequencies around 200–400 Hz — common in budget LCD units — the light-source cycling is slow enough for the visual cortex to partially resolve, dramatically increasing the risk of eye damage over time. Premium units marketed as a high-frequency monitor push PWM to 1000Hz or above, making the pulsing effectively imperceptible even to photosensitive individuals. The safest option, however, is a flicker free panel that uses continuous DC dimming across the entire brightness range, eliminating pulsed-light cycles altogether.

A key technical limitation worth understanding: a browser-based check can only generate on-screen variation up to half your panel's update rate because of how frames are rendered. A 60 Hz unit can produce at most 30 Hz of visible pattern change through software alone. This is why the on-screen animation functions as a reference pattern rather than a direct PWM measurement. To measure your panel's real pulse frequency, you need to record the output — using your phone camera — and observe whether dark horizontal bands appear in the viewfinder. Those bands are camera banding caused by the mismatch between your camera's capture interval and the light source cycling. A specialized photodiode tool or hardware oscilloscope would give the most precise reading, but phone recording is practical, accessible, and reliable for everyday checks.

PWM Dimming vs. DC Dimming: What the Test Tells You

Pulsed brightness control and DC dimming represent fundamentally different approaches to brightness control. A PWM-free display using DC dimming adjusts the current amplitude — meaning the light source is always on, just dimmer. This eliminates image flicker entirely and is the preferred method for users with eye health concerns, gamers who demand visual clarity, or anyone doing precision work such as photo editing or calibration. By contrast, a pulsed-illumination unit produces what is technically called screen flicker — a repeating light-dark cycle driven by the technology and its associated firmware. The flicker and pwm test here helps you identify which category your current screen falls into, with no specialist equipment and no software needed beyond a smartphone.

Run the Flicker and PWM Test: Step-by-Step Instructions for Any Screen

This tool offers two complementary motion assessments that together give a complete picture of your panel's PWM behaviour. Understanding which mode to use — and what to look for — makes the difference between a confident pass or fail result and a confusing outcome.

Choosing Between the Flicker Mode and Scrolling Line Mode

The scrolling line mode requires no camera at all. A bright sweeping stroke crosses the panel repeatedly. On a flicker-free unit using DC dimming, your eyes see one continuous, smooth blur as it passes. On a unit with active pulsed illumination, the stroke appears to split into several distinct ghost images spaced across the surface. This is because your visual system is seeing the element only during the brief moments the light source is on, creating a pattern of light positions rather than a single fluid sweep. This mode is best used as a quick first check and works at your panel's native update rate.

The flicker assessment mode generates a full-screen alternating pattern at a selectable frequency — this is the mode you use when you want to record the output with your smartphone and look for dark scrolling bands or strobing in the camera viewfinder. It is also useful for confirming results seen in the scrolling line mode.

Step-by-Step PWM Test Procedure

  1. Set your brightness level: Lower your panel brightness to between 30% and 75% — this is where pulsed dimming is most active. At 100% brightness, most units switch to DC dimming and show no flicker whatsoever.
  2. Choose your test mode: Start with the scrolling line mode for a camera-free result. Switch to flicker mode if you want to record the output.
  3. Enter fullscreen mode: Use the fullscreen button so the reference pattern fills your entire surface, maximising visibility of any banding or strobing.
  4. Open your phone camera: Point your smartphone at the panel. Use standard video mode or, for even clearer results, switch to slow-motion video (120fps or higher). A high-framerate camera resolves lower-frequency pulsing more clearly than standard 30 fps recording.
  5. Adjust your capture configuration: Disable auto-exposure to prevent the camera from compensating for the flickering light. On iPhone, tap and hold to lock AE/AF. On Android, open Pro mode and set a fixed exposure — a capture interval around 1/120s makes banding highly visible against a pulsing light source.
  6. Observe the viewfinder: Look for horizontal dark bands that scroll through the image. These scrolling bands confirm pulsed dimming is active. If the output appears uniformly lit with no banding, your unit is likely using DC dimming at this brightness level.
  7. Test at multiple brightness levels: Repeat at 30%, 50%, and 75% brightness. Some units activate pulsing only at very low illumination levels — testing across the range gives the full picture.

Worked Examples: Real-World Flicker Check Scenarios

Example 1 — Budget 60 Hz computer panel at 50% brightness: A user running a standard office PC points their smartphone at a budget LCD unit set to 50% brightness. In the camera viewfinder, three to four dark horizontal bands scroll steadily downward — a classic result caused by the unit's pulse frequency interacting with the camera's capture interval. Switching to scrolling line mode, the moving element breaks into four distinct ghost copies rather than a single smooth blur. Both results confirm that the PWM circuit is active. The light-source cycling is real, even though it was completely invisible to the naked eye. This is a failing result — the user should consider switching to a flicker free unit.

Example 2 — High-end OLED phone at low brightness: A user runs the PWM flicker test on a flagship self-emissive smartphone set to 20% brightness. Despite such units being commonly associated with pulsed dimming sensitivity, this particular device uses high-frequency cycling above 1000Hz, combined with DC dimming at low output levels. In the camera viewfinder: no banding, no strobing, uniform illumination across the entire frame. In scrolling line mode, the moving element produces a single consistent smooth blur with no ghost copies. This is a passing result — the technology is handling brightness reduction without visible light-source pulsing. Note that not all self-emissive units behave this way; always run the check on your specific device.

Example 3 — Laptop at maximum then reduced brightness: A user tests their laptop at 100% brightness and sees no banding in the camera viewfinder — the unit is in DC mode at maximum output. They then reduce brightness to 30%, re-run the check, and immediately see strobing lines appear in the viewfinder. The scrolling line mode similarly shows the element fragmenting into visible ghost copies. This illustrates perfectly why testing only at maximum brightness gives a false sense of security. The check should always be performed at the brightness level you actually use day-to-day. At reduced output, this laptop's pulsing is clearly active and potentially contributing to the user's afternoon discomfort during extended sessions.

Common Questions About Image Flicker, PWM Sensitivity, and Automatic Detection

Which test mode should I use — Flicker or Moving Line?

Start with the scrolling line mode. It requires no camera, works at your panel's native update rate, and produces an immediately readable result — either a single smooth blur (DC dimming, no flicker) or multiple ghost copies (pulsing active). If you want to record the output to confirm findings or share results, switch to flicker mode and use your phone camera with a fixed exposure setting enabled. The flicker assessment mode also allows you to vary the cycling rate to understand the range at which banding becomes visible on your specific unit. Run the subpixel test to check your display for this issue directly in your browser — no download required.

Is PWM harmful?

For most people, pulsed dimming at frequencies above 1000Hz poses no meaningful health risk. However, individuals with photosensitivity, a history of severe head pain, or light-sensitivity disorders may experience discomfort even at higher frequencies. The real concern lies with units operating below 1000Hz, particularly in the 100–400 Hz range common in budget units. Prolonged exposure during extended sessions can contribute to cumulative eye fatigue, tension, and in sensitive individuals, trigger episodes. Whether or not PWM causes permanent eye damage is still debated, but eliminating the risk by choosing a flicker-free or high-frequency unit is straightforward. It's not considered inherently dangerous for most users, but it is an avoidable stressor worth addressing.

My screen is always at maximum brightness — does PWM still apply?

At maximum brightness — 100% — the majority of units disable their PWM circuit and switch to continuous DC dimming, meaning no light-source cycling occurs. However, the moment you reduce brightness even slightly, many units reactivate pulsing. Always run the check at your actual working brightness, not at full output. The 50–75% brightness range is where pulsed dimming is most commonly and aggressively active.

Can this tool detect PWM automatically?

The most reliable method remains manual filming — pointing your phone camera at the panel and observing the viewfinder for scrolling dark bands. Automatic detection via a laptop webcam is experimentally possible but requires a high-framerate camera (120fps+), which most built-in webcams cannot achieve. A dedicated photodiode tool using a specialized photodiode delivers lab-grade accuracy and can measure pulse frequency with precision, but this is overkill for most users. The browser-based approach here runs in browser, requires no downloads, collects no data collected, and delivers reliable results using equipment everyone already owns — a smartphone. This tool uses photon capture via the phone camera rather than relying on any automatic webcam scanning, which makes results more dependable across different unit types.

What phone camera settings work best for the PWM flicker test?

Open your standard camera app — not portrait or bokeh mode. Disable auto-exposure so the camera doesn't compensate for the flickering light, which would mask the banding. On iPhone, tap the screen and hold to lock AE/AF. On Android, use Pro mode and set a fixed exposure with a capture interval of around 1/120s. Slower values make the dark bands wider and easier to read; faster values can make them too narrow to see. Slow-motion video (120fps or 240fps on compatible phones) is particularly effective for catching high-frequency pulsing. Avoid using a laptop webcam as your primary recording device — the frame rate is typically too low for reliable detection. A good exposure configuration will reveal banding clearly even on phones with smaller sensors.

Does the UFO test also show PWM flicker?

The UFO test and similar browser-based motion assessments are primarily designed to evaluate blur caused by pixel response times and update rates, not light-source pulsing. They reveal artefacts related to the unit itself rather than the PWM cycle. For specifically diagnosing pulsed-illumination issues and sensitivity to them, the dedicated flicker and pwm test here — combined with the phone filming method — is significantly more targeted and informative.

How to Find a PWM-Free Monitor or Phone After Your Flicker and PWM Test

Once your check confirms that your current unit is using pulsed dimming, the logical next step is finding a comfortable alternative that eliminates the problem entirely. Here is what to look for.

PWM-Free and Flicker-Free Certification Labels

The most trustworthy indicator of a genuinely pulse-free unit is the VESA certified Flicker-Free badge, which requires that a unit produce no detectable cycling at any brightness level using a calibrated photodiode measurement. Look for this certification in product listings — it is a hard specification, not a marketing claim. Some manufacturers use their own labels (such as TÜV Rheinland certification), which use similar photodiode tool testing and are equally reliable. A certified pulse-free unit uses continuous DC dimming throughout the entire brightness range, making it safe for users with sensitivity to pulsed light, those recovering from eye health issues, and anyone prioritising long-term ergonomics and visual comfort.

OLED Displays and PWM: What the Test May Reveal

Self-emissive units are frequently misunderstood in the context of pulsed dimming. Unlike LCD panels that use a separate light source, self-emissive pixels generate their own light. At lower brightness levels, many such units still use pulsed cycling to control output because current modulation alone can cause colour shift. However, premium models increasingly offer DC dimming modes or use extremely high-frequency cycling (above 1000Hz) that falls outside the range of human sensitivity. Always run the flicker assessment on any self-emissive device you are considering, especially at the brightness levels you use most frequently. The approach described above works identically on these units.

Practical Display Upgrade Checklist

  • Look for VESA certified Flicker-Free or equivalent third-party certification on the product spec sheet
  • Check whether the unit uses DC dimming across all brightness levels, not just at full output
  • For gaming units, confirm that any strobe feature — which reintroduces deliberate pulsing for motion clarity — can be disabled during everyday use
  • When evaluating a DC dimming phone, verify whether the DC dimming mode is a toggle in settings or a permanent design choice — some phones offer both pulsed and DC modes
  • Prioritise quality metrics beyond just pulsing: check native resolution, device pixel ratio, colour depth, and gamma accuracy to ensure the replacement is genuinely better across all parameters
  • Consider running a gamma check after any switch to verify that settings are calibrated correctly — a visually smooth gamma curve matters for accurate image rendering and proper calibration

Whether you are a gaming enthusiast pushing frames on a high-refresh LCD, a designer demanding colour-accurate output, or simply someone who wants to get through a long session without splitting head pain, understanding and acting on your pulsed-dimming results is one of the most impactful — and most overlooked — steps in building a genuinely comfortable viewing environment. Run the test, interpret the result, and if your unit fails, you now know exactly what to look for in its replacement.

Frequently Asked Questions

What phone camera settings work best for the PWM flicker test?
Use your phone's slow-motion mode at 120 fps or 240 fps for best results. Lower your display brightness before filming, as PWM is most pronounced at lower brightness levels. Avoid auto-exposure mode — lock the exposure manually so the camera doesn't compensate for the flicker automatically.
Which test mode should I use — Flicker or Moving Line?
The Moving Line test is better for detecting subtle flicker that's hard to see statically. The Flicker (fullscreen strobe) mode is best for confirming whether your eyes can perceive the frequency directly. Try both: if the moving line appears as a dotted or segmented trail, your monitor likely uses PWM.
My screen is always at maximum brightness — does PWM still apply?
At 100% brightness, many monitors disable PWM and use full DC power, meaning no flicker at all. PWM is most commonly engaged below 80% brightness to dim the backlight. However, some monitors use PWM even at full brightness — filming the screen at high camera fps is the only way to confirm.
Can this tool detect PWM automatically?
No browser-based tool can directly measure your monitor's PWM frequency — the display renders the test pattern and you film it with your phone to detect the flicker in the footage. The tool calculates the maximum detectable reference frequency based on your refresh rate (half the Hz), which you use as a guide when analyzing your camera footage.
Is PWM harmful to your eyes?
PWM flicker is not proven to cause permanent eye damage, but many users report eye strain, headaches, and fatigue after extended use of PWM-dimmed displays. Sensitivity varies widely between individuals. If you experience discomfort, keeping brightness high (above 80%), using a flicker-free monitor, or enabling DC dimming are effective solutions.
What PWM frequency is considered safe?
Generally, PWM frequencies above 1000 Hz are considered unlikely to cause perceptible discomfort for most users. Frequencies below 500 Hz — especially below 250 Hz — are where flicker sensitivity complaints are most common. OLED screens often use very high PWM frequencies (up to 60,000 Hz) which are imperceptible to the human eye.
What is the difference between PWM dimming and DC dimming?
PWM (Pulse Width Modulation) dimming rapidly switches the backlight on and off to simulate lower brightness — this switching causes flicker. DC dimming reduces the actual electrical current to the backlight, keeping it always-on at a lower intensity. DC dimming produces no flicker but can sometimes affect color accuracy at very low brightness levels.
Do OLED screens have PWM flicker?
Yes, most OLED displays use PWM for brightness control because they cannot use a separate backlight like LCDs. However, modern OLEDs often use very high-frequency PWM (above 30,000 Hz) which is imperceptible. Some flagship phones and monitors now offer a DC dimming option in settings to eliminate flicker entirely for sensitive users.