Most LED flicker is invisible — the light pulses far faster than you can consciously see. Your visual system still processes it, and the result shows up as eye strain, headaches after a long evening, or a vague sense that a room is tiring to be in.
Because you cannot see it directly, you need a way to test for it. Two work, and both use things you already own.
The phone camera test
Open the camera, point it at the light, and look at the screen rather than the bulb.
A phone sensor reads the image line by line rather than all at once. If the light is pulsing, different lines are captured at different points in the pulse, and you see horizontal bands rolling across the preview.
Clean image — good driver. Strong banding — the bulb is flickering.
Try it in slow-motion video for a more sensitive result, and test at full brightness and dimmed, since many bulbs are clean at full output and bad when dimmed.
The pencil test
Wave a pencil back and forth quickly under the light and watch the pencil, not the light.
Under a steady source you see a continuous blur. Under a flickering source you see a series of discrete frozen images — a stroboscopic effect, the same thing that makes a spinning wheel appear to stand still.
This test catches lower-frequency flicker, which is the kind most likely to cause symptoms. It works better in an otherwise dark room.
What causes it
An LED chip responds to current essentially instantly. The mains supply alternates, so the job of turning that into steady light belongs entirely to the driver — the electronics in the base of the bulb.
A good driver smooths the supply into near-constant current. A cheap one lets the ripple through, and the light output follows the ripple.
Cheap bulbs flicker because the smoothing components are the easiest place to save money, and the resulting fault is invisible on a shop shelf.
| Situation | Typical cause |
|---|---|
| Flicker at full brightness | Poor driver smoothing |
| Flicker only when dimmed | Dimmer incompatibility |
| Flicker when another appliance starts | Voltage sag on the circuit |
| One bulb in a row flickers | That bulb's driver, or a loose connection |
| Faint glow when switched off | Leakage through a switch with a neon or a smart module |
What to look for on the box
The useful specification is flicker percentage or percent flicker, sometimes given as a flicker index. Lower is better; under 5% is comfortable and under 1% is excellent.
Some manufacturers publish a frequency instead. Above about 3 kHz, flicker stops producing measurable effects in most people even at higher depth, so a high-frequency driver is a good sign.
Most domestic bulbs publish neither. Two practical proxies:
- Bulbs sold for video and photography use almost always have very low flicker, because a flickering light ruins footage. They cost more and are worth it in a room you spend evenings in.
- Very cheap multipacks are where flicker concentrates, along with poor color rendering and loose color tolerance. It tends to be the same bulbs failing all three.
Where it matters most
Rooms you spend hours in after dark — living room, home office, bedroom reading light. Symptoms are cumulative, so exposure time matters more than intensity.
Anywhere with rotating machinery, including a workshop or a garage. Stroboscopic effect can make a spinning blade look stationary, which is a genuine hazard rather than a comfort issue.
Anywhere children do homework. Flicker sensitivity varies a lot between individuals, and headaches are easy to attribute to something else.
If it only flickers on a dimmer
That is a different problem with a different fix. The bulb may be fine and the pairing wrong — LED loads are small and behave nothing like the incandescent filaments most older dimmers were designed for.
Start with why LED lights flicker when dimmed, then check dimmer compatibility. Flicker is one of three specifications that separate a good bulb from a cheap one — the others are covered in what CRI you need.