Two things called flicker
Visible flicker is a modulation slow enough for the eye to resolve directly — the flutter of a failing lamp, or a dimmed source that steps rather than glides. Invisible or non-obvious flicker is faster. It is not perceived as flashing, but it can appear as the stroboscopic effect: a moving object seems to jump between positions rather than travel smoothly, which is why it shows up around fans, tools, drills and anything that spins.
Both come from the same place. An LED responds almost instantly to the current through it, so any ripple in the current the driver supplies is reproduced in the light. A mains-frequency supply carries that ripple by nature; whether it reaches the light depends on how well the driver smooths it.
How it is measured
Two older measures describe the waveform directly: percent flicker, the depth of the modulation, and flicker index, which accounts for its shape as well. Newer measures describe the effect on a person instead — a short-term flicker indicator for visible flicker, and a stroboscopic visibility measure for the faster kind. European ecodesign rules for light sources set limits on both of these, which is the practical reason product data now quotes them.
A datasheet that quotes a flicker figure at all is telling you the manufacturer measured it. One that does not is not evidence of a problem, but it is a reasonable thing to ask about — particularly for a space where people work for long periods, or where the lighting will be dimmed.
Dimming is where it usually appears
Many lamps are quiet at full output and become far less so at twenty per cent. Dimming changes how the driver delivers current, and a driver that smooths well at full load may not at low load. Compatibility between the lamp, the driver and the dimmer is a system property: three components that are each fine on their own can flicker badly together.
- Test the actual lamp with the actual dimmer before ordering the whole quantity.
- Check behaviour at the bottom of the dimming range, not only at full output.
- Keep one control protocol per circuit rather than mixing dimming types.
- Point a phone camera at the lamp: a rolling band on the screen is a strong hint of modulation, though it is an indicator rather than a measurement.
Common mistakes
- Judging a lamp only at full brightness when it will spend most of its life dimmed.
- Assuming a dimmer works with an LED lamp because it worked with the halogen lamp it replaced.
- Buying the whole quantity before a sample has been tried on the actual control system.
- Treating a phone-camera test as a measurement rather than as a prompt to ask for data.