Stroboscopic effect and its prevention

Stroboscopic effect is the illusion in which a rotating or moving object appears to be stationary, moving slowly, or moving backwards when lit by a light source whose output flickers at a regular rate. Discharge lamps (fluorescent, sodium, mercury) and poor LED drivers on 50 Hz supply give light that rises and falls 100 times per second. If a machine part rotates at a speed that is a multiple of this flicker, it looks still, which is dangerous in factories because workers may touch a running lathe chuck, fan or saw.

Prevention in factory lighting

1. Distribute lamps over three phases: adjacent luminaires (or the tubes of a twin/three-lamp fitting) are connected to R, Y and B phases. The light dips of each phase occur at different instants, so the total light is nearly steady.
2. Lead–lag (twin-tube) circuit: in a twin fluorescent fitting, one tube is fed through an inductive ballast (current lags by about 60°) and the other through a ballast with a series capacitor (current leads by about 60°), so their currents are about 120° apart and their light pulses do not coincide.
3. High-frequency electronic ballasts: operate lamps at 20–40 kHz, so the flicker is far too fast to cause the effect.
4. Flicker-free LED drivers: use constant-current drivers with low output ripple (low flicker percentage) instead of cheap capacitor drivers.
5. Local lighting with incandescent/halogen lamps on machines with rotating parts; the filament's thermal inertia smooths out the variation.
6. Mix of light sources in the same area so that the flicker of one is filled by another.