21 Fluorescent Lamps: Stroboscopic Effect

Fluorescent lamps supplied from an AC source and powered by core and coil ballasts will experience the stroboscopic effect. In a 60 hertz AC supply, the AC sine wave goes through the zero axis twice in a cycle, and so there are instances where the arc is temporarily extinguished and no light is produced.

This happens too quickly for the human eye to perceive and is not normally an issue.  However, in certain industrial settings which have rotating equipment, such as motor-driven saws or lathes, also powered by the 60 hertz AC supply, the rate of flickering of the lamps can sync up with the rotation of the equipment and make them appear to be at a standstill.

This is the same idea as using a strobe light to test the rotational speed of an engine. Injuries have occurred where a worker thought that a particular piece of machinery was de-energized, when in fact it was rotating at a similar rate to the flickering of the lamp.

This can be avoided in several ways. One method involves installing incandescent lamps, which do not experience any flickering as the filament doesn’t have time to cool down between alternations, and so puts out a steady stream of light. Alternatively fluorescent lamps can be installed on differing phases of a three-phase system so that they flicker at different times, or a lead-lag ballast can be installed to alternate which individual lamps flicker at what time. Electronic ballasts will also eliminate the stroboscopic effect because they operate in the 20kHz range.