What is Stroboscopic Effect? | Stroboscopic effect is the phenomenon which makes moving objects like fan blades to appear to be standstill, and a wave of the hand to appear as if it occurred in a series of jumps. This effect is noticed when objects are lit by gas-discharge lamps. The reason for this is that the light from these lamps is not continuously emitted but in spurts, with short intervals of no light, occurring 100 times a second when working on a 50 c/s supply. Any object lit up, therefore, becomes visible only at intervals a hundredth of a second apart.
The stroboscopic effect causes some unusual effects, and is annoying. The blades of a revolving fan may appear stationary at some speed and to move in one direction or the other if the speed is raised or lowered, although it is all the time moving at a high speed in one direction alone. A filament lamp, on the other hand, has thermal capacity because of which the variations in the light output are greatly evened out, and no stroboscopic effect is observed.
Methods to Reduce Stroboscopic Effect
The stroboscopic effect is less marked when a choke is used than with a resistance for the ballast. The luminescence of phosphorus powders persists for a short time and bridges the short intervals when no arc current flows. Therefore, fluorescent lamps exhibit fewer flickers than pure gas discharge lamps. Flicker is also more pronounced at 25 c/s than at 50 c/s and is almost completely absent at a higher frequency like 400 c/s.
If three fluorescent lamps can each be connected to a different phase on a 3-phase supply, the light peaks will occur at different times and no flicker will be noticed. For single phase supply, twin lamp circuits have been developed in which the phase angle between potential and current is altered in the two lamps. The usual power factor improvement condenser C is connected here in series with one lamp instead of across the mains.
In normal operation, a voltage higher than 230V is developed across the condenser even through a applied voltage is 230V. The chokes too are dissimilar, the inductance of the choke and the capacity being so chosen that the power factor of one lamp leads as much as it lags in the other. Because of this phase difference between the currents, flicker is greatly reduced.
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