Stroboscopic Effect in Fluorescent Lamps

A fluorescent lamp or fluorescent tube is a low-pressure mercury vapor gas discharge lamp that uses fluorescence effect to produce visible light. In the fluorescent lamp, an electric current is passed through the gas which excites the mercury vapor to produce UV rays, these UV rays strike a phosphor coating on the inside of the lamp and causes it to glow.

What is Stroboscopic Effect?

The phenomenon which causes running or moving objects to appear stationary or appear to be operating slower than their actual speed, is known as stroboscopic effect.

The stroboscopic effect produced by the fluorescent lamps is due to the periodic fluctuations in the light output of the lamp caused by the cyclic variations of the supply current in AC circuits.

Consider a fluorescent lamp provided with a 50 Hz AC supply. When the lamp operating under this frequency, the lamp will be extinguished 100 times per second, i.e., it becomes zero double of the supply frequency. But, due to the persistence of vision, human eyes do not notice the flickering of the lamp. However, if the light falls on a moving object, then due to illusion, the object may appear to be either running slow, or in reverse direction or even may appear stationary. This effect is termed as stroboscopic effect.

Stroboscopic effect creates multiple image appearance of moving objects and makes the movement of the objects appear jerky. The most important points to note the stroboscopic effects are −

• Stroboscopic effect is more pronounced at lower frequencies.

• The frequency of the flickers is double of the supply frequency.

Stroboscopic effect is very troublesome in the following situations −

• Stroboscopic effect creates problems in case of rotating machines whose frequency of rotation happens to be a multiple of flicker frequency, and the machines appear to decrease in speed of rotation or be stationary. Sometimes the machines may appear to rotate in the reverse direction.

• Stroboscopic effect also causes problems, when an operator has to move objects very quickly, these objects would appear to move with jerky motion which over a long period would produce visual fatigue.

Methods to Avoid Stroboscopic Effect

The stroboscopic effect is not allowed in industries as this mat lead to accidents. This is the main reason fluorescent lamps are usually not used in the industry.

The method for avoiding the stroboscopic effect in three-phase supply and single-phase supply are given as follows −

• Stroboscopic effect can be minimized by using three lamps on the separate phases of 3-phase supply.

• When three-phase supply is used in the industry, the adjacent fluorescent lamps should be fed with different phases so that the zero crossing of the two lamps will not be the same.

• Stroboscopic effect can be minimized by using a twin lamp circuit on a one-phase supply. In this case, the connection of two adjacent fluorescent lamps is made such that the two lamps are connected in parallel with the supply and in connection of the one lamp, a capacitor is connected in series with the choke, this makes a phase shift and hence eliminating the stroboscopic effect.