Compact Fluorescent Lamp  Print page
 
Fig. 1   There are many kinds of compact fluorescent lamps available on the market.

A normal incandescent light bulb is very hot when in use and with a working lifetime of only 750 to 1000 hours on average. That is why they need to be replaced quite frequently. In contrast, because compact fluorescent lamps (CFL) have a much longer life and operate at a much lower temperature, they help us to save energy and do not need replacing often. Sounds good? Work on the following activity to discover more advantages of using CFL.


 
Activity: Comparing a compact fluorescent lamp and an ordinary incandescent lamp

How can a CFL operate at such a low temperature and save so much energy? The answer lies in its fluorescent operation which is quite different from that of an ordinary light bulb.

The operation of CFLs is very similar to that of fluorescent tubes. The advantage of a CFL is that it is compact and comparable in size to a regular light bulb; it can be used in most lighting fixtures.


Incandescent light bulb

The part of the light bulb that produces light is a long, thin and heavily coiled filament (usually made of tungsten), which is just a resistor coiled into a compact space. When you switch on the light bulb, current passes through the resistor and heats it up to a very high temperature, over 2000 oC. Because of this high temperature, the filament will glow and emit electromagnetic waves. A small part of the electromagnetic waves is visible light. Visible light is the part that is useful to us. At the same time, the filament also emits a large amount of invisible infrared radiation which carries away most of the energy from the light bulb. Infrared radiation can heat things, and that is why you feel very hot when you place your hand close to a light bulb. Light bulbs therefore have a low efficiency rate of about 5-10 % since most of their electrical energy is converted to heat instead of visible light.


Fluorescence

Fig. 2   In fluorescence, a material emits radiation with a wavelength different from that of the radiation it absorbs.
Fig. 3   A CFL tube has a phosphor inner coating, with the ballast at the base.
Fig. 4   The schematic of a typical CFL. Electrons are emitted at the electrode to excite the mercury atoms, which then emit ultraviolet radiation, causing the phosphor coating to glow by florescence.

The visible light emitted by a CFL does not come from material heating, but from a process called fluorescence. Fluorescence is a process in which radiation (electromagnetic waves) is emitted from a material (e.g., phosphor) at a wavelength different from that of the radiation absorbed. In the case of a fluorescent lamp, the wavelength the phosphors absorb is ultraviolet and the wavelength emitted is visible light. In the older models of fluorescent tubes, only a layer of single phosphor is used and the light produced is of a cool white colour. Nowadays a coating of three kinds of phosphor is applied inside the lamp cavity. When ultraviolet radiation is absorbed by the coating, each of the phosphors gives out visible light in one of the 3 colours: red, blue and green. With the right combination of the mixture of the three phosphors, the colour of visible light emitted can be made comfortable for human eyes.


Compact fluorescent lamps

For the fluorescence process to take place, electrical energy must be converted to ultraviolet radiation inside the CFL cavity. Inside a CFL cavity, there is a small amount of mercury vapour under low pressure in an inert gas environment, usually argon. A CFL has two electrodes which are small filaments at each end of the cavity, and a device called ballast to control the applied voltage between the electrode filaments.

During the operation of a CFL, a current is passed through the electrode filament to heat it up, making it easier for electrons to leave the filament. A voltage, controlled by the ballast, is applied between the electrodes and, together with the low pressure inside the cavity, causes electrons to leave the hot filament of one electrode and accelerates along the cavity. The electrons collide with the mercury atoms, so energy is transferred to the mercury atoms and they become excited. Because of the electron arrangement in mercury atoms, these excited mercury atoms will give out energy mainly in the form of ultraviolet radiation. The phosphor coating then converts the ultraviolet radiation to visible light by florescence, completing the conversion process of electrical energy to visible light.

Two processes are involved in the operation of a CFL: 1) Electrical energy is converted to ultraviolet radiation when electrons collide and excite the mercury atoms, and 2) ultraviolet radiation is converted into visible light by florescence occurring on the phosphor coating. Because both processes have a high efficiency, CFL is much more energy efficient than incandescent light bulbs. Typically, CFLs consume only about 20 % of the power of a regular light bulb to produce the same brightness [1]. Compact fluorescent lamps are included in the Energy Efficiency Labelling Scheme. Registered compact fluorescent lamps bear a Recognition Type energy label.


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