|Fig. 1 See the filament of the light bulb glowing.
If you take a close look at a light bulb, you will
see that the filament inside is in fact a very fine and heavily
coiled piece of wire. The filament is made of tungsten, a metal
with very high melting point. When a light bulb is switched on,
an electric current goes through the tungsten filament and heats
it up to over 2000 oC. At this temperature, the filament
will emit a large amount of radiation, about 90% of which is infrared
radiation and about 5 - 10% is visible light. As the human eye
is only sensitive to visible light, most of the electrical energy
input to the light bulb is wasted as infrared radiation and not
used for illumination.
Compact fluorescent lamps
|Fig. 2 A compact fluorescent lamp has phosphor coating on the inside of the glass tubes. The ballast helps control the electrons travelling inside the glass tubes.
The energy efficiency of a compact fluorescent
lamp (CFL) is much better than a light bulb. A CFL with brightness
similar to a light bulb typically consumes only 20% of the electrical
power ; this means
the power consumed by the light bulb can be used to power 5 compact
Why are CFLs so energy efficient? The reason is that a CFL does not convert electrical energy to visible light the same way as a light bulb, and as a result, much less infrared radiation is produced. This makes CFL much cooler during operation and also much more energy efficient. Look at Fig. 3 and the explanation below to find out how a CFL operates to produce visible light.
|Fig. 3 Inside the glass tube of a compact fluorescent lamp.
When a compact fluorescent lamp is switched on,
electrical energy causes electrons to travel along the glass tube.
The electrons collide with the mercury atoms sealed inside the
glass tube. This causes the mercury atoms to emit ultraviolet
radiation. The ultraviolet radiation is then converted to visible
light by a kind of material called phosphor. When you take a close
look at a compact fluorescent lamp like the one in Fig. 2, you
will observe a white coating inside each of the glass tubes. The
white coating is in fact the phosphor and it has the property
of absorbing ultraviolet radiation that shines on it and releasing
the energy in form of visible light.
See the EMSD website on Hong Kong Energy Efficiency Labelling Scheme for Compact Fluorescent Lamps.