Rural lighting
Practical Action
Methods of providing light
There are three main physical principles by which light may be produced:
incandescent
fluorescent
electroluminescence
The incandescent principle depends on heating a source to a temperature in the region of 2000,
4000 or 6000°C to obtain reddish, yellowish or white light, respectively. Typical examples are
candles and lamps utilising a bright flame (where white hot or incandescent particles of carbon in
the flame produce the light) and incandescent filament light bulbs where a fine coil of tungsten
wire is heated (in a vacuum or inert low pressure gas to prevent the filament oxidising or burning)
by an electric current passing through it. All incandescent light sources, whether flames or
electrically heated filaments, tend to produce more heat than light and are therefore relatively
inefficient in the rate of conversion of energy to light.
Fluorescence occurs under certain conditions when a
material can be made to glow with a 'cold light'.
Fluorescence is a phenomenon in which the atoms of a
gas, vapour or solid are excited in such a way that they
emit light. In some cases such as sodium and mercury
vapour discharge lamps (used commonly for street
lighting) vapour in a glass tube emits the light. In other
cases such as the commonly used fluorescent tube
lights, ultraviolet light, which is invisible to the eye, is
emitted by exciting mercury vapour atoms within the
tube, and this in turn causes a white translucent coating
in the tube to fluoresce with a whitish light. In other
words, the coating converts invisible ultraviolet light into
visible white (or near-white) light. Most fluorescent
processes involve some expenditure of energy, so they
are accompanied by the production of some heat. (Non-
electrical fluorescence is used where the mantles of
pressure lamps and gas lamps are heated and emit a
much brighter and whiter light than would occur simply Figure 2: A biogas powered lamp in
as a result of their temperature.)
Bandarawella, Sri Lanka. Biogas is
piped directly into the home and
White Light Emitting Diodes (WLEDs) produce
used for cooking and lighting. Photo:
electroluminescence when electrons recombine with
Practical Action / Zul.
holes releasing photons. The colour of the light depends
on the energy the photons have which is determined by the jump the electrons make in the
semiconductor.
The advantages of WLEDS over other
lighting sources include their very low
energy requirements, their long
working life, robustness and
reliability. WLEDs are becoming more
popular because of their low energy
demand although their availability is
still limited in many regions and they
are still relatively costly. Current high
performance WLEDs cost roughly
twice as much as regular LEDs and
more than incandescent bulbs.
Although WLEDs consume very little
energy compared to an incandescent
bulb, luminous efficiency (lumens /
2
Figure 3: A WLED lamp helps cook dinner, Nepal.
Photo: Practical Action / Rakesh Shrestha