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Lamp Life
Lamps are rated as to their average life in hours. Average life
rating means that 50% of a large number of lamps of a given specification
will have burned out by that time and 50% will still be operational. Typical
average lives for incandescent lamps range from 1000 to 3000 hours depending
upon the type.
How
Incandescent Lamps Work
The word incandescent means emitting visible
radiation from solid material as the result of being
heated. Figure 4.1 shows that an incandescent lamp
produces light from a length of heated wire. The
wire temperature can be as high as 2800 degrees
Celsius. The wire is called a filament and is
normally in the shape of a coil. The wire is made of
tungsten because of its very high melting point.
Figure 4.1. Incandescent lamp
operation.
Lamp Parts When
exposed to surrounding air, the heated filament
literally burns up and disintegrates in a short
time. This is solved by enclosing the filament in
a glass envelope or BULB. See Figure 4.2. Air is
removed from inside the bulb at the time of
manufacture to form a vacuum, or the bulb is
filled with an inert gas usually a mixture of
nitrogen and argon.
The LAMP CAP connects the lamp to the electrical
circuit made of aluminium or brass.
FILAMENT SUPPORTS are wires that hold the filament
in place - protect it during shipment and in
service.
LEAD WIRES are the internal wires that connect the
filament to the lamp cap.
BUTTON is the top of central glass stem which
holds the filament support wires.
HEAT DEFLECTING DISC (not shown) reduces the flow
of hot gases from around the filament into the
neck of the bulb protects the stem and cap -
usually only found in lamps of more than 250W.
PINCH is where the entry of the lead wires into
the glass bulb are sealed.
EXHAUST TUBE is connected to a hole in the pinch
and is where the air is removed and the bulb
filled with gas during manufacture.
FUSE is a protective device as part of the lead
wire in case an arc or short circuit develops
inside the lamp.
Figure 4.2. Incandescent lamp
parts.
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