Exercise 5: Fluorescent Lamp (Segmented)
Description Case 5: Fluorescent Lamp
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A fluorescent lamp is a long
straight glass tube that glows
when a current passing through
low-pressure gas within the tube
causes a coating on the glass to
emit white light.
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Fluorescent lamps were first
introduced commercially in 1938.
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A standard fluorescent lamp is a
cylindrical glass tube 1.5 inches
in diameter and 48 inches long
(other sizes are available).
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A 2-pin metal base or cap seals
each end of the tube (see Fig. 1).
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Inside each end cap, attached to
the pins, is a filament or
electrode, a thin thread of wire
from which electrons boil when it
is heated by an electric current.
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Sealed within the tube by the
caps is a drop of mercury and a
very low-pressure inert gas
(usually argon).
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A light-emitting chemical (see
the Operation section) called a
phosphor coats the entire inside
surface of the glass.
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Fluorescent lamps carry
standardized labels outside that
identify their internal physical
and electrical properties.
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For example, a lamp with the
black characters
F40-T12
stenciled on one end is a
fluorescent (F) tube (T) that
uses 40 watts of power and has
a diameter of 12 eighths of an
inch (12/8 = 3/2 = 1.5 inch).
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STARTING. When the lamp is off,
the mixture of mercury and gas
inside does not conduct
electricity.
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So every fluorescent lamp is
attached to a starting device
called a ballast, which combines
* a "transformer" to produce an
initial, high-voltage burst,
and
* an "inductor" to limit
current flow while the lamp
is on.
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EMITTING. When power is first
applied, a 250- to 400-volt burst
of electricity vaporizes the
mercury (see Fig. 1, left).
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Electrons in the mercury atoms
absorb energy and jump to
"higher," more energetic orbits
(Fig. 1, middle).
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They then fall back to less
energetic orbits (Fig.1, right).
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This repeating process, called
gas discharge, continuously
emits the absorbed energy as
light.
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Once started, only about 175 volts
are needed to maintain this
discharge in a 40-watt lamp.
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VISIBILITY. When an applied
voltage causes discharge in some
low-pressure, inert gases, they
emit visible light.
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Ionized neon gas emits red
light, for example, seen
directly in a glowing neon bulb.
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But in a fluorescent lamp, the
discharge comes almost entirely
from the mercury vapor, even
though it is only 1 percent of
the enclosed gas.
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And almost all of the mercury
discharge is ultraviolet (UV)
light, whose wavelength is too
short for human eyes to see.
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The phosphor that coats the
inside of the lamp tube converts
the UV mercury discharge into
useful light that people can see.
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The phosphor absorbs the
invisible, short-wave UV
emissions from the excited
mercury atoms (Fig. 1, right).
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It then emits other light with
a longer wavelength, almost all
of which is visible.
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The chemical composition of the
phosphor lining the tube controls
the color of the visible light
emitted, which may be
* "cool white" (partly blue), or
* "warm white" (partly pink), or
* other visible colors.
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HEAT/LIGHT RATIO. All lamps
convert current into visible
light and heat.
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Fluorescent lamps are about 2 to
4 times more efficient than
incandescent (glowing filament)
lamps.
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For the same power, they produce
2 to 4 times more light and less
heat.
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BULB LONGEVITY. Fluorescent
lamps also have longer lifetimes.
A typical incandescent bulb lasts
1000 hours before the filament
fails.
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But a typical fluorescent lamp
lasts 10,000 to 20,000 hours,
depending on how often it is
started.
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