Overview
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.
Fluorescent lamps were first introduced commercially in 1938.
Structure
Size
A standard fluorescent lamp is a cylindrical glass tube 1.5 inches in diameter
and 48 inches long (other sizes are available). A 2-pin metal base or cap
seals each end of the tube (see Fig. 1).
Contents
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. Sealed within the tube by the caps is a drop of mercury and a very
low-pressure inert gas (usually argon). A light-emitting chemical (see the
Operation section) called a phosphor coats the entire inside surface of the
glass.
Labels
Fluorescent lamps carry standardized labels outside that identify their
internal physical and electrical properties. 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).
Operation
Gas Discharge
STARTING. When the lamp is off, the mixture of mercury and gas inside
does not conduct electricity. 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.
EMITTING. When power is first applied, a 250- to 400-volt burst of
electricity vaporizes the mercury (see Fig. 1, left). Electrons in the mercury
atoms absorb energy and jump to "higher," more energetic orbits (Fig. 1,
middle). They then fall back to less energetic orbits (Fig.1, right). This
repeating process, called gas discharge, continuously emits the absorbed
energy as light. Once started, only about 175 volts are needed to maintain
this discharge in a 40-watt lamp.
VISIBILITY. When an applied voltage causes discharge in some lowpressure, inert gases, they emit visible light. Ionized neon gas emits red
light, for example, seen directly in a glowing neon bulb. 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. And almost all of the
mercury discharge is ultraviolet (UV) light, whose wavelength is too short for
human eyes to see.
Wavelength Conversion
The phosphor that coats the inside of the lamp tube converts the UV
mercury discharge into useful light that people can see.
The phosphor absorbs the invisible, short-wave UV emissions from the
excited mercury atoms (Fig. 1, right). It then emits other light with a longer
wavelength, almost all of which is visible.
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.
Efficiency
HEAT/LIGHT RATIO. All lamps convert current into visible light and heat.
Fluorescent lamps are about 2 to 4 times more efficient than incandescent
(glowing filament) lamps. For the same power, they produce 2 to 4 times
more light and less heat.
BULB LONGEVITY. Fluorescent lamps also have longer lifetimes. A typical
incandescent bulb lasts 1000 hours before the filament fails. But a typical
fluorescent lamp lasts 10,000 to 20,000 hours, depending on how often it is
started.