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Incandescent
There are three main kinds of light bulbs-incandescent, fluorescent, and high-
intensity discharge. They are categorized by how they work. Other important
differences among light bulbs are shape, size, wattage (the measure of how
much electricity they use), color, and voltage (the pressure of electricity).

Incandescent
Incandescent light bulbs come in many shapes and sizes and are used in a
variety of household devices. Any sealed glass bulb with fine wire inside could be
called incandescent. The wire is called a filament. Electricity heats the filament to
generate light. Finding just the right thing to use for the filament was the hardest
part of developing the light bulb. Thomas Edison tried hundreds of materials
including bamboo, silver, and thread. (Lewis Latimer developed a carbon
filament, which Edison used.) When the light is switched on, an electric current
heats up the filament, which glows when hot. Incandescent lights come in many
different coatings and colors. The most common shape for an incandescent light
is the one shown here.
Incandescent lights include globe, tubular, and reflector bulbs. Globe-shaped
bulbs are often used in a row around a bathroom mirror. Tubular bulbs are seen
in terrariums, piano lights, and green-shaded banker's desk lamps. Reflector
bulbs, used indoors for floodlighting, direct their light into a wide beam instead of
glowing evenly in all directions as most bulbs do. Parabolic aluminized reflector (PAR) bulbs direct their light in a more narrow beam for spot lighting and
floodlighting objects or areas both indoors and outdoors. They are similar to
automotive headlights and are made with a special hard glass that resists
breakage when they come in contact with water. Halogen lights are advanced
incandescent bulbs containing a tiny filament in a tube. Halogen lights are
advances incandescent bulbs containing a tiny filament in a tube. Halogen bulbs
are extremely energy-efficient and either produce more light or last longer than
regular incandescent lamps do. Their most common shape is the PAR shape,
and they are very popular in the retail stores you would find at a mall.
Fluorescent
Fluorescent light bulbs are long, thin glass tubes that are coated on the inside
with a white powder called "phosphor." In order to turn them on, you must
connect them to a device called a ballast. Most ballasts are about the size of a
brick and they're hidden inside the lighting fixture, usually behind a piece of sheet
metal. When you flip a switch to turn on the lights, what you are actually doing is
turning on the ballast. The ballast "turns on" the fluorescent lamp by passing
electricity through the tube. The electrical current causes the gas inside the lamp
to give off ultraviolet energy. The ultraviolet energy hits the phosphor and gets
converted to light. Fluorescent lamps use less electricity than incandescent
lamps and they are less expensive to operate. You can find them in the ceilings
of many schools, offices, stores, and large buildings.
You can now find small compact fluorescent light bulbs (CFLs) that can be used
in place of incandescent bulbs in table lamps at home. They have a very small
ballast built into the bulb and screw into the socket of the table lamp. A CFL uses
less electricity than an incandescent bulb that provides the same amount of light,
so they cost less to operate and last up to 13 times longer. HID
The powerful high-intensity discharge (HID) bulbs are used outdoors as street
and security lights and in large buildings, such as warehouses and sporting
arenas. These bulbs contain a small cylinder called an arc tube that is filled with
gases. Like the fluorescent lights, a ballast makes electricity flow through the
cylinder. This electric arc makes gases in the cylinder glow brightly. The HID bulb
is the most energy-efficient, and brightest bulb of the three types.