see fluorescent lamps everywhere -- in offices, stores,
warehouses, street corners -- even in peoples' homes. But even
though they're all around us, these devices are a total
mystery to most people. Just what is going on inside those
In this edition of HowStuffWorks,
we'll find out how fluorescent lamps emit such a bright glow
without getting scalding hot like an ordinary light bulb.
We'll also find out why fluorescent lamps are more efficient
than incandescent lighting, and see how this technology is
used in other sorts of lamps.
Let There Be Light To understand fluorescent
lamps, it helps to know a little about light itself. Light is
a form of energy that can be released by an atom. It is
made up of many small particle-like packets that have energy
and momentum but no mass. These particles, called light
photons, are the most basic units of light. (For more
information, see How Light
Atoms release light photons when their electrons
become excited. If you've read How Atoms
Work, then you know electrons are the negatively charged
particles that move around an atom's nucleus (which has a net
positive charge). An atom's electrons have different levels of
energy, depending on several factors, including their speed
and distance from the nucleus. Electrons of different energy
levels occupy different orbitals. Generally speaking,
electrons with greater energy move in orbitals farther away
from the nucleus.
When an atom gains or loses energy, the change is expressed
by the movement of electrons. When something passes energy on
to an atom -- heat, for example -- an electron may be
temporarily boosted to a higher orbital (farther away
from the nucleus). The electron only holds this position for a
tiny fraction of a second; almost immediately, it is drawn
back toward the nucleus, to its original orbital. As it
returns to its original orbital, the electron releases the
extra energy in the form of a photon, in some cases a light
of the emitted light depends on how much energy is released,
which depends on the particular position of the electron.
Consequently, different sorts of atoms will release different
sorts of light photons. In other words, the color of
the light is determined by what kind of atom is excited.
This is the basic mechanism at work in nearly all light
sources. The main difference between these sources is the
process of exciting the atoms. In an incandescent light
source, such as an ordinary light bulb or gas
lamp, atoms are excited by heat; in a light
stick, atoms are excited by a chemical reaction.
Fluorescent lamps have one of the most elaborate systems for
exciting atoms, as we'll see in the next section.
Down the Tubes The central element in a
fluorescent lamp is a sealed glass tube. The tube
contains a small bit of mercury and an inert gas,
typically argon, kept under very low pressure. The tube
also contains a phosphor powder, coated along the
inside of the glass. The tube has two electrodes, one
at each end, which are wired to an electrical circuit. The
electrical circuit, which we'll examine later, is hooked up to
an alternating current (AC) supply
When you turn the lamp on, the current flows through the
electrical circuit to the electrodes. There is a considerable
voltage across the electrodes, so electrons will migrate
through the gas from one end of the tube to the other. This
energy changes some of the mercury in the tube from a
liquid to a gas. As electrons and charged atoms move through
the tube, some of them will collide with the gaseous
mercury atoms. These collisions excite the atoms, bumping
electrons up to higher energy levels. When the electrons
return to their original energy level, they release light
As we saw in the last section, the wavelength of a photon
is determined by the particular electron arrangement in the
atom. The electrons in mercury atoms are arranged in such a
way that they mostly release light photons in the
ultraviolet wavelength range. Our eyes don't register
ultraviolet photons, so this sort of light needs to be
converted into visible light to illuminate the lamp.
This is where the tube's phosphor powder coating comes in.
Phosphors are substances that give off light when they
are exposed to light. When a photon hits a phosphor atom, one
of the phosphor's electrons jumps to a higher energy level and
the atom heats up. When the electron falls back to its normal
level, it releases energy in the form of another photon. This
photon has less energy than the original photon, because some
energy was lost as heat. In a fluorescent lamp, the emitted
light is in the visible spectrum -- the phosphor gives off
white light we can see. Manufacturers can vary the
color of the light by using different combinations of
Conventional incandescent light bulbs also emit a good bit
of ultraviolet light, but they do not convert any of it to
visible light. Consequently, a lot of the energy used to power
an incandescent lamp is wasted. A fluorescent lamp puts this
invisible light to work, and so is more efficient.
Incandescent lamps also lose more energy through heat emission
than do fluorescent lamps. Overall, a typical fluorescent lamp
is four to six times more efficient than an incandescent lamp.
People generally use incandescent lights in the home, however,
since they emit a "warmer" light -- a light with more red and
As we've seen, the entire fluorescent lamp system depends
on an electrical current flowing through the gas in the glass
tube. In the next section, we'll see what a fluorescent lamp
needs to do to establish this current.
Cooking with Gas In the last section, we saw
that mercury atoms in a fluorescent lamp's glass tube are
excited by electrons flowing in an electrical current. This
electrical current is something like the current in an
ordinary wire, but it
passes through gas instead of through a solid. Gas
conductors differ from solid conductors in a number of
Fluorescent lamps are
just one lighting application of a gas discharge
lights are essentially fluorescent lamps without a
phosphor coating. They mostly emit ultraviolent light,
which causes phosphors outside of the lamp to emit
visible light (click
here to learn more).
Neon lights are gas discharge lamps containing
gases, such as neon, that release colored visible light
when stimulated by electrons and ions. Many street
lights use a similar system, with different sorts of
In a solid
conductor, electrical charge is carried by free electrons
jumping from atom to atom, from a negatively-charged area to a
positively-charged area. As we've seen, electrons always have
a negative charge, which means they are always drawn toward
positive charges. In a gas, electrical charge is carried by
free electrons moving independently of atoms. Current
is also carried by ions, atoms that have an electrical
charge because they have lost or gained an electron. Like
electrons, ions are drawn to oppositely charged areas.
To send a current through gas in a tube, then, a
fluorescent light needs to have two things:
Free electrons and ions
A difference in charge between the two ends of the
tube (a voltage)
Generally, there are few ions
and free electrons in a gas, because all of the atoms
naturally maintain a neutral charge. Consequently, it is
difficult to conduct an electrical current through most gases.
When you turn on a fluorescent lamp, the first thing it needs
to do is introduce many new free electrons from both
There are several different ways of doing this, as we'll
see in the next couple of sections.
Start it Up The classic fluorescent lamp
design, which has fallen mostly by the wayside, used a special
starter switch mechanism to light up the tube. You can see how
this system works in the diagram below.
When the lamp first turns on, the path of least resistance
is through the bypass circuit, and across the starter
switch. In this circuit, the current passes through the
electrodes on both ends of the tube. These electrodes are
simple filaments, like you would find in an
incandescent light bulb. When the current runs through the
bypass circuit, electricity heats up the filaments. This boils
off electrons from the metal surface, sending them into the
gas tube, ionizing the gas.
At the same time, the electrical current sets off an
interesting sequence of events in the starter switch. The
conventional starter switch is a small discharge bulb,
containing neon or some other gas. The bulb has two electrodes
positioned right next to each other. When electricity is
initially passed through the bypass circuit, an electrical
arc (essentially, a flow of charged particles) jumps
between these electrodes to make a connection. This arc lights
the bulb in the same way a larger arc lights a fluorescent
One of the electrodes is a bimetallic strip that
bends when it is heated. The small amount of heat from the lit
bulb bends the bimetallic strip so it makes contact with the
other electrode. With the two electrodes touching each other,
the current doesn't need to jump as an arc anymore.
Consequently, there are no charged particles flowing through
the gas, and the light goes out. Without the heat from the
light, the bimetallic strip cools, bending away from the other
electrode. This opens the circuit.
Inside the casing of a conventional
fluorescent starter there is a small gas discharge
By the time this happens, the filaments have already
ionized the gas in the fluorescent tube, creating an
electrically conductive medium. The tube just needs a voltage
kick across the electrodes to establish an electrical arc.
This kick is provided by the lamp's ballast, a special
sort of transformer wired into the circuit.
When the current flows through the bypass circuit, it
establishes a magnetic field in part of the ballast.
This magnetic field is maintained by the flowing current. When
the starter switch is opened, the current is briefly cut off
from the ballast. The magnetic field collapses, which creates
a sudden jump in current -- the ballast releases its stored
The ballast, starter switch and fluorescent
bulb are all wired together in a simple
This surge in current helps build the initial
voltage needed to establish the electrical arc through the
gas. Instead of flowing through the bypass circuit and jumping
across the gap in the starter switch, the electrical current
flows through the tube. The free electrons collide with the
atoms, knocking loose other electrons, which creates ions. The
result is a plasma, a gas composed largely of ions and
free electrons, all moving freely. This creates a path for an
The impact of flying electrons keeps the two filaments
warm, so they continue to emit new electrons into the plasma.
As long as there is AC current, and the filaments aren't worn
out, current will continue to flow through the tube.
The problem with this sort of lamp is it takes a few
seconds for it to light up. These days, most fluorescent lamps
are designed to light up almost instantly. In the next
section, we'll see how these modern designs work.
Light Right Away Today, the most popular
fluorescent lamp design is the rapid start lamp. This
design works on the same basic principle as the traditional
starter lamp, but it doesn't have a starter switch. Instead,
the lamp's ballast constantly channels current through
both electrodes. This current flow is configured so that there
is a charge difference between the two electrodes,
establishing a voltage across the tube.
When the fluorescent light is turned on, both electrode
filaments heat up very quickly, boiling off electrons, which
ionize the gas in the tube. Once the gas is ionized, the
voltage difference between the electrodes establishes an
electrical arc. The flowing charged particles (red) excite the
mercury atoms (silver), triggering the illumination process.
Rapid start and starter switch fluorescent
bulbs have two pins that slide against two contact
points in an electrical
An alternative method, used in instant-start
fluorescent lamps, is to apply a very high initial voltage to
the electrodes. This high voltage creates a corona
discharge. Essentially, an excess of electrons on the
electrode surface forces some electrons into the gas. These
free electrons ionize the gas, and almost instantly the
voltage difference between the electrodes establishes an
No matter how the starting mechanism is configured, the end
result is the same: a flow of electrical current through an
ionized gas. This sort of gas discharge has a peculiar
and problematic quality: If the current isn't carefully
controlled, it will continually increase, and possibly explode
the light fixture. In the next section, we'll find out why
this is and see how a fluorescent lamp keeps things running
Ballast Balance We saw in the last section
that gases don't conduct electricity in the same way as
solids. One major difference between solids and gases is their
electrical resistance (the opposition to flowing
electricity). In a solid metal conductor such as a wire,
resistance is a constant at any given temperature, controlled
by the size of the conductor and the nature of the material.
In a gas discharge, such as a fluorescent lamp, current
causes resistance to decrease. This is because as more
electrons and ions flow through a particular area, they bump
into more atoms, which frees up electrons, creating more
charged particles. In this way, current will climb on its own
in a gas discharge, as long as there is adequate voltage (and
household AC current has a lot of voltage). If the current in
a fluorescent light isn't controlled, it can blow out
the various electrical components.
A fluorescent lamp's ballast works to control this.
The simplest sort of ballast, generally referred to as a
magnetic ballast, works something like an inductor.
A basic inductor consists of a coil of wire in a circuit,
which may be wound around a piece of metal. If you've read How
Electromagnets Work, you know that when you send
electrical current through a wire, it generates a magnetic
field. Positioning the wire in concentric loops amplifies this
This sort of field affects not only objects around the
loop, but also the loop itself. Increasing the current in the
loop increases the magnetic field, which applies a voltage
opposite the flow of current in the wire. In short, a coiled
length of wire in a circuit (an inductor) opposes change in
the current flowing through it (see How Inductors
Work for details). The transformer elements in a
magnetic ballast use this principle to regulate the current in
a fluorescent lamp.
A ballast can only slow down changes in current -- it can't
stop them. But the alternating current powering a fluorescent
light is constantly reversing itself, so the ballast
only has to inhibit increasing current in a particular
direction for a short amount of time. Check out this
site for more information on this process.
Magnetic ballasts modulate electrical current at a
relatively low cycle rate, which can cause a noticeable
flicker. Magnetic ballasts may also vibrate at a low
frequency. This is the source of the audible humming sound
people associate with fluorescent lamps.
Modern ballast designs use advanced electronics to more
precisely regulate the current flowing through the electrical
circuit. Since they use a higher cycle rate, you don't
generally notice a flicker or humming noise coming from an
electronic ballast. Different lamps require specialized
ballasts designed to maintain the specific voltage and current
levels needed for varying tube designs.
Fluorescent lamps come in all shapes and sizes, but they
all work on the same basic principle: An electric current
stimulates mercury atoms, which causes them to release
ultraviolet photons. These photons in turn stimulate a
phosphor, which emits visible light photons. At the most basic
level, that's all there is to it!
To learn more about this remarkable technology, including
descriptions of various lamp designs, check out the links on
the next page.