Are fluorescent bulbs really more efficient than
normal light bulbs?
A "normal light bulb" is also known as an incandescent light bulb. These bulbs
have a very thin tungsten filament that is housed inside a glass sphere. They
typically come in sizes like "60 watt," "75 watt," "100 watt" and so on.
The basic idea behind these bulbs is simple. Electricity runs through the filament.
Because the filament is so thin, it offers a good bit of resistance to the electricity,
and this resistance turns electrical energy into heat. The heat is enough to make
the filament white hot, and the "white" part is light. The filament glows because of
the heat -- it incandesces.
The problem with incandescent light bulbs is that the heat wastes a lot of
electricity. Heat is not light, and the purpose of the light bulb is light, so all of the
energy spent creating heat is a waste. Incandescent bulbs are therefore very
inefficient. They produce perhaps 15 lumens per watt of input power.
A fluorescent bulb uses a completely different method to produce light. There
are electrodes at both ends of a fluorescent tube, and a gas containing argon
and mercury vapor is inside the tube. A stream of electrons flows through the gas
from one electrode to the other (in a manner similar to the stream of electrons in
a cathode ray tube). These electrons bump into the mercury atoms and excite
them. As the mercury atoms move from the excited state back to the unexcited
state, they give off ultraviolet photons. These photons hit the phosphor coating
the inside of the fluorescent tube, and this phosphor creates visible light. It
sounds complicated, so lets go through it again in slow motion:
There is a stream of electrons flowing between the electrodes at both
ends of the fluorescent bulb.
The electrons interact with mercury vapor atoms floating inside the bulb.
The mercury atoms become excited, and when they return to an unexcited
state they release photons of light in the ultraviolet region of the spectrum.
These ultraviolet photons collide with the phosphor coating the inside of
the bulb, and the phosphor creates visible light.
The phosphor fluoresces to produce light.
A fluorescent bulb produces less heat, so it is much more efficient. A fluorescent
bulb can produce between 50 and 100 lumens per watt. This makes
fluorescent bulbs four to six times more efficient than incandescent bulbs.
That's why you can buy a 15-watt fluorescent bulb that produces the same
amount of light as a 60-watt incandescent bulb.
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 Works.)
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 orbital’s 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 photon.
The wavelength 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.
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 photons.
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 phosphors.
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.
Cooking with Gas
Sources of Light
Fluorescent lamps are just one lighting application of a gas discharge tube.
Black lights are essentially fluorescent lamps without a phosphor coating. They
mostly emit ultraviolet light, which causes phosphors outside of the lamp to emit
visible light.
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 gases.
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
ways.
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:
1. Free electrons and ions
2. 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
electrodes.
There are several different ways of doing this, as we'll see in the next couple of
sections
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 bulb.
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.
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 circuit.
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 electrical
arc.
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 smoothly.
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 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 field.
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 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.