You see fluorescent lighting everywhere these days -- in offices, stores, warehouses, street corners... You'll even find fluorescent lamps 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 white tubes?
In this article, 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.
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 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 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.
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 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. People generally use incandescent lights in the home, however, since they emit a "warmer" light -- a light with more red and less blue.
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 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:
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 electrodes.
There are several different ways of doing this, as we'll see in the next couple of sections.
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 (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 gases.
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 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.
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 energy.
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 electrical current.
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.
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.
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 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 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.