How Fluorescent Lamps Work

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How does a fluorescent starter work?A fluorescent light does not have the usual glowing filament of anincandescent bulb, butinstead contains a mercury vapor that gives off ultraviolet light when ionized. The

ultraviolet light makes particles that coat the inside of the tube, and these particles glow

or fluoresce (see How Fluorescent Lamps Workfor details).Fluorescent starters are used in several types of fluorescent lights. The starter is there to

help the lamp light. When voltageis applied to the fluorescent lamp, here's what happens:

1. The starter (which is simply a timed switch) allows current to flow through thefilaments at the ends of the tube.

2. The current causes the starter's contacts to heat up and open, thus interrupting the

flow of current. The tube lights.

3. Since the lighted fluorescent tube has a low resistance, the ballast now serves as acurrent limiter.

When you turn on a fluorescent tube, the starter is a closed switch. The filaments at the

ends of the tube are heated by electricity, and they create a cloud of electrons inside the

tube. The fluorescent starter is a time-delay switch that opens after a second or two.When it opens, the voltage across the tube allows a stream of electrons to flow across the

tube and ionize the mercury vapor.

Without the starter, a steady stream of electrons is never created between the two filaments, and the lampflickers. Without the ballast, the arc is a short circuit between the filaments, and this short circuit contains alot of current. The current either vaporizes the filaments or causes the bulb to explode.

According toSam's F-Lamp FAQ:The most common fluorescent starter is called a "glow tube starter" (or just starter) and contains a small gas(neon, etc.) filled tube and an optional radio frequency interference (RFI) suppression capacitor in acylindrical aluminum can with a 2 pin base. While all starters are physically interchangeable, the wattagerating of the starter should be matched to the wattage rating of the fluorescent tubes for reliable operationand long life.The glow tube incorporates a switch which is normally open. When power is applied, a glow discharge takesplace which heats a bimetal contact. A second or so later, the contacts close and provide current to thefluorescent filaments. Since the glow is extinguished, there is no longer any heating of the bimetal and thecontacts open. The inductive kick generated at the instant of opening triggers the main discharge in thefluorescent tube. If the contacts open at a bad time, there isn't enough inductive kick and the processrepeats.

How Fluorescent Lamps WorkIntroduction to How Fluorescent Lamps WorkYou see fluorescent lighting everywhere these days -- in offices, stores, warehouses, street corners... You'lleven find fluorescent lamps in peoples' homes. But even though they're all around us, these devices are atotal mystery to most people. Just what is going on inside those white tubes?

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Fluorescent lamps are anenergy-efficient option.

In this article, we'll find out how fluorescent lamps emit such a bright glow without getting scalding hot like anordinary 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 canbe released by an atom. It is made up of many small particle-like packets that have energy and momentumbut 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 theirelectrons become excited. If you've readHow Atoms Work, thenyou know electrons are the negatively charged particles that move around an atom's nucleus (which has anet 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 differentorbitals. 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. Whensomething passes energy on to an atom -- heat, for example -- an electron may be temporarily boosted to a

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higher orbital (farther away from the nucleus). The electron only holds this position for a tiny fraction of asecond; almost immediately, it is drawn back toward the nucleus, to its original orbital. As it returns to itsoriginal 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 theparticular position of the electron. Consequently, different sorts of atoms will release different sorts of lightphotons. In other words, the colorof 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 orgas lamp, atoms are excited by heat; in a light stick, atoms are excited by a chemical reaction. Fluorescentlamps have one of the most elaborate systems for exciting atoms, as we'll see in the next section.Down the TubesThe central element in a fluorescent lamp is a sealed glass tube. The tube contains a small bit ofmercuryand an inert gas, typically argon, kept under very low pressure. The tube also contains a phosphorpowder, coated along the inside of the glass. The tube has two electrodes, one at each end, which arewired to an electrical circuit. The electrical circuit, which we'll examine later, is hooked up to an alternatingcurrent (AC) supply

When you turn the lamp on, the current flows through the electrical circuit to the electrodes. There is aconsiderable voltage across the electrodes, so electrons will migrate through the gas from one end of thetube to the other. This energy changes some of the mercury in the tube from a liquid to a gas. As electronsand 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 returnto 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 electronarrangement in the atom. The electrons in mercury atoms are arranged in such a way that they mostlyrelease light photons in the ultraviolet wavelength range. Our eyes don't register ultraviolet photons, so thissort 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 lightwhen they are exposed to light. When a photon hits a phosphor atom, one of the phosphor's electrons jumpsto a higher energy level and the atom heats up. When the electron falls back to its normal level, it releasesenergy in the form of another photon. This photon has less energy than the original photon, because someenergy was lost as heat. In a fluorescent lamp, the emitted light is in the visible spectrum -- the phosphorgives offwhite light we can see. Manufacturers can vary the color of the light by using differentcombinations of phosphors.

Cooking with GasIn the last section, we saw that mercury atoms in a fluorescent lamp's glass tube are excited by electronsflowing in an electrical current. This electrical current is something like the current in an ordinarywire, but itpasses through gas instead of through a solid. Gas conductors differ from solid conductors in a number ofways.

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In a solid conductor, electrical charge is carried by free electrons jumping from atom to atom, from anegatively-charged area to a positively-charged area. As we've seen, electrons always have a negativecharge, which means they are always drawn toward positive charges. In a gas, electrical charge is carriedby free electrons moving independently of atoms. Current is also carried by ions, atoms that have anelectrical charge because they have lost or gained an electron. Like electrons, ions are drawn to oppositelycharged areas.To send a current through gas in a tube, then, a fluorescent light needs to have two things:

1. Free electrons and ions2. 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 aneutral charge. Consequently, it is difficult to conduct an electrical current through most gases. When youturn on a fluorescent lamp, the first thing it needs to do is introduce many new free electrons from bothelectrodes.There are several different ways of doing this, as we'll see in the next couple of sections.Start it UpThe classic fluorescent lamp design, which has fallen mostly by the wayside, used a special starter switchmechanism 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. Theseelectrodes are simple filaments, like you would find in an incandescent light bulb. When the current runsthrough 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. Theconventional starter switch is a small discharge bulb, containing neon or some other gas. The bulb has twoelectrodes positioned right next to each other. When electricity is initially passed through the bypass circuit,anelectrical arc(essentially, a flow of charged particles) jumps between these electrodes to make aconnection. This arc lights the bulb in the same way a larger arc lights a fluorescent bulb.

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One of the electrodes is a bimetallic strip that bends when it is heated. The small amount of heat from thelit bulb bends the bimetallic strip so it makes contact with the other electrode. With the two electrodestouching each other, the current doesn't need to jump as an arc anymore. Consequently, there are nocharged particles flowing through the gas, and the light goes out. Without the heat from the light, thebimetallic strip cools, bending away from the other electrode. This opens the circuit.

Inside the casing of a conventional fluorescent starter there isa small gas discharge lamp.

By the time this happens, the filaments have already ionized the gas in the fluorescent tube, creating anelectrically conductive medium. The tube just needs a voltage kick across the electrodes to establish anelectrical 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. Thismagnetic field is maintained by the flowing current. When the starter switch is opened, the current is brieflycut off from the ballast. The magnetic field collapses, which creates a sudden jump in current -- the ballastreleases its stored energy.

The ballast, starter switch and fluorescent bulb are all wiredtogether in a simple circuit.

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 electricalcurrent flows through the tube. The free electrons collide with the atoms, knocking loose other electrons,

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which creates ions. The result is a plasma, a gas composed largely of ions and free electrons, all movingfreely. 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 theplasma. As long as there is AC current, and the filaments aren't worn out, current will continue to flowthrough the tube.The problem with this sort of lamp is it takes a few seconds for it to light up. These days, most fluorescentlamps are designed to light up almost instantly. In the next section, we'll see how these modern designs

work.Light Right AwayToday, the most popular fluorescent lamp design is the rapid start lamp. This design works on the samebasic principle as the traditional starter lamp, but it doesn't have a starter switch. Instead, the lamp's ballastconstantly channels current through both electrodes. This current flow is configured so that there is a chargedifference 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 chargedparticles (red) excite the mercury atoms (silver), triggering the illumination process.

Rapid start and starter switch fluorescent bulbs have two pinsthat 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 theelectrodes. This high voltage creates a corona discharge. Essentially, an excess of electrons on theelectrode surface forces some electrons into the gas. These free electrons ionize the gas, and almostinstantly 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 currentthrough an ionized gas. This sort ofgas discharge has a peculiar and problematic quality: If the current isn'tcarefully controlled, it will continually increase, and possibly explode the light fixture. In the next section, we'llfind out why this is and see how a fluorescent lamp keeps things running smoothly.Ballast BalanceWe saw in the last section that gases don't conduct electricity in the same way as solids. One majordifference between solids and gases is theirelectrical resistance (the opposition to flowing electricity). In asolid metal conductor such as a wire, resistance is a constant at any given temperature, controlled by thesize 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 asmore electrons and ions flow through a particular area, they bump into more atoms, which frees upelectrons, creating more charged particles. In this way, current will climb on its own in a gas discharge, aslong as there is adequate voltage (and household AC current has a lot of voltage). If the current in afluorescent 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 amagnetic ballast, works something like aninductor. 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 thatwhen you send electrical current through a wire, it generates a magnetic field. Positioning the wire inconcentric loops amplifies this field.

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This sort of field affects not only objects around the loop, but also the loop itself. Increasing the current in theloop increases the magnetic field, which applies a voltage opposite the flow of current in the wire. In short, acoiled length of wire in a circuit (an inductor) opposes change in the current flowing through it (seeHowInductors Workfor details). The transformer elements in a magnetic ballast use this principle to regulatethe current in a fluorescent lamp.

A ballast can only slow down changes in current -- it can't stop them. But the alternating current powering afluorescent 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 sitefor more information on this process.Magnetic ballasts modulate electrical current at a relatively low cycle rate, which can cause a noticeableflicker. Magnetic ballasts may also vibrate at a low frequency. This is the source of the audible hummingsound people associate with fluorescent lamps.Modern ballast designs use advanced electronics to more precisely regulate the current flowing through theelectrical circuit. Since they use a higher cycle rate, you don't generally notice a flicker or humming noisecoming from an electronic ballast. Different lamps require specialized ballasts designed to maintain thespecific 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 electriccurrent stimulates mercury atoms, which causes them to release ultraviolet photons. These photons in turnstimulate a phosphor, which emits visible light photons. At the most basic level, that's all there is to it!Conventional incandescent light bulbs also emit a good bit of ultraviolet light, but they do not convert any ofit to visible light. Consequently, a lot of the energy used to power an incandescent lamp is wasted. Afluorescent lamp puts this invisible light to work, and so is more efficient. Incandescent lamps also losemore 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 gasin the glass tube. In the next section, we'll see what a fluorescent lamp needs to do to establish this current.

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How Fluorescent Lamps Work