Fluorescent lamp

A fluorescent lamp is a gas-discharge lamp that uses electricity to excite mercury vapor in argon or neon gas, resulting in a plasma that produces short-wave ultraviolet light. This light then causes a phosphor to fluoresce, producing visible light.

Unlike incandescent lamps, fluorescent lamps always require a ballast to regulate the flow of power through the lamp. In common tube fixtures (typically 4' or 8' in length) the ballast is enclosed in the fixture. Compact fluorescent light bulbs may have a conventional ballast located in the fixture or they may have ballasts integrated in the lamps, allowing them to be used in lampholders normally used for incandescent lamps.

History

The earliest ancestor of the fluorescent lamp is probably the device by Heinrich Geissler who, in 1856, obtained a bluish glow from a gas which had been sealed in a tube and excited with an induction coil. Though he is remembered as a physicist, Geissler was trained as a glassblower.

At the 1893 World's Fair, the World Columbian Exposition in Chicago, Illinois displayed Nikola Tesla's fluorescent lights.

In 1894, D. McFarlane Moore created the Moore lamp, a commercial gas discharge lamp meant to compete with the incandescent light bulb of his former boss Thomas Edison. The gases used were nitrogen and carbon dioxide emitting respectively pink and white light, and had moderate success.

In 1901, Peter Cooper Hewitt demonstrated the mercury-vapor lamp, which emitted light of a blue-green color, and thus was unfit for most practical purposes. It was, however, very close to the modern design. This lamp had some applications in photography where color was not yet an issue, thanks to its much higher efficiency than incandescent lamps.

Edmund Germer and coworkers proposed in 1926 to increase the operating pressure within the tube and to coat the tube with fluorescent powder which converts ultraviolet light emitted by a rare gas into more uniformly white-colored light. Germer is today recognized as the inventor of the fluorescent lamp.

General Electric later bought Germer's patent and under the direction of George Inman brought the fluorescent lamp to wide commercial use by 1938.

Principles of operation

The main principle of fluorescent tube operation is based around inelastic scattering of electrons. An incident electron (emitted from the coils of wire forming the cathode electrode) collides with an atom in the gas (such as mercury, argon or krypton) used as the ultraviolet emitter. This causes an electron in the atom to temporarily jump up to a higher energy level to absorb some, or all, of the kinetic energy delivered by the colliding electron. This is why the collision is called 'inelastic' as some of the energy is absorbed. This higher energy state is unstable, and the atom will emit a photon to allow the atom's electron to revert to a lower, more stable, energy level. The photons that are released from the chosen gas mixtures tend to have a wavelength in the ultra-violet part of the spectrum. This is not visible to the human eye, so must be converted into visible light. This is done by making use of fluorescence. This fluorescent conversion occurs in the phosphor coating on the inner surface of the fluorescent tube, where the ultra-violet photons are absorbed by electrons in the phosphor's atoms, causing a similar energy jump, then drop, with emission of a further photon. The photon that is emitted from this second interaction has a lower energy than the one that caused it. The chemicals that make up the phosphor are specially chosen so that these emitted photons are at wavelengths visible to the human eye. The difference in energy between the absorbed ultra-violet photon and the emitted visible light photon goes to heat up the phosphor coating.

Mechanism of light production

A fluorescent lamp bulb is filled with a gas containing low pressure argon (or more rarely argon-neon or sometimes even krypton) and mercury vapor. The inner surface of the bulb is coated with a fluorescent paint made of varying blends of metallic and rare-earth phosphor salts. The bulb's cathode is typically made of coiled tungsten which is coated with a mixture of barium, strontium and calcium oxides (chosen to have a relatively low thermionic emission temperature). When the light is turned on, the electric power heats up the cathode enough for it to emit electrons. These electrons collide with and ionize noble gas atoms in the bulb surrounding the filament to form a plasma by a process of impact ionization. As a result of avalanche ionization, the conductivity of the ionized gas rapidly rises, allowing higher currents to flow through the lamp. The mercury, which exists at a stable vapour pressure equilibrium point of about one part per thousand in the inside of the tube (with the noble gas pressure typically being about 0.3% of atmospheric pressure (1 atm)), is then likewise ionized, causing it to emit light in the ultraviolet (UV) region of the spectrum predominantly at wavelengths of 253.7 nm and 185 nm. The efficiency of fluorescent lighting owes much to the fact that low pressure mercury discharges emit about 65% of their total light at the 254 nm line (also about 10-20% of the light emitted in UV is at the 185 nm line). The UV light is absorbed by the bulb's fluorescent coating, which re-radiates the energy at lower frequencies (longer wavelengths) (see stokes shift) to emit visible light. The blend of phosphors controls the color of the light, and along with the bulb's glass prevents the harmful UV light from escaping.

Electrical aspects of operation

Fluorescent lamps are negative resistance devices. This means that as more current flows through them and more gas is ionized, the electrical resistance of the fluorescent lamp drops, allowing even more current to flow through them. Connected directly to a constant-voltage mains power line, a fluorescent lamp would rapidly self-destruct due to the unlimited current flow. Because of this, fluorescent lamps are always used with some sort of auxiliary device, commonly called a ballast, to regulate the current flow through the tube.

While the ballast could be (and occasionally is) as simple as a resistor, substantial power is wasted in a resistive ballast so ballasts usually use a reactance (inductor or capacitor) instead. For operation from mains voltage, the use of simple inductor (a so-called "magnetic ballast") is common. In countries that use 120 V AC mains, the mains voltage is insufficient to light large fluorescent lamps so the ballast for these larger fluorescent lamps is often a step-up autotransformer with substantial leakage inductance (so as to limit the current flow). Either form of inductive ballast may also include a capacitor for power factor correction.

More sophisticated ballasts may employ transistors or other semiconductor components to convert mains voltage into high-frequency AC while also regulating the current flow in the lamp. These are referred to as "electronic ballasts".

Fluorescent lamps which operate directly from mains frequency AC will flicker at twice the mains frequency, since the power being delivered to the lamp will drop to zero twice per cycle. This means that the light will flicker at the rate of 120 times per second (Hz) in countries which use 60-cycle-per-second (60 Hz) AC, and 100 times per second in those which use 50 Hz. This same principle applies to the occasional hum one hears from fluorescent lamps, which is primarily caused by the ballast. Both the annoying hum and flicker are eliminated in lamps which use a high-frequency electronic ballast, such as the increasingly popular compact fluorescent bulb.

Although most people cannot directly see 120 Hz flicker, some people ^*" target="_blank" >report that 120 Hz flicker causes eyestrain and headache. Dr. J. Veitch has found that people have better reading performance using high-frequency (20-60 kHz) electronic ballasts than magnetic ballasts (120 Hz)[http://irc.nrc-cnrc.gc.ca/ie/lighting/vision/flf_e.html.

In some circumstances, fluorescent lamps operated at mains frequency can also produce flicker at the mains frequency (50 or 60 Hz) itself, which is noticeable by more people. This can happen in the last few hours of tube life when the cathode emission coating at one end is almost run out, and that cathode starts having difficulty emitting enough electrons into the gas fill, resulting in slight rectification and hence uneven light output in positive and negative going mains cycles. Mains frequency flicker can also sometimes be emitted from the very ends of the tubes, as a result of each tube electrode alternately operating as an anode and cathode each half mains cycle, and producing slightly different light output pattern in anode or cathode mode. (This was a more serious issue with tubes over 40 years ago, and many fittings of that era shielded the tube ends from view as a result.) Flicker at mains frequency is more noticeable in the peripheral vision than it is in the centre of gaze.

Method of 'starting' a fluorescent lamp

The mercury atoms in the fluorescent tube must be ionized before the arc can "strike" within the tube. For small lamps, it does not take much voltage to strike the arc and starting the lamp presents no problem, but larger tubes require a substantial voltage (in the range of a thousand volts). In some cases, that is exactly how it is done: instant start fluorescent tubes simply use a high enough voltage to break down the gas and mercury column and thereby start arc conduction. These tubes can be identified by the facts that

they have a single pin at each end of the tube and
the lampholders that they fit into have a "disconnect" socket at the low-voltage end to ensure that the mains current is automatically removed so that a person replacing the lamp can not receive a high-voltage electric shock.

In other cases, a separate starting aid must be provided. Old fluorescent designs (preheat lamps) used a combination filament/cathode at each end of the lamp in conjunction with a mechanical or automatic switch (see photo) that would initially connect the filaments in series and thereby "preheat" the filaments prior to striking the arc. Because of thermionic emission, the filaments would readily emit electrons into the gas column, creating a glow discharge near the filaments. Then, when the starting switch opened up, the inductive ballast would create a voltage surge which would (usually) strike the arc. If so, the impinging arc then kept the filament/cathode warm. If not, the starting sequence was repeated. If the starting aid was automatic, this often led to the situation where an old fluorescent lamp would flash time and time again as the starter repeatedly tried to start the worn-out lamp. More advanced starters would "trip out" in this situation and not attempt another start until manually reset.

Newer lamp and ballast designs (known as rapid start lamps) provide true filament windings within the ballast; these rapidly and continuously warm the filaments/cathodes using low-voltage AC. Unfortunately, there is no inductive voltage surge produced so the lamps must usually be mounted near a grounded (earthed) reflector to allow the glow discharge to propagate through the tube and initiate the arc discharge. Electronic ballasts often revert to a style in-between the preheat and rapid-start styles: a capacitor or other electronic circuit may join the two filaments, providing a conduction path that preheats the filaments but which is subsequently shorted out by the arc discharge. Generally this capacitor also forms, together with the inductor that provides current limiting in normal operation, a resonant circuit, increasing the voltage across the lamp so that it can easily start. Some electronic ballasts use programmed start: the output AC frequency is started above the resonance frequency of the output circuit of the ballast, and after the filaments are heated the frequency is rapidly decreased. If the frequency approaches the resonant frequency of the ballast, the output voltage will increase so much that the lamp will ignite. If the lamp does not ignite an electronic circuit stops the operation of the ballast.

Mechanisms of lamp failure at end of life

The end of life failure mode for fluorescent lamps varies depending how you use them and their control gear type. There are 3 main failure modes currently, and a 4th which is starting to appear:

Emission mix runs out

The emission mix on the tube filaments/cathodes is necessary to enable electrons to pass into the gas via thermionic emission at the tube operating voltages used. The mix is slowly sputtered off by bombardment with electrons and mercury ions during operation, but a larger amount is sputtered off each time the tube is started with cold cathodes. (The method of starting the lamp and hence the control gear type has a significant impact on this.) Lamps operated for typically less than 3 hours each switch-on will normally run out of the emission mix before other parts of the lamp fail. The sputtered emission mix forms the dark marks at the tube ends seen in old tubes. When all the emission mix is gone, the cathode cannot pass sufficient electrons into the gas fill to maintain the discharge at the designed tube operating voltage. Ideally, the control gear should shut down the tube when this happens. However, some control gear will provide sufficient increased voltage to continue operating the tube in cold cathode mode, which will cause overheating of the tube end and rapid disintegration of the electrodes and their support wires until they are completely gone or the glass cracks wrecking the low pressure gas fill and stopping the gas discharge.

Failure of integral ballast electronics

This is only relevant to compact fluorescent lamps with integral electronic ballasts. Ballast electronics failure is a somewhat random process which follows the standard failure profile for any electronic devices. There is an initial small peak of early failures, followed by a drop and steady increase over lamp life. Life of electronics is heavily dependant on operating temperature—it typically halves for each 10C temperature rise. The quoted average life is usually at at 25C ambient (this may vary by country). In some fittings, the ambient temperature could be well above this, in which case failure of the electronics may become the predominant failure mechanism. Similarly, running a compact fluorescent lamp base-up will result in hotter electronics and shorter average life (particularly with higher power rated ones). Electronic ballasts should be designed to shut down the tube when the emission mix runs out as described above. In the case of integral electronic ballasts, since they never have to work again, this is sometimes done by having them deliberately burn out some component to permanently cease operation.

Failure of the phosphor

The phosphor drops off in efficiency during use. By around 25,000 operating hours, it will typically be half the brightness of a new lamp (although there are some lamps where manufacturers claim much longer lives). Lamps which are not hit by failures of the emission mix or integral ballast electronics will eventually get hit by this. They still work, but have become dim and inefficient. The process is slow, and often only becomes obvious when a new lamp is operating next to an old lamp.

Tube runs out of mercury

Mercury is lost from the gas fill throughout the lamp life, as it is slowly absorbed into glass, phosphor, and tube electrodes, where it can no longer function. Historically this hasn't been a problem because tubes have had an excess of mercury. However, environmental concerns are now resulting in low mercury content tubes which are much more accurately dosed with just enough mercury to last the expected life of the lamp. (In some jurisdictions, tubes with any more mercury than this minimum amount now have to be handled as hazardous waste.) This means that loss of mercury will take over from failure of the phosphor in some lamps. The failure symptom is similar, except loss of mercury initially causes an extended run-up time (time to reach full light output), and finally causes the lamp to go a dim pink when the mercury runs out and the argon base gas takes over as the primary discharge.

Phosphors and the spectrum of emitted light

Many people find the color spectrum produced by some fluorescent lighting to be harsh and displeasing. It is common for a healthy person to appear with a sickly bluish skin tone under fluorescent lighting. This is due in part to the presence of prominent blue and green lines emitted directly by the mercury arc and in part to the type of phosphor used. Many pigments appear a slightly different color when viewed under fluorescent light versus incandescent. This is mainly the case with fluorescent lamps containing the older halophosphate type phosphors (chemical formula Ca₅(P O₄)₃(F,Cl):Sb³⁺,Mn²⁺), usually labeled as "cool white". The bad color reproduction is due to the fact that this phosphor mainly emits yellow and blue light, and relatively little green and red. To the eye, this mixture looks white, but light reflected from surfaces has an incomplete spectrum. More expensive fluorescent lamps use a triphosphor mixture, based on europium and terbium ions, that have emission bands more evenly distributed over the spectrum of visible light. These phosphors give a more natural color reproduction to the human eye.

|| For an explanation of the origin of the peaks click on the image. Note that several of the spectral peaks are directly generated from the mercury arc. || For an explanation of the origin of the peaks click on the image.

Fluorescent lamp spectra

Usage

Fluorescent light bulbs come in many shapes and sizes. An increasingly popular one is the compact fluorescent light bulb (CF). Many compact fluorescent lamps integrate the auxiliary electronics into the base of the lamp, allowing them to screw into a regular light bulb socket. In the US, residential use of fluorescent lighting remains low (generally limited to kitchens, basements, hallways and other areas), but schools and businesses find the cost savings of fluorescents to be significant and only rarely use incandescent lights. Typical lighting arrangements may include fluorescent tubes sending different tints of white, in order to provide good color reproduction. In other countries, residential use of fluorescent lighting varies depending on the price of energy and the environmental concerns of the local population as well as the acceptability of the light output.

Because they contain toxic mercury,in many areas government regulations require special disposal of fluorescent lamps, separately from general and household wastes. (A typical 4 ft. T-12 fluorescent lamp contains about 12 milligrams of mercury^*). While this generally applies only to large commercial buildings which produce many waste bulbs, it is a good idea to find out if you can safely dispose of your waste bulbs in some manner.

Advantages over incandescent lamps

Fluorescent lamps are more efficient than incandescent light bulbs of an equivalent brightness. This is because more of the consumed energy is converted to usable light and less is converted to heat, (allowing fluorescent lamps to run cooler). An incandescent lamp may convert only 10% of its power input to visible light. A fluorescent lamp producing as much useful visible light energy may require only 1/3 to 1/4 as much electricity input. Typically a fluorescent lamp will last between 10 and 20 times as long as an equivalent incandescent lamp. Where lighting is used in air-conditioned spaces, all the lamp losses must also be removed by the air conditioning equipment, resulting in a double penalty for losses due to lighting.

The higher first cost of a fluorescent lamp can usually be offset by lower energy consumption over its life. The longer life may also reduce lamp replacement costs, providing additional saving especially where labour is costly.

Disadvantages

Fluorescent lamps require a ballast to stabilize the lamp and to provide the initial striking voltage required to start the arc discharge; this increases the cost of fluorescent luminaires, though often one ballast is shared between two or more lamps.

Conventional lamp ballasts do not operate on direct current. If a direct current supply with a high enough voltage to strike the arc is available, a resistor can be used to ballast the lamp but this leads to low efficiency. The lamps must be reversed at regular intervals since on direct current, only one end of the lamp will produce most of the light.

Fluorescent lamps operate best around room temperature (say, 68 degrees Fahrenheit or 20 degrees Celsius). At much lower or higher temperatures, efficiency decreases and at low temperatures (below freezing) standard lamps may not start. Special lamps may be needed for reliable service outdoors in cold weather.

Because the arc is quite long relative to higher-pressure discharge lamps, the amount of light emitted per unit of surface of the lamps is low, so the lamps are large compared with incandescent sources. This affects design of fixtures since light must be directed from long tubes instead of a compact source. However, in many case low luminous intensity of the emitting surface is useful because it reduces glare.

Fluorescent lamps do not give out a steady light, instead they flicker (fluctuate greatly in intensity) at a rate that depends on the frequency of the driving voltage. While this is not easily discernable by the human eye, it can cause a strobe effect posing a safety hazard in a workshop for example, where something spinning at just the right speed may appear stationary if illuminated solely by a fluorescent lamp. It also causes problems for video recording as there can be a 'beat effect' between the periodic reading of a camera's sensor and the fluctuations in intensity of the fluorescent lamp. Incandescent lamps, due to the thermal inertia of their element, fluctuate less in their intensity, although the effect is measurable with instruments. This is also less of a problem with compact fluorescents, since they multiply the line frequency to levels that are not visible. Installations can reduce the stroboscope effect by using lead-lag ballasts or by operating the lamps on different phases of a polyphase power supply.

The problems with color faithfulness are discussed above.

Unless rated as such, fluorescent lights cannot be connected to a standard dimmer switch used for incandescent lamps. Many installations require 4-pin fluorescent lamps and compatible controllers for successful fluorescent dimming.

Most old houses may contain Edison (screw base) lampholders, intended for incandescent lamps, into which only some compact fluorescent lamps can be fitted.

The disposal of phosphor and the small amounts of mercury in the tubes may also be an environmental problem, compared to the disposal of incandescent lamps. For large commercial or industrial users of fluorescent lights, recycling services are beginning to become available.

Tube designations

Note: the information in this section might be inapplicable outside of North America.

Lamps are typically identified by a code such as F##T##, where F is for fluorescent, the first number indicates the power in watts (or strangely, length in inches in very long lamps), the T indicates that the shape of the bulb is tubular, and the last number is diameter in eighths of an inch. Typical diameters are T12 (1½" or 38 mm) for residential bulbs with old magnetic ballasts, T8 (1 in or 25 mm) for commercial energy-saving lamps with electronic ballasts, and T5 (⁵⁄8" or 16 mm) for very small lamps which may even operate from a battery-powered device.

High-output lamps are brighter and draw more electrical current, have different ends on the pins so they cannot be used in the wrong fixture, and are labeled F##T12HO, or F##T12VHO for very high output. Since about the early to mid 1950's to today, General Electric developed and improved the Power Groove(R) lamp with the label F##PG17. These lamps are recognisable by their large diameter, grooved tubes.

U-shaped tubes are FB##T##, with the B meaning "bent". Most commonly, these have the same designations as linear tubes. Circular bulbs are FC##T#, with the diameter of the circle (not circumference or watts) being the first number, and the second number usually being 9 (29 mm) for standard fixtures.

Color is usually indicated by WW for warm white, EW for enhanced (neutral) white, CW for cool white (the most common), and DW for the bluish daylight white. BL is often used for blacklight (commonly used in bug zappers), and BLB for the common blacklight-blue bulbs which are dark purple. Other non-standard designations apply for plant lights or grow lights.

Philips uses numeric color codes for the colors:

Low color rendition
- 33 the ubiquitous cool white (4000 K)
- 32 warm white (3000 K)
- 27 living room warm white (2700 K)

High color rendition
- 840 cool white (4000 K)
- 830 warm white (3000 K)
- 827 warm white (2700 K)

Other
- 09 Sun tanning lamps
- 08 Blacklight
- 05 Hard UV (no phosphors used at all, using an envelope of fused quartz)

Odd lengths are usually added after the color. One example is an F25T12/CW/33, meaning 25 watts, 1.5" diameter, cool white, 33" or 84 cm long. Without the 33, it would be assumed that an F25T12 is the more-common 30" long.

Compact fluorescents do not have such a designation system.

Other fluorescent lamps

Blacklights are a subset of fluorescent lamps that are used to provide long-wave ultraviolet light (at about 360nm wavelength). They are built in the same fashion as conventional fluorescent lamps but the glass tube is coated with a phosphor that converts the short-wave UV within the tube to long-wave UV rather than to visible light. They are used to provoke fluorescence (to provide dramatic effects using blacklight paint and to detect materials such as urine and certain dyes that would be invisible in visible light) as well as to attract insects to bug zappers.

So-called blacklite blue lamps are also made from more expensive deep purple glass known as Wood's glass rather than clear glass. The deep purple glass filters out most of the visible colors of light directly emitted by the mercury-vapor discharge, producing proportionally less visible light compared to UV light. This allows UV-induced fluorescence to be seen more easily (thereby allowing blacklight posters to seem much more dramatic). The blacklight lamps used in bug zappers do not require this refinement so it is usually omitted in the interest of cost; they are called simply blacklite (and not blacklite blue).

Sun lamps contain a different phosphor that emits more strongly in medium-wave UV, provoking a tanning response in most human skin.

Grow lamps contain a phosphor blend that encourages photosynthesis in plants; they usually appear pinkish to human eyes.

Germicidal lamps contain no phosphor at all (technically making them gas discharge lamps rather than fluorescent) and their tubes are made of fused quartz that is transparent to the short-wave UV directly emitted by the mercury discharge. The UV emitted by these tubes will kill germs, ionize oxygen to ozone, and cause eye and skin damage. Besides their uses to kill germs and create ozone, they are sometimes used by geologists to identify certain species of minerals by the color of their fluorescence. When used in this fashion, they are fitted with filters in the same way as blacklight-blue lamps are; the filter passes the short-wave UV and blocks the visible light produced by the mercury discharge. They are also used in EPROM erasers.

Electrodeless induction lamps are fluorescent lamps without internal electrodes. They have been commercially available since 1990. A current is induced into the gas column using electromagnetic induction. Because the electrodes are usually the life-limiting element of fluorescent lamps, such electrodeless lamps can have a very long service life, although they also have a higher purchase price.

Cold-cathode fluorescent lamps (CCFL) are used as backlighting for LCD displays in laptop personal computers. They are also popular with case modders in recent years.

Fluorescent fun

If you live in a dry cold climate with lots of static electricity, try this: Put on your best static-gathering socks and take hold of a short fluorescent tube. Then shuffle about on the carpet to gather a robust static charge. Now discharge by gently touching the lamp electrodes to anything electrically grounded. Instead of the usual little spark the entire tube will flash as the electrons course (painlessly) out of your body. This also applies with Van de Graaff generators; simply touch the light to the sphere or touch the sphere while holding the light. Warning: This may produce a rather "jolty" shock.

Alternatively, if you happen to have a Tesla coil handy, you can fully illuminate the fluorescent lamp at quite a distance from the Tesla coil simply by holding the detached lamp in your hand and possibly touching one of its terminals. Do not touch the lamp to the coil, as this may result in injury and/or burning out the lamp (a hobbyist Tesla coil may operate at several kilowatts).

If you live near high-voltage power lines you might try standing underneath them at night while holding a fluorescent tube. The strong electric field created by power lines will cause a very small (harmless) current flow through the tube and it should give off at least a feeble glow.^* Obviously you should never do this during stormy weather and no attempt should ever be made to get closer than average standing height to the lines using, for instance, a ladder, for that may get you killed.

It is possible to tune a guitar or other string instrument to a fluorescent lamp.

External links

NASA: The Fluorescent Lamp: A plasma you can use
How Stuff Works: Are fluorescent bulbs really more efficient than normal light bulbs?
How Stuff Works: How Fluorescent Lamps Work
The Lighting Design Lab: Should I Turn Off Fluorescent Lighting When Leaving A Room?
Application notes for CCFL lamps.

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