Editing Fluorescent lamp (section)

==Principles of operation==
The fundamental mechanism for the conversion of electrical energy to light is the emission of a [[photon]] when an electron in a mercury atom falls from an excited state into a lower [[energy level]]. Electrons flowing in the arc collide with the mercury atoms. If the incident electron has enough [[kinetic energy]], it transfers energy to the atom's outer electron, causing that electron to temporarily jump up to a higher energy level that is not stable.  The atom will emit an ultraviolet [[photon]] as the atom's electron reverts to a lower, more stable, energy level. Most of the photons that are released from the mercury atoms have [[wavelength]]s in the [[ultraviolet]] (UV) region of the spectrum, predominantly at wavelengths of 253.7 and 185 [[nanometer]]s (nm). These are not visible to the human eye, so ultraviolet energy is converted to visible light by the [[fluorescence]] of the inner phosphor coating.  The difference in energy between the absorbed ultra-violet photon and the emitted visible light photon heats the phosphor coating.

Electric current flows through the tube in a low-pressure [[electric arc|arc discharge]]. Electrons collide with and ionize [[noble gas]] atoms inside the bulb surrounding the filament to form a [[Plasma (physics)|plasma]] by the process of [[impact ionization]]. As a result of [[electron avalanche|avalanche ionization]], the conductivity of the ionized gas rapidly rises, allowing higher currents to flow through the lamp.

The fill gas helps determine the electrical characteristics of the lamp but does not give off light itself. The fill gas effectively increases the distance that electrons travel through the tube, which allows an electron a greater chance of interacting with a mercury atom.  Additionally, argon atoms, excited to a metastable state by the impact of an electron, can impart energy to a mercury atom and ionize it, described as the [[Penning ionization|Penning effect]]. This  lowers the breakdown and operating voltage of the lamp, compared to other possible fill gases such as krypton.<ref>William M. Yen, Shigeo Shionoya, Hajime Yamamoto, ''Practical Applications of Phosphors'', CRC Press, 2006, {{ISBN|1-4200-4369-2}},  pp. 84–85</ref>

===Construction===
[[File: Germicidal Lamp 2.jpg|thumb|right|upright|Close-up of the cathodes of a [[germicidal lamp]] (an essentially similar design that uses no fluorescent phosphor, allowing the [[electrode]]s to be seen)]]

A fluorescent lamp tube is filled with a mix of [[argon]], [[xenon]], [[neon]], or [[krypton]], and mercury vapor. The pressure inside the lamp is around 0.3% of atmospheric pressure.<ref name="Alok">{{cite book
  | last = Kulshreshtha
  | first = Alok K.
  | title = Basic Electrical Engineering: Principles and Applications
  | publisher = Tata McGraw-Hill Education
  | date = 2009
  | location = India
  | page = 801
  | url = https://books.google.com/books?id=GWPfL5bxkAwC&q=pressure+in+%22fluorescent+tube%22&pg=PA801
  | isbn = 978-0-07-014100-1
  | access-date = 2020-10-17 
  | archive-date = 2023-04-20 
  | archive-url = https://web.archive.org/web/20230420064855/https://books.google.com/books?id=GWPfL5bxkAwC&q=pressure+in+%22fluorescent+tube%22&pg=PA801
  | url-status = live
  }}</ref> The partial pressure of the mercury vapor alone is about 0.8&nbsp;Pa (8&nbsp;millionths of atmospheric pressure), in a T12 40-watt lamp.{{sfn|Kane|Sell|2001|p=185}} The inner surface of the lamp is coated with a [[fluorescent]] coating made of varying blends of metallic and [[rare-earth]] [[phosphor]] salts. The lamp's electrodes are typically made of coiled [[tungsten]] and are coated with a mixture of barium, strontium and calcium oxides to improve [[thermionic emission]].

[[File:Germicidal UV discharge tube glow.jpg|thumb|left|upright|A [[germicidal lamp]] uses a low-pressure mercury-vapor glow discharge identical to that in a fluorescent lamp, but the uncoated [[fused quartz]] envelope allows ultraviolet radiation to transmit.]]

Fluorescent lamp tubes are often straight and range in length from about {{convert|100|mm|in|sp=us}} for miniature lamps, to {{convert|2.43|m|ft|sp=us}} for high-output lamps. Some lamps have a circular tube, used for table lamps or other places where a more compact light source is desired. Larger U-shaped lamps are used to provide the same amount of light in a more compact area, and are used for special architectural purposes. [[Compact fluorescent lamp]]s have several small-diameter tubes joined in a bundle of two, four, or six, or a small diameter tube coiled in a helix, to provide a high amount of light output in minimal volume.

Light-emitting phosphors are applied as a paint-like coating to the inside of the tube. The organic solvents are allowed to evaporate, then the tube is heated to nearly the melting point of glass to drive off remaining organic compounds and fuse the coating to the lamp tube. Careful control of the grain size of the suspended phosphors is necessary; large grains lead to weak coatings, and small particles lead to poor light maintenance and efficiency. Most phosphors perform best with a particle size around 10 micrometers. The coating must be thick enough to capture all the ultraviolet light produced by the mercury arc, but not so thick that the phosphor coating absorbs too much visible light. The first phosphors were synthetic versions of naturally occurring fluorescent minerals, with small amounts of metals added as activators. Later other compounds were discovered, allowing differing colors of lamps to be made.{{sfn|Van Broekhoven|2001|p=93}}

Fluorescent tubes can have an outer silicone coating applied by dipping the tube into a solution of water and silicone, and then drying the tube. This coating gives the tube a silky surface finish, and protects against moisture,  guaranteeing a predictable surface resistance on the tube when starting it.<ref>{{Cite book |last=Cayless |first=M. A. |url=https://books.google.com/books?id=x40rBgAAQBAJ&dq=fluorescent+tube+silicone+moisture&pg=PA208 |title=Lamps and Lighting |date=2012-08-21 |publisher=Routledge |isbn=978-1-135-13809-7 |language=en}}</ref>

===Ballasts ===
{{main|Electrical ballast}}
[[File:LAjaHID.jpg|thumb|Different ballasts for fluorescent and discharge lamps]]
Fluorescent lamps are [[negative differential resistance]] devices, so as more current flows through them, the electrical resistance of the fluorescent lamp drops, allowing for even more current to flow. Connected directly to a [[constant-voltage power supply]], a fluorescent lamp would rapidly self-destruct because of the uncontrolled current flow. To prevent this, fluorescent lamps must use a [[electrical ballast|ballast]] to regulate the current flow through the lamp.

The terminal voltage across an operating lamp varies depending on the [[electric arc|arc]] current, tube diameter, temperature, and fill gas.  A general lighting service {{Convert|48|in|mm|adj=on|abbr=out|sigfig=4|sp=us}} T12<ref>T12 specifies the bulb's diameter in 1/8 inch units; a T12 bulb is 12×(1/8)&nbsp;inches or {{Convert|1.5|in|mm|abbr=on|sp=us}} in diameter.</ref> lamp operates at 430&nbsp;mA, with 100 volts drop. High-output lamps operate at 800 mA, and some types operate up to 1.5&nbsp;A. The power level varies from 33 to 82 watts per meter of tube length (10 to 25 W/ft) for T12 lamps.<ref name=GE1978>General Electric, ''Fluorescent Lamps Technical Bulletin TP 111R'', December 1978</ref>

The simplest ballast for [[alternating current]] (AC) use is an [[inductor]] placed in series, consisting of a winding on a laminated magnetic core. The [[inductance]] of this winding limits the flow of AC current. This type of ballast is common in 220–240V countries (And in North America, up to 30W lamps). Ballasts are rated for the size of lamp and power frequency. In North America, the AC voltage is insufficient to start long fluorescent lamps, so the ballast is often a step-up [[autotransformer]] with substantial [[leakage inductance]] (to limit current flow). Either form of inductive ballast may also include a [[capacitor]] for [[power factor]] correction.
[[File:New GE Ballast.JPG|thumb|230&nbsp;V [[electrical ballast|ballast]] for 18–20&nbsp;W]]

Fluorescent lamps can run directly from a [[direct current]] (DC) supply of sufficient voltage to strike an arc. The ballast must be resistive, and would consume about as much power as the lamp. When operated from DC, the starting switch is often arranged to reverse the polarity of the supply to the lamp each time it is started; otherwise, the mercury accumulates at one end of the tube. Fluorescent lamps are (almost) never operated directly from DC for those reasons. Instead, an [[inverter (electrical)|inverter]] converts the DC into AC and provides the current-limiting function as described below for electronic ballasts.

===Effect of temperature===
[[File:Helical fluorescent lamp thermal image.jpg|thumb|Thermal image of a helical fluorescent lamp]]
The performance of fluorescent lamps is critically affected by the temperature of the bulb wall and its effect on the partial pressure of the mercury vapor within.{{sfn|Kane|Sell|2001|loc=c.f. 182}} Since mercury condenses at the coolest spot in the lamp, careful design is required to maintain that spot at the optimum temperature, around {{convert|40|C|F}}.

Using an [[amalgam (chemistry)|amalgam]] with some other metal reduces the vapor pressure and increases the optimum temperature range. The bulb wall "cold spot" temperature must still be controlled to prevent condensing. High-output fluorescent lamps have features such as a deformed tube or internal heat-sinks to control cold spot temperature and mercury distribution. Heavily loaded small lamps, such as compact fluorescent lamps, also include heat-sink areas in the tube to maintain mercury vapor pressure at the optimum value.{{sfn|Kane|Sell|2001|p=188}}

===Losses===
[[File:Fluorescent Energy.svg|thumb|right| A [[Sankey diagram]] of energy losses in a fluorescent lamp. In modern designs, the biggest loss is the [[quantum efficiency]] of converting high-energy UV photons to lower-energy visible light photons.]]
Only a fraction of the electrical energy input into a lamp is converted to visible light. The ballast dissipates some heat; electronic ballasts may be around 90% efficient. A fixed voltage drop occurs at the electrodes, which also produces heat. Some of the energy in the mercury vapor column is also dissipated, but about 85% is turned into visible and ultraviolet light.

Not all the UV radiation striking the phosphor coating is converted to visible light; some energy is lost. The largest single loss in modern lamps is due to the lower energy of each photon of visible light, compared to the energy of the UV photons that generated them (a phenomenon called [[Stokes shift]]). Incident photons have an energy of 5.5 electron volts but produce visible light photons with energy around 2.5 electron volts, so only 45% of the UV energy is used; the rest is dissipated as heat.{{sfn|Kane|Sell|2001|pp=196–197}}

===Cold-cathode fluorescent lamps===  <!-- target of redirect -->
{{Redirect|CCFL}}
{{Further|Cold cathode}}
[[File:Cold Cathode Fluorescent Lamp.JPG|thumb|A cold-cathode fluorescent lamp from an emergency-exit sign. Operating at a much higher voltage than other fluorescents, the lamp produces a low-amperage [[glow discharge]] rather than an arc, similar to a [[neon light]]. Without direct connection to line voltage, current is limited by the transformer alone, negating the need for a ballast.]]

Most fluorescent lamps use electrodes that emit electrons into the tube by heat, known as hot cathodes. However, [[cold cathode]]  tubes have cathodes that emit electrons only due to the large [[voltage]] between the electrodes. The cathodes will be warmed by current flowing through them, but are not hot enough for significant [[thermionic emission]]. Because cold cathode lamps have no thermionic emission coating to wear out, they can have much longer lives than [[hot cathode]] tubes. This makes them desirable for long-life applications (such as backlights in [[liquid crystal display]]s). Sputtering of the electrode may still occur, but electrodes can be shaped (e.g. into an internal cylinder) to capture most of the sputtered material so it is not lost from the electrode.

Cold cathode lamps are generally less efficient than thermionic emission lamps because the cathode fall voltage is much higher. Power dissipated due to cathode fall voltage does not contribute to light output. However, this is less significant with longer tubes. The increased power dissipation at tube ends also usually means cold cathode tubes have to be run at a lower loading than their thermionic emission equivalents. Given the higher tube voltage required anyway, these tubes can easily be made long, and even run as series strings. They are better suited for bending into special shapes for lettering and signage, and can also be instantly switched on or off.

===Starting===
{{main|Glow switch starter}}
The gas used in the fluorescent tube must be ionized before the arc can "strike" . 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). Many different starting circuits have been used. The choice of circuit is based on cost, AC voltage, tube length, instant versus non-instant starting, temperature ranges and parts availability.

====Preheating====
[[File:Fluorescent Light.svg|thumb|A ''preheat'' fluorescent lamp circuit using an automatic starting switch. A: Fluorescent tube, B: Power (+220 volts), C: Starter, D: Switch (bi-metallic thermostat), E: Capacitor, F: Filaments, G: Ballast]]
[[File:Fluorescent lamp-electronic ballast starter-movie VNr°0001.ogv|thumb|left| Starting a preheat lamp. The automatic starter switch flashes orange each time it attempts to start the lamp.]]
Preheating, also called switchstart, uses a combination [[electrical filament|filament]]–[[cathode]] at each end of the lamp in conjunction with a mechanical or automatic ([[Bimetallic strip|bi-metallic]]) switch (see circuit diagram to the right) that initially connect the filaments in series with the ballast to preheat them; after a short preheating time the starting switch opens. If timed correctly relative to the phase of the supply AC, this causes the ballast to induce a voltage over the tube high enough to initiate the starting arc.<ref name=philipsapp>{{cite book|chapter=Chapter 8. Lighting|title=Power Semiconductor Applications|chapter-url= http://www.nxp.com/documents/application_note/APPCHP8.pdf|archive-url= https://web.archive.org/web/20091122114429/http://www.nxp.com/documents/application_note/APPCHP8.pdf|url-status=dead|archive-date=22 November 2009|access-date=22 November 2009|publisher=Philips Semiconductors}}</ref> These systems are standard equipment in 200–240&nbsp;V countries (and in the United States lamps up to about 30&nbsp;watts).

[[File:Starterp.jpg|thumb|left|A ''preheat'' fluorescent lamp "starter" (automatic starting switch)]]
Before the 1960s, four-pin thermal starters and manual switches were used.{{Citation needed|date=June 2013}} A [[glow switch starter]] automatically preheats the lamp cathodes. It consists of a normally open [[bi-metallic strip|bi-metallic]] switch in a small sealed [[gas-discharge lamp]] containing inert gas (neon or argon). The glow switch will cyclically warm the filaments and initiate a pulse voltage to strike the arc; the process repeats until the lamp is lit.  Once the tube strikes, the impinging main discharge keeps the cathodes hot, permitting continued electron emission. The starter switch does not close again because the voltage across the lit tube is insufficient to start a glow discharge in the starter.<ref name=philipsapp/>

[[File:Pulsestarter.JPG|thumb|Electronic fluorescent lamp starters]]

With glow switch starters a failing tube will cycle repeatedly.  Some starter systems used a thermal over-current trip to detect repeated starting attempts and disable the circuit until manually reset.

A [[power factor]] correction (PFC) [[capacitor]] draws leading current from the mains to compensate for the lagging current drawn by the lamp circuit.<ref name=philipsapp/>

'''Electronic starters''' use a different method to preheat the cathodes.<ref name="electrostart">{{cite web|title=Datasheet of typical electronic starter (not fast start), with detailed explanation of operation|url=http://www.tabelek.co.uk/datasheets/UM2-UM3-Datasheet.pdf|access-date=2011-04-08 |archive-date=2012-03-22 |archive-url=https://web.archive.org/web/20120322120421/http://www.tabelek.co.uk/datasheets/UM2-UM3-Datasheet.pdf|url-status=live}}</ref> They may be plug-in interchangeable with glow starters. They use a semiconductor switch and "soft start" the lamp by preheating the cathodes before applying a starting pulse which strikes the lamp first time without flickering; this dislodges a minimal amount of material from the cathodes during starting, giving longer lamp life.<ref name="philipsapp" /> This is claimed to prolong lamp life by a factor of typically 3 to 4 times for a lamp frequently switched on as in domestic use,<ref name="faststart">{{cite web|title=Datasheet of typical fast start electronic starter, with detailed explanation of operation|url=http://www.tabelek.co.uk/datasheets/300C-Datasheet.pdf|access-date=2011-04-08 |archive-date=2012-03-22 |archive-url=https://web.archive.org/web/20120322120454/http://www.tabelek.co.uk/datasheets/300C-Datasheet.pdf|url-status=live}}</ref> and to reduce the blackening of the ends of the lamp typical of fluorescent tubes. While the circuit is complex, the complexity is built into an [[integrated circuit]] chip. Electronic starters may be optimized for fast starting (typical start time of 0.3 seconds),<ref name="faststart" /><ref name="type300c">{{cite web|title=Electronic Tube Starter 300C Fastlux for fluorescent strip lights|url=http://www.tabelek.co.uk/product-300C-fastlux-electronic-fluorescent-tube-starter.php|website=www.tabelek.co.uk|access-date=2019-04-26 |archive-date=2019-01-08 |archive-url=https://web.archive.org/web/20190108100850/http://www.tabelek.co.uk/product-300C-fastlux-electronic-fluorescent-tube-starter.php|url-status=live}}</ref> or for most reliable starting even at low temperatures and with low supply voltages, with a startup time of 2–4 seconds.<ref name="um2">{{cite web|title=Soft Start Electronic Starter for fluorescent tubes the UM2 Multi Pulse|url=http://www.tabelek.co.uk/product-um2-multipulse-electronic-fluorescent-tube-starter.php|website=www.tabelek.co.uk|access-date=2019-04-26 |archive-date=2019-04-23 |archive-url=https://web.archive.org/web/20190423140214/http://www.tabelek.co.uk/product-um2-multipulse-electronic-fluorescent-tube-starter.php|url-status=live}}</ref> The faster-start units may produce audible noise during start-up.<ref>{{cite web| title=Fast electronic starters for fluorescent lights| date=20 June 2004<!--from page source-->| quote=All three of the 'FAST' (< .5 seconds) starter brands caused an audible 'BURRRRRRRP' noise in some light fittings as they started and this is an inherent problem caused by their use of the faster 'DC' heating. It is worse with higher wattage tubes and if there is any loose metal in the light fitting.| website=users.tpg.com.au| url=http://users.tpg.com.au/pschamb/light.html| access-date=25 February 2023| archive-date=2012-11-04 | archive-url=https://web.archive.org/web/20121104034555/http://users.tpg.com.au/pschamb/light.html| url-status=live}}</ref>

Electronic starters only attempt to start a lamp for a short time when power is initially applied, and do not repeatedly attempt to restrike a lamp that is dead and unable to sustain an arc; some automatically stop trying to start a failed lamp.<ref name="electrostart" /> This eliminates the re-striking of a lamp and the continuous flashing of a failing lamp with a glow starter. Electronic starters are not subject to wear and do not need replacing periodically, although they may fail like any other electronic circuit. Manufacturers typically quote lives of 20 years, or as long as the light fitting.<ref name="type300c" /><ref name="um2" />

====Instant start====
[[File:T12 Fluorescent lamp electrodes - rapid start and instant strt.jpg|thumb|T12 fluorescent tubes. The first two are rapid start, (for "tombstone" and socket holders respectively) while the third is an instant-start lamp. The instant-start  has a characteristic, rounded, single pin, for plugging into the spring-loaded socket holders.]]
''Instant start'' fluorescent tubes were invented in 1944. Instant start simply uses a high enough voltage to break down the gas column and thereby start arc conduction. Once the high-voltage spark "strikes" the arc, the current is boosted until a [[glow discharge]] forms. As the lamp warms and pressure increases, the current continues to rise and both resistance and voltage falls, until mains or line-voltage takes over and the discharge becomes an arc. These tubes have no filaments and can be identified by a single pin at each end of the tube (for common lamps; compact cold-cathode lamps may also have a single pin, but operate from a transformer rather than a ballast). The lamp holders have a "disconnect" socket at the low-voltage end which disconnects the ballast when the tube is removed, to prevent [[electric shock]]. Instant-start lamps are slightly more energy efficient than rapid start, because they do not constantly send a heating current to the cathodes during operation, but the cold cathodes starting increases sputter, and they take much longer to transition from a glow discharge to an arc during warm up, thus the lifespan is typically about half of those seen in comparable rapid-start lamps.<ref>''Mechanical and Electrical Equipment for Buildings'' by Walter T Grondzik, Alison Kwok, Benjamin Stein, John S Reynolds, Wiley Publishing, 2010, pp. 545–546</ref>

====Rapid start ====
[[File:Magnetek Watt Reducer HPF Rapid Start Ballast.jpg|thumb|right|A rapid-start "iron" (magnetic) [[electrical ballast|ballast]] continually heats the [[cathode]]s at the ends of the lamps. This ballast runs two F40T12 lamps in series.]]
Because the formation of an arc requires the [[thermionic emission]] of large quantities of electrons from the cathode, ''rapid start'' ballast designs provide windings within the ballast that continuously warm the cathode filaments. Usually operating at a lower arc voltage than the instant start design; no inductive [[voltage spike]] is produced for starting, so the lamps must be mounted near a grounded (earthed) reflector to allow the glow discharge to propagate through the tube and initiate the arc discharge via [[capacitive coupling]]. In some lamps a grounded "starting aid" strip is attached to the outside of the lamp glass. This ballast type is incompatible with the European energy saver T8 fluorescent lamps because these lamps require a higher starting voltage than that of the open circuit voltage of rapid start ballasts.

====Quick-start====
Quick-start ballasts use a small auto-transformer to heat the filaments when power is first applied. When an arc strikes, the filament heating power is reduced and the tube will start within half a second. The auto-transformer is either combined with the ballast or may be a separate unit. Tubes need to be mounted near an earthed metal reflector in order for them to strike. Quick-start ballasts are more common in commercial installations because of lower maintenance costs. A quick-start ballast eliminates the need for a starter switch, a common source of lamp failures. Nonetheless, Quick-start ballasts are also used in domestic (residential) installations because of the desirable feature that a Quick-start ballast light turns on nearly immediately after power is applied (when a switch is turned on). Quick-start ballasts are used only on 240&nbsp;V circuits and are designed for use with the older, less efficient T12 tubes.

====Semi-resonant start====
[[File:65W SRS fluorescent lamp starting.ogv|thumb|left|A 65-watt fluorescent lamp starting on a semi-resonant start circuit]]
[[File:semi resonant start circuit.png|thumb|right|A semi-resonant start circuit diagram]]
The semi-resonant start circuit was invented by Thorn Lighting for use with [[fluorescent-lamp formats|T12]] fluorescent tubes. This method uses a double wound transformer and a capacitor. With no arc current, the transformer and capacitor [[electrical resonance|resonate]] at line frequency and generate about twice the supply voltage across the tube, and a small electrode heating current.<ref>Thorn Lighting Technical Handbook</ref> This tube voltage is too low to strike the arc with cold electrodes, but as the electrodes heat up to thermionic emission temperature, the tube striking voltage falls below that of the ringing voltage, and the arc strikes. As the electrodes heat, the lamp slowly, over three to five seconds, reaches full brightness. As the arc current increases and tube voltage drops, the circuit provides current limiting.

Semi-resonant start circuits are mainly restricted to use in commercial installations because of the higher initial cost of circuit components. However, there are no starter switches to be replaced and cathode damage is reduced during starting making lamps last longer, reducing maintenance costs. Because of the high open circuit tube voltage, this starting method is particularly good for starting tubes in cold locations. Additionally, the circuit power factor is almost 1.0, and no additional power factor correction is needed in the lighting installation. As the design requires that twice the supply voltage must be lower than the cold-cathode striking voltage (or the tubes would erroneously instant-start), this design cannot be used with {{nowrap|240 volt}} AC power unless the tubes are at least {{convert|1.2|m|abbr=on}} length.  Semi-resonant start fixtures are generally incompatible with energy saving T8 retrofit tubes, because such tubes have a higher starting voltage than T12 lamps and may not start reliably, especially in low temperatures. Recent proposals in some countries to phase out T12 tubes will reduce the application of this starting method.

====Electronic ballasts====
[[File:Fluorescent-lamp-electronic-ballast.webm|thumb|left|Fluorescent lamp with an electronic ballast]]
[[File:EL 2x58ngn.jpg|thumb|Electronic ballast for fluorescent lamp, 2×58&nbsp;W]]
[[File:Fluorescent Lamp Inverter.png|thumb|[[Electronic ballast]] basic schematic]]
[[File:TCS lamps2.jpg|thumb|[[Electronic ballast]]s and different compact fluorescent lamps]]Electronic ballasts employ [[transistor]]s to change the supply frequency into high-[[frequency]] [[alternating current|AC]] while regulating the current flow in the lamp. These ballasts take advantage of the higher efficacy of lamps, which rises by almost 10% at {{nowrap|10 kHz}}, compared to efficacy at normal power frequency. When the AC period is shorter than the relaxation time to de-ionize mercury atoms in the discharge column, the discharge stays closer to optimum operating condition.{{sfn|Kane|Sell|2001|p=182}} Electronic ballasts convert supply frequency AC power to variable frequency AC. The conversion can reduce lamp brightness modulation at twice the power supply frequency.

Low cost ballasts contain only a simple oscillator and series resonant [[LC circuit]]. This principle is called the [[Resonant inverter|current resonant inverter]] circuit. After a short time the voltage across the lamp reaches about 1&nbsp;kV and the lamp instant-starts in cold cathode mode. The cathode filaments are still used for protection of the ballast from overheating if the lamp does not ignite. A few manufacturers use positive temperature coefficient (PTC) [[thermistor]]s to disable instant starting and give some time to preheat the filaments.

More complex electronic ballasts use programmed start. The output 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.

Many electronic ballasts are controlled by a [[microcontroller]], and these are sometimes called digital ballasts. Digital ballasts can apply quite complex logic to lamp starting and operation. This enables functions such as testing for broken electrodes and missing tubes before attempting to start, detection of tube replacement, and detection of tube type, such that a single ballast can be used with several different tubes. Features such as dimming can be included in the embedded microcontroller software, and can be found in various manufacturers' products.

Since introduction in the 1990s, high-frequency ballasts have been used in general lighting fixtures with either rapid start or pre-heat lamps. These ballasts convert the incoming power to an output frequency in excess of {{nowrap|20 kHz}}. This increases lamp efficiency.<ref>{{cite web|title=Energy Conservation Standards for Fluorescent Lamp Ballasts|url=http://www1.eere.energy.gov/buildings/appliance_standards/residential/pdfs/flballast_preanalysis_ch3.pdf|archive-url=https://web.archive.org/web/20120803184327/http://www1.eere.energy.gov/buildings/appliance_standards/residential/pdfs/flballast_preanalysis_ch3.pdf|url-status=dead|archive-date=3 August 2012|publisher=US Department of Energy|access-date=29 January 2012|pages=3–23}}</ref>  These ballasts operate with voltages that can be almost 600 volts, requiring some consideration in housing design, and can cause a minor limitation in the length of the wire leads from the ballast to the lamp ends.

===End of life===
The life expectancy of a fluorescent lamp is primarily limited by the life of the cathode electrodes. To sustain an adequate current level, the electrodes are coated with an emission mixture of metal oxides.  Every time the lamp is started, and during operation, a small amount of the cathode coating is [[sputtering|sputtered]] off the electrodes by the impact of electrons and heavy ions within the tube. The sputtered material collects on the walls of the tube, darkening it. The starting method and frequency affect cathode sputtering.  A filament may also break, disabling the lamp. 
[[File:Fluorescent wear - sputter.png|thumb|right|This tube failed after it had been turned on many times. Too much of the thermionic emission mix had sputtered off the cathodes, sticking to and blackening the glass.]]
[[Image:Thermionic filament.jpg|thumb|left|The filament of a low-pressure mercury gas-discharge lamp, with white [[thermionic emission]] coating acting as [[hot cathode]]. A little of the coating is [[Sputtering|sputtered away]] at every start; the lamp ultimately fails.]]

Low-mercury designs of lamps may fail when mercury is absorbed by the glass tube, phosphor, and internal components, and is no longer available to vaporize in the fill gas. Loss of mercury initially causes an extended warm-up time to full light output, and finally causes the lamp to glow a dim pink when the argon gas takes over as the primary discharge.<ref>{{cite conference |last1=Corazza |first1=A. |last2=Giorgi |first2=S. |last3=Massaro |first3=V. |title= Mercury Dosing in Fluorescent Lamps |book-title=2008 IEEE Industry Applications Society Annual Meeting |pages=1–4 |conference=Industry Applications Society Annual Meeting |date=5–9 October 2008 |issn=0197-2618 |publisher=IEEE |doi= 10.1109/08IAS.2008.237}}</ref>

Subjecting the tube to asymmetric current flow, effectively operates it under a DC bias, and causes asymmetric distribution of mercury ions along the tube. The localized depletion of mercury vapor pressure manifests itself as pink luminescence of the base gas in the vicinity of one of the electrodes, and the operating lifetime of the lamp may be dramatically shortened. This can be an issue with some poorly designed [[inverter (electrical)|inverters]].<ref>{{cite web|url=http://www.htl.co.jp/img/p_pro4_p10.pdf|archive-url=https://web.archive.org/web/20071022003220/http://www.htl.co.jp/img/p_pro4_p10.pdf|url-status=dead|archive-date=22 October 2007|title=Cold Cathode Fluorescent Lamp|publisher=Harison Toshiba Corp.|access-date=22 October 2007}}</ref>

The phosphors lining the lamp degrade with time as well, until a lamp no longer produces an acceptable fraction of its initial light output.

Failure of the integral electronic ballast of a compact fluorescent bulb will also end its usable life.

[[File:Mercury Adsorbed CFL.jpg|thumb|right|Compact fluorescent lamp that has reached end of life because of mercury [[adsorption]]. Light is produced only by the base argon fill.]]