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Fluorescent lamp



      A fluorescent lamp or fluorescent tube 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 ft (120 cm) or 8 ft (240 cm) 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 bulbs, allowing them to be used in lampholders normally used for incandescent lamps.

Additional recommended knowledge

Contents

History

The history of the fluorescent lamp begins with early research into electrical phenomena. By the beginning of the 18th century, experimenters had observed a radiant glow emanating from partially evacuated glass vessels through which an electrical current passed. Little more could be done with this phenomenon until 1856 when a German glassblower named Heinrich Geißler (1815-1879) created a mercury vacuum pump that evacuated a glass tube to an extent not previously possible. When an electrical current passed through a Geissler tube, a strong green glow on the walls of the tube at the cathode end could be observed.

Because it produced some beautiful light effects, the Geissler tube was popular source of amusement. More important, however, was its contribution to scientific research. One of the first scientists to experiment with a Geissler tube was Julius Plücker (1801-1868) who in 1858 systematically described the luminescent effects that occurred in a Geissler tube. He also made the important observation that the glow in the tube shifted position when in proximity to an electromagnetic field.

Inquiries that began with the Geissler tube continued as even better vacuums were produced. The most famous was the evacuated tube used for scientific research by William Crookes (1832-1919) that was evacuated by the highly effective mercury vacuum pump created by Hermann Sprengel (1834-1906). Research conducted by Crookes and others ultimately led to the discovery of the electron in 1897 by J. J. Thomson (1856-1940). But the Crookes tube, as it came to be known, produced little light because it contained too good a vacuum and thus lacked the trace amounts of gas that are needed for electrically stimulated luminescence. An important stage on the long scientific path that lead to the fluorescent lamp was Alexandre Edmond Becquerel’s (1820-1891) observation in 1859 of the luminescence of certain substances when they were placed in a Geissler tube. He went on to apply thin coatings of luminescent materials to the surfaces of these tubes. Fluorescence occurred, but the tubes were very inefficient and had a short operating life. A few years earlier another scientist, George G. Stokes (1819-1903), had noted that ultraviolet light caused fluorspar to fluoresce, a property that would become critically important for the development of fluorescent lights many decades later.

While Becquerel was primarily interested in conducting scientific research into fluorescence, Thomas Edison (1847 – 1931) briefly pursued fluorescent lighting for its commercial potential. He invented a fluorescent lamp in 1896 which used a coating of calcium tungstate as the fluorescing substance, but although it received a patent in 1907, it was not put into production. As with a few other attempts to use Geissler tubes for illumination, it had a short operating life, and given the success of the incandescent light, Edison had little reason to pursue an alternative means of electrical illumination.

Although Edison lost interest in fluorescent lighting, one of his former employees was able to create a gas-based lamp that achieved a measure of commercial success. In 1895 Daniel McFarlan Moore (1869 - 1933) demonstrated electrically activated tubes 7 to 9 feet in length that used carbon dioxide or nitrogen to emit white or pink light, respectively. As with future fluorescent lamps, it was considerably more complicated than an incandescent bulb.

After years of work, Moore was able to extend the operating life of the lamps by inventing an electromagnetically controlled valve that maintained a constant gas pressure within the tube. Although Moore’s lamp was complicated, expensive to install, and required very high voltages, it was considerably more efficient than incandescent lamps, and it produced a more natural light than incandescents. From 1904 onwards Moore’s lighting system was installed in a number of stores and offices. Its success contributed to General Electric’s motivation to improve the incandescent lamp, especially its filament. GE’s efforts came to fruition with the invention of a tungsten-based filament. The extended lifespan of incandescent bulbs negated one of the key advantages of Moore’s lamp, but GE purchased the relevant patents in 1912. These patents and the inventive efforts that supported them were to be of considerable value when the firm took up fluorescent lighting more than two decades later.

At about the same time that Moore was developing his lighting system, another American was creating a means of illumination that also can be seen as a precursor to the modern fluorescent lamp. This was the mercury vapor lamp, invented by Peter Cooper Hewitt (1861-1921) and patented in 1901 (U.S. Pat. No. 889,692). As the name implies, Cooper-Hewitt’s lamp luminesced when an electric current was passed through mercury vapor at a low pressure. Unlike Moore’s lamps, those made by Cooper-Hewitt could be manufactured in standardized sizes and operated at low voltages. The mercury-vapor lamp was superior to the incandescent lamps of the time in terms of energy efficiency, but the blue-green light it produced limited its applications. It was, however, used for photography and some industrial processes.

Mercury vapor lamps continued to be developed at a slow pace, especially in Europe, and by the early 1930s they received limited use for large-scale illumination. Some of them employed fluorescent coatings, but these were primarily used for color correction and not for enhanced light output. Mercury vapor lamps also anticipated the fluorescent lamp in their incorporation of a ballast to maintain a constant flow of current.

Cooper-Hewitt had not been the first to use mercury vapor for illumination, as earlier efforts had been mounted by Way, Rapieff, Arons, and Bastian and Salisbury. Of particular importance was the mercury vapor lamp invented by Küch in Germany. This lamp used quartz in place of glass to allow higher operating temperatures, and hence greater efficiency. Although its light output relative to electrical consumption was better than other sources of light, the light it produced was similar to that of the Cooper-Hewitt lamp in that it lacked the red portion of the spectrum, making it unsuitable for ordinary lighting.

An electric current passing through a tube was the basis for another form of illumination, the neon light. While Moore used carbon dioxide, nitrogen, or atmospheric air to fill the tubes and Cooper-Hewitt and others employed mercury vapor, the next step in gas-based lighting took advantage of the luminescent qualities of neon, an inert gas that had been discovered in 1898. In 1909 Georges Claude (1870 – 1960), a French chemist, observed the red glow that was produced when running an electric current through a neon-filled tube. He also discovered that a blue glow resulted from the use of another inert gas, argon. The light could be used for general illumination, and in fact was used in France for this purpose beginning around 1930, but neon lighting was no more energy-efficient than conventional incandescent lighting, and it came to be used primarily for eye-catching signs and advertisements. Neon lighting was not irrelevant to the development of fluorescent lighting, however, as Claude’s improved electrode (patented in 1915) overcame “sputtering,” a major source of electrode degradation. Sputtering occurred when ionized particles struck an electrode and tore off bits of metal. Although Claude’s invention required electrodes with a lot of surface area, it showed that a major impediment to gas-based lighting could be overcome.

The development of the neon light also was significant for the last key element of the fluorescent lamp, its fluorescent coating. In 1926 Jacques Risler received a French patent for the application of fluorescent coatings to neon light tubes. The main use of these lamps, which can be considered the first commercially successful fluorescents, was for advertising, not general illumination. This, however, was not the first use of fluorescent coatings. As has been noted above, Edison used calcium tungstate for his unsuccessful lamp. Other efforts had been mounted, but all were plagued by low efficiency and various technical problems. Of particular importance for the story that was to follow was the invention in 1927 of a low-voltage “metal vapor lamp” by Friedrich Meyer, Hans-Joachim Spanner, and Edmund Germer, who at the time were employees of a German firm located in Berlin. A German patent was granted but the lamp never went into commercial production.

All the major features of fluorescent lighting were in place at the end of the 1920s. Decades of invention and development had provided the key components of fluorescent lamps: economically manufactured glass tubing, inert gases for filling the tubes, electrical ballasts, long-lasting electrodes, mercury vapor as a source of luminescence, effective means of producing a reliable electrical discharge, and fluorescent coatings that could be energized by ultraviolet light. At this point, intensive development was more important than basic research.

In 1934 Arthur Compton, a renowned physicist and GE consultant, sent a report to W.L. Enfield, manager of research and engineering at GE lamp department, outlining successful experiments with fluorescent lighting at the research laboratory of the General Electric Co., Ltd. in Great Britain (although it bore the GE moniker, this firm had no direct connection with General Electric in the United States). Stimulated by this report, and with all of the key elements available, a team led by George E. Inman built a prototype fluorescent lamp in 1934 at General Electric’s Nela Park (Ohio) engineering laboratory. This was not a trivial exercise; as noted by Arthur A. Bright, “A great deal of experimentation had to be done on lamp sizes and shapes, cathode construction, gas pressures of both argon and mercury vapor, colors of fluorescent powders, methods of attaching them to the inside of the tube, and other details of the lamp and its auxiliaries before the new device was ready for the public.”

In addition to having talented engineers and technicians along with excellent facilities for R&D work on fluorescents, General Electric controlled what it regarded as the key patents covering fluorescent lighting, including the patents originally issued to Cooper-Hewitt, Moore, and Küch. More important than these was a patent covering an electrode that did not disintegrate at the gas pressures that ultimately were employed in fluorescent lamps. This invention had been created by Albert W. Hull of GE’s Schenectady Research Laboratory, and was registered as U.S. Pat. No. 1,790,153.

While the Hull patent gave GE a basis for claiming legal rights over the fluorescent lamp, a few months after the lamp went into production the firm learned of a U.S. patent application had been filed in 1927 for the aforementioned “metal vapor lamp” invented in Germany by Meyer, Spanner, and Germer. The patent application indicated that the lamp had been created as a superior means of producing ultraviolet light, but the application also contained a few statements referring to fluorescent illumination. Efforts to obtain a U.S. patent had met with numerous delays, but were it to be granted, the patent might have caused serious difficulties for GE. At first, GE sought to block the issuance of a patent by claiming that priority should go to one of their employees, Leroy J. Buttolph, who according to their claim had invented a fluorescent lamp in 1919 and whose patent application was still pending. GE also had filed a patent application in 1936 in Inman’s name to cover the “improvements” wrought by his group. In 1939 GE decided that the claim of Meyer, Spanner, and Germer had some merit, and that in any event a long interference procedure was not in their best interest. They therefore dropped the Buttolph claim and paid $180,000 to acquire the Meyer, et al. application, which at that point was owned by a firm known as Electrons, Inc. The patent (U.S. Pat. No. 2,182,732) was duly awarded in December 1939. This patent, along with the Hull patent, put GE on what seemed to be firm legal ground, although it faced years of legal challenges from Sylvania Electric Products, Inc., which claimed infringement on patents that it held.

Even though the patent issue would not be completely resolved for many years, General Electric’s strength in manufacturing and marketing gave it a pre-eminent position in the emerging fluorescent light market. Sales of “fluorescent lumiline lamps” commenced in 1938 when four different sizes of tubes were put on the market. During the following year GE and Westinghouse publicized the new lights through exhibitions at the New York World’s Fair and the Golden Gate Exposition in San Francisco. Fluorescent lighting systems diffused rapidly during World War II as industrial manufacture, stimulated by wartime needs, gave rise to intensified lighting demands. The use of fluorescent lighting continued to spread in the years following the war, and by 1951 more light was produced in the United States by fluorescents than by incandescents.

Principles of operation

The main principle of fluorescent tube operation is based around inelastic scattering of electrons. An incident electron (emitted from the coating on 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 an ultraviolet photon as the atom's electron reverts to a lower, more stable, energy level. The photons that are released from the chosen gas mixtures tend to have a wavelength in the ultraviolet 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 ultraviolet 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 is filled with a gas containing low pressure mercury vapor and argon (or xenon), or more rarely argon-neon, or sometimes even krypton. The inner surface of the bulb is coated with a fluorescent (and often slightly phosphorescent) coating 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 vapor 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 standard atmospheric pressure), 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: two intense lines of 440nm and 546nm wavelength appear on commercial fluorescent tubes) (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, so as more current flows through them (more gas ionized), the electrical resistance of the fluorescent lamp drops, allowing even more current to flow. Connected directly to a constant-voltage mains power line, a fluorescent lamp would rapidly self-destruct due to the uncontrolled current flow. To prevent this, fluorescent lamps must use an 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 AC 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.

In the past, fluorescent lamps were occasionally run directly from a DC supply of sufficient voltage to strike an arc. In this case, there was no question that the ballast must have been resistive rather than reactive, leading to power losses in the ballast resistor. Also, when operated directly from DC, the polarity of the supply to the lamp must be reversed every time the lamp is started; otherwise, the mercury accumulates at one end of the tube. Nowadays, fluorescent lamps are essentially never operated directly from DC; instead, an inverter converts the DC into AC and provides the current-limiting function as described below for electronic ballasts.

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".

Flicker

Fluorescent lamps which operate directly from mains frequency AC will flicker at twice the mains frequency, since the power being delivered to the lamp drops to zero twice per cycle. This means the light flickers at 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 can also cause hum from fluorescent lamps, actually from its 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.

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 center of gaze.

Efficacy

The efficacy of fluorescent tubes ranges from about 16 lumens/watt for a 4 watt tube with an ordinary ballast to as high as about 95 lumens/watt for a 32 watt tube with modern electronic ballast, commonly averaging 50 to 67 lm/W overall. Most compact fluorescents 13 watts or more with integral electronic ballasts achieve about 60 lumens/watt. Due to phosphor degradation as they age, the average brightness over the entire service life is actually about 10% less.[1]

Starting

   

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

  1. they have a single pin at each end of the tube and
  2. 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 cannot receive a high-voltage electric shock.

In other cases, a separate starting aid must be provided. Some fluorescent designs (preheat lamps) use a combination filament/cathode at each end of the lamp in conjunction with a mechanical or automatic switch (see photo) that initially connect the filaments in series with the ballast and thereby preheats the filaments prior to striking the arc.

These systems are standard equipment in 240 V countries, and generally use a glowstarter. Before the 1960s, four-pin thermal starters and manual switches were also used. Electronic starters are also sometimes used with these electromagnetic ballast fittings.

During preheating, the filaments emit electrons into the gas column by thermionic emission, creating a glow discharge around the filaments. Then, when the starting switch opens, the inductive ballast and a small value capacitor across the starting switch create a high voltage which strikes the arc. Tube strike is reliable in these systems, but glowstarters will often cycle a few times before letting the tube stay lit, which causes objectionable flashing during starting. The older thermal starters behaved better in this respect.

Once the tube is struck, the impinging main discharge then keeps the filament/cathode hot, permitting continued emission.

If the tube fails to strike, or strikes then extinguishes, the starting sequence is repeated. With automated starters such as glowstarters, a failing tube will thus cycle endlessly, flashing time and time again as the starter repeatedly starts the worn-out lamp, and the lamp then quickly goes out as emission is insufficient to keep the cathodes hot, and lamp current is too low to keep the glowstarter open. This causes visually unpleasant frequent bright flashing and runs the ballast at above design temperature. Turning the glowstarter a quarter turn anticlockwise will disconnect it, opening the circuit.

Some more advanced starters time out in this situation, and do not attempt repeated starts until power is reset. Some older systems used a thermal overcurrent trip to detect repeated starting attempts. These require manual reset.

Newer rapid start ballast designs provide filament power windings within the ballast; these rapidly and continuously warm the filaments/cathodes using low-voltage AC. No inductive voltage spike is produced for starting, 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 sometimes an autodisconnecting circuit) may complete the circuit between the two filaments, providing filament preheating. When the tube lights, the voltage and frequency across the tube and capacitor typically both drop, thus capacitor current falls to a low but non-zero value. Generally this capacitor and the inductor, which provides current limiting in normal operation, form a resonant circuit, increasing the voltage across the lamp so 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.

Beginning in the 1990s a new type of ballast came into the mainstream, with a more expensive but significantly more efficient design: high frequency operation. These newer design high frequency ballasts have been used with either rapid start or pre-heat cathode/anode style lamps (with pins shorted at the lamp end), and use high frequency to excite the mercury within the lamp. These newer electronic ballasts convert the 50 or 60 hertz coming into the ballast to an output frequency in excess of 100 kHz. This allows for a more efficient system that generates less waste heat and requires significantly less power to light the lamp, and operates in a rapid starting manner. These are used in several applications, including new generation tanning lamp systems, whereby a 100 watt lamp (i.e., F71T12BP) can be lighted using 65 to 70 watts of actual power while obtaining the same lumens as traditional ballasts at full power. These 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. These ballasts run just a few degrees above ambient temperature, which is partly why they are more efficient and allows them to be used in applications that would be inappropriate for hotter running electronics.

End of life

The end of life failure mode for fluorescent lamps varies depending how they are used and their control gear type. There are three main failure modes currently, and a fourth which is starting to appear:

Emission mix

  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.

Ballast electronics

This is only relevant to compact fluorescent lamps with integral electrical ballasts. Ballast electronics failure is a somewhat random process which follows the standard failure profile for any electronic devices. Integral electronic ballasts suffer from shortened lifespans in high humidity applications. There is an initial small peak of early failures, followed by a drop and steady increase over lamp life. Life of electronics is heavily dependent on operating temperature—it typically halves for each 10 °C temperature rise. The quoted average life of a lamp is usually at 25 °C ambient (this may vary by country). The average life of the electronics at this temperature is normally greater than this, so at this temperature, not many lamps will fail due to failure of the electronics. 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.

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 some manufacturers claim much longer half-lives for their lamps). Lamps which do not suffer failures of the emission mix or integral ballast electronics will eventually develop this failure mode. 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.

Loss 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. 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 glow 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

Some people find the color spectrum produced by some fluorescent lamps to be harsh and displeasing. A healthy person can sometimes appear to have an unhealthy skin tone under fluorescent lighting. The extent to which this phenomenon occurs is related to the light's color rendering index (CRI).

CRI is a measure of how well balanced the different color components of the white light are. By definition, an incandescent lamp has a CRI of 100. Real-life fluorescent tubes achieve CRIs of anywhere from 50% to 99%. Fluorescent lamps with low CRI have phosphors which emit too little red light. Skin appears less pink and unhealthy compared to incandescent lighting. Colored objects appear muted. For example a low CRI 6800K halophosphate tube, which is about as visually unpleasant as they get, will make reds appear dull red or brown.

CCT Color temperature is a measure of the whiteness of a light source. Typical incandescent lighting is 2700K which is yellowish-white. Halogen lighting is 3000K. Fluorescent lamps are manufactured to a chosen CCT by altering the mixture of phosphors inside the tube. Warm-white fluorescents have CCT of 2700K and are popular for residential lighting. Neutral-white fluorescents have a CCT of 3000K or 3500K. Cool-white fluorescents have a CCT of 4100K and are popular for office lighting. Daylight fluorescents have a CCT of 5000K to 6500K, which is bluish-white.

High CCT lighting generally requires higher light levels. At dimmer illumination levels, the human eye perceives lower color temperatures as more natural. So, a dim 2700K incandescent lamp appears natural, and a bright 5000K lamp also appears natural, but a dim 5000K fluorescent lamp appears too pale. Daylight-type fluorescents look natural only if they are very bright.

Some of the least pleasant light comes from tubes containing the older halophosphate type phosphors (chemical formula Ca5(PO4)3(F,Cl):Sb3+,Mn2+). 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 appears white, but the light has an incomplete spectrum. The CRI of such lamps is only 60.

Since the 1990s, higher quality fluorescent lamps use either a higher CRI halophosphate coating, or a triphosphor mixture, based on europium and terbium ions, that have emission bands more evenly distributed over the spectrum of visible light. High CRI halophosphate and triphosphor tubes give a more natural color reproduction to the human eye. The CRI of such lamps is typically 82-100.

At least some of the early fluorescent lamps used compounds containing beryllium, a toxic element. However, it's very unlikely that one would encounter any such lamps.

Fluorescent lamp spectra
Typical fluorescent lamp with "rare earth" phosphor A typical "cool white" fluorescent lamp utilizing two rare earth doped phosphors, Tb3+, Ce3+:LaPO4 for green and blue emission and Eu:Y2O3 for red. For an explanation of the origin of the individual peaks click on the image. Note that several of the spectral peaks are directly generated from the mercury arc. This is likely the most common type of fluorescent lamp in use today.
An older style halophosphate phosphor fluorescent lamp Halophosphate phosphors in these lamps usually consist of trivalent antimony and divalent manganese doped calcium halophosphate (Ca5(PO4)3(Cl,F):Sb3+, Mn2+). The color of the light output can be adjusted by altering the ratio of the blue emitting antimony dopant and orange emitting manganese dopant. The color rendering ability of these older style lamps is quite poor. Halophosphate phosphors were invented by A.H. McKeag et al. in 1942.
"Natural sunshine" fluorescent light An explanation of the origin of the peaks is on the image page.
Yellow fluorescent lights The spectrum is nearly identical to a normal fluorescent bulb except for a near total lack of light below 500 nanometers. This effect can be achieved through either specialized phosphor use or more commonly by the use of a simple yellow light filter. These lamps are commonly used as lighting for photolithography work in cleanrooms and as "bug repellant" outdoor lighting (the efficacy of which is questionable).
Spectrum of a "blacklight" bulb There is typically only one phosphor present in a blacklight bulb, usually consisting of europium-doped strontium fluoroborate which is contained in an envelope of Wood's glass.
Spectrum of a "mercury" Fluorescent bulb Taken from a 'low cost' spectrometer (cost about $100). The results are similar if not better than traditional but far more expensive spectrometers

Usage

Fluorescent light bulbs come in many shapes and sizes. The compact fluorescent light bulb (CF) is becoming more popular. Many compact fluorescent lamps integrate the auxiliary electronics into the base of the lamp, allowing them to fit 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 rarely use incandescent lights.

Lighting arrangements use fluorescent tubes in an assortment of tints of white. Sometimes this is because of the lack of appreciation for the difference or importance of differing tube types. Mixing tube types within fittings improves the color reproduction of lower quality tubes. Tax incentives and environmental awareness result in higher use in places such as California.

In other countries, residential use of fluorescent lighting varies depending on the price of energy, financial and environmental concerns of the local population, and acceptability of the light output. In East and Southeast Asia it is very rare to see incandescent bulbs in buildings anywhere.

In February 2007, Australia enacted a law that will ban most sales of incandescent light bulbs by 2010.[2] While the law does not specify which alternative Australians are to use, compact fluorescents are likely to be the primary replacements. In April 2007, Canada announced a similar plan to phase out the sale of incandescent bulbs by 2012. Finnish parlament has been discussing banning sales of incandescent light bulbs by the beginning of 2011.[3]

Advantages

Fluorescent lamps are more efficient than incandescent light bulbs of an equivalent brightness. This is because a greater proportion of the power used is converted to usable light and a smaller proportion is converted to heat, allowing fluorescent lamps to run cooler. A typical 100 Watt tungsten filament incandescent lamp may convert only 2.6% of its power input to visible light, whereas typical fluorescent lamps convert between 6.6% and 15.2% of their power input to visible light - see the table in the luminous efficacy article. Typically a fluorescent lamp will last between 10 to 20 times as long as an equivalent incandescent lamp.[citation needed]

The higher initial cost of a fluorescent lamp is usually more than compensated for by lower energy consumption over its life. The longer life may also reduce lamp replacement costs, providing additional saving especially where labour is costly. Therefore it is widely used by businesses worldwide, but not so much by households.

 

Disadvantages

Health issues

Fluorescent lamps can cause problems among individuals with pathological sensitivity to ultraviolet light. They can induce disease activity in photosensitive individuals with Systemic lupus erythematosus; standard acrylic diffusers absorb UV-B radiation and appear to protect against this.[4] In rare cases individuals with solar urticaria (allergy to sunlight) can get a rash from fluorescent lighting.[5]

Elimination of fluorescent lighting is appropriate for several conditions. In addition to causing headache and fatigue,[6] and problems with light sensitivity,[7] they are listed as problematic for individuals with epilepsy,[8] lupus,[9] chronic fatigue syndrome, and vertigo[10] (related to cardiovascular problems, MS, and several other disorders.) Research on this is very limited. There seems to be even less evidence disputing the effects than confirming them.

Ballasts

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 light fixtures, though often one ballast is shared between two or more lamps. Electromagnetic ballasts with a minor fault can produce an audible humming or buzzing noise.

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 because of the power lost in the resistor. Also, the mercury tends to migrate to one end of the tube leading to only one end of the lamp producing most of the light. Because of this effect, the lamps (or the polarity of the current) must be reversed at regular intervals.

Power factor

Fluorescent lamp ballasts have a power factor of less than unity. For large installations, this makes the provision of electrical power more expensive as special measures need to be taken to bring the power factor closer to unity.

Power harmonics

Fluorescent lamps are a non-linear load and generate harmonics on the 50 Hz or 60 Hz sinusoidal waveform of the electrical power supply. This can generate radio frequency noise in some cases. Suppression of harmonic generation is standard practice, but imperfect. Very good suppression is possible, but adds to the cost of the fluorescent fixtures.

Optimum operating temperature

Fluorescent lamps operate best around room temperature (say, 20 °C or 68 °F). 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. A "cold start" electrical circuit was also developed in the mid-1970s.

Non-compact light source

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 tube lamps were large compared with incandescent sources. However, in many cases low luminous intensity of the emitting surface was useful because it reduced glare. The bulk created by this lamp affected the design of fixtures since light must be directed from long tubes instead of a compact source.

Recently, a new type of fluorescent lamp, the CFL, has been introduced to address this issue and allow regular incandescent sockets to be fitted with this type of lamp, thereby negating the need to mount it on special fixtures. However, some CFLs intended to replace incandescents will not fit some desk lamps, because the harp (heavy wire shade support bracket) is shaped for the narrow neck of an incandescent lamp. CFLs tend to have a wide housing for their electronic ballast close to the lamp's base, too wide to fit.

Flicker problems

Fluorescent fittings using a magnetic mains frequency ballast do not give out a steady light; instead, they flicker (fluctuate in intensity) at twice the supply frequency. While this is not easily discernible 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 to a lesser extent. 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, by operating the lamps on different phases of a polyphase power supply, or by use of electronic ballasts.

Electronic ballasts do not produce light flicker, since the phosphor persistence is longer than a half cycle of the higher operation frequency.

The non-visible 100–120 Hz flicker from fluorescent tubes powered by magnetic ballasts is associated with headaches and eyestrain. Individuals with high flicker fusion threshold are particularly affected by magnetic ballasts: their EEG alpha waves are markedly attenuated and they perform office tasks with greater speed and decreased accuracy. The problems are not observed with electronic ballasts.[11] Ordinary people have better reading performance using high-frequency (20–60 kHz) electronic ballasts than magnetic ballasts.[12]

The flicker of fluorescent lamps, even with magnetic ballasts, is so rapid that it is unlikely to present a hazard to individuals with epilepsy.[13] Early studies suspected a relationship between the flickering of fluorescent lamps with magnetic ballasts and repetitive movement in autistic children.[14] However, these studies had interpretive problems[15] and have not been replicated.

Color rendition

The issues with color faithfulness of some tube types are discussed above.

Dimming

Unless specifically designed and approved to accommodate dimming, most fluorescent light fixtures cannot be connected to a standard dimmer switch used for incandescent lamps. Two effects are responsible for this: the waveshape of the voltage emitted by a standard phase-control dimmer interacts badly with many ballasts and it becomes difficult to sustain an arc in the fluorescent tube at low power levels. Many installations require 4-pin fluorescent lamps and compatible controllers for successful fluorescent dimming; these systems tend to keep the cathodes of the fluorescent tube fully heated even as the arc current is reduced, promoting easy thermionic emission of electrons into the arc stream.

Disposal and recycling

The disposal of phosphor and particularly the mercury in the tubes is an environmental issue. Mercury poses the greatest hazard to pregnant women, infants, and children. Governmental regulations in many areas require special disposal of fluorescent lamps separate from general and household wastes. For large commercial or industrial users of fluorescent lights, recycling services are available in many nations, and may be required by regulation. In some areas, recycling is also available to consumers. The need for a recycling infrastructure is an issue with instituting proposed bans of incandescent bulbs.

The amount of mercury in a standard lamp can vary dramatically, from 3 to 46 mg. [16] Newer lamps contain less mercury and the 3-4 mg versions are sold as low-mercury types. A typical 2006-era 4 ft (122 cm) T-12 fluorescent lamp (i.e., F32T12) contains about 12 milligrams of mercury[17]. In early 2007, the National Electrical Manufacturers Association in the US announced that "Under the voluntary commitment, effective April 15, 2007, participating manufacturers will cap the total mercury content in CFLs under 25 watts at 5 milligrams (mg) per unit. CFLs that use 25 to 40 watts of electricity will have total mercury content capped at 6 mg per unit."[18]

A broken fluorescent tube is more hazardous than a broken conventional incandescent bulb due to the mercury content. Because of this, the safe cleanup of broken fluorescent bulbs differs from cleanup of conventional broken glass or incandescent bulbs. 99% of the mercury is typically contained in the phosphor, especially on lamps that are near their end of life.[19] Fluorescent lamps manufactured many decades ago had phosphors that contained beryllium, which is toxic. One is not likely to encounter lamps this old.

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" or 25 mm) for commercial energy-saving lamps with electronic ballasts, and T5 (58" or 16 mm) for very small lamps which may even operate from a battery-powered device.

Some lamps are designed with a reflector built inside the lamp. This is done by pouring an opaque coating into the lamp first, rotating the lamp to achieve the desired amount of coverage, then allowing it to dry before adding the traditional phosphors. With straight lamps, this is commonly poured in a fashion as to cover half the lamp as it is lying flat, with the lamp rated as to the amount of curvature that is covered in the opaque coating. A 180 degree lamp has 50% coverage, whereas a 210 degree lamp has 30 degrees more coverage. These are the most common type, although the reflector can vary from 120 degrees to well over 310 degrees. Lamps that have significantly more than 210 degrees of coverage are often referred to as "aperature lamps" as the amount of open area that light can escape is significantly less than the area that acts as an internal reflector. Often, a lamp is marked as a reflector lamp by adding the letter "R" in the model code, so a F71T12HO lamp with a reflector would be coded as "FR71T12HO". VHO lamps with reflectors may be coded as VHOR. No designation exists for the amount of reflector degrees the lamp has.

Reflector lamps are used in several application, particularly when light is only desired to be emitted in a single direction, or when an application requires the maximum amount of light. This can be as simple as in a higher end tanning bed or in some backlighting situation for electronics. An internal reflector is more efficient than standard external reflectors as there is less opportunity to lose light due to wave cancellation. Another example is color matched aperature lights (330 degrees of opening, give or take) are used in the food industry for quality control purposes, to allow robotic inspection of cooked goods.

Slimline lamps operate on an instant start ballast and are recognizable by their single-pin bases.

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 1950s 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 used for blacklight lamps commonly used in bug zappers. BLB is used for blacklight-blue lamps commonly used in nightclubs. 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
    • 9xy "Graphica Pro" / "De Luxe Pro" (xy00 K; eg "965" = 6500 K)
    • 8xy (xy00 K; eg "865" = 6500 K)
    • 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
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 with 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).
Tanning lamps
The lamps used in tanning beds contain a different phosphor blend (typically 3 to 5 or more phosphors) that emits both UVA and UVB, provoking a tanning response in most human skin. Typically, the output is rated as 3% to 10% UVB (5% most typical) with the remaining UV as UVA. These are mainly F71, F72 or F73 HO (100w) lamps, although 160w VHO are somewhat common.
Grow lamps
Grow lamps contain a phosphor blend that encourages photosynthesis in plants; they usually appear pinkish to human eyes.
Germicidal lamps
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
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.
Compact fluorescent lamps (CFL)
Also known as a compact fluorescent light bulb is a type of fluorescent lamp designed to replace an incandescent lamp. Many CFLs can fit in the existing incandescent light fixtures.
Cold-cathode fluorescent lamps (CCFL)
Cold-cathode fluorescent lamps are used as backlighting for LCD displays in personal computer and TV monitors. They are also popular with case modders in recent years.

Science demonstrations

Fluorescent lamps can be illuminated by means other than a proper electrical connection. These other methods however result in very dim or very short-lived illumination, and so are seen mostly in science demonstrations. With the exception of static electricity, these methods can be very dangerous if done improperly:

Film and video use

Special fluorescent lights are often used in film/video production. The brand name Kino Flos are used to create softer fill light and are less hot than traditional halogen light sources. These fluorescent lights are designed with special high-frequency ballasts to prevent video flickering and high color-rendition index bulbs to approximate daylight color temperatures.

See also

References

  1. ^ Klipstein, Donald L.. Light and Lighting Facts and Bits of Data!. Retrieved on 2007-12-29.
  2. ^ Australia Bans Traditional Light Bulbs to Combat Global Warming. green wombat blog (2007-02-20). Retrieved on 2007-12-31.
  3. ^ Ban on incandescent lamps discussed by Finnish Parliament. Helsingin Sanomat - International Edition (2007-09-27). Retrieved on 2007-12-31.
  4. ^ Rihner M, McGrath H Jr (1992). "Fluorescent light photosensitivity in patients with systemic lupus erythematosus". Arthritis Rheum 35 (8): 949–52. doi:10.1002/art.1780350816. PMID 1642660.
  5. ^ Beattie PE, Dawe RS, Ibbotson SH, Ferguson J (2003). "Characteristics and prognosis of idiopathic solar urticaria: a cohort of 87 cases". Arch Dermatol 139 (9): 1149–54. PMID 12975156.
  6. ^ Accommodation and Compliance Series: Employees with Chronic Fatigue Syndrome
  7. ^ Working with Light Sensitivity
  8. ^ Accommodation Ideas for Employees with Epilepsy
  9. ^ Accommodation and Compliance Series: Employees with Lupus
  10. ^ Accommodating People with Vertigo
  11. ^ Küller R, Laike T (1998). "The impact of flicker from fluorescent lighting on well-being, performance and physiological arousal". Ergonomics 41 (4): 433–47.
  12. ^ Veitch JA, McColl SL (1995). "Modulation of fluorescent light: flicker rate and light source effects on visual performance and visual comfort". Light Res Tech 27 (4): 243–256. Retrieved on 2007-08-12.
  13. ^ Binnie CD, de Korte RA, Wisman T (1979). "Fluorescent lighting and epilepsy". Epilepsia 20 (6): 725–7. PMID 499117.
  14. ^ Colman RS, Frankel F, Ritvo E, Freeman BJ (1976). "The effects of fluorescent and incandescent illumination upon repetitive behaviors in autistic children". J Autism Child Schizophr 6 (2): 157–62. doi:10.1007/BF01538059. PMID 989489.
  15. ^ Turner M (1999). "Annotation: Repetitive behaviour in autism: a review of psychological research". J Child Psychol Psychiatry 40 (6): 839–49. PMID 10509879.
  16. ^ Page 183 of http://www.chem.unep.ch/MERCURY/Toolkit/UNEP-final-pilot-draft-toolkit-Dec05.pdf
  17. ^ http://lightingdesignlab.com/articles/mercury_in_fl/mercurycfl.htm
  18. ^ NEMA Voluntary Commitment on Mercury in CFLs
  19. ^ Floyd, et al. (2002), quoted on page 184 of Toolkit for identification and quantification of mercury releases (PDF)
  20. ^ http://www.richardbox.com/
  • 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
  • Sam's F-Lamp FAQ
  • The Fluorescent Lighting System
  • The Lighting Design Lab: Should I Turn Off Fluorescent Lighting When Leaving A Room?
  • Application notes for CCFL lamps.
  • History, Science and Technology of Light Sources
  • R. N. Thayer (1991-10-25). The Fluorescent Lamp: Early U. S. Development. The Report courtesy of General Electric Company. Retrieved on 2007-03-18.
  • Dozens of raw visible spectra of fluorescent and other light bulbs
  • Fluorescent Lighting, UK.D-I-Y Wiki
  • What to Do if a Fluorescent Light Bulb Breaks
  • Fluorescent Tubes in the Workplace Article explaining the proper disposal of fluorescent tubes
  • 'Energy saving lightbulbs'
  This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Fluorescent_lamp". A list of authors is available in Wikipedia.
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