FluorescentFluorescentFluorescent LampLampLamp PhosphorsPhosphorsPhosphors Energy saving, long life, high efficiency fluorescent lamp products

by Alok M. Srivastava and Timothy J. Sommerer

luorescent lamp phosphors con- vert the ultraviolet emission of a rare-gas/mercury discharge plasma into visible (white) light. The phosphor is responsible for nearlyFF all the visible light produced by the lamp with the visible mercury lines contributing only a few percent to the total lamp light output. This article reviews fluorescent lamp phosphors that are used in general purpose illumination. Readers interested in applications of phosphor in specialized fluorescent lamp products such as skin-tanning lamps, ger- FIG. 1: Anatomy of a con- micidal lamps, lamps used in photo- ventional fluorescent lamp. copiers, to name a few, are encouraged to consult References 1 and 2. The article incandescent lamps with CFLs in most cury condenses at the coolest location in begins with an overview of fluorescent industrial, commercial and residential the tube (usually 40–45°C) and comes lamp use, construction, and operation, applications could thus yield substan- into equilibrium with saturated mercury and proceeds with an overview of fluores- tial savings in energy and overall lamp vapor; the mercury vapor pressure is typ- cent lamp phosphors. Finally, comments life cost with concurrent reduction in ically 5–10 mtorr and is chosen to maxi- on the energy and environmental issues energy use and greenhouse gas emis- mize luminous efficacy. Because the associated with the use of fluorescent sion from fossil-fuel power plants. lowest-lying atomic mercury energy lamp products in residential and com- However, the CFLs are presently used in levels are much lower than those of mercial markets are made. less than 2 percent of the approxi- argon, the argon atoms remain in their General-purpose lighting consumes mately 2.5 billion incandescent sockets ground state and emit negligible radia- about one-quarter of all electricity pro- available in residential application in tion. The discharge converts about two- duced in the United States. The incan- the United States. thirds of the input electric power into descent lamp, where a filament of In the residential market the low CFL atomic mercury line emission at 254 material is resistively heated to incan- penetration is mainly due to their high nm, corresponding to the 3P j 1S descence by the passage of electrical 1 0 purchase price. Residential customers are intercombination line. An additional current, is known for its pleasing extraordinarily sensitive to the initial 10–20 percent of the input power is con- appearance to the human eye and its purchase price. For example, a 40 watt verted to 185 nm radiation corre- low purchase price. It is also known for fluorescent lamp may consume $70 in sponding to the “true” mercury atom its drawbacks, particularly a relatively electricity over its life, yet consumer pur- resonance transition 1P j 1S . Details short operating life (usually 1000 h) 1 0 chasing decisions may be swayed by a of discharge lamp science and tech- and low luminous efficacy (15 few cents on its $1 retail cost. Commer- nology can be found in the book by lumens/watt, lm/W). The incandescent cial buyers, on the other hand, are more Waymouth (3). lamp is thus a very inefficient lamp conscious of the total operating cost of a The electrodes are coiled tungsten fil- source, converting only 3–5 percent of lamp, the so-called “cost of light.” aments coated with a material (typically the electric energy into visible light. a mixture of BaO-SrO-CaO) which emit Fluorescent lamps (linear and compact) Fluorescent Lamp Construction electrons by thermionic emission at rela- are known for their high efficacy tively low temperatures. The average arc (80–100 lm/W), long life The conventional fluorescent lamp current is regulated by an external bal- (10,000–20,000 h or 1–2.5 yr of contin- (FIG. 1) consists of a soda-lime glass tube last to a value of typically 200–400 mA. uous operation), and pleasing color. filled with a few torr of rare gas (typically The ballast is required because the fluo- Compact fluorescent lamps (CFLs) argon) and a drop of mercury. Metal rescent lamp has a negative differential usually are integrated with suitable electrodes sealed into the tube ends con- electrical impedance, and if it would be electrical controls and are designed to duct electric current from the external powered directly from the wall socket it fit into a conventional incandescent circuit to the interior gas. During opera- would draw a rapidly increasing current lamp socket. A typical 20 W CFL is tion a small fraction of the mercury until limited by a fuse, circuit breaker or rated at 1200 initial lm and 10,000 h atoms are ionized and a positive column some catastrophic failure. life as compared to a 75 W incandes- discharge forms which carries 200–400 Phosphor is coated on the inside of cent lamps which is rated at 1170 lm mA of electric current. The liquid mer- the tube; the coating is conveniently and 750 h life. The replacement of

28 The Electrochemical Society Interface • Summer 1998 characterized by the phosphor mass per The visible output of an F40T12 impact on general lighting is the focus unit area of the tube commonly denoted (these lamps consume 40 W of electric of the rest of our article. as “coating weight.” The optimum coat- power and have a tube diameter of During the 1970s a new generation of ing weight occurs where the phosphor 12/8ths inch) halophosphate (cool- low pressure mercury fluorescent lamps layer is sufficiently thick to absorb the white) fluorescent lamp is typically were developed where one could simul- incident ultraviolet radiation, but not so 3050 lumens (efficacy of 76 lm/W). The taneously combine markedly high color thick to cause unnecessary reflection and color rendition index (CRI) is a measure rendering (CRI ~ 80) with high efficacy absorption of the generated visible radia- of the accuracy with which white light (3400 lumens in an F40T12 fluorescent tion. The optimum thickness is about renders various colors. By definition, a lamp). Theoretical modeling had already four phosphor particles. blackbody source has a CRI of 100. The predicted high CRI and efficacy in a spec- trum having three emission bands: red at 610 nm, green at 550 nm and blue at 450 nm (4). These wavelengths are near peaks in the CIE tristimulus functions which are used to define colors. In the mid-1970s such fluorescent lamps were commercially available and contained the revolutionary blend of three rare- earth-activated phosphors (the blends are popularly known as tricolor or triphos- 3+ phor blends): Eu -activated Y2O3 (red 3+ FIG. 2: Emission spectrum emitting), Tb -activated CeMgAl11O19 of a typical cool white (green emitting) and Eu2+-activated halophosphate phosphor showing the Sb3+ emis- BaMgAl10O17 (blue emitting) (5). The sion around 480 nm and individual rare-earth based phosphors 2+ the Mn emission around which have played an important role in 580 nm. the development of tricolor lamps, will be discussed. Red emitting phosphor (Eu3+-acti- Conventional Lamp Phosphors CRI of “cool white” fluorescent lamp vated Y2O3).—The emission spectrum are in the upper 50s or lower 60s. The of this phosphor is ideal for red color The early fluorescent lamps used var- rather low CRI follows from the emis- generation (FIG. 3a). It consists of a ious combinations of naturally fluorescing sion spectrum which shows that the dominant peak at 611 nm which corre- minerals such as willimite (Mn2+-acti- two complementary emission bands do sponds to the electric dipole transition 5 j 7 vated Zn2SiO4) to generate white light. A not fill the visible region of the spec- D0 F2. The commercial formula- significant breakthrough in fluorescent trum and in particular are deficient in tion contains relatively high Eu con- lighting occurred in the 1940s with the the red spectral region. Hence, colors centrations (3–5 mole percent). At development of the calcium halophos- are distorted under these lamps com- lower Eu3+ concentrations emission in phate phosphor (Sb3+,Mn2+-activated pared to their appearance under black- the green-yellow, which originate from 5 5 Ca5(PO4)3(Cl,F)). This phosphor has two body radiation sources or sunlight. the higher energy states ( D1, D2), are 5 emission bands, one in the blue and the Improved CRI is obtained by blending enhanced at the expense of the red D0 j 7 other in the orange-red (FIG. 2). The blue halophosphate phosphor with a red F2 emission. 3+ 2+ 3+ band is due to the activator Sb ions emitting phosphor (typically Sn -acti- The Y2O3:Eu phosphor absorbs which absorb the 254 nm radiation of the vated strontium orthophosphate). the 254 nm mercury discharge emis- discharge and emit a part of this energy in These “deluxe” lamps display higher sion through a charge transfer transi- a band peaking near 480 nm. The excita- CRI (85) due to the continuous emis- tion involving the Eu3+ ion and the tion energy is also transferred from Sb3+ to sion across the visible region from the neighboring O2- ions. This charge Mn2+ resulting in the orange-red Mn2+ phosphor blend. The improvement in transfer transition peaks at about 230 emission peaking at about 580 nm. The CRI is, however, accompanied by a nm. Hence, the absorption of 254 nm ratio of the blue to orange emission can be marked drop in the lamp brightness radiation is not very high with plaques adjusted; increasing Mn2+ content, for (efficacy of 50 lm/W). of the commercial formulation re- example, suppresses the blue emission flecting about 25 percent of this radia- and enhances the orange emission. A Rare-Earth Triphosphors tion. The reflectivity can be decreased range of whitish color from near blue to with an increase in the Eu concentra- orange can therefore be attained from a The color of the halophosophate tion, but the high cost of Eu-based single material. A further variation in lamp, although acceptable in many phosphors prohibits such compositions color can be achieved by changing the applications, is not sufficiently pleasing for practical applications. The high F:Cl ratio. Generally, the increase in Cl for many people to install such lamps reflectivity and the fact that the green levels results in the peak position of the in the living areas of their home; hence and blue phosphors are stronger Mn2+ emission band shifting toward the they tend to be relegated to the base- absorbers of the 254 nm radiation 3+ orange. The synthesis of the halophos- ment or the garage. Much-improved requires the Y2O3:Eu phosphor to be phate phosphor is complex and is not dis- light is possible through the use of the dominant component (by weight) cussed in this article. Interested readers are modern rare-earth phosphors where of the triphosphor blend. encouraged to consult a monograph by white light is made by mixing red-, The quantum efficiency of this phos- Butler (2). green- and blue-emitting phosphors. phor is close to unity and constant in the The rare-earth phosphors and their (continued on next page)

The Electrochemical Society Interface • Summer 1998 29 range of Eu concentrations from 1 to 15 mole percent. The quantum efficiency is the highest of all lighting phosphors. The lumen maintenance (defined as the change in the lamp brightness with time) of this phosphor is exceptional and exceeds that of the other lighting phos- phors. It should be remarked that only a 3+ few years ago the Y2O3:Eu phosphor was the most expensive component of the triphosphor blend. This is no longer the case as less costly sources of this phos- phor have appeared in the market. (Refer- ence 6 provides an interesting outlook of the rare-earth industry and market.) Research aimed at replacing this phos- phor with less-expensive alternatives have not met with success. The high quantum efficiency, near perfect red emission and exceptional lumen mainte- FIG. 3: The emission spectra of rare earth phosphors used in triphosphor blend; (a) red emitting 3+ 3+ 3+ 2+ nance are the reasons for the application Y2O3:Eu , (b) green emitting LaPO4:Ce , Tb , and (c) blue emitting (Sr, Ca, Ba)5(PO4)3Cl:Eu . 3+ of the Y2O3:Eu phosphor in fluorescent lamps for more than two decades. tion between rather refractory starting luminescence in LaPO4. Green emitting phosphors.— materials. A reducing condition during The phosphor is manufactured at rel- 3+ (a) CeMgAl11O19:Tb .—This phosphor synthesis is required to maintain the atively low temperatures (1000°C) with crystallizes in the magnetoplumbite trivalent states of Ce and Tb species. the aid of a suitable flux and under a structure (PbAl12O19) in which alu- (b) LaPO4:Ce,Tb—This phosphor is reducing atmosphere. The phosphor dis- minum (magnesium)-oxygen spinel rapidly gaining popularity as an alter- plays excellent lumen output and main- blocks are separated by mirror planes native to the CeMgAl O :Tb3+ phos- tenance during lamp operation. containing the larger divalent (rare-earth) 11 19 phor. The advantages are the (c) GdMgB O :Ce, Tb—A new class ions. The large cation sites are ten-coordi- 5 10 manufacturing ease due to the lower of Ln(Mg,Zn)B5O10 pentaborate mate- nated. The composition CeMgAl11O19 is synthesis temperatures (about 1000°C) rials (derived from SmCoB5O10) have derived from PbAl12O19 by replacing and the lower Tb concentrations been synthesized and characterized (9). 2+ 3+ Pb by Ce and substituting one of the required for optimum performance. The Ln ions are ten-coordinated with Al3+ by Mg2+. The emission spectrum shown in FIG. the Mg/Zn ions in a distorted octahe- The compound CeMgAl11O19 is an 3b is dominated by the transition at dral coordination. The rare-earth poly- efficient ultraviolet emitter when excited 5 j 7 543 nm ( D4 F5). hedra share edges to form isolated by 254 nm radiation. The emission arises The first half of the lanthanide series zig-zag chains. The shortest intra- and from allowed transitions between the (Ln = La to Gd) crystallizes in the mono- inter-Ln-Ln distances are about 4 Å and 3+ ground and excited states of the Ce ion clinic monazite-type structure while the 6 Å, respectively. The structure thus 1 1 which are derived from the 4f and 5d later half (Ln = Tb to Lu, Y, Sc) exists as contains one dimensional Ln-Ln chains electronic configurations, respectively. the zircon type structure. In the mon- which has lead to interesting energy 3+ The introduction of Tb in azite structure the lanthanide ions are migration studies in this material (10). 3+ CeMgAl11O19 quenches the Ce emis- nine-coordinated. The efficient green The Gd3+ ions assist in the transport sion and generates the green Tb3+ lumi- luminescence of LaPO :Ce3+,Tb3+ has of energy from the sensitizer (Ce3+) to 3+ 3+ 4 nescence as a result of the Ce -to-Tb been known since the 1970s, but only the activator (Tb3+) ions. The Ce3+ energy transfer. The high efficiency of recently has the material gained impor- emitting levels are resonant with the 3+ the Ce luminescence in CeMgAl11O19 3+ 6 tance as a fluorescent lamp phosphor (8). lowest Gd excited states ( PJ). Energy suggests the absence of significant energy The role of Ce3+ in this phosphor is transfer from Ce3+ to Gd3+ energy 3+ migration among the Ce ions in this that of the sensitizer. The allowed Ce3+ transfer is an efficient process. Efficient lattice. The spectral overlap between the 4f j 5d transitions are in resonance green luminescence is generated when excitation and emission bands is small with the mercury discharge. In contrast the activator (Tb3+) ions trap the excita- due to the relatively large Stokes shift to CeMgAl O :Tb3+, however, con- tion energy that is percolating over the -1 3+ 11 19 (about 9400 cm ) of the Ce lumines- siderable energy migration occurs over Gd3+ ions. The quantum efficiency is 3+ 3+ cence. The Ce -to-Tb energy transfer the Ce3+ ions prior to the Ce3+ to Tb3+ high and the phosphor displays excel- is limited to the six nearest neighbors at transfer. This reduces the amount of lent stability in fluorescent lamps. 3+ 5.6 Å and this requires rather high Tb Tb3+ required for optimum perfor- A red-emitting phosphor can be concentration (33 mole percent) for the mance with the commercial formula- developed by substituting a part of 3+ complete quenching of the Ce emis- tion containing 27 percent Ce and 13 Mg2+/Zn2+ with Mn2+ ions. The Mn2+ sion (7). The optimum phosphor compo- 3+ percent Tb (La0.60Ce0.27Tb0.13PO4). ions efficiently trap the migrating Gd sition (Ce0.67Tb0.33)MgAl11O19 exhibits Energy migration among the Ce3+ ions excitation energy and display a broad high quantum efficiency and excellent results from the large spectral overlap band emission peaking at 620 nm. The lumen output and maintenance during between the excitation and emission phosphor is used in high-color-ren- lamp operation. bands due to the relatively small Stokes dering fluorescent lamps (CRI ~ 95), the Phosphor synthesis requires high shift (about 4700 cm-1) of the Ce3+ so called five-band lamps (these lamps temperatures (1500°C) to promote reac- have four or five phosphors instead of

30 The Electrochemical Society Interface • Summer 1998 the three of the triphosphor lamp). particles requires proportionally less of the 40 W “cool white” fluorescent Such fluorescent lamps with CRI close to material with decreasing particle size. lamps which has strongly encouraged a an incandescent lamp find application The reduction in particle size is limited move towards accepted alternatives in color critical applications such as dis- by the ultraviolet absorption strength such as 34 W “Wattmiser.” play lighting. The increase in CRI occurs of the individual particles. As an There is an increasing concern that at the expense of the lamp efficacy. example, the General Electric Company mercury, which is central to the operation Blue emitting phosphors.—Two blue introduced in 1993 a halophosphate of fluorescent lamps, could leach into emitting phosphors are commonly used phosphor with a median particle size of groundwater supplies when the spent in tricolor fluorescent lamps. One is Eu2+ 8–9 microns whereas the typical par- lamps are discarded in solid landfills. activated BaMgAl10O17, a material with ticle size is in the 11–13 micron range. Although only about 0.1 mg of mer- a beta-alumina structure. Efficient Eu2+ The concentration of rare-earth ions cury is required to generate the desired luminescence with emission maximum in a matrix may be affected by a change 5–10 mtorr of mercury vapor pressure, at 450 nm supplies the required narrow in the host lattice system. Cost may be chemical binding of mercury by glass, band blue emission in the triphosphor reduced by a decrease in the required phosphors and electrodes during lamp blend. The strong absorption in the UV rare-earth ion concentration, or in some operation requires dosing of the fluo- region is due to the allowed 4f75d j cases, by completely eliminating rare- rescent lamps with several milligrams 4f65d1 transitions. The synthesis is gen- earth ions. Consider the green emitting of mercury to ensure the availability of erally accomplished under a reducing phosphors that are based on the narrow sufficient free mercury to sustain the 3+ atmosphere with BaF2 or MgF2 as line emission of the expensive Tb ion. desired vapor pressure. fluxing agents. The second commonly Although narrow spectral emission in There is thus a significant effort to used blue phosphor is Eu2+-activated the green spectral region is important, it reduce the amount of mercury in fluores- (Sr,Ba,Ca)5(PO4)3Cl, a material with an is certainly not the dominant concern cent lamps. Note that incandescent lamps apatite (halophosphate) structure (FIG. for red-emitting phosphor. Hence, effi- are mercury-free lighting devices. We 3c). The phosphor displays strong ultra- cient broad band green luminescence of may argue that the environmental con- violet absorption with a narrow band Mn2+ ion can in principle be used in cerns associated with mercury-based fluo- emission peaking at 450 nm. The blue triphosphor lamps. An interesting rescent lamps can be eliminated by phosphors represent only a minor example is the broad band green emis- replacing them with energy-hungry and weight fraction of the triphosphor blend sion (peak at 550 nm) of Mn2+ in short-lived incandescent lamps. In the 2+ 2+ (about 10 percent for color temperature Sr3Gd2Si6O18:Pb ,Mn (11). As in global view, however, such a replacement 3+ 3+ 2+ of 4100 K). However, blends designed GdMgB5O10:Ce ,Tb , the Pb transfer would actually result in more mercury for higher color temperatures, say 6500 the energy to the Gd3+ ions with the entering the biosphere. This is because K, require higher amounts of the blue Mn2+ ions acting as acceptors of the per- fossil fuels, expected to provide the major emitting component. colating Gd3+ excitation energy. To portion of world’s electric energy in the account for the unusually long wave- foreseeable future, contain mercury. ■ Cost Issues length green emission, it is assumed that the Mn2+ ions occupy the Gd3+ sites of The development of high-efficacy the host. The quantum efficiency of 87 and high-color-rendering fluorescent percent is comparable to the quantum About the Author lamps would not have been possible efficiency of Tb3+ luminescence of rare- without the application of rare-earth earth phosphors. Unfortunately, the Drs. Srivastava and Sommerer are phosphors. Compact fluorescent lamps phosphor exhibits a efficiency loss of 25 Research Scientists at General Electric would not have been possible without percent when exposed to the full mer- Corporate Research and Development. the development of rare-earth phos- cury spectrum, thereby eliminating the phors, as their higher wall temperature phosphor as a practical material. and higher ultraviolet flux quickly degrades halophosphate materials. The Energy and Environmental Issues References main disadvantage of the rare-earth phosphors is their high cost which has Periodic energy crises can lead to the 1. K. H. Butler, Fluorescent Lamp Phosphors, The Pennsylvania State University Press (1980). more than doubled the cost of some introduction of new lighting tech- 2. G. Blasse and B. C. Grabmaier, Luminescent fluorescent lamps. As a compromise nology. For example, during the energy Materials, Springer-Verlag (1994). between phosphor cost and perfor- turmoil of 1970s the General Electric 3. J. F. Waymouth, Electrical Discharge Lamps, MIT Press, Cambridge, MA (1978) mance a double coating scheme is Company introduced the Wattmiser 4. M. Koedam and J. J. Opstelten, Lighting Res. widely used in the fluorescent lamp lamps (Wattmiser is a GE trademark; Tech., 3, 205 (1971). industry. In this scheme a relatively other lighting companies offer similar 5. A. L. N. Stevels, J. Lumin., 12/13, 97 (1976) light coat of the high performance rare- lamps under their own trademark). and references cited therein. 6. Marie Angeles Major-Sosias, Elements, earth phosphor blend is coated over a This modified fluorescent lamp, when March/April (1997), p. 10. base layer of inexpensive halophos- operated on an existing installed base 7. J. L. Sommerdijk and J. M. P. J. Verstegen, J. phate phosphor. The rugged rare-earth of an electromagnetic ballast designed Luminescence, 9, 415 (1974); J. M. P. J. Ver- stegen, J. Electrochem. Soc., 121, 1623 (1974). phosphor coating is directly exposed to to produce lamps at 40 W, induced the 8. R. C. Ropp, J. Electrochem. Soc., 115, 531 the discharge, generating white light ballast to operate the lamp at 34 W. (1968); J. C. Bourcet and F. K. Fong, J. with high efficacy and good CRI while This resulted in every fixture con- Chem. Phys., 60, 34 (1974). 9. B. Saubat, M. Vlasse and C. Foussier, J. Solid protecting the more-easily-damaged suming about 15 percent less power St. Chem., 34, 271 (1980). halophosphate phosphor. while also producing 15 percent less 10. C. Foussier, B. Saubat and P. Hagenmuller, Cost may also be reduced by light. The government can also legislate J. Lumin., 23, 405 (1981); W. van Schaik and G. Blasse, J. Lumin., 62, 203 (1994). decreasing the phosphor particle size. changes. Recent energy legislation has 11. H. C. G. Verhaar and W. M. P. van Keme- A covering of four layers of phosphor effectively outlawed the manufacturing nade, Mat. Chem. Phys., 31, 213 (1992).

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