Phosphor

For the chemical element, see phosphorus.

A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness (>1ms), and fluorescent materials, where the emission decay takes place over tens of nanoseconds. Phosphorescent materials are known for their use in radar screens and glow-in-the-dark toys, whereas fluorescent materials are common in CRT and plasma video display screens, sensors, and white LEDs.

Phosphors are often transition metal compounds or rare earth compounds of various types. The most common uses of phosphors are in CRT displays and fluorescent lights. CRT phosphors were standardized beginning around World War II and designated by the letter "P" followed by a number.

Phosphorus, the chemical element named for its light-emitting behavior, emits light due to chemiluminescence, not phosphorescence.[1]

Contents

Principles

A material can emit light either through incandescence, where all atoms radiate, or by luminescence, where only a small fraction of atoms, called emission centers or luminescence centers, emit light. In inorganic phosphors, these inhomogeneities in the crystal structure are created usually by addition of a trace amount of dopants, impurities called activators. (In rare cases dislocations or other crystal defects can play the role of the impurity.) The wavelength emitted by the emission center is dependent on the atom itself, and on the surrounding crystal structure.

The scintillation process in inorganic materials is due to the electronic band structure found in the crystals. An incoming particle can excite an electron from the valence band to either the conduction band or the exciton band (located just below the conduction band and separated from the valence band by an energy gap). This leaves an associated hole behind, in the valence band. Impurities create electronic levels in the forbidden gap. The excitons are loosely bound electron-hole pairs that wander through the crystal lattice until they are captured as a whole by impurity centers. The latter then rapidly de-excite by emitting scintillation light (fast component). In case of inorganic scintillators, the activator impurities are typically chosen so that the emitted light is in the visible range or near-UV where photomultipliers are effective. The holes associated with electrons in the conduction band are independent from the latter. Those holes and electrons are captured successively by impurity centers exciting certain metastable states not accessible to the excitons. The delayed de-excitation of those metastable impurity states, slowed down by reliance on the low-probability forbidden mechanism, again results in light emission (slow component).

Phosphor degradation

Many phosphors tend to lose efficiency gradually by several mechanisms. The activators can undergo change of valence (usually oxidation), the crystal lattice degrades, atoms - often the activators - diffuse through the material, the surface undergoes chemical reactions with the environment with consequent loss of efficiency or buildup of a layer absorbing either the exciting or the radiated energy, etc.

The degradation of electroluminescent devices depends on frequency of driving current, the luminance level, and temperature; moisture impairs phosphor lifetime very noticeably as well.

Examples:

Materials

Phosphors are usually made from a suitable host material with an added activator. The best known type is a copper-activated zinc sulfide and the silver-activated zinc sulfide (zinc sulfide silver).

The host materials are typically oxides, nitrides and oxynitrides,[9] sulfides, selenides, halides or silicates of zinc, cadmium, manganese, aluminium, silicon, or various rare earth metals. The activators prolong the emission time (afterglow). In turn, other materials (such as nickel) can be used to quench the afterglow and shorten the decay part of the phosphor emission characteristics.

Many phosphor powders are produced in low-temperature processes, such as sol-gel and usually require post-annealing at temperatures of ~1000 °C, which is undesirable for many applications. However, proper optimization of the growth process allows to avoid the annealing.[10]

Phosphors used for fluorescent lamps require a multi-step production process, with details that vary depending on the particular phosphor. Bulk material must be milled to obtain a desired particle size range, since large particles produce a poor quality lamp coating and small particles produce less light and degrade more quickly. During the firing of the phosphor, process conditions must be controlled to prevent oxidation of the phosphor activators or contamination from the process vessels. After milling the phosphor may be washed to remove minor excess of activator elements. Volatile elements must not be allowed to escape during processing. Lamp manufacturers have changed composition of phosphors to eliminate some toxic elements, such as beryllium, cadmium, or thallium, formerly used.[11]

The commonly quoted parameters for phosphors are the wavelength of emission maximum (in nanometers, or alternatively color temperature in kelvins for white blends), the peak width (in nanometers at 50% of intensity), and decay time (in seconds).

Applications

Lighting

Phosphor layers provide most of the light produced by fluorescent lamps, and are also used to improve the balance of light produced by metal halide lamps. Various neon signs use phosphor layers to produce different colors of light. Electroluminescent displays found, for example, in aircraft instrument panels, use a phosphor layer to produce glare-free illumination or as numeric and graphic display devices.

Phosphor thermometry

Main article: Phosphor thermometry

Phosphor thermometry is a temperature measurement approach that uses the temperature dependence of certain phosphors for this purpose. For this, a phosphor coating is applied to a surface of interest and, usually, the decay time is the emission parameter that indicates temperature. Because the illumination and detection optics can be situated remotely, the method may be used for moving surfaces such as high speed motor surfaces. Also, phosphor may be applied to the end of an optical fiber as an optical analog of a thermocouple.

Glow-in-the-dark toys

In these applications, the phosphor is directly added to the plastic used to mold the toys, or mixed with a binder for use as paints.

ZnS:Cu phosphor is used in glow-in-the-dark cosmetic creams frequently used for Halloween make-ups. Generally, the persistence of the phosphor increases as the wavelength increases. See also lightstick for chemiluminescence-based glowing items.

Radioluminescence

Main article: Radioluminescence

Zinc sulfide phosphors are used with radioactive materials, where the phosphor was excited by the alpha- and beta-decaying isotopes, to create luminescent paint for dials of watches and instruments (radium dials). Between 1913 and 1950 a radium-228 and radium-226 were used to activate a phosphor made of silver doped zinc sulfide (ZnS:Ag), which gave a greenish glow. The phosphor is not suitable to be used in layers thicker than 25 mg/cm², as the self-absorption of the light then becomes a problem. Furthermore, zinc sulfide undergoes degradation of its crystal lattice structure, leading to gradual loss of brightness significantly faster than the depletion of radium. ZnS:Ag coated spinthariscope screens were used by Ernest Rutherford in his experiments discovering atomic nucleus.

Copper doped zinc sulfide (ZnS:Cu) is the most common phosphor used and yields blue-green light. Copper and magnesium doped zinc sulfide (ZnS:Cu,Mg) yields yellow-orange light.

Tritium is also used as a source of radiation in various products utilizing tritium illumination.

Electroluminescence

Main article: Electroluminescence

Electroluminescence can be exploited in light sources. Such sources typically emit from a large area, which makes them suitable for backlights of LCD displays. The excitation of the phosphor is usually achieved by application of high-intensity electric field, usually with suitable frequency. Current electroluminescent light sources tend to degrade with use, resulting in their relatively short operation lifetimes.

ZnS:Cu was the first formulation successfully displaying electroluminescence, tested at 1936 by Georges Destriau in Madame Marie Curie laboratories in Paris.

Indium tin oxide (ITO, also known under trade name IndiGlo) composite is used in some Timex watches, though as the electrode material, not as a phosphor itself. "Lighttape" is another trade name of an electroluminescent material, used in electroluminescent light strips.

White LEDs

White light-emitting diodes are usually blue InGaN LEDs with a coating of a suitable material. Cerium(III)-doped YAG (YAG:Ce3+, or Y3Al5O12:Ce3+) is often used; it absorbs the light from the blue LED and emits in a broad range from greenish to reddish, with most of output in yellow. The pale yellow emission of the Ce3+:YAG can be tuned by substituting the cerium with other rare earth elements such as terbium and gadolinium and can even be further adjusted by substituting some or all of the aluminium in the YAG with gallium. However, this process is not one of phosphorescence. The yellow light is produced by a process known as scintillation, the complete absence of an afterglow being one of the characteristics of the process.

Some rare-earth doped Sialons are photoluminescent and can serve as phosphors. Europium(II)-doped β-SiAlON absorbs in ultraviolet and visible light spectrum and emits intense broadband visible emission. Its luminance and color does not change significantly with temperature, due to the temperature-stable crystal structure. It has a great potential as a green down-conversion phosphor for white LEDs; a yellow variant also exists. For white LEDs, a blue LED is used with a yellow phosphor, or with a green and yellow SiAlON phosphor and a red CaAlSiN3-based (CASN) phosphor.[12][13][14]

White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiency europium based red and blue emitting phosphors plus green emitting copper and aluminium doped zinc sulfide (ZnS:Cu,Al). This is a method analogous to the way fluorescent lamps work.

Cathode ray tubes

Cathode ray tubes produce signal-generated light patterns in a (typically) round or rectangular format. Bulky CRTs were used in the black-and-white household television ("TV") sets that became popular in the 1950s, as well as first-generation, tube-based color TVs, and most earlier computer monitors. CRTs have also been widely used in scientific and engineering instrumentation, such as oscilloscopes, usually with a single phosphor color, typically green.

White (in black-and-white): The mix of zinc cadmium sulfide and zinc sulfide silver, the ZnS:Ag+(Zn,Cd)S:Ag is the white P4 phosphor used in black and white television CRTs.

Red: Yttrium oxide-sulfide activated with europium is used as the red phosphor in color CRTs. The development of color TVs took a long time due to the long search for a red phosphor. The first red emitting rare earth phosphor, YVO4,Eu3, was introduced by Levine and Palilla as a primary color in television in 1964.[15] In single crystal form, it was used as an excellent polarizer and laser material.[16]

Yellow: When mixed with cadmium sulfide, the resulting zinc cadmium sulfide (Zn,Cd)S:Ag, provides strong yellow light.

Green: Combination of zinc sulfide with copper, the P31 phosphor or ZnS:Cu, provides green light peaking at 531 nm, with long glow.

Blue: Combination of zinc sulfide with few ppm of silver, the ZnS:Ag, when excited by electrons, provides strong blue glow with maximum at 450 nm, with short afterglow with 200 nanosecond duration. It is known as the P22B phosphor. This material, zinc sulfide silver, is still one of the most efficient phosphors in cathode ray tubes. It is used as a blue phosphor in color CRTs.

The phosphors are usually poor electrical conductors. This may lead to deposition of residual charge on the screen, effectively decreasing the energy of the impacting electrons due to electrostatic repulsion (an effect known as "sticking"). To eliminate this, a thin layer of aluminium is deposited over the phosphors and connected to the conductive layer inside the tube. This layer also reflects the phosphor light to the desired direction, and protects the phosphor from ion bombardment resulting from an imperfect vacuum.

Standard phosphor types

Standard phosphor types[17]
Phosphor Composition Color Wavelength Peak width Persistence Usage Notes
P1, GJ Zn2SiO4:Mn (Willemite) Green 528 nm 40 nm[18] 1-100ms CRT, Lamp Oscilloscopes and monochrome monitors
P2  ? Blue-Green - - Long CRT Oscilloscopes
P4 ZnS:Ag+(Zn,Cd)S:Ag White - - Short CRT Black and white TV CRTs and display tubes.
P4 (Cd-free) ZnS:Ag+ZnS:Cu+Y2O2S:Eu White - - Short CRT Black and white TV CRTs and display tubes, Cd free.
P4, GE ZnO:Zn Green 505 nm - 1-10µs VFD VFDs
P5 Blue - - Very Short CRT Film
P7 Blue with Yellow persistence - - Long CRT Radar PPI, old EKG monitors
P10 KCl green-absorbing scotophor - - Long Dark-trace CRTs Radar screens; turns from translucent white to dark magenta, stays changed until erased by heating or infrared light
P11, BE ZnS:Ag,Cl or ZnS:Zn Blue 460 nm - 0.01-1 ms CRT, VFD Display tubes and VFDs
P12 - Orange - - Medium/Long CRT Radar
P14 Blue with Orange persistence - - Medium/Long CRT Radar PPI, old EKG monitors
P15 Blue-Green - - Extremely Short CRT
P19, LF (KF,MgF2):Mn Orange-Yellow 590 nm - Long CRT Radar screens
P20, KA (Zn,Cd)S:Ag or (Zn,Cd)S:Cu Yellow-green - - 1-100 ms CRT Display tubes
P22R Y2O2S:Eu+Fe2O3 Red - - Short CRT Red phosphor for TV screens
P22G ZnS:Cu,Al Green - - Short CRT Green phosphor for TV screens
P22B ZnS:Ag+Co-on-Al2O3 Blue - - Short CRT Blue phosphor for TV screens
P26, LC (KF,MgF2):Mn Orange 595 nm - Long CRT Radar screens
P28, KE (Zn,Cd)S:Cu,Cl Yellow - - - CRT Display tubes
P31, GH ZnS:Cu or ZnS:Cu,Ag Yellowish-green - - 0.01-1 ms CRT Oscilloscopes
P33, LD MgF2:Mn Orange 590 nm - > 1sec CRT Radar screens
P38, LK (Zn,Mg)F2:Mn Orange-Yellow 590 nm - Long CRT Radar screens
P39, GR Zn2SiO4:Mn,As Green 525 nm - - CRT Display tubes
P40, GA ZnS:Ag+(Zn,Cd)S:Cu White - - - CRT Display tubes
P43, GY Gd2O2S:Tb Yellow-green 545 nm - - CRT Display tubes
P45, WB Y2O2S:Tb White 545 nm - Short CRT Viewfinders
P46, KG Y3Al5O12:Ce Green 530 nm - - CRT Beam-index tube
P47, BH Y2SiO5:Ce Blue 400 nm - - CRT Beam-index tube
P53, KJ Y3Al5O12:Tb Yellow-green 544 nm - Short CRT Projection tubes
P55, BM ZnS:Ag,Al Blue 450 nm - Short CRT Projection tubes
ZnS:Ag Blue 450 nm - - CRT -
ZnS:Cu,Al or ZnS:Cu,Au,Al Green 530 nm - - CRT -
(Zn,Cd)S:Cu,Cl+(Zn,Cd)S:Ag,Cl White - - - CRT -
Y2SiO5:Tb Green 545 nm - - CRT Projection tubes
Y2OS:Tb Green 545 nm - - CRT Display tubes
Y3(Al,Ga)5O12:Ce Green 520 nm - Short CRT Beam-index tube
Y3(Al,Ga)5O12:Tb Yellow-green 544 nm - Short CRT Projection tubes
InBO3:Tb Yellow-green 550 nm - - CRT -
InBO3:Eu Yellow 588 nm - - CRT -
InBO3:Tb+InBO3:Eu amber - - - CRT Computer displays
InBO3:Tb+InBO3:Eu+ZnS:Ag White - - - CRT -
(Ba,Eu)Mg2Al16O27 Blue - - - Lamp Trichromatic fluorescent lamps
(Ce,Tb)MgAl11O19 Green 546 nm 9 nm - Lamp Trichromatic fluorescent lamps[18]
BAM BaMgAl10O17:Eu,Mn Blue 450 nm - - Lamp, displays Trichromatic fluorescent lamps
BaMg2Al16O27:Eu(II) Blue 450 nm 52 nm - Lamp Trichromatic fluorescent lamps[18]
BAM BaMgAl10O17:Eu,Mn Blue-Green 456 nm,514 nm - - Lamp -
BaMg2Al16O27:Eu(II),Mn(II) Blue-Green 456 nm, 514 nm 50 nm 50%[18] - Lamp
Ce0.67Tb0.33MgAl11O19:Ce,Tb Green 543 nm - - Lamp Trichromatic fluorescent lamps
Zn2SiO4:Mn,Sb2O3 Green 528 nm - - Lamp -
CaSiO3:Pb,Mn Orange-Pink 615 nm 83 nm[18] - Lamp
CaWO4 (Scheelite) Blue 417 nm - - Lamp -
CaWO4:Pb Blue 433 nm/466 nm 111 nm - Lamp Wide bandwidth[18]
MgWO4 Blue pale 473 nm 118 nm - Lamp Wide bandwidth, deluxe blend component [18]
(Sr,Eu,Ba,Ca)5(PO4)3Cl Blue - - - Lamp Trichromatic fluorescent lamps
Sr5Cl(PO4)3:Eu(II) Blue 447 nm 32 nm[18] - Lamp -
(Ca,Sr,Ba)3(PO4)2Cl2:Eu Blue 452 nm - - Lamp -
(Sr,Ca,Ba)10(PO4)6Cl2:Eu Blue 453 nm - - Lamp Trichromatic fluorescent lamps
Sr2P2O7:Sn(II) Blue 460 nm 98 nm - Lamp Wide bandwidth, deluxe blend component[18]
Sr6P5BO20:Eu Blue-Green 480 nm 82 nm[18] - Lamp -
Ca5F(PO4)3:Sb Blue 482 nm 117 nm - Lamp Wide bandwidth[18]
(Ba,Ti)2P2O7:Ti Blue-Green 494 nm 143 nm - Lamp Wide bandwidth, deluxe blend component [18]
3Sr3(PO4)2.SrF2:Sb,Mn Blue 502 nm - - Lamp -
Sr5F(PO4)3:Sb,Mn Blue-Green 509 nm 127 nm - Lamp Wide bandwidth[18]
Sr5F(PO4)3:Sb,Mn Blue-Green 509 nm 127 nm - Lamp Wide bandwidth[18]
LaPO4:Ce,Tb Green 544 nm - - Lamp Trichromatic fluorescent lamps
(La,Ce,Tb)PO4 Green - - - Lamp Trichromatic fluorescent lamps
(La,Ce,Tb)PO4:Ce,Tb Green 546 nm 6 nm - Lamp Trichromatic fluorescent lamps[18]
Ca3(PO4)2.CaF2:Ce,Mn Yellow 568 nm - - Lamp -
(Ca,Zn,Mg)3(PO4)2:Sn Orange-Pink 610 nm 146 nm - Lamp Wide bandwidth, blend component[18]
(Zn,Sr)3(PO4)2:Mn Orange-Red 625 nm - - Lamp -
(Sr,Mg)3(PO4)2:Sn Orange-Pinkish White 626 nm 120 nm - Fluorescent Lamps Wide bandwidth, deluxe blend component[18]
(Sr,Mg)3(PO4)2:Sn(II) Orange-Red 630 nm - - Fluorescent Lamps -
Ca5F(PO4)3:Sb,Mn 3800K - - - Fluorescent Lamps Lite-white blend[18]
Ca5(F,Cl)(PO4)3:Sb,Mn White-Cold/Warm - - - Fluorescent Lamps 2600K to 9900K, for very high output lamps[18]
(Y,Eu)2O3 Red - - - Lamp Trichromatic fluorescent lamps
Y2O3:Eu(III) Red 611 nm 4 nm - Lamp Trichromatic fluorescent lamps[18]
Mg4(F)GeO6:Mn Red 658 nm 17 nm - High Pressure Mercury Lamps [18]
Mg4(F)(Ge,Sn)O6:Mn Red 658 nm - - Lamp -
Y(P,V)O4:Eu Orange-Red 619 nm - - Lamp -
YVO4:Eu Orange-Red 619 nm - - High Pressure Mercury and Metal Halide Lamps -
Y2O2S:Eu Red 626 nm - - Lamp -
3.5 MgO · 0.5 MgF2 · GeO2 :Mn Red 655 nm - - Lamp 3.5 MgO · 0.5 MgF2 · GeO2 :Mn
Mg5As2O11:Mn Red 660 nm - - High Pressure Mercury Lamps, 1960s -
SrAl2O7:Pb Ultraviolet 313 nm - - Special Fluorescent Lamps for Medical use Ultraviolet
CAM LaMgAl11O19:Ce Ultraviolet 340 nm 52 nm - Black-light Fluorescent Lamps Ultraviolet
LAP LaPO4:Ce Ultraviolet 320 nm 38 nm - Medical and scientific U.V. Lamps Ultraviolet
SAC SrAl12O19:Ce Ultraviolet 295 nm 34 nm - Lamp Ultraviolet
BSP BaSi2O5:Pb Ultraviolet 350 nm 40 nm - Lamp Ultraviolet
SrFB2O3:Eu(II) Ultraviolet 366 nm - - Lamp Ultraviolet
SBE SrB4O7:Eu Ultraviolet 368 nm 15 nm - Lamp Ultraviolet
SMS Sr2MgSi2O7:Pb Ultraviolet 365 nm 68 nm - Lamp Ultraviolet
MgGa2O4:Mn(II) Blue-Green - - - Lamp Black light displays

Various

Some other phosphors commercially available, for use as X-ray screens, neutron detectors, alpha particle scintillators, etc., are:

See also

References

  1. ^ Emsley, John (2000). The Shocking History of Phosphorus. London: Macmillan. ISBN 0330390058. 
  2. ^ Bizarri, G; Moine, B (2005). "On phosphor degradation mechanism: thermal treatment effects". Journal of Luminescence 113 (3–4): 199. Bibcode 2005JLum..113..199B. doi:10.1016/j.jlumin.2004.09.119. 
  3. ^ Lakshmanan, p.171
  4. ^ Tanno, Hiroaki; Fukasawa, Takayuki; Zhang, Shuxiu; Shinoda, Tsutae; Kajiyama, Hiroshi (2009). "Lifetime Improvement of BaMgAl10O17:Eu2+Phosphor by Hydrogen Plasma Treatment". Japanese Journal of Applied Physics 48 (9): 092303. Bibcode 2009JaJAP..48i2303T. doi:10.1143/JJAP.48.092303. 
  5. ^ Ntwaeaborwa, O. M.; Hillie, K. T.; Swart, H. C. (2004). "Degradation of Y2O3:Eu phosphor powders". Physica status solidi (c) 1 (9): 2366. Bibcode 2004PSSCR...1.2366N. doi:10.1002/pssc.200404813. 
  6. ^ Wang, Ching-Wu; Sheu, Tong-Ji; Su, Yan-Kuin; Yokoyama, Meiso (1997). "Deep Traps and Mechanism of Brightness Degradation in Mn-doped ZnS Thin-Film Electroluminescent Devices Grown by Metal-Organic Chemical Vapor Deposition". Japanese Journal of Applied Physics 36: 2728. Bibcode 1997JaJAP..36.2728W. doi:10.1143/JJAP.36.2728. 
  7. ^ Lakshmanan, pp. 51, 76
  8. ^ http://tubedevices.com/alek/pwl/luminofory/luminofory.ppt
  9. ^ Xie, Rong-Jun; Hirosaki, Naoto (2007). "Silicon-based oxynitride and nitride phosphors for white LEDs—A review" (free pdf). Sci. Technol. Adv. Mater. 8 (7–8): 588. Bibcode 2007STAdM...8..588X. doi:10.1016/j.stam.2007.08.005. http://www.iop.org/EJ/article/1468-6996/8/7-8/A08/STAM_8_7-8_A08.pdf. 
  10. ^ Li, Hui-Li; Hirosaki, Naoto; Xie, Rong-Jun; Suehiro, Takayuki; Mitomo, Mamoru (2007). "Fine yellow α-SiAlON:Eu phosphors for white LEDs prepared by the gas-reduction–nitridation method" (free pdf). Sci. Techno. Adv. Mater. 8 (7–8): 601. Bibcode 2007STAdM...8..601L. doi:10.1016/j.stam.2007.09.003. http://www.iop.org/EJ/article/1468-6996/8/7-8/A09/STAM_8_7-8_A09.pdf. 
  11. ^ Raymond Kane, Heinz Sell Revolution in lamps: a chronicle of 50 years of progress (2nd ed.), The Fairmont Press, Inc. 2001 ISBN 0-88173-378-4 . Chapter 5 extensively discusses history, application and manufacturing of phosphors for lamps.
  12. ^ Youn-Gon Park et al.. "Luminescence and temperature dependency of β-SiAlON phosphor". Samsung Electro Mechanics Co. http://www.science24.com/paper/15977. 
  13. ^ Hideyoshi Kume, Nikkei Electronics (Sept 15, 2009). "Sharp to Employ White LED Using Sialon". http://techon.nikkeibp.co.jp/english/NEWS_EN/20090915/175305/. 
  14. ^ Hirosaki Naoto et al. (2005). "New sialon phosphors and white LEDs". Oyo Butsuri 74 (11): 1449. http://sciencelinks.jp/j-east/article/200602/000020060205A1031052.php. 
  15. ^ Levine, Albert K.; Palilla, Frank C. (1964). "A new, highly efficient red-emitting cathodoluminescent phosphor (YVO4:Eu) for color television". Applied Physics Letters 5 (6): 118. Bibcode 1964ApPhL...5..118L. doi:10.1063/1.1723611. 
  16. ^ Fields, R. A.; Birnbaum, M.; Fincher, C. L. (1987). "Highly efficient Nd:YVO4 diode-laser end-pumped laser". Applied Physics Letters 51 (23): 1885. Bibcode 1987ApPhL..51.1885F. doi:10.1063/1.98500. 
  17. ^ Shigeo Shionoya (1999). "VI: Phosphors for cathode ray tubes". Phosphor handbook. Boca Raton, Fla.: CRC Press. ISBN 0849375606. http://books.google.com/?id=lWlcJEDukRIC&pg=PA469. 
  18. ^ a b c d e f g h i j k l m n o p q r s t u "Osram Sylvania fluorescent lamps". http://www.sylvania.com/BusinessProducts/MaterialsandComponents/LightingComponents/Phosphor/FluorescentLamps/. Retrieved 2009-06-06. 

External links