Chapter – 11 Fluorescent Lamps Materials Mechanism of light emission First generation – fourth generation phosphors Correlated color temperature (CCT)

Light emission mechanism in Lamp phosphors Fluorescent lamp phosphors convert the ultraviolet emission of a rare-gas (Ne, Ar, Xe)/mercury discharge plasma into visible (white) light. The discharge inside the tube converts about two thirds of the input electric power into atomic mercury line emission at 254 nm, corresponding to the 3P1  1S0 transition. The phosphor material is responsible for nearly all the visible light produced by the lamp. Usually rare earth elements in lamp materials are used for light emission.

Light generation in mercury low-pressure discharges

What are Rare Earth Metals? The name rare earth has its origins in the history of the discovery of these elements. They are never found as free metals in the Earth’s crust and pure minerals of individual rare earths do not exist. They are found as oxides. The word rare part of their name refers to the difficulty in obtaining the pure elements form. However, they exist in abundances in the Earth’s crust in oxide form.

Periodic Table Rare Earth Elements

First Generation Phosphors Zinc silicate mineral (Zn2SiO4) is highly fluorescent (green) under shortwave ultraviolet light. The first phosphors used in early 1935 were naturally occurring minerals long known, - zinc silicate (willemite) for green, cadmium silicate for pink, and calcium tungstate for blue. The search for the discovery of improved phosphors led to invention of Zinc Beryllium Silicate ((yellow-white color). The first material found to create white light with reasonable quality. However the efficacy of the popular 5-foot tube using these materials was just 35 lumens per watt. These phosphors were rapidly superseded once more efficient materials had been developed.

Emission spectra of Zinc Beryllium Silicate

Second Generation - The Halophosphates The most commonly found tubes on the global lighting market employ an internal coating of Calcium Halophosphate materials. This revolutionary material was invented in 1942 by a group led by A.H. McKeag of Osram-GEC in London, and succeeded in almost doubling lamp efficacy. The lamp efficacy is approximately 60 - 75 lm/W. The phosphor is a blend of two different materials which radiate broadly in the blue and orange parts of the spectrum. The halophosphate materials are relatively inefficient and deliver inferior lighting quality compared to newer technologies. Owing to their reduced energy efficiency, Halophosphate tubes were replaced with more efficient fluorescent phosphor materials.

Some Halophosphate Phosphors Color Wavelength Usage Ca5(F,Cl)(PO4)3:Sb,Mn White- Cold/Warm – Fluorescent Lamps Ca5F(PO4)3:Sb,Mn 3800K (Sr,Mg)3(PO4)2:Sn(II) Orange-Red 630 nm (Sr,Mg)3(PO4)2:Sn Orange-Pinkish White 626 nm

Calacium Halophosphate Phosphors

Europium activated calacium halophosphate hhosphors Emission spectra of halophosphate phosphors at room-temperature Excitation spectra of halophosphate phosphors at room-temperature

Third Generation - The Triphosphors In 1970s, the modern family of Triphosphor lamps was invented. This phosphor has a good colour rendering index. The lamp produces a full spectrum with light of all wavelengths. A color rendering index (CRI) is a quantitative measure of the ability of a light source to reveal the colors of various objects faithfully in comparison with an ideal or natural light source. The phosphor emits three bands peaking at 450nm (blue), 540nm (green) and 610nm (orange) photo-receptors in the eye. New materials were synthesized with emissions close to these wavelengths.

6. The desirable fluorescent lamp lamps are constructed with two layers of phosphor coatings. 7. The first or the base coat is an inexpensive halophosphate phosphor of the desired lamp color temperature. 8. The second or skin coat is comprised of three expensive rare earth activated phosphors, emitting in the red, green and blue spectral regions, blended to effect a composite white emission of desired color temperature. 9. In this configuration, the expensive tri-phosphor blend or skin coat absorbs the ultra- violet excitation energy of the Hg plasma. The halophosphate base coat absorbs the excitation energy that eludes the skin coat. 10. A skin coat or tri-phosphor blend that has been generally used is a red Y2O3:Eu+3, a green CeMgAl11O19:Tb+3, and a blue BaMg2 Al16O27:Eu+2.

Base Coat or pre-coats: Layer 12 - provided between the phosphor layers and the inner surface of the glass tube. Skin Coat: 13, 14, 15, 16 and 17 are rare earth phosphor layers Diagrammatically a representative glass envelope or tube of a low pressure mercury vapor discharge fluorescent lamp

11. tri-phosphor blend :- lanthanum cerium orthophosphate phosphor activated with terbium (La1-x-y Cex Tby PO4) - a blue color emission phosphor and another europium-activated yttrium oxide ((Y1-a Eua)2 O3 ) – red color emission phosphor. 12. Tri-phosphors (triband) for CFL,CCFL: - Sr5Cl(PO4)3: Eu2+  - Blue 447nm (Ca.Sr.Ba)3(PO4)2Cl2:Eu  - Blue 452nm BaMg2Al16O27:Eu2+  - Blue 450nm BaMg2Al16O27:Eu2+,Mn2+  - Blue 450/515nm (Ce,Tb)MgAl11O19:Ce,Tb  - Green 543nm (La,Ce,Tb)PO4:Ce,Tb  - Green 546nm Y2O3: Eu  - Red 611nm Triband blend Warm White – Color temperature 2700K Triband blend White - Color temperature 3500K Triband blend Cool White - Color temperature 4000K Triband blend Daylight - Color temperature 6400-6500K

Typical Triphosphor Component Materials Color Phosphor Name Chemical Formula Wavelength Blue Barium Aluminate (BAM) BaMg2Al16O27 : Eu2+ 450nm or  SrCaBaMg Chloroapatite (Sr,Ca,Ba,Mg)5(PO4)3Cl : Eu2+ 453nm Green Calcium Tungstate (CAT) Ce0.65Tb0.35MgAl11O19 543nm Lanthanum Phosphate (LAP) LaPO4 : Ce3+Tb3+ 544nm Orange-Red Yttrium Oxide (YOX) Y2O3 : Eu3+ 611nm By blending together the blue, green and red components in the correct proportions, a net white output can be realized.

Spectra of individual components are also presented in the following figures.

Blue emitting phosphors Two blue emitting phosphors are commonly used in tricolor fluorescent lamps. One is Eu2+ activated BaMgAl10O17. Efficient Eu2+ luminescence with emission maximum at 450 nm supplies the required narrow band blue emission in the triphosphor blend. The second commonly used blue phosphor is Eu2+ activated (Sr,Ba,Ca)5(PO4)3Cl, a material with an apatite (halophosphate) structure. The phosphor shows strong ultraviolet absorption with a narrow band emission peaking at 450 nm. Emission spectrum of BaMgAl10O17:Eu2+ (BAM) blue phosphors

Green emitting phosphors CeMgAl11O19:Tb3+ The compound CeMgAl11O19 is an efficient ultraviolet emitter when excited by 254 nm radiation. The introduction of Tb3+ in CeMgAl11O19 quenches the Ce3+ emission and generates the green Tb3+ luminescence as a result of the Ce3+to Tb3+ energy transfer. (b) LaPO4:Ce,Tb This phosphor is rapidly gaining popularity as an alternative to the CeMgAl11O19:Tb3+ phosphor. The advantages are the easy manufacturing due to the lower synthesis temperatures (about 1000°C) and the lower Tb concentrations required for optimum performance. The emission spectrum is dominated by the transition at 543 nm (5D4  7F5). (c) GdMgB5O10:Ce, Tb The Gd3+ ions assist in the transport of energy from the sensitizer (Ce3+) to the activator (Tb3+) ions.

Red emitting phosphor (Eu3+-activated Y2O3) The emission spectrum of this phosphor is ideal for red color generation. The commercial formulation contains relatively high Eu concentrations (3–5 mole percent). It consists of a dominant peak at 611 nm due to the electric dipole transition 5D0  7F2. The Y2O3:Eu3+ phosphor absorbs the 254 nm mercury discharge emission through a charge transfer transition involving the Eu3+ ion and the neighboring O2- ions. This charge transfer transition peaks at about 230 nm.

Fourth Generation Phosphor - Single white-light-emitting phosphors Current research into light and its effects on the human psyche are continuing to drive lamp technology into exciting new areas. Recent years have witnessed a number of special colour tubes which are intended to create an enhanced sense of well-being. Single-component white-light-emitting phosphors are useful for the development of white light-emitting diodes (W-LEDs). Single white-light-emitting phosphors have very high correlated color temperature (Very High CCT ). The actual colour temperature of these new tubes is pitched between 8000K and 17000K depending on the manufacturer.

The correlated color temperature (CCT) is a specification of the color appearance of the light emitted by a lamp, relating its color to the color of light from a reference source (black body emission) when heated to a particular temperature, measured in degrees Kelvin (K). According to lighting industry convention, lamps with low CCT values (2700 K to 3000 K) provide light that appears "warm," while lamps having high CCT values (4000 K to 6500 K) provide light that appears "cool." The CIE 1931 x,y chromaticity space, also showing the chromaticities of black-body light sources of various temperatures, and lines of constant correlated color temperature.

Commercial white LEDs are fabricated by combing a blue InGaN chip with the yellow-emitting Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor.

1. Luminescence of Ca2LiSiO4F: Ce3+, Tb3+ Phosphors Ce3+, Tb3+ singly-doped and co-doped Ca2LiSiO4F phosphors have been synthesized through conventional solid-state reaction. Under UV excitation, Ce3+ singly-doped Ca2LiSiO4F phosphor emit blue light (460 nm), while Tb3+ singly-doped phosphor presents yellowish green color with emission peaks at 414, 437, 486, 543, 588, and 620 nm.  The results indicate that these phosphors have potential as phosphor candidates used for white LEDs pumped by UV chips. 2. Tunable Luminescence and Energy Transfer Investigation in Sr8La2[(PO4)4.5(SiO4)2(BO4)0.5](BO2): Ce3+/Mn2+ for White-Light-Emitting Diodes Powder samples of Sr8La2[(PO4)4.5(SiO4)2(BO4)0.5](BO2) (SLP) doped with Mn2+and Ce3+ were prepared by solid-state reaction method.   The tunable emission colors from blue to orange have been achieved by tuning the relative ratio of the Ce3+ to Mn2+based on the principle of energy transfer, and the developed phosphor could be promising as a single-component white-light-emitting phosphor for white light-emitting diodes (W-LEDs).

3. A direct warm-white-light CaLa2(MoO4)4: Tb3+, Sm3+ phosphor with tunable color tone via energy transfer for white LEDs A series of Tb3+, Sm3+ co-doped CaLa2(MoO4)4 phosphors have been prepared via a solvothermal method without further sintering. Upon 277 nm excitation, the phosphors exhibit strong green emission of the Tb3+ ions and red orange emission of the Sm3+ ions. This kind of phosphor is a potential candidate for white LEDs. 4. Yellowish-Green Emitting Ba4Gd3Na3(PO4)6F2:Eu2+ Phosphor Ba4Gd3Na3(PO4)6F2:Eu2+ phosphor can act as a promising candidate for n-UV convertible white LEDs.