EFFICIENCY, AND HlGH LUMEN MAINTENANCE

BACKGROUND OF THF DISCLOSURE

[0001] The present disclosure relates to a discharge lamp, and more specifically to a High

Intensity Discharge (HID) metal halide lamp made of transparent ceramic arc chambei materials The HID lamp finds particular application in street-lighting, although it will be appreciated that selected aspects may find application in i elated discharge lamp environments encountering the same issues with regard to lumen efficacy and lumen maintenance

[0002] High Intensity Discharge (HID) lamps are high-efficiencs lamps that can generate large amounts of light from a relatively small source These lamps are widely used in many applications, including highway and road lighting, lighting of large venues such as sports stadiums, floodlighting of buildings, shops, industrial buildings, and projectors, to name but a few The term "HID lamp" is used to denote different kinds of lamps These include mercun vapor lamps, metal halide lamps, and sodium lamps Metal halide lamp's, in particular, are widely used in areas that require a high level of brightness at relamejy low cost HID lamps differ from other lamps because their functioning environment requires operation at high temperature and high pressure ox er a prolonged period of time Also, due to their usage and cost, it is desirable that these HID lamps have relatively long useful lives and produce a consistent level of brightness and coioi of light Although in principle, HID lamps can operate with either an alternating current (AC) supply or a direct-current (DC) supply, in practice, the lamps are usuall\ drtsen via an AC supply

[0003] Dischaige lamps produce light by ionizing a vapoi fill material, such as a mixture of rare gases, metal halides and mercury with an electric arc passing between two electrodes Hie electrodes and the till material are sealed within a translucent or transparent discharge vessel that maintains the pressure of the energized fill material and allows the emitted light to pass through it The fill material, also known as a "dose," emits a desired spectral energy distribution in response to being excited by the electric arc For example. haSides provide spectral energy distributions that offer a broad choice of light properties, e g color temperatures, color reπderirms and luminous efficacies [0004] With current technology, if the ratio between the distance separating the electrodes in the discharge vessel to the diameter of the chamber is less than four, the relative abundance of sodium between the arc and the discharge chamber walls produces greater absorption of generated light radiation by such sodium due to its absorption lines near the peak values of visible light. Additionally, if the ratio is less than five, the lamp being operated in a horizontal position, results in the arc established in the arc discharge chamber substantially bending upward due to the buoyancy of its vaporized chamber constituents. The upward bending of the arc draws it closer to the wall of the arc discharge chamber raising the temperature of the wall in that vicinity. Such temperature increases can reduce the operating life of the lamp when operated horizontally.

[0005] Therefore, it would not be obvious to a person skilled in the art for a lamp to achieve higher efficacies and better color performance having as aspect ratio of less than 4.0 with an operating voltage of less than 110.

[0006] Unexpectedly, the present invention achieves greater than about 1 10 lumens per watt (LPW) initially, at least about 80% lumen maintenance at 12,000 hours and a color rendering index (Ra) of about greater than 65, with less than about 5 mg of mercury present These benefits are achieved through a combination of an aspect ratio of greater than about 2.5 and less than about 4.0, a ratio of sodium to rare earth of greater than about 10 and less than about 40, a dose composition of sodium halide, a calcium halide, thallium halide, and lanthanum halide. and a tungsten-oxygen cycle.

SUMMARY OF THE DISCLOSURE

[0007] In an exemplary embodiment, a lamp includes a discharge vessel and electrodes extending into the discharge vessel. The vessel further includes an ionizable fill sealed within the vessel. The ionizable fill includes at least an inert gas and a halide fill. The halide fill includes a sodium halide, a thallium hatide, at least one of a calcium halide and a strontium halide, and at least one of a rare earth halide selected from the group consisting of lanthanum, cerium, praseodymium, samarium, and neodymium, and combinations thereof. [0008] In one embodiment of the invention, the foregoing combination includes lanthanum halide as the rare earth component.

[0009] According io the invention, the aspect ratio of the discharge tube is satisfied by:

2.0 < EA/Di < 4.8

where EA - a distance between the electrodes, and

Di - an inner diameter of the discharge vessel

[0010] In another embodiment of the invention, the molar ratio of sodium halide to rare earth halide in the fill is satisfied by: 8≤ Na/RE≤448.

[0011] In yet another embodiment of the invention, a method of forming a lamp is provided. The method includes providing a discharge vessel having sealed therein, an ionizing fill, this till including an inert gas and a halide component. The halide component includes a sodium halide, a thallium halide, at least one of a calcium halide and a strontium halide and at least one of a rare earth halide selected from the group consisting of lanthanum, cerium, praseodymium, samarium, and neodymium. The method further includes positioning electrodes within the discharge vessel to energize the fill in response to a voltage applied thereto. It may be appreciated the current invention is not limited to any particular manufacturing method or processing.

[0012] A primary benefit realized by the lamp according to the invention is high efficiency due to the combination of aspect ratio (AR) and wall loading

[0013] Another benefit realized by the lamp according to the invention is high efficiency due to dose composition and the amount of that dose that is added to the discharge vessel.

[0014] Still another benefit of the lamp according to the invention is high percent lumen maintenance due to the establishment of a tungsten-halogen wall cleaning cycle preventing tungsten from deposition on the translucent wall of the discharge vessel

[0015] Further still another benefit of the lamp according to the invention is high percent lumen maintenance due to maintaining a long separation distance between the electrode tips (EA) wherein a large percent of the translucent ceramic w alls do not receive am deposited tungsten thereby reducing wall blackening

[0016] Still another benefit of the lamp according to the invention is a long life due to dose composition and dose weight added to the discharge \essel

[0017] Still other features and benefits of the lamp according to the invention will become mote apparent from reading and understanding the following detailed description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIGURE 1 is a ctoss-scctional view of an HID lamp according to the exemplary embodiment

[0019] FIGURE 2 illustrates a theoretical plot of aspect ratio versus lumens per wait

(LPW) according to the exemplary embodiment

[0020] FIGURE 3 illustrates a theoretical plot of the solubility of a tungsten species versus temperature for 0 025 nig of WO ₃ as a source of oxygen according to the exemplary embodiment

DETAILED DESCRIPTION OF THE PREFERRED EMBDIMENTS

[0021] The inv ention prvides a lamp exhibiting a combination of an aspect ratio of greater than about 2 0 and less than about 4 8, an lomzablc fill including a halide component having a ratio of sodium halide to rare earth halide of greater than about 8 and less than about 48, and an oxygen cycle. This lamp provides for a high intensity discharge lamp having higher efficacies and better color performance than other similar lamps current!) available

[0022] Aspects of the exemplary embodiment relate to a lamp that includes a discharge vessel and electrodes extending into the discharge vessel The lamp further includes an ionizable fill sealed within the vessel The iυnizable fill includes an inert gas and a haiide component The halide component includes a sodium halide, a thallium halide. at least one of a calcium halide and a strontium halide, and at least one of a rare earth halide selected ftom the group consisting of lanthanum, cerium, praseodymium, samarium, and neodymium. and combinations thereof.

[0023] The exemplary embodiment provides a while light, high color rendering index

(Ra) lamp with 110 lumens per watt (LPW) and 80% lumen maintenance at 12,000 hours. It allows metal halide lamps to compete with High Pressure Sodium (HPS) lamps for street- lighting applications. It may be appreciated that these metal halide lamps would not be limited to street-lighting applications, but also to city beau tifica lion lighting and urban lighting due to their high Ra and white light.

[0024] With reference Io FIGURE 1, a cross-sectional view of an exemplary HID lamp

10 is shown. The lamp includes a discharge vessel or arc tube 12, which defines an interior chamber 14. The discharge vessel 12 has a wall 16, which may be formed of a ceramic material, such as alumina, or other suitable light-transmissive material. The exemplary discharge vessel 14 is formed of a high temperature resistant, light permeable material formed as a single component. The discharge vessel 14 may be coated with a UV or infrared reflective coating as appropriate. The exemplary lamp 10 may be a high intensity discharge (HID) lamp, which operates at wattage of at least about 45W and in one embodiment, at least about 200W, e.g.. up to about 250W. The lamp is supplied with current by a circuit (not shown) connected with a source of AC power. The lamp may be designed to run on low frequency square wave electronic ballast. Alternatively the lamp may run on an electromagnetic ballast.

[0025] An ionizable fill 18 is sealed in the interior chamber 14. Electrodes 20, 22. which may be formed from tungsten, are positioned at opposite ends of the discbarge vessel so as to energize the fill when an electric current is applied thereto. The two electrodes 20 and 22 are typically fed with an alternating electric current via conductors 24. 26 (e.g., from a ballast, not shown). Tips 28, 30 of the electrodes 20. 22 are spaced by a distance ϊiA which defines the arc gap It will be appreciated that other known electrode materials may alternatively be used.

[0026] When the lamp 10 is powered, indicating a flow of current to the lamp, a voltage difference is created across the two electrodes. This voltage difference causes an arc across the gap between the tips 28, 30 of the electrodes. The arc results in a plasma discharge in the region between the electrode tips 28, 30. Visible light is generated and passes out of the chamber 14. through the wall 16.

[0027] The electrodes become heated during lamp operation a«d tungsten tends to vaporize from the tips 28, 30. Some of the vaporized tungsten may deposit on an interior surface 32 of wall 16. Absent a regeneration cycle, the deposited tungsten may lead to wall blackening and a reduction in the transmission of the visible light which ultimately reduces useful lamp life.

[0028] The electrode tip separation EA is the distance between the electrode tips 28. 30.

The EA for a 50VV embodiment as measured along the lamp axis X can be, for example, from about 8 ram Io about 15 mm, e g , about 8 ram Io about 12 ram, and in one embodiment, about 10 mm. For a 7OW lamp embodiment, EA as measured along the lamp axis X can be, for example, from about 10 mm to about 15 mm, e.g., about 10 mm to about 14 mm. and in one embodiment, about 1 1 mm. For a lOOVV embodiment, EA as measured along the lamp axis X can be, for example, from about 13 mm to about 24 cm, e.g., about 13 mm to about 20 mm, and in one embodiment, about 13.5 mm. For a 150W lamp embodiment, EA as measured along the lamp axis X can be, for example, from about 15 mm to about 25 mm, e g., about 15 mm to about 22 mm, and in one embodiment, about 19 mm.

[0029] The arctube diameter Di is the internal diameter of the arctube, measured in a region between the electrodes 28, 30. The Di for a 50W embodiment, for example, Di can be for example, from about 3.5 mm to about 4 mm, e.g., about 3.6 mm to about 4.0 mm, and in one embodiment, about 3.8 mm For a 7OW embodiment, for example, Di can be for example, from about 3.5 mm to about 5 ram, e g , about 3.8 mm to about 4.9 nun, and in one embodiment, about 4.25 mm. For a IOOW embodiment, for example, from about 4.2 mm to about 6.0 mm, e.g., about 4 6 mm to about 5.8 mm, and in one embodiment, about 5.45 mm. For a 150W embodiment, for example, Di can be for example, from about 5 mm to about 7 mm, e.g., about 5.5 mm to about 7.0 mm, and in one embodiment, about 6.3 mm

[0030] According to the present invention, a lamp is provided having an aspect ratio AR that, along with the till and dosage requirements set forth herein, exhibits unexpected performance advantages The aspect ratio (EA/Di) is defined as the ratio of electrode tip separation EA divided by the internal arctube diameter Di. In one embodiment, the aspect ratio of the discharge tube according to the invention is satisfied by , for example, 2 0 -- EA/Di 4 8, and in another embodiment by, 2.5 < EA/Di < 4 0

[0018] Lumens (Im), as used herein, refer to the S3 unit of luminous flux, a measure of the peteehed power of light If a light source emits one candela of luminous intensif ^y , into a solid angle of one steradian, the total luminous flux emitted into that solid angle is one lumen Put another way, an isotropic one-candela light source emits a tota! luminous flux of exactly 4π lumens The lumen can be considered as a measure of the total "amount" of visible Sight emitted The output of a lamp can be defined in terms of lumens per Watt (LPW)

[0032] The exemplary lamp according to the invention exhibits about 80% luinen maintenance at 12,000 hours of operation For example, in one embodiment the lamp exhibits 110 lumens per watt at 100 hours using an electronic ballast This lamp exhibits at least about 80 lumens per watt (LPW) at 12,000 hours of operation, and in one specific embodiment, at least about 88 LP W in another embodiment, a lamp using an electromagnetic ballast and exhibiting at least about 105 I PW at 100 hours, then exhibits at least about 80 [.PW at 12,000 hours of operation, and in one specific embodiment, at least about 84 LPW at 12,000 hows of operation

[0033] FlGL RE 2 is a plot for a iOOW lamp of aspect ratio (AR) versus lumens per watt

( LPVV), based on a statistical model from experimental data FKJURE 2 illustrates a maximum LPW of about S B at an aspect ratio of about 2 8 The LA for the lamp was about S ^ t>6 mm and the Di about 5 45 mm Thus, the maximum LPW occurs at an aspect ratio of less than 4 0

[0034] As used herein, "Aretuhe Wall Loading" (WL) is the arctube power (watts) divided by the arctube surface area (square mm) For purposes of calculating WL, the surface area is the tota! interna! surface area and the arctube power is the total arctube pouei including electrode power According to the invention, WL can be ii 35 W cm ² In one embodiment, the uaϋ loading is from about 20 to 35 W/cm ² , few example, about 3 1 W/cm ² In general, the fill and wall loading are sufficient to maintain an external wall temperature of at least about 1 100K, e g , 1100- 1525K

[0035] Higher efficiency of the lamp according to the invention is in one exemplary embodiment achieved due to the combination of aspect ratio (AR) and wall loading (VVL) loo long of an aspect ratio, i.e. > 4.0, may lead to a low wall loading and insufficient vapor pressure of the metal halide additives. Conversely, a too short aspect ratio, i.e. < 2.5, may lead to a high wall loading and a higher metal halide additive vapor pressure causing a reduction in lumens.

[0036] With further reference to Figure 1. the interior space 14 has a volume commensurate with the operating voltage of the lamp and sustainable wall loading. For example, for a 50 W lamp, the volume may be about 0.125 cm ³ to about 0.17 cm ³ e.g., about 0.15 cm ³ For example, for a 70 W lamp, the volume may be about 0 16 cm ³ io about 0.26 cm\ e.g., about 0.20 cm ³ . For example, for a 100 W lamp, the volume may be about 0.26 cm' to about 0.54 cm ³ , e.g., about 0.40 cm ³ . For example, for a 150 W lamp, the volume may be about 0.5 CRY' to about 0.9 cm ³ e g., about 0 7 cm ³

[0037] The ionizable fill 18 includes an inert gas. free mercury CHg), a halide component, and a source of available oxygen. The components of the fill 18 and their respective amounts are selected to provide available oxygen at the wall surface 32 for reaction wilh, and removal of, any tungsten deposited there. The halide component includes a rare earth halide and may further include one or more of an alkali metal halide, an alkaline earth ineial halide, and a Group HIa halide (indium or thallium). In operation, the electrodes 20. 22 produce an arc between tips 28, 30 of the electrodes that ionizes the fill to produce a plasma in the discharge space. The emission characteristics of the light produced are dependent, primarily, upon the constituents of the fill material, the voltage across the electrodes, the temperature distribution of the chamber, ihe pressure in the chamber, and the geometry of the chamber. In the following description of the fill, the amounts of the components refer to the amounts initially sealed in the discharge vessel, i.e., before operation of the lamp, unless otherwise noted.

[0038] The inert gas, also known as a butter gas, may be. for example argon, xenon, krypton, or a combination thereof, and may be present in the fill at from about 2-20 micromoles per cubic centimeter (μmol/cm ³ ) of the interior chamber 14. The buffer gas may also function as a starting gas for generating light during the early stages of lamp operation. In one embodiment, suited to CMH lamps, the lamp is backfilled with Ar. In another embodiment, Xe or Ar with a small addition of Kr85 is used. The radioactive Kr85 provides ionization that assists in starting the lamp. The cold fill pressure may be about 60-300 Torr, although higher cold fill pressures are notexcluded. In oneembodiment,a cold fill pressure ofatleastabout 120 Torrisused. Inanotherembodiment,thecoldfillpressureofuptoabout240Torrisu sed.Toohighapressure,i.e.,aboveabout300Torr,maycompromisesta rting. Toolowapressure,i.e.,belowabout60Torr,canleadtoincreasedlume ndepreciationoverthelifeofthelamp. [0039] Themercurydosemaybepresentatfromabout2to15mg/ ^' cm*ofthearctubevolume.Themercuryweightisadjustedtoprov idethedesiredarctubeoperatingvoltage(Vop)fordrawingpowerfrom theselectedballast. [0040] Thehalidecomponentmaybepresentatfromabout5toabout80mg/cm*oft hearc lubevolume, e.g, about 10-60 mg/cm\ A ratio of halide doseto mercury can be, forexample, from about 1:1 to about 10:1, expressed by weight. The halide(s) in the halidecomponentcaneachbeselectedfromchlorides,bromides,iodid esandcombinationsthereof. Inone embodiment, the halides are all iodides. Iodides tend to provide longer lamp life, ascorrosionofthearctubeand/orelectrodesislowerwithiodidecomp onentsinthefillthanwithotherwise similar chloride or bromide components. The halide compounds are usuallyincorporatedtorepresentstoichiometricrelationships. [0041] Therareearthhalideofthehalidecomponentisonethatisselectedint ypeandconcentrationsuchthatitdoesnotformastableoxidebyreacti onwiththeoptionalsourceofoxygen,i.e.,itformsanunstableoxide. Bythisitismeantthatitpermitsavailableoxygentoexistinthefilld uringlampoperation.Exemplaryrareearthhalideswhichformunstabl eoxidesincludehalidesoflanthanum(La),praseodymium(Pr),neodym ium(Nd),cerium(Ce),samarium(Sm),andcombinationsthereof. Therareearth halide(s)ofthefill canhavethegeneral formREX ₃ ,whereREisselectedfromLa,Pr,Nd,Sm,andCe,andXisselected fromCl,Br,andLandcombinationsthereof.Therareearthhalidemaybe presentinthefillatatotalconcentrationof,forexample,fromabout 0.3toabout 13μmol/cm\ Anexemplaryrareearthhalidefromthisgroupislanthanumhalide,whi chmaybepresentatamolarconcentrationofatleastabout2%ofthehali desinthefill,e.g.,atleastabout2mol%ofthehalidesinthefill.Ino neembodiment,thefillcomprisesonly3areearthhalidesfromthislim itedgroup,incombinationwithsodiumiodide,calciumiodideandthal liumiodide. Thelampfillthusissubstantiallyfreeofotherrareearthhalides.by whichitismeantthatallotherrareearthhalidesarepresentinatotal amountofno more than about 0.01 μmol/cm ³ In particular the fill is free of halides of the following rare earth elements terbium, dysprosium, holmiυm, thulium, erbium, ytterbium, lutetium, and yttrium Other haiides which form stable oxides are also not present in the fill such as scandium halides and magnesium haiides

[0042] The alkali metal halide. where present, ma> be selected from sodium (Na), potassium ( K), and cesium (Cs) haiides, and combinations thereof In one specific embodiment, the aikali metal halide includes sodium halide The alkali metal halidc(s) of the fill can have the general form AX. where A is selected from Na, K, and Cs, and X is as defined above, and combinations thereof The alkali metal haiide may be present in the fill at a total concentration of, foi example, from about 10 to about 300μmol/cin

[0043] The alkaline earth metal halide, where piesem, may be selected from calcium

(Ca), and strontium (Sr) halides. and combinations thereof The alkaline earth metal halide(s) of the fill can ha\ e the general form MX ₂ , uheie M is selected from Ca and Sr, and X is as defined above, and combinations thereof In one specific embodiment, the alkaline earth metal halide includes calcium halt do The alkaline earth metal haiide may be present in the till at a total concentration of. for example, from about 3 to about 100μmol. cm ³

[0044 ] The Group IIIa halide, where present, may be selected from thallium (T1) and indium (In) haiides In one specific embodiment, the Group 31Ia halide includes thallium halide llie Group IIIa halide(s) of the fill may have the general form TlX or InX ₃ , where X is as defined above The Group IHa halide may be present in the fill at a total concentration of, for example, from about 0 15 to 15 Oμmol/cm ³

[0045] In one embodiment, the till comprises

69-76 mol% of sodium halide.

18 0-21 5 mol% of calcium halide,

2 0-7 5 mol% of rare earth halide. and

1 0-3 5 mol% of thallium halide

[0046] In one embodiment, a molar ratio of sodium halide to rare earth halide in the fill is satisfied by 8≤Na/RE≤48

where Na - moles of rare earth halide in the fill, and

Re = moles of rare earth halide in the fill.

[0047] In another embodiment, a molar ratio of j;odium halide to rare earth halide in the fill is satisfied by. 10 < Na/RE < 36

[0048] -The color rendering index (CRI) is an indication of a lamp's ability to show individual colors relative to a standard. This value is derived from a comparison of the lamp's spectral distribution compared to a standard (typically a black body) at the same color temperature. There are fourteen special color rendering indices (Ri where i ^:::: 1-14) which define the color rendering of the light source when used to illuminate standard color tiles. The general color rendering index (Ra) is the average of the first eight special color rendering indices (which correspond to non-saturated colors) expressed on a scale of 0-100. Unless otherwise indicated, color rendering is expressed herein in terms of the Ra. The color rendering index can be at least 50, in some embodiments, at least 55, and in specific embodiments, about 65 or greater.

[0049] Higher efficiency of the exemplary embodiment is achieved due to dose composition and amount added to the arc tube. The design requirement of a relatively low Ra allows for dose weight to be held to a minimum. Low halide dose weight and the resulting low halide vapor pressure may lead to an increase in efficiency. The requirement of a low Ra may allow for dose composition to favor higher amounts of the more efficacious species, such as sodium halide, and lower amounts of the less efficacious species, such as lanthanum halide.

[0050] The source of available oxygen is one which, under the lamp operating conditions, makes oxygen available for reaction with other fill components to form WO2X2 The source of available oxygen may be an oxide which is unstable under lamp operating conditions, such as an oxide of tungsten, free oxygen gas (O:), water, molybdenum oxide, mercury oxide, dioxides of lanthanum, cerium, neodymium, samarium, praseodymium, or combination thereof. The oxide of tungsten may have the general formula WO _n X _1n , where n is at least 1, m can be 0, and X is as defined above. Exemplary tungsten oxides include WO ₃ , WO2, and tungsten oxyhalides, such as WO ₂ I ₂ . The source of available oxygen present in the fill may be expressed in terms of its available O2 content at, for example, from about 0.1 μmol/cm', e.g., from 0.2-3.0 μmol/cm\ and in one embodiment from 0.2-2.0 μmol/cm ³ . As will be appreciated, certain oxides do not decompose readily to form available oxygen under lamp operating conditions, such as cerium oxide (Ce ₂ O ₃ ) and calcium oxide, and thus do not tend to act effectively as sources of oxygen, m general, most oxides of rare earth elements (RE ₂ O ₃ ) are not suitable sources of available oxygen as they are stable at lamp operating conditions.

[0051] In one embodiment, the tungsten electrode is partially oxidized to form tungsten oxide, e.g., a spot on its surface is thermally oxidized prior to insertion into the lamp, to provide the source of available oxygen. In other embodiments, comminuted tungsten oxide, such as tungsten oxide chips, may be introduced in the fill.

[0052] FIGURE 3 illustrates theoretical thermodynamic calculations for the solubility of tungsten species vs. temperature for 0.025 nig of WO ₃ added Io the fill as a source of available oxygen. SPW represents the summed pressures in atmospheres of all tungsten species present in vapor form. As can be seen from FIGURE 3, the plot passes through a trough where the solubility is lowest (e.g.. at SPW mint.). The present exemplary embodiment takes advantage of this trough by selecting a tungsten oxide concentration such ihat the electrode tip temperature falls closer to the trough, i.e., a lower SPW, than the wall. In general, the SPW at the electrode tip (or wherever on the electrode solubility is lowest) should be no more than 90% of the SPW at the wall to encourage regeneration. Thus, for example, with a WO ₃ dose of 0.025 mg, where the wall temperature is about 1200K during operation and the lip temperature is about 2900K, lhe SPW at the electrode tip 28, 30 is lower than at the wall 32.

[0053] As described in various aspects, the lamp is able to simultaneously satisfy photometric targets without compromising targeted reliability or lumen maintenance. Some additional photometric properties that are desirable in a lamp design include CCT. and dCCy.

[0054] Correlated Color Temperature (CCT) is defined as the absolute temperature, expressed in degrees Kelvin (K). of a black body radiator when the chromaticity (color) of the black body radiator most closely matches that of the light source. CCT may be estimated from the position of the chromatic coordinates (u, v) in the Commission Internationale de I'Eclairage (CIE) 1960 color space From this standpoint, the CCT rating is an indication of how "warm" or "coor the light source is. The higher the number, the cooler the lamp. The lower the number, the warmer the lamp. The exemplary lamp may provide a correlated color temperature (CCT) between for example, about 2700K and about 4500K, preferably between about 2900K and about 3200K, e.g., 3000K. For example, in one embodiment, the lamp includes a fill containing calcium halide, wherein in operation, the Samp operates at a correlated color temperature (CCT) of at least about 3000K. In another embodiment, the Samp includes a fill containing strontium halide, wherein in operation, the lamp operates at a correlated color temperature (CCT) of at least about 4000K. The present invention has been described by way of the above embodiments. It should be appreciated, however, that the present invention is in no way limited to the specific embodiments described above. Various modifications including fills and temperatures may be modified.

[0055] dCCy is the difference in chromaticity of the color point on the Y axis (CCY), from that of the standard black body curve The exemplary embodiment may have a dCCy of greater than about -0.015 but less than about +0.005 with respect to the black body locus, and in one specific embodiment, the lamp lies directly on the black body locus, i.e. dCCy ~ 0.000.

[0056] It has been determined, in light of the foregoing, that in order to achieve the desirable Samp operating characteristics of CCT (2850 K < CCT < 3250 K) and dCCy (-0.015 < dCCy < 0.005), which correspond to a lamp exhibiting a cooler, whiter light more characteristic of typical ceramic metaS halide lamps, then

10 < Na/RB < 18 and 1 < 111% < 3.5.

Similarly, when it is desirable to have lamp operating characteristics of CCT (2550 K < CCT < 2S50 K) and dCCy (-0.0 S 5 < dCCy < 0.005), corresponding to a wanner, more yellowish light, more typical of high pressure sodium lamps, then

18 < Na/RB < 36 and 1 < TIII % < 3 5.

[0057] The following Table I provides data with regard to Na/RE ratio, CCT, Tlϊ, and dCCy for lamps at varying powers and including varying fills.

TABLE I

[0058] Table 1 provides data and parameters for typical lamp embodiments Example 1, the first lamp is a 150W lamp with a NA/RE ratio of 36 and T1I% of 3%. This lamp operates al a CCT of 3056 K and dCCy of 0.01. Having these parameters, this lamp would emit light appearing to be slightly greenish-white.

[0059] Example 2 is another 150W lamp, however in this lamp the Na/RE ratio has been reduced to 18 and Tll% reduced to 1.5%. This lamp operates with CCT of 2826 K and dCCY of -.0012. This lamp produces a warmer white light than Example 1.

[0060] Example 3 is yet another 150W lamp. In this case, the Na/RE ratio has been reduced further, to 1 1 and TII% retained at 1.5% as in Example 2. This lamp operates with CCT of 3001 K and dCCy of 0.0008. The light emitted by this lamp is a cooler white light than example 2 and a whiter light than example I .

[0061] Based on the foregoing, it is seen that by altering the NaRE ratio and the III content, the lamp can be tailored to produce a certain emittance. as a function of CCT and dCCy.

[0062] Table I further illustrates that all of the parameter ranges discussed above may be simultaneously satisfied in the present lamp design Unexpectedly, this can be achieved without negatively impacting lamp reliability or lumen maintenance. Thus, for example, the exemplary lamp may have a lumen maintenance of approximately 80% or better at 12,000 hours, e.g.. at an external wall temperature which is no greater than 1525K Typical current art lamps exhibit a lumen maintenance of less than 65% at 12,000 hours.

[0063] The invention has been described with reference to the preferred embodiments.

Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations