Rare earth halide light source with enhanced red emission

Abstract: This invention teaches a novel high pressure electric discharge lamp (1) which has the desired properties of high efficacy, good color rendering, and a warm color temperature. These desired properties are attained by utilizing as fills the rare earth iodides in conjunction with calcium halides and, or sodium halides. Efficient visible emission from the rare earth atomic and molecular fragments in the discharge is combined with the red emission from calcium monohalide to provide an efficient, warm source.

Description: This invention relates to a high pressure electric discharge lamp. More particularly, this invention relates to a high pressure electric discharge lamp having an enhanced red emission. High pressure electric discharge lamps containing Hg and rare earth iodides are commercially available and used for studio lighting. These sources have high efficacy, greater than 80 lm/W (LPW), good colour rendering, CRI approx. equal to 85, and a high colour temperature, approx. 6000 K. The high colour temperature is compatible with photographic film. Sources for more general illumination should have the high efficacy and good colour rendering of the rare earth studio lamps, but a warm colour temperature, approximately 3,000 K, more representative of an incandescent source, would be desirable. The high efficacy and good colour rendering of rare earth halide lamps arises from both atomic and molecular emission from the arc. Many rare earth atomic emission lines in the visible region of the spectrum originate from the central core of the arc. Superimposed on the atomic emission spectrum is molecular emission from the rare earth subhalides, which comes from the mantle of the arc. Since the radiation from the rare earth halide sources is deficient in the red, compared to the blue and green, a high colour temperature results. One approach to lowering the colour temperature is the addition of alkali atoms, such as sodium or lithium. These are added as the iodides to reduce reaction with the lamp envelope. The discharge typically contains cesium iodide to help broaden and stabilize the arc, and provide a source of atoms with low ionization potential (cesium ionization potential = 3.9 eV). Ionized cesium provides the electrons necessary for maintaining the discharge and reduces the cesium neutral emission in the IR which lowers the efficacy of the lamp. Ionization of cesium also lowers the extent of ionization of the rare earth atoms. This is desirable because maximization of rare earth neutral atoms increases the visible emissions. Addition of sodium alone lowers the colour temperature and increases the efficacy, but at the expense of colour rendering. The sodium emission is predominantly located at 590 nm and tends to dominate the spectrum. Also, addition of the sodium can increase the rare earth ion to neutral ratio because of the higher ionization potential of sodium relative to cesium. Addition of lithium results in emission at 671 nm. Although emission from this line lowers the colour temperature, the emission is far outside the photopic response, and efficacy decreases. It is known from US-A-3852630 to provide a high-pressure discharge lamp with a fill gas comprising mercury, a rare gas, and one or more of the halides of sodium, thallium and indium. The present invention provides a high pressure electric discharge lamp having an enhanced red emission comprising a refractory inner envelope containing a fill gas consisting substantially of mercury, cerium iodide, thulium iodide, calcium iodide, an inert gas, and cesium iodide or sodium iodide. The fill gas is preferably contained within a refractory inner envelope, which together with a support frame, and electrical connectors are contained within an outer envelope. A base can be fixed to the outer envelope and connected to the electrical connectors, to couple electrical energy to the refractory inner envelope and the electrodes. In alternative embodiments the lamp can be of the high pressure electrodeless variety. Preferred embodiments of the present invention have a fill gas which comprises mercury in the range of 1.0 to 11.0 mg/cm³, cerium iodide in the range of 0.1 to 4.0 mg/cm³, thulium iodide in the range of 0.1 to 4.0 mg/cm³, calcium iodide in the range of 0.3 to 13.6 mg/cm³ and either cesium iodide in the range of 0.1 to 4.8 mg/cm³ or sodium iodide in the range of 0.1 to 11.2 mg/cm³. In even more preferred examples, the fill gas comprises mercury in the range of 4.0 to 9.0 mg/cm³, cerium iodide in the range of 0.5 to 2.5 mg/cm³, thulium iodide in the range of 0.5 to 2.5 mg/cm³, calcium iodide in the range of 1.8 to 8.5 mg/cm³ and either cesium iodide in the range of 0.6 to 3.0 mg/cm³ or sodium iodide in the range of 1.4 to 7.0 mg/cm³. In the most preferred embodiments, the fill gas comprises mercury of about 7.0 mg/cm³, cerium iodide of about 1.0 mg/cm³, thulium iodide of about 1.0 mg/cm³, calcium iodide of about 3.4 mg/cm³ and either cesium iodide of about 1.2 mg/cm³ or sodium iodide of about 2.8 mg/cm³. These ranges can be adapted to be utilized in electroded or electrodeless high pressure electric discharge lamps, with preferably using argon as the inert gas at pressures detailed in dependent claims 8 to 12. Preferred embodiments of the present invention will now be discussed by way of example only and with reference to the accompanying drawings, in which: FIG.1 is an elevational view of a preferred high-pressure electric discharge lamp. FIG.2 is an emission spectrum of an electrodeless high pressure electric discharge lamp containing a lamp fill of Hg/CeI₃/TmI₃/CsI and Ar. FIG.3 is an emission spectrum of a electrodeless high pressure electric discharge lamp containing a lamp fill of CaI₂ in addition to Hg/CeI₃/TmI₃/CsI and Ar in accordance with a preferred embodiment of the present invention. FIG.4 is an emission spectrum of an electrodeless high pressure electric discharge lamp containing a lamp fill of CaI₂ and NaI in addition to Hg/CeI₃/TmI₃ and Ar in accordance with a preferred embodiment of the present invention. FIG.5 is a schematic representation of a preferred high-pressure electrodeless discharge apparatus. There is shown in Fig. 1 one embodiment of the present invention, an electroded high pressure electric discharge lamp 1, which comprises an outer vitreous envelope 2 of generally tubular form having a central bulbous portion 3. Envelope 2 is provided at its end with a re-entrant stem 4 having a press through which extend relatively stiff lead-in wires 5 and 6 connected at their outer ends to the electrical contacts of the usual screw type base 7 and at their inner ends to the arc tube 8 and harness 9. Arc tube 8 is generally made of quartz although other types of material may be used such as alumina, yttria or Vycor™, the later being a glass of substantially pure silica. Sealed in the arc tube 8 at the opposite ends thereof are main discharge electrodes 10 and 11 which are supported on lead-in wires 12 and 13 respectively. Each main electrode 10 and 11 comprises a core portion which is made by a prolongation of the lead-in wires 12 and 13 and may be prepared of a suitable metal such as, for example, molybdenum and tungsten. The prolongations of these lead-in wires 12 and 13 are surrounded by molybdenum or tungsten wire helixes. An auxiliary starting probe or electrode 14, generally made of tantalum or tungsten is provided at the base and of the arc tube 8 adjacent the main electrode 11 and comprises an inwardly projecting end of another lead-in wire 15. Each of the current lead-in wires described have their ends welded to an intermediate foil section made of molybdenum which are hermetically sealed within the pinched sealed portions of arc tube 8. The foil sections are very thin, for example, approximately 20µm (0.0008'') thick and go into tension without rupturing or scaling off when the heated arc tube pulls. Relatively short molybdenum wires 15, 16, and 17 are welded to the outer ends of the foil sections foil and serve to convey current to the various electrodes 10, 11, and 14 inside the arc tube 8. Insulators 18 and 19 cover lead-in wires 15 and 16 respectively to preclude an electrical short between the lead-in wires 15 and 16. Molybdenum foil strips 20 and 21 are welded to lead-in wires 15 and 16. Foil strip 21 is welded to resistor 22 which in turn is welded to the arc tube harness 9. Resistor 22 may have a value, for example, 40,000 ohm and serves to limit current to auxiliary electrode 14 during normal starting of the lamp. Molybdenum foil strip 20 is welded directly to stiff lead-in wire 5. Lead in wire 17 is welded at one end to a piece of foil strip which is sealed in the arc tube 8. The other end of the foil strip is welded to lead-in wire 2 which is welded to electrode 10. Molybdenum foil strip 23 is welded to one end of lead-in wire 17 and at the ether end to the harness portion 24. The pinched or flattened end portions of the arc tube 8 form a seal which can be of any desired width and can be made by flattening or compressing the ends of the arc tube 8 while they are heated. The U-shaped internal wire supporting assembly or arc tube harness 9 serves to maintain the position of the arc tube 8 substantially coaxial with the envelope 2. To support the arc tube 8 within the envelope 2 lead-in wire 6 is welded to base 25 of harness 9. Because stiff lead-in wires 5 and 6 are connected to opposite sides of the power line, they must be insulated from each other, together with all members associated with each of them. Clamps 26 and 27 hold arc tube 8 at the end portions and fixedly attached to legs 28 of harness 9. Harness portion 24 bridges the free ends of harness 9 and is fixedly attached thereto by welding for imparting stability to the structure. The free ends of the harness 9 are also provided with a pair of metal leaf springs 29 frictionally engaging the upper tubular portion of lamp envelope 2. A heat shield 30 is disposed beneath the arc tube 8 and above resistor 22 so as to protect the resistor from excessive heat generated during lamp operation. The arc tube 8 is provided with a fill gas consisting substantially of mercury, cerium iodide, thulium iodide, calcium iodide, an inert gas, and cesium iodide or sodium iodide. The inert gas can be selected from the group consisting of neon, argon, krypton, xenon, and mixtures thereof. The preferred inert gas is argon. The fill gas of the present invention can be used in electrodeless lamps as well as electroded lamps. One particular fill of a preferred embodiment of the present invention might consist of mercury, argon, and the iodides of cerium, thulium, cesium, sodium, and calcium. Another preferred fill of the present invention might consist substantially of mercury, argon, and the iodides of cerium, thulium, sodium and calcium. Still another preferred fill of the present invention might consist substantially of mercury, argon, and the halides of cerium, thulium, cesium and calcium. In Figure 2, an emission spectrum is shown of an electrodeless high pressure electric discharge lamp containing a lamp fill of mercury, cerium iodide, thulium iodide, cesium iodide and argon. The emission spectrum shown in Fig. 2 has poor red colour rendition. However, in Figure 3, in accordance with a preferred embodiment of the present invention, an emission spectrum is shown of an electrodeless high pressure electric discharge lamp containing a lamp fill of calcium iodide in addition to mercury, cerium iodide, thulium iodide, cesium iodide and argon which has good red colour rendition. The emission spectrum shown in Figure 3 has an increased emission in the 620 nm to 650 nm region resulting in a warmer colour temperature and an increased red colour rendition as compared to the emission spectrum shown in Figure 2. Electroded lamp spectra are similar. In Figure 4, in accordance with a further preferred embodiment of the present invention, an emission spectrum of an electrodeless high pressure electric discharge lamp containing a lamp fill of calcium iodide and sodium iodide in addition to mercury, cerium iodide, thulium iodide and argon is shown. This lamp also shows an increased emission in the 620 nm to 650 nm region resulting in a warmer colour temperature and an increased red colour rendition. Electroded lamp spectra are similar. Figure 5 is a schematic representation of an embodiment of a preferred high-pressure electrodeless discharge apparatus for use in accordance with the present invention. Shown in Figure 5 is a high-pressure electrodeless discharge lamp 32 having a discharge chamber 33 made of a light transmitting substance, such as quartz. Chamber 33 contains a volatile fill material 34. Volatile fill material 34 of discharge chamber 33 preferably includes mercury, cerium iodide, thulium iodide, cesium iodide, calcium iodide and argon or maybe includes mercury, cerium iodide, thulium iodide, sodium iodide, calcium iodide and argon. An RF coupling arrangement includes a spiral coil electrode 35 disposed around discharge chamber 33 and attached to fixture 36. A grounded conductive mesh 37 surrounds the discharge chamber 33 and spiral coil electrode 35 providing an outer electrode which is transparent to radiation from the discharge chamber 33. Spiral coil electrode 35 and grounded conductive mesh 37 are coupled by a suitable coaxial arrangement 38,39 to a high frequency power source 40. The radio frequency electric field is predominantly axially directed coincident with the spiral axis of spiral coil electrode 35 and causes an arc to form within discharge chamber 33. As used herein, the phrase "high frequency" is intended to include frequencies in the range generally from 100 MHz to 300 GHz. Preferably, the frequency is in the ISM band (ie., industrial, scientific and medical band) which ranges from 902 MHz to 928 MHz. A particularly preferred frequency is 915 MHz. One of the many commercially available power sources which may be used is an AIL Tech Power Signal Source, type 125. Visible radiation is produced by the resulting arc discharge within the lamp as depicted by the emission spectrum depicted in Figs. 2, 3 and 4. Specific details of the structure of the apparatus of this general type are shown in US-A-4 178 534. The emission spectrum produced by the addition of calcium iodide is efficiently produced in a rare earth halide discharge and originates from the mantle of the discharge like the rare earth subhalide emission. There are relatively few atomic calcium emission lines in the visible, 423 nm being the strongest, and thus, atomic calcium emission does not significantly alter the emission spectrum of that discharge. In addition, the ionization potential of calcium at 6.1 eV is sufficiently high that little ionization of calcium occurs. The vapour pressures of all the rare earth iodides are very close at 1100K and the temperature dependences of their vapour pressures are also similar. Thus, it is possible to utilize several rare earth iodides in a lamp and derive additive properties from their emission. Lamps containing rare earth halide additives must be operated at higher wall loadings and subsequent higher wall temperatures than lamps containing more volatile metal halides. The vapour pressure of calcium iodide is similar to that of the rare earth iodides. Consequently, addition of calcium iodide to the lamp does not require a change in the wall loading of rare earth containing lamps. The high wall temperature can increase wall reactions and decrease the lifetime of the lamp. However, both electrodeless and electroded lamps made from quartz and containing fills as described above were run successfully for hundreds of hours. One electroded lamp was tested for over 800 hours. These lamps also started easily and repeatedly. Alternate envelope materials such as alumina or yltria, which are designed for higher temperature operation than quartz, could be utilized to increase the operating lifetime of the source. The chemistry described herein should be applicable to ceramic envelopes. Metal iodides are usually used as additives in high pressure discharge lamps because their vapour pressure is higher than the corresponding bromides or chlorides. When only atomic emission originates from the discharge there is no advantage to using a different halide. However, when molecular emission is present, an alternate halide or mixture of halides can shift the molecular emission and desirably alter the colour properties of the lamp. This is the case for the rare earth and calcium halides. The emission from the monobromide and monochloride of calcium, like calcium iodide, is also in the wavelength region 600nm to 640nm. Thus, CaX₂, where X represents a halide atom, should be a good red emitter independent of which halides are present in the lamp. The addition of CaX₂ and NaI is more effective in improving the desirous colour properties of the rare earth lamp than the addition of NaI alone. Na tends to dominate the spectrum at 590 nm (yellow) and produces red light due to broadening of the resonance line. This typically causes a decrease in the colour temperature and an increase in efficacy at the expense of colour rendition. More red in visually acute regions is added by the CaX₂ emission. The addition of small amounts of NaI increases the efficacy, decreases the colour temperature and even increases the colour rendering index in the presence of CaI₂ as shown in Table VII. Examples Table I entitled "Rare Earth Metal Halide Summary of Lamp Fill Ranges" list the lamp fills designated type B and type C. Fill type B contains Hg, CeI₃, TmI₃, CaI₂, CsI and Ar and Fill type C contains Hg, CeI₃, TmI₃, CaI₂, NaI and Ar. Table II entitled "Rare Earth Metal Halide Lamps Summary" illustrates specific examples of lamps having the fill type B as designated in Table I. The efficiency, colour temperature, colour rendition index, wall temperature, fill type, the wall loading, and additive molar ratios are listed. Table III shows lamp data from individual lamps made with fill type C as designated in Table I. Table IV shows lamp data from individual lamps with fill type B. The lamp performance as a function of rare earth concentration is shown. Table V shows lamp data from individual lamps made with fill type B. The lamp performance as a function of mercury concentration is shown. Table VI shows reproducibility of lamp performance for the optimized type B fill. And Table VII shows lamp data for individual electroded quartz lamps at 60 Hertz utilizing a type B and a type C fill. Preferred embodiments of the present invention provide a novel high pressure electric discharge lamp which has the desired properties of high efficacy, good colour rendition and a warm colour temperature. Preferred lamps of the present invention would be good sources for more general illumination especially those applications requiring high colour rendering (e.g. department store illumination).

Claims: A high pressure electric discharge lamp (1;32) having an enhanced red emission comprising a refractory inner envelope (8;33) containing a fill gas consisting substantially of mercury, cerium iodide, thulium iodide, calcium iodide, an inert gas, and cesium iodide or sodium iodide. A lamp as claimed in claim 1 wherein said fill gas comprises mercury in the range of 1.0 to 11.0 mg/cm³, cerium iodide in the range of 0.1 to 4.0 mg/cm³, thulium iodide in the range of 0.1 to 4.0 mg/cm³, calcium iodide in the range of 0.3 to 13.6 mg/cm³ and cesium iodide in the range of 0.1 to 4.8 mg/cm³. A lamp as claimed in claim 1 wherein said fill gas comprises mercury in the range of 1.0 to 11.0 mg/cm³, cerium iodide in the range of 0.1 to 4.0 mg/cm³, thulium iodide in the range of 0.1 to 4.0 mg/cm³, calcium iodide in the range of 0.3 to 13.6 mg/cm³ and sodium iodide in the range of 0.1 to 11.2 mg/cm³. A lamp as claimed in claim 1, wherein said fill gas comprises mercury in the range of 4.0 to 9.0 mg/cm³, cerium iodide in the range of 0.5 to 2.5 mg/cm³, thulium iodide in the range of 0.5 to 2.5 mg/cm³, calcium iodide in the range of 1.8 to 8.5 mg/cm³ and cesium iodide in the range of 0.6 to 3.0 mg/cm³. A lamp as claimed in claim 1, wherein said fill gas comprises mercury in the range of 4.0 to 9.0 mg/cm³, cerium iodide in the range of 0.5 to 2.5 mg/cm³, thulium iodide in the range of 0.5 to 2.5 mg/cm³, calcium iodide in the range of 1.8 to 8.5 mg/cm³ and sodium iodide in the range of 1.4 to 7.0 mg/cm³. A lamp as claimed in claim 1, wherein said fill gas comprises mercury of about 7.0 mg/cm³, cerium iodide of about 1.0 mg/cm³, thulium iodide of about 1.0 mg/cm³, calcium iodide of about 3.4 mg/cm³ and cesium iodide of about 1.2 mg/cm³. A lamp as claimed in claim 1, wherein said fill gas comprises mercury of about 7.0 mg/cm³, cerium iodide of about 1.0 mg/cm³, thulium iodide of about 1.0 mg/cm³, calcium iodide of about 3.4 mg/cm³ and sodium iodide of about 2.8 mg/cm³. A lamp as claimed in claims 2 or 3, wherein said lamp is electroded and said inert gas comprises argon in the range of about 1.33 kPa (10 torr) to 7.98 kPa (60 torr). A lamp as claimed in claims 2 or 3, wherein said lamp is electrodeless and said inert gas comprises argon in the range of about 66.6 Pa (0.5 torr) to 1.33 kPa (10.0 torr). A lamp as claimed in claims 4 or 5, wherein said lamp is electroded and said inert gas comprises argon in the range of about 2.00 kPa (15 torr) to 6.66 kPa (50 torr). A lamp as claimed in claims 4 or 5, wherein said lamp is electrodeless and said inert gas comprises argon in the range of about 373 Pa (2.8 torr) to 1.00 kPa (7.5 torr). A lamp as claimed in claims 6 or 7, wherein said lamp is electroded and said inert gas comprises argon of about 5.99 kPa (45 torr). A lamp as claimed in claims 6 or 7, wherein said lamp is electrodeless and said insert gas comprises argon of about 666 Pa (5.0 torr).

Inventors: KRAMER JERRY, LAPATOVICH WALTER P, KRAMER, JERRY, LAPATOVICH, WALTER P.
Applicants: GTE PROD CORP, GTE PRODUCTS CORPORATION
Application year: 1987
Main classification: H01J61
IPC codes: H01J61, H01J65