Electrodeless lamp bulb

Abstract

 A lamp bulb for an electrodeless lamp comprising a sealed envelope (3) containing a fill material (4) for supporting a gas discharge, said fill material comprising an inert starting gas, mercury, and alkali metal iodides, characterised in that the alkali metal iodides consist substantially of sodium iodide and lithium iodide, and in that the fill further comprises at least one iodide of a rare earth and scandium iodide.
Preferably the lamp bulb will have a colour rendering index, Ra, of a value greater than about 80 and result in a colour temperature between about 3000 to about 5000 degrees Kelvin.

Description

[0001] This invention relates a lamp bulb. More particularly, this invention relates to a lamp bulb for electrodeless lamps having a metal iodide fill with improved colour rendering.

[0002] High frequency electromagnetic field excitation of gas discharges has been studied and applied for many years. Originally, microwaves were applied in gas discharge devices such as Noise Sources, Transmit-Receive (TR) Tubes, and, generally, as Gas Discharge Circuit Elements. The interaction of microwaves with gas discharges was treated by S. C. Brown, Introduction to Electrical Discharges in Gases, John Wiley & Sons, Inc., New York, (1966). An early application to lamps is given in "Microwave Discharge Cavities Operating at 2450 MHz" by F. C. Fehsenfeld et al., Rev. Sci. Instruments, 36, No. 3, (March 1965), where,in a resonant discharge cavity power, is transferred from the source to the lamp. The lamp is substantially enclosed by the resonant cavity impeding the transmission of light from the gas discharge source.

[0003] The first practical microwave light sources, often called electrodeless lamps, were described by a group at GTE Laboratories in 1975. Using an electrodeless lamp and a termination fixture, having an inner and outer conductor, it is excited by high frequency power at 915 or 2450 MHz, or in the possible frequency range from 100 MHz to 300 GHz. This work is described and covered in the following patents: US-A-3,942,058; US-A-3,942,068; US-A-3,943,401; US-A-3,943,402; US-A-3,943,403; US-A-3,943,404; US-A-3,993,927; US-A-3,995,195; US-A-3,997,816; US-A-4,001,631; US-A-4,001,632; US-A-4,002,944; US-A-4,041,352; US-A-4,053,814; US-A-4,065,701; US-A-4,070,603; US-A-4,178,534; and US-A-4,266,162.

[0004] The possible frequency bands available for microwave lamp operation are regulated by the Federal Communications Commission, Rules and Regulations, Vol. II, Part 18, Industrial, Scientific, and Medical Equipment, Federal Communication Commission, July 1981. See 18.13, page 180. Guidelines for threshold limit values for microwave radiation are published by the American Conference of Governmental Industrial Hygienists, Threshold Limit Values and Biological Exposure Indices for 1989-1990; American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio, pp. 108-111.

[0005] Microwave-powered lamps are comprised of a gas discharge in a sealed envelope containing a chemical fill of mercury, metal halides and starting gas, such as argon. The microwave power from a (solid state, magnetron or other tube) power source is coupled over a transmission line (e.g. waveguide, coaxial line or microstrip line). Impedance matching efficiently couples the EM-field into the chemical fill to start, develop, and maintain the discharge for efficient generation of light. The light emitting plasma discharge, the lamp fill, the arc tube and the field coupler comprise the effective impedance matched load (field coupled lamp load) that the power supplying microwave transmission line sees.

[0006] US-A-4,427,921 describes such an application of high frequency power to an electrodeless lamp containing metal iodide or iodine. Optical emission is described as being dominated from excited iodine atoms which emit ultraviolet light at 206.2 pm. Additional emissions are described as being produced in the visible and ultraviolet portions of the spectrum from radiative transitions in I, I₂, HgI₂, HgI, Cd, CdI₂, CdI, etc., depending on the composition of the fill material.

[0007] US-A-4,206,387 al describes scandium iodide and sodium iodide chemical fills for an electrodeless lamp to provide high efficacy (about 100LPW) but only fair colour rendering (CRI=65). As set forth, the use of rare earth fills in electroded lamps results in "very high wall loadings ... resulting in a rapid decrease in colour temperature ... and a very short effective lifetime of about 200 hours". The improved electrodeless lamp, is set forth in column 6, lines 50 to 55, "mercury is needed for a high pressure discharge, argon is used to initiate the discharge, and a rare-earth halide is used to achieve atomic plus molecular emission". The results are described as being improved with the addition of cesium halide, but only mercury, argon, and a rare-earth halide are described as necessary. The improved fill includes a rare earth compound, i.e., dysprosium iodide, holmium iodide.

[0008] Because of their superior efficacy and operating life, conventional electroded lamps utilizing a chemical fill of alkali and scandium iodides are highly desirable. GTE's Metalarc M100/U lamp, with a NaIScI₃CsI chemistry, has a colour rendering index (CRI) of 65, an initial lumens per watt (LPW) of 85, and a 10,000 hour lifetime. The above chemistry can be modified by the replacement of the element cesium with lithium to form a chemistry of NaIScI₃LiI. The resulting lamp has an improved CRI of 73 while still maintaining the 10,000 hour life and the 85 LPW efficacy. However, a CRI of 73 must be further improved for the excellent colour rendering needed for showroom lighting, displays in stores, and decorative illumination, both for indoor and outdoor use. Without such further improvement, their colour rendering properties limit their commercial use in certain colour-critical applications.

[0009] Certain advantages are attendant with the electrodeless lamp, or microwave powered lamp as compared to the conventional electroded lamp. The absence of conduction current electrodes, i.e., the elimination of tungsten from the inside of the gas discharge tube, reduces significantly the limitations imposed by the high temperature chemical reactions of the active light producing lamp fill with the container and electrical lamp materials. The electrode feedthroughs (eg. press seals) that can lead to lamp defects are also not required. In addition, lamp efficacy is improved, compared to equivalent electroded lamps, by absence of the electrical and thermal conduction losses generated in lamps with conduction electrodes. Electrolysis of fill species, such as sodium, is reduced to give good colour stability. The improved lamp performance may be more easily achieved without increase of wall temperatures.

[0010] These advantages, of themselves, do not improve the CRI of electrodeless lamps. With rare earth fills they also do not allow low enough temperatures at the wall of the gas discharge tube to promote long life. Further improvements in chemical fills for high frequency microwave powered lamps are desirable, especially fills which desirably contribute to improved colour rendering, superior efficacy, and longer operating life.

[0011] Certain terms as used in this specification have meanings which are generally accepted in the lighting industry. These terms are described in the IES LIGHTING HANDBOOK, Reference Volume, 1984, Illuminating Engineering Society of North America. The colour rendering index of light source (CRI) is a measure of the degree of colour shift objects undergo when illuminated by the light source as compared with the colour of those same objects when illuminated by a reference source of comparable colour temperature. The CRI rating consists of a General Index, Ra, based on a set of eight test-colour samples that have been found adequate to cover the colour gamut. The colour appearance of a lamp is described by its chromaticity coordinates which can be calculated from the spectral power distribution according to standard methods. See CIE, Method of Measuring and Specifying Colour Rendering Properties of Light Sources (2nd ed.), Publ. CIE No. 13.2 (TC-3,2), Bureau Central de 1a CIE, Paris, 1974. The CIE standard chromaticity diagram includes the colour points of black body radiators at various temperatures. The locus of blackbody chromaticities on the x,y-diagram is known as the Planckian locus. Any emitting source represented by a point on this locus may be specified by a colour temperature. A point near but not on this Planckian locus has a correlated colour temperature (CCT) because lines can be drawn from such points to intersect the Planckian locus at this colour temperature such that all points look to the average human eye as having nearly the same colour. Luminous efficacy of a source of light is the quotient of the total luminous flux emitted by the total lamp power input as expressed in lumens per watt (LPW or lm/W).

[0012] Viewed from one aspect the present invention provides a lamp bulb for an electrodeless lamp comprising a sealed envelope containing a fill material for supporting a gas discharge, said fill material comprising an inert starting gas, mercury, and alkali metal iodides, characterised in that the alkali metal iodides consist substantially of sodium iodide and lithium iodide, and in that the fill further comprises at least one iodide of a rare earth and scandium iodide.

[0013] Preferably, the lamp bulb comprises a sealed transparent envelope with a continuous wall and containing a chemical fill. During operation of the bulb by energization of the chemical fill with a high frequency electromagnetic field, the bulb may be operable at a desirable wall temperature conducive to long life while emitting the visible radiation.

[0014] The chemical fill comprises an inert starting gas, mercury, alkali metal iodides, scandium iodide, and at least one iodide of a rare earth. The alkali metal iodides comprise substantially sodium iodide and lithium iodide. The iodide of a rare earth and scandium iodide are preferably present in amounts sufficient to form a complex for increasing the density of the rare earth in the discharge gas during lamp operation to effect a colour rendering index greater than about 80 and a colour temperature between about 3000 to about 5000 Kelvin. Due to the increased density of the rare earth in the discharge gas at lower temperatures of operation, the wall temperature of the gas discharge tube is desirably maintained at a temperature to enhance the life of the bulb.

[0015] An embodiment of the present invention will now be discussed by way of example only and with reference to the accompanying drawings, in which:

[0016] FIG. 1 shows a representation of a microwave lamp system showing a schematic representation of the bulb during operation.

[0017] FIGS. 2A-2D show a process for preparing bulbs by a three part construction. FIG. 2A shows bulb components. FIG. 2B and FIG. 2C show construction steps. FIG. 2D shows the completed tube.

[0018] FIG. 1 shows a microwave lamp bulb in accordance with an embodiment of the invention. A bulb 1 is a transparent envelope containing a chemical fill 4 within an exterior wall 3. The fill forms a gas discharge during lamp operation. The wall material is preferably a fused silica or ceramic alumina (PCA). Yttria or sapphire which is a single crystalline alumina may be used. Since the bulb 1 is utilized in an electrodless lamp, the continuous wall has an internal surface uninterrupted by an electrically conducting path extending through the wall 3 as is found in conventional electroded bulbs.

[0019] The purpose of the metal halide chemical fill 4 is to generate sufficient optical rare earth emissions without chemical interaction with wall 3. The arc tube can have various shapes, however, a cylindrical arc tube with hemispherical end chambers is most practical. A football shape is more difficult to fabricate, but it will have a desirable increased end-temperature.

[0020] A chemical fill which forms an electrical discharge sustaining gas for emitting radiation is disposed within the transparent envelope. The chemical fill contains a base chemistry of an inert starting gas, mercury, alkali metal iodides, and scandium iodide. The desired base chemistry contributes to the desirable lamp characteristics of low wall temperature, high LPW, moderate CRI, and long life. The lamp emission due to the base chemistry is approximately on the black body chromaticity locus.

[0021] In addition to the appropriate base chemistry, the chemical fill comprises at least one iodide of a rare earth element which is at least partially vapourized during lamp operation. The iodide of a rare earth and scandium iodide are present in a molar ratio sufficient to form a complex for increasing the concentration of the rare earth in the discharge gases during lamp operation at a low arc tube wall temperature. Due to the formation of the complex, the vapour phase concentration of the rare earth is increased at the arc tube wall temperature beyond what is obtainable using the rare earth iodide alone. The wall temperature of the arc tube in the lamp of the present invention is preferably maintained between about 690 and about 960 degrees Celsius, more preferably between about 690 and about 730 degrees Celsius.

[0022] In accordance with the principles of the present invention, the improved chemical fill comprising the base chemistry and at least one rare earth iodide may enhance the colour rendering index of the lamp. Due to the presence of the rare earth atoms in the discharge gas, the lamp has a colour rendering index greater than about 80. Preferably, the colour rendering index is greater than 85 and more preferably greater than 90.

[0023] High colour rendering indices, on the order of about 90, are easier to realize at high correlated colour temperatures (CCT). In a preferred embodiment, the present invention achieves high Ra at relatively low CCT between 3000 and 4000 Kelvin.

[0024] During lamp operation, the amount of rare earth in the gas discharge is sufficient to produce an enhanced colour rendering index while maintaining the relatively low arc tube wall temperature that is conducive to long lamp life. The formation of complex molecules of the rare earth with scandium iodide results in an increased density of rare earth atoms in the gas discharge.

[0025] In preferred embodiments of the present invention, rare earth is present in an amount sufficient to complex with scandium iodide in order to increase the density of the rare earth atoms in the vapour during lamp operation to the desired level. Preferably the molar ratio of the rare earth iodide to scandium iodide in the fill is between about 1:1 to about 30:1, and more preferably between about 5:1 to about 20:1.

[0026] Due to their many emission lines, all rare earths may enhance the arc performance of a lamp, at least to some degree and in some respect. The rare earths are selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and mixtures thereof. The choice of rare earth depends on the desired radiation characteristics. The preferred rare earths for enhanced CRI are the iodides of cerium (Ce), praseodymium (Pr), neodymium (Nd), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), and lutetium (Lu). According to one embodiment the rare earth iodide is present as a single rare earth iodide selected from the above preferred group. Even more preferred are the rare earth iodides of cerium, praseodymium, dysprosium, holmium, and thulium.

[0027] A charge of mercury is present in a sufficient amount so as to establish the electrical characteristics of the lamp by desirably increasing the electric field strength to sustain a desirable power loading. Such an amount should provide an operating mercury pressure between 10 kPa (1 atmosphere) to about 1 MPa (100 atmospheres), and preferably between about 10 kPa (1 atmosphere) to about 200 kPa (20 atmospheres).

[0028] In addition to mercury, a small charge of an inert ionizable starting gas such as argon is contained within the transparent envelope. It is contemplated that other noble gases can be substituted for argon provided an appropriate pressure is maintained that is conducive to starting the lamp.

[0029] To achieve the above discussed desirable lamp properties, the scandium iodide and the alkali metal iodides are present in the fill and in the discharge gas during lamp operation. These ingredients form a base chemistry which is conducive to the low wall temperature and long lamp life. These ingredients also improve colour quality by adding a variety of lines to the emission spectrum and are preferably present in amounts for producing emission with its colour substantially on the black body radiator chromaticity locus. The molar ratio of sodium iodide to scandium iodide is preferably between about 5:1 to about 25:1. The ratio of sodium iodide to lithium iodide is preferably between about 1:1 to about 5:1.

[0030] The alkali metal iodides adjust the current-voltage characteristics, improve the colour quality, and contribute to lumen output of the lamp through strong emissions. The scandium iodides significantly improves the "efficacy" in lumens per watt (LPW) and the CRI. The addition of rare earth iodides further improves the LPW to greater than 90 and preferably greater than 100, and, also, improves the CRI to greater than 80 while maintaining CCT between 3000 and 5000 Kelvin.

[0031] In embodiments of the present invention, the selection of fill ingredients results in a desirable colour temperature between 3000 K and 5000 K, more preferably between about 3000 to about 4000 Kelvin. The molar ratios of the ingredients are selected also so that the resulting emission colour is near the highly desirable black body (BB) chromaticity locus at this desired colour temperature.

[0032] In addition to the above-mentioned fill ingredients, scandium, thorium, cadmium, or zinc may be added to the fill as metals or alloys to adjust the metal/iodine ratio in the lamp and to getter oxygen impurities. The preferred additive is scandium. For a low wattage metal iodide discharge lamp with a lamp wattage less than 175 watts, e.g., between 40 to 150 watts, the scandium metal weight dosage is preferably about 100 micrograms per cubic centimeter of arc tube volume at all wattages. The total fill weight varies with lamp operating power between about 4 and about 20mg. For example, the 100 watt lamp fill is preferably between about 4mg and about 8 mg, and more preferably between about 5.5 and about 6.5 mg.

[0033] As illustrated schematicly in FIG. 1, a microwave power source 7 may be solid state, magnetron or some other tube coupled over a transmission line 9 in the form of a waveguide, coaxial line or microstrip line. The impedance matching network 11 and EM-field coupler 13 delivers power to the bulb 1. A - A is the impedance reference plane. The light emitting plasma discharge, the lamp fill 4, the bulb 1 and the field coupler 13 comprise the effective impedance matched load (field coupled lamp load) that the power supplying microwave transmission line sees.

[0034] The lamp is powered by high frequency (microwave) excitation of the discharge that is the matched load of a microwave circuit (for maximum power transfer) operating in the frequency range from 100 MHz to 300 GHz. The lamp is impedance matched to the impedance of the transmission line 9 from the driving source for such load circuit conditions the lamp represents when it is operating in equilibrium at lamp design input power. The range of design input power for the microwave lamps is typically from 10 Watt to 1 kWatt.

[0035] When the lamp is fully warmed up and operating in equilibrium at the design power, the arc tube wall temperature at the center is preferably in the range of 690 to 730 degrees Celsius. This, of course, depends on the lamp design parameters such as mercury pressure, arc tube wall thickness and wall-loading (W/cm²) of the arc tube.

[0036] Preferably, bulb 1 is a high purity fused silica with zero hydroxyl ion content, such as GTE-Sylvania water free fused silica or General Electric GE 214A hydroxyl free fused silica. The bulb 1 is formed from tubing having a size (I.D. and O.D.) which is determined according to the desired and allowable wall loading for the particular discharge lamp.

[0037] As shown in FIG. 2A, the quartz tube 43 is first attached to a quartz rod or support member 45. As shown in FIG. 2B, a funnel 47 is inserted into the resulting assembly. The charge of chemical fill is introduced through the funnel 47. FIG. 2C shows the quartz tube 43 necked down at a constriction 49 for evacuaation and sealing. FIG. 4D shows the completed bulb 1 which includes support 45 which may be used to hold and positioned the bulb 1 in the EM-field coupling structure.

[0038] Because the chemical fill is highly hygroscopic, the bulb blanks as shown in Fig. 2B are prepared for filling by baking them in a furnace at temperatures of 1000°C and ultra high vacuum by attaching them to a vacuum system. This is done by means of a NUPRO^R B series valve (SS 8BG TSW) that is equipped with a quick-connect (CAJON, Ultra-Torr) for attaching the fill tube of bulb blank to the valve. The baked bulb still under vacuum is put into an argon filled drybox and opened to the argon. The bulb is then is filled with the liquid and solid components of the fill, the valve is again closed, and then the bulb is transferred from the drybox and attached to the gas fill system. After the argon is pumped out, the bulb is filled to the desired pressure with a noble gas such as argon, xenon or a Penning mixture and then tipped off. The backfilled gas serves as the starting gas in the lamp. The following fill is for a bulb with a volume of 1.25 cm³ and the total fill weight is 19 mg. Typical fill weights are from about 4 to about 50 mg/cm³.

Hg	67.30	µmol
Li	4.03	µmol (as iodide)
Na	10.20	µmol (as iodide)
Sc	0.42	µmol (as iodide)
Tm	6.82	µmol (as iodide)
I	35.95	µmol (as metal iodide)
Sc	2.89	µmol (as metal)
Ar	0.5 to 50	torr (as gas)

[0039] A preferred embodiment of the present invention may provide a lamp bulb with increased colour rendering index for an electrodeless high intensity discharge lamp utilizing the NaIScI₃LiI chemistry while maintaining the efficacy and long life characteristic of such lamps; may improve the colour rendering properties of the emitted light while maintaining a long bulb life; may increase the density of the rare earth species above the density obtainable with a rare earth iodide alone; may increase the density of the rare earth atoms in the gas discharge by forming a complex molecule containing the rare earth element; and may have a low wall temperature of the gas discharge envelope which is conducive to a long lamp life.

Claims

1. A lamp bulb for an electrodeless lamp comprising a sealed envelope (3) containing a fill material (4) for supporting a gas discharge, said fill material comprising an inert starting gas, mercury, and alkali metal iodides, characterised in that the alkali metal iodides consist substantially of sodium iodide and lithium iodide, and in that the fill further comprises at least one iodide of a rare earth and scandium iodide.

2. A lamp bulb as claimed in claim 1, characterised in that the emission from said bulb has a colour temperature of between 3000-5000 Kelvin and a colour rendering index greater than about 80.

3. A lamp bulb as claimed in claim 2, characterised in that the emission from said bulb has a colour temperature of between 3000-4000 Kelvin.

4. A lamp bulb as claimed in claims 1, 2 or 3, characterised in that the iodide of a rare earth and the scandium iodide are present in a molar ratio which results in an increased density of rare earth in the discharge in use.

5. A lamp bulb as claimed in claim 4, characterised in that the iodide of a rare earth and the scandium iodide are present in amounts sufficient to form a complex molecule.

6. A lamp bulb as claimed in any of the preceding claims, characterised in that the molar ratio of said iodide of a rare earth to scandium iodide is between about 1:1 to about 30:1.

7. A lamp bulb as claimed in any of the preceding claims, characterised in that the molar ratio of said sodium iodide to said scandium iodide is between about 5:1 to about 25:1.

8. A lamp bulb as claimed in any of the preceding claims, characterised in that the molar ratio of said sodium iodide to said lithium iodide is between about 1:1 to about 5:1.

9. A lamp bulb as claimed in any of the preceding claims, characterised in that said iodide of a rare earth is selected from the group consisting of the iodides of cerium, praseodymium, neodymium, dysprosium, holmium, erbium, thulium, lutetium and mixtures thereof.

10. A lamp bulb as claimed in any of the preceding claims, characterised in that said scandium iodide and said alkali metal iodides are present in amounts for producing emission with its colour substantially on the black body radiator chromaticity locus.

11. A lamp bulb as claimed in any of the preceding claims, characterised in that in use the wall temperature is in the range of about 690 to about 960 degrees Celsius and said envelope (3) has a wall loading in the range of about 12 to 17 watts/cm².

12. A lamp bulb as claimed in any of the preceding claims, characterised in that in use the wall temperature is from about 690 to about 730 degrees Celsius.

13. A lamp bulb as claimed in any of the preceding claims, characterised in that said envelope (3) has a total concentration of fill between about 4 to about 50 mg/cm³.

14. An electrodeless lamp comprising a lamp bulb as claimed in any of the preceding claims.

Drawing