Abstract:
The invention provides an incandescent electromagnetic radiation source comprising a non-metallic emitter body that conducts electricity, and an emitting volume within the emitter body that has a thermal energy, optical absorption coefficients, and optical scattering coefficients, and that generates and externally emits electromagnetic radiation. An electric current is applied to the emitting volume such that a substantial portion of the thermal energy is generated by electrical resistive heating within the emitting volume. The optical absorption coefficients have significantly larger values within a predetermined high emissivity portion of the electromagnetic spectrum than within a predetermined low emissivity portion of the spectrum, and the optical scattering coefficients have much larger values than the optical absorption coefficients within the predetermined low emissivity portion of the spectrum. Also, to provide electrical stability and electrical switching, a resistance inverting switching device is used. The device comprises a variable resistance element, at least one output load, at least one resistance sensing device whereby changes in the resistance of the variable resistance element is sensed, and at least one electronic switching element that switches the load current on and off. Electrical interconnections between the switching element and the resistance sensing device causes the switching element to decrease the length of time that the load conducts current when the electrical resistance of the variable resistance element decreases, and to increase the length of time that the load conducts current when the electrical resistance of the variable resistance element increases.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to incandescent electromagnetic (E-M) radiation sources and electrical switching circuits. More specifically this invention relates to selective incandescent emitters that preferentially radiate within a selected portion of the E-M spectrum, and to electrical power controllers and switching.  
           [0003]    2. General Background and Description of Related Art  
           [0004]    For an emitter of thickness d that has spectral absorption coefficients a ν  at frequency ν, which is much larger than its optical scattering coefficient σ ν , the spectral emissivity, ε ν , at frequency νis given by, ε ν =(1−R)(1−T)/(1−RT a ). R is the surface reflectivity and T a , being the transmissivity, is given by exp(−a ν d). Therefore, by utilizing the appropriate first order expansions, for optically thin media (i.e. a ν d&lt;&lt;1, yielding minimal absorption of internally generated radiation) with negligible optical scattering, we get an emissivity equal to a ν d, and for optically thick media (i.e. a ν d&gt;&gt;1, yielding almost total absorption of internally generated radiation) we get an emissivity equal to 1−R. The spectral emissivity of an object that absorbs perfectly (i.e. R=0) at all wavelengths is a constant value of one. The object is called a blackbody, and its spectral intensity distribution is given by the Plank blackbody distribution.  
           [0005]    For an incandescent body radiating at a particular temperature, the power radiated as a function of wavelength is the product of the emissivity and the Plank blackbody spectral distribution. The Plank distribution varies strongly with temperature, and therefore, so does the radiated intensity. The hotter the blackbody, the shorter the median wavelength of its radiated spectrum. For example, up to about solar temperatures (5776 K), the visible-to-infrared (VIS/IR) radiant power ratio increases with temperature. Since thermal material properties limit practical incandescent lighting to temperatures less than about 3100 K (a standard 100 W tungsten bulb operates at about 2770 K), significant improvements in the VIS/IR ratio require making the emissivities within the near infrared (NIR) much smaller than those within the visible spectrum. Selective emitters are incandescent radiant bodies with emissivities that are substantially larger in a selected portion of the spectrum, thereby significantly shifting their radiated spectral distribution from that of a blackbody radiating at the same temperature.  
           [0006]    One means of attaining selective emissivity within the VIS is to construct optically thick emitters from materials with reflectivity R larger within the NIR than within the VIS (the emissivity of an optically thick emitter is 1−R). However, the relatively small variations in R exhibited by most refractory materials within the visible and NIR regions are not enough to provide significant selectivity. The tungsten-filament emitter used in standard incandescent light bulbs is an example. Its emissivity, which is almost two times greater within the VIS than within the NIR, provides very little selectivity because even at 2770 K, the total power within the NIR of the Plank distribution is an order of magnitude greater than that within the VIS.  
           [0007]    An optically thick emitter resulting in better selectivity than tungsten is the Nernst Glower (Ropp 1993, and Solomon 1912). Commercially produced from 1902 to 1912, it consists of a ceramic oxide composite (zirconia, thoria, ceria and yttria) filament that glows brightly when resistively heated to up to 2650 K by an electric current. Typical lamp life, which is limited by electrolysis of the oxides during operation, is about 800 hours. Thermal failure of the electrodes (i.e. the electrical leads), which are drawn from platinum, can also be a problem. Though its VIS/NIR radiant power ratio is grater than that of tungsten, the glower has a negative temperature coefficient of resistance, which, without adequate ballast, causes thermal runaway to catastrophically high temperatures. A wire-wound ballast resistor having a positive current vs. voltage curve is used. However, energy loss within the ballast decreases overall energy efficiency to about half that of tungsten bulbs, and while modem electronic ballast have been developed for fluorescent lighting, none have been developed for incandescent lighting. Moreover, since electrical conduction within the ceramic composition occurs only at high temperatures, a separate heater is required to attain “turn-on” temperatures (i.e. the minimum temperature at which the ceramic composition appreciably conducts).  
           [0008]    Another means of attaining selective emissivity is to utilize optically thin emitters. Optically thin selective emitters are important because their spectral emissivities are a direct function of their spectral absorptivities, which can vary by orders of magnitude. One well-known approach to exploiting the spectral selectivity of certain optically thin ceramic oxides is to heat the emitters within a gas flame that does not itself radiate extensively within the NIR. Known as the Welsbach mantle, a mixture of ceramic oxides (mainly zirconia, thoria and ceria) is impregnated within thin gauze strands and arranged within a cylindrical framework. When first lit, the gauze burns away, leaving the ceramic composition in the form of thin strands. Since, for zirconia, thoria and ceria, the spectral absorptivity is well over two orders of magnitude greater within the VIS than within the NIR, and since the ceramic strands constitute optically thin emitters with spectral emissivity proportional to spectral absorptivity, the mantles radiate at significantly greater VIS/NIR radiant power ratios than tungsten bulbs. But since gas flame heating is unsuitable for general lighting purposes the lanterns are limited to mainly outdoor recreational use. The patent of Fok (1970) is another example of a special purpose (i.e. miniature lighting) optically thin, selective emitter, but in this case, a semiconductor, instead of ceramic oxides compose the emitter body. The rear-earth oxide emitters discussed by Chubb et al. (1999), present other examples of special purpose (i.e. thermophotovoltaic energy conversion) optically thin selective emitters. In this case the emitters are optimized for selective emissivity within the NIR.  
           [0009]    A relatively recent approach to selective emissivity that combines the potentially high selectivity of optically thin emitters with the versatility of thick emitters is to utilize significant optical scattering within materials having large variations in spectral absorptivity (see Warren et al. 1976, Riseberg 1985, Chubb and Lowe 1993, or McIntosh, 2000). With this approach, an optically thick emitter can radiate as if optically thin because scattering limits the distance below the surface from which significant amounts of internally generated radiation can emerge. Unlike the case with no internal scattering, with scattering an optically thick medium can exhibit a selective emissivity that is a function of its spectral absorption coefficient, a ν . This is important because oxides such as zirconia and ceria have absorption coefficients that can be two to three orders of magnitudes greater within the VIS than within the NIR. However, a mathematical description of such emitters requires a radiation transfer model. A formulation of such a model was solved in closed form by Chubb and Lowe (1993) to obtain a general expression for the spectral emissivity. In FIG. 13, ε ν  (the spectral emissivity) is plotted as a function of z ν  (the scattering albedo) for an optically thick body with z ν =σ/(a ν +σ) (a ν  is the spectral absorption coefficient and σ is the scattering coefficient). As z ν  approaches 1, ε ν  decreases by many orders of magnitude. Therefore, for high selectivity, 1−z ν  should be roughly two to three orders of magnitude smaller than 1 in the desired low emissivity portion of the emission spectrum, and a ν  should have values roughly two to three orders of magnitude greater within the desired high emissivity portion of the spectrum than its values within the low emissivity portion of the spectrum. Since σ does not vary significantly with wavelength, this requires a substantial decrease in a ν  as ν transitions from the VIS to the NIR (assuming the VIS is the desired high emissivity portion of the spectrum). For zirconia and ceria, a ν  decreases by approximately three orders of magnitude.  
           [0010]    Only a few published reports describe attempts to enhance spectral selectivity by introducing significant optical scattering within incandescent emitters (Warren et al. 1976, Riseberg 1985, McIntosh 2000). Riseberg discloses a candoluminescent filament with a carbonized resistive core, wherein the sheath surrounding the core contains a porous structure that one supposes could provide some degree of optical scattering. However, nowhere within the disclosure is there mention of utilization of the porous structure to provide any optical scattering or enhancement of spectral selectivity. Moreover, due to the carbon-thoria and the carbon-ceria makeup of the filament, and the fact that the maximum temperature at which phase stability at the carbon interfaces exists is only about 2250 K, sufficiently high temperatures cannot be maintained to provide the desired efficiency improvements.  
           [0011]    In Warren et al. (1976), the core of the emitter contains a metal-ceramic oxide composite that is resistively heated via an electric current and that conducts heat to the outer emitting portion, which has a plurality of spaced minute optical scattering discontinuities and optical absorption coefficients such that visible radiation is substantially absorbed while traversing the distance between scattering discontinuities. However, similarly to Riseberg (1985), phase instabilities at the metal-ceramic interface do not allow stable operation above 2200 K. Another fundamental problem for Warren (as well as for Riseberg) is the reliance on thermal conduction between a heating component (the emitter core) and an emitting component (the outer sheathe), which are chemically different, and therefore cannot maintain interface stability at sufficiently high temperatures. This problem is a result of being unable to directly heat the emitting layer via stable electrical resistive heating.  
           [0012]    McIntosh (2000) describes a selective emitter having absorption and scattering coefficients consistent with the radiative transfer design suggested by FIG. 13 and described above. The body of the disclosed Multi-Element Selective Emitter (MESE) is structured in the form of a hollow bi-layer tube with a tungsten heating coil enclosed within. The coil does not physically contact the tube, thereby avoiding thermally activated surface-to-surface corrosion. Heating is accomplished by radiant energy transfer; however, this approach yields maximum outer layer temperatures of less than 2200 K. Consequently, the VIS/NIR radiant power ratio is no greater than that of a standard tungsten bulb operated at 2770 K.  
         SUMMARY OF THE INVENTION  
         [0013]    The invention provides an incandescent selective emitter having an electrically conducting externally emitting body that is directly resistively self-heated, and that contains significant optical discontinuities such that the relative values of its optical scattering and absorption coefficients allow substantial selectivity within the relevant E-M spectrum. In the preferred embodiment, direct resistance heating of the emitter body is accomplished by connecting electrodes across and conducting a current through the emitter. This approach overcomes the need to depend on radiant heating, which proved insufficient with the MESE (McIntosh 2000), and overcomes the need to depend on thermal conduction between two dissimilar materials, which proved unstable at high temperatures with the emitters disclosed by Warren et al. (1976) and Riseberg (1985). Selective emissivity is accomplished by utilizing, for the emitter body, a refractory material with spectral absorption coefficients that are much larger within the desired high emissivity portion of the spectrum (i.e. the selected spectrum) than that within the desired low emissivity portion of the spectrum. Significant scattering is introduced by incorporating many minute pores within a multicrystalline body. Wide band-gap materials such as the ceramic oxides zirconia, ceria and thoria, are used for selectivity within the UV-VIS, and a wide band-gap semiconductor such as silicon carbide or rare earth doped ceramics such as ytterbium and thulium doped zirconia (Chubb et al.) are used for selectivity within the VIS-NIR. However, because the conductivity of such materials increases with temperature, without a means of electro-thermal stabilization, thermal runaway to catastrophically high temperatures occur.  
           [0014]    Different methods for limiting the emitter current can be used to prevent thermal runaway. For instance, a variety of electronic, magnetic or resistive ballast, which are well known within the art, can be used. Additionally, a novel electronic ballast utilizing a triac to switch off electrical power for longer durations in response to a load with a decreasing resistance is disclosed. This provides a simplified electronic ballast design that is more efficient and cost-effective that one based on fluorescent lamp ballast designs. Also provided is an efficient resistive ballast design obtained by mounting a metal coil resistor within the cylindrical cavity of a tube-shaped emitter body without physically contacting the cavity walls. This allows recovery by the emitter of the heat dissipated by the resistor. A further stabilization approach provided involves applying additional radiant heating to the emitter body during operation. The absorbed radiant power raises the emitter temperature to significantly greater values than would otherwise be possible at that particular emitter current and voltage. Since the radiated power, which is proportional to (temperature){circumflex over ( )}4 is now substantially greater (or, from the other perspective, the resistively generated power, which is proportional to (voltage){circumflex over ( )}2, is now substantially less), thermal power fluctuations are quickly radiated away and do not result in heat buildup and thermal runaway. While an externally positioned electrical coil heater is conceivable for this task, a heater mounted concentrically within a tubular emitter is more efficient.  
           [0015]    In oxygen rich atmospheres, ceramic oxides such as zirconia and thoria are solid-state electrolytes that conduct electricity primarily via oxygen ion charge carriers. This can yield oxygen evolution at, and oxidation of the electrodes. But at high temperatures and very low oxygen partial pressures, the oxygen ion component is essentially eliminated and conduction is via electron hopping between stationary oxygen sites within the crystalline lattice. The invention facilitates electronic condition by providing an evacuated or an inert gas enclosure (i.e. a glass bulb) for the emitter, allowing the use of inexpensive metal electrodes such as molybdenum and tungsten (platinum electrodes are used with the Nernst Glower). An oxygen getter is provided to maintain negligibly low oxygen levels.  
           [0016]    To minimize electrode-emitter interface instabilities, the electrodes are spatially isolated from the emitter by electrically conducting spatial isolation terminals positioned between the electrodes and the electrical contact points on the emitter body. The isolation terminals are formed from materials exhibiting stable interfaces with both the emitter material and the electrode material at temperatures somewhat below that of the emitter center. This includes terminals formed from the emitter material, in which case the major function is providing thermal insulation between emitter and electrode, or terminals formed from an inert metal, in which case the major function is electrochemical buffering.  
           [0017]    At room temperature, ceramic oxides such as zirconia and thoria have high electrical resistances and must be preheated to minimum “turn-on” temperatures, at which point electrical conduction ensues. For the embodiments involving an internally mounted electrical coil, this arrangement allows using the coils as pre-heaters. The other embodiments are heated with externally mounted heating coils. The need for preheating requires a resistance change sensing device that signals a switching device to modify the heater current (typically to shut it off) once electrical conduction within the emitter body ensues. Such devices, which are well known within the art, include solid-state relays, electromagnetic relays, bimetallic switches, and electronic switching circuits. A novel electronic switching circuit utilizing triacs to decrease the on-time of electrical power in response to an electrical component having a decreasing resistance is disclosed. Prior art triac switching circuits of comparable simplicity can only increase instead of decrease the on-time. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a perspective view of physical layout- 1  of the invention.  
         [0019]    [0019]FIG. 2 is a perspective view of physical layout- 2  of the invention.  
         [0020]    [0020]FIG. 3 is a perspective view of physical layout- 3  of the invention.  
         [0021]    [0021]FIG. 4 is a functional diagram showing functional relationships applicable to layout- 1  or layout- 2 .  
         [0022]    [0022]FIG. 5 is a functional diagram showing an additional functional relationship applicable to layout- 1 .  
         [0023]    [0023]FIG. 6 is a functional diagram showing a functional relationship applicable to layout- 3 .  
         [0024]    [0024]FIG. 7 is a functional diagram showing an additional functional relationship applicable to layout- 3 .  
         [0025]    [0025]FIG. 8 is a schematic circuit diagram applicable to the FIG. 4 functional diagram.  
         [0026]    [0026]FIG. 9 is a schematic circuit diagram applicable to the FIG. 5 functional diagram.  
         [0027]    [0027]FIG. 10 is a schematic circuit diagram applicable to the FIG. 6 functional diagram.  
         [0028]    [0028]FIG. 11 is a schematic circuit diagram applicable to the FIG. 7 functional diagram.  
         [0029]    [0029]FIG. 12 is a functional diagram that highlights the resistance inversion function of the stabilization circuits.  
         [0030]    [0030]FIG. 13 is a plot of emissivity as a function of z ν  for optically thick scattering media. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]    [0031]FIG. 1 shows a perspective view of physical layout- 1  of the invention, which is a first physical layout of the thermal components of the invention. An internal tungsten heating coil  102  is positioned within a tubular emitter body  104  such that there is no physical contact between the two by threading coil leads  110  and  110 ′ concentrically through fixed end-caps  108  and  108 ′. To ensure no sagging, the coil is mounted in a stretched position and fixed in place by utilizing molybdenum crimps  120  applied between a bend in the leads  121  and the end-caps. The end caps help contain radiation within the emitter cavity  106 . To prevent electrical conduction between the emitter body and the coil leads, the end-caps are made from a high electrical resistivity refractory oxide such as magnesia or alumina using standard powder pressing techniques. Electrodes  112  and  112 ′, attached roughly 5 mm from the end of the emitter body, provide electrical current to the middle two thirds of the emitter body without significantly heating the ends. Annular isolation terminals  114  and  114 ′, formed from the emitter material by extrusion into rings of width greater than the emitter body thickness, are positioned between annular electrode contacts  116  and  116 ′ and the emitter body to provide thermal insulation between emitter and electrode (the electrode contacts distribute the current from the electrodes to the emitter body).  
         [0032]    For all the drawing figures, the emitter body is extruded from a paste obtained by mixing a sucrose solution with a micron grain size powder mixture comprised of 32% by volume yttria stabilized zirconia doped with about 1 volume percent ceria and mixed with 33% by volume each of carbon-black and graphite powder and subsequently sintered at about 1300 C to form a tubular body roughly 30 mm long, 4 mm in diameter, and 0.5 mm thick. The carbon black and graphite powder vaporize during sintering leaving a porous microstructure, and as with the outer layer of the emitter described by McIntosh (2000), yields  1 −z ν  values of roughly 0.60 within the VIS and 0.0013 within the IR.  
         [0033]    [0033]FIG. 2 shows a perspective view of physical layout- 2  of the invention, which is a second physical layout of the thermal components of the invention. In this layout, an external tungsten heating coil  224  is positioned externally outside the tubular emitter body  204  such that there is no physical contact between the two. Electrodes  212  and  212 ′ connected to annular electrode contacts  216  supply electrical current to the emitter body. Annular isolation terminals  214 , formed from the emitter material by extrusion, are positioned between annular electrode contacts  216  and the emitter body to provide thermal insulation between emitter and electrode. Bi-layer spacing rings  226  and  226 ′ positioned between the heating coil&#39;s end hoops  222  and  222 ′, and the electrode contacts  216  maintain concentricity and spacing of the heating coil. The outer layer  227  and  227 ′ of the spacing rings are thin molybdenum rings whose electrical contact with the end hoops  222  ensure high electrical conductivity in these areas, thereby generating minimal resistive heating in these regions. The inner layers  225  and  225 ′ of the spacing rings are extruded from alumina or magnesia or other high electrically resistive refractory oxide. End-caps  208  are used to help contain radiation within the emitter cavity (not shown). The external heating coil is connected to electrical power via leads  228  and  228 ′.  
         [0034]    [0034]FIG. 3 shows a perspective view of layout- 3  of the invention, which is essentially layout- 1  with the externally mounted heating coil of layout- 2 . Internal tungsten heating coil  302  is positioned within a tubular emitter body  304  such that there is no physical contact between the two by threading coil leads  310  and  310 ′ concentrically through fixed end-caps  308 , which are identical to  108 . The internal coil is mounted in a stretched position and fixed in place by tubular molybdenum crimps  320  positioned between the end caps and a bend  321  in the coil leads. Electrodes  312  and  312 ′ attach to ring-shaped electrode contacts  316  roughly 5 mm from the end of the emitter body. Annular isolation terminals  314  are positioned between the electrode contacts  316  and the emitter body. Bi-layer spacing rings  326  positioned between the end hoops  322  and  322 ′ of external heating coil  324  and the electrode contacts  316  maintain concentricity and spacing of the heating coil. As described for spacing rings  226 , the outer layer  327  of the spacing rings are thin molybdenum rings whose electrical contact with the end hoops  322  ensure high electrical conductivity in these areas. The inner layer  325  of the spacing rings is extruded from alumina or magnesia or other high electrically resistive refractory oxide. The external heating coil is connected to electrical power via leads  328  and  328 ′.  
         [0035]    [0035]FIG. 4 is a functional diagram showing a first and a second functional layout of the thermal and electrical components applicable to physical layout- 1  and physical layout- 2  respectively. For functional layout- 1 , electrical power for the emitter body  404  and the heating coil (in this case heating coil  402  is mounted internally and corresponds to internal coil  102 ) is derived from voltage source  452 . One end of the emitter body is electrically connected to resistance sensing device  440 , which senses the emitter body&#39;s increase in electrical conductivity when heated to its turn-on temperature by heating coil  402 , and signals switching module  442  (which is connected to heating coil  402 ), via interconnection  444 . In response, the switching module switches terminal  411  from a high power to a low power. Ballast  450 , through which electrical power to the emitter body is routed, via electrode  412 , ensures stable emitter operation. Functional layout- 2  is exactly the same as for functional layout- 1  except that coil  402  now corresponds to outer coil  224 , and the low power switched to by switching device  442  corresponds to zero power.  
         [0036]    [0036]FIG. 5 is another functional diagram showing a third functional layout of the thermal and electrical components applicable to physical layout- 1 . Prior to the emitter body  504  attaining its turn-on temperature, terminals  541  and  539  are electrically connected via switching module  542  such that internal heating coil  502  is connected directly across the input power source  552 . Electrode  512  connects emitter body  504  to resistance sensing device  540 , which senses the emitter body&#39;s increase in electrical conductivity when heated to its turn-on temperature by internal heating coil  502 , and signals switching module  542  via interconnection  544 , at which point the switching device severs electrical contact between terminals  539  and  541  and connects terminal  539  to terminal  543  instead. This provides a series connection between the emitter body and the heating coil, and allows use of the internal heating coil as both an emitter body pre-heater and as ballast.  
         [0037]    [0037]FIG. 6 is a functional diagram showing a fourth functional layout of the thermal and electrical components applicable to physical layout- 3 . Electrical power for the emitter body  604 , external heating coil  624 , and internal heating coil  602  is derived from voltage source  652 . One end of the emitter body is electrically connected to resistance sensing device  640 , which senses the emitter body&#39;s increase in electrical conductivity when heated to its turn-on temperature by the heating coils, and signals switching module  642 , which is connected to internal heating coil  602 , and switching module  643 , which is connected to external heating coil  624 . In response, switching module  642  switches terminal  611  from a high power to a low power, and switching module  643  disconnects terminal  629  from electrical power. As described above, this configuration does not require separate ballast because of the increase of emitter body temperature attributable to inner heating coil  602 .  
         [0038]    [0038]FIG. 7 is another functional diagram showing a fifth functional layout of the thermal and electrical components applicable to physical layout- 3 . Prior to the emitter body  704  attaining its turn-on temperature, terminals  741  and  739  are electrically connected via switching module  742  such that external heating coil  724  is connected directly across the input power supply  752 . Electrode  712  connects emitter body  704  to internal heating coil  702  in series with input power supply  752 . The change in voltage at terminal  743  due to the emitter body&#39;s increase in electrical conductivity when heated to its turn-on temperature by external heating coil  724 , is communicated to switching device  742  via interconnection  744 , at which point the switching module disconnects terminal  739  from electrical power. The internal heating coil functions as ballast in its series connection with the emitter body.  
         [0039]    [0039]FIG. 8 is a schematic circuit diagram showing a first and a second electrical schematic applicable to functional layout- 1  and functional layout- 2  respectively of FIG. 4. For functional layout- 1  resistor  824  represents internal heating coil  102 , and for functional layout- 2  resistor  824  represents external heating coil  224 . Before emitter body  804  is heated to its turn-on temperature by heating coil  824 , capacitor  874  charges quickly enough through resistor  866  to cause diac  862  to fire relatively early in the phase of the AC supply voltage  852  as the phase increases from zero degrees or from 180 degrees. This causes the length of time that triac  843  conducts electricity to be relatively long, which causes heating coil  824  to dissipate a relatively large electrical power.  
         [0040]    After emitter body  804  attains its turn-on temperature, its conductivity increase causes a decrease in the voltage between nodes  884  and  886  via resistor  870  (which functions as a resistance sensing device) during the period of time when triac  842  is switched off. This causes slower charging of capacitor  874 , and for functional layout- 1  where resistor  824  is the internal heating coil, resistor  866  is chosen such that diac  862  fires relatively late in the phase of the supply voltage so as to decrease the power dissipated by heating coil  824  by a predetermined amount. For functional layout- 2  where resistor  824  is the external heating coil, resistor  866  is chosen such that capacitor  874  charges so slowly that diac  862  never fires, effectively turning off heating coil  824 . For both layout- 1  and layout- 2 , the circuit arrangement yielding an effective decrease in electrical power caused by the increase in emitter conductivity constitutes a resistance inverting switching device that decreases the length of time current flows through the load (i.e. heating coil  824 ) in response to the resistance decrease of a variable resistance electrical component (i.e. the emitter body  804 ). In this case the load is distinct from the variable resistance electrical component.  
         [0041]    After emitter body  804  attains its turn-on temperature, but before self-heating to its predetermined operating temperature, capacitor  872  charges quickly enough through resistor  864  to cause diac  860  to fire relatively early in the phase of the AC supply voltage as the phase increases from zero degrees or from 180 degrees. This causes the length of time that triac  842  conducts electricity to be relatively long, which causes the emitter body to dissipate a relatively large electrical power. If the emitter body  804  self-heats past its predetermined operating temperature, its conductivity increase causes a larger decrease in the voltage between nodes  884  and  880  via resistor  868  (which functions as another resistance sensing component) during the period of time when triac  842  is switched off. This larger voltage decrease causes slower charging of capacitor  872  such that diac  860  fires relatively late in the phase of the supply voltage so as to decrease the electrical power dissipated by the emitter body and return it to its predetermined operating temperature, thereby providing ballast. In this case the load is the same as the variable resistance electrical component, and the resistance inverting switching circuit is employed as ballast.  
         [0042]    [0042]FIG. 9 is a schematic circuit diagram showing a third electrical schematic applicable to functional layout- 3  of FIG. 5. Resistor  902  represents internal heating coil  102 . Before emitter body  904  is heated to its turn-on temperature by heating coil  902 , capacitor  974  charges quickly enough through resistors  970  and  968  (triac  942  is off) to cause diac  962  to fire relatively early in the phase of the AC supply voltage  952 . This causes the length of time that triac  943  conducts electricity to be relatively long, which causes heating coil  902  to dissipate a relatively large electrical power. Meanwhile, capacitor  972  is chosen large enough such that it charges too slowly to allow diac  960  to fire, thereby maintaining triac  942  in its off state. After emitter body  904  is heated to its turn-on temperature, its conductivity increase causes a decrease in the voltage between nodes  984  and  980 . This causes capacitor  974  to charge so slowly that diac  962  never fires, effectively severing the heating coil&#39;s direct connection, via triac  943 , across the supply voltage. However, because the voltage at node  980  is now much closer to that at node  984 , capacitor  972  can now charge fast enough to cause diac  960  to fire early enough in the phase of the supply voltage to turn on triac  942  for a substantial length of time. This essentially connects the emitter body in series with the heating coil across the supply voltage. In this case, in addition to utilizing a resistance inverting switching arrangement to disconnect the heating coil  902  from direct connection (via triac  943 ) across the power supply  952 , a non-inverting switching arrangement is employed to connect it in series with the emitter body.  
         [0043]    [0043]FIG. 10 is a schematic circuit diagram showing a fourth electrical schematic applicable to functional layout- 4  of FIG. 6. Resistor  1002  represents internal heating coil  102 , and resistor  1024  represents external heating coil  224 . Before emitter body  1004  is heated to its turn-on temperature by heating coils  1024  and  1002 , capacitors  1074  and  1072  charge quickly enough through resistors  1066  and  1064  respectively to cause diac  1062  and  1060  respectively to fire relatively early in the phase of the AC supply voltage  1052 . This causes the length of time that triacs  1043  and  1042  conduct electricity to be relatively long, which causes heating coils  1024  and  1002  to dissipate relatively large amounts of electrical power. After emitter body  1004  attains its turn-on temperature, its conductivity increase causes a decrease in the voltage between nodes  1084  and  1086  via resistance sensing resistor  1070 , and between nodes  1084  and  1080  via resistance sensing resistor  1068  during the period of time when diac  1040  is not conducting. This causes slower charging of capacitors  1074  and  1072 , such that diac  1062  never fires, effectively turning off heating coil  1024 , and such that diac  1060  fires substantially later, effectively decreasing electrical power to heating coil  1002 . In this case two different switching modules are used to decrease and disconnect the power from the internal and external heating coils respectively.  
         [0044]    [0044]FIG. 11 is a schematic circuit diagram showing a fifth electrical schematic applicable to functional layout- 5  of FIG. 7. Resistor  1102  represents internal heating coil  102 , and resistor  1124  represents external heating coil  224 . Before emitter body  1104  is heated to its turn-on temperature by heating coils  1124 , capacitor  1172  charges quickly enough through resistor  1168  and heating coil  1102  to cause diac  1160  to fire relatively early in the phase of the AC supply voltage  1152 . This causes the length of time that triac  1142  conducts electricity to be relatively long, which causes heating coil  1124  to dissipate a relatively large amount of electrical power. After emitter body  1104  attains its turn-on temperature, its conductivity increase causes a decrease in the voltage between nodes  1184  and  1180 . This causes slower charging of capacitor  1172  such that diac  1160  never fires, effectively turning off heating coil  1124 .  
         [0045]    Nominal values of the various circuit elements are:  
                                                                 Triacs (All):   Trigger and latching currents ˜15 mA               Trigger and on-state voltage ˜1 V           Diacs (All):    Breakover voltage ˜35 V               Breakover current ˜.1 mA                Capacitors (All except 972 and 1072): - .1 μF           Capacitor (972): - .15 μF           Capacitor (1072): - .075 μF           Resistor (868): ˜10 kΩ           Resistor (968 and 1168): ˜50 kΩ           Resistors (864, 866, 970, 1062, 1064): ˜100 kΩ           Resistors (870, 1068, and 1070): ˜200 kΩ           Resistor (Internal heating coil): ˜50 Ω           Resistor (External heating coil): ˜150 Ω           Resistor (Emitter body): ˜50 Ω                      
 
         [0046]    [0046]FIG. 12 is a functional diagram that illuminates the relationships described above between the variable resistance element (i.e. the emitter body)  1204 , the resistance inverting switching device  1250 , comprising at least one resistance sensing device and at least one switching module, and the output loads  1202  and  1203 . Increased conduction in the variable resistance element  1204  causes the switching device  1250  to decrease the length of time that load current flows between nodes  1280  and  1290 , thereby effectively decreasing the time-averaged current (the opposite action occurs for increased conduction in the variable resistance element) and providing ballast to the variable resistance element as described in FIG. 8. Increased conduction in the variable resistance element  1204  also causes the switching device to decrease the length of time that load current flows between nodes  1281  and  1291 , or between nodes  1283  and  1293 , thereby providing the power control functions described in FIGS. 8, 10 and  11 . Further switching is also provided to connect or disconnect nodes  1280   b ,  1281   b , and  1283  to any one of nodes  1290 ,  1291  and  1293   b , thereby providing changes in circuit topology similarly to that described in FIG. 9.  
         [0047]    The invention is not limited to the particular physical layouts shown in FIGS.  1  to  3 . Any layout that allows radiant heating and direct electrical resistive heating of the emitting volume is contemplated by the invention. For instance, the emitter body could be fabricated as a bi-layer tube, either to obtain a particularly absorbing inner layer as with the MESE (McIntosh 2000) or to obtain a thinner emitting outer layer with a low emissivity inner layer, thereby incorporating the advantages of optically thin emitters. Also, the emitter cavity could be pressurized with an inert gas such as argon to extend the life of the internal heating coil. A further example is to incorporate several support rods for the external heating coil that are attached at either end to the inner layer  225  of the bi-layer spacing rings so as to ensure stability of the heating coil. Moreover, the mounting of the emitter need not be constrained to be within a bulb enclosure. As with the Nernst Glower, the utilization of platinum or other stable electrode allows operation within air.  
         [0048]    The functional interrelations of the electrical components of the invention are not limited to those shown in FIGS.  4  to  7 , instead all configurations are contemplated by the invention that allow various heating coils to radiantly heat the emitter body, and that allow the emitter to operate stably at elevated temperatures. For instance, a constant current source can be used instead of the ballast in FIG. 4, or a separate tungsten incandescent filament with associated switching module could be used to provide near-instant-on lighting until the emitter body heats up, or the external coil in FIG. 5 could be eliminated. The resistance sensing device  440  and the switching module  442  could likewise be eliminated. Also, direct electrical connections to the emitter body could be eliminated by inductively coupling microwave energy to the emitter body similarly to the induction approach used in electrode-less high intensity discharge lighting.  
         [0049]    The electronic implementation of the functional diagrams shown in FIGS.  4  to  7  are not limited to the switching circuits shown in FIGS.  8  to  11 . For instance, instead of the electronic switching described, electromagnetic relays or bimetallic switches could be used. Other types of ballast such as the resonant designs used with fluorescent lamps can also be utilized. Any electrical arrangement capable of supplying the emitter with a stable current and modifying the current conducted by the heating coils is contemplated by the invention. For instance, a timed switching of the electrical power supplied to the heating coils instead of one triggered by changes in the emitter body&#39;s conductivity is an additional possibility.  
         [0050]    The electronic implementations of the resistance inverting switching circuits are not limited to those shown in FIGS.  8  to  11 . Instead, any implementation such that the function described for FIG. 12 is retained is contemplated by the invention. For instance, the further switching that is provided to connect or disconnect nodes  1280   b ,  1281   b , and  1283  to any one of nodes  1290 ,  1291  and  1293   b  could be via electromagnetic relay instead of electronic switching. Moreover, the switching circuits are not limited to the number of input and output devices shown in FIG. 12. More variable resistance elements can be added and the number of loads can be changed.  
         [0051]    It can thus be appreciated that the objectives of the present invention have been fully and effectively accomplished. The foregoing specific embodiments have been provided to illustrate the structural and functional principles of the present invention and is not intended to be limiting. To the contrary, the present invention is intended to encompass all modifications, alterations, and substitutions within the spirit and scope of the appended claims.  
       REFERENCES  
       [0052]    Chubb, D. L. and Lowe, R. A., J. Appl. Phys. 74, (9), 5687 (1993).  
         [0053]    Chubb, D. L., Pal, A. T., Patton, M. O., and Jenkins, P. P.,  J. European Ceramic Soc.  19, 2551, (1999).  
         [0054]    Fok, M. V., Incndescent Lamp With a Glower Made of an Alloyed Semiconductor Material, U.S. Pat. No. 3,502,930, (Mar. 24, 1970).  
         [0055]    McIntosh, D. R., Multielement Selective Emitter, U.S. Pat. No. 6,018,216, (Jan. 25, 2000).  
         [0056]    Riseberg, L. A., Candolumiscent Electric Light Source, U.S. Pat. No. 4,539,505, (Sep. 3, 1985).  
         [0057]    Ropp, R. C.,  The Chemistry of Artifical Lighting Devices  (Elsevier, N.Y., 1993).  
         [0058]    Solomon, M.,  Electric Lamps,  P. 138-175 (D. van Nostrand, N.Y., 1912).  
         [0059]    Warren, R. W., Feldman, D. W., Incandescent Source of Visible Radiation, U.S. Pat. No. 3,973,155, (Aug. 3, 1976).