Electrodeless coupled discharge lamp having reduced spurious electromagnetic radiation
BACKGROUND OF THE INVENTION 
     This invention relates to electrodeless discharge lamps and more particularly to an improved design for such a lamp which yields higher efficiency, lower thermal loading and reduced radio interference as compared to lamps found in the prior art. 
     An electrodeless discharge lamp is described in U.S. Pat. No. 4,010,400 issued to Donald D. Hollister on Mar. 1, 1977. The lamp of that patent includes a sealed envelope, an ionizable medium within the envelope and a coil of wire wrapped around a non-magnetic core and positioned adjacent the envelope in close physical proximity to the ionizable medium to supply radio frequency (RF) energy to the medium. The ionizable medium emits radiant energy when subjected to the radio frequency field. 
     The lamp disclosed in the Hollister patent has the general overall shape of a conventional incandescent lamp, the coil being positioned in an open cylindrical cavity which extends through part of the distance of the envelope. This design has several resultant disadvantages. First, it does not have an optimum shape for discharge efficiency nor for coupling of the radio frequency energy to the ionizable medium. Additionally, there is a relatively high amount of thermal loading, i.e. a large amount of heat is generated inside of the lamp envelope. Although the frequency of the RF energy is chosen so that the base frequency and several higher harmonics do not interfere with FCC allotted broadcast frequencies, the energy from the high frequency coil produces a substantial amount of radio frequency interference within the immediate environment of the home or office. This can cause objectionable local radio interference with radio, T.V., microwave ovens, and the human body. 
     U.S. Pat. No. 3,521,120 issued to J. M. Anderson on July 21, 1970 shows an electrodeless fluorescent lamp having a hermetically sealed toroidal envelope containing an ionizable medium with the envelope surrounding a radio frequency coil and the ionizable medium is activated by the coil. This patent does not teach a design which maintains the RF field within the discharge volume, nor does it teach an arrangement for cooling the toroidal envelope. It therefore has two of the disadvantages of the Hollister lamp, i.e. the problems of radio frequency interference and high thermal loading. 
     It is an object of the present invention to provide an electrodeless discharge lamp having a geometry optimized for discharge efficiency and radio frequency coupling. 
     A further object is to provide an electrodeless discharge lamp having increased cooling efficiency as compared to lamps of the prior art. 
     Another object is to provide an electrodeless discharge lamp which substantially reduces radio frequency interference. 
     An additional object is to provide an electrodeless discharge lamp in which the heat distribution is more uniform than that found in lamps of the prior art. 
     Still a further object is to provide an electrodeless discharge lamp which is cooled by convection. 
     In accordance with the invention, an electrodeless discharge lamp is provided which utilizes an envelope having a toroidal shape. A toroid is defined as any planar shape which is rotated about an axis in the same plane, the axis not intersecting the planar shape. The envelope is hollow and is filled with an ionizable medium which is capable of emitting radiant energy when subjected to and ionized by the energy of a radio frequency field. In the preferred embodiment, transparent windings are coated on the interior, top and exterior surfaces of the envelope. The windings on the top and exterior surfaces confine the radio frequency field almost principally to within the toroid, thereby substantially eliminating radio interference while producing a more efficient coupling of radio frequency energy to the discharge. The free space near the bottom interior of the envelope is used to mount and cool the electronics needed to drive the radio frequency windings and provides convection to the interior of the toroid. 
     The foregoing brief description as well as further objects, features and advantages of the present invention are best appreciated by reading the following detailed description of several preferred embodiments in accordance with the invention while referring to the accompanying drawings. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a lamp in accordance with the present invention; 
     FIG. 2 is a cross-sectional elevation of the lamp of FIG. 1; 
     FIG. 3 is a perspective view of a further embodiment of a lamp in accordance with the present invention; 
     FIG. 4 is a cross-sectional elevation of the lamp of FIG. 3; and 
     FIG. 5 is a cross-sectional elevation view of an alternate embodiment of the lamp of either FIG. 1 or FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, the lamps of the present invention are designed to fit into a socket of the type used by a conventional incandescent bulb. They have a distinctive envelope which is shaped like a hollow toroid in the form of an elipse rotated about an axis of revolution external to the elipse. This envelope is fitted into a base, and is filled with an ionizable medium. In the preferred embodiment transparent windings are coated on the interior, top and exterior surfaces of the toroidal envelope. An air gap provided in the envelope near the base of the lamp allows for convection cooling. 
     A perspective view showing the toroidal shape of envelope 12 is shown in FIG. 1. The outside dimensions are similar to those of a conventional incandescent bulb. The diameter, i.e. the distance between the outermost walls of the toroid, is chosen to optimize the electron temperature. Its height, i.e. the distance from top to bottom of the toroid, spreads out the current density to provide more uniform loading and allow for the maintenance of a lower current density throughout the envelope. This lower current density is favorable to high efficiency. 
     A cross-section of the toroid taken through a plane containing the axis of revolution, as shown in FIG. 2, shows that the discharge region 20 of the envelope is elliptical in shape in order to prevent discharge constriction. In one preferred embodiment of the invention, the maximum width of the ellipse, i.e. the distance between the interior and exterior walls of the envelope for one ellipse, is approximately 11/2 inches. An overall outside diameter of the envelope or about 31/2 inches will provide adequate space for electronics to be positioned near the bottom of the interior opening of the toroid and for convection currents. A height of about 4 inches is desirable. These dimensions, are merely illustrative suggestions and, of course, a toroidal envelope of any reasonable size would be suitable. 
     The envelope is filled with any suitable ionizable medium. For example, the envelope may be charged with mercury vapor and an inert gas, such as argon. A layer of a fluorescent light emitting phosphor such as any of the standard halophosphates or fluorophosphates, is also preferably on the surface of the discharge region 20. The mercury vapor and inert gas, when ionized, will produce ultraviolet radiation. The fluorescent light emitting phosphor layer effectively converts the ultraviolet radiation to visible radiation, although for some applications this may be not desirable and the layer may be omitted. The type of radiation emitted, e.g. ultraviolet, visible, etc., is dependent on the particular ionizable medium used, and one skilled in the art will be capable of making an appropriate choice. 
     In a preferred embodiment windings are coated on the surface of the toroid using a transparent conductive coating. Tin oxide may be used for this purpose. These windings consist of exterior windings 14, a top winding 10 and interior windings 22. 
     The windings 14 and 22 on the exterior and interior of the envelope, respectively, are helically shaped and are coated in opposed directions. The windings serve two functions. They couple the electric field to the medium and initiate ionization. Simultaneously, they couple a radio frequency magnetic induction field to the medium for maintaining the ionization. The peak magnitude and frequency of the magnetic induction field is selected to optimize the efficiency of conversion of radio frequency energy to emitted radiant energy. 
     The windings 14 on the exterior of the lamp force the radio frequency field almost wholly within the toroidal envelope. The top winding 10 eliminates any stray end fields. It is these windings which cause the radio frequency interference to be substantially eliminated and efficient coupling of radio frequency power to the ionizable medium to be effected. 
     The ratio of the number, N 2 , of the turns of the exterior windings 14 to the number, N 1 , of turns of the interior windings 22 can be determined by setting the flux of the magnetic field flowing up in the interior opening 28 of the toroidal envelope to equal the flux flowing down within the discharge region 20. If the windings have the same axial length this requires: 
     
         φ.sub.1 =φ.sub.2 
    
     
         (B.sub.1 -B.sub.2)A.sub.1 =B.sub.2 (A.sub.2 -A.sub.1) 
    
     
         B.sub.1 -N.sub.1 i; B.sub.2 -N.sub.2 i 
    
     
         (N.sub.1 -N.sub.2)A.sub.1 i=N.sub.2 (A.sub.2 -A.sub.1)i 
    
     
         (N.sub.1 -N.sub.2)A.sub.1 =N.sub.2 (A.sub.2 -A.sub.1) 
    
     
         N.sub.1 A.sub.1 -N.sub.2 A.sub.1 =N.sub.2 A.sub.2 -N.sub.2 A.sub.1 
    
     
         N.sub.1 A.sub.1 =N.sub.2 A.sub.2 
    
     
         A.sub.1 /A.sub.2 =N.sub.2 /N.sub.1 
    
     where: 
     A 2  is the total cross-sectional area normal to the axis of symmetry enclosed between the exterior windings; A 1  is the total cross-sectional area normal to the axis of symmetry enclosed between the interior windings; B 2  induction field created by the exterior winding. B 1  is the magnetic induction field created by the interior windings. 
     Additionally, ##EQU1## since 
     
         N.sub.1 A.sub.1 =N.sub.2 A.sub.2, 
    
     
         M.sub.1 =N.sub.2 A.sub.2 i-N.sub.2 A.sub.1 i 
    
     
         M.sub.1 =M.sub.2 
    
     The M&#39;s are the dipole moments of the windings. M 2  relates to exterior windings; M 1  to the interior ones. With M 2  =M 1 , the net magnetic dipole moment, M 2  -M 1 , is zero. In that case, there is no radiation field associated with the complete set of windings. 
     The envelope is securely positioned in base 18, which is preferably an adapter designed to fit into a socket for a conventional incandescent bulb. 
     The electronics 30 for activating the solenoidal windings are connected to the base 18. They are preferably positioned within a region bounded by the interior opening of the toroid and the base. The electronics are of the solid state variety, i.e. transistors(s) and/or IC&#39;s. If AC is supplied to the lamp socket, the electronics include a suitable rectifier. The electronics can also be located separately from the lamp in which case the RF energy is supplied to the socket and the electronics are made to contact the socket. 
     An air gap 16, in the envelope near the base, allows for convection cooling of the interior surface, the exterior surface and the electronics of the lamp. The air gap is simply an aperture through the envelope which permits air to flow from the external environment of the lamp into the interior of the toroid, permitting the air which is heated by the interior electronics to rise through the interior of the toroid into the external environment of the lamp. 
     In use, the base of the lamp of the present invention is screwed into a standard socket of they type used for a conventional incandescent bulb. When the switch necessary to close the circuit is turned &#34;on&#34;, the windings act to couple RF energy to the ionizable medium within the toroidal envelope. The ionizable medium is ionized and emits radiation. Simultaneously, a radio frequency magnetic induction field is emitted by the same windings and is coupled to the medium for maintaining the ionization. If non-visible radiation, e.g. ultraviolet radiation, is emitted by the ionizable medium it is necessary to coat a suitable phosphor of other material on the surface of discharge region 20 inside the toroid so that visible radiation is produced. If visible radiation is produced directly by the ionizable medium an additional coating will not be necessary. The type of radiation produced is dependent on the particular ionizable medium which is used, as is described above, and one skilled in the art will be well qualified to make an appropriate selection. For some applications ultraviolet radiation, without conversion to visible radiation may be desirable. 
     The interior electronics and interior surface of the toroid are cooled by convection currents. Air, which is heated by the electronics in the bottom interior of the toroid, flows upward in the interior of the toroid in accordance with the well-known law of nature that hot air rises. As the warm air rises, cooler air from the external environment of the toroid flows through the air gap near the base of the lamp. This continual replacement of warm air by cooler air acts in two ways. First, the ambient temperature of the air in the interior of the lamp is lower than would otherwise be the case. Additionally, the convection currents both in the interior of the toroid and in the external environment of the lamp act in the manner of gentle breeze to cool the windings and surface of the lamp. 
     In a further embodiment shown in FIG. 3 a shielding plane 24 is coated on the exterior surface of the envelope in place of the exterior windings 14. The shielding plane preferably comprises a transparent coating of tin oxide. Any form or shape of plane which would confine the radio frequency field within the discharge region is acceptable, one form being shown in the drawings. The interior may be provided with either a coated set of interior windings 22 as shown in FIG. 4, or a conventional solenoid 26 as shown in FIG. 5. In either case, the currents produced in the shielding plane by the radio frequency field act similarly to the currents in the exterior winding to confine the magnetic and electric fields. 
     Although the invention has been described in terms of specific embodiments for illustrative purposes, it will be appreciated by one skilled in the art that numerous additions, substitutions, and modifications are possible without departing from the scope and spirit of the invention as defined in the accompanying claims.