Electrodeless fluorescent lamp

Abstract: An electrodeless fluorescent lamp comprises a sealed lamp vessel (G) containing a luminescent layer (P) and a fill capable of sustaining a discharge when suitably excited by an electric field. A coating (FTO) of electrically conductive light transparent material is provided on the external surface of the vessel to confine the electric field within the vessel (G). The conductive coating (FTO) is covered with a transparent insulative layer (2) and is electrically connected to a mains decoupling capacitor for electrical safety and to reduce conducted interference. Preferably the layer 2 is a silicone sheath.

Description: The present invention relates to an electrodeless fluorescent lamp. Such a lamp is disclosed in US-A-4727294 (U.S. Philips Corporation). The lamp of US-A-4727294 comprises an externally spherical lamp vessel which is sealed and which contains a fill capable of sustaining a discharge when suitably excited. The discharge excites a phosphor coating on the inside of the vessel. The fill is excited by a winding which is energised by a high frequency, e.g. RF, oscillator. The winding surrounds a core of magnetic material in US-A-4727294. The core and winding project into a cylindrical sealing member of the vessel which projects, in re-entrant fashion, into the spherical vessel. The lamp vessel is further provided with a light transparent, electrically conductive layer within the vessel to substantially confine the electric field generated by the core and winding within the vessel. In order to reduce conducted interference in US-A-4727294, a portion of the external surface of the vessel is also provided with a conductive coating capacitively coupled to the conductive layer inside the vessel. The external coating is connected by a conductor to a lamp cap, i.e. a power mains terminal, of the lamp. In US-A-4727294 an electrically insulative, generally cylindrical, housing supports the spherical lamp vessel and the re-entrant sealing member. The housing has a diameter smaller than the spherical lamp vessel. The housing contains the oscillator circuit and mechanically connects the lamp vessel to the lamp cap. The portion of the external surface of the vessel which is provided with the conductive coating is inside the housing for electrical safety limiting the area available for the capacitive coupling and thus limiting the impedance of the coupling to an undesirably high value. Providing the conductive coating on the inner surface of the lamp vessel produces two problems. Firstly, the actual coating process is difficult and secondly, it is difficult to arrange a satisfactory electrical coupling between the RF ground and the inner conductive layer. EP-A-0512622 discloses an electrodeless low-pressure mercury vapour discharge lamp whose discharge vessel is provided with a core of magnetic material and a coil surrounding the core which coil connected to a highfrequency supply unit. An interference-suppressing, transparent, electrically conductive layer is present on the outside of the discharge vessel, which layer can be connected to the supply mains through an electrical coupling. The electrical coupling comprises one or several capacitors connected in series to keep the conductive layer safe to touch during operation. Providing the conductive coating on the outer surface of the vessel reduces the difficulty of the coating process and avoids the problem of electrical coupling to an inner conductive layer. However, with the arrangement of EP-A-0512 622 an excessive contact current may flow from the lamp to a user who touches it. In addition, the coating may be easily damaged. According to the invention, there is provided an electrodeless fluorescent lamp as disclosed in claim 1. In an embodiment of the invention, means are provided for electrically coupling the outer coating to an electrical ground point within the lamp to reduce conducted interference. In an embodiment of the invention, in which there is provided a mains powered means for producing the electric field, there is provided a mains decoupling capacitor electrically connected to the said external electrically conductive coating. For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawings in which: FIGURE 1 is a schematic sectional side view of an illustrative electrodeless fluorescent lamp in accordance with the invention; FIGURE 2 is a schematic circuit diagram of the lamp of Figure 1; FIGURE 3 is a schematic side view of another electrodeless fluorescent lamp in accordance with the invention; and FIGURE 4 is a schematic side view of yet another lamp in accordance with the invention. Referring to FIGURE 1, the illustrative electrodeless lamp comprises a sealed glass vessel G shown as generally spherical but which may be of any suitable shape. A re-entrant cylinder 3 also of glass is fused to the vessel G. The vessel contains a fill (not shown) e.g. of mercury and a rare gas, which, when excited, produces a discharge of ultraviolet (UV) light. On the internal surface of the vessel is a layer of phosphor P which converts the UV light into visible light as in a conventional fluorescent lamp. The phosphor P covers not only the internal surface of the vessel G but also the surface of the cylinder 3. A further coating (not shown) may be provided between the phosphor layer and the glass to reduce blackening of the vessel with age, as is known in the art. The fill is excited by an electromagnetic field produced by a winding which comprises turns of copper wire. The turns are preferably arranged around a magnetic core of e.g. ferrite. The winding and core 4 are arranged in the re-entrant cylinder 3. The winding is excited at high frequency e.g. 2.65 MHz by RF excitation means comprising for example an oscillator 5 powered from the power mains via a rectifier 6 and smoothing capacitor 6a (shown in Figure 2). The RF excitation means is housed in an electrically insulative housing H to which a lamp cap C is fixed. In order to substantially confine the high frequency field to the lamp vessel, a light transparent, electrically conductive coating FTO is provided over the entire external surface of the lamp vessel but not over the surface of the cylinder 3. The coating has sufficient resistance e.g. at least 10 ohms per square so that it does not present a short-circuit to the winding 4. 300 ohms per square may be used. The coating FTO is preferably of fluorine-doped tin oxide but may be of other materials known to be suitable in the art. In order to reduce or eliminate conducted interference the coating FTO is coupled to RF ground, via a decoupling capacitor 7 having capacitance Cp which provides high impedance to mains frequency but low impedance to the RF. The value of Cp is such that the reactance at the RF frequency is much less than the resistance of the coating (so that it provides insignificant impedance to the flow of current when compared with the coating itself). It must also be high impedance at 50 Hz such that mains contact currents are limited to less than 500 µA [National Radiological Protection Board (NRPB) - Board Statement on Restrictions on Human Exposure to standard time varying electromagnetic fields and radiation) Documents of NRPB, Vol. 4 No. 5 1993]. In addition capacitor 7 must be Class Y (supply voltage less than 250V) or Class U (supply voltage less than 125V). Such capacitors are defined in IEC 384-14 (1981) as being "of a type suitable for use in situations where failure of the capacitor could lead to danger of electric shock". There are many ways of making connection between the capacitor and the external coating FTO. Examples are : A metal strip attached to coating FTO with conducting cement to which capacitor is welded, soldered or crimped. Spring fingers that slip over the seal area lip (in Fig. 3). The spring finger is used to retain the lamp vessel in the housing. A conducting coating on the housing. Contact is made by snap fitting the lamp vessel into housing. The capacitor 7 is bonded or crimped to a lug on the housing. Providing the coating FTO on the external surface of the vessel G makes the connection of the decoupling capacitor 7 to the coating simpler. Also, the decoupling capacitor 7 can then be chosen for its electrical requirements without other constraints. Providing the coating FTO on the external surface of the vessel also reduces the difficulty of the coating process. The coating however is easily damaged. Furthermore, as shown in Figure 2, the coating FTO is connected to RF zero via the capacitor 7 which - because of the use of a rectifier bridge has mains voltage 50Hz embedded on it. To provide the user with additional isolation from mains and to protect the coating FTO, the external FTO coating is protected by a transparent insulative layer 2. The layer may be a coating chosen from: inorganic material; glass-frit; plastics; polytetrafluoroethylene (PTFE); silicone; and latex; an example being "Modified Silicone Conformal coating". The chosen material may be sprayed, painted, dipped or otherwise deposited on the lamp vessel. A preferred transparent insulative layer is a cover or sheath of liquid injection molded silicone providing greater than 4KV insulation throughout the life of the lamp in accordance with the IEC standard 968. The cover is preformed and slipped over the glass vessel. It may have a thickness of 0.5mm. The silicone material of the cover is sold by GE Plastics (a division of General Electric Company) under the Trade Mark LIM. Suitabe covers are disclosed in WO88/03327 of Colourcover Limited and are available from Colourcover Limited. FIGURE 3 is a schematic view of another embodiment of a lamp in accordance with the invention. The lamp of FIGURE 3 comprises a glass vessel G, a re-entrant cylinder 3, a winding and core 4, an oscillator 5, a rectifier 6, a capacitor 7, a housing H and a cap C generally as described with respect to FIGURE 1. The vessel G contains a fill, and has on its internal surface at least a layer of phosphor P as described with reference to FIGURE 1. The vessel G has on its external surface a light transparent coating FTO of electrically conductive material covered by a light transparent layer 2 of electrically insulative material as generally described with reference to Figure 1. Preferably the layer 2 is the cover of liquid injection molded silicone. The mains decoupling capacitor 7 is electrically connected between the coating FTO and an RF zero point on the rectifier board within the housing H. Within the housing is a substantially closed metal box having a generally cylindrical side wall portion S1 between upper and lower end walls E1 and E2, and an extension S2 of the side wall which extends towards the lamp cap. The closed box S1,E1,E2 contains the oscillator 5, provides electrical shielding for the oscillator, and also acts as a heat sink. The extension S2 supports the rectifier 6. Terminals T extending through end wall E1 connect the oscillator 6 to the winding and core 4, the circuit board 41 of which is supported by the end wall E1. The lamp vessel G is supported by and glued to, the circuit board 41 of the core and winding, although other support arrangements may be used. The core and winding 4 forms a hollow cylinder through which extends a tube 8 which re-entrantly extends through the cylinder 3. The tube 8 extends into the box S1,E1,E2. The tube 8 contains mercury amalgam 10 retained by a dimple 12 within the end portion of the tube inside the box. The lamp of Figure 3 described hereinbefore may be modified as shown in Figure 4 to act as a reflector lamp by the addition of a reflective layer R under part of the phosphor layer P. The reflective layer may be of titania (TiO 2 ) for example. Components of the lamp of Figure 4 identified by reference numerals used also in Figure 3 are equivalent to the components of the lamp of Figure 3. The electrically insulative housing of Figure 4 comprises two opaque parts H' and H". Part H' is similar to housing H of Figure 3 and contains the substantially closed metal box S1,S2,E1,E2 the oscillator 5 and rectifier 6, and supports the circuit board 41, and the winding and core 4. Part H" is connected to part H' by a snap-fit 16 but may be connected by any other suitable means. Part H" extends from part H' to the zone Z of maximum diameter of the mushroom-shaped glass vessel G. The reflective layer R also extends from adjacent the circuit board 41 to the zone Z of maximum diameter to reflect light to the face 40 of the glass vessel. The conductive transparent coating FTO extends over the whole external surface of the glass vessel G including the face 40 thereof. The electrically insulative housing part H" protects and electrically isolates part of the coating FTO. To protect and electrically isolate the part of the coating FTO on the face 40 of the vessel G, a light transparent electrically insulative layer 2' is provided over the face 40 and extends part the zone Z towards the housing part H' so that the part H" overlaps the layer 2'. The layer 2' is as described with reference to layer 2 of Figures 1 to 3. Preferably the layer 2' is the cover of liquid injection molded silicone. The lamps described hereinbefore may be modified in various ways. For example, the ballast, i.e. the core and winding 4 oscillator 5 and rectifier 6, may be made and sold separately from the lamp vessel in which case suitable means for connecting the lamp vessel to the ballast must be provided. Such means are within the skill of those skilled in the art. The decoupling capacitor may, in theory, be omitted in which case the coating FTO is connected directly to RF zero and the insulative layer 2 or 2' must be provided for electrical safety. However, in the circuit of Figure 2, RF zero is coupled to the mains supply via the rectifier 6 and RF zero thus has mains voltage embedded on it. In this situation safety requires that the insulative layer 2 or 2' must be designed to outlast the lamp, remaining insulative under all conditions of use. Preferably the liquid injection molded silicone cover is used in this situation. It is possible to use an isolating transformer between mains and the rectifier providing an RF zero point isolated from mains. The light transparent electrically insulative layer 2 or 2' may be replaced by a light translucent, or otherwise light transmissive, electrically insulative layer. In addition to the light transmissive electrically insulative coatings 2, 2' mentioned hereinbefore, suitable silicone coating materials are also disclosed in: US-A-4,382,057 US-A-4,379,902 US-A-4,328,137 and US-A-5,034,061 all assigned to General Electric Company. US-A-5,034,061 discloses coatings suitable for electric incandescent Lamps. If such coatings are applied to lamps according to the present invention, they must conform to the safety requirements mentioned hereinbefore. The FTO coating in the embodiments described above is thick enough to alone provide low resistance for RF to ground. The FTO could be made thinner and covered in a fine mesh of conductive material, e.g. metal wire, to provide the low resistance without obstructing the light output.

Claims: An electrodeless fluorescent lamp comprising a sealed lamp vessel (G) containing a luminescent layer (P) and a fill capable of sustaining a discharge when suitably excited by an electric field, and a coating (FTO), on the external surface of the vessel, of electrically conductive light transmissive material for confining the electric field within the vessel, characterized by electrically insulative means 2;2';H;H';H") comprising an electrically insulative housing (H;H") which houses part of the external surface of the lamp vessel, and a light transmissive electrically insulative layer (2;2') overlying the electrically conductive coating over at least the remainder of the external surface of the lamp vessel. A lamp according to claim 1, further comprising means (4) for producing the said electric field. A lamp according to claim 2, wherein the field producing means is mains powered and further comprising a mains decoupling capacitor (7) electrically connected to the said external electrically conductive coating (FTO). A lamp according to claim 2 wherein the means for producing the electric field; comprises means for producing an RF electric field; and including means for coupling the electrically conductive coating to a RF ground of the field producing means. A lamp according to claim 4, wherein a mains decoupling capacitor (7) provides the said means for coupling the electrically conductive coating to a RF ground. A lamp according to claim 4, wherein the said means for coupling the electrically conductive coating to a RF ground comprises a conductive connection of the coating to the said RF ground. A lamp according to claim 4 comprising a mains isolating transformer for energising the RF field producing means. A lamp according to any preceding claim, wherein the light transmissive layer (2;2') is a sheath of silicone, or is glass-frit, or is a layer of polytetrafluoroethylene.

Inventors: GIRACH MOHAMED HANIF, WHARMBY DAVID OSBORNE, GIRACH, MOHAMED HANIF, WHARMBY, DAVID OSBORNE
Applicants: GE LIGHTING LTD, GE LIGHTING LIMITED
Application year: 1995
Main classification: H01J65
IPC codes: H01J61, H05B41, H01J65