A PHOSPHOR COATING ARRANGEMENT FOR AN ELECTRODELESS DISCHARGE LAMP

Abstract: An electrodeless discharge lamp (10) having a lamp envelope (12) containing a fill of mercury and a rare gas is excitable to a discharge state by introduction of RF energy in close proximity thereto. A ballast circuit contained within the housing base portion (22) of the lamp is effective for generating the RF energy from conventional line power. The ballast circuit includes a core portion which extends within a re-entrant cavity (16) formed within the lamp envelope. During operation of the lamp, the re-entrant cavity portion is exposed to higher wall loading conditions than is the outer surface of the lamp envelope. These higher wall loading conditions require the use of comparatively more expensive rare earth phosphors (30) at the re-entrant cavity portion than are needed on the outer portion of the lamp envelope where it would be appropriate to utilize conventional halophosphate materials (32). In order to provide for two distinct phosphor coating materials in a single lamp envelope configuration and at a reasonable cost, such phosphor coating is done by coating the re-entrant cavity and the outer portion separately and then joining these components together to produce the finished lamp envelope. The juncture point between the re-entrant portion and the outer envelope portion resides under an upper rim portion of the housing and is therefore not visible in the end lamp product.

Description: A PHOSPHOR COATING ARRANGEMENT FOR AN ELECTRODELESS DISCHARGE LAMP FIELD OF THE INVENTION This invention relates to an arrangement for providing phosphor coating to a lamp envelope portion of an electrodeless discharge lamp and more particularly, to such a phosphor coating arrangement as provides a more practical solution to the differences in wall loading characteristics that are present in an electrodeless discharge lamp. BACKGROUND OF THE INVENTION The electrodeless discharge lamp is projected as being a large contributor in efforts to reduce electricity demand thereby allowing electric utilities to forego the construction of costly power generating facilities. The contribution of the electrodeless discharge lamp is expected to result from the increased energy efficiency of such device as well as the expected long life resulting from the elimination of life inhibiting electrode elements. An example of an electrodeless discharge lamp can be found in US Patent No. 4,010,400 in which it is disclosed that an ionizable medium can be disposed in a lamp envelope and excited to a discharge state by introduction of an RF signal in close proximity thereto such that by use of a suitable phosphor, visible light can be produced by such discharge. In order to generate this RF signal, the electrodeless discharge lamp includes a ballast circuit arrangement disposed in the lamp base, such ballast circuit arrangement including a resonant tank circuit which utilizes a coil member extending into the lamp envelope to inductively couple the RF signal to the ionizable medium. As with any conventional fluorescent lamp, the electrodeless discharge lamp requires a phosphor layer to convert the discharge from the ionized medium into visible light. It is the typical practice in fluorescent lamp manufacture to use halophosphates and to obtain the required final color by blending phosphates together or by the addition of relatively small quantities of phosphate. Halophosphates are relatively inexpensive and are used extensively because of their good efficacy, low cost and wide range of acceptable colors. In a compact fluorescent application however, wall loading characteristics are typically of such a high value as to make the use of halophosphates inappropriate because of their tendency to deteriorate quickly under such high wall loading conditions. In such cases, it is necessary to use comparatively more expensive rare earth phosphors. In an electrodeless discharge lamp, it has been found that the wall loading characteristics vary along different portions of the lamp envelope but that near the region of -the- cavity sur.rounding the coil member from which the RF signal is inductively coupled, the wall loading characteristics are sufficiently high as to preclude the use of halophosphates for such area. One way to alleviate the risk of using halophosphates and suffering degradation of the phosphor material would be to coat the entire surface of the lamp envelope inner wall with the rare earth phosphate. Such a measure would allow for a long life light source however, the cost of such a lamp will have been increased significantly by the use of the more expensive rare earth phosphor. One other way to alleviate the problem is mask off certain portions of the inner wall of the lamp envelope and use a separate coating step for each of the different phosphors used. Such an arrangement would also prove costly in that the manufacturing operation needed to implement the masking and multiple coating approach would be prohibitive. Accordingly, it would be advantageous if a phosphor coating arrangement could be provided for an electrodeless discharge lamp which was both cost effective as a result of using different phosphors based on the wall loading requirements and was easily implemented in a manufacturing process that did not include the addition of multiple steps such as masking and plural coating operations. SUMMARY OF THE INVENTION Based on the principles of the present invention, there is provided in an electrodeless discharge lamp having a lamp envelope containing a fill of mercury and a rare gas excitable to a discharge state upon the introduction of an RF signal generated by a ballast circuit disposed in a lamp base portion, and wherein the lamp envelope is disposed in at least a partially surrounding relation to a coil member from which the RF signal is output, a phosphor coating arrangement on the lamp envelope in a cost effective manner and one which is easily implemented in a mass production manufacturing process. An outer portion of the lamp envelope is disposed in a spaced apart relation to the coil member whereas an inner cavity portion extending into the lamp envelope along the central axis thereof, is sized to accommodate insertion of the coil member therein and is in close proximity to the coil member thereby subjecting the inner cavity portion to higher wall loading values than that of the outer portion of the lamp envelope. A connection area of the lamp envelope is formed where the inner cavity portion is joined to the outer portion of the lamp envelope, this connection area being disposed at the lowermost portion of the lamp envelope which is inserted into the lamp base for securing the lamp envelope to the lamp base. A rare earth phosphate coating is applied to the inner cavity portion whereas one of either a halophosphate or fluorescent phosphate material is applied to the inner surface of the outer portion of the lamp envelope. The manufacturing operation of joining the inner cavity portion to the outer portion of the lamp envelope is performed after the phosphate coatings are applied thereby obviating the need for masking procedures during the coating applications procedure. BRIEF DESCRIPTION OF THE DRAWINGS This invention will be described more fully with reference to the drawings in which: Figure 1 is an elevational view in section of an electrodeless lamp constructed in accordance with the present invention. Figure 2 is an elevational view in section of the lamp envelope portion of the lamp of Fig. 1 showing the phosphor coating arrangement of the present invention. DETAILED DESCRIPTION OF THE INVENTION As seen in Fig. 1 , a low pressure electrodeless fluorescent lamp 10 includes a lamp envelope 12 having an outer envelope portion 14 and an inner portion identified as re-entrant cavity 16. The re-entrant cavity 16 is essentially cylindrical in shape and extends within the outer envelope portion 14 along the central axis thereof. Extending through the central axis of the re-entrant cavity 16 is an exhaust tube 18 which is shown extending beyond the point at which the re-entrant cavity 16 and the outer envelope portion 14 join to complete the lamp envelope 12, such juncture being identified as reference 20. The outer envelope portion 14 is illustrated as essentially the same shape as a conventional incandescent reflector lamp product. However, other configurations of the outer lamp envelope 14 can be utilized equally as well; for instance, the outer envelope 14 can be configured in the shape of a conventional A-line lamp product or a decorative globe lamp product. Regardless of the shape of the outer envelope portion 14, the lower regions 12b and 14b to each of the outer envelope and re-entrant cavity portions of the lamp envelope 12, defined as juncture 20 will reside inside of the upper rim region of the base housing 22. As will be discussed in hereinafter in further detail, by location of the juncture 20 at this point, the precision of the coating process for the fluorescent material can be somewhat relaxed. Moreover, it can be appreciated that by use of a lamp manufacturing process whereby the outer envelope portion 14 is formed separately from the forming process for the re-entrant portion 16, the coating application can be accomplished for each portion separately before such portions are joined in the final lamp manufacturing step. In this manner, the present invention allows for the use of a separate phosphor material application to the outer envelope portion 14 than is used for the re-entrant portion thereby avoiding the need for masking off one portion while the other is being coated with a different phosphor material. As further seen in Fig. 1 , the electrodeless discharge lamp 10 of the present invention generates a torroidally shaped discharge 23 within lamp envelope 12. Such discharge 23 is generated upon the introduction of radio frequency (RF) energy to the fill contained within the lamp envelope, such fill being of the conventional type used in standard fluorescent lamps. The RF energy is produced by a resonant circuit portion of a ballast circuit 24 disposed within the housing base 22. The resonant circuit portion includes an excitation coil 26 having a core portion 26a and a winding 26b, and a capacitor 28. The ballast circuit drives the resonant circuit portion with a conditioned signal developed from line power. By excitation and maintenance of the discharge 23 within the lamp envelope 12, it has been observed that the re-entrant portion 16 of lamp envelope 12 experiences higher wall loading values (measured as Watts/cm 2 ) than does the outer envelope portion 14. In order to provide a phosphor coating on the re-entrant cavity 16 that will not deteriorate over the life of the lamp 10 particularly given the expected long life of such an electrodeless discharge lamp 10, it is necessary to utilize a tri- phosphor coating material on this region. As seen in Fig. 2, a tri- phosphor coating material 30 is applied to the lamp envelope in the region covering the re-entrant cavity 16 facing the interior space of lamp envelope 12. The tri-phosphor material 30 can be that material which is typically utilized on conventional compact fluorescent lamps, such material being readily available in the marketplace. Unlike the wall loading conditions experienced by the re-entrant cavity 16, the outer envelope portion 14 of lamp envelope 12 are significantly lower and can accommodate the use of the lesser expensive halophosphate material, such material being the same phosphor material as can be utilized in larger conventional fluorescent lamps such as 2 and 4 foot versions of such lamps. As seen in Fig. 2, the different phosphor material 32 is illustrated as having a different grain size as that material 30 used on the re-entrant cavity. As further seen in Fig. 2, the illustrated lamp 10 is a reflector lamp and as such, includes a reflective coating 34 disposed on the re-entrant cavity 16 and a portion of the outer envelope extending to approximately the equatorial region of the lamp envelope, the dividing line being designated as reference line A-A. The coating process utilized for coating each of the re-entrant and outer envelope portions 14, 16 of lamp envelope 12 does not require a precision operation inasmuch as the portion of the lamp envelope 12 at which the re-entrant cavity 16 and outer envelope 14 join, juncture 20, is not visible in the end product but is covered by the upper rim of housing base 22. Furthermore, in the manufacturing operation utilized for produced finished, coated lamp envelopes 12 which are assembled with the housing and ballast portions 22, 24, it can be appreciated that such manufacturing operation is facilitated by the fact that each of the lamp envelope portions 14, 16 can be separately coated without the need for a masking process to accommodate the separate coating materials. After such separate coating process, the lamp envelope portions 14, 16 are joined to form the finished lamp envelope thereby achieving a more cost effective component in that the expensive tri-phosphor material is only used on the area where the wall-loading characteristics require the use of such expensive material. Although the above described embodiment constitutes the preferred embodiment, it should be understood that modifications can be made thereto without departing from the scope of the invention as set forth in the appended claims.

Claims: C L A I M S 1. In an electrodeless discharge lamp having a lamp envelope containing a fill of mercury and a rare gas excitable to a discharge state upon the introduction of an RF signal generated by a ballast circuit disposed in a base portion of the discharge lamp, and wherein the lamp envelope is disposed in at least a partially surrounding relation to a coil member from which the RF signal is output, a phosphor coating arrangement on said lamp envelope comprising: an outer portion of said lamp envelope disposed in a spaced apart relation to said coil member; an inner cavity portion extending into said lamp envelope along the central axis thereof and being sized to accommodate insertion of said coil member therein, wherein said inner cavity portion is subjected to higher wall loading values than those to which said outer portion of said lamp envelope is subjected; a connection area of said lamp envelope connecting said inner cavity portion with said outer portion of said lamp envelope; wherein a rare-earth phosphor coating is applied to said inner cavity portion of said lamp envelope; and wherein at least one of a halophosphate and a fluorescent phosphate material is applied to the inner surface of said outer portion of said lamp envelope. 2. A phosphor coating arrangement for a lamp envelope as set forth in claim 1 wherein during manufacture of said electrodeless discharge lamp, said rare-earth phosphor coating is applied to said inner cavity portion and said at least one of a halophosphate and a fluorescent phosphor material is applied to said outer portion of said lamp envelope prior to said inner cavity portion being joined to said outer portion of said lamp envelope at said connecting area thereby avoiding a masking operation of any area of said lamp envelope during application of said phosphor materials on any other surface area. 3. A phosphor coating arrangement for a lamp envelope as set forth in claim 2 wherein said connection area between said inner cavity portion is joined to said outer portion of said lamp envelope resulting in a juncture point therebetween, and wherein said juncture point is disposed within said lamp base so as to be covered by an upper rim portion of said lamp base.

Inventors: ROWLEY ANDREW TERENCE, ROWLEY, ANDREW, TERENCE
Applicants: GE LIGHTING LTD, ROWLEY ANDREW TERENCE, GE LIGHTING LIMITED, ROWLEY, ANDREW, TERENCE
Application year: 1995
Main classification: H01J61
IPC codes: H01J61, H01J65