Fluorescent Lamp Heat Shield

Abstract

A fluorescent lamp protective assembly employing a plastic shield over the fluorescent tube glass envelope is adapted for use with high output fluorescent lamps that exhibit high intensity localized heating in the vicinity of the lamp cathode. A wire mesh screen formed into a cylindrical roll is inserted with the plastic shield at each end of the lamp. The wire mesh protects the plastic shield from the heat output of the lamp provided the mesh is of the woven wire cloth type in which the crossing wire elements have been deformed during the weaving process so as to be in intimate contact with each other at each intersection and not of the seamless "stocking" type. With the wire cloth mesh, polycarbonate plastic tube shields may be employed to provide implosion protection for high output and very high output fluorescent lamps without subjecting the plastic tube shield to discoloration, blistering, cracking or other heat damage from the filament.

Description

BACKGROUND OF THE INVENTION

This invention relates to fluorescent lamp protection apparatus and more particularly to an improved reuseable protective assembly for high output fluorescent lamps.

Jacketing of fluorescent lamps has heretofore been used for two principal purposes, to improve the lamp operation under low temperature ambient conditions and to contain the explosion products--particularly the glass and fluorescent lamp powders which otherwise would be scattered about when the glass lamp envelope breaks. In the recently granted P. R. DuPont U. S. Pat. No. 3,673,401 issued June 27, 1972, an improved fluorescent lamp jacket assembly is shown in which the glass envelope fluorescent lamp is jacketed within a dilateable plastic cylinder fitted with end caps whose webbed ends are provided with openings for the electrical contacts but otherwise provide a fairly lightly sealed assembly. The resilient walls of the dilateable plastic cylinder undergo a flexure during an implosion of the lamp envelope so that the assembly absorbs the accompanying pressure change and prevents separation of the end caps from the plastic envelope thereby containing the shards of glass and fluorescent powders and preventing them from being scattered about.

While the aforementioned P. R. DuPont arrangement satisfactorily accomplishes the aforementioned two-fold objectives, a problem has been noted when the plastic jacket assembly is employed with very high output fluorescent lamps. A high output and very high output fluorescent lamps, as the terms are employed in the fluorescent lamp art, denote lamps which draw upwards of 800 and 1500 milliamperes, respectively, in normal operation. These types of fluorescent lamps produce an unusual amount of heating in the region of the glass envelope adjacent to the lamp cathodes. Fluorescent lamps cathodes used by different lamp manufacturers take varying shapes and may be positioned at varying distances from the ends of the lamp depending upon the length of the stem presses employed. The cathode itself may in some lamps be fitted with metallic plate anodes. Nevertheless, it has been found that the excessive heating of the lamp envelope is confined to a zone within between 1 inch and 3 inches of either side of the lamp cathode filament. This concentrated heat production effect causes discoloration, blistering and, in severe cases cracking and charring of the plastic protective cylinder jacket.

While under unusual circumstances, it might be possible to provide a larger diameter plastic jacket or to provide a jacket of more heat-resistant plastic, these alternatives are not practical alternatives. Commercially available fluorescent lamp luminares and the electrical sockets therein are designed to provide only just enough room to allow for the insertion and removal of the lamp alone. When the lamp is used with a standard size protective jacket most if not all of the available space in the luminare is take up. Consequently, increasing the jacket diameter is not permissible. While heat-resistant plastic materials may some day become economical, none are known presently that can withstand the temperatures of (350.degree.) F. that have been observed in the vicinity of lamp cathodes in high output and very high output fluorescent lamps. In addition, it appears that in some cases as the temperature resistance of the plastic is improved, the degree of jacket flexure tends to decrease. As described in the aforementioned P. R. DuPont patent, jacket flexure is necessary to prevent the protective assembly from coming apart should the lamp implode.

Accordingly, it is an object of this invention to provide an improved protective assembly which may be employed with high output and very high output fluorescent lamps, i.e., those lamps which tend to have unusually large heat generation in the vicinity of the lamp cathode.

SUMMARY OF THE INVENTION

In accordance with my invention, I provide at each end of the fluorescent lamp, a woven wire cloth mesh insert, each such insert having an axial length disposed to extend throughout the high heat producing region adjacent a lamp cathode and having a diameter proportioned to fit outside of the fluorescent lamp glass envelope and small enough to fit inside the protective plastic cylinder jacket. It is an aspect of my invention that the type of wire cloth found suitable as a heat shield must be one which is woven in such a manner that the wires of the mesh cross over each other in a substantially orthogonal manner and which have been deformed during the weaving process so as to permanently be in intimate contact with each other. Advantageously the contact may be evidenced by a small degree of local flattening of the normally round wires at each point of cross-over. I have found that this degree of local flattening at each point of intersection appears to evidence a sufficient area of contact between the crossing wires of the mesh to provide good heat transfer and distribution among the wires of the mesh. When a wire mesh or "cloth" of this type is inserted between the glass lamp envelope and the protective jacket, the heat produced by the lamp filament is prevented from injuring the plastic protective jacket.

Accordingly, it is a feature of my invention to employ a woven wire heat shield between the fluorescent lamp glass envelope and the plastic protective jacket, the wire mesh being of the type having efficient contact areas at each point of intersection of its constituent wire elements.

DESCRIPTION OF THE DRAWING

The foregoing and other features of my invention may become more apparent by referring now to the drawing in which:

FIG. 1 shows an end view of a prior art fluorescent lamp protective assembly exhibiting the heat damage caused by a high output or very high otuput fluorescent lamp filament;

FIG. 2 shows a wire mesh of the type found unsuitable for use as a heat shield;

FIG. 3 shows a wire mesh of appropriate type;

FIG. 4 shows an end view of a fluorescent lamp protective assembly employing the wire mesh of FIG. 3; and

FIG. 5 shows an enlarged view of the point of intersection of the wires comprising the mesh of FIGS. 3 and 4.

Referring now to FIG. 1, a high output fluorescent lamp is shown encased within a protective jacket 1 and end cap 7. The lamp filament 2 is of the type equipped with plate anodes 3 and is spaced from the lamp base 7' by a stem press 6 having a length of a few inches. The region of maximum lamp cathode heat output 5-5' is evidenced by blistering and discoloration of the plastic jacket 1 in the area adjacent the lamp filament 2 and plate anodes 3. Among fluorescent lamps, made by different lamp manufacturers or having output capacity the length of the stem press 6 may vary and consequently the area 5-5' of maximum heat concentration may be located closer to or farther from lamp base 7.

In the initial attempt to provide a heat shield, I employed a knit type of wire mesh sleeve 21 FIG. 2 wherein a single continuous wire is braided in the fashion of a seamless knit stocking. It was though that this type of wire mesh sleeve would be desirable because it would be seamless and hence more esthetically attractice. Although seamless wire mesh cylinders of various mesh were tried and though different types of wire, both copper and aluminum were tried in various gauge wires from 0.008 to 0.020 diameter, it was found that the seamless tubular stocking type of wire mesh never completely eliminated heat damage to the plastic lamp jacket 20. Accordingly, the braided or continuous element seamless stocking type of mesh shown in FIG. 2 has been found to be unsuitable for use as a heat shield.

I have found that a satisfactory heat shield may be obtained by employing the common variety of copper or aluminum window screening notwithstanding that such screening may employ the very same gauge wire as the unsatisfactory seamless stocking sleeve of FIG. 2 and notwithstanding that the same mesh or number of "window" opening per inch be employed. The wire mesh 31 FIG. 3, FIG. 5 which I have found to be satisfactory may employ anywhere from 0.008 to 0.012 inch diameter copper or aluminum wire and is commonly known as 18 by 16 mesh aluminum or copper wire cloth.

To fabricate the heat shield of my invention, a wire cloth is cut to a suitable axial length L (see FIG. 4) dimensioned so as to extend from just inside the end cap annular recesses to approximately 1 to 2 inches beyond the lamp filament 2. The wire mesh 31 is advantageously coiled into a cylinder so as to have a diameter which fits loosely over the lamp glass envelope 32. The inner surface 43 of the plastic end cap 44 is dimensioned to be large enough to retain the end of the wire mesh cylinder in addition to receiving the end 45 of the plastic jacket 41. Advantageously, the end cap recess 46 may be dimensioned to wedge the end of screen 31 against the glass envelope and so prevent the wire screen from sliding about.

I have found that wire meshs having the asdescribed dimensions do not noticeably reduce the light output available from the fluorescent lamp but have been uniformly effective in preventing discoloration, blistering and cracking of the plastic protective jackets and have been effective in reducing the temperature of the protective jacket in the area immediately adjacent the lamp cathode to a level below that which will damage plastics. Accordingly, the polycarbonate tubing of which the jacket 41 may advantageously be made in accordance with the teaching of the aforementioned P. R. DuPont patent can safely be used with high output 800 milliamperes and very high output 1500 milliamperes fluorescent lamps with indefinite useful life whereas without such a wire mesh heat shield, or with the seamless "stocking" heat shield of FIG. 2, discoloration and blistering of the polycarbonate tubing occurs within 15 to 20 minutes of lamp usage. With the wire cloth of my arrangement, high output and very high output fluorescent lamps have been in continuous operation for months with no apparent deterioration of the plastic protective jacket.

Claims

What is claimed is:

1. In a protective assembly for a plastic jacketed fluorescent lamp having a lamp cathode that produces a locallized heating of the fluorescent lamp glass envelope of sufficient magnitude normally to damage the plastic protective jacket, the combination comprising

a cylindrical wire mesh grating interposed between said glass lamp envelope and said plastic protective jacket, said wire mesh grating having an axial length extending throughout the region of locallized lamp envelope heating and being formed of wire cloth wherein the woven wire elements cross each other substantially orthogonally and at each point of crossing are in sufficiently intimate contact to produce wire deformation.

2. In a protective assembly according to claim 1, the combination wherein said wire deformation is evidenced by localized flattening of the wire elements at each point of cross over.

3. In a protective assembly according to claim 1, the combination wherein said wire mesh grating employs copper or aluminum wire of from 0.008 to 0.012 diameter.

4. In a protective assembly according to claim 1, the combination wherein said wire mesh is of aluminum or copper wire cloth having an 18 by 16 mesh.