CA1143777A - Fluorescent lamp having a tubular envelope of compact tridimensional configuration, and method of making such envelope - Google Patents

Abstract

48,105 ABSTRACT OF THE DISCLOSURE

An efficient fluorescent lamp of high brightness and compact size is provided by forming the envelope from vitreous tubing that is bent into three U-shaped sections which are disposed in mutually perpendicular planes and define a single convoluted discharge channel. The U-shaped sections are so oriented that the legs of the U's are disposed in quadrangular columnar array with the ends of the tubing located adjacent one another and opposite one of the U-bends. Conventional electrode-stem assem-blies are sealed into the ends of the bent tubing and the sealed legs are coupled to each other and the proximate U-bend by brace means which rigidifies the multi-section envelope. An efficient method of manufacturing the enve-lope employing a pair of arcuate molding components and two bending operations that form the three U-bends with only a single tube-heating operation is also disclosed.

Description

1 l~8,105 FI,UORESCENT LAMP HAVING A TUBULAR ENVELOPE
OF COMPACT TRIDI~F.NSIONAL CONFIGURATION~
AND rlETHOD OF M~KING SUCH RNVELOPE
CROSS-REFE-RE,NCE TO RELATED PATENT
This application discloses and claims a compact fluorescent lamp that is of triple-bent tub~lar config~
uration an~ is employed as the light source in the light-ing apparatus disclosed and claimed in U.S. Patent No.
4,300,07~,lssued November 10, 1981, of H. Skwirut et al.
entitled "Screw~In Type I.ighting Unit Having Convoluted Tridimensional Fluorescent Lamp,~ which patent is assigned to -the assignee of this app1ication.
BACKGROI~D OF THE INVENTION
Field of the Invention:
This invention generally relates to electric '``~
, ~, 3~ 7 ~ 48,105 discharge lamps and has particular reference to an im-proved fluorescent lamp of compact size and high bright-ness that is suitable for use as a light source in various kinds of residential and co~ercial lighting fixtur~s that are designed for incandescent type lamps.
~c~e~lon of the Prior ~rt:
Electric discharge lamps having tubular enve-loDes which provide "three-dimensional" type light sources are generally well known in the art. U.S. Patent

2,652,483 granted September 15, 1953 to Laidig et al.
discloses fluorescent lamp units of helical configuration (Figs. 7 and 15 embodiments) that are formed by coupling several semi-circular lamps together to ~form one operative assembly. Another fluorescent lamp of three-dimensional configuration that is formed by four sections of conven-tional fluorescent ]amp tubing which are "bundled" to-gether and interconnected by suitable apertures in the side walls of the respective tubes is disclosed in U.S.
Patent 3,501,662 issued March 17, 1970 to Plagge.
2 Electric discharge lamps having tubular enve-lopes that are bent into various shapes to provide concen-trated light sources are also known in the art. A double-ended type sodium-vapor discharge lamp having an envelope which is formed from a single tube that is folded or bent upon itself twice to provide three straight tubular seg-ments that are disposed in parallel triangular-spaced relationship is disclosed in British Patent 854,745 pub-lished November 23, 1960 (Figs. 3 and 4 embodiment). A
luminous discharge tuhe de3igned for advertising and

3 48,105 display purposes (or as a beacon light) and having an envelope formed from glass tubing which is bent upon itself eleven times to provide a corresponding number of U-shaped sections is disclosed in U.S. Patent 1,898,615 : granted February 21, 1933 to Byrnes. Gaseous discharge vi.Aes consisting of a heat-conserving housing that encloses a lamp having a tubular envelope that is bent u?or itself three times to provide four straight sections which are spaced from one another and arranged so that the lC electrode-containing ends of the envelope are located on opposite sides of one of the curved segments or bends is disclosed in U.S. Patents Nos. 2,001,511 and 2,200,940 granted to Uyterhoeven et al.
In accordance with the more recent proposal, a screw-in type flourescent lamp of compact size is provided by suitably partitioning the interior of a cylindrical envelope or by making the envelope from tubing that is bent upon itself to provide a U-shaped bulb, and which may be additionally twisted into spiral shape or redoubled on itself to provide a generally M-shaped envelope. A fluor-escent lamp constructed in this manner is disclosed in U.S. Patent 3,551,736 granted December 29, 1970 to Doehner (t'ne M shaped tubular envelope embodiment being shown in Fig. 5 and described along with the other aforementioned shapes at lines 24-30, column 2).
A method of making a circular-shaped ~luorescent lam~i~ by bending a straight tubular glass envelope around a suitable curved jig is disclosed in U.S. Patent 3,993,465 granted November 23, 1976 to Hurx et al.

3'7'77

4 48,105 SIJMMARY OF THE INVENTION
. .
While the prior art electric discharge lamps were functionally satisfactory insofar as they provided the desired physical compactness and light concentration by employing "three-dimensional" type envelopes, they were ~ifficult and expensive to manufacture since they either required the use of a number of separate lamp or bulb c~mponents that were joined together, or utilized enve-, lopes made from tubing which was bent in such a way that lr~ the electrode~containing ends of the tubing were located at opposite ends of the convoluted envelope or on opposite sides of one of its U-bent segments. This not only com-plicated the base and terminal structures required to con-nect the lamp electrodes with a suitable power source, but made it very difficult or impossible to provide the lamp with a screw-in type base which would permit the lamp to be used in lighting fixtures designed for incandescent type lamps. In addition, some of the prior art lamps also required tubular envelopes of such complex shape that their manufacture on a mass-production basis would be impractica] from both a cost and quality standpoint.
The foregoing difficulties and problems are solved in acco-rdânce with the present invention by fabri-` cating the envelope of the discharge lamp from a single piece of vitreous ~ubing that is bent into three conjoined U-shaped sections which are so oriented that the ends of the tubing which contain the electrodes are disposed adjacer.t one another in a common plane that does not intersect any of the U-be~d segments of the envelope.
~.

~ 3'7~7 : 5 48,105 This is accomplished by orienting the U-shaped sections of the envelope in such a manner that two of them are dis-posed in paired side-by-side relationship and are joined by the third U-shaped section which is inverted and lo-cated in a plane that is parallel to the plane containing ttle ~ree ends of the tubing. The spatial relationship of the three U-shaped sections of the novel envelope is ~cco~di;lgly such that two of them form a pair of spaced . hooks or "crooks" that are aligned with one another and have their tips laterally bent toward each other and joined by a U-bend which forms the third U-shaped section.
The straight tubu].ar legs of the U-shaped sections are thus disposed in rectangular columnar a~rray and the free ehds of the envelope are located proximate and on the same side of the U-bend which connects the tips of the paired : "crooks."
The resulting triple-U-bent envelope is thus of , three-dimensional. configuration and, while very compact, still provides a single discharge channel that is more 2~ than four times the length of one of the straight legs of the convoluted envelope. The u~ e shape of the "dual-C~C-~ 'r~ Y~g ~r crooked" envelope t~h~s provides a tortuous discharge path which is not only of sufficient length to permit the lamp to be operated efficj.ently at a practical power loading but is so confi.gured that the electrodes are located at the same end of the en~elope and so oriented that the enve1ope can readily be fitted with a screw-type base and s.ili provide a lamp that is sma~.1 enough to be used in lightin,n fi.xtures de~igned fo-r incandescent type lamps.

7~7 6 48,105 me spaced tubular legs of the triple-U-bent envelope also provide an air space between each of ths U-shaped sections and a "chimney-like" central opening which prevent~ the lamp from becoming overheated, especially if it is oper~ted at hlgh power loadings in a confined environ-ment. The "free-standing" orientation of the U-shaped sections also permits the envelope to be readily manufac-tured ~rom glass tubing of the t~pe employed for conven-tional flourescent lamps by heating the tubing to make it plastic and then sub~ectîng it to two bending operations which rapidly form all of the U-bends while the glass tubing is still soft and pliable.
Since the electrode-containing ends of the envelope are located ad~acent to one another and on the same side of the medial U-shaped section, these parts of the envelope can al~o be read~ly coupled to each other by suitable bracing means to rigidify the lamp and prevent the envelope from breaking while the finished lamp is being handled or shlpped.
: 20 ~RIEF DESCRIPTION OF THE DRAWINGS
A better under~tanding of the invention wlll be obtained from the exemplary embodiment shown in the ~ccom-panying drawings, wherein:
Figure 1 is a pictorial view of a compact fluor-escent lamp that embodies the invention and employs the triple-~-bent envelope of the type produced by the invention;
Figure 2 is an enlarged fr~nt elevational view of the lamp shown in Figure 1, portions o~ the envelope being removed for illustrative purpo~es; and ,. ...
~, 7~'7 7 43,105 Figures 3 and 4 are simplified pictor~al views illustrating the manner in which a straight piece of vitreous tubing is formed in~o a triple-U-bent lamp enve-lope by only two sequential bending operations.

While the present invention can be advantageous-ly employed in various kinds of electr~c discharge lamps that are suited by virtue of their small physical size and high brightness ~or lighting homes or offices, it is particularly adapted for use in conjunction with low-pressure type dlscharge lamps such as fluorescent lamps and it has, accordingly, been so illustrated and will be so described.
The compact fluorescent lamp L shown in Figs. 1 and 2 employs an envelope 10 that is compo~ed of suitable light-tran~mitting vitreous mater1al, such as glas~ tubing7 that is shaped to provide a tortuous discharge channel which ` extends between a pair of electrodes 12 and 14 (see Fig.
2) sealed within the ends of the en~elope. As will be noted, the tubular envelope 10 is bent upon itself three times in such a fashion that it has three generally U-shaped sections which are located in three dif~erent planes and oriented so that two of the sections are aligned with one another in paired side-by-side relation-ship and joined by the other U-shaped section which is inverted. One of the paired U-~haped sections is defined by a substantially straight tubular segment 16 that is joined by arcuate segment 17 to another straight tubular ~ .

3~ 7 8 48,105 segment 18. Tubular segment 18, in turn, is joined by another arcuate segment l9 to another straight segment 20---thus forming a second U-shaped section that is lo-cated in a plane which is substantially normal to the plane of the first U-shaped section. Straight segment 20 s Joined by a third arcuate segment 21 to another straight segment 22 and thereby forms a third U-shaped section that is located in a plane which is substantially normal to the plane of the second o medial U-shaped section and is substantially parallel ~o the plane of the first U-shaped section.
As shown mosr clearly in Fig. 1, each of the arcuate segments 17, 19 and 21 of the c~onvoluted envelope 10 are U~bends of such curvature that the straight tubular segments 16, 18, 20~ and 22 extend in the same direction and are disposed substantially parallel to one another in quadran~ular and columnar-spaced array. The straight segments 16 and 2~ which contain the lamp electrodes (and thus terminate the discharge channel and the envelope 10) - 20 are located proximate one another in a common plane that is substantially parallel to the plane of the inverted U-shaped section which is formed by the straight segments 1.~ and 20 and the medial U-bend 19. The straight tubular segments 16, 18, 20, and 22 are thus arranged in pairs that are located in fou- different planes which are so oriented that adjacent planes are mutually perpendicular to one another. As a result, the overall configuration of ~he triple-U-bent envelope 10 is such that it is generally c~r~ical or tetrahedrai in character. When the fluorescent 3'7~7 9 48,105 lamp L is energized, the envelope 10 thus provides a three-dimensional light source which i5 quite compact but still provides a single discharge channel that i~ consider-ably longer than four times the overall height dimension "h" of the convoluted envelope 10 and thus permits the lamp L to be operated very efficiently.
As will also be noted in Fig. 1, the straight leg segments 16 and 22 which terminate the envelope 10 and the discharge channel are fitted with suitable base mem-bers 24 and 25 that are secured to the sealed ends of theenvelope by cement or other suitable means and carry a pair o~ terminals such as metal pins 26 and 27 that are connected to the respective electrodes. me orientation o~
the three U-shaped envelope sections is such that the based ends o~ the straight segments or legs 16 and 22 of the envelope 10 are located on the same side of the U-bend segment 19 that constitutes part of the inverted U-envelope section. This spat~al arrangement not only e~fectively makes the lam~ L of single-ended construction (and thus greatly simplifies connecting the lamp to a suitable power source) but permits the envelope 10 to be easily rigidified and protected from breakage by a suit-able bracing means such as a strut 28 fashioned from rigid wire or other suitable material. The strut is secured to the base members 24 and 25 and laterally extends to U-bend segment 19 to which it is coupled, as by a slip-interlock type of fit effected by an arcuate hook 2g that is pro-vided at the end of the strut and grips the U bend 19. As shown, the bracing strut 28 can be fabricated from a ~:,f 48,105 sin~le piece of wire (or other rigid material) that is bent upon itself and prevented from fle~ing by a cross-piece 30 that is joined to the ends of the strut which are secured to the base members 2~, 25.
As shown in the enlarged view of the fluorescent l-~nlp L illustrated in Fig. 2, the triple-U-bent envelope 10 is provided with the customary inner coating 11 of a suitable phosphor material that converts the ultraviolet radiation produced by the discharge into visible radiation The terminating legs or segments 16 and 22 of the envelope are hermetically sealed by stem assemblies 13 and 15 of conventional construction that are fused to the envelope and carry the thermionic electrodes 12 and 14. The elec-trodes comprise the usual tungsten wire coils that are coated with electron-emission material and are electri-cally connected by lead wires 35 and 36 to the pin termi-nals 26 and 27 of the respective base members 24, 25 fastened to the sealed ends of the envelope. In accord-ance with standard lamp-making practice, the envelope 10 contains a suitable ioniæable medium such as a fill gas and a dose of mercur~77 thac are introduced into the enve-lope through a tubulat:ion 23 that extends within stem 15 and is subsequently tipped-off and sealed in the usual manner. A suitable fiil gas is argon at a pressure below about 10 Torr, and preferably about 3 Torr. The mercury dosage will vary accordin~ to the physical size of the envelope 10 and the power loading at which the lamp L is operated The mercury dosage is sufficient to provide ~,~rcury vapor at a partial pressure of from about 6 to 10 '7'~7 11 48,105 milli~orrs when the lamp is operated at its rated wattage.
While any suitable phosphor or admixed phosphors can be used to form the inner luminescent coating 11, in lighting applications where optimum visual clarity and color rendition of the illuminated objects or scene are requi.red, coatings which contain a blend of three phos-phors that emit visible radiations in three different se1ected regions of the spectrum (specifically, the wave-leng~h regions of about 450 nm, 540 nm and 610 nm) are desirably employed to provide a so-called "prime color"
.luorescent lamp, pursuant to the teachings of W. A.
Thornton in the article entitled "Luminosity And Color-Rendering Capability Of White Light"~ Journal of the Optical Society of Anlerica~ Volume 61, ~o. 9 (September 1971), pages 1155-1163. As a specific example, a suitable phosphor blend for a fluorescent lamp having such an enhanced light output contains manganese-activated zinc silicate, europium-activated strontium chlorophosphate, and europium-act:ivated yttrium oxide phosphors---all of whi^h are well known to those skilled in the art.
As iliustrated in ~ig. 2, the wire brace 28 has two laterally deper.ding legs 31 and 33 that extend through suitable openings in thf base members 24, 25 and are terminaLed by arcuate segments 32 and 34 which nestingly seat within the base collars and securely couple the brace to the based ends of the envelope 10. The hooked end 2f?
of rhe brace 28 slips over and interlocks with the U-bend segment 19 l:ha~ is located opposi~e the based ends of the enve'.o}e, as previously described.

3'77~7 12 48,105 ; _VELOPE MAN7~FACTURE (FIGS. 3 AND 4) An important feature of the invention is that the configuration of the convoluted en~7elope 10 is such that it can be readily manufactured from a straight piece of glass tubing by only two bending operations (illus-trated in Figs. 3 and 4) and a single heating operation, all of which can be performed efficiently with automated ,~?chines. The first tube-bending operation is shown in ~ig. 3 and consists of first placing a straight piece 10' of suitable glass tubing of predetermined length in a pair of holders 38, 40 which grip the end segments of the tubing and are swingable toward one another along arcuate paths (as indicated by the arrows) in~ a plane that is substantially aligned wiLh and contains the tube axis. As a specific example~ soda-lime glass tubing of the type conventionally used in the manufacture of fluorescent lamps can be employed. Such glasses are well known in the art and typically have a softening point in the order of 693C, an annealing point of about 515C, and a strain point of around 470C.
After the glass tubing 10' is placed in the holders 38 and 40~ the medial portion of the tubing ex-tendillg between the holders 38 and 40 is heated by gas fires (or other suitable means) to a uniform temperature which softens the glass sufficiently to permit the tubing to be ~ent without fracturing or collapsing. A tempera-ture of around 780C is suitable in the case of the afore-Mentioned soda-lime type glasses. The center of the ~ubing 10' is then place~ against the arcuate work surface 3~7~
13 48,105 of a preheated mold colllrc,nent 42, which surface is con-toured to define a peripheral groove 43 that nestingly accommodates and effects a mechanical interlock with the tubing, While the glass is in its heat-softened condition, . holders 38 and 40 are rapidly swung through an arc of ~pproxima-tely 90, in the manner indicated by the arrows, thus bending the central part of the heated tubing 10' - around the mold 42 and forming a U-bend 19 that transforms the straight tubing into a U-shaped tubular component 10"
which has long leg sections that are substantially paral-lel to one another and CI the same length, As indicated, the segments of the U-shaped tubing 10" immediately adja-cent the U-bend 19 constitute the tubular segments 18 and 20 of the finished en~7elope 10, and the rigid end segments 16 and 22 of the tubing that are gripped by the holders 38 and 40 constitute the corresponding tubular segments which terminate the finlshed envelope.
The second bending operation is depicted in Fig.
4 and is begun immediately after the first bending opera-tion is completed while the glass tubing is still in itsheat-softened ~nd plastic condition. As shown, the legs of the embryonic U-shaped envelope component 10 " are sea~ed ~gainst the edge of the working surface of another prehea~eG mold component 44 that is reciprocally movable (as indicated b~7 the arrows) and indexed into bridging posit-on with both legs after the first bending operation ha, beer, cc:mpleted. As will be noted, the working surface o ~ seccr.d mold 44 is also of arcuate configuration and ccn.~urec, to pro~7ide a pair of channels or grooves 45 and 3 1 ~
14 48,105 46 that are spaced to nestingly accommodate the legs of the U-shaped tube component 10" and thus serve as guides during the second bending operation.
Immediatel~7 after the U-shaped tube 10 " is seated against the rim of mold 44, t7r.- holders 38 and 40 ng with the gripped tube ends 16 and 22) are heid stationary and mold component 42 and the U-bent end 19 of the tube 10 " are swung through an arc of approximately 1~0 (indicated by the arrow) along a plane that is sub-stantially normal to the plane of the first bending opera-tion, thereby effecting the second bending operation. As shown, the bending action is such that selected medial portions of both legs of the tube 10 " are wrapped around the grooved portions of the second mold component 44 and formed :into two additional U-bend segments 17 and 21 that are spaced from each other and disposed in substantially parallel-paired relationship. The mechanical interloclc with the tube lC " effected by the grooved working surface of mold component 42 is such that mold component 42 grips the U-bend 19 sufficientl~ to permit the mold to apply the necessary tor~ue to the tubing for the second bending operation when mold 42 is swung through the aforesaid arc.
.~s will be noted from the phantom showing in Fig. 4, the second bending opera~ion places the first-formed U-bend 19 adjacent the end segments 16 and 22 (the latter being cut way for illustrative purposes) and completes the forma-tior. o~ the finished convoluted tubular envelope 10.
~ tile the glass tubing is still hot and plastic, a s~ream of inert gas (such as compressed air) is prefer-~ 7~ 7 48,105ably introduced into one end of the tubing to produce a positive pressure and "round out" any deformities in the circular shape of the tubing which may have occurred during the bending operations, especially in the U-bent segments.
If the tubular envelGpe lO is still in heat-softened condition, it is then subjected to a stream of co~l air to set the tubing in its new shape. Mold com-ponent 42 is then lowered, rotated 180 and withdrawn.
Molc' component 44 is also manipulatecl. to free it from U-'bends 17 and 21 and withdrawn from the envelope lO.
Holders 38 and 40 are opened and the released triple-U-bent envelope lO is then transferred to a suitable anneal-irg apparatus and subsequently subjected to the sealing-in operations, etc., required to transform it into a fluores-cent lamp.
For ease of manufac~ure, the straight piece of glass tubing 10 ! iS provided with an inner coating of phosphor (not shown in Figs. 3 or 4) before it is heated and sub~lected to the two bending operations. However, if optir.l~1m light output from the finished fluorescent lamp is desired, then the phosphcr-coating operation is performed after a- clear glass length 5f tubing is reshaped into the desirecr~ trip1e-lT-be~t configuration since this would increase the lumen output of the finished lamp by approxi-matel ~
If the envelope lO is ~o be made in the manner .c~ ed abov--, it is imperative that the glass tubing ~:' be uni formlJr heated to the proper temperature before 7~ 7 16 48,105 the bending operations, and that all of the U-bends be made very rapidly before the glass begins to cool and r, rigidify.
- Uniform heading of the medial portion of the tubing lO' is best accomplished by continuous rotation of t.~e tu~ing on rollers over an open burner or in an oven whlch are designed to keep both ends of the tubing cool and rigid so that the heated tubing can be transferred to arld gripped by the movable pair of holders 38 and 40.
The fact that the last two U-bends 17 and 21 are made simultaneously, pursuant to the invention~ enables the tube-reshaping process to be performed in a very short period of time (in the order of from~ about lO to 20 seconds)---before the glass has cooled to a point that the tubing begins to rigidify. In addition, the tube-holders 38 and 40 and the mold components 42 and 44 are so con-structed that they can readily be made parts of a single machine having drive means, indexing units, etc., which actuate and move the holders and molds in the proper ~equence automatically in a very precise and controlled manner---thus reducing the time required for the tube-bending operations to a minimum.
_PECIFIC EXAMPLE
Fo].lowing is a specific example of a compact tridimens~onal fluorescent lamp which embodies the fea-tures of the present invention.
A conventional 40 watt fluorescent lamp having a 'IlO t~pe envelope (approxi.mately 32 mm. outside diameter) a~d a-- overalI lengrh (unbased) of about 4 feet (approxi-17 48,105mately 122 cms.), when bent into three U-shaped sections of substantially the same length and curvature, forms a generally rectangular convoluted lamp which has an overall height of about 9-3/4 inches (24.76 cms.~ and a width dimension of about 5-1/2 inches (13.97 cms.) along each ~;.r.'e with a 3 inch (7.62 cms.) spacing between each of the adjacent legs of the U-bent sections. Since the lumen OU~p~lt (published) of a conventiolla]. 40 watt T10 rapid st:art lamp is 3200 lumens and the lamp has a published lC life of 18,000 hours, it ~ill provide a triple-U-bent lamp of the foregoing dimensions that has the same useful life and only a sl.ight decrease in lumen output (perhaps a loss of 350 lumens) due to the masking or light-blocking effect of the juxtaposed legs of t;rie U-sections.
Hence, despite its very com.pact size (approxi-mately ~4-1/2 cms. by 14 cms.~, the aforementioned triple-U-bent lamp provides a light source which will produce about 2850 lumens at an efficacy of around 57 lumens per watt (when operated on a conventional rapid start ballast) that consumes about 10 watts) and have a useful life of appro.Y~marely 18,Q00 hours. The cost savings and power-conscrvation advartages derived from such a convoluted ~;luol-escent lall;p are accvrdingly quite substantial com-pared to a ].50 ~,ati incandescent type lamp that produces around 28')() lumeIIs at an efficacy of about 18 or 19 lumens per watt and ha~ an average life (published) of only 750 ours .
As wil~ be apparent to those skilled in the art, riple~ bent lamps of the present invention can be 37~77 18 48,105 fabricated from glass tubing of various diameters and lengths to provide a new family of compact tridimensional low-pressure discharge lamps that can be used as cost-saving and energy-saving replacements for incandescent lamps now used for general lighting applications in many d-fferent kinds of fixtures (table lamps, floor lamps, etc.).

Claims (17)

19 48,105 I claim as my invention:

1. A fluorescent lamp adapted to be operated at a predetermined power loading and having a phosphor-coated tubular envelope of convoluted tridimensional con-figuration that contains a pair of spaced electrodes and an ionizable medium which sustains an electric discharge between the electrodes when the lamp is energized, said convoluted envelope comprising;
a single piece of glass tubing that is sealed at each end and conformed to provide four substantially straight segments that are joined by three curved segments and together therewith form three generally U-shaped sec-tions that define a single discharge channel of elongated tortuous configuration, each of the conjoined generally U-shaped sec-tions of said convoluted tubular envelope being disposed in a different plane and being so oriented that the four substantially straight segments extend in the same general direction and the discharge channel is terminated by two of the said substantially straight segments that are proximate one another and to the generally U-shaped sec-tion which comprises the medial part of the envelope so 48,105 that the substantially straight segments of said envelope are disposed in substantially quadrangular columnar array and provide a tridimensional light source of compact size and high brightness when the lamp is energized, said electrodes being located within the sub-stantially straight segments of the envelope which term-inate the discharge channel and are proximate one another so that the tridimensional fluorescent lamp is of single-ended construction, base means secured to the sealed ends of the glass tubing and having exposed terminals that are con-nected to the respective electrodes, the three generally U-shaped sections being of such size that the convoluted tubular envelope has (a) an overall height dimension which does not exceed 24.76 cms., (b) an overall width dimension which does not exceed 13.97 cms., and (c) defines an arc path of such length that the compact tridimensional fluorescent lamp generates up to about 2850 lumens at an efficacy of up to about 57 lumens per watt when the lamp is operated at said predetermined power loading.

2. The compact tridimensional fluorescent lamp of claim 1 wherein the curved segments of the convoluted tubular envelope are of such size and configuration that the substantially straight segments are spaced from one another by a central air space which provides a chimney-like cooling effect that prevents the lamp from becoming overheated when operated in a substantially upright posi-21 48,105 tion at high power loadings in a confined environment.

3. The compact tridimensional fluorescent lamp of claim 1 wherein;
the substantially straight segments which con-stitute the sealed ends of said convoluted tubular envel-ope and contain the electrodes are located on the same side of the generally U-shaped section which comprises the medial portion of the envelope, and said base means comprises base members that are fastened to the envelope-terminating segments of the convoluted envelope and have pin type terminals.

4. The compact tridimensional fluorescent lamp of claim 1 or 3 wherein said electrodes are mounted on glass stems that are sealed to the respective ends of the glass tubing.

5. The compact tridimensional fluorescent lamp of claim 1 wherein;
said ionizable medium comprises a gaseous fill-ing at a pressure below about 10 Torr and a predetermined amount of mercury, and the phosphor coating on said envelope includes three phosphors that emit visible radiations in three different regions of the spectrum comprising the wave-length regions of about 450 nm, 540 nm and 610 nm, respec-tively.

6. The compact tridimensional fluorescent lamp of claim 5 wherein the phosphors in said coating comprise manganese-activated zinc silicate phosphor, europium-acti-22 48,105 vated strontium chlorophosphate phosphor, and europium-activated yttrium oxide phosphor.

7. The compact tridimensional fluorescent lamp of claim 3 wherein;
the electrodes are sealed within the ends of the glass tubing, two of the generally U-shaped sections of the convoluted tubular envelope are disposed in matching paired relationship, and the third generally U-shaped section of said envelope is inverted relative to said paired sections so that the three curved segments of the envelope are joined to one another by two of the substantially straight seg-ments of the convoluted envelope.

8. The compact tridimensional fluorescent lamp of claim 7 wherein;
the curved segments of the convoluted tubular envelope are of such size and configuration that the substantially straight segments are spaced from one another, and the convoluted tubular envelope is reinforced by brace means that couples both of the substantially straight end segments of the envelope to each other and to the curved segment of the generally U-shaped section which constitutes the medial portion of the envelope.

9. The compact tridimensional fluorescent lamp of claim 7 wherein the three generally U-shaped envelope sections and the two substantially straight segments that terminate the discharge channel and envelope are disposed 23 48,105 in four different planes adjacent ones whereof are sub-stantially mutually perpendicular.

10. The compact tridimensional fluorescent lamp of claim 9 wherein;
said glass tubing is of substantially circular cross-section, the generally U-shaped sections of the convo-luted envelope are of substantially the same length, the based ends of the convoluted tubular envel-ope are located substantially opposite the bend of the generally U-shaped section which constitutes the central part of the convoluted envelope, and said base members are mechanically coupled to each other and to the bend of the oppositely-disposed generally U-shaped section of the envelope by brace means which has a hook-shaped portion that is in slip-locked relationship with said bend.

11. In the manufacture of an electric lamp, the method of forming an envelope of compact triple-U-bent configuration from vitreous tubing that is substantially straight, which method comprises the steps of;
heating a predetermined length of such substan-tially straight vitreous tubing in a manner such that the end segments thereof are rigid and the remainder of the tubing is uniformly softened sufficiently to be bent without fracturing or collapsing, gripping the rigid end segments of said tubing 24 48,105 and swinging them toward one another along a common plane until the heat-softened medial portion of the tubing is bent through an arc such that the tubing is U-shaped, immediately placing heated mold means of arcuate configuration against the heat-softened leg portions of the U-shaped tubing and, while maintaining the gripped end segments of the tubing stationary, swinging the U-bent medial part of the tubing through an arc of approximately 180° along a path which is substantially normal to the plane of the first bending operation and thereby simultaneously bending both of the heat-softened leg portions around said mold means and forming two additional U-bends in the tubing which are aligned with one another and position the neck of the first-formed U-bend proximate the gripped end segments of the tubing, and then removing said mold means and cooling the result-ing triple-U-bent tubular envelope to rigidify and set the vitreous tubing in such configuration.

12. The envelope-forming method of claim 11 wherein;
the first tube-bending operation is performed by placing the heat-softened medial portion of the vitreous tubing against another heated arcuate-shaped mold means before the end segments of the tubing are swung toward one another, the mold means which is employed in the first bending operation constituting a first mold means and the mold means which is employed in the second bending opera-tion constituting a second mold means, and 48,105 said first mold means and the U-bent part of the tubing formed therearound are both swung as a unit through said arc during the second bending operation.

13. The envelope-forming method of claim 11 or 12 wherein;
said tubing is of substantially circular cross-section, and a pressurized stream of inert gas is introduced into the tubing, immediately after both bending operations have been completed and the tubing is still in heat-softened condition, to subject the walls of the tubing to positive pressure and thereby substantially correct any out-of-round deformities in the U-bent portions of the tubing that may have been produced by the heating and bending operations.

14. The envelope-forming method of claim 11 wherein a coating of phosphor material is deposited on the inner surface of the vitreous tubing before the tube bending operations are performed.

15. The envelope-forming method of claim 11 wherein a coating of phosphor material is deposited on the inner surface of the vitreous tubing after both of the tube bending operations have been performed and the result-ing triple-U-bent envelope has cooled.

16, The envelope-forming method of claim 11 wherein;
the arcuate surface of the first mold means which contacts the vitreous tubing defines a peripheral 26 48,105 groove that nestingly receives the tubing and thus estab-lishes a mechanical interlock between said first mold means and the associated U-bent part of the tubing that is produced by the first bending operation, and the tube-gripping action resulting from said mechanical interlock is sufficient to permit the second tube-bending operation to be accomplished by utilizing the first mold means as the component by which the bending force for the second bending operation is applied to the first-formed U-bent pottion of the tubing.

17. The envelope-forming method of claim 16 wherein;
said second mold means comprises a single mold component that bridges both leg portions of the U-shaped tubing, and the arcuate surfaces of the second mold com-ponent which contact the tubing also define peripheral grooves that nestingly receive the respective leg portions of the U-bent tube component which is produced by the first bending operation and thus serve as guide means for properly reshaping said leg portions during the second bending operation.