An incandescent lamp and a lighting apparatus using the lamp

Abstract: An incandescent lamp has an envelope (1) having a bulbous portion (4) including a wall defining a space, a thin tube (5) extending outwardly along a central axis (O1-O1) of the bulbous portion (4) so as to communicate with the space, and a sealed portion (2) located on an opposite side to the thin tube (5). The wall has a slope (7) being inclined to the thin tube (5) near the thin tube. A filament (13) is provided along said central axis of the bulbous portion (4) for generating heat during lamp operation. An optical interference layer (6) is arranged on a surface of the wall for reflecting infrared rays toward the filament (13). A first lead wire (9) has a first end portion (9a) fixed in the sealed portion (2) of the envelope, a second end portion (9b) coupled to one end of the filament (13) and an intermediate U-shaped portion (8) inserted into the thin tube (5) for regulating the position of the lead wire. A second lead wire (12) has a first end portion (12a) fixed in the sealed portion (2) of the envelope and a second end portion (12b) coupled to other end of the filament.

Description: BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The present invention relates to incandescent lamps. In particular, the invention relates to an incandescent lamp having an optical interference layer arranged on a surface thereof. 2. Description of the Related Art In a recently developed and sold incandescent lamp including a halogen lamp, an infrared ray reflecting film through which visible light passes is formed on the surface of the envelope. Of the light emitted by the filament, infrared rays are reflected from the reflecting film and returned to the filament. Thus, the returning infrared rays heats the filament and the emitting efficacy of the incandescent lamp is increased. At the same time, the amount of infrared rays emitted outside the incandescent lamp is reduced. In order to increase the emitting efficacy of an incandescent lamp having a reflecting film, the filament is preferably disposed at the centre of the envelope. Because the conventional envelope of an incandescent lamp has a spherical shape, and elliptical shape or a cylindrical shape, the returning infrared rays to the filament increase when the filament of the incandescent lamp is located at the centre of the envelope. Therefore, when the filament is located on the centre line of the envelope the infrared rays which are reflected by the reflecting film return with certainty to the filament. The percentage of the infrared rays returned is known as the geometrical gain factor and this geometrical gain factor increases the nearer the filament is located to the centre line of the envelope. When the envelope of the lamp has only one sealed portion, known as a single sealed type, a pair of lead wires is fixed only in the sealed portion. Accordingly, at the opposite side from the sealed portion, at the top of the envelope, the filament tends to get out of position from the centre line of the envelope and as a result the geometrical gain factor decreases. In general incandescent lamps having no reflecting film, in order to maintain mechanical strength against a shock there has been proposed a lamp having means for supporting the lead wire at the opposite side to the sealed portion of the envelope. Japanese Laid-Open Patent Application No. 57-38557/1982 discloses such a lamp, having a U-shaped support wire inserted into an exhaust tube located at the opposite side to the sealed portion. SUMMARY OF THE INVENTION The present invention seeks to provide an incandescent lamp in which a difference of location between the filament and the centre line of the envelope is substantially avoided so as to achieve emission efficiency. According to the present invention there is provided an incandescent lamp, comprising: an envelope having a bulbous portion including a wall defining a space, a thin tube extending outwardly along a central axis of said bulbous portion so as to communicate with said space, and a sealed portion located on an opposite side to said thin tube; a filament provided along said central axis of said bulbous portion for generating heat during operation of the lamp; an optical interference layer arranged on a surface of said wall for reflecting infrared rays toward said filament; a first lead wire having a first end portion fixed in said sealed portion of said envelope and a second end portion coupled to one end of said filament; and, a second lead wire having a first end portion fixed in said sealed portion of said envelope and a second end portion coupled to other end of said filament; said incandescent lamp being characterized in that said wall of said bulbous portion adjacent to said thin tube has a curved transition part, whereby a tangent to the curve at any point on the curved transition part near said thin tube is inclined at an angle of 70° or less to the axis of said thin tube, and said first lead wire further has an intermediate U-shaped portion inserted into said thin tube for controlling the position of the lead wire such that said second end portion is positioned along said central axis. According to a favourable embodiment, the bulbous portion of the envelope comprises a spheroidal portion having two focuses, each focus being positioned near a respective end of the filament. In a further preferred embodiment according to the invention, the filament is formed of a coiled coil. According to another embodiment, the second end portion of the first lead wire is inserted into the primary coil of the coiled coil for coupling therebetween. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention and to illustrate how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings, in which: FIG. 1 is a partial front view of an incandescent lamp according to a first embodiment of the present invention; FIG. 2 is a perspective view of an incandescent lamp shown in FIG. 1; FIG. 3 is an illustration of a manufacturing process for a mount used in the first embodiment of the present invention; FIG. 4 is a perspective view of an incandescent lamp according to a second embodiment of the present invention; FIG. 5 is a greatly enlarged detailed elevation and broken view of a filament of an incandescent lamp in accordance with the second embodiment of the present invention; and FIG. 6 is a cross sectional and broken view of a lighting apparatus using the incandescent lamp shown in Figs. 1 and 2. DETAILED DESCRIPTION OF THE INVENTION Referring now to Figures 1 and 2, a first embodiment of this invention will be explained. Fig. 1 shows an incandescent lamp 100. An envelope 1 of the incandescent lamp 100 is formed with fused silica. The envelope 1 is continuously formed with a sealed portion 2 at one end of the envelope 1, a cylindrical neck portion 3, a bulbous portion 4 and a thin tube 5 provided at the opposite end side of the envelope 1 from the sealed portion 2 and cylindrical portion 3. The bulbous portion 4 of the envelope 1 is generally spheroidal with a major axis between the sealed portion 2 and the thin tube 5. This major axis corresponds to a central axis of the envelope 1 (O1-O1). The bulbous portion 4 of the envelope 1 is so formed that it has a first focal point F1 and a second focal point F2 on the central axis O1-O1 of the envelope 1, and a wall of the bulbous portion 4 is coated with an optical interference layer 6. Near the thin tube 5 the wall of the bulbous portion 4 has a slope 7 inclined to the thin tube 5 to enable easy insertion therein of an intermediate U-shaped bend portion 8 of a first lead wire 9, described below. The slope 7 of the wall near the thin tube 5 has an inclination of about 70 degrees to the central axis O1-O1. Preferably the angle of inclination indicated by  is under 70 degrees with respect to the central axis O1-O1. That is, a tangent X-X (or Y-Y) to the curve at any point near the thin tube 5 is inclined at an angle of 70 degrees or less to the axis 01-01. If the angle of inclination exceeds 70 degrees, it is difficult to insert smoothly the intermediate U-shaped bend portion 8 into the thin tube 5. As shown in Figure 2, the sealed portion 2 of the envelope 1 is attached with adhesives (not shown) to a ceramic base 10 having a metal cap 11. In this embodiment, the metal cap 11 is utilized an Edison screw type. As can be seen in Figure 1, the thin tube 5 of the envelope 1 extends outwardly along the central axis O1-O1 of the envelope 1. The thin tube 5 is typically an exhaust tube for pumping all the air from the envelope 1 and for refilling the envelope 1 with a mixture of an inert gas and a halogen gas. The thin tube 5 communicates with the space defined by the wall of the bulbous portion 4. If the inner diameter of the thin tube 5 is less than 2.5 mm, it can be difficult to pump the air efficiently from the envelope 1 through tube 5. On the other hand if the inner diameter of the thin tube 5 is greater than 6.0 mm, the effective area of the optical interference film 6 becomes small. Accordingly, the diameter of the thin tube 5 is preferably selected to be between 2.5 mm and 6.0 mm. The optical interference layer 6, indicated by a dot line, is arranged on the external surface of the wall of envelope 1. The optical interference layer 6 includes a high refractive index layer made of a metal compound, for example metal oxides or metal sulfides, preferably titanium oxide (TiO 2 ), tantalum oxide (Ta 2 O 5 ), zirconium oxide (ZrO 2 ) or zinc sulfide (ZnS), and a low refractive index layer made of a metal compound, preferably silicon oxide (SiO 2 ) or magnesium fluoride (MgF 2 ) (not shown separately in the Figures). The high refractive index layer and the low refractive index layer are alternately stacked on each other with 6 to 80 layers of appropriate thickness in total. The dimensions of the optical interference layer 6 are chosen so that the layer functions to transmit visible light and reflect infrared rays as a result of optical interference. The first lead wire 9 and a second lead wire 12, each connecting to a filament 13, are made of tungsten wire. Each has one end, 9a, 12a, connected to thin molybdenum foils 14, 14 which are in turn connected to outer lead wires 15, 15 respectively, and fixed in the sealed portion 2 of the envelope 1. The outer lead wires 15, 15 are connected to a top contact 11a and a cap shell 11b of the cap 11, respectively (shown in Figure 2). The first and second lead wires 9, 12 penetrate through a glass support bridge 16 at the cylindrical neck portion 3 of the envelope 1 so as to be held isolated from each other. The first lead wire 9 is shaped such that it has an overhang 17 near the filament 13 so as to avoid heating. The first lead wire 9 further has an intermediate bend portion 8 inserted into the thin tube 5. The intermediate bend portion 8 is bent so as to form a U-shaped configuration. The intermediate U-shaped bend portion 8 prevents an inclination of the first lead wire 9, whereby the position of the first lead wire 9 is controlled. The first lead wire 9 preferably has an outer diameter between 0.2 mm and 0.55 mm. If the outer diameter of the first lead wire 9 is less than 0.2 mm, it is difficult to maintain sufficient mechanical strength for the first lead wire 9 spanning the filament 13. On the other hand if the outer diameter of the fist lead wire 9 is more than 0.55 mm, the first lead wire 9 impedes a pumping all of the air from the envelope 1 through the thin tube 5. The other end portion 9b of the first lead wire 9 has a straight configuration extending along the central axis O1-O1 of the envelope 1. Similarly, the one end portion 12b of the second lead wire 12 has a straight configuration extending along the central axis O1-O1 of the envelope 1. The filament 13 is shaped into a straight configuration, which is formed with a coiled coil made of tungsten. The secondary coil of the filament 13 is extended between the first and the second focal points F1, F2 of the bulbous portion 4. Therefore, the infrared rays emitted form the filament 13 between the focal points F1, F2 can be reflected back towards the filament 13 between the focal points F1, F2 by the optical interference film 6. Each of the other end portions 9b, 12b of the first and second lead wires 9, 12 is inserted into a respective end of the filament 13, into the primary coils of the filament, whereby the filament 13 spans or connects both lead wires 9, 12 as mentioned above. Therefore, an axis of the filament 13 corresponds to the axis O1-O1 of the envelope 1. Moreover, since the wall of the bulbous portion 4 has a slope inclined to the thin tube 5, the intermediate U-shaped bent portion 8 of a first lead wire 9 is easily and smoothly inserted into the thin tube 5 and the intermediate U-shaped bent portion 8 restrains deformation thereof. Additionally, when manufacturing the incandescent lamp 100, the first lead wire 9 is prevented from sliding. In this embodiment for the thin tube 5 of the envelope 1 there is utilized a conventional exhaust tube, however, a separate thin tube separate from the exhaust tube may be used as the tube to hold the bent portion 8. When the incandescent lamp 100 is energized, the filament 13 heats up to a high temperature to generate light including infrared rays and visible light. When the light generated by the filament 13 reaches the optical interference film 6 through the envelope 1, the optical interference film 6 transmits visible rays and reflects infrared rays. The infrared rays reflected by the optical interference film 6 are returned to the filament 13 and reabsorbed by the filament 13. The absorbed infrared rays reduces the input energy needed to maintain the temperature of the filament 13. As a result, the geometrical gain factor becomes high and the luminous efficiency of the incandescent lamp 100 is greatly improved. According to this embodiment the first lead wire 9 is regulated its location at the opposite side to the sealed portion 2 of the envelope 1, therefore the filament 13 spanning first lead wire 9 and lead 12 is located on the central axis O1-O1 of the envelope 1 exactly. As a result, the geometrical gain factor becomes high. Next, a manufacturing process for the composite unit including the first lead wire 9, the second lead wire 12 and the filament 13, which is known as a mount, will be described. As shown in Figure 3(A), at the first manufacturing process step, one straight tungsten wire is bent so as to form the intermediate U-shaped bent portion 8 and the overhang 17 which will be located near the filament 13. Secondly, as shown in Figure 3(B) both ends of the tungsten wire are coupled by the glass support bridge 16. Next, as shown in Figure 3(C), the tungsten wire is cut at predetermined positions to form a space which will be spanned the filament 13 (shown in Figure 3(C)). Whereby the first lead wire 9 and the second lead wire 12 are formed, separated from each other. Finally, the filament 13 is inserted spanning between the positions, whereby the mount is completed (shown in Figure 4(D)). Another embodiment in accordance with the present invention is shown in Figures 4 and 5 and explained next. Like reference numerals designate identical or corresponding element of the above disclosed first embodiment. The construction and operation of the following embodiments are substantially the same as the first embodiment and, therefore, a detailed explanation of its operation is not provided. Figure 4 shows an incandescent lamp 101 according to a second embodiment of the present invention. The incandescent lamp 101 has an anchor wire 18 fixed in the glass support bridge 16. The anchor wire 18 supports the filament 13 at an intermediate position of the filament. According to this embodiment the filament 13 is given mechanical strength against a shock. Figure 5 illustrates one end of a filament 13 and one end portion 9b of the first lead wire 9 according to another embodiment of the present invention. In this embodiment, the end portion 9b of the first lead wire 9 is inserted into the primary coil of the filament 13, and has a wave or corrugated form in order to prevent or resist disconnection from the filament 13. Figure 6 shows a lighting apparatus 200 according to an embodiment of the invention, which uses the incandescent lamp 100 shown in Figures 1 and 2. The lighting apparatus 200 is assembled with the incandescent lamp 100, a reflector 201, a front cover 202 and a base 203 having a metal cap 204. The reflector 201 includes a PAR (parabolic aluminized reflector) type. The incandescent lamp 100 is arranged in the reflector 201 made of a soft glass. The front cover 202 is fixed on the front edge of the reflector 201 by an epoxy resin glue. Glass welding may be used to fix the front cover 202 and the reflector 201. The reflector 201 is coated with an aluminized reflection film on the inner surface thereof, however, a conventional dichroic mirror may be used. The base 203 is mounted on the rear edge of the reflector 201 and the metal cap 204 is electrically connected to the outer lead wires (not shown) of the incandescent lamp 100. When the incandescent lamp 100 is energized, the filament 13 heats up to a high temperature to generate light including infrared rays and visible rays. When the light generated by the filament 13 enters the optical interference film 6 through the envelope 1, the optical interference film 6 transmits visible light rays and reflects infrared rays. The visible light rays transmitted through the optical interference film 6 are reflected by the reflector 201, and thus, radiate toward the outside of the lighting apparatus 200 through the front cover 202. Infrared rays reflected by the optical interference film 6 return to the filament 13 and heat up the filament 13. As a result, the luminous efficiency of the incandescent lamp 100 is greatly improved. While the invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

Claims: An incandescent lamp, comprising: an envelope (1) having a bulbous portion (4) including a wall defining a space, a thin tube (5) extending outwardly along a central axis (01-01) of said bulbous portion (4) so as to communicate with said space, and a sealed portion (2) located on an opposite side to said thin tube (5); a filament (13) provided along said central axis (01-01) of said bulbous portion (4) for generating heat during operation of the lamp; an optical interference layer (6) arranged on a surface of said wall for reflecting infrared rays toward said filament (13); a first lead wire (9) having a first end portion (9a) fixed in said sealed portion (2) of said envelope (1) and a second end portion (9b) coupled to one end of said filament (13); and, a second lead wire (12) having a first end portion (12a) fixed in said sealed portion (2) of said envelope (1) and a second end portion (12b) coupled to other end of said filament (13); said incandescent lamp being characterized in that said wall of said bulbous portion (4) adjacent to said thin tube has a curved transition portion (7), whereby a tangent to the curve at any point on the curved transition part near said thin tube is inclined at an angle of 70° or less to the axis (01-01) of said thin tube (5), said first lead wire (9) further has an intermediate U-shaped portion (8) inserted into said thin tube (5) for controlling the position of the lead wire (9) such that said second end portion (9b) is positioned along said central axis (01-01). An incandescent lamp according to claim 1, wherein said bulbous portion (4) comprises a spheroidal portion having two focuses (F1, F2), each focus being positioned near a respective end of said filament (13). An incandescent lamp according to claim 1 or 2, wherein said first lead wire (9) is made of inseparable wire. An incandescent lamp according to any preceding claim, wherein said filament (13) is formed of a coiled coil. An incandescent lamp according to claim 4, wherein said second end portion (9b) of said first lead wire (9) is inserted into the primary coil of said coiled coil for coupling therebetween. An incandescent lamp according to claim 4 or 5, wherein said second end portion (12b) of said second lead wire (12) is inserted into the primary coil of said coiled coil for coupling therebetween. An incandescent lamp according to any preceding claim, wherein said thin tube (5) of said envelope (1) has an inner diameter of 1.0 to 3.0 mm and said first lead wire (9) has a diameter of 0.2 to 0.55 mm. An incandescent lamp according to any preceding claim, further comprising a glass support bridge (16) disposed between said filament (13) and said sealed portion (2) of said envelope (1), said first and second lead wires (9, 12) passing through said glass support bridge (16). An incandescent lamp according to claim 8, further comprising an anchor wire (18) fixed in said glass support bridge (16) for supporting said filament (13) at an intermediate portion thereof. An incandescent lamp according to claim 8 or 9, wherein said envelope (1) includes a cylindrical neck portion (3) disposed between said filament (13) and said glass support bridge (16). A lighting apparatus comprising: an incandescent lamp according to any preceding claim; and a luminaire (201) housing said incandescent lamp (100, 101) for directing the light transmitted through said envelope (1).

Inventors: BESSHO MAKOTO, MOCHIZUKI HIDETO, SUGANO TETSUYA, BESSHO, MAKOTO, MOCHIZUKI, HIDETO, SUGANO, TETSUYA
Applicants: TOSHIBA LIGHTING & TECHNOLOGY
Application year: 1995
Main classification: H01K1
IPC codes: H01K1