Double bulb halogen lamp

Abstract: In a double bulb type halogen lamp, a sealed space (49) between the inner surface of an outer bulb (13) and the outer surface of the inner bulb (15) in which a coiled filament (43) is disposed is filled with a weakly oxidising gas to avoid reduction of an optical interference film, including metal oxide substances,exposed to the sealed space.

Description: The present invention relates, in general, to halogen lamps. In particular, the invention relates to a double bulb type halogen lamp wherein an optical interference film composed of a selected metal oxide substance is coated on at least either the outer surface of an inner bulb thereof or the inner surface of an outer bulb to transmit visible rays and to reflect infrared rays from a light source thereof. A double bulb type halogen lamp in which a halogen lamp is airtightly enveloped by an outer bulb is used as a reflector lamp, e.g., a sealed beam lamp, to achieve a high output. A prescribed amount of halogen element, e.g., iodine, bromine, etc., or a compound thereof is sealed in the halogen lamp. The temperature of the inner bulb of the halogen lamp greatly increases during the operation to perform a halogen cycle. In recent years, an improved double bulb type halogen lamp has been developed to achieve a high efficiency and to radiate light having a small amount of infrared rays, i.e., a so-called cool beam. In such an improved double bulb type halogen lamp, an optical interference film is formed on the outer surface of the inner bulb to transmit visible rays and to reflect infrared rays. Furthermore, air in a sealed space between inner and outer bulbs is exhausted, or a prescribed amount of inert gas is sealed in the sealed space. The above-described optical interference film includes a high refractive index layer made of a metal oxide substance, e.g., titanium oxide (TiO₂), tantalum oxide (TaO₂), or zirconium oxide (ZrO₂), and a low refractive index layer made of another oxide substance, e.g., silicon oxide (siO₂), or selenium oxide (SeO₂), alternately stacked one on the other in six to twenty one layers on the bulb. The light from a specific wavelength range may be transmitted or reflected by controlling the thickness of each refractive index layer. When the optical interference film transmits visible rays and reflects infrared rays in light radiated from the light source, i.e., a coil filament, infrared rays reflected by the interference film return toward the filament and heat up the filament, resulting in a high luminous efficiency in the halogen lamp. However, since the heat discharge from the inner bulb of the double bulb type halogen lamp is not effective, the temperature of the inner bulb increases excessively during the operation. The temperature of the base portion of the halogen lamp also increases close to that of the inner tube. In the above-described conventional double bulb type halogen lamp, the temperature of the optical interference film reaches 500 ∼ 800 °C during the operation. The optical interference film becomes black as the temperature of the optical interference film frequently increases to such a high temperature during an extended operation period, and thus, the optical interference film loses the original optical characteristics. The light output of the halogen lamp is greatly reduced. Accordingly, it is an object of the present invention to maintain original characteristics of the optical interference film of a double bulb type halogen lamp for an extended operation period. According to the invention, there is provided a double bulb type halogen lamp comprising: an outer bulb; an inner bulb, arranged in the outer bulb, thereby defining a sealed space within the outer bulb, the inner bulb having a filament therein for radiating light; and an optical interference film arranged on at least one of the inner surface of the outer bulb and the outer surface of the inner bulb for reflecting radiation in a prescribed wavelength range of the electromagnetic spectrum, the optical interference film including a component liable to be reduced at elevated temperatures; wherein the sealed space contains an oxidising atmosphere to impede reduction of the optical interference film. These and other objects and advantages of this invention will becomes more apparent and readily appreciated from the following detailed description of the presently preferred exemplary embodiments of the invention, taken in conjunction with the accompanying drawings, wherein like reference numerals throughout the various figures denote like structure elements and wherein: FIGURE 1 is a side view, partly in section, illustrating a sealed beam type double bulb halogen lamp of one embodiment of the present invention; FIGURE 2 is an enlarged cross sectional view illustrating the inner bulb of the halogen lamp shown in FIGURE 1; and FIGURE 3 is an enlarged cross sectional view illustrating the optical interference film coated on the inner bulb of the halogen lamp shown in FIGURE 2. The inventors of the present invention conducted a chemical experiment with regard to changes in characteristics of metal oxide substances of an optical interference film used in a double bulb type halogen lamp. A chemical process of losing original optical characteristics of the optical interference film will be described based on the experiment. Deoxidation, or reduction, of a metal oxide substance, e.g., TiO₂, TaO₂, SeO₂, etc., of the optical interference film occurs if the temperature of the optical interference film exceeds 500 ° C while the optical interference film has been heated in an inert gas, e.g., argon, nitrogen, etc., or in a vacuum for an extended period. The metal oxide substance is gradually degraded to become a low grade oxide substance and is finally reduced to the metal. If such a deoxidation of a metal oxide substance occurs in the optical interference film of the double tube type halogen lamp, the wavelength range at which it reflects or transmits light varies, and thus, the original characteristics of the optical interference film are lost. Based on the above-description, a preferred embodiment of the present invention will be described with reference to the accompanying drawings. As shown in FIGURE 1, a double bulb type halogen lamp 11 includes a funnel shaped outer bulb 13 and an inner bulb arranged in outer bulb 13. Outer bulb 13 includes a PAR (parabolic aluminized reflector) type sealed beam bulb element 17 and a lens element 19 fixed on the front edge of bulb element 17 by an epoxy resin glue 21. Glass welding may be used to fix lens element 19 and bulb element 17. An aluminized reflection film 23, indicated by a dot and dashed line, is formed on the inner surface of sealed beam bulb element 17. A pair of ferrules (not shown) which form part of the electrical contact structure of the lamp are mounted on a closed rear wall of bulb element 17 to fix a pair of support wires 24, shown in FIGURE 2, by welding. The pair of support wires 24,whose diameter is 1 mm and length is 40 mm, are made of nickel and are used to support inner bulb 15 at an appropriate position in outer bulb 13. A screw type cap 25 is mounted on the rear edge of bulb element 17 and is electrically connected to support wires 24. An optical interference film 27, indicated by a dotted line, is formed on the outer surface of inner bulb 15. The structure of inner bulb 15 will now be described. As shown in FIGURE 2, inner bulb 15 includes a tube 31 made of a heat-resisting glass, e.g., quartz glass, etc. Opposite ends of tube 31 are closed, and one of the opposite ends thereof is pinched to form a flat base 33. A pair of molybdenum foil plates 35 are arranged in parallel in flat base 33. One of the ends of a first inner lead wire 37 is connected to one of the molybdenum foil plates 35, and the other end thereof extends to the other end of tube 31 along the inner surface of tube 31. One end of a second inner lead wire 39 is connected to the other molybdenum foil plate 35, and the other end thereof is positioned in the vicinity of flat base 33. A base portion of each inner lead wire 37, 39 is supported by an insulating supporter 41 inside tube 31. The other end of first inner lead wire 37 and the other end of second inner lead wire 39 are opposed at a prescribed distance along an elongated center line of tube 31. A tungsten coiled filament 43 extends between the opposed ends of first and second inner lead wires 37 and 39. A middle portion of tungsten coiled filament 43 is supported by one of the ends of a bar anchor 45, the other end of which is fixed to insulating supporter 41. One of the ends of each outer lead wire 47 is connected to molybdenum foil plate 35, respectively, and the other end thereof extends to the outside of tube 31. The extended end of each outer lead wire 47 is fixed to support wire 24 to perform an electric connection and a mechanical support. A prescribed amount of halogen is sealed in tube 31 together with a given amount of an inert gas, e.g., argon or nitrogen. The amount of halogen and the inert gas are similar to those in the conventional halogen lamp. Optical interference film 27 coated on the outer surface of tube 31 is formed such that it reflects infrared rays and transmits visible rays. The construction of optical interference film 27 is schematically shown in FIGURE 3. Optical interference film 27 includes a high refractive index layer 27H made of a metal oxide substance, preferably TiO₂, TaO₂ or ZrO₂, and a low refractive index layer 27L made of an oxide substance, preferably SiO₂ or SeO₂. The layers 27H, 27L are alternately stacked on the outer surface of tube 31 of inner bulb 15. In this case, high refractive index layer 27H is formed on the outer surface of tube 31, and then low refractive index layer 27L is stacked on high refractive index layer 27H. In this manner, high and low refractive index layers 27H and 27L are alternately stacked, with 6 to 21 layers in total. The thickness of each refractive layer 27H, 27L is regulated at a suitable value to reflect radiation e.g., infrared rays, from the electromagnetic spectrum in a prescribed wavelength range by the interference of light. In this embodiment, a space 49 defined by outer and inner bulbs 13 and 15 is maintained with a weakly oxidising atmosphere. An inert gas including argon of 90% by volume and nitrogen of 10% by volume is sealed in space 49 at 40-203 kPa (300-1520 Torr) together with oxygen of 37-80 Pa (0.28 = 0.6 Torr) (approx. 500 - 1000 ppm). In the above-described double bulb type halogen lamp, when tungsten coiled filament 43 is energized, filament 43 heats up to a high temperature to generate light including infrared rays and visible rays. When the light generated by filament 43 enters into optical interference film 27 through inner bulb 15, optical interference film 27 transmits visible rays and reflects infrared rays by the light interference. Visible rays transmitted from optical interference film 27 are reflected by aluminized reflection film 23, and thus, radiated toward the outside of outer bulb 13 through lens element 19. Infrared rays reflected by optical interference film 27 return to filament 43 and heat up filament 43. As a result, the luminous efficiency of lamp 11 is greatly improved. During the above-described operation of lamp 11, tube 31 including base portion 33 is heated to a high temperature by filament 43. However, cooling of tube 31 is not effective because of the double bulb construction, and thus, temperatures of both tube 31 and optical interference film 27 increase to about 800 °C. Temperature of base portion 33 of tube 31 also increases to substantially the same temperature as tube 31. However, neither oxidation nor reduction (deoxidation) occurs in the metal oxide substance of each refractive index layer and the original composition of the metal oxide substance of each refractive index layer, e.g., the degree of oxidation of TiO₂ or SiO₂, is maintained for an extended operation period. This is because space 49 is filled with an inert gas including a small amount of oxygen and the pressure of decomposing oxygen from each metal oxide substance substantially balances with the pressure of oxygen in space 49. Thus, the metal oxide substance of each refractive index layer 27H, 27L is reoxidized by oxygen in space 49 even if the metal oxide substance of each refractive index layer 27H, 27L is decomposed and oxygen therein escapes into space 49. The original optical characteristics of optical interference film 27 can be maintained for an extended operation period. The transmission/reflection wavelength range of optical interference film 27 does not change and the light transmission factor thereof does not greatly decrease. Furthermore, since the degree of oxidation in space 49 is low, molybdenum foil plates 35 are hardly oxidized even though foil plates 35 are subject to a high temperature when the gas filled in space 49 enters into flat base 33 of inner tube 15 along each outer lead wire 47. Thus, the airtightness of inner tube 15 is not adversly affected for an extended operation period. In the present invention, it is required that the weak oxidation in space 49 does not cause deoxidation of optical interference film 27 or oxidation of both molybdenum foil plates 35 and outer lead wires 47. For these reasons, it is preferable to include oxygen at a partial pressure of 1.01 Pa - 10.1 kPa (0.0076 - 76 Torr) in the inert gas, e.g., argon or nitrogen, filled in space 49 at a conventional sealed pressure, i.e., 40-203 kPa (300-1520 Torr). However, in view of the accuracy of mixing a proper amount of oxygen into the inert gas, it is more preferable to include oxygen at a partial pressure of 5.33 - 440 Pa (0.04 - 3.3 Torr) (approx. 100 - 5000 ppm) in the inert gas filled in space 49 at a pressure of 60 - 87.7 kPa (450 - 650 Torr). In the above-described embodiment, the present invention is applied to a sealed beam type halogen lamp. However, the invention may be applied to a double tube type halogen lamp wherein a tube type inner bulb is coaxially supported in a tube type outer bulb and an optical interference film is coated on at least either the inner surface of the outer bulb or the outer surface of the inner bulb. A similar result to the above-described embodiment may be achieved even if temperature of the double tube type halogen lamp increases above 500 °C. The present invention has been described with respect to a specific embodiment. However, other embodiments based on the principles of the present invention will be obvious to those of ordinary skill in the art. Such embodiments are intended to be covered by the claims.

Claims: A double bulb type halogen lamp comprising: an outer bulb (13); an inner bulb (15), arranged in the outer bulb, thereby defining a sealed space within the outer bulb, the inner bulb having a filament therein for radiating light; and an optical interference film (27) arranged on at least one of the inner surface of the outer bulb and the outer surface of the inner bulb for reflecting radiation in a prescribed wavelength range of the electromagnetic spectrum, the optical interference film including a component liable to be reduced at elevated temperatures; wherein the sealed space contains an oxidising atmosphere to impede reduction of the optical interference film. A lamp according to claim 1, wherein the oxidising atmosphere includes a predetermined amount of inert gas at a sealed pressure of 40-203 kPa (300 - 1520 Torr). A lamp according to claim 1 or 2, wherein the oxidising atmosphere also includes a prescribed amount of oxygen having a partial pressure in the range of 1.01 Pa-10.1 kPa (0.0076 - 76 Torr). A lamp as claimed in claim 3, wherein the oxidising atmosphere includes oxygen having a partial pressure in the range of 5.33-440 Pa (0.04-3.3 Torr). A lamp as claimed in claim 4, wherein the oxidising atmosphere includes oxygen having a partial pressure in the range of 37-80 Pa (0.28-0.6 Torr). A lamp according to one of claims 2 to 5, wherein the inert gas includes argon. A lamp according to one of claims 2 to 6, wherein the inert gas includes nitrogen. A lamp according to any preceding claim, wherein the outer bulb includes a funnel shaped body (17) having a first edge of a first diameter and a second edge of a second diameter greater than the first diameter, and a lens element (19) integrally fixed on the second edge of the funnel shaped body. A lamp according to claim 8, wherein the outer bulb also includes an optical reflection film (23) on the inner wall of the funnel shaped body. A lamp according to one of claims 8 or 9, wherein the outer bulb includes means (24) for supporting the inner bulb arranged opposite to the optical reflection film. A lamp according to one of claims 8 to 10, wherein the outer bulb includes a cap (25) formed on the first edge of the funnel shaped body thereof. A lamp according to any preceding claim, wherein the optical interference film includes a first layer (27H) having a prescribed refractive index on one of the outer surface of the inner bulb and the inner surface of the outer bulb, and a second layer (27L), having a predetermined refractive index lower than that of the first layer, provided on the first layer, the first layer including a metal oxide substance, and the second layer including a second oxide substance different from the first metal oxide substance.

Inventors: SUWA TAKUMI, WATANABE TSUTOMU, SUWA, TAKUMI, WATANABE, TSUTOMU
Applicants: TOSHIBA LIGHTING & TECHNOLOGY, TOSHIBA LIGHTING & TECHNOLOGY CORPORATION
Application year: 1989
Main classification: H01K1
IPC codes: H01K1