US2931933A - Incandescent lamp - Google Patents

Description

April 5, 1960 c. F. ROBINSON 2,931,933

INCANDESCENT LAMP Filed March 28, 1958 INVENTOR. CHAR! 5.5 f RAB/N60 g Jae/2% United States Patent ()fifice INCANDESCENT LAMP Charles F. Robinson, Pasadena, Calif., assignor to Bell & Howell Company, Chicago, 111., a corporation of Illinois Application March 28, 1958, Serial No. 724,693 4 Claims. (Cl. 313-113) The present invention relates to lamps and more particularly to an incandescent lamp.

The surface brightness of an optical radiating source such as an incandescent lamp is determined largely by the temperature of the filament of the lamp and is comparatively independent of the nature of the material used in the filament. In order to obtain an incandescent lamp which will operate as a high-intensity source of optical radiation, it is necessary to utilize a material as the filament thereof which can be operated at high temperatures. The material generally used for such filaments is tungsten because of its high-melting point (3,370 centigrade). A filament of tungsten may be operated up to a temperature of approximately 2700 centigrade as a simple self-heated helix. When such a tungsten filament is operated for a considerable time at such high temperatures, failure generally occurs as a result of evaporation of the tungsten reducing the cross-section at some point in the helix to such an extent that excessive heating occurs at that point which results in a burn-out or breaking of the helix. If it is attempted to circumvent this difliculty by using helices of thicker cross-section, failure of thelamp then occurs primarily by the deposition of the evaporated material from the filament onto the window of the lamp envelope which reduces the transparency of the window to such an extent that the intensity of the light transmitted through the window becomes too low to be useful.

Certain other materials such as tantalum carbide, hafnium carbide and zirconium carbide have appreciably higher melting points than tungsten and can also be utilized as the filaments for incandescent lamps to obtain a higher surface brightness than can be achieved with a tungsten filament. However, the boiling points of these materials are lower than that of tungsten so that the useful life of a lamp employing such materials for a filament is limited to a great extent by evaporation of the material from the filament onto the window of the lamp envelope.

In accordance with the present invention an incandescent lamp for use as an optical radiating source is provided which overcomes the above disadvantages by preventing direct evaporation of the material from the filament of the lamp onto the window or transparent portion of the lamp envelope. A lamp in accordance with the present invention includes a filament composed of a reasonable specular metal or other specular material disposed within an envelope. A portion of the envelope is transparent to light and functions as a window. Reflecting means are secured to the envelope for reflecting the light emitted from the filament to the transparent portion or window of the envelope and a shield means is disposed within the envelope between the filament and the transparent portion thereof for preventing direct evaporation of the material in the filament onto the transparent portion. Since the filament is composed of a reasonably good specular material the deposition of it onto the reflecting means has little effect on the operation of 2,931,933 iatented Apr. 5, 1960 the lamp since it in turn acts as a reflector when so deposited. By preventing appreciable amounts of the material that is evaporated from the filament from depositing onto the transparent portion or window of the envelope the life of the lamp is greatly increased. There is no intrinsic limit on the amount of material which may be evaporated from the filament of such a lamp without reducing the apparent brightness of the lamp.

The invention is described in more detail in reference to the accompanying drawing which is a sectional elevation of one embodiment of a lamp employing the principles of this invention.

Referring now to the drawing, there is shown an'incandescent lamp which includes an envelope 10 having a transparent po'rtion or window 11 at one side thereof. The envelope It? may be evacuated or filled with an inert gas as'is well known in the art. A filament 12 is disposed within the envelope adjacent the window 11. A pair of conductors 13 and 14- are connected to the filament 12 at one end and extend through a base portion 15 of the envelope 10. The conductors 13 and 14 are hermetically sealed to the base portion 15 in a well known manner.- To energize the filament 12 a suitable source of energizing potential (not shown) may be connected to the ends of the conductors that are disposed outside of the envelope 10. A light reflecting coating 16 is disposed on the inner surface of the top portion of the envelope 1!) which is diametrically opposed to the window 11. The coating 16 and the portion of the envelope on which it is disposed are arranged to reflect the light emitted from the filament 12 onto the window 11 or to project the image of the filament 12 onto the window 11 as may be seen in the drawing, the image of the filament being designated by the reference numeral 12 A shield plate or baffle means 17 which is formed integrally with the envelope 10 is disposed between the filament 12 and the Window 11 to block the line of sight between these two elements of the lamp so that material will not be evaporated from the filament 12 directly onto the window 11.

The filament 12 should be composed of a reasonably good specular metal or other material so that the material that is evaporated from the filament and deposited onto the coating 16 during the operation of the lamp will itself act as a reflector. If the lamp is to be operated at high temperatures to provide a high surface brightness of the filament 12, the filament should also be composed of. a material that has a relatively high melting point. There are many materials that fulfill both criteria, i.e., tungsten, tungsten carbide, tantalum, tantalum carbide, hafnium carbide, zirconium carbide and rhenium. Where tungsten or tungsten carbide is utilized as a filament it is necessary to prevent anyappreciable amount of oxvgen from coming into contact with the filament by maintaining a very high vacuum or an atmosphere of inert gas within the envelope because tungsten readily oxidizes and the oxide of tungsten does not depo'sit to form a good reflector. Preferably the filament is formed of rhenium which has a melting point slightly lower than the melting point of tungsten and is a very good specular metal.

There has thus been disclosed an incandescent lamp which may be operated at very high temperatures for long periods of time without noticeable change in the apparent brightness thereof.

I claim:

1. An incandescent lamp comprising an evacuated envelope having a transparent portion in a first side thereof, a filament composed of a high melting point specular material disposed within the envelope adjacent the transparent portion, a light reflecting coating disposed on a portion of the inner surface of asecond side of the envelope that is diametrically opposed to the first side, the

light reflecting coating being arranged to reflect light emitted from the filament to the transparent portion, and a plate secured to the envelope adjacent the transparent portion thereof and extending into the interior of the envelope between the filament and the transparent portion for preventing material evaporated from the filament from depositing directly onto the transparent portion, the light reflecting coating being exposed directly to the filament whereby material evaporated from the filament during operation of the lamp will be deposited onto the light reflecting coating, the material from the filament deposited on the coating being arranged to act as a refiecting coating.

2. An incandescent lamp as defined in claim 1 wherein the filament is composed of rhenium.

3. An incandescent lamp as defined in claim 1 wherein the plate is formed integrally with the envelope.

4 4. An incandescent lamp as defined in claim 1 wherein the filament iscornposed of a material chosen from the group of tungsten carbide, tantalum, tantalum carbide, hafnium carbide and zirconium carbide.

References Cited in the file of this patent UNTTED STATES PATENTS 2,494,916 Van Horn Jan. 17, 1950 2,592,102 Alexander Apr. 8, .1952

2,699,515 Williams Jan. 11, 1955 2,822,495 Albright Feb. 4, 1 958 FOREIGN PATENTS 762,566 Germany a Feb. 22, 1954

Images