Patent Publication Number: US-6661164-B2

Title: Incandescent lamp

Description:
BACKGROUND OF THE INVENTION 
     Such an incandescent lamp is disclosed, for example, in the European laid-open specification EP 0 986 093 A1. This specification describes an incandescent lamp whose lamp vessel has an interference filter coating with a locally differing layer thickness. The layer thickness of the interference filter varies in such a way that all regions of the lamp vessel which is coated with the interference filter emit light of the same color composition in the switched-on state of the incandescent lamp. The incandescent lamp is designed as an automobile signal lamp emitting orange or red light. 
     BRIEF SUMMARY OF THE INVENTION 
     It is the object of the invention to provide an incandescent lamp of the generic type having an improved interference filter for producing red light. An incandescent lamp may be made having a transparent, essentially rotationally symmetrical lamp vessel ( 20 ), an incandescent filament surrounded by the lamp vessel ( 20 ), and an interference filter ( 30 ) which is arranged on the lamp vessel ( 20 ) and designed as an edge filter. The interference filter ( 30 ) has layers of low optical refraction and high optical refraction for setting the edge of the interference filter ( 30 ) in the red spectral region. The layer thicknesses of the layers of low optical refraction and high optical refraction differ locally as a function of the angle of incidence of the light emitted by the incandescent filament and impinging on the interference filter, and the interference filter ( 30 ) has absorber layers for absorbing blue and violet light. The interference filter further has at least two of these absorber layers with, in each case, an intermediate layer of low optical refraction arranged therebetween, and additional layers of low optical refraction and high optical refraction for further suppressing light from the violet and blue spectral regions. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 shows a side view of an incandescent lamp in accordance with the preferred exemplary embodiment of the invention, 
     FIG. 2 shows an enlarged detail of the lamp vessel of the incandescent lamp illustrated in FIG. 1, in a sectional, schematic illustration, and 
     FIG. 3 shows transmission curves of the interference filter and the individual stacks of the interference filter of the incandescent lamp in accordance with the preferred exemplary embodiment. 
     FIGS. 4-8 show the layered coatings. 
     FIG. 9 shows a side view of an incandescent lamp with a coating having an exaggerated thickness variation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The incandescent lamp according to the invention is fitted with a transparent, essentially rotationally symmetrical lamp vessel, an incandescent filament surrounded by the lamp vessel, and an interference filter which is arranged on the lamp vessel and designed as an edge filter, the interference filter having layers of low optical refraction and of high optical refraction for setting the edge of the interference filter in the red spectral region. The layer thicknesses of the layers of low optical refraction and high optical refraction differ locally as a function of the angle of incidence of the light emitted by the incandescent filament and impinging on the interference filter. According to the invention, the interference filter also has at least two absorber layers with, in each case, an intermediate layer of low optical refraction arranged therebetween for absorbing blue and violet light, as well as additional layers of low optical refraction and high optical refraction for further suppressing light from the violet and blue spectral regions. These measures ensure that the incandescent lamp according to the invention emits essentially red light and is suitable for use as a stop light lamp or tail light lamp of an automobile. 
     The interference filter advantageously comprises at least four stacks of layers, the first stack being arranged directly on the lamp vessel and including the at least two absorber layers with in each case an intermediate layer of low optical refraction arranged there between for absorbing blue and violet light, and at least one of the subsequent stacks including the additional layers of low optical refraction and high optical refraction, the layer thicknesses thereof being optimized in such a way that this at least one stack has a low transmission for light from the violet and blue spectral regions and a high transmission for light from the red spectral region, and the other stacks including the layers of low optical refraction and high optical refraction for setting the edge of the interference filter in the red spectral region. The layer thicknesses of the layers of low optical refraction and high optical refraction in these stacks are optimized in such a way that the edge of the interference filter is situated in the wavelength region from 580 nm to 600 nm. In this way, an interference filter with comparatively few layers can be produced which has in the wavelength region from 580 nm to 600 nm a steep transition from the spectral region of low transmission to the spectral region of high transmission. 
     The first stack advantageously includes at least two absorber layers made from iron oxide Fe 2 O 3  with in each case a layer of low optical refraction arranged therebetween. Iron oxide is a material with a comparatively high index of optical refraction. Given a sufficiently thin layer thickness, the iron oxide layers have metallic properties in the violet and blue spectral regions and dielectric properties in the red spectral region. Given the respective intermediate layer of low optical refraction, it is possible by adapting and optimizing its layer thickness to make use of the interference effect in combination with the iron oxide layers of high optical refraction in order to achieve a high transmission of the first stack for light from the red spectral region, and a high reflection of the first stack for light from the blue spectral region. 
     The preferred exemplary embodiment of the invention concerns an incandescent lamp with an electric power consumption of approximately 25 W, which can be used, for example, as a light source in the tail lamp for producing the tail light or stop light. This incandescent lamp has a bayonet-type lamp base  10  and a pear-shaped glass lamp vessel  20  which is rotationally symmetrical about the lamp axis A—A and surrounds an incandescent filament (not illustrated). The outer surface of the lamp vessel  20  is coated with an interference filter  30  which has a high transmission for red light and is virtually opaque to light of other spectral regions. The layer thickness of the interference filter  30  varies locally as a function of the angle of incidence of the light emitted by the incandescent filament and impinging on the interference filter  30 . The interference filter  30  has the least layer thickness  100  on the crest  102  of the lamp vessel  20  and the greatest layer thickness  104  in the vicinity of the base. The layer thickness of the interference filter  30  increases continuously from the crest  102  to the base  10 . The difference between the least  100  and the greatest layer thickness  104  is approximately 7 percent. The layer thickness of the interference filter  30  is constant along concentric rings about the lamp axis A—A. FIG. 9 shows a side view of an incandescent lamp with a coating having an exaggerated thickness variation. The interference filter  30  comprises a total of  28  layers which are arranged in five stacks  31 - 35 . 
     The first stack  31 , FIG. 4, which is applied directly on the lamp vessel  20 , comprises a first absorber layer made from Fe 2 O 3  with a physical layer thickness of approximately 8 nm, and a second absorber layer made from Fe 2 O 3  with a physical layer thickness of approximately 14 nm, as well as an intermediate layer, made from SiO 2 , of low optical refraction which is arranged between the two absorber layers and has a physical layer thickness of approximately 87 nm. The transmission response of the first stack  31  is illustrated in FIG. 3 as a function of the optical wavelength by the curve  1 . 
     The second stack  32 , FIG. 5, is formed from a layer sequence which is repeated once and comprises a layer of high optical refraction made from TiO 2  with a physical layer thickness of approximately 12 nm, a layer of low optical refraction made from SiO 2  with a physical layer thickness of approximately 40 nm, and a layer of high optical refraction made from TiO 2  with a physical layer thickness of 25 nm. The second stack  32  is optional. It brings about an additional reduction in the transmission of the interference filter  30  in the violet spectral region. Its transmission response is not illustrated in FIG.  3 . 
     The third layer  33 , FIG. 6, is formed by a layer sequence which is repeated twice and comprises a layer of high optical refraction made from TiO 2  with a physical layer thickness of approximately 14 nm, a layer of low optical refraction made from SiO 2  with a physical layer thickness of 77 nm, and a layer of high optical refraction made from TiO 2  with a physical layer thickness of approximately 14 nm. This third stack  33  has a low transmission for light from the violet and blue spectral regions, and a high transmission for light from the red spectral region. In addition to the absorption filter it serves the purpose of additionally suppressing violet and blue light. The transmission response of the third stack  33  is illustrated in FIG. 3 as a function of the optical wavelength by the curve  2 . 
     The fourth stack  34 , FIG. 7, is formed by a layer sequence which is repeated twice and comprises a layer of high optical refraction made from TiO 2  with a physical layer thickness of approximately 24 nm, a layer of low optical refraction made from SiO 2  with a physical layer thickness of 79 nm, and a layer of high optical refraction made from TiO 2  with a physical layer thickness of 24 nm. The curve  3  in FIG. 3 shows the transmission response of the fourth stack  34  as a function of the optical wavelength. 
     The fifth stack  35 , FIG. 8, is formed from a layer sequence which is repeated three times and comprises a layer of high optical refraction made from TiO 2  with a physical layer thickness of approximately 25 nm, a layer of low optical refraction made from SiO 2  with a physical layer thickness of 86 nm, and a layer of high optical refraction made from TiO 2  with a physical layer thickness of 24 nm. The curve  4  in FIG. 3 shows the transmission response of the fifth stack  35  as a function of the optical wavelength. All data on layer thickness relate to the crest of the lamp vessel  20 . 
     The fourth stack  34  and fifth stack  35  serve to set the edge of the interference filter  30  at approximately 590 nm. The layer thicknesses of the SiO 2  and TiO 2  layers of these two stacks are optimized in such a way that the interference filter  30  has a steep transition from the short-wave spectral region of low transmission to the long-wave spectral region of high transmission in the case of an optical wavelength of approximately 590 nm. The transmission response of the overall interference filter  30  is illustrated in FIG. 3 as a function of the optical wavelength by the curve  5 . The five stacks  31 - 35  follow one another seamlessly. The interference filter  30  therefore has  28  layers.