Electric lamp and discharge devices – With gas or vapor – Envelope with particular structure
Reexamination Certificate
1999-06-22
2002-03-12
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Envelope with particular structure
C313S112000, C313S580000, C359S588000
Reexamination Certificate
active
06356020
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an electric lamp comprising a light-transmitting lamp vessel which accommodates a light source, at least a part of the lamp vessel being provided with an interference film, the interference film including layers of alternately a first layer which is made predominantly of silicon oxide and a second layer which is made of a material having a refractive index which is high as compared to a refractive index of silicon oxide which is 1.45.
An electric lamp of this type is known from U.S. Pat. No. 5,138,219. In the known discharge lamp, the layers having a high refractive index are made of tantalum oxide. These layers may alternatively consist of niobium oxide or titanium oxide. The layers may also be composed of a combination of two layers, for example TiO
2
.ZrO
2
, TiO
2
.HfO
2
, TiO
2
.Nb
2
O
5
, TiO
2
.Ta
2
O
5
or Ta
2
O
5
. 2TiO
2
.
It is a drawback of lamps having an interference film on the lamp vessel that the film is exposed to substantially varying temperatures (above 500° C.). As a result, diffuse scattering of the interference film during the service life of the electric lamp increases. Diffuse scattering leads to an unclear transparent lamp. Diffuse scattering additionally causes the direction in which (for example infrared) radiation is reflected by the interference film to be changed relative to the desired direction, thus causing the efficacy of the lamp to be reduced.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an electric lamp, in which diffuse scattering of the interference film is reduced.
In accordance with a first aspect of the invention, one of the second layers comprises an odd number of sub-layers of alternately a first sub-layer of the material with the high refractive index (over 1.45) and a second sub-layer of a further material with a high refractive index (over 1.45).
During the service life of the electric lamp, the lamp vessel of which is provided with an optical interference film, crystallization occurs of, in particular, the (high-refraction) material of the second layer of the interference film. In addition, as a result of temperature changes, the crystallization state of the material of the second layer may change. Crystallization is regarded as one of the causes of diffuse scattering occurring during the service life of a lamp provided with a lamp vessel covered with an interference film. It has been found that the crystals can grow bigger as the thickness of the layer of the high-refraction material is larger. The effect on diffuse scattering increases as the crystals become bigger. If, in accordance with a first aspect of the invention, the (relatively thick) second layers of the high-refraction material in the interference film are built up of a stack of an odd number of sub-layers, whereby between two first sub-layers of the material with the high refractive index a second sub-layer of a further material with a further high refractive index is sandwiched, the possibilities of crystal growth in the second layer are effectively reduced by 50%. By virtue of the measure in accordance with the invention, crystals developing in the odd sub-layers can only grow within said sub-layer and will consequently remain relatively small, so that the effect of such crystals on diffuse scattering is reduced. By sandwiching a (second) sub-layer of the further material with the further high refractive index between two (first) sub-layers of the material with the high refractive index, the relatively thick layer of the material with the high refractive index is interrupted as it were, which has a favorable effect on the reduction of undesirable diffuse scattering.
Preferably, the further material of the second sub-layer is selected from the group formed by tantalum oxide, zirconium oxide, hafnium oxide and combinations of these materials. Particularly the high-refraction optical material tantalum oxide exhibits favorable properties at higher temperatures. It is further desirable that the refractive index of the further material forming the second sub-layer should correspond at least substantially to the refractive index of the material of the first sub-layer. If so, the spectrum of the interference film in accordance with the invention hardly differs from that of the known interference film. A suitable choice of the material of the second sub-layer (for example a combination of two high-refraction materials) enables the refractive indices of the materials of the first and the second sub-layer to be properly matched. Suitable combinations of high-refraction materials are TiO
2
.ZrO
2
, TiO
2
.HfO
2
and TiO
2
.Nb
2
O
5
. A further suitable combination of high-refraction materials is TiO
2
.Nb
2
O
5
. In a very favorable, alternative embodiment of the electric lamp in accordance with a first aspect of the invention, the further material of the second sub-layer includes a combination of titanium oxide and tantalum oxide. Such a combination of materials (for example TiO
2
.Ta
2
O
5
or Ta
2
O
5
.2TiO
2
) unites the favorable property that the refractive index of titanium oxide is relatively high relative to that of silicon oxide with the favorable behavior of tantalum oxide at relatively high temperatures.
Preferably, the stack of an odd number of alternately a first and a second sub-layer comprises three sub-layers, namely two (first) sub-layers of the high-refraction material between which a (second) sub-layer of the further material with the further high refractive index is sandwiched.
In accordance with a second aspect of the invention, one of the second layers comprises an odd number of sub-layers of alternately a first sub-layer of the material with the high refractive index and a second sub-layer of silicon oxide, the optical layer thickness d
op
of the second sub-layer lying in the range 1≦d
op
≦20 nm.
By incorporating such a second sub-layer of silicon oxide having a relatively low refractive index, the optical layer thickness of which is chosen so that these second sub-layers contribute little, or not at all, to the optical effect of the interference film, the possibilities of crystal growth in the second (high-refraction) layer are effectively reduced by 50%. By virtue of the measure in accordance with the invention, crystals which develop in the odd sub-layers can grow only within said sub-layer and, as a result, will remain relatively small, so that the effect of such crystals on diffuse scattering is reduced. By sandwiching a (second) sub-layer of silicon oxide between two (first) sub-layers of the material with the high refractive index, the relatively thick layer of the material with the high refractive index is interrupted as it were, which has a favorable effect on the reduction of undesirable diffuse scattering.
An expression which in connection with the term optical layer thickness is known to those skilled in the art is QWOT (=“Quarter Wave Optical Thickness”), which is defined as the wavelength at which the optical thickness of a layer is equal to a quarter (0.25) of the design wavelength of the stack of the interference film, that is:
QWOT=4nd
ph
cos &agr;
where n×d
ph
is the product of the (complex) refractive index n and the physical layer thickness d
ph
, and &agr; is the angle at which the light is incident on the interference film (if light is incident transversely to the stack of layers, then &agr;=0°). As a result of the relative simplicity of stacks of such so-called “quarter-wave” optical layer thicknesses, designs of interference films are often referred to in terms of fractions of “quarter-waves” at a reference wavelength.
Preferably, the optical layer thickness of the second sub-layer is smaller than or equal to 10 nm (d
op
≦10 nm). Such relatively thin sub-layers with such a small optical layer thickness practically have no effect on the spectral characteristic of the interference film. An existing design of an interference film does not have to be adapted if the optical layer thickness of the second sub-layer is
Halajian Dicran
Patel Nimeshkumar D.
Quarterman Kevin
U.S. Philips Corporation
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