Electric lamp and discharge devices – With optical device or special ray transmissive envelope – Polarizer or special ray transmission
Reexamination Certificate
2001-06-13
2003-10-28
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With optical device or special ray transmissive envelope
Polarizer or special ray transmission
C313S113000, C313S114000
Reexamination Certificate
active
06639342
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric lamp employing a light-transmitting lamp vessel in which a light source is arranged, the electric lamp employing a light-absorbing medium exhibiting a spectral transition in the visible range, where at least a part of the lamp vessel being provided with an interference film.
Such lamps are used in automotive applications, for example as a (halogen) headlamp which, in operation, emits yellow light, as an amber-colored light source in indicators (also referred to as vehicle signal lamps) or as a red-colored light source in brake lights. Such electric lamps are also used for general illumination purposes. The electric lamps are further used in traffic and direction signs, contour illumination, traffic lights, projection illumination and fiber optics illumination. Alternative embodiments of such lamps employ lamps wherein the color temperature is increased by a suitable combination of a light-absorbing coating and an interference film.
2. Description of the Related Art
In a known electric lamp, an interference film reflecting blue light is provided on the lamp vessel of a (halogen) headlamp, which lamp, in operation, emits yellow light. Besides, a light-absorbing medium for absorbing the blue light reflected by the interference film is provided on an outer surface of the lamp vessel, between the lamp vessel and the interference film.
A drawback of the known lamps is that the appearance of the lamp is insufficiently color-neutral.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an electric lamp to obviate the aforementioned drawback with the prior art.
This object is achieved, in accordance with the invention, in that the light-absorbing medium exhibits a comparatively steep spectral transition in the visible range.
To achieve this, the electric lamp in accordance with the present invention comprises a light-absorbing medium wherein a spectral transmission T of light transmitted by the light-absorbing medium changes from T≦0.15 to T≧0.75 in a wavelength range having a width &lgr;≦75 nm.
The application of a light-absorbing medium having such a comparatively steep spectral characteristic in an electric lamp whose lamp vessel is provided with an interference film results in an electric lamp having an improved color-neutral appearance. In the known electric lamp, a light-absorbing medium, such as Fe
2
O
3
, is used whose spectral characteristic gradually changes over a comparatively large wavelength range. As a result, the spectral transmission of the combination of interference film and light-absorbing medium is subject to change over a comparatively large range of the visible region, which is accompanied by color effects, which are generally undesirable. The known electric lamp emits yellow light in operation, but in the off state, it is blue in appearance.
By applying, in accordance with the invention, a light-absorbing medium having a comparatively steep spectral characteristic, the spectral changes in the visible region are limited to a comparatively small wavelength range, which is favorable for obtaining a color-neutral appearance of the electric lamp. In operation, the electric lamp in accordance with the invention emits light of the desired color. In the offstate, the electric lamp in accordance with the invention is color-neutral in appearance.
Preferably, in the above-mentioned wavelength range, the spectral emission of the light transmitted by the light-absorbing medium changes from T≦0.10 to T≧0.80. Preferably, there is a change in the spectral transmission T of light transmitted by the light-absorbing medium in a wavelength range having a width &lgr;≦50 nm. The color-neutral appearance of the electric lamp is better as the spectral transition in the light-absorbing medium is steeper. It is particularly favorable for the change in spectral emission in the light-absorbing medium to occur in a wavelength range from 30 to 40 nm.
In a preferred embodiment, the electric lamp is provided with an interference film having an at least substantially flat reflection spectrum over at least substantially the entire visible region. To achieve this, the electric lamp in accordance with the invention is characterized in that the variation in the reflection R of the interference film in the wavelength range from 400≦&lgr;≦690 nm is less than 10%.
In this preferred embodiment, within the given variation range, the reflection spectrum exhibits no negligible peaks or dips in the relevant range of the visible spectrum. As the reflection by the interference film is uniform throughout the visible region, the reflection is at least substantially independent of the wavelength in the relevant part of the visible region. The interference film in accordance with the invention will reflect all colors in the visible spectrum in the same manner and provides the electric lamp with a color-neutral appearance.
In the known electric lamp, a combination of a light-absorbing medium and an interference film is applied, said interference film comprising a so-called step filter. The term “step filter” is to be taken to mean in the description of the current invention that the reflection spectrum exhibits a comparatively sharp spectral transition (from R≈100% to R≦10%) in a comparatively narrow wavelength range (≦20 nm). The positioning of the spectral transition of this step filter is very sensitive to process variations. Small variations readily lead to a shift of the spectral transition, as a result of which the known electric lamp no longer meets legal requirements. In the known electric lamp, it is further necessary to apply an interference film having a comparatively high reflection in the relevant spectral range, thus rendering necessary a stack of comparatively many optical layers. These high reflection values are necessary to sufficiently enhance the comparatively small effect of the comparatively thin light-absorbing medium. In addition, the optical layers in the interference film of the known electric lamp must be at least substantially absorption-free in order to realize the high reflection values of the step filter. In the known electric lamp, the spectral transition of the step filter lies in a wavelength range from approximately 530 to approximately 540 nm.
It is to be noted further that the visible region globally comprises the wavelength range from 380≦&lgr;≦780 nm. Taking into account the sensitivity of the human eye and the fact that the eye sensitivity curve decreases rapidly at the edges of the visible region, it is sufficient, in practice, if the reflection spectrum in the electric lamp in accordance with the invention is at least substantially flat in the wavelength range from 400≦&lgr;≦690 nm. In a preferred embodiment of the electric lamp, the variation of the reflection R of the interference film in the wavelength range from 380≦&lgr;≦780 nm is less than 10%. Experiments have shown that a 5-10% variation in the reflection of the interference film can be readily achieved throughout the visible region.
A preferred embodiment of the electric lamp in accordance with the invention is characterized in that the reflection R of the interference film lies in the range from 0.50≦R≦0.90. In this preferred embodiment, the interference film has a metallic or silvery appearance. As a result thereof, the electric lamp in accordance with the invention can very suitably be used as an indicator lamp for automotive applications. Statutory regulations define a range, in the 1931 C.I.E. color triangle known to those skilled in the art, for the color point of the light emitted by such indicator lamps. A suitable combination of a light-absorbing medium and an interference film applied to an outside surface of the lamp vessel enables the appearance of the electric lamp to be changed. This particularly enables a distinction to be made between the appearance of the electric lamp in the off state and the color of the light emitted by the
Boonekamp Erik Paul
Doetter Wolfgang
Peek Johannes Adrianus Frederikus
Koninklijke Philips Electronics , N.V.
Patel Ashok
Phinney Jason
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