EP0981831B1 - Discharge lamp with dielectrically impeded electrodes - Google Patents

Abstract

A discharge lamp (1) having an electrically conducting screen (12, 13) which at least partially surrounds the discharge vessel (2). The electrodes (3-5) are separated from the interior of the discharge vessel (2) by a dielectric barrier (6-8). Moreover, this screen (12, 13) is electrically separated from the electrodes (3-5) by a dielectric (2). In order largely to prevent the electric power fed to the lamp electrodes (3-5) during operation from being capacitatively coupled to the electrically conducting screen (12, 13), the thickness dB and the dielectric constant ∈D of the dielectric (2), as well as the thickness dB and the dielectric constant ∈D of the barrier (6-8), which separates the electrodes (3-5) from the gas filling, are specifically mutually coordinated such that the following relationship is fulfilled:

Description

The invention is based on a discharge lamp according to the preamble of claim 1.

This discharge lamp has a discharge vessel including a gas filling, at least parts of the discharge vessel being used for radiation of a desired spectral range, in particular light, i.e. visible electromagnetic radiation, or also ultraviolet (UV) and vacuum ultraviolet (VUV) radiation are transparent. With a suitable electrical supply, a number of electrodes produce a discharge in the gas filling. The discharge either generates the desired radiation directly or the radiation emitted by the discharge is converted into the desired radiation with the aid of a phosphor.

In particular, it is a discharge lamp which is suitable for operation by means of dielectric barrier discharge. For this purpose, either the electrodes of one polarity or all electrodes, that is to say both polarities, are separated from the gas filling or from the discharge during operation by means of a dielectric layer (one-sided or two-sided dielectric barrier discharge, see for example WO 94/23442 or EP 0 363 832). The term “dielectric barrier” is also used for this dielectric layer and the term “barrier discharge” is also used for discharges produced in this way.

It must also be clarified that the dielectric barrier does not have to be a layer applied to an electrode specifically for this purpose, but can also be formed, for example, by a discharge vessel wall if electrodes are arranged on the outside of such a wall or inside the wall.

Presentation of the invention

It is an object of the present invention to provide a discharge lamp with reduced electromagnetic interference radiation (EMI).

This object is achieved in a lamp with the features of the preamble of claim 1 by the features of the characterizing part of claim 1. Particularly advantageous configurations can be found in the dependent claims.

The invention proposes that the discharge lamp comprise an electrically conductive shield which at least partially surrounds the discharge vessel. In addition, the shield is electrically isolated from at least one electrode, depending on the electrical potential, and possibly from all electrodes. In order to largely prevent the electrical power supplied to the lamp electrodes from being capacitively coupled to the electrically conductive shield during operation, the thickness d _D and the dielectric constant ε _{D of} the dielectric as well as the thickness d _B and the dielectric constant ε _{B of} the barrier, which are the electrodes separates from the gas filling, specifically coordinated so that the following relationships are fulfilled:
$\frac{{\text{d}}_{\text{D}}}{{\text{ε}}_{\text{D}}}$ ≥ F $\frac{{\text{d}}_{\text{B}}}{{\text{ε}}_{\text{B}}}$ and F ≥ 1.5, preferably F ≥ 2.0, particularly preferably F ≥ 2.5.

Below the lower limit, i.e. if the factor F is approximately 1.5, the electrical power is already coupled to the shield to an unacceptable degree. Reliable operation of the dielectric barrier discharge within the discharge vessel of the lamp is then no longer reliably guaranteed under all operating conditions.

In principle, the capacitive decoupling of the shield from the dielectric barrier discharge also increases with increasing factor F. In this respect, relatively high factors F are aimed for. In the event that the dielectric numbers of the dielectric and the barrier are approximately the same, high factors F mean a large ratio between the thicknesses of the dielectric and the barrier. In other words, the thickness of the dielectric must be correspondingly greater than the thickness of the barrier in this case. However, the thickness of the dielectric is limited for cost and design reasons. Consequently, there is only the possibility of reducing the thickness of the barrier, which in turn places high demands on the precision of the barrier in order not to negatively influence the uniformity of the dielectric barrier discharge. In a specific individual case, a suitable compromise may have to be made here.

If, however, the dielectric constant ε _{B of} the barrier is larger or even significantly larger than the dielectric constant ε _{D of} the dielectric, correspondingly high factors F can also be realized.

Numerous specific configurations are conceivable under the aforementioned premises.

In a particularly advantageous embodiment, the dielectric, which separates the shield from the electrodes, is formed by the wall of the discharge vessel itself. For this purpose, at least the electrodes with an electrical potential different from the shielding are targeted on the inner wall of the discharge vessel arranged. This procedure has the advantage, among other things, that the above-mentioned relationships can be easily fulfilled as long as ε _{B is} not chosen too small compared to ε _D , since the wall of the discharge vessel is generally thicker than the barrier of the electrodes for mechanical reasons.

On the other hand, the dielectric between the shield and the electrodes can also be constructed from two or more layers with different dielectric numbers. Under certain circumstances, this can be particularly useful in the area of the electrodes in order to be able to reliably meet the above-mentioned conditions there even with a relatively thin discharge vessel wall. In principle, the barrier can also be constructed from several layers with different dielectric numbers.

When using several layers, however, it must be taken into account that in the above inequality the two quotients by the sums respectively. · To be replaced, where d _Di , ε _Di , d _Bi, ε _Bi denote the thicknesses or dielectric constants of the respective layer i. The index i assumes the value 1 for a single-layer system, the values 1 and 2 for a two-layer system and accordingly the values 1, 2, ... n for an n-layer system.

It is also possible to arrange at least the electrodes with an electrical potential different from the shielding within the wall of the discharge vessel. In this case, the electrodes are arranged such that the layer of the vessel wall facing the interior of the discharge vessel is thinner than the layer facing the shield.

The shield is designed, for example, as a metallic jacket with an opening. The opening defines the effective radiation area of the lamp.

In a particularly advantageous variant, at least part of the jacket is further developed into cooling fins. As a result, the jacket performs a double function, namely, on the one hand, the shielding effect and, on the other hand, the dissipation of the heat loss generated by the discharge and / or possibly the electronics for operating the lamp. Since the lamp is expediently in particularly close contact with the jacket, a good homogenization of the temperature distribution along the contact zone between lamp and jacket is also ensured.

The shielding effect can be further improved if at least the part of the outer wall of the discharge vessel facing the jacket opening is covered by an electrically conductive, transparent layer, e.g. made of indium tin oxide (ITO). In addition, the jacket and the transparent layer are electrically contacted with one another.

Furthermore, the jacket can also be realized entirely by the electrically conductive, transparent layer. However, the cooling effect of the jacket must then be dispensed with in this variant.

The shield can be at floating electrical potential, but is advantageous with a ground, e.g. Earth, connected potential to prevent possible electromagnetic radiation from the shield itself.

Description of the drawings

In the following, the invention will be explained in more detail using an exemplary embodiment.

The figure shows a cross section of a rod-shaped aperture fluorescent lamp with shielding in a schematic representation.

It is an aperture fluorescent lamp 1 for OA (Office Automation) applications. The lamp 1 consists essentially of a tubular discharge vessel 2 with a circular cross-section, which is surrounded by a shield, and three strip-shaped electrodes 3-5, which are applied to the inner wall of the discharge vessel 2 parallel to the longitudinal axis of the tube. Each of the inner wall electrodes 3-5 is covered with a dielectric layer 6-8. Furthermore, with the exception of a rectangular aperture 9, the inner wall of the discharge vessel _{2 is} provided with a reflection double layer 10 made of Al ₂ O ₃ and TiO ₂ . A phosphor layer 11 is applied to the reflection double layer 10 and also to the inner wall of the vessel in the area of the aperture 9. The reflection double layer 10 reflects the light generated by the phosphor layer 11. In this way, the luminance of the aperture 9 is increased.

The outer diameter of the tubular discharge vessel 2 is approximately 9 mm. Xenon with a filling pressure of 21.3 kPa (160 torr) is located in the discharge vessel 2.

The electrodes 3-5 are passed gas-tight to the outside through a first end of the discharge vessel 2 and pass there into an external power supply (not shown). At its other end, the discharge vessel 2 is also sealed gas-tight with a dome (not shown) formed from the vessel.

A first 5 of the three electrodes 3-5 is provided for a first polarity of a supply voltage, while the other two electrodes 4, 5 are provided for the second polarity. The first electrode 5 is diametrical to the aperture 9 and the other two electrodes 4, 5 are in the immediate vicinity arranged on both long sides of the aperture 9. The width and the length of the aperture 5 are approximately 6.5 mm and 255 mm, respectively.

The barrier consists of glass solder with a dielectric constant of approx. 8 and a thickness of approx. 250 µm. This results in a quotient from the barrier thickness to the dielectric constant of approx. 0.031 mm.

The discharge vessel 2 consists of low-alkali soda-lime glass (Schott # 8350) with a dielectric constant of approx. 7 and a wall thickness of approx. 0.6 mm. This results in a quotient from wall thickness to dielectric constant of approx. 0.086 mm. This quotient is approx. 2.77 times larger than the corresponding quotient for the barrier. Consequently, the relationship required in the general description is fulfilled here.

The shielding of the lamp 1 consists of a solid, essentially cuboid, metallic jacket 12 and a transparent layer 13. The jacket 12 has an opening corresponding to the lamp aperture 9 in such a way that only the aperture 9 of the lamp is visible from the outside. The transparent layer 13 consists of indium tin oxide (ITO) and covers the outer wall of the discharge vessel 2 only in the area of the aperture 9. The transparent layer 13 is electrically contacted with the jacket 12 along its opening and therefore completes the shielding effect of the jacket 12 versus EMI. The jacket 12 has a number of cooling fins 14 on its side opposite the opening. A thermal paste 15 improves the heat transfer between the discharge vessel 2 and the jacket 12.

The phosphor layer 11 is a three-band phosphor. It consists of a mixture of the blue component BaMgAl ₁₀ O ₁₇ : Eu, the green component LaPO ₄ : Ce, Tb and the red component (Y, Gd) BO ₃ : Eu.

The resulting color coordinates are x = 0.395 and y = 0.383, i.e. the UV radiation generated by the discharge is converted into white light.

Claims (6)

1. Discharge lamp (1)

   • having a discharge vessel (2) which is at least partially transparent and filled with a gas filling,

   • a number of electrodes (3-5) which are arranged on or in walls of the discharge vessel (2),

   • and at least one dielectric barrier (6-8) made from one or more layers with

   - the thicknesses d_Bi and with

   - the dielectric constants ε_Bi

   between at least one electrode (3-5) and the gas filling, suitable for a dielectrically impeded discharge in the discharge vessel (2) between electrodes of different polarity,
   characterized by

   • an electrically conducting screen (12, 13) which surrounds the discharge vessel (2) at least partially,

   • at least one dielectric (2) made from one or more layers with

   - the thicknesses d_Di and with

   - the dielectric constants ε_Di,

   which at least one dielectric (2) electrically separates the screen (12, 13) from at least one electrode (3-5),

   • and the relationship
2. Discharge lamp according to Claim 1, in which the screen (12) is connected to a potential at frame, for example earth.
3. Discharge lamp according to Claim 1 or 2, in which the screen comprises a transparent layer (13) which is arranged at least on a subregion (9) of the outer wall of the discharge vessel (2).
4. Discharge lamp according to Claim 3, in which the transparent layer (13) consists of indium tin oxide (ITO).
5. Discharge lamp according to one of the preceding claims, in which the electrodes (3-5) are arranged on the inner wall of the discharge vessel (2), and in which the dielectric, which separates the screen (12, 13) from the electrodes (3-5), is formed by the wall of the discharge vessel (2).
6. Discharge lamp according to one of the preceding claims, in which at least a part of the screen (12) is further formed into cooling ribs (14).