This invention relates to a dielectric barrier discharge lamp.
Of the various low pressure discharge lamps known in the art, the majority are the so-called compact fluorescent lamps. These lamps have a gas fill which also contains small amounts of mercury. Since mercury is a highly poisonous substance, novel types of lamps are being recently developed. One promising candidate to replace mercury-filled fluorescent lamps is the so-called dielectric barrier discharge lamp (shortly DBD lamp). Besides eliminating the mercury, it also offers the advantages of long lifetime and negligible warm-up time and independence of ambient temperature. Concerning these latter two features, a DBD lamp is comparable to an incandescent lamp.
As explained in detail, for example, in U.S. Pat. No. 6,060,828, the operating principle of DBD lamps is based on a gas discharge in a noble gas (typically Xenon). The discharge is maintained through a pair of electrodes, of which at least one is covered with a dielectric layer. An AC voltage of a few kV with a frequency in the kHz range is applied to the electrode pair. Often, multiple electrodes with a first polarity are associated to a single electrode having the opposite polarity. During the discharge, excimers (excited molecules) are generated in the gas, and electromagnetic radiation is emitted when the meta-stable excimers dissolve. The electromagnetic radiation of the excimers is converted into visible light by suitable phosphors, in a physical process similar to that occurring in mercury-filled fluorescent lamps. This type of discharge is also referred to as dielectrically impeded discharge.
As mentioned above, DBD lamps must have at least one electrode set which is separated from the discharge gas by a dielectric. It is known to employ the wall of the discharge vessel itself as the dielectric. Various discharge vessel-electrode configurations have been proposed to satisfy this requirement. U.S. Pat. No. 5,994,849 discloses a planar configuration, where the wall of the discharge vessel acts as a dielectric. The electrodes with opposite polarities are positioned alternating to each other. The arrangement has the advantage that the discharge volume is not covered by electrodes from at least one side, but a large proportion of the energy used to establish the electric field between the electrodes is dissipated outside the discharge vessel. On the other hand, a planar lamp configuration can not be used in the majority of existing lamp sockets and lamp housings, which were designed for traditional incandescent bulbs.
In order to increase the efficiency, it has been proposed to put the electrodes within the discharge vessel, to lower the dissipation losses occurring outside the discharge vessel. U.S. Pat. Nos. 6,034,470 and 6,304,028 disclose two different DBD lamp configurations, where both set of electrodes are located within the discharge vessel, which confines the discharge gas atmosphere. The electrodes are covered with a thin layer of dielectric. However, none of these lamp configurations are suitable for a low-cost mass production, because the thin dielectric layers need an additional process step, and they are prone to premature aging, which quickly destroys their insulating properties.
U.S. Pat. No. 5,763,999 and U.S. patent application Publication No. US 2002/0067130 A1 disclose DBD light source configurations with an elongated and annular discharge vessel. The annular discharge vessel is essentially a double-walled cylindrical vessel, where the discharge volume is confined between two concentric cylinders having different diameters. A first set of electrodes is surrounded by the annular discharge vessel, so that the first set of electrodes is within the smaller cylinder, while a second set of electrodes is located on the external surface of the discharge vessel, i.e. on the outside of the larger cylinder.
This known arrangement has the advantage that none of the electrode sets need any particular insulation from the discharge volume, because the walls of the discharge vessel provide stable and reliable insulation. However, the external electrodes are visually unattractive, block a portion of the light, and also need to be insulated from external contact, due to the high voltage fed to them.
U.S. Pat. No. 6,246,171 B1 also discloses discharge vessel-electrode configurations where both the first and second sets of electrodes are located on the same side of a discharge vessel wall, similar to that proposed in U.S. Pat. No. 5,994,849. However, this configuration has the inherent disadvantage that the intensity of the electric field within the discharge volume is relatively small, and this negatively affects the efficiency of the lamp. On the contrary, the stray electric field (i.e. the field which is outside of the discharge volume, and hence useless for the purposes of the discharge) is relatively large. Therefore, U.S. Pat. No. 6,246,171 B1 also proposes to place the electrodes on two opposing surfaces of the discharge vessel, enclosing the discharge volume between the opposing surfaces, similarly to the solutions described above, albeit not for an annular discharge vessel but for a flat radiator. In this manner, a larger portion of the electric field will penetrate the discharge volume, and will contribute more effectively to the discharge. However, this arrangement again has the disadvantage that the electrodes will be visible from that side onto which they were applied.
Therefore, there is a need for a DBD lamp configuration with an improved discharge vessel-electrode configuration, which does not interfere with the aesthetic appearance of the lamp. There is also need for an improved discharge vessel-electrode configuration which ensures that the electric field within the discharge volume is homogenous and strong, and thereby effectively contributes to the barrier discharge. It is sought to provide a DBD lamp, which, beside having an improved electrode-discharge vessel arrangement, is relatively simple to manufacture, and which does not require expensive thin-film dielectric layer insulations of the electrodes and the associated complicated manufacturing facilities. Further, it is sought to provide a discharge vessel which readily supports electrode sets which are easy to apply directly onto the discharge vessel walls, but which will still have a reduced stray electric field.
In an embodiment of the present invention, there is provided a dielectric barrier discharge (DBD) lamp. The DBD lamp comprises a discharge vessel, which encloses a discharge volume filled with discharge gas. The discharge vessel further comprises a phosphor layer within the discharge volume. The discharge vessel comprises an outer tubular portion having an internal surface, and an inner tubular portion having an outward surface. The outer tubular portion surrounds the inner tubular portion. In this manner, a substantially annular discharge volume is enclosed between the internal surface of the outer tubular portion and the outward surface of the inner tubular portion. The inner tubular portion comprises a multitude of protrusions around its circumference. The protrusions extend into the substantially annular discharge volume. There is also provided a first set of interconnected electrodes and a second set of interconnected electrodes. The electrodes are isolated from the discharge volume by at least one dielectric layer, and at least one of the dielectric layers is constituted by the wall of the inner tubular portion.
According to another aspect of the invention, there is provided a discharge vessel for a DBD lamp. The discharge vessel encloses a sealed discharge volume, which may be filled with discharge gas. The discharge vessel comprises an outer tubular portion having an internal surface and an inner tubular portion having an outward surface. The outer tubular portion surrounds the inner tubular portion, so that a substantially annular discharge volume is enclosed between the internal surface of the outer tubular portion and the outward surface of the inner tubular portion. The inner tubular portion comprises a multitude of protrusions around its circumference. The protrusions extend into the substantially annular discharge volume.
The disclosed DBD lamp ensures that the electrodes also protrude into the discharge volume, so that the lines of force of the electric field will extend into the discharge volume, and the lamp will have a good efficiency. The electrodes may be located external to the discharge vessel, and yet do not cover the external surface of the lamp. Further, no sealed lead-through or any dielectric covering layer film for the electrodes is required. More importantly, the electrodes remain within the inner tube, being essentially unnoticeable, so the overall aesthetic appearance of the lamp is undisturbed. The lamp can provide a uniform and large illuminating surface.
The invention will be now described with reference to the enclosed drawings, where
Referring now to
The AC power source 7 is of a known type, which delivers an AC voltage of 1-5 kV with 50-200 kHz AC frequency, and need not be explained in more detail. The operation principles of power sources for DBD lamps are disclosed, for example, in U.S. Pat. No. 5,604,410. As shown in the embodiment of
It must be noted that the proposed DBD lamp need not include the AC power source, in case it is a so-called plug-in type lamp, where the essential electronic components (which may have a longer lifetime than the discharge tube itself) are included in a socket receiving a plug-in-type lamp base. Typically, the so-called electronic ballast needed for the start-up of the lamp is often separated from the lamp.
The internal structure of the discharge vessel 2 of the DBD lamp 1 is explained with reference to
As best seen in
The discharge vessel 2 is made of glass. The wall thickness dd of the inner tubular portion 9 is approx. 0.5 mm. As it will be explained below, the wall of the inner tubular portion 9 also plays a role as the dielectric in the dielectric barrier discharge. Therefore, it is desirable to use a relatively thin wall for the inner tubular portion 9. The inner tubular portion 9 of the discharge vessel 2 is corrugated, as will be shown in more detail below, and it may be manufactured with the help of a suitably shaped mould, into which a softened glass cylinder is pressed with the help of vacuum or overpressure.
In order to be able to manufacture the discharge vessel 2 with standard glass bulb manufacturing technology, the inner tubular portion 9 may also comprise an exhaust tube 10, such as shown in
In order to provide a visible light, the internal surface 15 and also the internal surface of the end portion 11 is covered with a phosphor layer 25. This phosphor layer 25 is within the sealed discharge volume 13. The efficiency of the lamp may be improved if also the outward surface 17 is covered with a phosphor layer, or, as shown in the
The dielectric barrier discharge (also termed as dielectrically impeded discharge) is generated by a first set of interconnected electrodes 16 and a second set of interconnected electrodes 18. The term “interconnected” indicates that the electrodes are on a common electric potential, i.e. they are connected with each other within a set.
The first set of the electrodes 16 and the second set of electrodes 18 are formed as elongated conductors. For example, these elongated conductors may be formed of metal stripes or metal bands, which extend substantially parallel to the principal axis of the inner tubular portion 9. Such electrodes may be applied onto the glass surface of the inner tubular portion 9 with any suitable method, such as tampon printing or by gluing thin foil strips onto the glass surface. However, the electrodes 16,18 may be formed of thin wires as well.
In the proposed discharge vessel design, the inner tubular portion 9 comprises a multitude of protrusions 20 around its circumference. The protrusions 20 extend into the substantially annular discharge volume 13. In the embodiment shown in FIGS. 2 to 4, the inner tubular portion 9 comprises a corrugated surface. The protrusions 20 are actually formed by a multitude of corrugations 21. As best seen in
As it is best perceived from
Due to the sinusoidal form, the protrusions 20, more properly the corrugations 21, have a convex surface 22 and a concave surface 23. The convex surface 22 turns towards the annular discharge volume 13, while the concave surface 23 turns towards the inside of the inner tubular portion 9. As best seen in the enlarged detail of
The smallest distance between the internal surface 15 of the outer tubular portion 8 and the outward surface 17 of the inner tubular portion 9 is approx. 5 mm (not considering the region around the ends 12), but in other embodiments it may vary, preferably between 3-11 mm. The “smallest distance” is meant as the average distance between the top of the protrusions 20 and the internal surface 15.
Every protrusion 20 supports an electrode alternating from the first set and the second set. In this manner, the electrodes 16 and 18 are distributed along the internal surface of the inner tubular portion 9 substantially uniformly and alternating with each other. In the shown embodiment, the distance De between two neighboring electrodes of opposite sets is approx. 3-5 mm. This distance is also termed as the discharge gap, and its value also influences the general parameters of the discharge process within the discharge vessel.
On the other hand, the electrodes 16 and 18 are isolated from the discharge volume 13 by the wall of the discharge vessel 2. More precisely, it is the wall of the inner tubular portion 9 which serves as the dielectric layer. As best seen in
As mentioned above, in a possible embodiment, the wall thickness dd of the discharge vessel 2 at the inner tubular portion 9 is approximately 0.5 mm. This thickness is a trade-off between the overall electric parameters of the lamp 1 and the mechanical properties of the discharge vessel 2.
As shown in
In the embodiment shown in
In all embodiments shown, it is preferred that the wall thickness of the inner tubular portion should be substantially constant, mostly from a manufacturing point of view.
A really effective increase in the electric field strength within the discharge volume 13 may be achieved if the height h of the protrusions is larger than the wall thickness d, as shown in
Finally, it must be noted that the parameters of the electric field and the efficiency of the dielectric barrier discharge within the discharge volume 13 also depend on a number of other factors, such as the excitation frequency, exciting signal shape, gas pressure and composition, etc. These factors are well known in the art, and do not form part of the present invention.
The invention is not limited to the shown and disclosed embodiments, but other elements, improvements and variations are also within the scope of the invention. For example, it is clear for those skilled in the art that a number of other forms of the protrusions may be suitable for the purposes of increasing the electric field and homogeneity. The general shape of the discharge vessel need not be strictly cylindrical, for example, a conical or frusto-conical design is also suitable. Even lamps more resembling a classical bulb form may be manufactured with the proposed discharge vessel design, as long as the inner tubular portion fits into the outer bulb at its narrower end. For example, it is not at all necessary that the outer tubular portion and the inner tubular portion have the same general form. The form of the discharge vessel may be any form that is feasible to manufacture, though it is preferred to keep the average “thickness” of the annular discharge volume—i.e. the distance between the inner and outer tubular portion—more or less constant. The exhaust tube of the discharge vessel may also have a different form and location, for example it may be located at the top of the outer tubular portion of the discharge vessel, and be cut off leaving only a short stub. Also, the shape and material of the electrodes may vary.