Tubular discharge lamp and method of preparing such a lamp

FIELD OF THE INVENTION

The invention relates to a tubular gas discharge lamp and a method of preparing such a lamp.

BACKGROUND OF THE INVENTION

Tubular gas discharge lamps, generally known as TL lamps, are used in various environments. Typically, the lamp comprises a translucent glass discharge vessel filled with an inert gas and mercury vapor that emits UV radiation upon discharge. The UV light is transformed into visible light by fluorescent compounds located in a luminescent layer deposited on the inner surface of the tube. Usually, the tubular light is a straight tube, but alternative forms such as circular tubes are also known.

In order to increase the amount of light at a desired location, the holder of the lamp usually contains a reflector, for instance, directed downwards in order to increase the percentage of light on a floor or desk. Recently, TL lamps have been developed that contain a built-in reflector. An additional light-reflecting layer is added between the luminescent layer and the glass surface of the lamp, which layer covers 30° to 270° of the arc of the tube, leaving a portion open. As a result, for a lamp with a reflector covering 200°, only 20% of the light is radiated through the reflector-coated portion, and 80% is radiated through the window which is only covered by the luminescent coating, without the reflecting layer. The reflector is typically made from aluminum oxide particles. Lamps with integrated reflectors and typical manufacturing processes are known from, for instance, EP 0,270,866 and EP 0,639,852.

Known TL lamps with an integrated reflector have the drawback that, without close inspection, the TL lamps with a built-in reflector cannot be distinguished from other TL lamps without a reflector. Thus, without a package indicating the type of TL lamp, it is difficult to distinguish the lamps without turning them on.

Another drawback is that the symmetrical connectors of most TL lamps may cause an unskilled person to accidentally place the lamp with a reflector directed in the

wrong direction. The difference is only noticeable when the lamp is turned on, when compared with a lamp placed in the correct orientation.

It is an object of the invention to provide a TL lamp with an integrated reflector that aims to overcome the problems indicated above.

SUMMARY OF THE INVENTION

The invention provides a tubular gas discharge lamp comprising a tubular glass discharge vessel, wherein an inner surface of the discharge vessel is provided with a luminescent layer, and a reflector layer covering part of the inner surface, wherein the discharge vessel is provided with visible identification means for locating the reflector from the outside of the vessel. Thus, it is easy to distinguish the TL lamp according to the invention from TL lamps without a reflector. Furthermore, it is easy to see the location of the reflector, ensuring that the lamp according to the invention can be placed in a lamp holder with the reflector directed in a desired direction. The glass discharge vessel and luminescent layer may have any suitable composition known in the art. The visible identification means may be used to indicate the whole of the reflector, but it is also possible to only indicate in general terms the location of the reflector and/or the window not covered by the reflector.

The visible identification means may be accomplished, for instance, by adding a suitable distinguishing color pigment to the reflector layer, with a color intensity which is sufficient to see the different color through the translucent glass discharge vessel. However, a colored reflector has the disadvantage that it will also absorb some light, which decreases the efficiency of the lamp and influences the emitted light color.

In a preferred embodiment, at least part of the visible identification means is located on the outer surface of the discharge vessel. This may be, for instance, a visual indication of the position of the reflector applied to the outer surface of the discharge vessel, for instance, a sticker, by printing or other deposition methods. Identification means on the outside of the vessel have the drawback that they are susceptible to wear.

It is preferred if at least part of the visible identification means is located on the inner surface of the discharge vessel. Such identification means are visible through the translucent glass of the discharge vessel, and are not susceptible to wear. Furthermore, colored layers seen through the translucent glass are perceived by users as having a metallic shine, which is easily associated with a reflector.

At least part of the identification means is preferably located between the inner surface of the discharge vessel and the reflector layer. At such a location, the identification

means have a very good visibility from the outside and a minimum effect on the light output of the lamp.

It is preferred if the identification means line the boundaries of the inner surface covered by the reflector layer. The location of the reflector layer can thus be precisely known to a user.

It is advantageous if the identification means cover the part of the inner surface covered by the reflector layer. This makes it possible to very easily establish the location of the reflector in the lamp.

In a preferred embodiment, the identification means comprise a colored layer provided with a pigment contrasting with the color of the luminescent layer. Highly visible identification means are thus obtained.

The pigment is preferably an inorganic pigment. Inorganic pigments are most stable during production and use of the lamp. In particular, inorganic pigments which are capable of withstanding elevated temperatures are useful, as most organic pigments are degraded at temperatures above 220 ⁰ C.

The pigment preferably comprises at least one colored metal oxide. Colored metal oxides are capable of withstanding the oxidative conditions that occur in lamps during manufacture and use. Colored metal oxides are particularly stable within matrix oxide materials which are commonly used for layers in TL lamps, in particular aluminum oxide. In a preferred embodiment, the colored metal oxide is selected from the group of cobalt aluminum oxide, cobalt chromium oxide, cobalt iron oxide, iron (II) titanium oxide and iron oxide. These colored metal oxides have an excellent stability as well as a relatively high color intensity, so only a relatively small percentage (<10% by weight) of pigment needs to be used to obtain a good color contrast between the white luminescent layer and a colored layer comprising the pigment. Various pigments may be mixed. Cobalt aluminum oxide and mixtures thereof are preferred, as cobalt aluminum oxide provides a desirable metallic shine that is associated with reflectors.

Preferably, the colored layer substantially consists of aluminum oxide comprising at least 0.1% by weight of pigment. Such a colored layer is compatible with commonly used luminescent layers in TL lamps, and offers sufficient color contrast to identify the location of the reflector. For a very clear contrast, at least 1% of pigment is preferred. More than 10% of pigment may influence the binding properties of the colored layer. In a preferred embodiment, the colored layer comprises 2 to 5% by weight of pigment. The weight percentages given are the percentages with respect to aluminum oxide. The 2 to

5% by weight is related to cobalt aluminum oxide as the pigment: the percentage is to be adjusted accordingly for other pigments with different molecular weights.

The invention further provides a method of preparing a tubular gas discharge lamp according to the invention, the method comprising the process steps of: A) providing a translucent tubular discharge vessel, B) applying a colored layer covering at least part of the inner surface of the vessel , C) applying a reflector layer covering at least part of the colored layer, D) applying a precoat layer covering at least the reflector layer and the colored layer, and E) applying a luminescent layer covering the precoat layer. Steps A, C, D and E are known in the art and typically involve the application of a slurry comprising precursors of the layer and an organic binder to the glass vessel or a layer lying underneath, followed by drying and sintering of the layers in order to remove water and organic materials including the organic binder.

In a preferred embodiment, in step B), the colored layer is applied by treating the inner surface of the vessel with an aqueous mixture of pigment, aluminum oxide and an organic binder, followed by sintering.

In a preferred embodiment, the organic binder comprises ammonium poly(methacrylate). Ammonium poly(methacrylate) is, for instance, sold under the brand name Technipol. This organic binder gives a good dispersion of pigment and is easily removed by sintering, yielding an essentially carbon- free layer. Ammonium poly(methacrylate) may be mixed with other binders, but the binder preferably consists essentially of ammonium poly(methacrylate).

The invention will now be elucidated with reference to the following embodiments.

BRIEF DESCRIPTION OF THE DRAWING

Figs. Ia- Id show the preparation of a tubular gas discharge lamp according to the invention.

Figs. 2a-2c show preferred embodiments of the tubular gas discharge lamp according to the invention.

Fig. Ia is a cross-section of a transparent glass tube 1 suitable for preparing a tube light. The glass may be any type of glass suitable for tube lights. The electric connectors 2 of the tube, connecting to the discharge electrodes, are installed at a later stage of the manufacturing process, but are indicated here as a reference position.

DESCRIPTION OF EMBODIMENTS

The process starts with the application of a colored layer 3 extending to part of the inner surface of the vessel 1, as shown in Fig. Ia. The colored layer 3 is applied by an aqueous mixture of aluminum oxide (AI2O3, preferably of the CR6 type), comprising 2 to 5% by weight of cobalt blue (cobalt aluminum oxide, CoAl ₂ O ₄ ) with an average particle size of approximately 50 nm, and ammonium poly(methacrylate) (Technipol) as an organic binder. Other pigments or mixtures of pigments may also be used. Also for the binder, it is possible to use other binders such as poly (ethylene) oxide. However ammonium poly(methacrylate) is preferred as a binder, as it is the easiest to remove in subsequent sintering steps. The arc A of the circular cross-section covered may vary from a single line to a coverage at larger angles, for instance, 60°, 170° or 270°. This will enable a user to easily identify the location of the reflector that is to be applied in the next step. The covered arc A of the colored layer 3 is preferably equal to or smaller than the arc covered by the reflector (still to be applied in Fig. Ia), in order to have an optimum light output of the lamp.

In Fig. Ib, a reflector layer 4 is applied over a second arc B, covering at least part of the colored layer 3. The arc A covered by the colored layer 3 is preferably smaller than the arc B covered by the reflector in order to minimize loss of light due to light absorption by the colored layer 3. More preferably, the colored layer 3 is completely covered by the reflector layer 4.

A typical reflector is prepared from an aqueous mixture of aluminum oxide and a suitable binder such as poly( ethylene) oxide. Suitable aluminum oxide particle sizes and ways of preparation are known from, for example, EP 0,270,866 and EP 0,639,852. Typically, the window left open by the reflector spans an arc C from 20° to 90°; consequently, the arc B covered by the reflector is typically between 340° and 270°. In the end product, depending on the angles A,B,C covered, approximately 80% of the light output is emitted on the window side of the tube, and only 20% is emitted from the reflector side.

Fig. Ic shows the application of a precoat layer 5 over the reflector layer 4 and colored layer 3, covering the inner surface of the tube 1. The mixture used to apply the precoat comprises aluminum oxide (preferably the Alon-C type) and butyl acetate. The precoat layer 5 is used as a basis for the luminescent phosphor layer that is applied subsequently. The precoat layer 5 has a relatively low visible light absorbance.

In Fig. Id, the precoat layer 5 is covered with a luminescent layer 6, essentially consisting of phosphors. It is applied by using a mixture of phosphors, butyl

acetate, and a suitable organic binder. Methods of applying luminescent layers, with or without using a precoat layer, are known in the art. The applied layers are sintered at a typical sintering temperature (typically up to 55O ⁰ C), removing water and organic matter such as butyl acetate and organic binders. After sintering, the tube is filled with an inert gas and mercury vapor, provided with electrodes and connectors, and subsequently sealed to form a gastight vessel, ready for use as a lamp.

Fig. 2a shows a TL lamp 10 with a colored layer, obtainable by the process described in Figs. Ia- Id. The colored layer is visible as a colored zone 11 from outside the layer, making it easy to distinguish the light-emitting window side 12 and the reflector side 13. About 80% of the light generated by the lamp is emitted from the window side. In this case, the reflector zone 14 extends beyond the colored zone 11 (indicated by the broken line). When the lamp is turned off, the reflector zone 14 itself is indistinguishable from the window 15 which is only covered by a pre-coat layer and a luminescent layer. Due to the colored zone 11, it is easier to connect the symmetrical electric connectors 16 in a lamp holder (not shown) in such way that the light 17 is emitted in the desired direction. In this example, the colored zone 11 extends throughout the length of the lamp 10, but for identification of the reflector side 13, it would be sufficient if only part of the length of the tube 10 was covered.

Fig. 2b shows an alternative embodiment in a cross-section of a tubular lamp 20 comprising a glass vessel 21, the inner surface of which is provided with a reflector layer 22 and a luminescent phosphor layer 23. The luminescent phosphor layer 23 may be applied on top of a pre-coated layer 5 as shown in Figs. Ic and Id. Three colored lines 24 are printed on the outer surface of the tube. The lines may be made of a pigment material and may be applied after sintering. If they are applied after sintering, not only inorganic but also organic materials may be applied, such as printing ink or a sticker. The lines visually indicate the side of the lamp on which the reflector 22 is located. The lines 24 protrude from the outer surface of the tube, thus making them not only visible but also tactile indication means. Consequently, it is possible to locate the side of the reflector even in poor light conditions, which may occur when a broken lamp needs to be replaced.

Fig. 2c shows another embodiment of a circular tubular gas discharge lamp 30 which is technically comparable with straight lamps and comprises an electric connector 31. The circular tube 30 is provided with a line 32 in a contrasting color, applied to either the inner or the outer surface of the lamp 30. This line 32 indicates the side on which the reflector 33 (broken line) is integrated in the lamp. Although the reflector 33 itself is not distinguishable from the outside, the line 32 indicates the side on which the reflector 33 is

located. In an alternative embodiment, the line 32 is located on the window, opposite to the reflector side. As the line 32 is relatively thin (smaller than 5 mm, preferably about 1-2 mm thick), the light absorption by the pigments in the line is relatively small.