WO2009027919A2

WO2009027919A2 - Low-pressure gas discharge lamp with improved color stability

Info

Publication number: WO2009027919A2
Application number: PCT/IB2008/053396
Authority: WO
Inventors: Gerardus A. R. Van Dijk; Aafko H. Tuin; Bart-Hendrik Huisman; Wouter J. M. Schrama
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-08-30
Filing date: 2008-08-25
Publication date: 2009-03-05
Also published as: TW200929319A; WO2009027919A3

Abstract

The invention relates to a low-pressure gas discharge lamp (10). The invention further relates to a compact fluorescent lamp and to a backlighting system comprising the low-pressure gas discharge lamp according to the invention. The low-pressure gas discharge lamp comprises a light-transmitting discharge vessel having discharge means (18) for maintaining a discharge emitting light substantially comprising ultraviolet light, a wall of the discharge vessel being provided with a luminescent layer (20) comprising a luminescent material for converting part of the ultraviolet light into visible light emitted by the low- pressure gas discharge lamp. The luminescent layer, in operation, has at least locally a temperature above 80 degrees Celsius and/or has at least locally a wall- load above 0.1 watt per square centimeter. The luminescent layer (20) further comprises a luminescent material (Y1-a-bLnaRb)2O3, where Ln is selected from a group comprising Gadolinium, Samarium, and Dysprosium, R is selected from a group comprising Europium, Terbium and Cerium, and a > 0.10 defining a contribution of Ln to a lattice of the luminescent material (Y1-a-bLnaRb)2O3. The effect achieved when using (Y1-a-bLnaRb)2O3 is that the low-pressure gas discharge lamp has improved color stability.

Description

Low-pressure gas discharge lamp with improved color stability

FIELD OF THE INVENTION:

The invention relates to a low-pressure gas discharge lamp having improved color stability.

The invention further relates to a compact fluorescent lamp, and a backlighting system comprising the low-pressure gas discharge lamp.

BACKGROUND OF THE INVENTION:

Low-pressure gas discharge lamps generally comprise a discharge vessel having a luminescent layer comprising a luminescent material. The luminescent layer generally is applied to an inner wall of the discharge vessel. The luminescent material converts UV light emitted from the discharge space into light of increased wavelength, typically visible light, which is subsequently emitted by the low-pressure gas discharge lamp. Such discharge lamps are also referred to as fluorescent lamps. Low-pressure gas discharge lamps for general illumination purposes usually comprise a mixture of luminescent materials, the combination of the luminescent materials determining the color of the light emitted by the fluorescent lamp. Examples of commonly used luminescent materials are: a blue-luminescent europium-activated barium magnesium aluminate, BaMgAlioOi7:Eu²⁺ (also referred to as BAM), a green- luminescent cerium-terbium co-activated lanthanum phosphate, LaPθ₄:Ce,Tb (also referred to as LAP) and a red- luminescent europium-activated yttrium oxide, Y2θ3:Eu (also referred to as YOX).

The discharge vessel of the low-pressure gas discharge lamp is usually constituted by a light-transmitting envelope enclosing a discharge space in a gastight manner. The discharge vessel is generally circular and comprises both elongate and compact embodiments. The elongated embodiments typically comprise a straight cylindrical envelope also commonly known as tubular light. The envelope in compact embodiments of the low- pressure gas discharge lamp typically is circular, U-shaped or multiple U-shaped. Generally, the means for generating and maintaining a discharge in the discharge space are electrodes arranged near the discharge space. Alternatively, the low-pressure gas discharge lamp is a so- called electrodeless low-pressure gas discharge lamp, for example an induction lamp, where energy required for generating and/or maintaining the discharge is transferred through the discharge vessel by means of an induced alternating electromagnetic field.

Compact embodiments of the low-pressure gas discharge lamps are, for example, compact fluorescent lamps. Compact fluorescent lamps often replace incandescent lamps to provide the same light output at increased efficiency. This replacement has become more important recently due to the current effort to reduce energy consumption for environmental reasons. However, the current compact fluorescent lamps are still relatively large and bulky, and thus cannot replace all incandescent lamps or do not provide the same aesthetic appearance as conventional incandescent lamps. Thus, the current drive in compact fluorescent lamps is to further miniaturize the low-pressure gas discharge lamp inside the compact fluorescent lamp.

A drawback when miniaturizing the compact embodiments of the known low- pressure gas discharge lamps is that these low-pressure gas discharge lamps are not color stable.

SUMMARY OF THE INVENTION:

It is an object of the invention to provide a low-pressure gas discharge lamp having improved color stability.

According to a first aspect of the invention, the object is achieved with a Io w- pressure gas discharge lamp comprising: a light-transmitting discharge vessel enclosing, in a gastight manner, a discharge space comprising a gas filling, the discharge vessel comprising discharge means for maintaining a discharge in the discharge space emitting light substantially comprising ultraviolet light, a wall of the discharge vessel being provided with a luminescent layer comprising a luminescent material for converting part of the ultraviolet light to visible light emitted by the low-pressure gas discharge lamp, the luminescent layer, in operation, at least locally having a temperature above 80 degrees Celsius and/or at least locally having a wall- load above 0.1 watt per square centimeter, the luminescent layer further comprising a luminescent material (Yi__a_bLnaRb)2θ3, where Ln is selected from a group comprising Gadolinium, Samarium, and Dysprosium, R is selected from a group comprising Europium, Terbium and Cerium, and a > 0.10 defining a contribution of Ln to a lattice of the luminescent material (Yi__a_ 1,LnJIb)₂O₃. The effect of the measures according to the invention is that by replacing at least 10% of the Yttrium in the YOX-lattice by Lanthanides selected from the group comprising Gadolinium, Samarium and Dysprosium, the low-pressure gas discharge lamp provides improved color stability over time. The inventors have observed that in known Io w- pressure gas discharge lamps in which the luminescent layer, in operation, has a temperature above 80 degrees Celsius and/or in which the wall- load of the luminescent layer is above 0.1 watt per square centimeter, the color of the light emitted by the known low-pressure gas discharge lamp changes over time. This color shift may even exceed 10% of the xy chromaticity diagram (CIE 1931) coordinates between 100 hours of use and 300 hours of use of the low-pressure gas discharge lamp. While not wishing to be held to any particular theory, the inventors have found that the change of the color of the emitted light seems to be caused by a difference of the degradation over time of the luminescent material YOX compared to the degradation over time of the other luminescent materials in the low-pressure gas discharge lamp. In the low-pressure gas discharge lamp according to the invention at least 10% of the Yttrium is replaced by Lanthanides selected from the group comprising Gadolinium, Samarium and Dysprosium. The addition of Gadolinium, Samarium and/or Dysprosium reduces the difference in degradation of the luminescent material which provides the primary color red to the emission spectrum compared to the remaining known luminescent materials. This reduced difference in degradation improves the color stability of the low-pressure gas discharge lamp according to the invention.

Wall-load is defined as the input (electric) power provided to the low-pressure gas discharge lamp divided by π. D.I, where 1 is the arc length (in centimeters) of the low- pressure gas discharge lamp, and D is an average inner diameter (in centimeters) of the low- pressure gas discharge lamp. The wall-load is expressed in watt per square centimeters. For miniaturized compact embodiments of the low-pressure gas discharge lamps, the wall- load typically ranges from 0.1 to 0.2 watt per square centimeter. For elongated embodiments of the low-pressure gas discharge lamps, the wall-load is typically lower than 0.5 watt per square centimeter.

A primary color comprises light of a predefined spectrum, for example, having a central wavelength with a spectral bandwidth around a central wavelength. In low-pressure discharge lamps typically three different luminescent materials are mixed, each absorbing ultraviolet light and converting the absorbed ultraviolet light to light of a primary color, for example, Red, Green and Blue. By using Red, Green and Blue, substantially white light can be mixed. Also other combinations of primary colors may be used. In an embodiment of the low-pressure gas discharge lamp, the contribution of Ln to the lattice of the luminescent material

is within a range: 0.45 < a < 0.55. The inventors have found that when using the luminescent material

where 0.45 < a < 0.55, instead of YOX in the luminescent mixture in a compact fluorescent lamp CFL 26W/830 the color stability is improved such that color instability is reduced to below 1% of the xy chromaticity diagram (CIE 1931) coordinates between 100 hours of use of the CFL lamp and 300 hours of use of the CFL lamp.

In an embodiment of the low-pressure gas discharge lamp, a contribution of R to the lattice of the luminescent material (Yi__a_bLriaRb)2θ3 is defined by b > 0.07. Generally, Europium, Terbium and Cerium are used in luminescent materials to absorb ultraviolet radiation from the discharge and convert the absorbed ultraviolet radiation into visible light. When using Europium in the luminescent material, the luminescent material absorbs ultraviolet radiation and subsequently emits the primary color red. When using Terbium and Cerium in the luminescent material, the luminescent material absorbs ultraviolet radiation and subsequently emits the primary color green. Generally, an increase of the concentration of Europium, Terbium or Cerium in the lattice of the luminescent material increases the efficiency of the luminescent material. However, above a maximum concentration it has been found that the efficiency of the luminescent material decreases due to concentration quenching. The inventors have found that an increase of the contribution of Europium, Terbium and/or Cerium in the luminescent material has a further effect in that it also improves the color stability of the low-pressure gas discharge lamp. So, although the efficiency of the luminescent material may be slightly reduced, an increased concentration of Europium, Terbium and/or Cerium will further improve the color stability of the low- pressure gas discharge lamp. In an embodiment of the low-pressure gas discharge lamp, a contribution of R to the lattice of the luminescent material (Yi__a_bLriaRb)2θ3 is within a range: 0.07 < b < 0.10. The inventors have found that when using a luminescent material in which the concentration of Europium, Terbium and/or Cerium is within the range 0.07 < b < 0.10, the efficiency of the luminescent material is still relatively high while a significant improvement of the color- stability of the luminescent material is achieved.

In a preferred embodiment of the low-pressure gas discharge lamp, Ln represents the rare earth metal Gadolinium and R represents the rare earth metal Europium. The presence of Europium causes the luminescent material to absorb ultraviolet light and convert the absorbed ultraviolet light to light of the primary color red. A benefit of this embodiment is that Gadolinium is relatively cheap. When replacing YOX by the luminescent material (Yi__a_bGd_aEub)2θ3, the low-pressure discharge lamp has relatively good color stability while it can be manufactured relatively cost effectively.

In an embodiment of the low-pressure gas discharge lamp, the gas filling of the discharge space comprises mercury. A benefit of this embodiment is that an emission of ultraviolet light is relatively efficient, which results in a low-pressure gas discharge lamp having a relatively high efficiency.

The invention also relates to the use of the luminescent material

(Yi__a_bLnaRb)2θ3, where a > 0.10, in a luminescent layer of a low-pressure gas discharge lamp to improve color stability. The invention further relates to a compact fluorescent lamp and to a backlighting system comprising the low-pressure gas discharge lamp according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS: These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings:

Fig. 1 shows a cross-sectional view of a low-pressure gas discharge lamp according to the invention, Fig. 2 shows the xy chromaticity diagram (CIE 1931),

Figs. 3 A and 3B show two embodiments of compact embodiments of the low- pressure gas discharge lamps according to the invention, and

Fig. 4 shows a display device comprising a backlighting system comprising the low-pressure gas discharge lamp according to the invention. The Figures are purely diagrammatic and not drawn to scale. Particularly for clarity, some dimensions are exaggerated strongly. Similar components in the Figures are denoted by the same reference numerals as much as possible.

DETAILED DESCRIPTION OF EMBODIMENTS: Fig. 1 shows a cross-sectional view of a low-pressure gas discharge lamp 10 according to the invention. The low-pressure gas discharge lamp 10 according to the invention comprises a light transmitting discharge vessel 14 which encloses a discharge space 16 in a gas-tight manner. The discharge space 16 comprises a gas filling, for example, comprising a metal compound and a buffer gas. The low-pressure gas discharge lamp 10 further comprises coupling elements. The coupling elements couple energy into the discharge space 16, for example, via capacitive coupling (not shown), inductive coupling (not shown), microwave coupling (not shown), or via electrodes 18 to obtain a gas discharge in the discharge space 16. The discharge vessel 14 comprises a wall 12 having a luminescent layer 20 comprising luminescent material. The luminescent material, for example, absorbs ultraviolet light emitted from the discharge and, for example, converts the absorbed ultraviolet light to visible light.

In an embodiment shown in Fig. 1, the discharge vessel 14 comprises a set of electrodes 18. In Fig. 1 only one electrode 18 of the set of electrodes 18 is shown. The electrodes 18 are electrical connections through the discharge vessel 14 of the low-pressure gas discharge lamp 10. By applying an electrical potential difference between the two electrodes 18 a discharge is initiated between the two electrodes 18. This discharge is generally located between the two electrodes 18 and is indicated in Fig. 1 as the discharge space 16. Alternative coupling elements such as capacitive couplers (not shown), inductive couplers (not shown), or microwave couplers (not shown) may be used. Because the electrodes 18 typically limit the lifetime of the low-pressure gas discharge lamp 10, the use of these alternative coupling elements may be beneficial.

In general, light generation in the low-pressure gas discharge lamp 10 is based on the principle that charge carriers, particularly electrons but also ions, are accelerated by an electric field applied between the electrodes 18 of the low-pressure gas discharge lamp 10. Collisions of these accelerated electrons and ions with the gas atoms or molecules in the gas filling of the low-pressure gas discharge lamp 10 cause these gas atoms or molecules to be dissociated, excited or ionized. When the atoms or molecules of the gas filling return to a ground state, a substantial part of the excitation energy is converted to radiation. When the gas filling comprises mercury, the light emitted by the excited mercury atoms is mainly ultraviolet light at a wavelength of approximately 254 nanometer. This ultraviolet light is subsequently absorbed by luminescent material in the luminescent layer 20, which converts the absorbed ultraviolet light, for example, to visible light of a predetermined color.

In the low-pressure gas discharge lamps 10, generally the luminescent layer 20 comprises a mixture of luminescent materials which is used to be able to emit substantially white light. In the known low-pressure gas discharge lamps often a mix of the luminescent materials BAM (emitting the primary color blue), LAP (emitting the primary color green) and YOX (emitting the primary color red) is used to obtain substantially white light. In the embodiment of the low-pressure gas discharge lamp 10 shown in Fig. 1, the luminescent layer 20 is applied to the inside of the wall 12 of the discharge vessel 14. Alternatively, the luminescent layer 20 may be applied to the outside (not shown) of the wall 12 of the discharge vessel 14. In the latter embodiment, the discharge vessel 14 must be made of a material which is transparent to ultraviolet light, such as quartz-glass. Fig. 2 shows the xy chromaticity diagram (CIE 1931). The point indicated with reference A represents a color point of the known low-pressure gas discharge lamp after 100 hours of use. The point indicated with reference B represents a color point of the known low-pressure gas discharge lamp after 300 hours of use. The known low-pressure gas discharge lamp used to obtain the reference points A and B has a luminescent layer which, in operation, has a temperature above 80 degrees Celsius. The color change in the known low- pressure gas discharge lamp is clearly visible between 100 hours of use and 300 hours of use. In Table 1, the measured color change is shown of the known low-pressure gas discharge lamp (indicated with Known). The known low-pressure gas discharge lamp comprises a mixture of luminescent material comprising BAM, CAT and YOX, wherein CAT represents CeMgAIi iOi9:Tb³⁺. The contribution of the Europium in YOX is less than 0.07. Table 1 further shows a first embodiment (indicated with Inv.I) of the low-pressure gas discharge lamp according to the invention and a second embodiment (indicated with Inv.II) of the low pressure gas discharge lamp according to the invention. The first embodiment (Inv.I) comprises the same mixture of luminescent materials as in the known low-pressure gas discharge lamp in which YOX is replaced by (Yi__a_bLriaRb)2θ3 in which Gadolinium is used as Ln, Europium is used as R, the contribution of Gadolinium in the lattice of (Yi__a_bLnaRb)2θ3 is 0.45 < a < 0.55 and the contribution of Europium in the lattice is b < 0.07 (same as in YOX in the known low-pressure gas discharge lamp). The second embodiment (Inv.II) comprises the same mixture of luminescent materials as in the known low-pressure gas discharge lamp in which YOX is replaced by

in which Gadolinium is used as Ln, Europium is used as R, the contribution of Gadolinium in the lattice of (Yi__a__bLri_aR_b)₂θ₃ is 0.45 < a < 0.55 and the contribution of Europium in the lattice is 0.07 < b < 0.10. As can be clearly concluded from Table 1, the replacement of part of the Yttrium of YOX by Gadolinium improves the color stability and the increase of the contribution of Europium in the lattice further improves the color stability of the low- pressure gas discharge lamps.

Table 1 color point at 100 hours and 300 hours.

Figs. 3 A and 3B show two embodiments of compact embodiments 30, 32 of the low-pressure gas discharge lamp 34 according to the invention. The compact fluorescent lamps 30, 32 comprise a plurality of u-shaped discharge vessels which are packed closely together. The compact fluorescent lamps 30, 32 further comprise an additional electronic circuit 38 hidden in a base of the compact fluorescent lamp 30, 32. The additional electronic circuit 38 regulates the switching on of the low-pressure gas discharge lamp 34 in the compact fluorescent lamp 30, 32. Due to the compact arrangement of the low-pressure gas discharge lamps 34, the temperature, in operation, of the luminescent material of the low- pressure gas discharge lamp 34 increases at least locally to above 80 degrees Celsius. Furthermore, the wall-load in these u-shaped low-pressure gas discharge lamps 34 at least locally exceeds 0.1 watt per square centimeter. Due to these locally high temperatures and/or high wall- load, the degradation of the luminescent material YOX seems to be different compared to the other luminescent materials in the luminescent layer. When using the low- pressure gas discharge lamp 34 according to the invention in the compact fluorescent lamps 30, 32, the color of the light emitted by the compact fluorescent lamps 30, 32 over time remains more stable. In the embodiment of the compact fluorescent lamp 32 shown in Fig. 3B, the low-pressure gas discharge lamp 34 is arranged within a cover 36. This cover 36 may have any shape, for example, mimicking the shape of an incandescent lamp. In the embodiment shown in Fig. 3B, the cover 36 has a cylindrical shape. This cover 36 further increases the temperature of the luminescent material of the low-pressure gas discharge lamp 34 of the compact fluorescent lamp 32. When using the known low-pressure gas discharge lamp in this compact arrangement, the color of the light emitted by the compact fluorescent lamp will change between 100 hours of use and 300 hours of use. When using the low-pressure gas discharge lamp 34 according to the invention in the compact fluorescent lamp 32, the stability of the color of the light emitted by the compact fluorescent lamp 32 is improved. Fig. 4 shows a display system 40 having a backlighting system 42 according to the invention. The display system 40 comprises a display 44, for example a well known liquid crystal display 44. The liquid crystal display device generally contains a polarizer (not shown), an array of light valves (not shown) and an analyzer (not shown). Each light valve typically comprises liquid crystal material which can alter a polarization direction of incident light, for example, by applying an electrical field across the liquid crystal material. The arrangement of polarizer, light valve and analyzer is such that when the light valve is switched to, for example, "bright", the light emitted from the backlighting system 42 will be transmitted. When the light valve is switched to, for example, "dark" the light emitted from the backlighting system 42 will be blocked. In that way an image can be produced on the display 44.

The backlighting system 42 comprises the low-pressure gas discharge lamp 10 according to the invention. In some embodiments of the backlighting system 42, low-pressure gas discharge lamps are used to illuminate a display 44. This is mainly because of the relatively high efficiency of the low-pressure gas discharge lamps. The backlighting system 42 typically comprises an array of low-pressure gas discharge lamps or comprises a low- pressure gas discharge lamp which meanders parallel to the display 44. The low-pressure gas discharge lamp used in backlighting systems 42 usually has a miniaturized diameter to generate a relatively thin backlighting system 42. Furthermore, the low-pressure gas discharge lamp or the array of low-pressure gas discharge lamps is typically arranged in a confined space of the backlighting system 42, within which the operating temperature may be relatively high. Due to the reduced diameter and/or the arrangement in a confined space, the temperature, in operation, of the luminescent layer of the low-pressure gas discharge lamps at least locally exceeds 80 degrees Celsius and/or the wall-load at least locally exceeds 0.1 watt per square centimeter. The color stability of the light emitted by the known low-pressure gas discharge lamp or by the array of known low-pressure gas discharge lamps is especially critical, because this determines the color stability of the light emitted by the backlighting system 42 towards the display 44. If the color of the light emitted by the backlighting system 42 to the display 44 is not stable over time, the display 44 may not display the true colors of the image. The display 44 may obtain information on the color shift of the light emitted by the backlighting system 42 over time, so that it may compensate for the color shift to still be able to display the image in its true colors. This, however, is a very expensive correction. Applying the low-pressure gas discharge lamp 10 according to the invention to the backlighting system 42 improves the color stability of the light emitted over time by the backlighting system 42 towards the display 44, which substantially continuously enables the display 44 to display the image in its true colors without the need for relatively expensive compensation arrangements.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A low-pressure gas discharge lamp (10, 34) comprising: a light-transmitting discharge vessel (14) enclosing, in a gastight manner, a discharge space (16) comprising a gas filling, the discharge vessel (14) comprising discharge means (18) for maintaining a discharge in the discharge space (16) emitting light substantially comprising ultraviolet light, a wall (12) of the discharge vessel (14) being provided with a luminescent layer (20) comprising a luminescent material for converting part of the ultraviolet light to visible light emitted by the low-pressure gas discharge lamp (10, 34), the luminescent layer (20), in operation, having at least locally a temperature above 80 degrees Celsius and/or having at least locally a wall- load above 0.1 watt per square centimeter, the luminescent material comprising

where Ln is selected from a group comprising Gadolinium, Samarium, and Dysprosium, R is selected from a group comprising Europium, Terbium and Cerium, and a > 0.10 defining a contribution of Ln to a lattice of the luminescent material

2. Low-pressure gas discharge lamp (10, 34) as claimed in claim 1, wherein the contribution of Ln to the lattice of the luminescent material (Yi__a__bLri_aR_b)₂θ₃ is within a range: 0.45 < a < 0.55.

3. Low-pressure gas discharge lamp (10, 34) as claimed in claim 1 or 2, wherein a contribution of Re to the lattice of the luminescent material (Yi__a_bLriaRb)2θ3 is defined by b > 0.07.

4. Low-pressure gas discharge lamp (10, 34) as claimed in claim 1 or 2, wherein a contribution of Re to the lattice of the luminescent material (Yi__a_bLriaRb)2θ3 is within a range: 0.07 < b < 0.10.

5. Low-pressure gas discharge lamp (10, 34) as claimed in any of the previous claims, wherein Ln represents the rare earth metal Gadolinium and R represents the rare earth metal Europium.

6. Low-pressure gas discharge lamp (10, 12) as claimed in any of the previous claims, wherein the gas filling of the discharge space (16) comprises mercury.

7. Use of a luminescent material

where a > 0.10, in a luminescent layer (20) of a low-pressure gas discharge lamp (10, 34) for improving color stability, the low-pressure gas discharge lamp (10, 34) comprising: a light-transmitting discharge vessel (14) enclosing, in a gastight manner, a discharge space (16) comprising a gas filling, the discharge vessel (14) comprising discharge means (18) for maintaining a discharge in the discharge space (16) emitting light substantially comprising ultraviolet light, - a wall (12) of the discharge vessel (14) being provided with the luminescent layer (20) comprising the luminescent material for converting part of the ultraviolet light to visible light emitted by the low-pressure gas discharge lamp (10, 34), the luminescent layer (20), in operation, having at least locally a temperature above 80 degrees Celsius and/or having at least locally a wall-load above 0.1 watt per square centimeter.

8. Compact Fluorescent Lamp (30, 32) comprising the low-pressure gas discharge lamp (10, 34) according to any of the previous claims.

9. Compact Fluorescent lamp (30, 32) as claimed in claim 8, wherein the Compact Fluorescent lamp (30, 32) comprises a cover (36).

10. Backlighting system (42) for illuminating a display (44), the backlighting system (42) comprising the low-pressure gas discharge lamp (10, 34) as claimed in claims 1 to 6.