FIELD OF THE INVENTION:

The invention relates to a lamp having improved efficiency.

The invention further relates to a luminaire, a backlighting system and a display device.

BACKGROUND OF THE INVENTION:

Lamps generally comprise a light-transmitting vessel enclosing a space in which light is generated. The lamp may, for example, be a halogen lamp comprising the light-transmitting vessel which comprises, for example, the halogen gas-filling and a filament for emitting light. The lamp may, for example, be a discharge lamp in which the light- transmitting vessel is a discharge vessel which encloses a discharge space comprising a gas filling. The discharge lamp also generally comprises discharge means for maintaining, in operation, a discharge in the discharge space. The discharge emits light which may be emitted by the discharge lamp. Alternatively, the discharge lamp may comprise luminescent material for converting at least a part of the light emitted by the discharge into light of a different color. For example, low-pressure mercury vapor discharge lamps typically have a gas filling comprising mercury. The mercury present in the discharge generates ultraviolet light which is emitted from the discharge and impinges on luminescent material arranged at the inner wall of the discharge vessel. The luminescent material converts the ultraviolet radiation into visible light which is subsequently emitted by the low-pressure mercury vapor discharge lamp. Such discharge lamps are also referred to as fluorescent lamps. Low-pressure mercury vapor discharge lamps usually comprise a mixture of luminescent materials, such that a combination of the luminescent materials determines the color of the light emitted by the fluorescent lamp. Examples of commonly used luminescent materials are, for example, a blue-luminescent europium-activated barium magnesium aluminate, BaMgAlioOi ₇ :Eu ²⁺ (also referred to as BAM), a green- luminescent cerium-terbium co-activated lanthanum phosphate, LaP0 ₄ :Ce,Tb (also referred to as LAP) and a red-luminescent europium- activated yttrium oxide, Y ₂ 0 ₃ :Eu (also referred to as YOX). The discharge vessel of the 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 elongated 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 is typically circular, U-shaped, multiple U-shaped or twister shaped. Generally, the means for generating and maintaining a discharge in the discharge space are electrodes arranged near the discharge space. Alternatively, the discharge lamp is a so-called electrodeless 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.

A drawback of the known lamps is that the efficiency of the lamp is still relatively low.

SUMMARY OF THE INVENTION:

It is an object of the invention to provide a discharge lamp having improved efficiency.

According to a first aspect of the invention the object is achieved with a lamp as claimed in claim 1. According to a second aspect of the invention the object is achieved with a luminaire as claimed in claim 13. According to a third aspect of the invention, the object is achieved with a backlighting system as claimed in claim 14. According to a fourth aspect of the invention, the object is achieved with a display device as claimed in claim 15.

The lamp according to the first aspect of the invention comprises a light- transmitting vessel enclosing, in a gastight manner, a space comprising a gas filling, the space comprising light generating means for generating and/or maintaining, in operation, light emission from the space through the light-transmitting vessel,

an outer surface of the light-transmitting vessel comprising an anti-reflection layer for improving a transmission of light from the space through the light-transmitting vessel, the outer surface of the light-transmitting vessel being a surface of the light-transmitting vessel facing away from the space.

An effect of the lamp according to the invention is that the application of the anti-reflection layer to the outer wall of the light-transmitting vessel allows a very efficient anti-reflection coating. The inventors have found that the efficiency of the anti-reflection layer is improved when a thickness of the anti-reflection layer is controlled relatively accurately. The thickness of the anti-reflection layer is a dimension of the anti-reflection layer in a direction substantially perpendicular to the surface comprising the anti-reflection layer. Typically, the processes used to apply, for example, a luminescent material to the inner surface of a discharge lamp are relatively inaccurate - especially in relation to the layer thickness obtained by these known processes. Variations in thicknesses of several microns are reported. Using such known processes to generate an anti-reflection layer to an inner surface of the light-transmitting vessel would result in a relatively poor anti-reflection layer. The inventors have found that the application of the anti-reflection layer to the outer surface of the light-transmitting vessel has the benefit that the thickness of the anti-reflection layer across a major part of the anti-reflection layer can be controlled relatively accurately, thus significantly improving the efficiency of the anti-reflection layer and of the lamp as a whole.

Especially with discharge lamps, the anti-reflection layer is very efficient. In such an embodiment the discharge lamp comprises 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, in operation, the discharge emitting light, an outer surface of the discharge vessel comprising an anti-reflection layer for improving the transmission of light from the discharge through the discharge vessel, the outer surface of the discharge vessel being a surface of the discharge vessel facing away from the discharge. Discharge lamps have been on the market for a relatively long time. Efficiency of the discharge lamps has always been an important drive in the improvement of the discharge lamps. To the surprise of the inventors, no lamps and especially no discharge lamps have been found in which the outer surface of the discharge vessel or light-transmitting vessel comprises an anti-reflection layer to reduce the reflection of light from the discharge lamp back into the discharge lamp, and hence promote emission of light. The inventors have realized that the application of the anti-reflection layer to the outer surface of the discharge vessel may be done in a much easier and more accurate manner to obtain a significant improvement of the efficiency of the discharge lamp.

Furthermore, the application of an anti-reflection layer to the inner surface of the discharge vessel of discharge lamps is often less effective. The inner surface of the discharge vessel often comprises a layer comprising luminescent materials to convert part of the light emitted by the discharge into light of a different wavelength. The refractive index of this luminescent material is somewhere between the refractive index of the gas filling and the refractive index of the discharge vessel material. Therefore, the difference in refractive index between the luminescent material and the discharge vessel is not so large, causing the efficiency of the application of an anti-reflection coating to the inner surface of the discharge vessel to be less efficient compared to the application of the anti-reflection layer to the outer surface of the discharge vessel. At the outer surface the difference between the refractive index of the discharge vessel and the surrounding environment, typically air, is relatively large, resulting in reflection of part of the light generated by the discharge lamp back into the discharge lamp - said part typically being lost due to re-absorption in the discharge lamp.

A further benefit of applying the anti-reflection layer to the outer surface of the discharge vessel or the light-transmitting vessel is that the material of the anti-reflection layer is not exposed to the relatively harsh environment within the discharge vessel or the light-transmitting vessel. Typically, the environment inside the discharge vessel or light-transmitting vessel often is relatively harsh, due to both the relatively high temperature resulting from the discharge or filament and the chemically harsh environment caused by the gas fillings. The anti-reflection layer, when applied to the inner surface of the discharge vessel or light-transmitting vessel, has to be able to withstand these harsh environmental conditions, as a result of which relatively few materials can be chosen to generate the anti-reflection layer. When applying the anti-reflection layer to the outer surface of the light- transmitting vessel or discharge vessel according to the current invention, the environmental conditions which the material of the anti-reflection layer has to be able to withstand are less severe, which allows a choice from a broader range of materials to generate the anti- reflection layer. Thus, a more efficient anti-reflection layer may be generated, because the material of the anti-reflection layer may be chosen so as to better match the transmission requirements of the light-transmitting vessel or discharge vessel.

A further surprising effect was observed by the inventors due to the application of the anti-reflection layer to the outer surface of the discharge vessel: without wishing to be held to any particular theory, experiments have shown that the application of the anti-reflection layer to the outer surface of the discharge vessel has hardly any influence on the spectrum of the light emitted by the discharge lamp. Often anti-reflection layers have different transmissive characteristics for different wavelengths of light. The anti-reflection layer may comprise a transmission maximum at specific wavelength peaks within the spectrum of light emitted by the discharge lamp. Therefore, the light emitted at or near such a transmission maximum of the transmission spectrum of the anti-reflection layer may have less reflection compared to light emitted by the discharge lamp outside these transmission peaks of the anti-reflection layer. This may alter the spectrum of the light emitted by the discharge lamp. Experiments have shown that when applying the anti-reflection layer to the outer surface of the discharge vessel, the change in the emission spectrum of the discharge lamp due to the presence of the anti-reflection layer is almost negligible. As such, a significant improvement may be achieved in emission efficiency of the discharge lamp without a correction for a color shift due to a change in the emission spectrum being required. Consequently, it seems to be sufficient to apply the anti-reflection layer to the outer surface of the discharge vessel to improve the efficiency of the discharge lamp while the remainder of the lamp characteristics remains substantially unchanged.

Finally, the application of the anti-reflection layer to the outer surface of the light-transmitting vessel or discharge vessel reduces the confinement of light inside the vessel wall. Especially in discharge lamps having luminescent material, for example, applied to the inner surface of the discharge vessel, the light generated by the luminescent material is emitted in substantially all directions near the vessel wall. Thus, a significant part of the light emitted from the luminescent material may be confined within the wall of the discharge vessel as it may be captured in the discharge vessel via total internal reflection. Such a confinement typically reduces the efficiency of the discharge lamp. The application of an anti-reflection layer between the luminescent material and the discharge vessel would enhance the in-coupling of light into the discharge vessel wall and would promote the confinement and/or reflection of light. Adding the anti-reflection layer to the outer surface of the discharge vessel according to the invention would reduce the confinement of light inside the discharge vessel wall and would thus improve the efficiency of the discharge lamp according to the invention.

In an embodiment of the lamp, a variation of the thickness of the anti-reflection layer over more than 80 percent of the area of the outer surface covered by the anti-reflection layer is less than 5 micrometer and/or less than 2 micrometer and/or less than 0.5 micrometer. The inventors have found that a layer thickness range of 100 - 300 nanometer is optimal. Accurate layer thicknesses depend on the processing and the desired wavelength for optimal anti-reflection.

In an embodiment of the lamp, the anti-reflection layer comprises a graded refractive index layer. Such a graded refractive index layer allows a relatively big

improvement of the efficiency. However, such a graded refractive index layer must be applied with a relatively high accuracy. Especially when such a graded refractive index layer is used, the application thereof to the outer surface of the light-transmitting vessel is preferred. In an embodiment of the lamp, the graded refractive index layer comprises a nano-porous layer and/or a layer comprising periodic structures of varying refractive index. A thickness of a layer comprising periodic structures is defined as the average thickness of the layer comprising the periodic structures. Thus, a variation of the thickness of the anti- reflection layer as previously defined is related to the average thickness of the layer when referring to the graded refractive index layer having periodic structures of varying refractive index. Especially when the graded refractive index layer comprising periodic structures is applied in discharge lamps, such a graded refractive index layer is preferably applied to the outer surface of the discharge vessel. As the luminescent material is typically applied to the inner surface of the discharge vessel, such luminescent material may fill the periodic structures of the graded refractive index layer when the graded refractive index layer is applied to the inner wall. Such filling of the periodic structures reduces the anti-reflective characteristics of such a graded refractive index layer.

In an embodiment of the lamp, the graded refractive index layer comprises adhesion material for improving the adhesion of the graded refractive index layer to the outer surface. The graded refractive index layer may, for example, comprise the nano-porous layer which is constituted of powder-like material having dimensions preferably smaller than the wavelength of the light. Such a nano-porous layer typically has a thickness smaller than the wavelength of the light for which it acts as an anti-reflection layer. The nano-porous layer may be constituted of a powder like material in which the powder particles and the voids between the powder particles both have dimensions smaller than the wavelength of the light. Alternatively, the nano-porous layer may be constituted by a film comprising a specific distribution of voids. Both the thickness of the film and the dimensions of the voids should be smaller than the wavelength of the light. The film comprises a substantially uniform distribution of a specific material. When such a nano-porous layer is applied to the outer surface of the light-transmitting vessel, care should be taken not to touch the light- transmitting vessel when handling the lamp, as this may damage the anti-reflection layer. For example, when the lamp is a low-pressure mercury vapor discharge lamp, such a discharge lamp is typically handled by grasping it at the outer surface of the discharge vessel. When the graded refractive index layer comprises nano-porous material applied to the outer surface of the discharge vessel, touching the discharge lamp may damage the anti-reflection layer, which reduces the efficiency of the anti-reflection layer. For this reason, the graded refractive index layer may comprise adhesion material for improving the adhesion of the graded refractive index layer to the outer surface. The nano-porous material may, for example, be distributed in the adhesion material to obtain the graded refractive index layer.

In an embodiment of the lamp, the anti-reflection layer comprises a material chosen from the list comprising: Zr0 ₂ , Y ₂ O ₃ , MgO, AI ₂ O ₃ , Si0 ₂ , KMgF ₃ , and MgF ₂ .

Especially when the periodic structures are used for generating the graded refractive index layer, the refractive index of the material constituting the anti-reflection layer is preferably relatively close to the refractive index of the light-transmitting vessel to ensure a gradual variation of the refractive index due to the periodic structures. When, for example, the light- transmitting vessel is produced of glass, the use of Si0 ₂ as material for generating the graded anti-reflection layer is preferred as its refractive index is relatively close to the refractive index of glass. Similar effects can be expected for A1 ₂ 0 ₃ when the light-transmitting vessel is produced of ceramics.

In an embodiment of the lamp, the lamp comprises a luminescent layer applied to the inner surface of the light-transmitting vessel comprising luminescent material for converting at least some of the light generated by the light generating means in the space into light of a predefined color, the inner surface being a surface of the light-transmitting vessel facing towards the space. Low-pressure mercury vapor discharge lamps typically emit ultraviolet light. Such low-pressure mercury vapor discharge lamps typically comprise the luminescent layer to convert the ultraviolet light into visible light. In alternative discharge lamps, the discharge emits, for example, light of a primary color blue. A discharge lamp comprising such a blue-emitting discharge may comprise a luminescent layer which converts a part of the light of the primary color blue into light of the color red and/or light of the color green. The specific luminescent material of the luminescent layer or the specific mixture of luminescent materials in the luminescent layer contribute to the spectrum of the light emitted by the discharge lamp and thus contribute to the perceived color of the light emitted by the discharge lamp. The luminescent layer typically is applied to the inner wall of the discharge vessel, which would protect the luminescent layer from damage due to handling of the discharge lamp and the cost of the discharge vessel in the low-pressure mercury vapor discharge lamps, as no ultraviolet light needs to pass the discharge vessel. The luminescent layer may comprise a single luminescent material, or a mixture of luminescent materials each contributing a specific part of the emission spectrum of the luminescent material to the overall emission spectrum of the discharge lamp.

In this context, light of a predefined color typically comprises light having a predefined spectrum. The predefined spectrum may, for example, comprise a primary color having a specific bandwidth around a predefined wavelength, or may, for example, comprise a plurality of primary colors. The predefined wavelength is a mean wavelength of a radiant power spectral distribution. In this context, light of a predefined color also includes invisible light, such as ultraviolet light or infrared light. The light of a primary color, for example, includes Red, Green, Blue, Yellow, Amber, and Magenta light. Light of the predefined color may also comprise mixtures of primary colors, such as Blue and Amber, or Blue, Yellow and Red. By choosing, for example, a specific combination of the Red, Green and Blue light emitting luminescent materials, substantially every color can be generated by the discharge lamp, including white. Also other combinations of primary colors may be used in the discharge lamp, which enables the generation of substantially every color, for example, Red, Green, Blue, Cyan and Yellow.

In an embodiment of the lamp, the luminescent material is selected for compensating for a color shift due to the presence of the anti-reflection layer to obtain a predefined color of the light emitted by the lamp. Although the experiments of the inventor have shown that the color shift due to the anti-reflection layer applied to the outer surface of the discharge vessel in discharge lamps seems to be negligible, the luminescent material in the luminescent layer may be used to correct for any remaining color shift due to the anti- reflection layer. As indicated before, anti-reflection layers typically have different transmissive characteristics for different wavelengths of light. The anti-reflection layer may comprise a transmission maximum at specific wavelength peaks within the spectrum of light emitted by the discharge lamp. Therefore, the light emitted at or near such a transmission maximum of the transmission spectrum of the anti-reflection layer may have less reflection compared to light emitted by the discharge lamp outside these transmission peaks of the anti- reflection layer. This may alter the spectrum of the light emitted by the lamp. Such a change in the spectrum of the light emitted by the lamp due to the anti-reflection layer may be anticipated by choosing a specific luminescent material such that the combination of the specific luminescent material and the anti-reflection layer generates light comprising the required spectrum. The specific luminescent material may comprise a different luminescent material than the standard lamp emitting the required spectrum without the anti-reflection coating, or, the specific luminescent material may comprise a different mixture of

luminescent materials than the standard lamp emitting the required spectrum without the anti- reflection coating.

In an embodiment of the lamp, the lamp further comprises an outer bulb at least partially surrounding the discharge vessel, the outer bulb comprising a further anti-reflection layer on an inner surface of the outer bulb and/or on an outer surface of the outer bulb, the inner surface of the outer bulb facing towards the light-transmitting vessel and the outer surface of the outer bulb facing away from the light-transmitting vessel. This further anti-reflection layer may be a similar anti-reflection layer which is applied to the outer surface of the light-transmitting vessel when the outer bulb is made of similar material to that used for the light-transmitting vessel. Still, the choice of suitable materials for the further anti-reflection layer applied to the outer bulb may be larger, because the temperature of the outer bulb typically remains much lower compared to the temperature of the light- transmitting vessel. When the refractive index of the filling of the space between the light- transmitting vessel and the outer bulb is similar to the refractive index of air, the

anti-reflection layer on the inner surface of the outer bulb may be the same as the

anti-reflection layer applied to the outer surface of the outer bulb. Consequently, the complexity of the processing and production of the anti-reflection layer on the lamp may be reduced. Alternatively, the anti-reflection layer on the inner surface of the outer bulb may be different compared to the anti-reflection layer applied to the outer surface of the outer bulb. This difference between the two anti-reflection layers may be due to different materials used and/or due to different gradients in the graded refractive index layer.

In an embodiment of the lamp, the lamp comprises another anti-reflection layer applied to the inner surface of the light-transmitting vessel. Said another anti-reflection layer is different from the anti-reflection layer applied to the outer surface of the light- transmitting vessel. Typically the average refractive index of an anti-reflection layer should be in-between the refractive indices of the materials on either side of the

anti-reflection layer. The anti-reflection layer applied to the outer surface of, for example, a discharge vessel preferably has a refractive index which is in-between the refractive index of the discharge vessel material and the refractive index of the environment - typically air. Said another anti-reflection layer is arranged between the gas filling of the discharge vessel and the discharge vessel material, or between the luminescent material and the discharge vessel material. As the gas filling of the discharge vessel and the luminescent material of the discharge vessel typically have a different refractive index compared to the environment - being air - the preferred material from which said another anti-reflection layer is produced is different compared to the material from which the anti-reflection layer is produced.

Alternatively, the anti-reflection layer applied to the outer surface of the light-transmitting vessel may comprise a similar material to said another anti-reflection coating applied to the inner surface - however, still a density or a density gradient of the material constituting the anti-reflection coating and/or said another anti-reflection coating may cause said another anti- reflection coating to be different compared to the anti-reflection coating applied to the outer surface of the light-transmitting vessel.

In an embodiment of the lamp, the lamp is a low-pressure discharge lamp, a high-pressure discharge lamp, a compact fluorescent lamp, a compact fluorescent lamp comprising a cover, a cold-cathode compact fluorescent lamp, or a halogen lamp. A benefit of these embodiments is that the anti-reflective coating can be separated from the harsh environment inside the light-transmitting vessel physics and chemistry. Furthermore, the outer surface of such a lamp is usually more smoothly shaped and therefore easier to coat in the production process.

In the luminaire according to the second aspect of the invention, the luminaire comprises the lamp according to any one of the preceding claims. A luminaire is a lighting fixture which is a combination of a light source and electrical circuitry for connection to a power source. The luminaire may also optionally comprise a reflector for directing the light emitted by the light source, an aperture (with or without a lens), a housing for the alignment and protection of the light source, and further electrical circuitry which may, for example, include a ballast. A benefit of the luminaire according to the invention is that the luminaire comprises an improved intensity of light from the luminaire due to the improved efficiency of the lamp. Alternatively, a reduction of the energy of the lamp may be obtained at the same light intensity.

In the backlighting system according to the third aspect of the invention, the backlighting system comprises the lamp according to the invention.

In the display device according to the fourth aspect of the invention, the display device comprises the backlighting system according to the invention and/or the 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 schematic cross-sectional view of a discharge lamp according to the invention,

Figs. 2 A and 2B show a detailed schematic representation of a graded refractive index layer as anti-reflection layer, Fig. 3 shows transmission versus wavelength of a KMgF ₃ interference anti-reflection layer applied to the outer surface of the discharge vessel,

Figs. 4 A and 4B show two embodiments of compact embodiments of the low-pressure gas discharge lamp according to the invention,

Fig. 5 shows a schematic representation of a high-pressure discharge lamp according to the invention,

Fig. 6 shows a schematic representation of a halogen lamp according to the invention,

Fig. 7 shows a schematic representation of a luminaire according to the invention, and

Fig. 8 shows a display system having a backlighting system 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 schematic cross-sectional view of a discharge lamp 10A, 10B according to the invention. The discharge lamp 10A, 10B as schematically shown in Fig. 1 is a low-pressure gas discharge lamp 10A, 10B which comprises a light transmitting discharge vessel 12 which encloses a discharge space 14 in a gas-tight manner. The discharge space 14 comprises a gas filling, for example, comprising a metal compound and a buffer gas. The low-pressure gas discharge lamp 10A, 10B further comprises discharge means 17 for coupling energy into the discharge space 14, for example, via capacitive coupling (not shown), inductive coupling (not shown), microwave coupling (not shown), or via electrodes 17 to obtain, in operation, a gas discharge 16 in the discharge space 14. The discharge vessel 12 comprises a wall having an inner surface and an outer surface. The inner surface of the discharge vessel 12 faces towards the discharge 16 and the outer surface of the discharge vessel 12 faces away from the discharge 16. The low-pressure gas discharge lamp 10A, 10B further comprises a luminescent layer 18 comprising luminescent material. The luminescent material, for example, absorbs ultraviolet light emitted from the discharge and, for example, converts the absorbed ultraviolet light into visible light.

In an embodiment shown in Fig. 1 the discharge vessel 14 comprises a set of electrodes 17. In Fig. 1 only one electrode 17 of the set of electrodes 17 is shown. The electrodes 17 are electrical connections through the discharge vessel 14 of the low-pressure gas discharge lamp 10A, 10B. By applying an electrical potential difference between the two electrodes 17 a discharge 16 is initiated between the two electrodes 17. This discharge 16 is generally located between the two electrodes 17 and is indicated in Fig. 1 as dashed area 16. Alternative coupling elements such as capacitive couplers (not shown), inductive couplers (not shown), or microwave couplers (not shown) may be used.

In the low-pressure gas discharge lamps 10A, 10B, generally the luminescent layer 18 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 BaMgAlioOi7:Eu ²⁺ (also often indicated as BAM, emitting the primary color blue), LaP0 ₄ :Ce,Tb ( also often indicated as LAP, emitting the primary color green) and Y ₂ C"3:Eu (also often indicated as YOX, emitting the primary color red) is used to obtain substantially white light. In the embodiment of the low-pressure gas discharge lamp 10A, 10B shown in Fig. 1, the luminescent layer 18 is applied to the inner surface of the discharge vessel 12 over the entire area of the inner surface. Alternatively, the luminescent layer 18 may be applied to the outer surface (not shown) of the discharge vessel 12.

In Fig. 1 two different embodiments of the discharge lamp 10A, 10B are shown which are separated by the dashed straight line. The embodiment of the discharge lamp indicated as 10A comprises the luminescent layer 18 applied to the inner surface of the discharge vessel 12 and comprises the anti-reflection layer 20 applied to the entire area of the outer surface of the discharge vessel 12. In the embodiment of the discharge lamp indicated as 10B another anti-reflection layer 24 is applied between the discharge vessel 12 and the luminescent layer 18. Typically, said another anti-reflection layer 24 is different from the anti-reflection layer 20, as the environment surrounding said another anti-reflection layer 24 is different. This difference between the anti-reflection layer 20 and said another

anti-reflection layer 24 may be caused by the material from which the anti-reflection layer 20 and said another anti-reflection layer 24 is manufactured, or may be caused by the density or varying density of said another anti-reflection layer 24 compared to the anti-reflection layer 20. A variation of a thickness of said another anti-reflection layer over more than 80 percent of the area of the outer surface covered by said another anti-reflection layer is less than 5 micrometer, preferably less than 0.5 micrometer.

Figs. 2 A and 2B show a detailed schematic representation of a graded refractive index layer 20A, 20B as anti-reflection layer 20A, 20B as applied to the outer surface of the discharge vessel 12 as shown in the dash-dotted circle of Fig. 1. In Fig. 2A a first embodiment of the graded refractive index layer 20A is schematically shown in which the graded refractive index layer 20A comprises a patterned surface having a wavelength λ periodic structure applied to the surface. This pattern applied to the surface causes the refractive index of the graded refractive index layer 20A to gradually vary through the thickness T of the graded refractive index layer 20A. In Fig. 2B a second embodiment of the graded refractive index layer 20B is schematically shown in which the graded refractive index layer 20B comprises a nano-porous layer 20B in which a gradient in density of the nano-porous layer 20B causes the refractive index to vary gradually through the thickness T of the graded refractive index layer 20B.

Preferably, the anti-reflective layer will cover the area of the surface of the light transmitting vessel that actually serves as transmitting surface. In most practical lamps this area corresponds to the entire surface of the light emitting vessel.

Fig. 3 shows transmission versus wavelength of a KMgF ₃ interference anti-reflection layer 20 applied to the outer surface of the discharge vessel 12 as shown in Fig. 1 for layers having a mutually different thickness. As can clearly be seen from the graph of Fig. 3, the layer thickness at which an optimum transmission is obtained is approximately 100 nanometer or 150 nanometer. When the thickness of the interference anti-reflection layer 20 is at 1150 nanometer, the transmission of the anti-reflection layer 20 varies for light of different wavelengths. Such a varying transmission which is obtained at the anti-reflection layer 20 of 1150 nanometer may alter the emission spectrum of the discharge lamp 10A, 10B, 30A, 30B, 40. By using a specific mixture of luminescent material 18, this varying transmission of the anti-reflection layer 20 may be anticipated upon and may be compensated such that the spectrum emitted by the discharge lamp 10A, 10B, 3 OA, 30B, 40 according to the invention corresponds to the required spectrum.

Figs. 4 A and 4B show two compact embodiments 3 OA, 30B of the low- pressure gas discharge lamp 3 OA, 30B according to the invention. The compact fluorescent lamps 30A, 30B comprise a plurality of U-shaped discharge vessels 32 which are packed closely together. The U-shaped discharge vessels 32 comprise the discharge space 34 comprising, in operation, a discharge 36 and typically comprise a luminescent layer (not indicated). The outer wall of the discharge vessel 32 comprises the anti-reflection layer 20 according to the invention. The compact fluorescent lamps 3 OA, 30B further comprise an additional electronic circuit 39 hidden in a base of the compact fluorescent lamp 3 OA, 30B. The additional electronic circuit 39 regulates the switching on of the compact fluorescent lamp 30A, 30B. Due to the compact arrangement of the compact fluorescent lamp 30A, 30B, the temperature, in operation, of the luminescent material 38 of the compact fluorescent lamp 30A, 30B increases at least locally above 80 degrees Celsius.

In the embodiment of the compact fluorescent lamp 30B shown in Fig. 4B, the compact fluorescent lamp 3 OB is arranged within an outer bulb 38. This outer bulb 38 may have any shape, for example, mimicking the shape of an incandescent lamp. In the embodiment shown in Fig. 3B, the outer bulb 38 has a cylindrical shape. The inner surface and/or outer surface of the outer bulb 38 may comprise a further anti-reflection layer 22 for further reducing reflection back into the discharge lamp 10A, 10B, 30A, 30B, 40.

Fig. 5 shows a schematic representation of a high-pressure discharge lamp 40 according to the invention. The high-pressure discharge lamp 40 comprises a discharge vessel 42 having a discharge space 44 in which, in operation, a discharge 46 is present emitting light. The outer wall of the discharge vessel 42 comprises an anti-reflection layer 20 for reducing the reflection of light generated inside the discharge vessel 42 back into the discharge vessel 42. In the embodiment shown in Fig. 5, optionally an outer bulb 48 is present. This outer bulb 48 may comprise a further anti-reflection layer 22 either on the outer wall of the outer bulb 48 facing the discharge vessel 42, or on the inner wall of the outer bulb 48 facing away from the discharge vessel 42, or on both the inner and outer wall of the outer bulb 48.

Fig. 6 shows a schematic representation of a halogen lamp 50 according to the invention. The halogen lamp 50 comprises a light-transmitting vessel 52 having a space 54 in which, in operation, a filament 57 is present for emitting light. The outer wall of the light- transmitting vessel 52 comprises an anti-reflection layer 20 for reducing the reflection of light generated inside the light-transmitting vessel 52 back into the light-transmitting vessel 52.

Fig. 7 shows a schematic representation of a luminaire 60 according to the invention. A luminaire 60 is a lighting fixture which is a combination of a light source 10A, 10B and electrical circuitry for connection to a power source (not shown). The luminaire 60 may also optionally comprise a reflector for directing the light emitted by the light source, an aperture (with or without a lens), a housing for the alignment and protection of the light source, and further electrical circuitry which may, for example, include a ballast. The luminaire 60 according to the invention comprises a lamp 10A, 10B, 30A, 30B, 40, 50 according to the invention, which provides either an improved intensity of light from the luminaire 60 due to the improved efficiency of the lamp 10A, 10B, 30A, 30B, 40, 50 or which provides a reduction of the energy of the lamp 10A, 10B, 30A, 30B, 40, 50 while emitting the same light intensity.

Fig. 8 shows a display system 70 having a backlighting system 72 according to the invention. The display system 70 comprises a display 74, for example, a well known liquid crystal display 74. 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 electric 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 72 will be transmitted. When the light valve is switched to, for example, "dark" the light emitted from the backlighting system 72 will be blocked. In that way an image can be produced on the display 74.

The backlighting system 72 comprises a lamp 10A, 10B, 30A, 30B, 40, 50 according to the invention, for example, a low-pressure mercury vapor discharge lamp 10A, 10B. The backlighting system 72 typically comprises an array of low-pressure mercury vapor discharge lamps 10A, 10B or comprises a low-pressure gas discharge lamp which meanders parallel to the display 74. As the intensity of the light emitted by a backlighting system 72 can always be improved, the use of the lamp 10A, 10B, 30A, 30B, 40, 50 according to the invention in the backlighting system 72 will beneficially improve the efficiency of the backlighting system 72 - and thus of the display device 70.

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.