WO2006114743A2

WO2006114743A2 - Backlighting system and display system

Info

Publication number: WO2006114743A2
Application number: PCT/IB2006/051242
Authority: WO
Inventors: Marco Van As; Jean P. Jacobs; Jan P. Dekker
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-04-27
Filing date: 2006-04-21
Publication date: 2006-11-02
Also published as: WO2006114743A3

Abstract

The invention relates to a backlighting system (Bl) combining a low-pressure mercury vapor discharge lamp (Fl, F3) with a plurality of Light Emitting Diodes (LEDs) (Ll), in which system the light emitted by the LEDs is mixed inside the discharge vessel (V) of the low-pressure mercury vapor discharge lamp. In a preferred embodiment, the low- pressure mercury vapor discharge lamp comprises a diffuser coating with an aperture (Al, A3) through which the light emitted by the LEDs is admitted to the discharge vessel. In another preferred embodiment, the plurality of LEDs are side-emitting LEDs, the light being emitted predominantly in a direction parallel to a light exit window (EW) of the backlighting system. In a further preferred embodiment, the low-pressure mercury vapor discharge lamp provides a first and a second primary color, and the plurality of LEDs provide a third primary color such that the combination of the three primary colors provides an improved covering of the EBU color standard.

Description

Backlighting system and display system

FIELD OF THE INVENTION:

The invention relates to a backlighting system for illuminating a display device comprising a low-pressure mercury vapor discharge lamp and a plurality of light emitting diodes. The invention further relates to a display system and a Liquid Crystal Display device comprising such a backlighting system.

BACKGROUND OF THE INVENTION:

Backlighting units typically comprise low-pressure mercury vapor discharge lamps for providing a uniform illumination of a display device. The low-pressure mercury vapor discharge lamps comprise a discharge vessel in which an inner wall of the discharge vessel is provided with a luminescent layer comprising a luminescent material. The luminescent material comprises a mix of different phosphors. Typically a mix of three different phosphors is used in which each one of the different phosphors is associated with a primary color. The mix of different phosphors inside the low-pressure mercury vapor discharge lamp determines the color of the low-pressure mercury vapor discharge lamp and thus the color of the backlighting system. When the backlighting system is used together with, for example, a Liquid Crystal Display (further also referred to as LCD), the color gamut which can be displayed by the LCD is determined by a combination of the mix of phosphors used in the low-pressure mercury vapor discharge lamp together with the transmission characteristics of the color filters used in the LCD. A display system in which a backlighting system comprising currently commercially used phosphors in the low-pressure mercury vapor discharge lamp is combined with an LCD comprising currently commercially available color filters typically displays a color gamut which does not fully cover the color gamut defined in, for example, the European Broadcasting Union (EBU) color standard.

Alternative backlighting units comprise a plurality of Light Emitting Diodes (further also referred to as LED) which provide a uniform illumination to the display device. The plurality of LEDs comprises different colors to cover the complete color gamut. The use of LEDs provides more freedom in choosing a combination of colors of the LEDs, such that the backlighting unit together with an LCD provides a display system which is capable of substantially covering the full standardized EBU color gamut. A drawback in using a plurality of LEDs as light sources in a backlighting unit is that LEDs provide a low luminous efficacy, which results in a relatively high power consumption. A further drawback is that the current market price of LEDs is much higher than that of low-pressure mercury vapor discharge lamps, and thus a solution in which the backlighting unit comprises a plurality of LEDs as light sources would result in a relatively expensive backlighting system. Finally, the backlighting unit which comprises LEDs requires an additional thickness for achieving a good color mixing of the individual LEDs for providing a uniformly illuminated display. The Japanese patent application JP 2004/294984 discloses a backlighting unit which provides an improved color gamut by combining a low-pressure mercury vapor discharge lamp with an array of LEDs. The light of the low-pressure mercury vapor discharge lamp provides the green and blue contributions to the color gamut. The array of LEDs provides the red contribution to the color gamut. In one embodiment of the backlighting unit, the low-pressure mercury vapor discharge lamp and the array of LEDs are arranged at a same rim of a light-mixing element of the backlighting unit. The light-mixing element comprises a sandwich arrangement of two optical waveguides, one for the array of LEDs and one for the low-pressure mercury vapor discharge lamp.

In the known backlighting unit an additional optical waveguide is necessary for mixing the light of the array of LEDs.

SUMMARY OF THE INVENTION:

It is an object of the invention to provide a backlighting system which combines a low-pressure mercury vapor discharge lamp with a plurality of LEDs and which comprises an improved arrangement for mixing the light of the plurality of LEDs.

According to a first aspect of the invention, the object is achieved with a backlighting system for illuminating a display device via a light exit window, the backlighting system comprising: a low-pressure mercury vapor discharge lamp having a discharge vessel, a wall of the discharge vessel being provided with a luminescent layer comprising a luminescent material, and a plurality of light emitting diodes, the discharge vessel of the low-pressure mercury vapor discharge lamp being arranged as a light-mixing chamber for light emitted by the light emitting diodes. The effect of the measures according to the invention is that the use of the discharge vessel of the low-pressure mercury vapor discharge lamp as the light-mixing chamber renders it possible to omit an additional optical waveguide for mixing the light of the plurality of LEDs. The omission of the additional optical waveguide results in a reduced cost of the backlighting system. A further benefit of omitting the additional optical waveguide for the plurality of LEDs is that the weight of the backlighting system can be reduced.

In an embodiment of the system, the discharge vessel further comprises a diffuser coating, and the light emitted by the light emitting diodes is coupled into the discharge vessel via an aperture in the diffuser coating. The benefit of this embodiment is that the apertures provide a good coupling of the light of the LEDs into the discharge vessel, which improves the mixing of the light inside the discharge vessel.

In an embodiment of the system, the low-pressure mercury vapor discharge lamp is arranged for direct incidence of light emitted by the low-pressure mercury vapor discharge lamp on the display device via the light exit window. Direct illumination of the display device by the low-pressure mercury vapor discharge lamp is typically effected in backlighting systems which have large light exit windows, for example an LCT TV system. The optical waveguides necessary for mixing the light of the plurality of LEDs and uniformly illuminating large light exit windows typically are relatively large and typically are relatively thick. Therefore, the optical waveguides for use in backlighting systems having large light exit windows are relatively expensive and substantially contribute to the weight of the display device, he use of the discharge vessel of the low-pressure mercury vapor discharge lamp as a light-mixing chamber for mixing the light of the LEDs typically reduces the cost and the weight of the backlighting system. In an embodiment of the system, the backlighting system comprises a plurality of low-pressure mercury vapor discharge lamps, the light emitting diodes being arranged between the low-pressure mercury vapor discharge lamps. The benefit of this embodiment is that the backlighting system can be made relatively thin. A further benefit of this embodiment is that the electronics necessary for the low-pressure discharge lamps and the LEDs can be produced on a single printed circuit board (further also referred to as PCB), which further reduces the cost of the backlighting system.

In an embodiment of the system, the backlighting system comprises a mirror arranged between the plurality of light emitting diodes and the light exit window for preventing direct incidence of light emitted by the plurality of light emitting diodes on the display device via the light exit window. The benefit of this embodiment is that the mirror ensures that the light is first mixed inside the discharge vessel of a low-pressure mercury vapor discharge lamp before it is emitted via the light exit window of the backlighting system. This embodiment improves the uniformity of the light emitted by the backlighting system.

In an embodiment of the system, the backlighting system comprises an optical waveguide for illuminating the display device, the optical waveguide comprising the light exit window and further comprising a light entrance window for coupling the light from the low-pressure mercury vapor discharge lamp and from the plurality of light emitting diodes into the optical waveguide, the light entrance window being arranged substantially perpendicularly to the light exit window. The benefit of this embodiment is that a single entrance window can be used for coupling both the light from the low-pressure mercury vapor discharge lamp and from the plurality of LEDs into the optical waveguide. Since the discharge vessel of the low-pressure mercury vapor discharge lamp is used as a mixing chamber for the plurality of LEDs, only a single optical waveguide having a single guide entrance window is sufficient for achieving a uniform illumination of the light exit window of the backlighting system.

In an embodiment of the system, the low-pressure mercury vapor discharge lamp is arranged between the light exit window and the plurality of light emitting diodes. The benefit of this embodiment is that this arrangement ensures that the light from the plurality of LEDs is mixed in the discharge vessel of the low-pressure mercury vapor discharge lamp before illuminating the light exit window of the backlighting system.

In an embodiment of the system, the low-pressure mercury vapor discharge lamp is arranged between the light exit window and the plurality of light emitting diodes, and optical elements are arranged adjacent to the plurality of light emitting diodes for preventing direct incidence of light emitted by the plurality of light emitting diodes on the display device via the light exit window. The benefit of this embodiment is that the optical elements further improve the uniformity of the light emitted by the light exit window by preventing direct incidence of light from the plurality of LEDs on the display device via the light exit window of the backlighting system.

In an embodiment of the system, the plurality of light emitting diodes comprise a side emission pattern for predominantly emitting light in a direction parallel to the light exit window. The benefit of this embodiment is that the emission pattern of the plurality of LEDs ensures that the light is first mixed inside the discharge vessel of the low-pressure mercury vapor discharge lamp before illuminating the display device via the light exit window of the backlighting system.

In an embodiment of the system, the luminescent material of the low-pressure mercury vapor discharge lamp comprises Europium-activated Barium Aluminate (further also referred to as BAL). In known low-pressure mercury vapor discharge lamps typically Europium-activated Barium Magnesium Aluminate (further also referred to as BAM) is used. When BAM is used in a low-pressure mercury vapor discharge lamp of a backlighting system of an LCD device the color gamut which can be displayed by the LCD device does not fully cover the color gamut as defined in, for example, the European Broadcasting Union (EBU) color standard. This is typically due to the combination of the luminescent material used in the low-pressure mercury vapor discharge lamp combined with the color filters typically used in the LCD device. A benefit when using BAL in a low-pressure mercury vapor discharge lamp of the backlighting system of an LCD device is that the combination of the light emitted by the BAL together with the color filters enable an improved color saturation of the LCD device which results in an improved color gamut which can be displayed by the LCD device having an improved coverage of the EBU color standard.

In an embodiment of the system, the luminescent material of the low-pressure mercury vapor discharge lamp comprises Europium-activated Yttrium Oxysulfide (further also referred to as YOS). In known low-pressure mercury vapor discharge lamps typically Europium-activated Yttrium Oxide (further also referred to as YOX) is used. When YOX is used in a low-pressure mercury vapor discharge lamp of a backlighting system of an LCD device the color gamut which can be displayed by the LCD device does not fully cover the color gamut as defined in, for example, the European Broadcasting Union (EBU) color standard. Again, this is typically due to the combination of the luminescent material used in the low-pressure mercury vapor discharge lamp combined with the color filters typically used in the LCD device. A benefit when using YOS in a low-pressure mercury vapor discharge lamp of the backlighting system of an LCD device is that the combination of the light emitted by the YOS together with the color filters enable an improved color saturation of the LCD device which results in an improved color gamut which can be displayed by the LCD device having an improved coverage of the EBU color standard.

In an embodiment of the system, the low-pressure mercury vapor discharge lamp is a Hot Cathode Fluorescent Lamp (further also referred to as HCFL). The benefit of using an HCFL as the low-pressure mercury vapor discharge lamp is that it renders possible a fast on/off switching of the low-pressure mercury vapor discharge lamp, which is especially beneficial when the backlighting system is used in a scanning mode of operation.

In an embodiment of the system, the backlighting system comprises a plurality of low-pressure mercury vapor discharge lamps, wherein the low-pressure mercury vapor discharge lamps are used in a scanning mode during operation and wherein the plurality of light emitting diodes are used in a continuous mode during operation. A driving circuit necessary for driving the low-pressure mercury vapor discharge lamps in a scanning mode of operation is typically different from a driving circuit for driving the plurality of LEDs in a scanning mode of operation. Driving the plurality of LEDs in a continuous mode of operation renders it possible to simplify the driving circuit for driving the plurality of LEDs in a scanning mode because of the omission of a scanning part of the driving electronics. The benefit of this embodiment is that the simplification of the driving electronics for the plurality of LEDs reduces the cost of the backlighting system. This embodiment is especially beneficial when the plurality of LEDs contribute the primary color red to the backlighting system. The sensitivity of the human eye to the primary color red is relatively low, so the impact on the reproduction of motion on an LCD when the LEDs are driven in a continuous mode of operation and the low-pressure mercury vapor discharge lamps are driven in a scanning mode of operation is expected to be minor compared with the situation in which all colors are driven in a scanning mode of operation. In an embodiment of the system, the low-pressure mercury vapor discharge lamp comprises a first luminescent material that is associated with a first primary color and a second luminescent material that is associated with a second primary color, while the plurality of light emitting diodes provide a third primary color which is the primary color red. Commercially available low-pressure mercury vapor discharge lamps typically comprise a third luminescent material associated with a third primary color, i.e. the color red. In commercially available third luminescent materials the conversion of ultraviolet light from the mercury vapor discharge into light of the primary color red is relatively inefficient owing to a large Stokes shift. Because of this relatively inefficient conversion, a relatively large percentage of the third luminescent material is necessary in commercially available low- pressure mercury vapor discharge lamps compared with the first and second luminescent materials if a good color gamut is to be provided. Another drawback of the relatively inefficient conversion in commercially available third luminescent materials is that the relatively inefficient conversion results in an additional heat generation which may require additional cooling arrangements inside the backlighting system. The efficiency of the backlighting system is improved, and additional cooling arrangements can be omitted in that the third luminescent material is not present in the low-pressure mercury vapor discharge lamp and LEDs are added in the backlighting system for providing the third primary color red.

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 backlighting system according to the invention in which LEDs emit light into the discharge vessel of a low-pressure mercury vapor discharge lamp via an aperture,

Fig. 2 shows a backlighting system according to the invention in which the LEDs are side-emitting LEDs, Fig. 3 shows a backlighting system according to the invention in which light emitted by the low-pressure mercury vapor discharge lamp and emitted by the LEDs is distributed across the display device via an optical waveguide,

Fig. 4 shows a backlighting system according to the invention in which additional optical elements are provided which prevent a direct incidence of light emitted by the plurality of LEDs on the display device,

Fig. 5 shows a display device.

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 THE PREFERRED EMBODIMENTS:

Fig. 1 shows a backlighting system Bl according to the invention in which light is generated by a plurality of low-pressure mercury vapor discharge lamps Fl, F3 together with a plurality of LEDs Ll. The low-pressure mercury vapor discharge lamps Fl, F3 comprise a discharge vessel V having a luminescent layer P typically arranged at the inner wall of the discharge vessel V. The discharge vessel V typically further comprises a diffuser coating Cl, C3, for example at the outer wall of the discharge vessel V. This diffuser coating Cl, C3 may be used to adapt the light emission distribution of the low-pressure mercury vapor discharge lamps Fl, F3. The discharge vessel V encloses a low-pressure mercury vapor environment. A discharge generated through the low-pressure mercury vapor environment causes the mercury vapor inside the discharge vessel V to emit ultraviolet light. The ultraviolet light is absorbed by the luminescent layer P and converted into visible light of a predefined color. The luminescent layer P comprises a first luminescent material which is associated with a first primary color. Ultraviolet light which illuminates the first luminescent material is converted into the first primary color, for example the primary color blue, which is emitted by the low-pressure mercury vapor discharge lamp Fl, F3. The luminescent layer P further comprises a second luminescent material which is associated with a second primary color. Ultraviolet light which illuminates the second luminescent material is converted into the second primary color, for example the primary color green, which is emitted by the low- pressure mercury vapor discharge lamp Fl, F3. The combination of the first and second luminescent materials determines the predefined color of the low-pressure mercury vapor discharge lamp Fl, F3. The backlighting system Bl further comprises a plurality of LEDs Ll which emit light of a third primary color, for example the primary color red. The plurality of LEDs Ll shown in Fig. 1 emit light in a substantial Lambertian pattern, indicated as the emission pattern EPl, substantially towards the low-pressure mercury vapor discharge lamp Fl, F3. The combination of the first, second, and third primary colors enables the backlighting system Bl to provide a full color gamut to the display device Di, for example substantially covering the full EBU color standard. Fig. IA is a cross-sectional view of the backlighting system Bl and Fig. IB shows the backlighting system Bl viewed through the display device Di of the backlighting system Bl.

In the arrangement shown in Figs. IA and IB, the plurality of LEDs Ll predominantly emit the third primary color via an aperture Al, A3 in the diffuser coating Cl, C3 into the discharge vessel V of the low-pressure mercury vapor discharge lamp Fl, F3.

Typically, the aperture Al, A3 still comprises luminescent material P for preventing UV light from issuing via the aperture Al, A3 into the backlighting system Bl. In a first embodiment of the low-pressure mercury vapor discharge lamp Fl, the aperture Al is a longitudinal aperture Al arranged at a side of the low-pressure mercury vapor discharge lamp Fl facing away from the display device Di. In a second embodiment of the low-pressure mercury vapor discharge lamp F3, the low-pressure mercury vapor discharge lamp F3 comprises a plurality of circular apertures A3 arranged at a side of the low-pressure mercury vapor discharge lamp F3 facing away from the display device Di. In both the first and the second embodiment shown in Fig. 1, the light emitted by the plurality of LEDs Ll is predominantly emitted through an aperture Al, A3 into the discharge vessel V of the low-pressure mercury vapor discharge lamp Fl, F3 and mixed inside the discharge vessel V before being emitted via the light exit window EW to illuminate the display device Di. The discharge vessel V of the low-pressure mercury vapor discharge lamp Fl, F3 is used as light-mixing chamber for the light emitted by the plurality of LEDs Ll, which provides a reduction in complexity and cost owing to the omission of a separate optical waveguide for mixing the light of the plurality of LEDs.

Fig. 2 shows a backlighting system B2, B3 according to the invention in which the plurality of LEDs are side-emitting LEDs L2. The plurality of side-emitting LEDs L2 are arranged in the backlighting system B2, B3 to emit light predominantly in a direction parallel to the light exit window EW, as shown in Fig. 2 as an emission pattern EP2. The backlighting system B2, B3 further comprises a plurality of the low-pressure mercury vapor discharge lamps F2 comprising a discharge vessel V having a luminescent layer P typically arranged at the inner wall of the discharge vessel V. The luminescent layer P comprises the first and second luminescent materials, which are associated with the first and second primary colors, for example the primary colors blue and green. In contrast to the diffuser coatings Cl, C3 shown in Fig. 1, the diffuser coating C2 of the low-pressure mercury vapor discharge lamp F2 of the backlighting system B2, B3 as shown in Fig. 2 does not comprise an aperture for admitting the light from the plurality of side-emitting LEDs L2 into the discharge vessel V. Instead, the light emitted by the plurality of side-emitting LEDs L2 impinges on an outer wall of the discharge vessel V and is admitted through the diffuser coating C2 into the discharge vessel V. The discharge vessel is again used as a mixing chamber for the light emitted by the plurality of side-emitting LEDs L2. The diffuser coating C2 acts as a diffusing layer for the light from the plurality of side-emitting LEDs.

In Fig. 2A, the low-pressure mercury vapor discharge lamp F2 is arranged between the light exit window EW and the plurality of side-emitting LEDs L2. In this embodiment, any leakage of light that might be emitted perpendicularly to the light exit window EW by the plurality of LEDs L2 and that might directly impinge on the display device (not shown) via the light exit window EW is diffused by the presence of the low- pressure mercury vapor discharge lamp F2. The dotted arrows indicate the general direction of the light emitted by the plurality of side-emitting LEDs L2.

In Fig. 2B, the plurality of side-emitting LEDs L2 are arranged between the low-pressure mercury vapor discharge lamps F2. To ensure that no light that might be emitted perpendicularly to the light exit window EW by the plurality of LEDs L2 is directly incident on the display device (not shown) via the light exit window EW, a mirror M is inserted between the light exit window EW and the plurality of side-emitting LEDs L2. This embodiment reduces the thickness T2 of the backlighting system B3 compared with the thickness Tl of the backlighting system B2 shown in Fig. 2A (T2 < Tl).

Fig. 2 shows a preferred embodiment of the low-pressure mercury vapor discharge lamp F2 which comprises the diffuser coating C2, typically arranged at the outer wall of the discharge vessel V of the low-pressure mercury vapor discharge lamp F2. In an alternative embodiment, a low-pressure mercury vapor discharge lamp may be used which does not comprise a diffuser coating C2, without departing from the scope of the invention.

Fig. 3 shows a backlighting system B4 according to the invention in which light emitted by the low-pressure mercury vapor discharge lamp Fl and emitted by the plurality of LEDs Ll is distributed across the display device Di via an optical waveguide LG. The optical waveguide LG comprises a light exit window EW for coupling out the light from the optical waveguide LG towards the display device Di. The light may be coupled out from the optical waveguide LG via, for example, lenses, diffusers, prisms, scratches, deformations, or other out-coupling elements well known in the art. The optical waveguide LG further comprises a light entrance window IW for coupling the light from the low-pressure mercury vapor discharge lamp Fl and from the plurality of LEDs Ll into the optical waveguide LG. In the embodiment shown in Fig. 3, the light entrance window IW is arranged substantially perpendicularly to the light exit window EW of the optical waveguide LG. Typically, the light is retained inside the optical waveguide LG by total internal reflection. The low- pressure mercury vapor discharge lamp Fl is identical to the low-pressure mercury vapor discharge lamp Fl of Fig. 1 and comprises an aperture Al for receiving the light of the plurality of LEDs Ll . In the embodiment shown, the low-pressure mercury vapor discharge lamp is arranged between the plurality of LEDs Ll and the light entrance window IW. The light of the plurality of LEDs Ll is mixed inside the discharge vessel V of the low-pressure mercury vapor discharge lamp Fl before it is coupled into the optical waveguide LG via the light entrance window IW. Alternatively, the low-pressure mercury vapor discharge lamp F3 as shown in

Fig. 1 may be used, in which the plurality of circular apertures A3 are arranged in the diffuser coating C3 for admitting the light from the plurality of LEDs Ll to the discharge vessel V. The low-pressure mercury vapor discharge lamp F2 as shown in Fig. 2 may also be used, in which the light emitted by the plurality of LEDs Ll impinges on the diffuser coating C2 of the discharge vessel V and is admitted to the discharge vessel V through the diffuser coating C2. Furthermore, the position of the LEDs Ll with respect to the light entrance window IW or with respect to the low-pressure mercury vapor discharge lamp Fl, F2, F3 may be altered.

Fig. 4 shows a backlighting system B 5 according to the invention in which additional optical elements S are provided which prevent direct incidence of light emitted by the plurality of LEDs L3 on the display device (not shown) via the light exit window EW. The plurality of LEDs L3 shown in this embodiment is, for example, a plurality of Suria.ce Mounted Devices (SMD) with? Lambertian emitter LEDs. The low-pressure mercury vapor discharge lamps F2 shown in this embodiment are identical to the low-pressure mercury vapor discharge lamps F2 shown in Figs. 2A and 2B. The low-pressure mercury vapor discharge lamp F2 is arranged between the light exit window EW and the plurality of LEDs L3. The light emitted by the plurality of LEDs L3 impinges on the diffuser coating C2 of the discharge vessel V and is admitted to the discharge vessel V through the diffuser coating C2. The discharge vessel is used as a mixing chamber for the light emitted by the plurality of LEDs L3. The diffuser coating C2 acts as a diffusing layer for the light from the plurality of LEDs L3.

Alternatively, the low-pressure mercury vapor discharge lamps Fl, F3 as shown in Fig. 1, in which the diffuser coatings Cl, C3 comprise apertures Al, A3 for admitting the light from the plurality of LEDs L3 to the discharge vessel V, may be used in this embodiment.

The optical elements S are, for example, quarter circular dielectric rods, for example made of PolyMethylMethAcrylate (PMMA) or Glass. The optical elements S are positioned adjacent to the plurality of LEDs L3 and are arranged for changing the emission profile of light emitted by the LEDs L3 but not transmitted through the diffuser coating C2 into the discharge vessel V located directly above the plurality of LEDs L3. The optical elements S redirect the light, for example, into a substantially parallel beam of light predominantly emitting light in a direction parallel to the light exit window EW, shown as an emission profile EP3 in Fig. 4. The substantially parallel beam of light generated by the optical elements S is, for example, directed so as to impinge on an adjacent low-pressure mercury vapor discharge lamp F2, is transmitted through the diffuser coating C2 of the adjacent low-pressure mercury vapor discharge lamp F2, and is mixed inside the discharge vessel V of the adjacent low-pressure mercury vapor discharge lamp F2.

In an alternative embodiment, the optical elements S may have a different geometry and may also comprise diffusing elements for diffusing the light emitted by the plurality of LEDs L3 which was not mixed inside the discharge vessel V located directly above the plurality of LEDs L3. The optical elements S may also be used to fix the position of the low-pressure mercury vapor discharge lamps F2 inside the backlighting system B5.

In an embodiment of the backlighting system Bl, B2, B3, B4, B5, the backlighting system Bl, B2, B3, B4, B5 comprises low-pressure mercury vapor discharge lamps Fl, F2, F3 comprising Europium-activated Barium Aluminate (BAL) and/or comprising Europium-activated Yttrium Oxysulfide (YOS). A benefit when using BAL and/or YOS in a low-pressure mercury vapor discharge lamp of a backlighting system Bl, B2, B3, B4, B5 of a Liquid Crystal Display device is that the combination of the light emitted by the BAL and/or the light emitted by the YOS together with the typical color filters of the Liquid Crystal Display device provide an improved color saturation of the Liquid Crystal Display device compared to the conventional luminescent materials used in low-pressure mercury vapor discharge lamps Fl, F2, F3. The use of BAL and/or YOS typically results in an improved coverage of the EBU color standard by the Liquid Crystal Display device.

Fig. 5 shows a display device, for example a Liquid Crystal Display device comprising the backlighting system Bl, B2, B3, B4, B5 according to the invention.

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. The 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. 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. Backlighting system (Bl, B2, B3, B4, B5) for illuminating a display device (Di) via a light exit window (EW), the backlighting system (Bl, B2, B3, B4, B5) comprising a low-pressure mercury vapor discharge lamp (Fl, F2, F3) having a discharge vessel (V), a wall of the discharge vessel (V) being provided with a luminescent layer (P) comprising a luminescent material, and a plurality of light emitting diodes (Ll , L2, L3), the discharge vessel (V) of the low-pressure mercury vapor discharge lamp (Fl, F2, F3) being arranged as a light-mixing chamber for light emitted by the plurality of light emitting diodes (Ll, L2, L3).

2. Backlighting system (Bl, B2, B3, B4, B5) as claimed in claim 1, the discharge vessel (V) further comprising a diffuser coating (Cl, C2, C3), wherein the light emitted by the plurality of light emitting diodes (Ll) is coupled into the discharge vessel (V) via an aperture (Al, A3) in the diffuser coating (Cl, C3).

3. Backlighting system (Bl, B2, B3, B5) as claimed in claim 1 or 2, wherein the low-pressure mercury vapor discharge lamp (Fl, F2, F3) is arranged for direct incidence of light emitted by the low-pressure mercury vapor discharge lamp (Fl, F2, F3) on the display device (Di) via the light exit window (EW).

4. Backlighting system (B3) as claimed in claim 1 or 2, having a plurality of low- pressure mercury vapor discharge lamps (Fl, F2, F3), wherein the plurality of light emitting diodes (L2) are arranged between the low-pressure mercury vapor discharge lamps (Fl, F2, F3).

5. Backlighting system (B3) as claimed in claim 4, wherein the backlighting system (B3) comprises a mirror (M) arranged between the plurality of light emitting diodes (L2) and the light exit window (EW) for preventing direct incidence of light emitted by the plurality of light emitting diodes (L3) on the display device (Di) via the light exit window (EW).

6. Backlighting system (B4) as claimed in claim 1 or 2, wherein the backlighting system (B4) comprises an optical waveguide (LG) for illuminating the display device (Di), the optical waveguide (LG) comprising the light exit window (EW) and further comprising a light entrance window (IW) for coupling the light from the low-pressure mercury vapor discharge lamp (F2) and from the plurality of light emitting diodes (Ll) into the optical waveguide (LG), the light entrance window (IW) being oriented substantially perpendicularly to the light exit window (EW).

7. Backlighting system (Bl, B2, B4, B5) as claimed in claim 3 or 6, wherein the low-pressure mercury vapor discharge lamp (Fl, F2, F3) is arranged between the light exit window (EW) and the plurality of light emitting diodes (Ll, L2, L3).

8. Backlighting system (B5) as claimed in claim 7, wherein optical elements (S) are arranged adjacent to the plurality of light emitting diodes (L3) for preventing direct incidence of light emitted by the plurality of light emitting diodes (L3) on the display device (Di) via the light exit window (EW).

9. Backlighting system (B2, B3) as claimed in claim 1 or 2, wherein the plurality of light emitting diodes (L2) comprise a side emission pattern (EP2) for emitting light predominantly in a direction parallel to the light exit window (EW).

10. Backlighting system (Bl, B2, B3, B4, B5) as claimed in claim 1 or 2, wherein the luminescent material comprises Europium-activated Barium Aluminate and/or comprises Europium-activated Yttrium Oxysulfide.

11. Backlighting system (Bl, B2, B3, B4, B5) as claimed in claim 1 or 2, wherein the low-pressure mercury vapor discharge lamp (Fl, F2, F3) is a Hot Cathode Fluorescent Lamp.

12. Backlighting system (Bl, B2, B3, B4, B5) as claimed in claim 1 or 2, comprising a plurality of low-pressure mercury vapor discharge lamps (Fl, F2, F3), wherein the low-pressure mercury vapor discharge lamps (Fl, F2, F3) are used in a scanning mode during operation

13. Backlighting system (Bl, B2, B3, B4, B5) as claimed in claim 12, wherein the plurality of light emitting diodes (Ll, L2, L3) are used in a continuous mode during operation.

14. Backlighting system (Bl, B2, B3, B4, B5) as claimed in claim 1 or 2, wherein the low-pressure mercury vapor discharge lamp (Fl, F2, F3) comprises a first luminescent material that is associated with a first primary color and a second luminescent material that is associated with a second primary color, and wherein the plurality of light emitting diodes provide a third primary color.

15. Backlighting system (Bl, B2, B3, B4, B5) as claimed in claim 14, wherein the third primary color is a primary color red.

16. Display system (Ds) comprising a backlighting system (Bl, B2, B3, B4, B5) as claimed in claim 1 or 2.

17. Liquid Crystal Display device comprising a backlighting system (Bl, B2, B3, B4, B 5) as claimed in claim 1 or 2.