Scanning backlight system

FIELD OF THE INVENTION

The invention relates to a scanning backlight system for illuminating a display device via a light exit window, the scanning backlight system comprising a plurality of first light sources emitting at least a first primary color towards the light exit window and at least one second light source emitting at least a second primary color towards the light exit window.

The invention further relates to a display system and a Liquid Crystal Display device.

BACKGROUND OF THE INVENTION

Display systems using light modulation or switching technology are increasingly employed for displaying moving pictures, for example in television sets, beamers and computer monitors. A typical example of such a display system is a liquid crystal display device (further also referred to as LCD). The LCD device typically comprises a display device comprising an arrangement of light modulators, which constitute pixels of the LCD device. The LCD device further comprises a backlighting system for illuminating the pixels. In an LCD device, the light originating from the backlighting system is modulated by means of a switch or modulator in which various types of liquid crystal effects are used. Other display systems may be based on, for example, electrophoretic or electromechanical effects.

A known problem with such a display system is that moving images tend to exhibit motion artifacts as a result of blurring caused by the fact that the light modulators cannot instantly respond to changes in transmission levels.

It has been shown that the use of a so-called scanning or blinking backlight unit reduces the blurring problem. Scanning backlight units typically comprise a plurality of light sources in a panel arrangement. The scanning backlight unit is typically scanned in that the light sources are switched on and off in conformity with video signals provided to the display device. The video signals include image data to be displayed and synchronization data, which enable synchronous scanning of the display device to form an image. The light

sources are addressed sequentially in accordance with the synchronization data, illuminating a group of light modulators typically after the required transmission levels in the light modulators have been achieved.

In one embodiment of the scanning backlight unit, the array of light sources is, for example, a row of low-pressure mercury vapor discharge lamps arranged in parallel and addressed sequentially. Alternative scanning backlight units comprise, for example, a two- dimensional array of light emitting diodes (further also referred to as LEDs) as light sources. The two-dimensional array of LEDs may comprise white-light LEDs which emit substantially white light or may comprise a mixture of LEDs emitting different primary colors, for example three different types of LEDs emitting the primary colors red, green, and blue.

The patent application US 5,461,397 discloses a backlighting structure which allows color light pulses to be generated in synchronization with the scanning operation of the display device. The combined operation of the backlighting structure and of the display device produces color images via a time sequential color mixing principle.

Known scanning backlight units comprise complex electronics for driving the different color light sources in the array of light sources.

SUMMARY OF THE INVENTION It is an object of the invention to provide a scanning backlight system in which the cost of the electronics for driving the different color light sources is reduced.

According to a first aspect of the invention, the object is achieved with a scanning backlight system for illuminating a display device via a light exit window, the scanning backlight system comprising: - a plurality of first light sources emitting at least a first primary color towards the light exit window; at least a second light source emitting at least a second primary color towards the light exit window, the first primary color being distinct from the second primary color; the scanning backlight system comprising a first driving circuit for driving the plurality of first light sources in a scanning mode of operation for sequentially illuminating groups of pixels of the display device; and the scanning backlight system further comprising a second driving circuit for driving the at least one second light source in a continuous mode of operation for continuously illuminating groups of pixels of the display device.

The effect of the measures according to the invention is that a driving circuit for driving light sources in a scanning mode of operation is typically more expensive than a driving circuit for driving light sources in a continuous mode of operation. This is typically due to the fact that in the scanning mode of operation relatively large currents must be switched on and off at a relatively high frequency. If part of the light sources are driven in a continuous mode of operation, the driving circuit for driving light sources in a scanning mode of operation can be replaced by the less expensive circuit for driving light sources in a continuous mode of operation, which reduces the overall cost of the driving circuits. The invention is also based on the recognition that a scanning mode of operation typically requires light to be produced within a relatively short time period. This typically results in the use of light sources, which are able to emit a substantially higher luminous intensity and which can be driven at the required frequency. The light sources typically used in the scanning mode of operation usually dissipate more power and in general are more expensive than light sources typically used in the continuous mode of operation. In the scanning backlight system according to the invention, the at least one second light source is operated in a continuous mode of operation, thus reducing the power consumption and the cost of the scanning backlight system.

In an embodiment of the system, the plurality of first light sources further emits a third primary color, or the at least one second light source further emits the third primary color, the third primary color being distinct from the first primary color and from the second primary color. A benefit of this embodiment is that the addition of the third primary color to the already emitted first primary color and second primary color enables the scanning backlight system to emit a full color gamut towards the light exit window.

In an embodiment of the system, the plurality of first light sources and the at least one second light source are light emitting diodes. The benefit of the use of LEDs as the light sources for the scanning backlight system is that the range of different colors that can be emitted by LEDs is relatively broad. A specific backlight color gamut can be emitted towards the display device through the choice of a first, second, and third primary color from the broad range of different colors. A display color gamut, being a color gamut that can be displayed on a display system, depends on the backlight color gamut in combination with the color filters used in the display device. A backlight color gamut can thus be chosen from the broad range of different colors that can be emitted by the LEDs so as to cover, in combination with the color filters of the display device, a color gamut of a commercial color standard like EBU or NTSC.

In a favorable embodiment of the system, the plurality of first light sources are low-pressure mercury vapor discharge lamps, and the at least one second light source is a light emitting diode. The benefit of this embodiment is that the combination of low-pressure mercury vapor discharge lamps with LEDs typically improves the backlight color gamut of the scanning backlight system over a scanning backlight system comprising only low- pressure mercury vapor discharge lamps. Commercially available low-pressure mercury vapor discharge lamps typically comprise several different luminescent materials which convert ultraviolet light emitted by the mercury vapor discharge into visible light. The combination of different luminescent materials creates a specific color gamut, which is emitted by the low-pressure mercury vapor discharge lamp. However, commercially used low-pressure mercury vapor discharge lamps, when used together with the color filters of the display device, currently do not fully cover color standards like EBU or NTSC. This is typically due to the luminescent materials currently present in the low-pressure mercury vapor discharge lamps. When LEDs are combined with low-pressure mercury vapor discharge lamps, the backlight color gamut can be adapted owing to the broad range of different colors that can be emitted by the LEDs, thus changing the display color gamut and providing an improved match with the commercial available color standards.

In an embodiment of the system, each low-pressure mercury vapor discharge lamp comprises a discharge vessel, a wall of the discharge vessel being provided with a luminescent layer comprising a luminescent material, the discharge vessel of at least one of the plurality of low-pressure mercury vapor discharge lamps being arranged as a light-mixing chamber for light emitted by the at least one second light source. 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 LEDs. The omission of the additional optical waveguide results in reduced cost of the scanning backlight system. A further benefit when omitting the additional optical waveguide is that the weight of the scanning backlight system can be reduced.

In an embodiment of the system, the discharge vessel comprises a diffuser coating, and the light emitted by the at least one second light source is coupled into the discharge vessel via an aperture in the diffuser coating. The benefit of this embodiment is that the aperture ensures that the light emitted by the at least one second light source is coupled into the discharge vessel of the low-pressure mercury vapor discharge lamp and mixed inside the discharge vessel before it is emitted via the light exit window of the scanning backlight system.

In an embodiment of the system, the plurality of low-pressure mercury vapor discharge lamps are arranged between the light exit window and the at least one light emitting diode. 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 scanning backlight system.

In an embodiment of the system, the scanning backlight system comprises a first optical waveguide for illuminating the display device via the light exit window, the first optical waveguide having different sections for illuminating a group of pixels of the display device, each section comprising light in-coupling elements for admitting light emitted by at least one of the plurality of first light sources into the section of the first optical waveguide. In another embodiment of the system, the scanning backlight system comprising a second optical waveguide for illuminating the display device via the light exit window, the second optical waveguide comprising light in-coupling elements for admitting light emitted by the at least one second light source into the second optical waveguide. The benefit of using an optical waveguide in the scanning backlight system is that the light emitted by the plurality of first light sources or by the at least one second light source is mixed inside the optical waveguide before being emitted towards the display device via the light exit window. A further benefit of using the optical waveguide in a scanning backlight system is that the light sources can be arranged at an edge of the scanning backlight system, a so called edge arrangement of the light sources. This edge arrangement typically reduces the thickness of the scanning backlight system. Furthermore, in the embodiment where the light sources are LEDs, the edge arrangement of the LEDs typically leads to a more efficient cooling of the LEDs, so that high power LEDs can be used, typically increasing the light output of the scanning backlight system or typically reducing the number of LEDs to be used in the scanning backlight system.

In an embodiment of the system, the scanning backlight system comprises low-pressure mercury vapor discharge lamps having luminescent material which 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 scanning backlight 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 scanning backlight 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 scanning backlight system comprises low-pressure mercury vapor discharge lamps having luminescent material which 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 scanning backlight 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 scanning backlight 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 an HCFL as the low-pressure mercury vapor discharge lamp is that it renders possible a fast on and off switching of the low-pressure mercury vapor discharge lamp, which is especially beneficial when the low-pressure mercury vapor discharge lamp is used a scanning mode. In an embodiment of the system, the first primary color is green. In another embodiment of the system, the second primary color is red. The sensitivity of the human eye to the primary color green is relatively high and to the primary color red relatively low. In a scanning backlight system in which the first primary color, being the primary color green, is emitted in a scanning mode of operation, motion artifacts caused by blurring will be significantly reduced. Although motion artifacts in the color red are still present, the

perceptive impact thereof is minor, so that the main advantage of scanning, i.e. the reduction of motion artifacts, is still maintained.

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 scanning backlight system according to the invention in which low-pressure mercury vapor discharge lamps are arranged as the plurality of first light sources and in which LEDs are arranged as second light sources;

Fig. 2 shows a scanning backlight system according to the invention in which LEDs emit light into the discharge vessel of a low-pressure mercury vapor discharge lamp;

Fig. 3 is a plan view of the scanning backlight system according to the invention in which a first optical waveguide comprises different sections for illuminating a group of pixels of the display device;

Fig. 4 shows a scanning backlight system according to the invention in which a second optical waveguide comprises light out-coupling elements for emitting light towards the light exit window; and

Fig. 5 shows a display system. 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 scanning backlight system Bl according to the invention in which light is generated by a plurality of light sources TLl, Ll, comprising a plurality of low-pressure mercury vapor discharge lamps TLl and a plurality of LEDs Ll. In the arrangement shown in Fig. 1, the plurality of LEDs Ll are, for example, arranged adjacent to the low-pressure mercury vapor discharge lamps TLl. Both the low-pressure mercury vapor discharge lamps TLl and the LEDs Ll emit light towards the display device Di via a light exit window EW of the scanning backlight system Bl. Typically, the side of the walls of the scanning backlight system Bl facing the light sources TLl, Ll comprise reflective layers R which reflect light emitted by the light sources TLl, Ll in a direction away from the light exit window EW back towards the light exit window EW. The backlighting system Bl comprises

a first driving circuit ECl for driving the plurality of low-pressure mercury vapor discharge lamps TLl, for example in a scanning mode of operation wherein groups of pixels (not shown) of the display device Di are sequentially illuminated. The backlighting system Bl further comprises a second driving circuit EC2 for driving the plurality of LEDs Ll, for example in a continuous mode of operation wherein groups of pixels (not shown) of the display device Di are continuously illuminated. Although the first driving circuit ECl and the second driving circuit EC2 are drawn as separate units in the arrangement shown in Fig. 1, the two driving circuits may, for example, also be combined within a single driving circuit or, for example, integrated within a single integrated circuit. Fig. IA is a cross-sectional view of the scanning backlight system Bl, and Fig.

IB shows the scanning backlight system Bl viewed through the display device Di of the scanning backlight system B 1.

In a preferred embodiment of the invention, the low-pressure mercury vapor discharge lamps TLl emit the first primary color, for example the primary color green, and emit the third primary color, for example the primary color blue. The LEDs Ll emit the second primary color, for example the primary color red. The combination of the low- pressure mercury vapor discharge lamps TLl and the LEDs Ll enable the scanning backlight system Bl to emit a color gamut which, when combined with the color filters (not shown) of the display device Di, substantially covers the color gamut of a commercial color standard like EBU or NTSC. Since the sensitivity of the human eye to the primary color red is relatively low, the impact of driving the LEDs Ll in the continuous mode of operation in the reproduction of motion by the display system Ds (Fig. 5) is expected to be minor. The benefit of this preferred embodiment is that the second driving circuit EC2 for driving the LEDs Ll in a continuous mode of operation typically will be less expensive than a comparable driving circuit (not shown) for driving LEDs in a scanning mode of operation.

In a further preferred embodiment, the low-pressure mercury vapor discharge lamps TLl are Hot Cathode Fluorescent Lamps (HCFL). The benefit of using HCFL is that they enable a fast on and off switching of the low-pressure mercury vapor discharge lamps TLl, which is especially beneficial when the low-pressure mercury vapor discharge lamps TLl are used the scanning mode of operation.

Fig. 2 shows a scanning backlight system B2 according to the invention, in which light is generated via a plurality of low-pressure mercury vapor discharge lamps TLl, TL2, TL3 together with a plurality of LEDs Ll. The low-pressure mercury vapor discharge lamps TLl, TL2, TL3 each comprise a discharge vessel V having a luminescent layer P

typically arranged at an inner wall of the discharge vessel V and comprising a diffuser coating Cl, C2, C3, typically arranged at an outer wall of the discharge vessel V. The discharge vessel V further comprises a low-pressure mercury vapor environment (not shown). The generation of a discharge in the low-pressure mercury vapor environment causes the mercury vapor inside the discharge vessel V to emit ultraviolet light (not shown). The ultraviolet light is absorbed by the luminescent layer P and converted into visible light of a predefined color. The luminescent layer P comprises, for example, 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 green, which is emitted by the low-pressure mercury vapor discharge lamp TLl, TL2, TL3. The luminescent layer P further comprises, for example, a second luminescent material which is associated with a third primary color. Ultraviolet light which illuminates the second luminescent material is converted into the third primary color, for example the primary color blue, which is emitted by the low-pressure mercury vapor discharge lamp TLl, TL2, TL3. The combination of the first and second luminescent materials determines the predefined color of the low-pressure mercury vapor discharge lamp TLl, TL2, TL3. The scanning backlight system B2 further comprises a plurality of LEDs Ll which emit light of a second primary color, for example the primary color red. The plurality of LEDs Ll shown in Fig. 1 emit light substantially towards the low-pressure mercury vapor discharge lamp TLl, TL2, TL3. The combination of the first, second, and third primary colors enables the scanning backlight system B2 to provide a color gamut to the display device Di which, when combined with the color filters (not shown) of the display device Di, substantially covers, for example, the color gamut defined in the EBU color standard. As was noted above with reference to Fig. 1, the side of the walls of the scanning backlight system B2 facing the light sources TLl, TL2, TL3, Ll typically comprise reflective layers R which reflect light emitted by the light sources TLl, TL2, TL3, Ll in a direction away from the light exit window EW back towards the light exit window EW. The backlighting system B2 also comprises a first driving circuit ECl and a second driving circuit EC2 as explained for Fig. 1. In the arrangement shown in Fig. 2, the first driving circuit ECl and the second driving circuit EC2 are drawn as a single unit (ECl; EC2), indicating that the driving circuits have been combined within a single driving electronics. Two separate driving circuits may obviously also be used, as in Fig. 1.

Fig. 2 A is a cross-sectional view of the scanning backlight system B2, and Fig. 2B shows the scanning backlight system B2 viewed through the display device Di of the scanning backlight system B2.

Figs. 2A and 2B show several different examples of low-pressure mercury vapor discharge lamps TLl, TL2, TL3. A second low-pressure mercury vapor discharge lamp TL2, for example, comprises an aperture A2 in the diffuser coating C2. The plurality of LEDs Ll predominantly emit the second primary color via the aperture A2 into the discharge vessel V of the second low-pressure mercury vapor discharge lamp TL2. In the second low- pressure mercury vapor discharge lamp TL2 shown here, the aperture A2 is, for example, a longitudinal aperture A2 arranged at a side of the second low-pressure mercury vapor discharge lamp TL2 facing away from the light exit window EW. A third low-pressure mercury vapor discharge lamp TL3, for example, comprises a plurality of apertures A3 in the diffuser coating C3. The plurality of LEDs Ll again predominantly emit the second primary color via the plurality of apertures A3 into the discharge vessel V of the third low-pressure mercury vapor discharge lamp TL3. Typically, the apertures Al, A3 still comprise luminescent material P for preventing UV light from issuing through the aperture Al, A3 into the backlighting system B2. In the third low-pressure mercury vapor discharge lamp TL3 shown here, the plurality of apertures A3 are, for example, circular apertures arranged at a side of the third low-pressure mercury vapor discharge lamp TL3 facing away from the light exit window EW. A first low-pressure mercury vapor discharge lamp TLl, for example, comprises no apertures in the diffuser coating Cl and is substantially identical to the low- pressure mercury vapor discharge lamp shown in Fig. 1. The plurality of LEDs Ll predominantly emit the second primary color through the diffuser coating Cl into the discharge vessel V of the first low-pressure mercury vapor discharge lamp TLl. In the first low-pressure mercury vapor discharge lamp TLl shown here, the light emitted by the plurality of LEDs Ll is diffused by the diffuser coating Cl before being mixed inside the discharge vessel V of the first low-pressure mercury vapor discharge lamp TLl. In the arrangement of the scanning backlight system B2 as shown in Fig. 2, the low-pressure mercury vapor discharge lamps TLl, TL2, TL3 are, for example, driven in a scanning mode of operation, and the plurality of LEDs Ll are, for example, driven in a continuous mode of operation. The light emitted by the plurality of LEDs Ll is mixed inside the discharge vessels V of the low-pressure mercury vapor discharge lamps TLl, TL2, TL3 before the light is emitted via the light exit window EW towards the display device Di.

Fig. 3 shows a scanning backlight system B3, B4 according to the invention comprising a first optical waveguide LGl having different sections Sl, S2,...Sn for illuminating groups of pixels (not shown) of the display device Di. Each section Sl, S2, ...Sn comprises light in-coupling elements (not shown) for receiving light emitted by at least one

of the plurality of LEDs L2, L3. The scanning backlight system B3, B4 shown in Fig. 3 further comprises low-pressure mercury vapor discharge lamps TL4, TL5, the first and second driving circuits ECl, EC2, and a display device Di as described with reference to the previous Figures. Side walls of the scanning backlight system B3, B4 have been omitted to give a clearer picture of the arrangement of the light sources TL4, L2, L3 inside the scanning backlight system B3, B4.

In the arrangement shown in Fig. 3A, the first optical waveguide LGl comprises, for example, a plurality of second LEDs L2 emitting a first primary color, for example the primary color green. The first optical waveguide LGl further comprises, for example, a plurality of third LEDs L3 emitting a third primary color, for example the primary color blue. The low-pressure mercury vapor discharge lamp TL4 typically emits a second primary color, for example the primary color red. The plurality of second LEDs L2 and third LEDs L3 are, for example, driven via the first driving circuit ECl in a scanning mode of operation. The plurality of low-pressure mercury vapor discharge lamps TL4 are, for example, driven via the second driving circuit EC2 in a continuous mode of operation. Fig. 3 A is a plan view of the scanning backlight system B3, in which the first optical waveguide LGl is, for example, arranged between the plurality of low-pressure mercury vapor discharge lamps TL4 and the light exit window EW of the scanning backlight system B3. The benefit of this embodiment is that the light emitted by the low-pressure mercury vapor discharge lamps TL4 is mixed inside the first optical waveguide LGl before being emitted via the light exit window EW towards the display device Di.

In the arrangement shown in Fig. 3B, the first optical waveguide LGl comprises, for example, a plurality of second LEDs L2 emitting a first primary color, for example the primary color green. The low-pressure mercury vapor discharge lamp TL5 typically emits a second primary color, for example the primary color red, and a third primary color, for example the primary color blue. The plurality of second LEDs L2 are, for example, driven via the first driving circuit ECl in a scanning mode of operation. The plurality of low-pressure mercury vapor discharge lamps TL5 are, for example, driven via the second driving circuit EC2 in a continuous mode of operation. Fig. 3B is a plan view of the scanning backlight system B4, in which the low-pressure mercury vapor discharge lamp TL5 is, for example, arranged at an edge of the first optical waveguide LGl. The benefit of this embodiment is that typically the thickness of the scanning backlight system B4 can be reduced while the light emitted by the low-pressure mercury vapor discharge lamp TL5 can

still be mixed inside the first optical waveguide LGl before being emitted via the light exit window EW towards the display device Di.

Fig. 4 shows a scanning backlight system B5 according to the invention comprising a second optical waveguide LG2. The scanning backlight system B5 shown in Fig. 4 further comprises the first, second, and third low-pressure mercury vapor discharge lamps TLl, TL2, TL3, the first and second driving circuits ECl, EC2, and a display device Di as described in Fig. 2. The second optical waveguide LG2 comprises light in-coupling elements (not shown) for receiving light emitted by the LEDs Ll into the second optical waveguide LG2. The second optical waveguide LG2 further comprises out-coupling elements OCl, OC2, OC3 which, for example, emit light from the second optical waveguide LG2 substantially towards the low-pressure mercury vapor discharge lamps TLl, TL2, TL3. The low-pressure mercury vapor discharge lamps TLl, TL2, TL3 are arranged between the second optical waveguide LG2 and the light exit window EW.

Fig. 4A is a cross-sectional view of the scanning backlight system B5, and Fig. 4B shows the scanning backlight system B5 viewed through the display device Di of the scanning backlight system B5.

Fig. 4A is a cross-sectional view of a scanning backlight system B5, which comprises different low-pressure mercury vapor discharge lamps TLl, TL2, TL3, the second low-pressure mercury vapor discharge lamp TL2 comprising, for example, the longitudinal aperture A2 and the third low-pressure mercury vapor discharge lamp TL3 comprising, for example, the plurality of circular apertures A3 as shown in Fig. 2. The second optical waveguide LG2 of the scanning backlight system B5 comprises different light out-coupling elements OCl, OC2, OC3 in the vicinity of the low-pressure mercury vapor discharge lamps TLl, TL2, TL3, respectively. The emission pattern of the second light out-coupling elements OC2 and the third light out-coupling elements OC3 are, for example, chosen such that the light emitted by the second and the third light out-coupling elements OC2, OC3 is admitted via the longitudinal apertures A2 and the circular apertures A3 into the discharge vessels V of the second low-pressure mercury vapor discharge lamp TL2 and the third low-pressure mercury vapor discharge lamp TL3, respecively. The emission pattern of the first light out- coupling element OCl is, for example, chosen such that the light emitted by the first light out-coupling elements OCl impinges on the diffuser coating Cl of the first low-pressure mercury vapor discharge lamp TLl and, for example, is diffused by the diffuser coating Cl before being admitted into the discharge vessel V of the first low-pressure mercury vapor discharge lamp TLl. The first light out-coupling elements OCl are arranged, for example, in

an array of separate first out-coupling elements OCl parallel to the first low-pressure mercury vapor discharge lamp TLl. The second light out-coupling elements OC2 are, for example, arranged in a substantially continuous line parallel to the longitudinal aperture A2 of the second low-pressure mercury vapor discharge lamp TL2. The third light out-coupling elements OC3 are, for example, arranged in an array of separate third light out-coupling elements OC3 such that the distribution of the third light out-coupling elements OC3 over the second optical waveguide LG2 coincides with the distribution of the plurality of circular apertures A3 over the third low-pressure mercury vapor discharge lamp TL3.

In the scanning backlight system B5 as shown in Fig. 4, the light emitted by the plurality of LEDs Ll is, for example, mixed inside the second optical waveguide LG2 and emitted via light out-coupling elements OCl, OC2, OC3. The light emitted by the second optical waveguide LG2 is admitted into the discharge vessel V of the low-pressure mercury vapor discharge lamp TLl, TL2, TL3 either via the longitudinal aperture A2, or via the plurality of circular apertures A3, or via the diffuser coating Cl. The discharge vessel V of the low-pressure mercury vapor discharge lamp TLl, TL2, TL3 acts, for example, as an additional mixing chamber for the light emitted by the second optical waveguide LG2, thus improving the uniformity of the light emitted by the scanning backlight system B5. Any combination of low-pressure mercury vapor discharge lamps TLl, TL2, TL3 with light out- coupling elements OCl, OC2, OC3 and/or apertures A2, A3 may obviously be used without departing from the scope of the invention.

In an embodiment of the scanning backlight system Bl, B2, B3, B4, B5, the scanning backlight system Bl, B2, B3, B4, B5 comprises a low-pressure mercury vapor discharge lamp TLl, TL2, TL3, TL4 having luminescent material 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 TLl, TL2, TL3, TL4 applied in a scanning backlight 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 TLl, TL2, TL3, TL4. 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 system Ds, for example a Liquid Crystal Display device comprising the scanning backlight system Bl, B2, B3, B4 according to the invention. Light out-coupling elements OCl, OC2, OC3 are, for example, optical elements like lenses, diffusers, or prisms. In another embodiment of the invention, the light out-coupling elements OCl, OC2, OC3 are, for example, scratches or deformations. In yet another embodiment of the invention, the light out-coupling elements OCl, OC2, OC3 are, for example, reflective elements.

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.