Backlight unit

Backlight unit comprising a supply source (4), a discharge lamp (2) having a tubular discharge vessel (6) with an ionizable filling, three or more electrodes (8, 10, 12) arranged in a row along and external of the vessel at spaced relation to each other. The supply source comprises lamp drivers (14, 16) and data processing means (18). Each pair of different pairs of electrodes is coupled to a corresponding lamp driver. Each lamp driver is suitable to supply a high frequency supply voltage to the pair of electrodes to which the lamp driver is coupled. The data processing means is suitable to process input display data (D) and to control the lamp drivers such that their supplied supply voltages are dependent on the input display data.

FIELD OF THE INVENTION

The invention relates to a backlight unit as described in the preamble of claim1. In particular, the backlight unit is for use in a liquid crystal display (LCD) device.

BACKGROUND OF THE INVENTION

A backlight unit of the above type is disclosed by WO99/25001. The prior art backlight unit comprises a single lamp, which is bent to follow a serpentine path. There are three electrodes provided external to the lamp: one outer electrode at each extremity of the lamp and one center electrode in the middle between the outer electrodes. A supply source generates a high frequency supply voltage which is supplied to a primary winding of a transformer. The transformer has two secondary windings with two outer terminals and one common center terminal. Each secondary winding is connected to a different pair of an outer electrode and the center electrode, such, that the electrodes at the extremities of the lamp have opposite phases. Accordingly, the supply source provides lamp drivers for driving different pairs of electrodes in one manner. A goal thereof is to provide a uniform brightness distribution of light emitted by the lamp.

OBJECT OF THE INVENTION

An object of the invention is to extend the possibilities for use of a backlight unit of the above type.

SUMMARY OF THE INVENTION

The above object of the invention is achieved by providing a backlight unit as described in claim1.

Accordingly, the pairs of electrodes divide the lamp into sections which can be supplied by different supply voltages, which are dependent on input display data, so that a display by a display device comprising the backlight unit can be altered by changing the amount of emitted light locally as well, that is, apart from individual controlling pixels of a display panel of the display device by the input display data. By changing the amount of emitted light locally a contrast between certain areas of the display can be increased. This may be particular advantageous when using a LCD panel and/or when the display contains an important message.

DETAILED DESCRIPTION OF EXAMPLES

The diagram of an electrical circuit of a backlight unit according to the invention shown inFIG. 1, comprises basically a tubular gas discharge lamp2and a supply source4. The lamp2comprises a vessel6which contains an ionizable filling and an luminescent layer at its inner surface. At two different locations, in particular at the extremities of the vessel6, the lamp2has external electrodes8and10. These electrodes8,10are called extremity electrodes in here. The extremity electrodes8,10are connected to the supply source4. The supply source4is suitable to generate and to supply an alternating supply voltage of, for example 200 Vrms and 2.65 MHz. Dependent on regional requirements other values may be used, such as a frequency of 13.56 MHz. The filling of the vessel6is such that it is ionized if the supply source4supplies said supply voltage to the extremity electrodes8,10. For example, the filling is a mixture of argon and mercury. If ionized, the filling is electrically conducting via an arc between the electrodes8,10. The luminescent layer will be activated then, so that the lamp2will emit light.

Between the extremity electrodes8,10the lamp2is provided with M additional external electrodes12a,12b,12c, . . . ,12M (12in general), called intermediate electrodes in here (by “intermediate” “somewhere between neighboring electrodes” is meant here). In the drawing ofFIG. 1the electrodes8,10,12are spaced equally. However, different spaces between the electrodes8,10,12are allowed as well.

The supply source4has a main lamp driver14, which is connected to the extremity electrodes8,10.

Further, the supply source4has N=M+1 local lamp drivers16a,16b,16c, . . . ,16N (16in general), which are connected individually to the intermediate electrodes12a,12b,12c, . . . ,12M, respectively, and in series to the extremity electrodes8,10.

In addition, the supply source has processing means18, which is supplied with display data D from a data source (not shown), such as an external computer system. The lamp drivers14,16are connected to the processing means18. The processing means18controls the lamp drivers14,16to let each of them generate a supply voltage of which at least one property, for example its magnitude, is made dependent on the input display data D. The lamp drivers14,16supply their supply voltages to the electrodes8,10,12connected therewith. Accordingly, different voltages may be applied to the electrodes8,10,12, so that different voltage potentials may occur between neighboring electrodes, that is, between electrode pairs8and12a;12aand12b;12band12c; . . . ;12M and10. As a result, different light intensities may occur between those pairs of electrodes dependent on the input display data D. In other words, by providing suitable display data D the data source has full control over a brightness distribution along the lamp2. This may be advantageous for several reasons. One of said reasons may be that it allows to compensate for not-uniform brightness distribution of the lamp2as it is. A more important reason is that it allows to emphasize areas of a display (text and/or graphics) with respect to other areas, in particular surrounding areas, on a display screen of the display device comprising the backlight unit. This may be the case if one wants to emphasize a displayed warning message. In addition, it is very useful to improve a contrast property of a liquid crystal display (LCD) device, as it is known that a LCD panel of such a device provides a poor contrast ratio itself. For example, when applying the backlight unit according to the invention to such a LCD device, for a dark display area one may minimize the supply voltage supplied to a pair (or to pairs) of electrodes8,10,12of which a lamp section in between covers said dark area to completely blacken said area.FIG. 2shows a circuit diagram of a part of the supply source4ofFIG. 1. In particular,FIG. 2shows a row of N=M+1 transformers20a,20b,20c, . . . ,20N (20in general), which are each a component of a local lamp driver16a,16b,16c, . . . ,16N, respectively. The secondary windings of the transformers20are connected in series to the extremity electrodes8,10. Connection nodes between each pair of connected secondary windings of the transformers20are connected to the intermediate electrodes12a,12b,12c, . . . ,12M, respectively. The primary windings of the transformers20a,20b,20c, . . . ,20N are supplied by an alternating voltage which is generated by the local lamp drivers16a,16b,16, . . . ,16N, respectively, under control of the processing means18. By applying the configuration ofFIG. 2the burden of using voltage level shifters in the local lamp drivers16is avoided.

FIG. 3shows a diagram of a part of a mechanical construction of the backlight unit according to the invention.FIG. 3shows, in cross section, a printed circuit board (PCB)22. On a surface of the PCB22an electrically conducting wiring24is provided. On specific parts of the wiring24a number of tubular connection studs26are soldered. Each stud26contains a spring28and a ball30. The spring28and the ball30are electrically conducting. The spring28forces the ball30against an electrode8,10,12of the lamp2. Displacement of the ball30towards the lamp2is limited by an inwardly bent edge part of the stud26. By applying such a configuration, the lamp2can easily be installed, electrical connections between the PCB22can be made easily, so that it is possible to arrange, to electrically connect and to drive a large number of intermediate electrodes12.

Preferably, at the side where the lamp2is arranged, the PCB22is provided with a light reflective layer (not shown), which may take over a reflection property of an inner surface of a back wall of a usual box containing a backlight unit. In fact, the PCB22may carry the supply source4and it may even make said usual box unnecessary.

It is noted that the invention is not limited to the illustrated embodiment but only by the following claims. For example, the backlight unit may comprise any number of lamps2, with each lamp2having its own external electrodes8,10,12which are coupled to corresponding lamp drivers14,16for said lamp2. Also, instead of using studs26containing spring loaded balls30other flexible contacting means, such as flexible tongues can be used. Also, a flexible PCB22can be used which is bent to and from the electrodes8,10,12to contact the electrodes directly.