Illumination system for illuminating display devices, and display device comprising such an illumination system

The invention relates to an illumination system for illuminating display devices, the system comprising: a light emission window for emitting light in the direction of a display device, a reflector for reflecting light, at least a part of which reflector is arranged opposite the light emission window, and a plurality of light sources arranged between said light emission window and said reflector, each light source comprising: an at least partially light-transmissive elongated discharge vessel filled with an ionisable substance, and multiple electrodes connected to said vessel, between which electrodes a discharge extends during lamp operation. The invention also relates to a display device comprising such an illumination system. Fluorescent light sources are commonly known and are applied in, for example, illumination systems. Such an illumination system is referred to as "direct-lit" backlight or "direct-under" type of backlight illumination system. The illumination systems are used, inter alia, as back or side lighting of (image) display devices, for example, for television receivers and monitors. Such illumination systems can particularly be used suitably as a backlight for non-emissive displays, such as liquid crystal display devices, also referred to as LCD panels, which are used in (portable) computers or (cordless) telephones. The illumination system is particularly suitable for application in large-screen LCD display devices for television and professional applications.

Said display devices generally include a substrate provided with a regular pattern of pixels, each driven by at least one electrode. In order to reproduce an image or a datagraphic representation in a relevant area of a (display) screen of the (image) display device, the display device employs a control circuit. In particular, in an LCD device, the light originating from the backlight is modulated by means of a switch or a modulator, while applying various types of liquid crystal effects. In addition, the display may be based on electrophoretic or electromechanical effects.

In the fluorescent light source mentioned above, a tubular low-pressure mercury- vapor discharge lamp, for example, one or more cold-cathode fluorescent lamps, hot-cathode fluorescent lamps, or external electrode fluorescent lamps (EEFL) may usually be employed as discharge lamps in the illumination system. A phosphorous coating is

generally applied to enable low-pressure mercury vapor discharge lamps to convert UV light to other wavelengths, for example, to UV-B light and UV-A light for tanning purposes (sun panel lamps) or to visible radiation for general illumination purposes. Such discharge lamps are therefore also referred to as fluorescent lamps. However, conventional fluorescent lamps have several drawbacks. In operation, a fluorescent lamp is also an infrared (IR) radiator causing a local temperature rise near the electrodes due to a so-called hot spot generated by both electrodes. Optical (plastic) foils used in illumination systems, generally used for polarizing or collimating visible light generated by the fluorescent lamps, have a limited temperature resistance and can barely survive heat radiated by the electrodes. Furthermore, a display device, such as an LCD, has a maximum operation temperature and is relatively sensitive to temperature gradients. Exposing both the optical foils and the display device to the heat generated by conventional discharge lamps is detrimental for both the optical foils and the display device and will generally reduce the life span of these components significantly. It is an object of the invention to provide an improved illumination system with which thermal radiation can be limited selectively.

The object of the invention can be achieved by providing an illumination system as described in the opening paragraph, which is characterized in that the illumination system further comprises thermally conductive means for conducting heat generated by said electrodes to said reflector. By conducting heat, caused by infrared radiation, in a direction opposite the relatively heat-sensitive light emission window, thermal radiation towards the light emission window can be limited selectively, as a result of which distribution of heat generated by the light sources can be managed efficiently. By preventing heat from being radiated substantially towards heat-sensitive components, such as the light emission window and a display device, the life span of these components can be improved significantly. Heat conducted by the thermally conductive means is removed substantially and relatively efficiently via the reflector to the local atmosphere. The use of a thermally conductive means has the further advantage that this means can be applied in a relatively easy and quick manner, wherein the degree and direction of heat reflection can be predetermined in a relatively accurate manner. The reflector is generally formed by a carrier structure on top of which a reflective layer (or coating) is formed. The heat conducted by said thermally conductive means is preferably led to said carrier structure, and usually less preferably to said reflective layer, to achieve a substantial heat dissipation from the discharge lamp. It may also be conceivable for a person skilled in the art to apply a reflector, wherein the carrier structure

and the reflective layer are mutually integrated so as to form a single layer or single structure. It is noted that infrared (IR) radiation is considered as electromagnetic radiation with a wavelength longer than visible light, but shorter than microwave radiation, which has typical wavelengths of between 700 nm and 1 mm. In order to effectively shield the light sources, in particular the so-called hot spots formed by the electrodes of said light sources, by the thermally conductive means, the thermally conductive means is preferably at least partially positioned between the light emission window and the light sources. In this manner, heat radiated by the light sources towards the light emission window can be absorbed by the thermally conductive means and can subsequently be guided backwards to the reflector. To optimize heat transfer from the thermally conductive means to said reflector, the thermally conductive means is preferably thermally coupled to the reflector. Physical contact between the thermally conductive means and the reflector to transfer heat is not necessary, though thermal coupling and in particular physical coupling is preferred. In a particularly preferred embodiment, the thermally conductive means is coupled to the reflector by means of a separate thermally conductive layer. Said layer is adapted to secure a satisfying thermal (by physical) coupling between the thermally conductive means and the reflector, wherein said separate thermally conductive layer may be formed by a thermally conductive paste. It is generally advantageous to increase the contact surface between the reflector and the thermally conductive means, as a result of which it is preferred that the thermally conductive means is coupled to multiple parts of the reflector. To this end, it is particularly preferred that the thermally conductive means is coupled to the reflector between (each set of) neighboring light sources.

In a preferred embodiment, the thermally conductive means covers portions of the vessel enclosing both electrodes. Since a substantial rise of temperature of a conventional illumination system is caused by the electrodes of the light sources, wherein each electrode is able to generate a hot spot, it is advantageous to position the thermally conductive means in such a way that this means can absorb and subsequently conduct a substantial part of the heat generated by the electrodes, thereby preventing, or at least counteracting overheating of the light emission window and possibly a display device neighboring said emission window. Although it is preferable to cover portions of the vessel enclosing or surrounding the electrodes to cover the hot spots of each light source, it is also preferable that both ends of the elongated vessel are left uncovered by the thermally conductive means. The ends of the elongated vessel generally form the so-called cold spot, i.e. an area of the light source where the temperature is relatively low during operation. The temperatures of these

cold spots determine the partial pressure of the ionisable substance, usually mercury, within the vessel, and hence the lumen output of each light source. To prevent these cold spots formed at both ends of each light source from (over)heating, they are left uncovered by the thermally conductive means. The thermally conductive means may have various dimensions and designs.

However, the thermally conductive means preferably comprises at least one strip, with which the hot spots (generated by the electrodes) can be covered in a relatively efficient way without influencing the lumen output of each light source significantly. The strip will generally have a non-flat deformed, in particular substantially curved, geometry to optimize heat absorption on one side and heat conduction and transfer to the reflector on the other side.

The material of which the thermally conductive means is made may be of various types. Since a suitable material must have a relatively good thermal conductivity, the thermally conductive means is preferably made substantially of a metal, such as copper. However, it may be conceivable to a person skilled in the art to apply a type of thermally conductive material other than metal.

To avoid substantial material stresses of the vessel due to pinching of said vessel with the thermally conductive means during operation, said thermally conductive means is preferably substantially positioned at a distance from each light source. In this manner, thermal expansion of the vessel during operation of each light source can be established in a relatively unhindered way.

The thermally conductive means preferably comprises apertures for accommodating at least one portion of each light source. In this manner, portions of each light source can be surrounded by the thermally conductive means so as to optimize heat absorption. To this end, the thermally conductive means may comprise one or more plates, each plate being provided with multiple openings or recesses for accommodating respective parts of the light sources.

To maximize heat conduction towards the reflector by minimizing heat emission towards the light emission window by the thermally conductive means, a side of the thermally conductive means opposite the reflector is preferably provided with a thermal insulation layer. In this manner, emission of infrared radiation towards the light emission window can be kept to a minimum, which is in favor of the operation temperature of both the light emission window and a display device, as a result of which the life span of these components can be increased significantly.

The invention also relates to a display device comprising an illumination system according to the invention. Besides Liquid Crystal Displays (LCD), any type of display requiring active illumination by an external illumination system according to the invention can be used.

The invention will be elucidated with reference to the following non- limiting embodiments, wherein:

Figure 1 is a top view of an illumination system according to the invention, Figure 2a is a first possible cross-section of the illumination system shown in

Figure 1,

Figure 2b is a second possible cross-section of the illumination system shown in Figure 1, and

Figure 2c is a third possible cross-section of the illumination system shown in Figure 1.

Figure 1 is a top view of an illumination system 1 according to the invention. The illumination system 1 comprises a reflective backing plate 2, a light-transmissive emission window 3, and multiple HCFL light sources 4 arranged between said backing plate 2 and said emission window 3. Each light source 4 comprises an elongated, substantially cylindrical discharge vessel 5 enclosing a discharge space for an ionisable substance, and two electrodes (not shown) positioned in the end regions of each vessel 5. During operation of the light sources 4, a hot spot is generated by the electrodes at a position of between 25 and 45 mm from each end surface 6 of the discharge vessel 5. Only these regions, where the hot spots are formed, are covered with two thermally conductive elements 7. The thermally conductive elements 7 may be of various types as shown in Figures 2a-2c. The thermally conductive elements 7 are coupled to the reflective backing plate 2 at positions between each pair of light source 4 so as to efficiently transfer heat absorbed by the conducting elements 7 to the reflective backing plate 2, as a result of which overheating of the light emission window 3 can be prevented. By positioning two thermally conductive elements 7 in the illumination system 1, thereby covering specific (relatively hot) parts of the light sources 4, the life span of the relatively heat-sensitive light emission window 3 can be increased significantly.

Figure 2a is a first possible cross-section of the illumination system 1 shown in Figure 1. Figure 2a clearly shows that each thermally conductive element 7 is formed by a curved strip 8. The strip 8 is adhered to the reflective backing plate 2 by means of a thermally conductive adhesive 9 or paste. The strip 8 is preferably spaced apart from each light source 4 so as to prevent generation of substantial material stresses during heat-up and during operation of the illumination system 1. Both the light emission window 3 and a display device 10, such as an LCD, are spaced apart from the strip 8.

Figure 2b is a second possible cross-section of the illumination system 1 shown in Figure 1. In the embodiment of the illumination system 1 shown in Figure 2b, each thermally conductive element 7 is formed by a metal, preferably copper, plate 11 provided with multiple recesses 12 for accommodating parts of each light source 4. The plate 11 is coupled to the reflective backing plate 2 by means of conventional fastening means (not shown).

Figure 2c is a third possible cross-section of the illumination system 1 shown in Figure 1. Each thermally conductive element 3 according to the embodiment shown in Figure 2c is formed by a thermally conductive plate 13 provided with multiple apertures 14 for accommodating critical heat-generating parts of the light sources 4. Moreover, a side of the plate 13 opposite the reflective backing plate 2 is provided with an insulating layer 15 or coating so as to minimize heat transfer from the conducting plate 13 towards the light emission window 3. Heat generated by the electrodes of the light sources 4 is absorbed substantially by the thermally conductive plate 13, said heat being subsequently conducted backwards to the reflective backing plate 2, by means of which backing plate 2 this heat can be radiated to the local atmosphere surrounding the reflective backing plate 2. By efficiently dissipating a substantial part of heat generated by the light sources 4 from the illumination system 1, overheating or at least serious heating of heat-sensitive parts of the illumination system 1, such as the light emission window 3 can be prevented or at least counteracted, thereby increasing the life expectancy of these components significantly.

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. Use of the article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. 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.