WO2006136970A2

WO2006136970A2 - Illumination system for illuminating display devices, and display device

Info

Publication number: WO2006136970A2
Application number: PCT/IB2006/051885
Authority: WO
Inventors: Jan P. Dekker
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-06-21
Filing date: 2006-06-13
Publication date: 2006-12-28
Also published as: WO2006136970A3

Abstract

The invention relates to an illumination system (14) for illuminating display devices, comprising a light emission window for emitting light in the direction of a display device, a reflective base portion for reflecting light, at least a part of which base portion is arranged substantially opposite to the light emission window, said light emission window and said base portion thereby enclosing a receiving space, a plurality of fluorescent lamps (1) substantially arranged within said receiving space, each fluorescent lamp (1) 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 further relates to a display device comprising such an illumination system (14) . The illumination system further comprises a separating means (17) for partitioning the receiving space into a closed space nad an open space, wherein the discharge vessel of at least one lamp is situated substantially in said closed space, and wherein both an electrode and a coldest spot of said lamp are situated in said open sace.

Description

Illumination system for illuminating display devices, and display device

The invention relates to an illumination system for illuminating display devices, comprising a light emission window for emitting light in the direction of a display device, a reflective base portion for reflecting light, at least a part of which base portion is arranged substantially opposite to the light emission window, said light emission window and said base portion thereby enclosing a receiving space, a plurality of fluorescent lamps substantially arranged within said receiving space, each fluorescent lamp 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 further relates to a display device comprising such an illumination system.

Fluorescent light sources are commonly known and are applied, amongst others, in illumination systems. Such an illumination system is referred to as a so-called "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 suitably be used 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, which are 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 in the opening paragraph, customarily a tubular low-pressure mercury- vapor discharge lamp, for example one or more cold-cathode fluorescent lamps(CCFL), hot-cathode fluorescent lamps HCFL), or external electrode fluorescent lamps (EEFL) may be employed as discharge lamps in the illumination system. Commonly, a phosphorus coating is applied for allowing low-pressure mercury vapour 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 commonly used in illumination systems 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 for temperature gradients. Exposing both the optical foils and the display device to the heat generated by conventional discharge lamps is detrimental toboth the optical foils and the display device and will commonly reduce the life span of these components significantly. Another major drawback of conventional fluorescent lamps is that the efficacy of these lamps is determined to a great extent by the temperature of the so-called 'cold spot' of the lamp, as this temperature plays an important role in the equilibrium of the vapor pressure of the mercury. Several advantages can be obtained by positioning the cold spot at an extremity of the lamp, that is in the vicinity of a hot electrode. However, this desired positioning of the cold spot increases the need to prevent significant heating up of the cold spot by the vicinal hot spot generated by the electrode.

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 according to the preamble, characterized in that the illumination system further comprises a separating means for partitioning the receiving space into a closed space and an open space, wherein the discharge vessel of at least one lamp is situated substantially in said closed space, and wherein both an electrode and a coldest spot of said lamp are situated in said open space. By mutually separating the (relatively hot) electrode and the heat-sensitive components of the illumination system, in particular the light emission window, thermal radiation towards this light emission window can be limited selectively, as a result of which distribution of heat generated by the electrodes of the lamps 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 generated within the open space can be dissipated substantially and relatively efficiently to the local atmosphere. Application of the separating means has the further advantage that this means can be applied in a relatively easy and quick manner, while the degree and direction of heat-reflection can be predetermined in a relatively accurate manner. A further major advantage of the illumination system according to the invention is that the so-called cold spot, positioned at an extremity of the discharge vessel, is also situated in the open space, which enables improved cooling of the cold spot. In this manner, the temperature of the cold spot of the lamp can be regulated relatively accurately, while overheating of the cold spot of the lamps can be avoided or at least counteracted, as a result of which the equilibrium of the partial vapor pressure of mercury within the discharge vessel, and hence the lumen output of each fluorescent lamp, can be optimised for specific purposes of the illumination system. Preferably, the discharge vessel of each lamp is situated substantially in the closed space, and both an electrode and a coldest spot of each lamp are situated in the open space. In case the illumination system is mounted vertically, e.g. to a wall, the open space is preferably oriented such that the amount of heat generated within said open space and conducted into said closed space is minimised. To this end, the open space is either oriented substantially vertically or at an upper section of the illumination system. It is noted that the cold spot is commonly formed at a single extremity of the lamp, while the electrodes are commonly positioned at opposite extremities of the lamp. For this reason, it may be advantageous to partition the receiving space into a closed space and multiple open spaces in order to be able to separate the cold spot as well as both electrodes of each lamp with respect to the closed space. In this context it is noted that it is also conceivable for a person skilled in the art to apply fluorescent lamps comprising a curved and/or angular discharge vessel, wherein the electrodes of each lamp are positioned on the same side of the lamp. To this end, commonly a bridge is used to interconnect multiple vessel segments. Application of this kind of lamps may be advantageous, since both the electrodes and the cold spot of each lamp can simply be situated within a single open space.

Preferably, the separating means are adapted to enclose portions of each discharge vessel at least partially. To this end, the separating means preferably comprise multiple apertures for receiving (portions of) respective discharge vessels. The apertures are preferably of circular or oval design so as to be able to fit tightly around the respective discharge vessels. In a particularly preferred embodiment, the apertures are provided with a sealing means enabling substantially medium-tight passage of the respective discharge vessels. By positioning a sealing means, e.g. one or more sealing rings, between the separating means and the discharge vessel led through the apertures, a medium-tight connection can be formed between the separating means and the discharge channels, as a result of which the closed space can be closed substantially medium-tightly. Sealing the closed space is commonly preferable in order to prevent dust and other small particles from entering the closed space, which could be detrimental to the light performance of the illumination system according to the invention.

Although different kinds of fluorescent lamps may be used within the illumination system according to the invention, it is preferable that each fluorescent lamp is formed by a Hot Cathode Fluorescent Lamp (HCFL), since this kind of lamp is ideally suitable for backlighting purposes. The major drawback of HCFL- lamps is that the electrodes of these lamps generate a significant amount of heat during lamp operation, which is detrimental to certain heat-sensitive components, in particular the light emission window, of the illumination system. To counteract this major drawback, it is considerably advantageous to position these electrodes outside the closed space and in the open space to allow improved cooling of these electrodes and, moreover, to decrease thermal radiation of the electrodes towards the heat-sensitive light emission window in a drastic manner.

The separating means can be of various design and dimensioning. Preferably, the separating means comprises at least one reflective plate. In a particularly preferred embodiment, a surface of the reflective plate facing the closed space is substantially light- reflective, and/or a surface of the reflective plate facing the open space is substantially heat- reflective. By making the (upper) surface facing the closed space reflective to visible light, the amount of light radiated and reflected towards the light emission window can be optimised. This upper surface may be specular and/or diffuse reflectively. By making the (lower) surface facing the open space reflective to infrared radiation, the amount of heat conducted from the open space into the closed space can be minimised. 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 optimise reflection of light by the reflective plate towards the light emission window, the light emission window and the reflective plate preferably mutually enclose an angle. This angle may vary, dependent on the situational circumstances, but is preferably between 0 en 90 degrees, and is more preferably such that the bisector of the reflective plate is directed towards the light emission window. The reflective plate may be flat or curved or even profiled, dependent on the specific circumstances. As mentioned afore, the open space allows improved cooling while leaving the atmosphere within the closed space mainly unaffected. For this reason, the open space is preferably provided with cooling means for cooling lamp parts situated within said open space. The cooling means can be adapted for passive cooling and/or for active cooling. There is commonly a distinctive preference for passive cooling, since moving and hence noise- producing and maintenance- sensitive parts are not required to achieve the cooling effect. Passive cooling can be achieved by means of convection and can be forced, e.g., by means of one or more heat sinks, in particular one or more cooling ribs. In order to facilitate convection current, the open space preferably communicates with the local atmosphere surrounding the illumination system. Preferably, the open space communicates with the surrounding atmosphere via multiple openings to allow generation of a relatively unhindered convention current through the open space. To prevent that a lamp extremity positioned at a relatively low position will be cooled significantly more intensively than a lamp extremity positioned at a higher position, the open space is preferably subdivided into multiple subspaces, each subspace containing an electrode and a coldest spot of at least one lamp. In this manner, a satisfying cooling effect can commonly be achieved for every lamp extremity. Although commonly less preferable, it is also conceivable for a person skilled in the art to apply active cooling means, for example by means of e.g. a fan or a cooling- water circuit. The invention also relates to a display device comprising an illumination system according to the invention. Besides Liquid Crystal Displays (LCD) all kinds of displays can be used which require active illumination by an external illumination system according to the invention.

The invention can further be illustrated by way of the following non- limitative embodiments, wherein:

Figure 1 shows a cross-sectional view of an end portion of a longitudinal HCFL- lamp which is used to explain the function of the invention,

Figure 2 shows a schematic front view of a backlight unit for an LCD-display (not shown),

Figure 3 shows a horizontal cross sectional view of the combination of the LCD-display and the backlight unit depicted in Figure 2, and

Figure 4 shows a detail of the view shown in Figure 3. Figure 1 shows an end portion of an HCFL- lamp 1, comprising a glass envelope 2 at the inside of which a fluorescent coating is applied. The inner cavity of the lamp is filled with an ionisable gas mixture comprising mercury. At the end of the lamp 1, the envelope 2 is closed off with a flange 3 in which a stem 4 is located. The stem 4 protrudes from the flange 3. The stem or exhaust tube 3 is used during the production of the lamp to evacuate the lamp 1, after which the stem is closed by fusing. Around the ends of the lamp 1, a cap 5 is arranged which is commonly made from metal. This cap 5 is preferably in good thermal contact with a part of the glass envelope 2, like the rims 6 of the cylindrical part of the envelope 2. To improve the thermal contact between the rims 6 of the envelope 2 and the lamp cap 5, a thermal paste 7 has been applied in between. It is, however, also possible to make use of other parts of the glass envelope, like the stem 4 with or without the application of thermal paste. The lamp 1 further comprises a hot electrode 8, of which the leads 9 are guided through the glass envelope 2 by pinch connections 10. The leads 9 are connected to connecting pins 11 incorporated into the lamp caps 5, but electrically insulated therefrom.

An important factor in the efficacy of the lamp is the temperature of the so-called cold spot, i.e. the point of the envelope which has the lowest temperature. To avoid a thermal short-circuit, the cold spot is advantageously located, which is determined by the external cooling means, as far away from the hot cathode as possible and outside the discharge area in the lamp. Due to other considerations, it is attractive to locate the cold spot not too far away from one of the lamp ends. This is illustrated in Figure 1, which shows that the cold spot is arranged at the rim 6 of the envelope 2 (see arrow C).

Figure 2 shows a schematic front view of a backlight unit 14 for an LCD- display (not shown). This backlight unit 14 comprises an array of lamps 1, arranged mutually substantially parallel. They are mounted in an outer housing 12, 13 which comprises a rear wall 12 extending at the rear side of the lamps 1, and wall parts 13 extending to the front of the structure, that is the side of the backlight unit 14. At the side walls 13 provisions have been made to mount the lamps 1 by their lamp caps 5. Preferably, the outer housing 12, 13 is made of thermally well-conducting material like metal. Within the outer housing 12, 13 an inner housing has been provided which is preferably made of thermal insulating material, like a plastic. Again, this inner housing comprises a rear wall 16 and two side walls 17 extending from the rear wall to the front. In these side walls 17 apertures have been provided through which the tubular lamps 1 extend. These side walls 17 partition the backlighting unit 14 into a substantially closed space for accommodating substantial parts of the envelopes 2 of the lamps 1, and two open spaces for accommodating the extremities of the lamps 2, in particular the electrodes 8 and the cold spot C of each lamp 1. In this case, channels 20 are formed extending between the inner side walls 17, the outer side walls 13 and the outer rear wall 12. At their front sides, these channels 20 may be closed off from the environment by any possible structure, like a front part of the housing in which the display is employed, a side part of the display unit or a part of a light diffuser 18. The diffuser is a plate which is transparent to light, but which has diffusing properties. It is commonly located between the array of lamps and the LCD-display. These channels 20 may be used for ventilation purposes to cool the electrodes 8 and the cold spot C by an air flow. Preferably, the cooling surface area of the channels 20 can be enlarged by providing heat sinks or other structures in the channels 20. It is however also possible to make use of other cooling mechanisms, like a solid thermal conductor which may conveniently be located in the channels.

In the depicted embodiment, these channels 20 are present at both sides of the lamps. As only one cold spot is required for each of the lamps 1, it is feasible to make use of only one side channel 20 for cooling purposes. Then the other side channels may be disposed of, leading to a smaller and simpler construction.

An effect of this construction is that a possible air (convection) flow in the channel or channels 20 is confined to the channels. The area between the inner side walls formed by the closed space is not touched. This has the advantage that the areas of the lamps in which the hot cathodes 8 and the cold spot C are located are subjected to cooling while the further problem of soiling of the lamps, which would lead to reduction of efficacy, is avoided.

As mentioned afore, the dimensioning of the lamps 1 and the housing is such that the hot electrodes 8 and the cold spot of each of the lamps are located outside the inner housing 16, 17 but within the outer housing 12, 13. The advantage of this feature is that an improved cooling facility can be achieved for selectively cooling and/or dissipating heat generated within the outer housing 12, 13. The result of this advantage is twofold. In this manner, substantial heating of heat-sensitive parts of the backlight unit 15 by the hot electrodes 8 can be avoided, or at least counteracted, while the cooling requirements for the cold spot are lower, as the flowing of heat towards the cold spot can be counteracted.

It is possible to implement the invention in other forms; it is for instance possible to make use of other locations for the cold spot C, like the stem 4. In this case, a sleeve is located around the stem 4, with thermal conducting paste between the glass of the stem and the sleeve. The sleeve is then in thermal contact with the lamp cap 5. It is however also possible to make use of a metal part extending through the envelope 2 of the lamp 1 at one of the ends. The metal part is different from the electrical connections to the electrode, and it serves another purpose. Its aim is to provide easy provisions for the establishment of a cold spot C. At the outer end, the metal part should be thermally connected to the cooling means. This feature leads to an efficient way of removing heat from the cold spot, as metal has better heat conducting properties than glass. Another possibility is the provision of a heat sink with a different shape. It has been found that a heat sink 21 with the shape of a wire, as depicted in Figure 4, may be very effective. The above-mentioned structures make use of the lamp cap 5 as a component in the thermal path from the part of the glass envelope 2 to the cooling means in the channel 20. It is very well possible to make use of other constructions, like a single element which is in intimate thermal contact with the glass envelope and the air flow within the channel. The structures described above define one or two channels 20 which function as a confinement for an air stream. It is possible to use other media for transport of heat from the heat sink to the environment, like another gas or a fluid like water. Besides, it is even conceivable to make use of solid media for heat transport. Any possible thermal conducting means can be located in the channels. It is however appreciated that the advantages of the present invention are most apparent when a gas, like air, is used as a cooling medium.

Figure 3 shows a horizontal cross-sectional view of the combination of the LCD-display and the backlight unit 14 depicted in Figure 2. In Figure 3, it is shown clearly that the side wall(s) 17 of the inner housing 16, 17 serve(s) as (a) partition wall(s) for separating the open space from the closed space. Each side wall 17 additionally serves as a reflective layer for reflecting light generated within the envelopes 2 of the lamps 1 on one side and for reflecting heat generated by the electrode 8 on the other side. As can be seen clearly in Figure 4, the side walls 17 are positioned such that both an electrode 8 and the cold spot C are situated in the outer housing 12, 13 (defining the open space) of the backlight unit 14, and that a substantial part of the envelope 2 of each lamp 1 is situated in the inner housing 16, 17 (defining the closed space). The light diffuser 18 and the side wall(s) 17 mutually enclose an angle α, which can be optimised for specific circumstances. The closed space defined by the inner housing 16, 17 is made substantially medium-tight by means of sealing rings 19 positioned between the envelope 2 and a corresponding aperture 22 of the side wall(s) 17 to prevent dust and other particles from entering this closed space. Optionally, one or more partition elements 23 are provided within the open space between one or more neighbouring (groups of) lamps 1 to secure sufficient cooling of every lamp extremity. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The 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. Illumination system for illuminating display devices, comprising: a light emission window for emitting light in the direction of a display device, a reflective base portion for reflecting light, at least a part of which base portion is arranged substantially opposite to the light emission window, said light emission window and said base portion thereby enclosing a receiving space, a plurality of fluorescent lamps substantially arranged within said receiving space, each fluorescent lamp 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, characterized in that the illumination system further comprises a separating means for partitioning the receiving space into a closed space and an open space, wherein the discharge vessel of at least one lamp is situated substantially in said closed space, and wherein both an electrode and a coldest spot of said lamp are situated in said open space.

2. System according to claim 1, characterized in that the discharge vessel of each lamp is situated substantially in the closed space, and that both an electrode and a coldest spot of each lamp are situated in the open space.

3. System according to claim 1 or 2, characterized in that the separating means comprises multiple apertures for receiving respective discharge vessels.

4. System according to claim 3, characterized in that the apertures are provided with a sealing means enabling substantially medium-tight passage of the respective discharge vessels.

5. System according to any one of claims 1 to 4, characterized in that each fluorescent lamp is formed by a Hot Cathode Fluorescent Lamp (HCFL).

6. System according to any one of claims 1 to 4, characterized in that the separating means comprises at least one reflective plate.

7. System according to claim 6, characterized in that a surface of the reflective plate facing the closed space is substantially light-reflective, and that a surface of the reflective plate facing the open space is substantially heat-reflective.

8. System according to claim 6, characterized in that the light emission window and the reflective plate mutually enclose an angle.

9. System according to claim 1, characterized in that the open space is provided with cooling means for cooling lamp parts situated within said open space.

10. System according to claim 9, characterized in that the cooling means comprises at least one heat sink, in particular a cooling rib.

11. System according to any one of the preceding claims, characterized in that the open space communicates with the local atmosphere surrounding the illumination system.

12. System according to any one of the preceding claims, characterized in that the open space is subdivided into multiple subspaces, each subspace containing an electrode and a coldest spot of at least one lamp.

13. Display device comprising an illumination system as claimed in any one of the claims 1 to 12.