WO2007060566A1

WO2007060566A1 - Illumination system for illuminating display devices, and display device

Info

Publication number: WO2007060566A1
Application number: PCT/IB2006/054152
Authority: WO
Inventors: Rob F. M. Van Elmpt; Petrus A. J. G. Brouwer
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-11-22
Filing date: 2006-11-07
Publication date: 2007-05-31

Abstract

The invention relates to an illumination system (1) for illuminating display devices, comprising: a plurality of fluorescent lamps (2), in particular hot cathode fluorescent lamps, a light emission window for emitting light generated by the lamps in the direction of a display device, and a reflective base portion (4) for reflecting light generated by the lamps in the direction of said light emission window, at least a part of which base portion is arranged substantially opposite the light emission window, said light emission window and said base portion thereby enclosing said lamps. The invention also relates to a display device comprising an illumination system according to the invention.

Description

Illumination system for illuminating display devices, and display device

The invention relates to an illumination system for illuminating display devices, comprising: a plurality of fluorescent lamps, in particular hot cathode fluorescent lamps, a light emission window for emitting light generated by the lamps in the direction of a display device, and a reflective base portion for reflecting light generated by the lamps in the direction of said light emission window, at least a part of which base portion is arranged substantially opposite the light emission window, said light emission window and said base portion thereby enclosing said lamps. The invention also relates to a display device comprising an illumination system according to the invention.

Fluorescent light sources are commonly known and are amongst others applied 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 illumination system mentioned in the opening paragraph, hot cathode fluorescent lamps (HCFL) are employed as discharge lamps in the illumination system. Although application of this kind of lamp in illumination systems for illuminating display devices is relatively suitable, application of these lamps in an illumination system also has several drawbacks. An important drawback of an HCFL is the non- lighting length (NLL) of an HCFL, wherein the non-lighting length is the distance between the lamp electrode and lamp extremity, in which area no light is generated. This leads to non-lighting, and hence during use relatively dark, lamp-ends. When using HCFL lamps in backlight systems, the available space outside the backlight is commonly limited; therefore a significant part of the lamp ends, and in particular of the non- lighting length(s), of each lamp is situated within the illumination system, and in particular within the lamp receiving space of the illumination system defined by the reflective base portion and the light emission window. This leads to undesired dark edges, as a result of which a display device cannot be illuminated in a uniform and satisfactory manner. Currently HCFL lamps are commonly held in place by a rail (lamp rail), made of highly diffuse reflective material. This is done to reflect as much light as possible back into the lamp receiving space to minimize dark edges; however it is not sufficient to eliminate all effects of the non- lighting-length. Hence also, the non- lighting lengths cause a visible luminance fall-off at opposite edges of the backlight. Especially, a periodic pattern of dark spots is visible above the lamp ends. It is an object of the invention to provide an improved illumination system with which a relatively homogeneous illumination of a display device can be achieved.

This object can be achieved by providing an illumination system according to the preamble, characterized in that the illumination system further comprises at least one optical structure positioned between at least two neighbouring lamp ends for directing a fraction of light generated by the lamps towards a part of the light emission window facing said lamp ends. Application of the optical structure between neighbouring lamp ends leads to an improved distribution of light within the illumination system, wherein in particular surrounding rays of light generated by the lamps are directed to a part of the light emission window in the vicinity of the relatively dark lamp ends, and in particular above the non- lighting area of the lamp ends. In this manner formation of dark spots or dark areas on the light emission window above the lamp ends can be prevented, or at least counteracted, which leads to an illumination system with which a relatively homogeneous (uniform) illumination of the display device can be achieved. Although also different fluorescent lamps are known in the prior art, such as cold cathode fluorescent lamps (CCFL) and external electrode fluorescent lamps (EEFL), the problem of insufficient lighting of the light emission window due to non-lighting lamp ends presently in particular occurs with HCFL lamps. However, for a person skilled in the art it may be conceivable to apply (future) discharge lamps with equivalent non-lighting areas other than HCFL lamps. For this reason, the invention is by no means limited to illumination systems wherein HCFL lamps are applied to generate light. Equivalent lamps to be developed may also be applied.

Selectively directing a fraction of light towards relatively dark zones within the illumination system by means of the optical structure can be realised in different manners. Rays of light can for example be refracted by the optical structure in a predetermined direction. However, preferably incident light is reflected by the optical structure in a predetermined direction. To this end, a surface of the optical structure opposite the reflective base portion is preferably made at least partially reflective. The reflectivity of the optical structure can be achieved by manufacturing the optical structure from a reflective material. Though, for a person skilled in the art it is also conceivable that the optical structure is provided with a reflective foil. Preferably, the optical structure is highly reflective, in particular over 95%. In a particular preferred embodiment the surface of the optical structure opposite the reflective base portion is substantially diffuse reflective. Although application of a specular reflective upper surface of the optical structure would also be conceivable for a person skilled in the art, it is preferred to apply a (highly) diffuse reflective surface of the optical structure to optimize distribution of light within the illumination system, and hence to optimize the relatively uniform illumination of the display device.

To optimise direction, and in particular reflection, of light towards (formerly) relatively poorly illuminated peripheral areas of the light emission window, the optical structure preferably has a three-dimensional shape. The exact dimensioning and geometry of the optical structure can be of various kinds. In particular, preferably the optical structure comprises symmetrical parts for reflecting a substantially equal amount of light towards the respective (neighbouring) lamp ends, in particular towards areas above the respective lamp ends. In a preferred embodiment the illumination system comprises at least three fluorescent lamps, wherein an optical structure is positioned between each set of neighbouring lamp ends. More preferably, the illumination system comprises eight lamps which are arranged substantially in parallel and substantially equidistant from each other. According to this embodiment, the illumination system thus comprises multiple optical structures, wherein each optical structure is positioned between at least two neighbouring lamp ends. In an alternative embodiment a reduced number of optical structures is applied, wherein each optical structure is positioned between multiple sets of neighbouring lamp ends. In this manner, an extended optical structure is applied, parts of which optical structure being simultaneously positioned in different spaces defined by different sets of lamp ends. Preferably, the at least one optical structure partially covers the neighbouring lamp ends. In this manner, a relatively dark (non- lighting) area of the lamp can be covered at least partially, as a result of which light generated by the lamps, and possibly directed by the optical structure, will not easily be absorbed by and/or conducted into these non-lighting lamp areas. This will commonly lead to an improved relatively efficient lumen output of the illumination system according to the invention. In another preferred embodiment the at least one optical structure comprises at least one aperture for receiving at least part of a lamp end. In this way a lamp end can be surrounded, and hence covered, in an optimum way, wherein a loss of light due to absorption by the non- lighting lamp ends of each lamp can be kept to a minimum. More preferably, each optical structure comprises multiple apertures for receiving multiple lamp ends respectively. The apertures are preferably of circular or oval design to be able to fit tightly around the respective lamp ends. In a particular preferred embodiment the apertures are provided with a sealing means for substantially mediumtight lead-through of the respective lamp ends. By positioning a sealing means, e.g. one or more sealing rings, within each aperture a mediumtight connection can be generated between the optical structure and the lamp(s) received by the aperture(s) of this optical structure, as a result of which loss of light generated by the lamps via the non-lighting lamp ends can be kept to a minimum, and hence an efficient lumen output can be optimised considerably. Sealing the illumination system, and in particular the lamp receiving space defined by the light emission window and the reflective base portion, is commonly preferable to prevent dust and other small particles from entering this space, which could be detrimental to the light performance of the illumination system according to the invention.

The one or more optical structures applied within the illumination system according to the invention can be formed by separate structures, which may be positioned between two neighbouring lamp ends in a removable manner. However, in a preferred embodiment the at least one optical structure forms (an integral) part of the reflective base portion. In this manner, both the reflective base portion and the one or more optical structures can be manufactured in a single process step. The reflective base portion may be made of a single piece. However, it is also conceivable that the reflective base portion comprises a reflective backing plate and multiple lamp rails for holding the extremities of the respective lamps. In this embodiment the one or more optical structures applied can be an integral part of the reflective backing plate and/or a lamp rail.

The invention also relates to a display device comprising an illumination system according to the invention. Besides Liquid Crystal Displays (LCDs), all kinds of displays which require active illumination by an external illumination system according to the invention can be used. However, it must be made clear that the illumination system may also be used for other purposes. To this end, the illumination system may for example also be used for direct lighting, or may be applied in light boxes or as part of tanning equipment.

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

Figure 1 is a top view of an illumination system according to the invention; Figure 2 is a graphical drawing of the difference in relative luminance between a conventional illumination system and an illumination system according to the invention;

Figure 3 is a cross section of an alternative illumination system according to the invention, wherein multiple embodiments of the illumination system are incorporated;

Figure 4 is a longitudinal section of yet another illumination system according to the invention, wherein multiple embodiments of the illumination system are incorporated; and

Figure 5 is a detailed top view of another illumination system according to the invention.

Figure 1 is a top view of an illumination system 1 according to the invention. The illumination system 1 comprises eight hot cathode fluorescent lamps (HCFL) 2, which are arranged substantially in parallel and equidistant Iy. Two lamp rails 3 hold the lamps 2 in position by clamping the extremities of the lamps 2. The lamp rails 3 are connected to a backing plate 4. Both the lamp rails 3 and the backing plate 4 are at least partially made of diffuse reflective material, and may for example comprise a reflective top layer, such as a highly reflective foamed sheet of micro cellular polyethylene terephthalate (MCPET), to reflect light generated by the lamps 2 towards a light emission window (not shown) of the illumination system 1. Said light emission window is commonly positioned on top of the lamp rails 3 opposite the reflective backing plate 4, and is adapted to direct light towards a display device in a relatively efficient and effective manner. A problem of the present hot cathode fluorescent lamps 2 is that each lamp 2 has over a certain non-lighting length (arrows A) two non- lighting lamp ends 5 due to the required positioning of the electrodes (not shown) within the lamp 2. Due to these non- lighting ends 5, dark areas, in particular dark spots, will commonly appear during illumination of a display device. However, to counteract this disadvantage the illumination system 1 according to the invention comprises multiple optical structures 6, wherein each multiple structure 6 is positioned between two neighbouring lamp ends 5. The optical structures 6, or at least an upper surface of the optical structures 6, is made of a reflective material, so as to reflect light generated by the lamps 2 selectively towards an area of the light emission window facing the relatively dark lamp ends 4 of the lamps 2. In this manner a relatively uniform outcoupling of light in the direction of a display device can be achieved without suffering clearly visible dark areas at opposite edges of the illumination system 1, and hence of a display device. Figure 2 is a graphical drawing 7 of the difference in relative luminance between a conventional illumination system (lower graph line I) and an illumination system according to the invention (upper graph line II), wherein in the illumination system according to the invention one or more optical structures are applied to realise a relatively uniform lumen profile as elucidated above. The X-axis of the drawing 7 represents the relative (width) position within the illumination system, wherein a simplified cross section of HCFL lamps 8 of the illumination system is shown. The Y-axis represents the relative luminance which is emitted via a light emission window (not shown) of the illumination system to illuminate a display device. In this graphical drawing 7 it is clearly shown that the relative luminance of the illumination system according to the invention (graph line II) is substantially, in particular about 10-20%, higher than the relative luminance of the conventional illumination system.

Moreover, it is clearly shown in this graphical drawing 7 that the intensity of dark spots in the relative luminance, present due to the non-lighting lengths of the lamp ends of each lamp 8, is significantly higher in the conventional illumination system (see arrows B) with respect to the intensity of these dark spots in the illumination system according to the invention (see arrows C). Thus, with the illumination system according to the invention both the relatively luminance and the uniformity of the luminance can be increased significantly, which will lead to an improved illumination of a display device.

Figure 3 is a cross section of an alternative illumination system 9 according to the invention, wherein multiple embodiments of the illumination system 9 are incorporated. The illumination system 9 comprises a highly reflective (>95%) backing plate 10, a light emission window 11, multiple HCFL lamps 12 arranged between said backing plate 10 and said light emission window 11, said lamps 12 being clamped by reflective lamp rails 13 (of which only one is shown). To counteract the effect of reduced lighting of the light emission window in the vicinity of the non- lighting lamp ends of the lamps 12, in this embodiment multiple different highly reflective optical structures 14, 15, 16 are applied to reflect light generated by the lamps 12 towards an area of the light emission window 11 in the vicinity of, and in particular above, the non-lighting lamp ends of the lamps 12. The optical structure 14 shown on the left is positioned between two neighbouring lamp ends, and comprises two flat (non-curved) surfaces 17. The optical structure 15 shown in the middle is also positioned between two neighbouring lamp ends, and comprises two concave surfaces 18, and a flat top surface 19. The optical structure 16 shown on the right encloses two neighbouring lamp ends at least partially. To this end this optical structure 16 comprises two apertures 20 through which the two lamps 12 are led. Between the optical structure 16 and each lamp 12 a sealing ring 21 is applied. By positioning a sealing ring 21 within each aperture 20 a mediumtight connection can be generated between the optical structure 16 and the lamps 12 received by the apertures 20 of this optical structure 16, as a result of which loss of light generated by the lamps 12 via the non- lighting lamp ends can be kept to a minimum, and hence an efficient lumen output can be optimised considerably. The optical structure 16 shown on the right is designed to reflect an optimum quantity of light to selective - commonly relatively dark - areas of the light emission window 11 to obtain a relatively homogenous lumen output for satisfactory illumination of a display device.

Figure 4 is a longitudinal section of yet another illumination system 22 according to the invention, wherein multiple embodiments of the illumination system 22 are incorporated. The illumination system 22 comprises a reflective backing plate 23, a light emission window 24 and an array of HCFL lamps 25 arranged in between the backing plate 23 and the light emission window 24. The lamps 25 are held by two reflective lamp rails 26. Since HCFL lamps 25 are used, the non- lighting lengths of these lamps 25 play an important role in uniform illumination of a display device, such as an LCD. In this embodiment the non- lighting lengths extend from each extremity of each lamp 25 to the dotted line. In particular in this non- lighting area of the lamps 25, highly diffuse reflective optical structures 27, 28 are present to reflect light generated by the lamps 25 in the direction of the light emission window 24. In this figure, certain rays of light are visualised by means of arrows. Therefore, it is clearly shown that light generated by the lamps 25 will be reflected by the optical structures 27, 28 towards the light emission window 24 to secure a relatively uniform illumination of a display device. In the illumination system 22 the optical structure 27 is a separate optical structure 27, which can be easily removed from the backing plate 23, the lamp rail 26, and from the illumination system 22 as such. This makes it relatively easy to replace and/or adapt the optical structure to specific optical circumstances in a given situation. The optical structure 28 forms an integral part of the corresponding lamp rail 26, and cannot be removed from this lamp rail 26. In this way, fewer separate elements are required to provide a satisfactory illumination system, which can lead to a reduced cost price of the illumination system 22. Moreover, the positioning of the optical structure(s) 27, 28 with respect to both the lamp rail 26 and the lamps 25 can be predetermined relatively easily, which can lead to improved aligning of the optical structures 27, 28 so as to optimise reflection of light towards relatively dark peripheral zones of the light emission window 24. Figure 5 is a detailed top view of another illumination system 29 according to the invention. The illumination system 29 comprises a reflective backing plate 30, and two lamp rails 31 (of which only one lamp rail 31 is shown). The lamp rails 31 are adapted to hold (the extremities of) multiple discharge lamps 32 in a substantially parallel and substantially equidistant manner. The illumination system 29 is closed by means of a light emission window (not shown) positioned opposite the reflective backing plate 30. Each lamp rail 31 comprises multiple diffuse reflective structures 33 to reflect light generated by the lamps 32 to a location of the light emission window facing the (non- lighting) end portions of the respective lamps 32. In this manner, formation of dark spots or dark areas on the light emission window above the lamp ends can be prevented, or at least counteracted, which leads to an illumination system 29 with which a relatively homogeneous (uniform) illumination of a display device can be achieved. In this embodiment of the illumination system 29, each lamp rail 31 comprises a bottom section 31a, and a top section 31b to facilitate positioning and clamping of the lamps 32, wherein a bottom section 33a of each reflective structure 33 forms (an integral) part of the bottom section 31a of the lamp rail 31, and wherein a top section 33b of each reflective structure 33 forms (an integral) part of the top section 31b of the lamp rail 31. 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 plurality of fluorescent lamps, in particular hot cathode fluorescent lamps; a light emission window for emitting light generated by the lamps in the direction of a display device; and - a reflective base portion for reflecting light generated by the lamps in the direction of said light emission window, at least a part of which base portion is arranged substantially opposite the light emission window, said light emission window and said base portion thereby enclosing said lamps; characterized in that the illumination system further comprises at least one optical structure positioned between at least two neighbouring lamp ends for directing a fraction of light generated by the lamps towards a part of the light emission window facing said lamp ends.

2. System according to claim 1, characterized in that a surface of the optical structure opposite the reflective base portion is at least partially reflective.

3. System according to claim 2, characterized in that the surface of the optical structure opposite the reflective base portion is substantially diffuse reflective.

4. System according to one of the foregoing claims, characterized in that the optical structure has a three-dimensional shape.

5. System according to one of the foregoing claims, characterized in that the optical structure comprises symmetrical structure parts for reflecting a substantially equal amount of light towards respective parts of the light emission window facing the respective lamp ends.

6. System according to one of the foregoing claims, characterized in that the illumination system comprises at least three fluorescent lamps, wherein an optical structure is positioned between each set of neighbouring lamp ends.

7. System according to one of claims 1-5, characterized in that the optical structure is positioned between multiple sets of neighbouring lamp ends.

8. System according to one of the foregoing claims, characterized in that the at least one optical structure partially covers the neighbouring lamp ends.

9. System according to one of the foregoing claims, characterized in that the at least one optical structure comprises at least one aperture for receiving at least part of a lamp end.

10. System according to claim 9, characterized in that each aperture is provided with a sealing means for substantially mediumtight lead-through of at least part of the lamp end.

11. System according to one of the foregoing claims, characterized in that the at least one optical structure forms part of the reflective base portion.

12. System according to one of the foregoing claims, characterized in that the reflective base portion comprises a reflective backing plate and multiple lamp rails for holding the extremities of the respective lamps.

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