WO2006077518A1

WO2006077518A1 - Illumination system and display device comprising such an illumination system

Info

Publication number: WO2006077518A1
Application number: PCT/IB2006/050147
Authority: WO
Inventors: Erik Boonekamp
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-01-20
Filing date: 2006-01-16
Publication date: 2006-07-27

Abstract

The invention relates to an illumination system (1) comprising: a light-guiding structure (3), said light-guiding structure comprising multiple channels (4), wherein each channel is defined partially by an upper surface (5) and two side surfaces (6), said upper surface being covered with an at least partially reflective layer (12), and at least one light source for illuminating said light-guiding structure, wherein light can be coupled into the light-guiding structure via said side surfaces. The invention also relates to a display device comprising such an illumination system.

Description

Illumination system and display device comprising such an illumination system

The invention relates to an illumination system comprising: a light-guiding structure, said light-guiding structure comprising multiple channels, wherein each channel is defined partially by an upper surface and two side surfaces, said upper surface being covered with an at least partially reflective layer, and at least one light source for illuminating said light-guiding structure, wherein light can be coupled into the light-guiding structure via said side surfaces. The invention also relates to a display device comprising such an illumination system.

Conventional direct-lit backlights for large-size LCD-TV modules are composed of an array of fluorescent tubes in a light box. Using Cold Cathode Fluorescent Lamps (CCFLs), External Electrode Fluorescent Lamps (EEFLs) or Hot Cathode Fluorescent Lamps (HCFLs), backlights can be constructed with a typical external thickness of between 25 and 35 mm. For backlights based on hot cathode lamps with a relatively large diameter of approximately 16 mm, specific optical constructions are required to make thin backlights with a good uniformity. There is a drive to design and construct much thinner backlights than indicated above.

To this end, an illumination system as described in the opening paragraph is disclosed in European patent application EP 1 231 429, wherein a light source, such as an array of Light Emitting Diodes (LEDs), is positioned within each channel. In this manner, illumination systems, and hence LCD-displays, can be realized with a reduced thickness. However, the design of the light-guiding structure and the channels formed therein is determined by the dimensioning and geometry of the light sources available in the prior art, which significantly limits the freedom of design, in particular the freedom of designing the dimensioning and geometry of the light-guiding structure.

It is an object of the invention to provide an efficient illumination system with a relatively high degree of freedom of design.

This object can be achieved by providing an illumination system as described in the opening paragraph, which is characterized in that said light source is a discharge lamp having a discharge space that is at least partially limited by an upper surface and corresponding side surfaces of at least one channel. By using the channels as a discharge space for the discharge lamp, thereby integrating the discharge lamp as a light source with the light-guiding structure, a relatively high degree of freedom of designing the light-guiding structure can be obtained. Since the design, in particular the dimensioning and geometry, of the light-guiding structure is no longer determined by the dimensioning and geometry of conventional light sources, a relatively compact and thin (flat) illumination system can be obtained. Moreover, in this manner, the design of the illumination system according to the invention can be optimized for the specific application of the illumination system. By integrating the light source with the light-guiding structure, a relatively simple construction of the illumination system can be obtained, which can commonly be manufactured at relatively low cost. The discharge space is at least partially limited by the surfaces surrounding and defining the channel acting as discharge space. It may be clear that, in this functionality, the channel is filled with an ionizable substance and is sealed in a (substantially) medium-tight manner to prevent leakage of the ionizable substance from the channel. The mutual orientation of the surfaces of the light-guiding structure is commonly found to be relatively critical if one wants to achieve a uniform and controlled emission of light from the light-guiding structure to e.g. an LCD display via a light-emitting surface of the light-guiding structure, said light-emitting surface being positioned opposite a surface in which the channels are formed. The side surfaces are preferably oriented substantially perpendicularly to the light-emitting surface, while the upper surface of each channel is preferably oriented substantially parallel to the light-emitting surface.

In a preferred embodiment, each channel is further defined by a lower surface provided with a reflective layer. In this manner, the lower surface can reflect light generated within said channel towards the light-emitting surface of the light-guiding structure. It is conceivable that the channels are formed as through-holes in said light-guiding structure, wherein each channel is fully surrounded by the light-guiding structure. However, the lower surface will commonly be part of a separate means. Furthermore, the illumination system preferably comprises a supporting structure coupled to said light-guiding structure for forming said lower surfaces. In this manner, it will be relatively easy to manufacture the illumination system, wherein the design of both side surfaces, the upper surface, and the lower surface can be optimized relatively easily before assembling the light-guiding structure and the supporting structure.

The lower surface may have a flat (planar) geometry. However, in a preferred embodiment, each lower surface has a non-planar geometry. To this end, the lower surface may have, for example, a curved geometry, wherein the non-planar lower surface either increases or decreases the channel volume. By applying a non-planar lower surface, the channel volume can be adjusted on one side, and the inner surface of the channel can be increased. The latter aspect is commonly advantageous when applying a phosphorous coating on (at least a part of) the inner surface of the channel to convert UV radiation into visible light. By increasing the inner surface of the channel by applying a non-planar lower surface, an increased quantity of phosphorus may be affixed to the inner surface of the channel, as a result of which an improved luminous intensity can be generated. It may be clear that a non- planar geometry of the lower surface is in no way limited to a curved surface. It is, for example, also possible to apply a lower surface provided with one or more protruding elements or recesses to enlarge the lower surface. In an alternative preferred embodiment, each lower surface has an angular geometry. In this manner, the channel can be split up into an upper channel part forming part of the light-guiding structure, and a lower channel part forming part of the supporting structure, wherein both channel parts may have an identical dimensioning and/or geometry. Preferably, the light-guiding structure and the supporting structure are mutually coupled by means of a seal for securing a medium-tight sealing of the channels so as to prevent leakage of a discharging medium (ionizable substance) from the channels. Each seal may be formed by a seal frit.

The discharge space will be commonly limited to a single channel, wherein a single channel thus actually forms a single light source. However, it is also conceivable that multiple channels are mutually connected, such that these channels jointly form a single discharge space. In this manner, a single light source will be formed by multiple channels. It is commonly known that, when displaying moving image material on an active matrix LCD, the picture becomes blurred because of what is called the 'sample and hold' effect and the slow response of the LC pixels. A scanning backlight creates a stroke of light that scrolls from top to bottom of the screen at the same speed as the row-addressing speed and reduces motion blur significantly. To this end, it may be advantageous not to combine more than four channels to form a single light source, thereby securing the scanning backlight principle as indicated above. The system preferably comprises multiple discharge lamps having a discharge space in a respective channel. As mentioned above, the light source can be formed by or within a single channel. However, it is also conceivable that a single light source can be formed by a (limited) number of mutually connected channels. In the illumination system according to the invention, it is feasible to apply a discharge lamp by means of which visible light can be generated. An example of these visible light-generating discharge lamps is a neon-based discharge lamp. However, discharge lamps which generate substantially UV light are commonly applied. To convert this UV light into visible light, at least a part of each channel is preferably provided with a phosphorous coating. The thickness of the coating may vary, but a coating with an average thickness of about 10 micrometers will be commonly applied. By exceeding this thickness of about 10 micrometers, the luminescent or phosphorous coating becomes less and less suitable for light transmission and has a more and more reflective nature. For this reason, it may be advantageous to apply a relatively thick coating with a thickness of over 10 micrometers on the lower surface so as to give the coating a bilateral functionality. This coating will act as a light-converting coating for converting UV light into visible light on one side, and will also act as a reflective layer, so that an additional reflective layer may be omitted. As mentioned above, the discharge lamp may be of various types. Said discharge space preferably comprises at least one of the following components: mercury, sodium, argon, xenon and neon. The discharging principles relating to these components are commonly known in the art. These compounds are generally retained within the channel in a controlled atmosphere, for example, by a noble gas (mixture). Said compounds may be charged by multiple electrodes, which are preferably applied within the channel, for applying a voltage within said discharge space.

To secure an optimal coupling of light out of the light-guiding structure, the light-emitting surface is preferably provided with optical outcoupling means. Such means are commonly known in the art and may be formed by concrete engraved structures or the like so as to direct the light in a controlled direction away from the illumination system, for example, towards a display device. The outcoupling means preferably has a single or multiple scattering, refractive and/or diffractive microstructure. To ensure luminance uniformity, the density of the outcoupling structures may be varied with the position of the light-guiding structure. Said outcoupling structures may optionally be covered by one or more optical foils, such as brightness enhancements films, diffusor foils, or reflective polarizing foils. All of these foils are known to a person skilled in the art.

In a preferred embodiment, said light-guiding structure is made of glass, preferably quartz glass. Glass is a highly transparent and durable material which is also resistant to relatively high temperatures. If a phosphorous coating is applied, the coating is commonly applied by means of a suspension of phosphorus in a binding solution. After applying the suspension on (a part of) the inner surface of each channel, the binder is burned out at an increased temperature of about 400°C, resulting in the desired phosphorous coating. Quartz glass is ideally suitable to resist this relatively high temperature. In an alternative embodiment, wherein the temperature resistance of the light-guiding structure is of minor importance, the light-guiding structure is made of a transparent synthetic material, such as poly (methyl methacrylate) (PMMA).

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 can be used, such as luminaires, which require active illumination by an external illumination system according to the invention. The illumination system can even be applied as a luminaire to illuminate rooms, for example, in offices.

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

Figure 1 is a cross-section of an illumination system according to the invention,

Figure 2a is a cross-section of an alternative embodiment of an illumination system according to the invention, Figure 2b is a cross-section of another embodiment of an illumination system according to the invention,

Figure 2c is a cross-section of yet another embodiment of an illumination system according to the invention,

Figure 2d is a cross-section of a particular embodiment of an illumination system according to the invention, and

Figure 2e is a cross-section of another particular embodiment of an illumination system according to the invention.

Figure 1 is a cross-section of an illumination system 1 according to the invention. The illumination system 1 is particularly suitable for backlighting an LCD display 2. The illumination system 1 comprises a light-guiding element 3 made of quartz glass. The light-guiding element 3 has multiple rectangular recesses 4 to form channels. Each channel 4 is partially defined by an upper surface 5 and two side surfaces 6. The channels 4 are closed by a supporting structure 7 coupled to the light-guiding element 3 by means of seal frits 8. The supporting structure 7 thereby defines a lower surface 9 for each channel 4, wherein the channels 4 are closed in a medium-tight manner. Each channel 4 is provided with mercury and adapted to function as a low-pressure discharge lamp. An electric field is produced between two electrodes 10 in the gas-filled channel 4. This electric field causes mercury atoms to radiate ultraviolet (UV) energy. A phosphorous coating 11 inside each channel 4 transforms said ultraviolet energy into visible light. The visible light can enter the light- guiding element 3 via the side surfaces 6 (see arrow A). To prevent light from entering the light-guiding element 3 via the upper surface 5, the upper surface 5 is provided with a specular reflector 12, which is preferably completely (100%) specularly reflective. In this illustrative embodiment, the lower surface 9 is also provided with a reflecting layer 13. Light that has entered the light-guiding element 3 can only be emitted by a light-emitting surface 14 of the light-guiding element 3 (see arrow B). This light-emitting surface 14 is therefore provided with a light-outcoupling structure 15. Emission of light via lateral end surfaces 16 of the light-guiding element 3 can be prevented, because the reflecting layer 13 of the supporting structure 7 extends along these lateral end surfaces 16. The relatively thin illumination system 1 shown in this Figure has a height H of about 5.5 mm, wherein the light-guiding element 3 has a height hi of 5 mm and the supporting structure 7 has a height h₂ of approximately 0.5 mm. The depth d of each channel 4 is about 3 mm, while the width w of each channel 4 is about 2 mm. The pitch p of the channels is about 4 mm. The length (not shown) of each channel 4 may vary, and may be, for example, 750 mm. In general, it is noted that the optical coupling between the phosphor particles of the coating 11 and the side surfaces 6 is preferably minimized. By minimizing the mutual contact of the phosphor and the side surfaces 6, the light emitted by the phosphor particles will be refracted at the side surfaces 6 due to air-to-glass transition.

Figure 2a is a cross-section of an alternative embodiment of an illumination system 17 according to the invention. The illumination system 17 has a top structure 18 and a bottom structure 19, both adapted for mutual cooperation. The top structure 18 and the bottom structure 19 comprise multiple rectangular grooves 20, 21, wherein corresponding grooves 20, 21 of the structures 18, 19 form a rectangular channel 22. Each channel 22 is adapted to function as a discharge space to generate UV light within each channel 22. The channels 22 are mutually isolated by means of multiple seal frits 23. The UV light generated within a channel 22 will be converted into visible light by means of a phosphorous coating 24 which is applied in each groove 21 of the bottom structure 19. This phosphorous coating 24 has a thickness which is sufficient to act as a reflective layer at the same time. The visible light can be coupled into said top structure via lateral walls 25 of each groove 20 of the top structure 18. An upper surface 26 of said groove 20 is also provided with a reflective coating 27. Under circumstances, this reflective coating 27 may be replaced by a transflective coating having a reflectivity of at least 80%. The light can be coupled out of the top structure 18 in a controlled manner by means of conventional outcoupling elements 28. In principle, the reflectivity of the transflector may be used to tune the uniformity of the light emitted by the illumination system instead of varying the density of the outcoupling structures.

Figure 2b is a detailed cross-section of another embodiment of an illumination system 29 according to the invention. The illumination system 29 comprises a light-guiding plate 30 provided with multiple elongated first recesses 31. The illumination system 29 further comprises a base plate 32 provided with multiple elongated second recesses 33 (only a single second recess 33 is shown in this Figure). The light-guiding plate 30 and the base plate 32 are mutually coupled in a medium-tight manner, such that two first recesses 31 of the light-guiding plate 30 and one second recess 33 of the base plate 32 jointly form a single discharge space for generating UV light. The discharge space, in particular the base plate 32, is provided with a set of opposite electrodes 34 for applying a voltage within said discharge space to generate UV light. This UV light can be converted by means of a phosphorous coating 35 which is applied on the whole circumferential surface of the discharge space. Both the light-guiding plate 30 and the base plate 32 are provided with a reflective layer 36, 37 for allowing light generated within said discharge space to enter the light-guiding plate 30 only via specific upstanding side walls 38 of the light-guiding plate 30. Light can subsequently be coupled out of the light-guiding plate 30 via an optical flat surface 39 provided with an outcoupling structure (not shown). The path of the light generated within said discharge space is indicated by arrows in this Figure.

Figure 2c is a cross-section of yet another embodiment of an illumination system 40 according to the invention. The illumination system 40 again comprises two major elements, namely, a light-guiding plate 41 and a base plate 42. The light-guiding plate 41 is provided with multiple linearly extending channels 43 enclosing a discharge space for generating UV light. The base plate 42 thereby closes the channels 43. The channels 43 are sealed by means of multiple seal frits 44. The base plate 42 is provided with multiple protruding beams 45 for increasing the circumferential surface of each channel 43. In this embodiment, this aspect is advantageous because a phosphorous coating 46 is required to convert the UV light generated within the discharge space into visible light. In this manner, a larger amount of a (thin- layer) phosphorous coating 46 can be applied, resulting in an increased luminous intensity of the illumination system 40. To prevent bright spots arising on a light-emitting surface 47 of the illumination system 40, each channel 43 is unilaterally provided with a reflective layer 48. A surface of the base plate 42 opposite the light-guiding plate 41 is further provided with a specularly reflective coating 49.

Figure 2d is a cross-section of a particular embodiment of an illumination system 50 according to the invention. The illumination system 50 comprises a light-guiding upper structure 51, and a reflective lower structure 52 coupled to said upper structure 51 by means of multiple seals 53. The upper structure 51 is provided with multiple elongated recesses 54, thereby forming channels acting as discharge space for generating UV light. A non-planar lower surface 55, 56, 57 of each channel 54 is formed by the lower structure 52. In order to illustrate the diversity of possible lower surfaces 55, 56, 57, three types of lower surfaces 55, 56, 57 are shown by way of example. A first lower surface 55 is a curved, convex surface. A second lower surface 56 is a curved, concave surface, and a third lower surface 57 is a wrinkled surface provided with an upstanding wrinkled element 58. All lower surfaces 55, 56, 57 have in common that they increase the inner surface of the channel 54 (with respect to a flat lower surface), thereby making it possible to apply an increased amount of phosphorous coating 59 to improve the lumen output of the illumination system 50. Light generated within the channels 54 can be coupled into said upper structure 51 via standing side walls 60 of each channel 54. An intermediate upper surface 61 of each channel 54 is provided with a reflective layer 62. Advantages of these reflective layers 62 have been comprehensively elucidated above.

Figure 2e is a cross-section of another particular embodiment of an illumination system 63 according to the invention. The system 63 comprises a light-guiding plate 64, which is provided with multiple through-channels 65 acting as discharge space for a neon gas contained within said channels 65. Each channel 65 is provided with electrodes 66 to apply a voltage within each channel 65 to generate visible light. The light-guiding plate 64 is covered trilaterally with a reflective coating 67. An upper surface of the channel is also provided with such a reflective coating 68. An upper side 69 of the light-guiding plate 64 is left uncovered and is adapted to couple out light. The illumination system 63 is primarily adapted to backlight a display device (not shown), such as an LCD display device. However, it is also conceivable that the illumination system 63 per se can act as a display device, such as a luminary. 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. An illumination system comprising: a light-guiding structure, said light-guiding structure comprising multiple channels, wherein each channel is defined partially by an upper surface and two side surfaces, said upper surface being covered with an at least partially reflective layer, and - at least one light source for illuminating said light-guiding structure, wherein light can be coupled into the light-guiding structure via said side surfaces, characterized in that said light source is a discharge lamp having a discharge space that is at least partially limited by an upper surface and corresponding side surfaces of at least one channel.

2. An illumination system according to claim 1, characterized in that each channel is further defined by a lower surface provided with a reflective layer.

3. An illumination system according to claim 2, characterized in that the illumination system further comprises a supporting structure coupled to said light-guiding structure for forming said lower surfaces.

4. An illumination system according to claim 3, characterized in that each lower surface has a non-planar geometry.

5. An illumination system according to claim 4, characterized in that each lower surface has an angular geometry.

6. An illumination system according to claim 3, characterized in that the light- guiding structure and the supporting structure are mutually coupled by means of a seal.

7. An illumination system according to claim 1, characterized in that multiple channels are mutually connected, such that these channels jointly form a single discharge space.

8. An illumination system according to claim 1, characterized in that the system comprises multiple discharge lamps having a discharge space in a respective channel.

9. An illumination system according to claim 1, characterized in that at least a part of each channel is provided with a phosphorous coating.

10. An illumination system according to claim 1, characterized in that said discharge space comprises at least one of the following components: mercury, sodium, argon, xenon, and neon.

11. An illumination system according to claim 1 , characterized in that each discharge space is provided with multiple electrodes for applying a voltage within said discharge space.

12. An illumination system according to any one of the preceding claims, characterized in that a surface of the light-guiding structure, opposite the surface of the light- guiding structure provided with the multiple channels, is provided with an optical outcoupling means.

13. An illumination system according to any one of the preceding claims, characterized in that said light-guiding structure is made of glass, preferably quartz glass.

14. A display device comprising an illumination system as claimed in any one of claims 1 to 13.