WO2006051475A1

WO2006051475A1 - Fluorescent light source and a display device comprising such a light source

Info

Publication number: WO2006051475A1
Application number: PCT/IB2005/053650
Authority: WO
Inventors: Ludo Haenen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-11-15
Filing date: 2005-11-07
Publication date: 2006-05-18

Abstract

The invention relates to a fluorescent light source (1) comprising an elongate light-transmissive envelope (3) enclosing a discharge space (4), wherein an elongate surface of said envelope is at least partly provided with: a phosphor coating (14) for converting UV light generated within said envelope into visible light, light-directing means (8) for a directional emission of light, and reflecting means (13) for reflecting light generated within said envelope towards said light-directing means. The invention also relates to an assembly of at least one of such light source and at least one optical waveguide (2). The invention further relates to a display device comprising at least one such light source.

Description

Fluorescent light source and a display device comprising such a light source

The invention relates to a fluorescent light source comprising an elongate light-transmissive envelope enclosing a discharge space, wherein an elongate surface of said envelope is at least partly provided with: a phosphor coating for converting UV light generated within said envelope into visible light, light-directing means for a directional emission of light, and reflecting means for reflecting light generated within said envelope towards said light-directing means. The invention also relates to an assembly of at least one such light source and at least one optical waveguide. The invention further relates to a display device comprising at least one such light source.

Fluorescent light sources are commonly known and are used in illumination systems among other uses. Such an illumination system is referred to as a "direct-lit" back light or "direct-under" type of back light illumination system. The illumination systems are used inter alia as back- or side-lighting units for (image) display devices, for example for television receivers and monitors. Such illumination systems are particularly suitable for use as back lights for non-emissive displays, such as liquid crystal display devices, also denoted LCD panels, which are used in (portable) computers or (cordless) telephones. The illumination system is particularly suitable for use 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. The display device uses a control circuit for reproducing an image or a datagraphic representation in a relevant area of a (display) screen of the (image) display device. In particular, in an LCD device, the light originating from the back light 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. The fluorescent light source mentioned in the opening paragraph is usually formed by a tubular low-pressure mercury- vapour discharge lamp, for example one or more cold-cathode fluorescent, hot-cathode fluorescent lamps, or external-electrode fluorescent lamps (EEFL), in the illumination system. United States patent US 6,402,343 discloses a fluorescent light source according to the openings paragraph, wherein not only a conventional phosphor coating but also a reflecting coating and a prismatic coating are applied to improve the light efficiency and luminance of said light source for back lighting applications, such that the light generated within the light source can be concentrated and can be efficiently radiated in a desired direction. However, it continues to be the aim to improve the known fluorescent light sources still further in order to achieve an improved visibility and luminance of a display device to be illuminated.

It is therefore an object to provide an improved light source with which light can be emitted more efficiently.

This object can be achieved by providing a fluorescent light source according to the preamble, characterized in that said phosphor coating is provided with at least one longitudinal uncoated aperture, said uncoated aperture being at least substantially covered by . the light-directing means. Omitting a longitudinal portion of the phosphor coating will cause less UV light generated within the discharge space to be converted into visible light, thereby resulting de facto in a certain loss of light. However, it has been found that the omission of a longitudinal portion of the phosphor coating, creating a longitudinal uncoated aperture, makes the amount of light exiting the light source via de light-directing means significantly , larger than the amount that can be obtained with the known fluorescent light source. The resulting significantly improved luminance is due to the partial elimination of certain unfavorable properties of the phosphor layer, which layer acts not only as a light converter but also to a certain extent as a light-diffusing, -absorbing, and -reflecting element, with the result that light generated within the discharge space can be concentrated and directed more efficiently and effectively, providing a significantly improved light emission via the light- directing means. Set against this major advantage of the light source according to the invention, the loss of light mentioned above is (substantially) negligible. This relatively efficient light output of the light source according to the invention can therefore lead to a significantly improved illumination of, for example, a display device. Moreover, light with a small divergent angle affords more opportunities to concentrate the light afterwards on a certain area, for example in general lighting fixtures. An aperture is commonly advantageous if the uncoated area accounts for less than 20% of the total surface area (10-mm slit for a 16- mm tube). Light losses can be even less if a non-diffuse selective UV reflection layer is used at the inner side of the glass tube. This layer may be used all over the tube, but at least at the aperture. Commonly, the envelope will be formed by a (cylindrical) glass tube, and the discharge space enclosed by the tube is filled with mercury, possibly mixed with a fraction of neon. However, it is also conceivable for a person skilled in the art to design a lamp with a different geometry and/or comprising a different ionizable substance. To improve the covering of the uncoated aperture by the light-directing means, said light directing-means preferably overlap the uncoated aperture at least on one side, more preferably on both sides, as a result of which (substantially) all light generated within the envelope will exit the light source via the light-directing means without being able to pass the light-directing means by, thereby optimizing the light output of the light source according to the invention.

Since the diffusing phosphor coating is no longer provided over a portion of the envelope covered by the light directing means, an angle α - enclosed by a segment formed by the center and the uncoated surface portion of the envelope - can be decreased with respect to this same angle present in the known fluorescent lamps, resulting in an enhanced concentration of light emitted by the light-directing means. Preferably, said angle α measures between 30° and 75°, and is preferably substantially 40°. For example, when a TL5 with a 16-mm tube with a 5-mm aperture is used, the angle α will be 36°. For an aperture of 8 mm the angle α will be 57°, and for an aperture of 10 mm the angle α will be 72°.

The reflective means may be either partly reflective (transflective) or fully reflective. Since the conventional reflective layers applied onto an inner surface of the envelope commonly transmit part of the light generated within said envelope, it is advantageous to apply a second reflective layer onto an outer surface of the envelope to achieve a complete reflection of the light, as a result of which this light will (substantially) solely be emitted via the light-directing means. This is particularly advantageous if the light source is used for side lighting applications. However, if the light source is used for back lighting applications, it is commonly advantageous to apply a transflective coating, the latter preferably with a reflectance of between 30% and 70%. In order to be able either to reflect or to direct light generated within the envelope, it is advantageous that either an inner surface or a corresponding outer surface of said envelope is covered substantially with either the light- directing means or the reflecting means. In this case an emission of light via the phosphor coating itself (away from the light source) can be prevented, such that this light will be forced to be emitted via the light-directing means, resulting in a relatively efficient light management within said light source. As was noted above, the envelope is formed by a cylindrical tube enclosing the discharge space in a conventional fluorescent light source. This tube, however, usually cannot be fitted tightly to an optical waveguide plate with a flat end surface. For this reason it is advantageous to adapt the geometry of either the light source or the end face of the optical waveguide plate. Therefore, said envelope is preferably flattened at least partly to be able to engage more tightly to the end face of the optical waveguide plate. In another embodiment it is also conceivable to make the end surface of the optical waveguide plate more concave to achieve this improved mutual connection.

The light-directing means may be of various kinds. Examples of suitable light- directing means are interference layers and holographic layers. In a preferred embodiment, however, said light-directing means comprise multiple micro-prisms. More preferably, every prism has an apex angle which measures (about) 90°, the pitch between every two prisms being (about) 50 micrometer. It has been found that such a prism configuration is advantageous for emitting light in a comparatively satisfactory beam shape and in a desired direction. The micro-prisms may be applied onto or into an inner and/or outer surface of the envelope. It is also conceivable, however, to apply said light-directing means as a separate layer in the form of at least one prismatic foil which may be (detachably) fixed to the envelope or directly incorporated into the glass tube (negative printing).

In an alternative embodiment, said phosphor coating is provided with multiple longitudinal uncoated apertures, said uncoated apertures being each at least substantially covered by the light-directing means. Depending on the mutual orientation of the apertures, a light source according to this embodiment can be advantageously used in both back lighting and side lighting for illuminating a display device. For direct backlighting, it is commonly advantageous to have two apertures roughly opposite to each other. In this way most of the light is directed sideways, preventing too much light directly above the lamp. A homogeneous direct back light is more easily obtained by means of a tuning of lamp distances and aperture directions.

The invention also relates to an assembly of at least one light source according to the invention and at least one optical waveguide. More preferably, said assembly comprises multiple light sources to achieve an improved illumination of said optical waveguide. Preferably, said optical waveguide is formed by a light-transmitting plate. The mutual orientation of the light source and the light-transmitting plate can be variable. In a preferred embodiment, however, the mutual orientation of the light source and the light- transmitting plate is such that said light source is substantially adapted to emit light towards an end side of said light-transmitting plate. This orientation is better known as side lighting, which can be used for illumination of monitors, etc. In another preferred embodiment, the mutual orientation of the light source and the light-transmitting plate is such that said light source is substantially adapted to emit light towards a lateral side of said light-transmitting plate. This orientation is better known as (direct) back lighting and can be used for illumination of televisions and the like.

To improve the mutual optical coupling between said light source and said optical waveguide, at least a part of said light source and at least a part of said optical waveguide are preferably formed mutually congruously. In this manner the loss of light emitted by the light source but not coupled into said optical waveguide can be minimized.

The coupling between said light source and said light-directing means can be improved by using an optical adhesive, for example an adhesive better known under the trade name 'Lens Bond'^® (obtainable from Summers Laboratories, Fort Washington, PA). Alternatively, the light-directing means may be formed by or provided with a so-called pressure-sensitive foil and/or pressure-sensitive adhesive (PSA) such as, for example, a polyacrylic acid ester pressure foil obtainab Ie from Lintec Corporation to establish a solid coupling between the light source and the foil. A pressure-sensitive foil or adhesive is commonly characterized by the fact that the coupling between two components can be realized by applying a certain pressure on the pressure-sensitive foil. In this manner, a firm adhesion can be reali-ed between the light source and the light-directing means without the use of a separate adhesion agent.

The invention further relates to a display device comprising at least one light source according to the invention. Besides Liquid Crystal Displays (LCDs), all kinds of displays can be used which require active illumination by an external illumination system constructed along the priciples of in the assembly according to the invention.

The invention will be further illustrated with reference to the following non- limitative embodiment and the drawings, wherein: Figure 1 shows a cross-section of a first embodiment of a fluorescent light source according to the invention and an optical waveguide plate,

Figure 2 shows a cross-section of a second embodiment of a fluorescent light source according to the invention, and Figure 3 shows a cross-section of an alternative embodiment of a fluorescent light source according to the invention and an optical waveguide plate.

Figure 1 shows a cross-section of a first embodiment of a fluorescent light source 1 according to the invention and an optical waveguide plate 2. The light source 1 comprises a (precoated) glass tube 3 enclosing a discharge space 4. An inner surface of the tube 3 is partly provided with a laminate of successively a first reflective layer 5 and a phosphor layer 6, thereby defining an uncoated aperture 7. This uncoated aperture 7 is covered by a prismatic foil 8 which is applied to an outer surface of the tube 3. The prismatic foil 8 is adapted to concentrate and to direct light 9 towards said optical plate 2. Another part of the outer surface of the tube 3 is partly covered with a second reflecting layer 10 in order to secure a complete reflection of light generated within said tube 3. The lumen output of the light source 1 can be optimized thereby. The aperture 7 can be kept relatively small, since generated light can exit the light source 1 relatively unhindered, at least not hindered by the phosphor layer omitted in this part of the light source 1. In this illustrative embodiment, the aperture 7 together with the center of the tube 3 encloses an angle α of 40°. The outer diameter d of the light source 1 is 16 mm, and the thickness D of the optical plate 2 is 6 mm. The optical plate 2 is made of a transparent material, preferably PMMA, and is adapted to illuminate a display device (not shown), such as a liquid crystal device. The light source 1 is. adapted to side-light a display device, wherein the light source 1 generates an illumination beam 9 which is relatively clear and intense with respect to such beams generated with conventional fluorescent light sources. Other advantages of the light source 1 as shown have been elucidated above in a comprehensive manner. Figure 2 shows a cross-section of a second embodiment of a fluorescent light source 11 according to the invention. The light source 11 comprises a cylindrical envelope 12, wherein an inner surface of the envelope 12 is partly covered by successively a first reflective layer 13, in particular a transflective layer, and a phosphor coating 14, thereby defining two uncoated apertures 15 through which generated light can exit the light source 11 relatively unhindered and easily. The phosphor coating 14 is adapted to convert UV light generated within said envelope 12 into visible light. The first reflective layer 13 is provided for partly reflecting incident light generated within said envelope 12. To secure a complete reflection, an additional (second) reflective layer 16 is applied to an outer surface of the light source 11. A remaining part of the outer surface of the light source 11 is covered with two prismatic foils 17, wherein each prismatic foil 17 overlaps an (underlying) uncoated aperture 15 on both sides. The prismatic foils 17 are adapted to direct light 18 into a desired direction. The light source 11 has a double mirrored symmetry, while both apertures 15 are of uniform size. Each aperture 15 encloses an angle β, which is 40° in the embodiment shown. The light source 11 shown may be used in various applications, in particular illumination systems, more preferably for back-lighting display devices. In an alternative embodiment of the light source 11, the second reflective layer 16 is omitted, thus allowing a fraction of the light to exit the light source 11 via the transflective layer 13 (instead of via the prismatic foils 17) to improve .the lumen output for back-lighting purposes. Figure 3 shows a cross-section of an alternative embodiment of a fluorescent light source 19 according to the invention and an optical waveguide plate 20. In this illustrative embodiment, the diameter of the light source 19 is smaller than the thickness of the optical plate 20. The light source 19 comprises a glass tube 21 which is partly flattened and is therefore provided with a truncation 22 which is congruous with an end surface 23 of the optical plate 20. In this way light emitted by said light source 19 can be coupled into said plate 20 in a relatively efficient manner, whereby the loss of light (not coupled into said plate 20) can be minimized. An inner curved surface of the tube 21 is provided with a reflective coating 24, and a phosphor coating 25 is applied on top of said reflective coating 24. An inner surface of the truncation 22 of the tube 21 is provided with a prismatic layer 26 enabling controlled emission of light out of said light source 19. However, it is also conceivable to apply the prismatic layer 26 on the outer surface of the tube 21, between the tube 21 and the plate 20.

In this manner an assembly of a light source 19 and an optical waveguide plate 20 can be provided wherein the optical coupling between these components 19, 20 can be optimized relatively easily, and wherein the luminance of the light source 19 is significantly better than the luminance obtained by means of a conventional fluorescent light source. Also, the amount of light coupled- in is more, and together with more directed light inside the optical waveguide this has several advantages for backlighting and general lighting with optical waveguides. Further advantages have been comprehensively elucidated above. 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 invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer: In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. 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. Fluorescent light source comprising an elongate light-transmissive envelope enclosing a discharge space, wherein an elongate surface of said envelope is at least partly provided with: a phosphor coating for converting UV light generated within said envelope into visible light, light-directing means for a directional emission of light, and reflecting means for reflecting light generated within said envelope towards said light-directing means, characterized in that said phosphor coating is provided with at least one longitudinal uncoated aperture, said uncoated aperture being at least substantially covered by the light- directing means.

2. Light source according to claim 1, characterized in that the light-directing means overlap the uncoated aperture at least on one side.

3. Light source according to claim 1 or 2, characterized in that the uncoated aperture encloses an angle α, enclosed by a segment formed by the center of the envelope and the uncoated aperture, of between 30° and 40°, preferably substantially equal to 35°.

4. Light source according to one of the foregoing claims, characterized in that either an inner surface or a corresponding outer surface of said envelope is covered substantially with either the light-directing means or the reflecting means.

5. Light source according to one of the foregoing claims, characterized in that said envelope is provided with said reflective means on both its sides.

6. Light source according to one of the foregoing claims, characterized in that said envelope is flattened at least partly.

7. Light source according to one of the foregoing claims, characterized in that said light-directing means comprise multiple micro-prisms.

8. Light source according to claim 7, characterized in that said light-directing means are formed by at least one prismatic foil.

9. Light source according to one of the foregoing claims, characterized in that said phosphor coating is provided with multiple longitudinal uncoated apertures, said uncoated apertures being each at least substantially covered by the light-directing means.

10. Assembly of at least one light source according to one of the claims 1 to 9 and at least one optical waveguide.

11. Assembly according to claim 10, characterized in that said optical waveguide is formed by a light-transmitting plate.

12. Assembly according to claim 11, characterized in that the mutual orientation of the light source and the light-transmitting plate is such that said light source is substantially adapted to emit light towards an end side of said light-transmitting plate.

13. Assembly according to claim 11, characterized in that the mutual orientation of the light source and the light-transmitting plate is such that said light source is substantially adapted to emit light towards a lateral side of said light-transmitting plate.

14. Assembly according to one of the claims 9 to 13, characterized in that at least a portion of said light source and at least a portion of said optical waveguide are formed mutually congruously.

15. Assembly according to one of the claims 9 to 14, characterized in that said assembly further comprises intermediate optical coupling means positioned between said light source and said optical waveguide.

16. Display device comprising at least one light source as claimed in one of the claims 1 to 9.