Patent Application
20120206938
The invention relates to a light-guide for an illumination system.
The invention also relates to a luminaire, a scanning backlight system and a display device.
Light-guides are known per se, for example, for illuminating a light output window of an illumination system such as a backlight system. They are used, inter alia, in backlighting systems for (picture) display devices, for example, for TV sets and monitors. Such light-guides are particularly suitable for use as backlighting systems for non-emissive display devices such as liquid crystal display devices, also denoted as LCD panels, which are used in, for example, (portable) computers or, for example, (portable) telephones.
Said non-emissive display devices usually comprise a substrate provided with a regular pattern of pixels which are each controlled by at least one electrode. The display device utilizes a control circuit for achieving a picture or a data graphical display in a relevant field of a (picture) screen of the (picture) display device. The light originating from the illumination system in an LCD device is modulated by means of a switch or modulator in which, for example, various types of liquid crystal effects may be used. In addition, the display may be based on electrophoretic or electromechanical effects.
To reduce motion artifacts scanning backlight systems are developed and are being further improved. Scanning backlight systems are configured to sequentially illuminate a predefined group of pixels of the display device. Currently, there are two commonly used configurations for scanning backlight systems for non-emissive display devices: the edge-lit configuration and the direct-lit configuration. In the edge-lit configuration, the scanning backlight system generally comprises a light-guide arranged parallel to the light output window and having an edge wall through which an (array) of light sources emit light into the light-guide. The light is guided substantially parallel to the light output window and is distributed throughout the light-guide. The light-guide comprises a plurality of light-guide segments which correspond to the area of pixels of the predefined group of pixels which are sequentially illuminated by the scanning backlight system. In such an edge-lit configuration, the predefined group of pixels constitutes a (plurality of) line(s) of pixels across the display device which generates a one-dimensional scanning backlight system. The light is emitted through the light output window by redirecting part of the guided light via light-extraction means.
In the direct-lit configuration, the light sources are arranged in an array substantially parallel to the light output window of the scanning backlight system. In the direct-lit configuration, the scanning backlight system may be a two-dimensional scanning backlight system in which the predefined group of pixels of the display device which are sequentially illuminated may be part of a two-dimensional array of predefined groups of pixels constituting the display device.
A drawback of known light-guides having a plurality of light-guide segments is that an image displayed by a display device having such light-guides may comprise local image imperfections.
It is an object of the invention to provide a light-guide for an illumination system in which local image imperfections are reduced.
According to a first aspect of the invention the object is achieved with a light-guide for an illumination system comprising a light output window. The light-guide comprises a light output window and comprises a plurality of light-guide segments, each light-guide segment being substantially optically separated from a neighboring light-guide segment. Each light-guide segment comprises a front wall arranged opposite a rear wall and being configured for distributing the light within the light-guide segment. Each light-guide segment further comprises light-extraction means for extracting at least part of the distributed light via the front wall to illuminate in operation the light output window. A distance between the front wall and the light output window is smaller at a center of the front wall compared to a predefined edge of the front wall. The predefined edge of the front wall is an edge at which the light-guide segment is arranged adjacent to the neighboring light-guide segment.
The distance between the light output window and the front wall is a dimension which is measured in a direction substantially perpendicular to the light output window. The light output window may be a window constituted by translucent material through which the light-guide emits its light away from the light-guide. Alternatively, the light output window may, for example, comprise an imaginary planar surface which, for example, may be a tangent plane contacting the front wall substantially at the center of the front wall or an imaginary planar surface which is substantially parallel to the tangent plane which contacts the front wall substantially at the center of the front wall. The front wall of the light-guide segment is configured for emitting the light guided by the light-guide segment. The front wall comprises predefined edges being a border or line separating neighboring light-guide segments. The front wall comprises a shape such that the distance between the front wall and the light output wall at the center of the front wall is smaller compared to the predefined edge. The center of the front wall is a part of the front wall which has substantially equal distance to the edges of the front wall measured along the surface of the front wall. In case the light-guide segment has a symmetry axis parallel to the front wall, the front wall typically only has a maximum of two predefined edges and has a center substantially coinciding with a line of the front wall surface which runs substantially parallel to this symmetry axis. If the light-guide segments are arranged in the illumination system in a one-dimensional array of light-guide segments, each light guide segment comprises a maximum of two predefined edges. If the light-guide segments are arranged in the illumination system in a two-dimensional array of light-guide segments, each light-guide segment comprises a maximum of four predefined edges.
The effect of the light-guide according to the invention is that the distance between the front wall and the light output window is larger at the predefined edge where the light-guide segment adjoins the neighboring light-guide segment compared to the center of the front wall of the light-guide segment. Due to this increased distance, local variations in intensity and/or color and/or distribution of the light at the interface between adjacent light-guide segments are averaged out due to mixing of the light originating from both of the adjacent light-guide segments before the light impinges on the light-output window. Consequently, due to the increasing of the distance at the predefined edge, visibility of variations at the interface between neighboring light-guide segments are reduced, improving the uniformity across the light output window of the illumination system. When the illumination system is a scanning backlight system, the increasing of the distance at the predefined edge improves the quality of the image produced on a display device comprising the scanning backlight system comprising the light-guide.
Especially light artifacts which result in substantially straight lines typically are clearly visible by a user, because the human eye is relatively sensitive to small brightness variations. Imperfections at the edges of the front wall may cause unwanted scatter effects which may cause visible intensity variations in, for example, an image illuminated via the illumination system such as the scanning backlight system comprising the light-guide. Also variations in intensity and/or color and/or distribution of the light between the light emitted by the light-guide segment and its neighboring light-guide segment may be visible as a line or area of deviating intensity and/or color and/or distribution in the image illuminated via illumination system such as the scanning backlight system comprising the light-guide. The inventors have found that by increasing the distance of the predefined edge to the light output window compared to the center of the front wall, any light artifacts caused by imperfections at the edge of the front wall are reduced due to additional mixing of the light emitted at the predefined edge caused by the additional distance. The increased distance at the predefined edge also causes a predefined overlap of the light emitted by the light-guide segment and the neighboring light-guide segment. This predefined overlap reduces any variations in intensity and/or color and/or distribution of the light between adjacent light-guide segments in the light-guide which also improve the quality of, for example, the image produced on the display device comprising the scanning backlight system comprising the light-guide. Also misalignment between two neighboring light-guide segments may generate local intensity variations which typically result in visible straight lines of deviating intensity and which may be significantly reduced by increasing the distance at the predefined edge to generate the predefined overlap.
The light-guide segment may, for example, be constituted by solid material substantially transparent for the light which is to be guided by the light-guide. The guiding and mixing of the light may, for example, occur via total internal reflection which generates a substantially loss-less confinement of the light inside the light-guide segment. The light extraction means ensure that the light is extracted from each light-guide segment such that the light output window of the illumination system is illuminated substantially homogeneously. When the illumination system is a scanning backlight system the light intensity which impinges on the light output window is homogeneous across the light output window during a predefined time interval which may, for example, be a frame time of a display device such as a television.
In an embodiment of the light-guide, the light-guide is constituted by a continuous material, the increased distance at the predefined edge of each of the light-guide segments constituting the optical separation between the light-guide segments. This embodiment has as an advantage that the cost of manufacturing of the light-guide is reduced. For example, well known molding techniques such as injection-molding or extrusion techniques may be used to generate the light-guide from a continuous material. During injection-molding or extrusion a solvable or deformable material is pressed into a mold after which the solvable or deformable material is cured to form the light-guide. The shape of the front wall may be constituted to have the increased distance between the front wall and the light output window at the predefined edge compared to the center of the front wall. Also well known stamping-techniques for adapting the shape of the front wall to generate the increased distance between the front wall and the light output window at the predefined edge may be used to generate the light-guide from a continuous material.
A further advantage of this embodiment is that neighboring light-guide segments do not need to be aligned during construction of the light-guide. When the light-guide is constituted by physically separate light-guide segments, the light-guide is constructed from the separate light-guide segments by aligning the separate light-guide segments adjacent to each other. Any misalignment between two adjacent light-guide segments may cause visible light artifacts reducing the quality of the light-guide, illumination system, scanning backlight system and of the display device. Although the increased distance between the front wall at the predefined edge and the light output window reduces any light artifacts which may result from this misalignment, the current embodiment enables to construct the light-guide without the need to align the individual light-guide segments. Consequently, light artifacts due to misalignment are substantially avoided which further improve the quality of the light-guide, illumination system, scanning backlight system and display device.
An even further advantage of this embodiment is that it results in a light-guide have improved mechanical stiffness.
In an embodiment of the light-guide, a thickness of the light-guide segment at the center of the front wall is at least 3 times larger than the thickness at the predefined edge of the front wall for generating the increased distance at the predefined edge. The thickness of the light-guide segment is a dimension between the front wall and the rear wall, measured in a direction substantially perpendicular to the light output window. Having a thickness at the predefined edge which is 3 times thinner than at the center of the front-wall ensures that the light in the light-guide segment is efficiently confined within the light-guide segment. This efficient confinement is still present when the light-guide is constituted by a continuous material.
In an embodiment of the light-guide, a shape of the front wall comprises a smoothly curved shape configured for increasing the distance between the front wall and the light output window at a predefined edge compared to the distance at the center. This embodiment has as an advantage that the smoothly curved shape reduces scattering elements from occurring which may cause non-uniformities in the emission profile. Any sharp edge at or near the front wall may comprise imperfections from which light may be scattered uncontrollably and which may generate the intensity variations. A further advantage of the smoothly curved shape is that no sharp transitions occur in the angular distribution of the light which is emitted from the front wall. Such transition in angular distribution may, for example, occur between two facets which have a different orientation with respect to each other. Furthermore, the smoothly curved shape may be beneficial when using injection molding techniques to manufacture the light-guide or to manufacture the individual light-guide segments. When the front wall comprises sharp edges, these sharp edges may be locally deformed or damaged when the light-guide or the light-guide segment is released from the mold. Such deformations or damages may cause scattering of light or uncontrolled redistribution of light emitted from the light-guide or light-guide segment which may be visible in the image. Alternatively, the light-guide or light-guide segment which has relatively sharp edges may need to be cured before removing the light-guide or light-guide segment from the mold, which increases the manufacturing time of the light-guide or light-guide segment and consequently the cost of the light-guide or light-guide segment. When using a smoothly curved front wall, the sharp edges at or near the front wall may be avoided, thus improving the image quality.
In an embodiment of the light-guide, a shape of the front wall comprises a plurality of facets constituting the front wall, wherein the plurality of facets comprises an edge-facet comprising the predefined edge, the edge-facet being configured for being arranged at an angle with the light output window for increasing the distance. This embodiment has as an advantage that the increased distance between the center of the front wall and the predefined edge is generated gradually defined by the angle between the edge-facet and the light output window. Such gradual increase of the distance further reduces the visibility of any remaining differences in intensity and/or color and/or distribution of the light of neighboring light-guide segments because the mixing between the light from the light-guide segment and the light of the neighboring light-guide segment also occurs gradually. The angle between the edge-facet and the light output window is, for example, within a range between (and including) 10 degrees and 30 degrees (10 to 30, in which represents the angle) to ensure sufficient distance between the predefined edge and the light output window, while preventing a too steep variation of the distance.
Each light-guide or light-guide segment may, of course also comprise an edge wall which may be constituted by a wall or facet arranged substantially perpendicular to the light output window and arranged between, for example, the edge-facet and the rear wall. Such an edge wall increases the thickness and the strength of the light-guide or light-guide segment near the edge of the light-guide or light-guide segment which makes the light-guide or light-guide segment less vulnerable for damages at the edge of the light-guide or light-guide segment. If no edge wall would be present at the edge of the front wall or at the edge-facet of the front wall, the thickness of the light-guide or light-guide segment at the edge of the front wall may become too narrow which would increase the risk that parts of the light-guide or light-guide segment may be damaged and break off, for example, during assembly of the light-guide.
In an embodiment of the light-guide, the plurality of light-guide segments is arranged in a one-dimensional array of light-guide segments. The light-guide segments in the current embodiment typically comprise an edge wall arranged between the front wall and the rear wall in which the edge wall comprises a light-entrance window for enabling light from a light source to enter the light-guide segment. Light emitted through the light-entrance window is distributed within the light-guide segment and is subsequently used to illuminate a predefined line or a predefined number of lines of pixels in the display device. This light-guide may be used in a one-dimensional scanning backlight system in which each light-guide segment comprises an associated light source emitting light into light-guide segment via the edge wall. Alternatively, other means of coupling the light of a light source into the light-guide segment may be used, for example, by applying a concavely shaped part bulging inward into the light-guide segment, for example, from the rear wall to accommodate room for a light source, for example, a side-emitting light emitting diode in which the light from the light source is emitted in a direction substantially parallel to the light output window.
Alternatively, the plurality of light-guide segments is arranged in a two-dimensional array of light-guide segments. In such an embodiment, the light-guide segment may comprise a light-entrance window which is arranged on an edge of the light-guide segment which extends from the light-guide segment to behind the rear wall of a neighboring light-guide segment. Alternatively, the light-guide segment may comprise the concavely shaped part bulging inward into the light-guide segment, for example, from the rear wall to accommodate room for the light source, for example, a side-emitting light emitting diode in which the light from the light source is emitted in a direction substantially parallel to the light output window.
In an embodiment of the light-guide, the light-guide segment comprises a light-entrance window for enabling light from a light source to enter the light-guide segment, at least one of the light-guide segments in the plurality of light-guide segments being configured for having the light-entrance window located at a side of the rear wall of the neighboring light-guide segment facing away from the neighboring light-guide segment. In backlight systems, the light source is often arranged at an edge wall of the light-guide segment. However, such an arrangement of the light source often results in a relatively broad and thick rim around the display device which, next to the less aesthetic appearance of the display, also requires additional space when, for example, integrating the display device in a further application or housing. The current embodiment has as an advantage that the light source may be hidden away behind the rear wall of a neighboring light-guide segment.
In an embodiment of the light-guide, each light-guide segment is configured for illuminating a corresponding part of the light output window, the corresponding parts of neighboring light-guide segments are configured to partially overlap. Especially such partial overlap may be used to the advantage that any differences between the intensity and/or color and/or distribution of the light emitted by the front wall by adjacent light-guide segments is averaged out across the overlapping region to make the transition in intensity and/or color and/or distribution of light between adjacent light-guide segments to reduce gradually, thus reducing the visibility of these transition to the human eye.
In an embodiment of the light-guide, a distribution, dimension and/or density of the light-extraction means in the light-guide segment is configured for reducing an intensity of the light extracted by a single light-guide segment at the overlap between neighboring parts illuminated by neighboring light-guide segments to generate a substantially uniform illumination of the output window. By reducing the intensity at the overlap region which is contributed by a single light-guide segment, the overall intensity of the overlap may be chosen such that the overall intensity across the whole light-output window is substantially homogeneous. In the embodiment in which the light-guide is part of a scanning backlight system, the different parts together with the overlap regions are illuminated by the scanning backlight system in a sequential manner. Over time, the light emitted across the light-output window is substantially uniform.
In an embodiment of the light-guide, the light-guide comprises luminescent material or comprises a mixture of luminescent materials for converting at least part of the light guided through the light-guide into light having a longer wavelength. The luminescent material may, for example, be arranged on the front wall and/or on the rear wall of the light-guide or may be arranged on a separate substrate arranged between the light source and the light output window. Alternatively the luminescent material may be arranged on the light output window or the luminescent material may be distributed within the light-guide segment. Even further alternatively, the luminescent material may constitute the light-extraction means. Any of the previously mentioned arrangements of the luminescent material in the light-guide is also known as a remote phosphor arrangement. The benefit when having the luminescent material remote from the light source is that the efficiency of the luminescent material is improved, the range of luminescent materials to choose from is improved due to the lower temperature requirements of the luminescent material in the remote phosphor arrangement, and the remote luminescent material also acts as a diffuser which diffuses the light emitted by the light source avoiding the use of a separate diffuser.
The invention also relates to an illumination system comprising a plurality of light sources and the light-guide according to the invention.
In an embodiment of the illumination system, the plurality of light sources each emit substantially white light, and/or the plurality of light sources each comprise a plurality of light-emitters emitting light of a plurality of colors. A benefit of this embodiment is that the plurality of colors may be used to tune a color emitted by the individual light-guide segments by tuning an intensity of the light-emitters in the light source. Furthermore, when the illumination system is a scanning backlight system, the scanning backlight system may be configured for scanning each color of the plurality of colors separately. Consequently, optimized scanning settings may be applied for the different colors to reduce, for example, motion artifacts in the image. Such optimized scanning settings may, for example, comprise different scan-speed for different colors due to the use of, for example, different luminescent materials for generating the different colors which have a substantially different decay-time, or, for example, by only applying the scanning mode of operation of the scanning backlight system for a specific color rather than for all colors as motion artifacts are only to be expected at a specific color of the range of available colors or are only visible at a specific color in the range of available colors.
The invention also relates to a luminaire comprising the light guide according to the invention or comprising the illumination system according to the invention.
The invention also relates to a scanning backlight system comprising a plurality of light sources and comprising the light guide according to the invention or comprising the illumination system according to the invention. The illumination system and/or scanning backlight system may further comprise a diffuser and/or a brightness enhancement foil and/or a redirection foil to further improve the uniformity of the light emitted from the illumination system and/or scanning backlight system.
The invention also relates to a display device comprising the light-guide according to the invention, or comprising the scanning backlight system according to the invention.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.
In the drawings:
The figures are purely diagrammatic and not drawn to scale. Particularly for clarity, some dimensions are exaggerated strongly. Similar components in the figures are denoted by the same reference numerals as much as possible.
The embodiments of the light-guide segments 10, 12A, 12B as shown in
The light-guide segment 10 as shown in
The light-guide segments 12A, 12B as shown in
The light-guide segment 10, 14, 16 may comprise light extraction means 40 to ensure that the light confined inside the light-guide segment 10, 14, 16 may be extracted from each light-guide segment 10, 14, 16 such that the light output window 220 of the illumination system 200, 202, 204 is illuminated substantially homogeneously. When the illumination system 200, 202, 204 is a scanning backlight system 200, 202, 204 the light intensity which impinges on the light output window 220 is substantially homogeneous across the light output window 220 during a predefined time interval which may, for example, be a frame time of a display device 300 such as a television 300.
The illumination system 200, 202, 204 as shown in
The illumination system 200 as shown in
The illumination system 202 as shown in
The illumination system 204 as shown in
As can be clearly seen from the cross-sectional views of
The light-guides 100, 102, 104 as shown in
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. 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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 09174124.9 | Oct 2009 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB10/54811 | 10/25/2010 | WO | 00 | 4/23/2012 |
