The present invention refers to diffusely reflecting displays, such as electrophoretic (E-ink) displays and Nanomat, which are often also called paper-like displays due to their physical character.
At low ambient light levels a display that has a reflective character needs a front light to illuminate the display. Traditional reflective displays, such as LCDs, require the illuminating light to reach the display surface at near normal angles therewith. Since the light source cannot be placed straight above that surface, but has to be located to the side of the display, there has been a development of transparent light guides, to be positioned above (in front of) the display surface, and to guide and redirect the light onto the display. In order to obtain a good light distribution and direction light guides have been provided with microstructures, such as saw tooth like microstructures at the front surface thereof The microstructures must be of high quality, and put high demands on the manufacturing thereof.
The introduction of diffusely reflecting displays, has relaxed the demands on the light guide of the front light, since they allow the illuminating light to reach the display surface at larger angles therewith. This advantage has triggered the development of simple wedge shaped light guides, which are easier to manufacture. Such a wedge shaped light guide is shown in EP 1 220 015, though for a reflection type LCD, which has a complicated additional reflector plate embedded in the display in order to simulate a more diffuse reflection character that makes it possible to use a wedge shaped light guide. However, those wedge shaped light guides have some disadvantages. First, such a light guide will be increasingly thick, at the end facing the light source, and undesirably heavy, when larger displays are to be illuminated or larger wedge angles are required. In addition to the problems of thickness and weight, integration of the front light with other parts is impeded. Further, the uniformity of the illumination cannot be tuned when a simple wedge is used.
The object of the present invention is to provide a diffusely reflecting display apparatus, which eliminates the front light drawbacks described above and provides for a front light which combines a substantially uniform thickness with the relaxed quality demands and with a tuneable illumination uniformity.
The object is achieved by means of a display apparatus according to claim 1.
Thus, according to an aspect of the invention, there is provided a display apparatus comprising a diffusely reflecting display panel, a front light comprising a light guide and a light source arranged at a first end of the light guide and emitting light, which enters into the light guide through said first end, and a transparent intermediate layer, which is arranged between a front surface of said display panel and a rear surface of said light guide, such that the display panel is in optical contact with said light guide. The rear surface of the light guide is provided with a microstructure comprising wedge-facets, which are arranged such that, for each wedge-facet, the distance between a first edge of the wedge-facet and said front surface of the display panel is shorter than the distance between a second edge of the wedge-facet and said front surface of the display panel, where said first edge is positioned closer to the light source than said second edge.
The expression “optical contact” is well known by a man skilled in the art and is defined as physical contact between different transparent materials that results in the zero or dark fringe of Newton's Rings experiment. This occurs at sufficiently small difference in refractive indices of the different materials. By “wedge-facet” is meant a facet that is inclined to a center plane of the light guide such that a portion of the light guide that defines the facet can be regarded as wedge shaped. This will be thoroughly exemplified below. By forming a microstructure comprising wedge-facets on the rear surface, or bottom, of the light guide and connecting it optically with the display panel by means of the intermediate transparent layer, the light diffusing properties of the diffusely reflecting display are advantageously used. Further, the optical quality of the microstructure need not be as high as when the microstructure is provided on the front surface, or top, of the light guide. In addition, the light guide as a whole need not be wedge shaped, thus eliminating the problem of large sized displays described above. However, within the scope of the invention, it is nevertheless possible, if desired, to make the front surface of the light guide non-parallel with the front surface of the display panel. Another advantage of the inventive display apparatus is that the microstructure is not exposed to the surrounding environment, which subject the top to dust particles, smear, scratching objects, etc. Further, the optical contact between the display and the microstructure of the light guide reduces the scattering of light due to non-sharp edges, scratches or other irregularities that may occur. Thus, the optical contact results in a better overall display performance.
In EP 1 220 015 indeed an embodiment of a light guide having a saw tooth shaped pattern on the rear surface of the light guide is shown. However, this prior art document basically is concerned with LCDs, and in particular with a modified variant of an LCD, as mentioned above, that does not imply the combination of a similar structure with a diffusely reflecting display.
According to an embodiment of the display apparatus the wedge-facets are arranged consecutively along the rear surface of the light guide, between the first end and a second end thereof, which second end is opposite to said first end. According to this embodiment the wedge-facets are arranged in a way that contributes to a uniform illumination.
According to an embodiment of the display apparatus, each two neighboring wedge-facets are connected by means of a connection surface, which is non-parallel with said front surface of the display panel. In other words, each connection surface extends from the second edge of a wedge-facet to the first edge of a consecutive wedge-facet. Due to the non-parallelism of the connection surfaces in combination with the orientation of the wedge-facets no light will hit these surfaces. Thus, the out coupling of light is only performed by the wedge-facets. Consequently, the finishing of the connection surfaces will be even less important.
According to an embodiment of the display apparatus, each two neighboring wedge-facets are connected by means of a connection surface, which consists of a first portion that is parallel with said front surface of the display panel and a second portion that is non-parallel with said front surface of the display panel. In this embodiment the wedge-facets are in a sense more separated from each other, due to the parallel portion. This means that the pitch of the wedge-facets is easily tuneable, which can be utilized in order to further improve the uniformity of the illumination.
According to an embodiment of the display apparatus, at least one wedge-facet property varies with the distance from said first end. This provides for further possibilities of tuning the illumination.
According to an embodiment of the display apparatus, said at least one wedge-facet property is chosen out of a group of wedge-facet properties consisting of the wedge angle, the distance between two consecutive connection surfaces, and the wedge pitch. In this embodiment several properties are possible to utilize for refining the distribution of the illumination on the display panel. For example this embodiment has a capability of obtaining a gradient in the out coupling rate by means of a proper positioning and shaping of the wedge-facets by varying one or more of the properties.
According to an embodiment of the display apparatus, said at least one wedge-facet property increases from said first end towards said second end. This embodiment defines one way of varying the property(ies).
According to embodiments of the display apparatus, an end surface of said first end comprises a first facet being non-perpendicular to a front surface of said light guide, or the end surface comprises also a second facet, which is adjacent to said first facet, wherein said first and second facets form a V-shaped groove extending along said end surface. In these embodiments the light entrance of the light guide is modified. Thereby, these embodiments provide an improvement to the incoupling of light into the light guide, which improvement aims at contributing to a uniform illumination.
According to an embodiment of the display apparatus, the intermediate layer is an adhesive. It is advantageous to apply an adhesive between the light guide and the display panel in order to obtain the intermediate layer, since it is easy to manufacture and since the adhesive easily fills up the cavities in the light guide that are obtained due to the microstructure.
According to an embodiment of the display apparatus it further comprises a touch screen and a second transparent intermediate layer, which is arranged between the touch screen and a front surface of said light guide, and which brings the touch screen into optical contact with the light guide. This embodiment extends the inventive concept for the basic display structure to touch screens, i.e. active displays, where input can be made on the display. This results in a significant improvement of the contrast and other properties of prior art touch screens.
According to an embodiment of the display apparatus the second transparent intermediate layer has a refractive index that is lower than that of the first transparent intermediate layer. This relation between the refractive indices of the first and second intermediate layers prevents the light from the light source from leaking into the touch screen, and thereby the light is out coupled towards the display panel only.
The present invention will be described in greater detail with reference made to the accompanying drawings, in which:
In
In a first embodiment of the display apparatus 20 according to the present invention, as illustrated in
The refraction index of the intermediate layer 29 should be lower than that of the light guide 23. The refraction index of the intermediate layer 29 is adjustable in order to promote a uniform illumination, and, typically, this results in a refractive index substantially lower than that of the light guide 23. Consequently, the intermediate layer 29 is also called low index layer.
An exemplifying light ray introduced into the light guide 23, at a first end 24 thereof, from the light source 22, is propagated and reflected through the light guide 23 as shown in
In
A second embodiment of the display apparatus is shown in
Since the light source has to be positioned at an end of the light guide, there are difficulties in obtaining a uniform illumination of the display panel. In accordance with the invention it is possible to adapt the microstructure in such a way as to compensate for the side mounting of the light source. There are several different properties of the wedge-facets that are possible to adjust in dependence on the position of individual wedge-facets in relation to the light source. A major factor is the distance from an individual wedge-facet to the light source, above all the distance as measured in the direction of propagation of the light Wedge-facet properties of particular interest are the wedge angle, wedge length, the distance between two consecutive connection surfaces, and the wedge pitch. By wedge angle is meant the angle at which a wedge-facet is inclined in relation to the geometric plane of the rear surface of the light guide, as indicated by a in
In accordance with embodiments of the display apparatus as shown in
In
In order to further improve the illumination properties, in an sixth embodiment as shown in
In accordance with a seventh embodiment, as shown in
Thus, in accordance with the present invention, a display apparatus is provided, which takes advantage of the inherent properties of a diffusely reflecting display panel, and includes a front light, which combines a substantially uniform thickness with relaxed quality demands, and which provides a tuneable illumination uniformity.
Number | Date | Country | Kind |
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031012289 | May 2003 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB04/50538 | 4/28/2004 | WO | 10/28/2005 |