EP1646904A1

EP1646904A1 - Display device with multiple channel wave guide

Info

Publication number: EP1646904A1
Application number: EP04735081A
Authority: EP
Inventors: Nijs C. Van Der Vaart; Frank A. Van Abeelen; Anthonie H. Bergman; Peter A. Duine; Ramon P. Van Gorkom; Albericus A. M. Hoevenaars; Boris Skoric
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2003-06-03
Filing date: 2004-05-27
Publication date: 2006-04-19
Also published as: TW200506792A; WO2004107015A1

Abstract

A display device (10) comprising a wave guide having a plurality of parallel channels (12) for conducting light, a back plate (22), and a plurality of light extracting elements (24) positioned orthogonal to the channels (12). The display further comprises actuating means including piezoelectric elements (28, 28' ,36 ,38), which are arranged to move the light extracting elements (24) into and out of physical contact with the wave guide. When contact is made between the channels (12) and the light extracting element (24), light can be coupled out of the wave guide and an image may be displayed.

Description

Display device with multiple channel wave guide

The present invention relates to a display device comprising a wave guide with a plurality of parallel channels for conducting light, a back plate, and a plurality of light extracting elements positioned across said channels.

Multiple channel optical wave guide displays are known in the prior art. One example is given in the article "Waveguide Panel Display Using Electromechanical Spatial Modulators", by Xiaochuan Zhou and Erdogan Gulari (ISSN0098-0966X/98/2901 (SID 1998)), and comprises a plurality of LEDs coupled to a multiple channel wave guide forming the columns of the display. The display further comprises rows of light switches comprising an electro-mechanically operated light scattering foil that can be electro-statically attracted towards the light guide one row at a time. When the foil makes optical contact with the light guide, light is coupled out of the light guide. When this operation is completed, the light switch is detached and a different row may be selected. In this way, it is possible to display an image with a line at a time addressing scheme. A problem with the use of an electro- mechanically operated foil is that the detachment of a row is very difficult since the foil tends to stick to the wave guide.

Another display is known from Japanese Patent Abstract No. 2001-265265, which discloses a display device comprising optical fibres, which fibres contain a liquid. An ultrasonic wave, which is generated by a piezoelectric element, is provided in order to generate air bubbles in the fibres. The air bubbles enable a laser beam to be taken out of the fibre, thus enabling an image to be displayed.

An object of the present invention is to provide a display device with a multiple channel wave guide which is improved compared with known display devices.

A particular object is to provide a display device which avoids sticking problems. These and other objects which will become apparent in the following description have now been achieved by a display device having the features defined in the accompanying claims.

The present invention is based on the understanding that an improved multiple channel wave guide display may be achieved by using piezoelectric elements as actuators in the display. The piezoelectric elements are used for bringing light extracting elements into and out of physical contact with the wave guide in the display, and thereby enable light to be coupled out from the wave guide. A light extracting element is simply selected by activating the piezoelectric elements associated with that particular light extracting element. The activation is achieved by applying a drive voltage to the appropriate piezoelectric element. One advantage with the use of piezoelectric actuators is that the aforementioned sticking problems are avoided. This is because of the ability to have large pull forces. This means that the detachment of the light extracting element away from the wave guide, i.e. after contact has been made and light has been coupled out, is improved. Another advantage is that it has been found that piezoelectric actuators make it possible to use smaller drive voltages in order to achieve a sufficient displacement action of the light extracting elements. Thus, the power efficiency of the display is improved.

The channels of the wave guide can be essentially parallel, and the light extracting elements can be orthogonal to the channels. Such a geometical arrangement corresponds to the conventional rows and columns of a display.

From US 5,862,275 it is known to use piezo actuators in combination with a large optical wave-guide plate. Each pixel is addressed with a separate actuator: when the piezo is actuated, optical contact is established and light is coupled out from the wave-guide plate. Further it is known from US 5,106,181 to couple out light from fibers by methods like electro-optics, thermo-optics, acoustic-optics and liquid crystal elements. But this invention, disclosing a combination of a display with the fibers for instance arranged as columns having the possibility of line-at-a-time addressing, and piezo actuators, for instance arranged as rows, which can be brought into optical contact with the fibers, is not known and offers advantages over the prior art. Preferably, the light extracting element has essentially the same dimensions as a row or column of the display, thereby facilitating addressing.

In one embodiment of the invention, the actuating means comprises one large piezoelectric element for each light extracting element. Each piezoelectric element is mounted between the columns of the optical wave guide and a large back plate. One large piezoelectric element per light extracting element enables straightforward construction of the display device.

In another embodiment, each large piezoelectric element is replaced by several smaller piezoelectric elements for each light extracting element. This results in smaller capacitive losses during switching. The several piezoelectric elements are electrically connected and may be mounted on a support plate, which plate, or e.g. a scattering layer provided thereon, can be brought into contact with the wave guide by means of the piezoelectric elements.

The piezoelectric elements mentioned above may be so called in-plane, or bending, piezoelectric elements. These elements are much thinner than standard piezoelectric elements that can provide the required displacement, thus making the in-plane elements easier to align compared to standard piezoelectric elements. Each in-plane piezoelectric element is fixed in relation to the back plate on each side of a light extracting element. When a drive voltage is applied to the piezoelectric element, the dimension of the activated element changes causing it to push the light extracting element away from the back plate and against the wave guide.

A particular type of in-plane piezoelectric elements is the piezoelectric foil, which is a low-cost alternative for the actuator.

The piezoelectric foil may for example increase its dimension when a drive voltage is applied, causing the foil to bend between two points of attachment and thus push the light extracting element against the wave guide. The foil and the drive.voltage may also be arranged so that the foil contracts when the drive voltage is applied. The contraction causes the light extracting element to establish contact with the wave guide.

However, one problem with piezoelectric foil is that in order to achieve a sufficient displacement of the foil perpendicular to the wave guide at a reasonable drive voltage, the foil has to be thin. And a thin foil tends to fold like a harmonica, which of course causes unwanted effects. Even if the foil does not fold, it is still difficult to get a reasonably large displacement when using in-plane piezoelectric elements such as for example a piezoelectric foil. In order to avoid folding problems with the piezoelectric foil, the display device according to the invention may further comprise a biasing element. The biasing element is arranged in order to put the foil under constant stress or give it more rigidity.

The biasing element, which for example may be an elastic foil or another piezoelectric foil, can be arranged either behind or in front of the piezoelectric foil. In order to accomplish a reasonable displacement of the in-plane piezoelectric element, the distance between the points where the in-plane piezoelectric element is fixed can be made larger than the width of a light extracting element, and in particular larger than the c-c distance between neighbouring light extracting elements. By c-c distance is meant the distance centre-to-centre between two adjacent light extracting elements. A section of the in- plane piezoelectric element may for example extend over several rows of the display. In this case the in-plane piezoelectric element cannot extend over the entire length of the row, but must be interrupted to leave room for in-plane piezoelectric elements that control other light extracting elements. Thus, each light extracting element is actuated by several piezoelectric elements, which are attached to a common support plate as mentioned above.

Currently preferred embodiments of the invention will now be further described in reference to the accompanying drawings wherein: Fig. la is a front view of a display device according to one embodiment of the invention;

Fig. lb is a side view taken along the line lb-lb in Fig. la; Fig. lc-ld is a top view taken along the line Ic-Ic in Fig. la; Fig. 2a-2b is a top view of another embodiment wherein the actuating means comprises only one piezoelectric element;

Fig. 3 is a side view wherein the piezoelectric elements are in-plane piezoelectric elements;

Fig. 4 is a side view of a display device comprising foil actuators and biasing elements; and Fig. 5 is a schematic perspective view of another embodiment of the invention.

The same reference numerals are used in Fig. 1-5 for corresponding details.

Fig. 1 shows a display device 10 according to one embodiment of the invention. In this case, the multiple channel optical wave guide comprises a plurality of parallel optical fibres 12 for conducting light. The fibres 12, which are made of for example glass or plastic, are arranged as columns and mounted on a transparent front plate 14. After mounting, a part of the fibres is removed by for example grinding and polishing so that the light guiding core 16 of the fibre comes to the surface, see Fig. lc. The open core surface is optically flat. Each fibre 12 is associated with a light source 18, for example a LED, whereby light coming from the LED is coupled into the fibre and travels via total internal reflection through the fibre 12 towards the other end of the display. The LEDs may be mounted at the bottom and/or top ends of the fibres 12. The display 10 further comprises spacers 20, which spacers have a boundary towards the fibres 12 with suitable optical properties (low index of refraction) in order to preserve the total internal reflection of light in the fibres 12, and a back plate 22, which back plate is arranged essentially parallel to the front plate. On the back plate 22, a plurality of light extracting elements 24 is attached. The light extracting elements 24 are arranged as rows orthogonal to the fibre columns. Each light extracting element 24 further includes actuating means, which means comprise several piezoelectric elements 28, see Fig. lc. The light extracting element is also preferably provided with a light scattering medium 26, which is mounted on the side of the light extracting element 24 facing the fibres 12. The light scattering medium 26 has preferably the same index of refraction as the core 16 of the fibres 12 and it should contain either scattering particles or it should be optically rough on the side facing the fibres 12. The embodiment shown in Fig. 1 also includes a support plate 30 on which said piezoelectric elements 28 are arranged to act. The support plate 30 has essentially the dimensions of a row of the display.

The display device 10 also includes a common electrode 32 and row electrodes 34. The common electrode 32 is mounted in between the back plate 22 and the piezoelectric elements 28 and it is connected to all light extracting elements 24 in the display. The row electrodes 34 are mounted in between the piezoelectric elements 28 and the support plate 30. Each row electrode 34 is associated to a particular light extracting element 24.

The display device 10 is operated by moving the light extracting elements 24 towards the fibres 12 one row at a time. When the light scattering medium 26 is brought into contact with the fibres 12, see Fig. Id, light is coupled out of the fibres 12 and leaves the display device 10 via the front plate 14. Thus, it is possible to display an image using a line- at-a-time addressing scheme. To move the light extracting elements 24, a voltage difference is applied between the common electrode 32 and the separate row electrode 34. This causes the piezoelectric elements 28 in that row to activate and push the light extracting element 24 in that row towards the fibres 12. The light scattering medium 26 is forced to contact the core 16 of all fibres 12, and the row is now selected. By using pulse width or pulse amplitude modulation of the LEDs 18, the light intensity in a fibre 12 can be adjusted, and the light output of the corresponding pixel in the selected row is then determined. After the generation of the required light output in the selected row has been completed, the light extracting element 24 is moved in a direction away from the fibres by the piezoelectric elements 28, and the light scattering medium 26 is detached, see Fig. lc. Now the next row can be selected and the required light intensity is adjusted in each of the LEDs 18. The displacement of the upper surface of the light extracting element 24 perpendicular to the fibres 12 is as mentioned above caused by the activation of the piezoelectric elements 28. A displacement of 1 μm can be obtained when a multi-layer piezoelectric material with a sheet thickness of 3 mm is used. The required drive voltage swing is typically 20 V. In order to facilitate alignment of the display 10 during manufacturing, a compressible support plate may be used and the display should then be assembled with activated piezoelectric elements 28. After assembly, the voltages are brought back to zero, and the required spacing between the fibres 12 and the light scattering medium 26 is obtained. In this way, problems with alignment and tolerances of the fibres 12 and the piezoelectric elements 28 may be circumvented.

It may be advantageous to reduce the pressure in the interior of the device in order to facilitate and improve switching of the light extracting elements. This can also prevent noise during switching.

Fig. 2 shows a top view of another embodiment of the invention where each light extracting element 24 is associated with only one large piezoelectric element 28', i.e. there is one large piezoelectric element 28' in each row. In this embodiment, the support plate 30 is not required. Instead, the light scattering medium 26 is attached directly on the row electrode 34 of the piezoelectric element 28'. Similar to Fig. lc-ld, Fig. 2a shows a situation when a row is not selected and the light scattering medium 26 is not in contact with the fibres 12, while Fig. 2b shows a situation when a row is selected and the light scattering medium 26 is in contact with the fibres 12.

The piezoelectric elements 28, 28' in Fig. 1 and Fig. 2 can be in-plane or bending piezoelectric elements 36, 38 as shown in Figs 3 and 4. One example of an in-plane piezoelectric element is disclosed in US-patent 6,091,182, herewith enclosed by reference. In figs 3 and 4, the piezoelectric elements 36, 38 are piezoelectric foils, which is a particular type of in-plane piezoelectric element.

With reference to fig 3, the piezoelectric foil 36 is attached to the back plate 22 in two points of attachment. The back plate 22 can comprise cavities 41 so that the part of the foil 36 between the points of attachment does not adhere to the back plate 22. Conductive layers 32, 34 are provided on both sides of the foil 36. On the front side, i.e. the side facing the fibres 12, the layer 34 is divided into sections as wide as the distance between the points of attachment of the foil. This sectioned front side serves as row electrode 34, while the conductive layer 32 on the back side serves as common electrode 32. When a drive voltage is applied to the electrodes 32, 34, the piezoelectric element 36 stretches and therefore bends to form an arch causing the light scattering medium 26 to be pushed into contact with the fibres 12.

According to a preferred embodiment, the piezoelectric foil is further put under constant stress or given more rigidity by a biasing element 40. The biasing element can be for example an elastic foil or another piezoelectric foil, arranged to apply stress to on the piezoelectric foil. Alternatively, it can be a sheet- like section of material attached so that it can vibrate, for example a thin section of ceramic material, as described in US 6,091,182. By using a biasing element 40, it is possible to avoid folding problems which otherwise may occur when using a thin foil. The piezoelectric foil 36 is preferably attached as a layer to the biasing element 40. The biasing element 40 may be behind the piezoelectric foil 36 as shown in Fig. 3, in which case it pulls the piezoelectric foil away from the fibres.

Alternatively, the biasing element may be in front of the in-plane piezoelectric element. In this case, the biasing element is arranged to push the light extracting element towards the fibres. When no drive voltage is applied, this movement is prevented by the piezoelectric foil. When the drive voltage is applied, the piezoelectric foil expands, and the biasing element moves the light extracting element so it makes contact with the fibres.

In Fig. 4, the aforementioned spacers 20 are now divided into two parts 37, 39, a lower part 39, attached to the back plate 22, and an upper part 37, attached to the front plate 14. A piezoelectric foil 38 is clamped between the spacers 37, 39, and the biasing element 40 is clamped between the spacers 39 and the back plate 22. The foil 38 and the biasing element 40 are attached to each other by for example adhesive. The foil 38 is further arranged to contract when the drive voltage is applied. When no drive voltage is applied, the elastic foil 40 pulls the piezoelectric foil 38 and the light extracting element 24 away from the fibres 12. When a drive voltage is applied, the contraction pushes the light extracting element 24 into contact with the optical fibres 12 against the force of the biasing element.

Alternatively, the biasing element 40 is on the front side, while the piezoelectric foil 38 is on the back side. In this case, the biasing element 40 is arranged to pull the.light extracting element 24 in the direction of the fibres 12. When no drive voltage is applied, this movement is prevented by the piezoelectric foil 38. When the drive voltage is applied, the piezoelectric foil 38 expands, and the biasing element 40 moves the light extracting element 24 so it makes contact with the fibres 12.

Fig. 5 shows a perspective view of another embodiment in which the distance 42 between the areas 44, 46 where the piezoelectric foil is attached to the back plate 22 is larger than the width of a row of the display. This results in a larger displacement of the light extracting element 24 perpendicular to the fibres 12.

In the embodiment shown in Fig. 5, a section of a foil between two fixed points extends over five rows of the display. The foils 38 that control one in every five rows do not extend over the entire length of the row in order to leave room for foils that control the other rows of the display. Therefore, the display is of the type shown in Fig. 1, with a support plate 30 actuated by several piezoelectric foils 38, for example two or three foils.

In Fig. 5, the piezoelectric foil 38 is arranged to expand when a drive voltage is applied, but it is of course also possible to use a foil which is arranged to contract when a drive voltage is applied and/or to combine the foil with a biasing element, as discussed above.

The invention is not limited to the embodiments described above. Those skilled in the art will recognize that variations and modifications can be made without departing from the scope of the invention as claimed in the accompanying claims. For example, other types of multiple channel wave guides may be employed.

Claims

CLAIMS:

1. A display device comprising: a wave guide having a plurality of parallel channels (12) for conducting light, a back plate (22), and a plurality of parallel light extracting elements (24) arranged in between said wave guide and said plate (22) and positioned across said channels (12), wherein each light extracting element comprises actuating means for bringing said light extracting element (24) into and out of physical contact with said wave guide, characterized in that said actuating means comprises at least one piezoelectric element (28, 28' ,36 ,38).

2. A display device according to claim 1, wherein said channels (12) are essentially parallel, and wherein said light extracting elements (24) are essentially orthogonal to said channels (12).

3. A display device according to claim 1 or 2, wherein the channels (12) and the light extracting elements (24) are individually addressable.

4. A display device according to claim 1, 2 or 3, wherein said actuating means comprises only one piezoelectric element (28').

5. A display device according to claim 1, 2 or 3, wherein said actuating means comprises several electrically connected piezoelectric elements (28).

6. A display device according to claim 5, wherein each light extracting element further includes a support plate (30), on which said piezoelectric elements (28) are arranged to act.

7. A display device according to any one of claims 1-6, wherein said piezoelectric element is an in-plane piezoelectric element (36) which is fixed in relation to the back plate (22) on each side of the light extracting element.

8. A display device according to claim 7, wherein said piezoelectric element is a piezoelectric foil (38).

9. A display device according to claim 8, wherein said actuating means further includes a biasing element (40), which biasing element is arranged to keep the piezoelectric foil (38) under constant stress or to give the piezoelectric foil (38) more rigidity.

10. A display device according to claim 8 or 9, wherein said biasing element (40) is an elastic foil.

11. A display device according to claim 8 or 9, wherein said biasing element (40) is a piezoelectric foil.

12. A display device according to any one of claims 7-11, wherein a distance between areas of attachment of the piezoelectric element is larger than the centre to centre distance between adjacent light extracting elements (24).