Abstract:
A pixel of a display device is controlled by a tilting mirror. When the mirror is in a first position, light from a light source is directed to a light diffusing structure for viewing. If, however, the mirror is tilted, the light is directed along a path which prevents it from entering the light diffusing structure. Thus, the brightness of a pixel is controlled by tilting a mirror. In one embodiment, the colour of an individual pixel can be controlled by controlling the degree of tilt of a mirror.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a display device in which pixels are controlled by respective mirrors. 
     BACKGROUND TO THE INVENTION 
     U.S. Pat. No. 5,517,347 discloses a directly viewable display device, that is it does not project an image onto a screen, in which the appearance of each pixel is controlled by the state of a respective mirror. However, it suffers from the problem that the image has a smaller area than the display device because light enters and leaves the device obliquely. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome the afore-mentioned problem of a prior art display device and provide a display device, in which each pixel is controlled by the state of a respective mirror, that can be viewed face on. 
     According to the present invention, there is provided a display device comprising a front face, means defining an array of directly viewable pixels and a respective mirror for each pixel, the state of which controls the appearance of the pixel, wherein light beams are reflected by said mirrors through diffusing means to a viewing position, emerging from the front face in a direction substantially perpendicular thereto, the diffusing means expanding said beams for viewing. 
     Preferably, light source means is provided for supplying pixel illuminating light to the mirrors and the light paths from the light source means to the mirrors are substantially equal in length. 
     Preferably, a display device according to the present invention includes a light source layer and a control layer containing the mirrors, wherein the light source layer and the control layer are substantially co-extensive. 
     The preferred lighting arrangements of the present invention overcome the subsidiary problem of non-uniform pixel illumination that arises with the side-lit arrangement disclosed in U.S. Pat. No. 5,517,347. 
     Preferably, the control layer includes focusing means, associated with each mirror, for reducing the cross-sectional areas of light beams that are reflected by the mirrors to be less that the cross-sectional areas of the visible pixels and a diffusion layer is provided for expanding the beams for viewing. The control layer may sandwiched between the light source layer and diffusion layer or the light source layer may sandwiched between the diffusion layer and the control layer. 
     Preferably, each pixel is defined by a cell, each cell comprising means defining a light path selectively extending between a light source and a viewing position. Each light path may selectively be interruptable by changing the state of the respective mirror. 
     Preferably, each mirror is tiltable between a first position in which its pixel is bright and a second position in which its pixel is dark. However, the mirror could be translated or have their reflectivities changed. 
     In one embodiment, each cell is provided with a source of light and the light path, when uninterrupted, extends from the source of light through a lens to a tiltable mirror and then to a fixed mirror, which directs light received via the lens and the tiltable mirror through an aperture in the tilting mirror to the diffusing means and thence to a viewing position, the lens focusing light from the light source at a point between the fixed mirror and the viewing position. Conveniently, the lens comprises a fresnel lens. 
     In another embodiment, each cell is provided with a source of light and the light path, when uninterrupted, extends from the source of light to a fixed concave parabolic mirror and then to a tiltable mirror which directs light in the light path to a viewing position through the diffusing means, the parabolic mirror focusing light from the light source to a point between the tiltable mirror and the viewing position. 
     The diffusing means may for instance comprise a hemispherical lens and a planoconcave lens. 
     Advantageously, the light path of at least one cell passes through a colour filter. In this way a colour image can be produced. Preferably, different colour filters are provided in different light paths. A full-colour display is most desirable and this can be achieved by providing different colour filters, e.g. red, green and blue, in different light paths. 
     In a further embodiment, each cell is provided with a source of light and the light path, when uninterrupted, extends from the source of light via a convex lens to a tiltable mirror which directs light in the light path to a viewing position through the diffusing means, the lens focusing light from the light source to a point between the tiltable mirror and the viewing position. Conveniently, the lens comprises a fresnel lens. 
     In a yet further embodiment, each cell is provided with a source of a narrow beam of collimated light and the light path, when uninterrupted, extends from the source of light via a fixed planar mirror to a tiltable planar mirror which directs light in the light path to a viewing position through the diffusing means. Preferably, each cell is provided with a plurality of colour filters and the tiltable mirror is controllable to direct light in the light path selectively through the filters. 
     An electroluminescent polymer may be used as the light source. 
     The depth of the control layer is conveniently defined by a spacer structure, the spacer structure defining a matrix of cells. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of two cells of a first embodiment of the present invention; 
     FIG. 2 is a partial front view of the embodiment of FIG. 1 illustrating the arrangement of the cells; 
     FIG. 3 is a sectional view of two cells of a second embodiment of the present invention; 
     FIG. 4 is an exploded perspective view of seven cells of the second embodiment; 
     FIG. 5 is a sectional view of two cells of a third embodiment of the present invention; 
     FIG. 6 is a side view of a cell of a fourth embodiment of the present invention; 
     FIG. 7 is another side view, orthogonal to that of FIG. 6, of the cell of FIG. 6; 
     FIG. 8 shows a first source of collimated light; and 
     FIG. 9 shows a second source of collimated light. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings. 
     In the following description, terms such as “upper”, “lower”, “lefthand” and “righthand” refer to aspects of the described devices in the orientation shown in FIGS. 1,  3  and  5 , i.e. screen uppermost. These terms are used solely in the interests of clarity and do not imply that the devices are only operational in the orientation used for FIGS. 1,  3  and  5 . 
     Referring to FIG. 1, a first embodiment of a display according to the present invention comprises a light source section  1 , a control section  2  overlying the light source section  1 , and a diffusion section  3  overlying the control section  2 . 
     The structure of the light source section  1  is not critical as long as it produces substantially collimated light for the control section  2 . The light source section  1  comprises a serpentine fluorescent lamp  4  above which is located a collimating structure  5 . The collimating structure  5  comprises a transparent sheet having a plain face directed to the lamp  4 . The other face of the transparent sheet comprises 45° prisms. Below the lamp  4  is a reflective back plate  6 . Further information on the light source section  1  can be obtained from U.S. Pat. No. 5,161,041. 
     FIG. 1 shows two cells of the display, the lefthand cell is shown producing a bright pixel and the righthand cell is shown producing a dark pixel. The control section  2  comprises for each cell a fresnel lens  7 , a mirror  8  located in the centre of the fresnel lens  7  and a tiltable mirror  9  opposite the fresnel lens  7 . The fresnel lens  7 , the mirror  8  and the tiltable mirror  9  are all rectangular. The tiltable mirror  9  has an aperture  10  opening in its mirrored surface opposite the fresnel lens  7 . Control signal lines (not shown) are provided so that each tiltable mirror  9  can be individually controlled. The fabrication and detailed structure of the control section  2  is described in U.S. Pat. No. 5,579,149 where the same structure is used for modulating light signals. It should be noted, however, that U.S. Pat. No. 5,579,149 does not disclose a device used to produce a directly viewed image. 
     The diffusion section  3  comprises a sheet of plastics material  11  affixed to the upper face of the control section  2  and a face panel of plastics material  12  overlying the sheet of plastics material  11 . The sheet of plastics material  11  and the face panel  12  are transparent and have integrally formed lenses. The sheet  11  has hemispherical lenses  13  aligned with the apertures  10  in the tiltable mirrors  9  of the control section  2 . The sheet  11  is coloured in the regions of the hemispherical lenses  13  so that different cells produce variously red, green and blue colour outputs. The upper surface of the sheet  11  is coloured black between the hemispherical lenses  13 . The face panel  12  has a plurality of plano-convex lenses  14  coaxially aligned with respective lenses of the sheet  11 . The convex sides of the plano-convex lenses  14  are directed towards the sheet  11 . 
     The cell structure shown in FIG. 1 is repeated many times in two dimensions in the complete display device. 
     Referring to FIG. 2, the cells are arranged in a honeycomb pattern. The colouration of the sheet  11  (FIG. 1) is arranged such that any group of three mutually adjacent cells will contain a red cell, a green cell and a blue cell, as indicated by R, G and B in FIG.  2 . 
     The operation of the device of FIGS. 1 and 2 will now be described. 
     If a bright pixel is required no control signals are applied to the tiltable mirror  9  for the pixel&#39;s cell. Consequently, the tiltable mirror  9  lies in a plane parallel to the cell&#39;s fresnel lens  7 . This arrangement is illustrated by the lefthand cell in FIG.  1 . Collimated light from the light source section  1 , is refracted by the fresnel lens  7  and then reflected from the tiltable mirror  9 . The light from the tiltable mirror  9  is reflected back by the mirror  8  on the fresnel lens  7 . The fresnel lens  7  focuses the light to a point such that substantially all of the light reflected by the tiltable mirror  9  passes through the aperture  10  to the hemispherical lens  13  of the diffusion section  3 . In FIG. 1, the focal point P is shown within the aperture  10 . However, this need not be the case. For optimum efficiency, the aperture  10 , fresnel lens  7  and the hemispherical lens  13  should be arranged such that all of the light passes through the aperture  10  and the beam has a width no greater than the diameter of the hemispherical lens  13  at the base of the hemispherical lens  13 . 
     The hemispherical lens  13  spreads the light beam which is then incident on the convex side of one of the plano-convex lenses  14 . The plano-convex lens  14  reduces the angular width of the beam to balance the requirements for a bright image and a wide viewing angle. 
     If a dark pixel is required, control signals are sent to the tiltable mirror  9  of the pixel&#39;s cell. The control signals cause an electrostatic field to be produced which tilts the mirror  9 , as shown by the righthand cell in FIG.  1 . Since the tiltable mirror  9  is tilted, the light reflected thereby is not incident on the mirror  8  and so does not pass through the aperture  10  in the tiltable mirror  9  instead, it is directed back to the light source section  1 . Consequently, the pixel is darkened. 
     The generation of images by controlling individual pixels is well known. The skilled person will readily be able to provide a circuit for providing control signals for the mirrors in dependence on an image to be displayed. 
     Referring to FIGS. 3 and 4, a second embodiment of a display according to the present invention comprises a light source section  1 , a control section  2  overlying the light source section  1 , and a diffusion section  3  overlying the control section  2 . The light source section is the same as that of the first embodiment described above. 
     The control section  2  comprises a transparent substrate  20  and a honeycomb spacer structure  21  mounted to the substrate  20 . The individual cells of the spacer structure  21  have an elongated hexagonal cross-section and the walls of each cell present a light absorbing surface. The light absorbency may be a characteristic of the material used to form the spacer structure  21  or a surface coating applied to the spacer structure  21 . An array of strip-like mirrors  22  is arranged on the substrate  20  so that each mirror  22  is centrally located in a respective cell. Each mirror  22  is supported on the substrate by a small electromechanical actuator  23  for tilting it. The actuators  23  are controlled via conductors formed on the upper surface of the substrate  20 . The cells of the control section  2  are closed by the bottom face of a panel  24  which forms the boundary between the control section  2  and the diffusion section  3 . The panel  24  is moulded from transparent plastics material. The lower surface of the panel  24  is scalloped and provided with a reflective coating to form a plurality of oval parabolic mirrors  25 . Each parabolic mirror  25  closes a respective control section cell. The reflective coating is interrupted in a central strip  26  of each parabolic mirror  25  so that light can pass from the control section  2  to the diffusion section  3 . 
     The diffusion section  3  comprises the upper surface of the panel  24  and a face panel  12 . The upper surface of the panel  24  is provided with a plurality of semi-cylindrical lenses  27 , each of which is aligned with an interruption in the reflective coating on the lower surface of the panel  24 . The upper surface of the panel  24  is coloured black between the semi-cylindrical lens. The face panel  12  is similar to that of the first embodiment save that the plano-convex lenses  14  are oval rather than circular. 
     The cell structure shown in FIG. 3 is repeated many times in two dimensions in the complete display device. 
     The operation of the device of FIGS. 3 and 4 will now be described. 
     If a bright pixel is required no control signals are applied to the mirror actuator  23  for the pixel&#39;s cell. Consequently, the mirror  22  lies in a plane parallel to the substrate  20 . This arrangement is illustrated by the lefthand cell in FIG.  3 . Collimated light from the light source section  1 , is reflected from the parabolic mirror  25  towards the tiltable mirror  22 . The tiltable mirror  22  reflects the light from the parabolic mirror  25  through the uncoated central region  26  of the parabolic mirror  25  to the diffusion section  3 . The parabolic mirror  25  focuses the light to a point such that substantially all of the light reflected by the tiltable mirror  22  passes through the uncoated region  26  to the semi-cylindrical lens  27  of the diffusion section  3 . In FIG. 3, the focal point P is shown in an upper region of the control section cell. However, this need not be the case. For optimum efficiency, the uncoated region  26 , the parabolic mirror  25  and the semi-cylindrical lens  27  should be arranged such that all of the light passes through the uncoated region  26  and the beam&#39;s cross-section matches the footprint of the semi-cylindrical lens  13  at the base of the semi-cylindrical lens  13 . 
     The semi-cylindrical lens  27  spreads the light beam which is then incident on the convex side of one of the plano-convex lenses  14 . The plano-convex lens  14  reduces the angular width of the beam to balance the requirements for a bright image and a wide viewing angle. 
     If a dark pixel is required, control signals are sent to the mirror actuator  23  of the pixel&#39;s cell. This causes the actuator  23  to tilt the mirror  22 , as shown by the righthand cell in FIG.  3 . Since the tiltable mirror  22  is tilted, the light reflected thereby is not incident on the uncoated region  26  and so does not pass through to the diffusion section  3 . Instead the light is directed onto a wall of the spacer structure  21  where it is absorbed. Consequently, the pixel is darkened. 
     Colour filters may be added for the production of colour images. 
     Referring to FIG. 5, a third embodiment of the present invention comprises a light source section  1  located between a control section  2  and a diffusion section  3 . 
     The light source section comprises a transparent substrate  30 , a electroluminescent polymer film  31 , mounted to the lower surface of the substrate  30 , and a collimating structure  32  comprising a transparent sheet whose lower face consists of 45° prisms. Gaps  33  are provided in the electroluminescent polymer film  31  and the collimating structure  32 . The gaps  33  are arranged to be centrally located in respective cells. The upper surface of the substrate  30  is coloured black save for regions overlying and coextensive with the gaps  33 . Each cell of the control section  2  comprises a fresnel lens  34  for focusing collimated light from the light source section  1 , a spacer structure  35  comprising a matrix of square section cells, and an array of micromirror devices  36  as disclosed in, for example, EP-A-0690329, spaced from the fresnel lens  34  by the spacer structure  35 . The walls of the spacer structure  35  are light absorbing. 
     The diffusion section  3  comprises a sheet  37  of light diffusing material. 
     The cell structure shown in FIG. 5 is repeated many times in two dimensions in the complete display device. 
     The operation of the device of FIG. 5 will now be described. 
     If a bright pixel is required no control signals are applied to the micromirror device  36  for the pixel&#39;s cell. Consequently, the mirror of the micromirror device  36  lies in a plane parallel to the electroluminescent polymer film  31 . This arrangement is illustrated by the lefthand cell in FIG.  5 . Collimated light from the light source section  1 , is directed towards the micromirror device  36  by the fresnel lens  34 . The micromirror device  36  reflects the light from the fresnel lens  34  through the gap  33  to the diffusion section  3 . The fresnel lens  34  focuses the light to a point such that substantially all of the light reflected by the micromirror device  36  passes through the gap  33 . In FIG. 3, the focal point P is shown in an upper region of the control section cell. However, this need not be the case. For optimum efficiency, the gap  33 , the fresnel lens  34  and the micromirror device  36  should be arranged such that all of the light passes through the gap  33  and presents as large a cross-section as is convenient on the undersurface of the sheet  37  of light diffusing material. 
     If a dark pixel is required, control signals are sent to the micromirror device  36  of the pixel&#39;s cell. This causes the mirror of the micromirror device  36  to tilt, as shown by the righthand cell in FIG.  5 . Since the mirror is tilted, the light reflected thereby does not pass through the gap  33  to the diffusion section  3 . Instead, the light is directed onto a wall of the spacer structure  35  where it is absorbed. Consequently, the pixel is darkened. 
     A colour image may be produced by the third embodiment by arranging for the electroluminescent polymer film  31  to emit light of different colours into different cells. 
     In the foregoing, the provision of bright and dark pixels has been described. It will be appreciated that intermediate brightness levels may be achieved by tilting the tilting mirror sufficient to reduce the amount of light passing to the diffusion layer. 
     Referring to FIGS. 6 and 7, a cell of a display comprises a light source section  1 , a control section  2  overlying the light source section  1  and a light diffusing section  3  overlying the control sections  2 . The light source section  1  comprises a source  40  of a narrow beam of collimated light. The control section  2  comprises an apertured substrate  41  on which is mounted a tiltable mirror  42  and a fixed, angled mirror  43  mounted to the bottom of the light diffusion section  3 . The light diffusion section comprises a panel  44  having a semi-cylindrical lens  45  and a front panel  46  of light diffusing material  46 . Three filter regions  47 ,  48 ,  49  respectively red, green and blue, are formed in the panel  44  below the lens  45 . 
     Light from the light source section  1  passes through an aperture in the substrate  41  and is incident on the fixed mirror  43 . The fixed mirror  43  reflects the light towards the tiltable mirror  42 . The light is then reflected towards the diffusion region  3  by the tiltable mirror  42 . The tiltable mirror  42  is controllable to direct the light from the fixed mirror  43  to one of the filter regions  47 ,  48 ,  49  to produce a coloured pixel or to a region outside of the filter regions  47 ,  48 ,  49 . Light directed through one of the filter regions  47 ,  48 ,  49  is spread by the semi-cylindrical lens  45  and then further diffused by the front panel  46 . The upper surface of the panel  44  is black between the lenses  45 . Consequently, if light is not directed through one of the filter regions  47 ,  48 ,  49 , the pixel appears dark. 
     Referring to FIG. 8, a source of a narrow beam of collimated light comprises a light source  50 , a panel  51  whose upper surface is covered with prisms, a bi-convex lens  52  for receiving light from the panel  51  and a biconcave lens  53  for collimating light focused by the bi-convex lens  52 . 
     Referring to FIG. 9, another source of a narrow beam of collimated light comprises a light source  50 , a panel  51  whose upper surface is covered with prisms, a concave parabolic mirror  54  having a hole axially through its centre and a convex parabolic mirror  55  axially aligned with the concave parabolic mirror  54 . Light from the panel  51  is focused by the concave parabolic mirror  54  and directed towards the convex parabolic mirror  55 . The convex parabolic mirror  55  collimates the focused light and directs it through the hole in the concave parabolic mirror  54 . 
     It will be appreciated that many modifications can be made to the embodiments described herein. For instance, colour displays may be produced by placing colour filters at any point in the light paths. The colour filters may be formed using a pigment or optical thin film interference coatings on mirrors.