Abstract:
A light source device for projection displays is disclosed, comprising a plurality of Light Emitting Diode (LED) devices. The plurality of LED devices are arranged to sequentially operate. A light combining means are arranged to convey light from the LED devices to a light output of the light source. The light combining means comprises controllable polarisation means arranged such that the light is polarized by a structure of the light combining means. Further, a projection display system comprising a projection lens, a controller, and an image generating means, using the light source above is disclosed.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to a light source for a projection display system, and particularly to a light source using sequentially operating light emitting diodes. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is an aim to use light emitting diodes (LEDs) as light source for projection displays due to small size, high durability, and long life of LEDs. However, in projection displays, brightness of the light source is crucial for the image quality and the usability of the projection system for different environments. 
         [0003]    In US 2003/0218723 A1, it is disclosed that the emission output of a LED drops due to heating of the LED during operation. This reduces the brightness of the light source either by implying operation at lower power, or by reduced emission as the light source is heated. In US 2003/0218723 A1, this is solved by introducing a non-emission time for each LED by placing the LEDs on a movable section, wherein the LEDs are in an illumination state during a shorter period when in illumination position with respect to the movable section, and in a non-illumination state when in a non-illumination position with respect to the movable section. Thus, the LEDs are not heated to such an extent that the light emission drops significantly. 
         [0004]    A problem with the solution disclosed in US 2003/0218723 A1 is that the movable parts imply a plurality of mechanical constraints. Further, production of mechanically complex moving structures also imply a problem. Summing up, a problem with the prior art solution is the provision of mechanically moving parts. 
       SUMMARY OF THE INVENTION 
       [0005]    It is an object of the present invention to provide a bright light source without mechanically moving parts. 
         [0006]    The above object is achieved according to a first aspect of the present invention by a light source device for projection displays, comprising a plurality of Light Emitting Diode (LED) devices. The plurality of LED devices are arranged to sequentially operate. Light combining means are arranged to convey light from the LED devices to a light output of the light source. The light combining means comprises controllable polarisation means arranged such that the light is polarized by a structure of the light combining means. 
         [0007]    Sequential operation of LED devices means that one or more LED devices are switched off while one or more other LED devices are switched on at a time instant to allow LED devices to work with a duty cycle that is lower than 50%, depending on the number of redundant LED devices. This will allow LED devices to cool down during off-state, which will improve light emission during on-state. 
         [0008]    A LED device may comprise one or more LEDs. Light combining means are a structure without moving parts that enable conveying light from LED devices that are active. 
         [0009]    The controllable polarisation means may comprise a switchable retarder. The switchable retarder may comprise a liquid crystal cell. 
         [0010]    The light combining means may comprise a polarization conversion system and/or a polarizing beam splitter. 
         [0011]    A polarization conversion system is a structure for directing all light in one direction, with a uniform polarization. 
         [0012]    A polarizing beam splitter is a structure that will transmit the p-polarized light and reflect the s-polarized light component in a perpendicular direction. 
         [0013]    The light source device may be configured such that a first LED device is arranged on a first side of a first polarizing beam splitter, a second LED device is arranged on a second side of the polarizing beam splitter perpendicular to the first side of the polarizing beam splitter, and a first controllable polarizer is arranged on a third side of the polarizing beam splitter opposite to the first side of the first polarizing beam splitter. 
         [0014]    The light source device may further be configured such that a second controllable polarizer is arranged on a fourth side of the first polarizing beam splitter opposite to the second side of the first polarizing beam splitter, a second polarizing beam splitter is arranged next to the second controllable polarizer, and a third controllable polarizer is arranged on a second side of the second polarizing beam splitter perpendicular to a side of the second polarizing beam splitter facing the second controllable polarizer, wherein the third controllable polarizer is arranged to convert s-polarized light to p-polarized light when the first LED device is active, and the first and third controllable polarizers are arranged to convert s-polarized light to p-polarized light when the second LED device is active, and the second controllable polarizer is arranged to convert p-polarized light to s-polarized light when the second LED device is active. 
         [0015]    The light source device may further be configured such that a third polarizing beam splitter is arranged next to the first controllable polarizer, wherein a third LED device is arranged on a side of the third polarizing beam splitter perpendicular to a first side of the third polarizing beam splitter facing the first controllable polarizer, a fourth controllable polarizer is arranged on a second side of the third polarizing beam splitter perpendicular to a side of the third polarizing beam splitter facing the first controllable polarizer, a fifth controllable polarizer is arranged on a side of the third polarizing beam splitter opposite to the first side of the third polarizing beam splitter, a fourth polarizing beam splitter is arranged next to the third and fourth controllable polarizers, and a sixth controllable polarizer is arranged on a side of the fourth polarizing beam splitter perpendicular to a side of the fourth polarizing beam splitter facing the fourth controllable polarizer and opposite to a side of the fourth polarizing beam splitter facing the third controllable polarizer, wherein the third controllable polarizer is active when the first LED device is active, the first, second, and third controllable polarizers are active when the second LED device is active, and the fourth, fifth, and sixth controllable polarizers are active when the third LED device is active. 
         [0016]    An active controllable polarizer is arranged to convert p-polarized light to s-polarized light, and s-polarized light to p-polarized light. 
         [0017]    The light source device may also be configured such that a second polarizing beam splitter is arranged next to the controllable polarizer, wherein a third LED device is arranged on a side of the second polarizing beam splitter perpendicular to a first side of the second polarizing beam splitter facing the first controllable polarizer, and a second controllable polarizer is arranged on a side of the second polarizing beam splitter opposite to the first side of the second polarizing beam splitter, wherein the first controllable polarizer is arranged to convert s-polarized light to p-polarized light when the second LED device is active and the second controllable polarizer is arranged to convert s-polarized light to p-polarized light when the third LED device is active. 
         [0018]    The light combining means may comprise a light guide arranged along the plurality of LED devices, wherein the controllable polarizer may be arranged between the LED devices and the light guide, and a reflective polarizer may be arranged along the light guide, between the controllable polarizer and the LED devices. The light combining means may further comprise a reflective layer arranged along the light guide opposite to the LED devices. A section of the controllable polarizer corresponding to an active LED device may be arranged to convert polarization of light. 
         [0019]    The above object is achieved according to a second aspect of the present invention by a projection display system comprising a projection lens, a controller, and an image generating means, using a light source according to the first aspect of the present invention. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0020]    The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, wherein: 
           [0021]      FIG. 1  shows a projection display system; 
           [0022]      FIG. 2  shows a light source according to one embodiment of the present invention; 
           [0023]      FIG. 3  shows an alternative light source according to the present invention; 
           [0024]      FIG. 4  shows a light source according to a further embodiment of the present invention; 
           [0025]      FIG. 5  shows a light source according to a further embodiment of the present invention; 
           [0026]      FIG. 6  shows a light source according to a further embodiment of the present invention; 
           [0027]      FIG. 7  shows a polarization conversion system; 
           [0028]      FIG. 8  shows an alternative polarization conversion system; 
           [0029]      FIG. 9  shows a light source according to a further embodiment of the present invention; and 
           [0030]      FIG. 10  shows a light source according to a further embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0031]      FIG. 1  shows a projection display system  100  comprising a light source  102 , a controller  104 , an image generating means  106 , and a projection lens  108 . The projection display system  100  projects an image on a screen  110 . The image generating means  106  preferably comprises a liquid crystal panel  112  and an analyzer  114 . The light source  102  provides polarized light to the liquid crystal panel  112  of the image generating means  106 . The liquid crystal panel  112  modulates the light in a plurality of pixels. It is an effect of the liquid crystal panel  112  that light of the modulated pixels will change polarization, while non-modulated pixels will not. Therefore, the analyzer  114  is a polarizing filter with a polarization direction for transmission that is perpendicular to the polarization direction of the light illuminating the liquid crystal panel  112  and will cancel light from non-modulated pixels to improve definition of the image. The controller  104  controls light generation of the light source  102 , and image generation of the image generating means  106 . For example, the controller can control sequential colour divided image generation, where the red, green, and blue image is generated sequentially, and displayed rapidly, such that a viewer experiences a full-colour image. 
         [0032]    For producing a large projected image, that can be viewed also in daylight, the brightness of the light source is crucial. To improve the brightness of the LEDs used in the light source  102 , the LEDs are only driven with a low duty cycle, to avoid the effects of decrease of light emission as the LEDs get hot. Instead, the LEDs are driven sequentially, to let the LEDs have a period of off-state. Thereby, the emission of the LEDs can be improved significantly during on-state. 
         [0033]      FIG. 2  shows a light source  200  comprising a first LED device  202  and a second LED device  204 . The LED devices  202 ,  204  are arranged to alternately emit light to enable an improved emission. A polarizing beam splitter (PBS)  206  is arranged to direct polarized light from the LED devices  202 ,  204  towards a light output of the light source. The LED devices  202 ,  204  produce unpolarized light, i.e. light polarized in both s-state and p-state. When the first LED device  202  emits light, unpolarized light is emitted to the PBS  206 . The PBS will transmit the p-polarized light towards the light output and reflect the s-polarized light component (downwards in  FIG. 2 ). Similarly, when the second LED device  204  emits light, unpolarized light is emitted to the PBS  206 . The PBS will transmit the p-polarized light (downwards in  FIG. 2 ) and reflect the s-polarized light component towards the light output. Thus is p-polarized light provided when the first LED device  202  is active, and s-polarized light when the second LED device is active. To achieve a uniformly polarized light output, a switchable retarder  208  is provided. The switchable retarder  208  rotates the polarization of linearly polarized light from p-state to s-state and vice versa when in an on-state. Thus, uniformly polarized light can be achieved at the output of the light source  200  by activating the switchable retarder  208  when the second LED device  204  is active, and deactivating the switchable retarder  208  when the first LED device  202  is active. Thus is p-polarized light achieved. It is also possible to achieve s-polarized light by activating the switchable retarder  208  when the first LED device  202  is active, and deactivating the switchable retarder  208  when the second LED device  204  is active. 
         [0034]    For some applications, there is no need for polarized light. Then, a switchable mirror  306  can be used instead of a PBS, as is shown in  FIG. 3 . A light source  300  then comprises a first LED device  302 , a second LED device  304 , and the switchable mirror  306  to provide unpolarized light at an output of the light source  300 . The switchable mirror  306  is operated to reflect the light from the second LED device  304  when active, and to transmit light from the first LED device  302  when active, towards the output of the light source  300 . 
         [0035]      FIG. 4  shows a light source  400  comprising a plurality of LED devices  402 ,  404 ,  406 ,  408 . The LED devices  402 ,  404 ,  406 ,  408  are arranged to alternately emit light to enable an improved emission. A plurality of polarizing beam splitters (PBSs)  410 ,  412 ,  414  are arranged to direct polarized light from the LED devices  402 ,  404 ,  406 ,  408  towards a light output of the light source. A plurality of switchable retarders  416 ,  418 ,  420  are provided to achieve uniformly polarized light at the output of the light source  400 . 
         [0036]    To achieve p-polarized light at the output, the switchable retarders  416 ,  418 ,  420  are in off-state when the first LED device  402  is active, the first switchable retarder  416  is in on-state when the second LED device  404  is active, while the other switchable retarders  418 ,  420  are in off-state. Similarly, when the third LED device  406  is active, the second switchable retarder  418  is in on-state, while the other switchable retarders  416 ,  420  are in off-state, and when the fourth LED device  408  is active, the third switchable retarder  420  is in on-state, while the other switchable retarders  416 ,  418  is in off-state. Thus, a lower duty-cycle is achieved. The number of alternating LED devices can be arbitrary by this structure, where more alternating LED devices result in a lower duty-cycle, which implies lower temperature of the LED devices, which improves light emission. 
         [0037]      FIG. 5  shows one embodiment of a light source according to the present invention. The light source comprises banks of light sources  502 ,  504 ,  506  similar to the light source of  FIG. 4 . Each of the banks  502 ,  504 ,  506  provide a color, e.g. bank  502  provides red, bank  504  provides green, and bank  506  provides blue light. Each of the banks  502 ,  504 ,  506  comprises a plurality of LEDs, a plurality of PBSs to direct the light, and a plurality of switchable retarders to get the right polarization of the light. The switchable retarders are preferably arranged in groups  508 ,  510 ,  512 ,  514  for better control, production and cost. A retarder group  508 ,  510 ,  512 ,  514  is preferably made of a single piece that is segmented in three parts. The light from the light source banks  502 ,  504 ,  506  are directed by light linking means  516 ,  518 ,  520 ,  522 , and light guides  524 ,  526 ,  528 ,  530  to a PBS  532 . The light that reaches the PBS  532  which is p-polarized will thus be transmitted to a liquid crystal on silicon (LCOS) device  534 , which will change the polarization to s-state and reflect the light back to the PBS  532 , which will transmit the light towards the output of the light source. It should be noted that it is advantageous if the area of the LCOS device  534  facing the PBS  532  is smaller than the corresponding area of the PBS  532 . This will avoid that light hits the borders of the PBS  532 , which will degrade image quality. To ensure that no light hits the border areas of the PBS  532 , a mask (not shown) can be inserted between the LCOS device  534  and the PBS  532 . It should also be noted that it can be advantageous to position the LCOS device  534 ′ on another side of the PBS  532  such that it requires illumination with s-polarized light. In this case, the switchable retarders  514  are used to ensure that only s-polarized light can leave each of the banks  502 ,  504 ,  506 . It should also be noted that it can be advantageous to insert additional passive optical elements, such as lenses (not shown), in between light guide  530  and PBS  532  and in between LCOS device  534  and PBS  532 , or alternatively between LCOS device  534 ′ and PBS  532 . 
         [0038]    The embodiment is shown by example only with three colours, but can be used for any number of colours. 
         [0039]      FIG. 6  shows another embodiment of a light source and image generator according to the present invention, where light is generated in banks of light sources  602 ,  604 ,  606 , each similar to the one in  FIG. 4 . Each of the banks  602 ,  604 ,  606  provide a color, e.g. bank  602  provides red, bank  604  provides green, and bank  606  provides blue light. Each of the banks  602 ,  604 ,  606  comprises a plurality of LEDs, a plurality of PBSs to direct the light, and a plurality of switchable retarders to get the right polarization of the light. The switchable retarders are preferably arranged in groups  608 ,  610 ,  612  for better control, production and cost. A retarder group  608 ,  610 ,  612  is preferably made of a single piece that is segmented in three parts. The light from the light source banks  602 ,  604 ,  606  are directed by light guides  614 ,  616 ,  618  and light guiding means  620 ,  622  to image generating means  624 ,  626 ,  628 , respectively. The light generating means preferably comprise a liquid crystal panel and an analyzer. The light source banks  602 ,  604 ,  606  provide polarized light to the liquid crystal panels of the image generating means  624 ,  626 ,  628 . The liquid crystal panels modulate the light in a plurality of pixels. It is an effect of the liquid crystal panels that light of the modulated pixels will change polarization, while non-modulated pixels will not. Therefore, the analyzers are polarizing filters with a polarization direction for transmission that is perpendicular to the polarization direction of the light illuminating the liquid crystal panels and will cancel light from non-modulated pixels to improve definition of the image. For example, red light from light source bank  602  is provided to image generating means  624  to generate the red component of the colour image, green light from light source bank  604  is provided to image generating means  626  to generate the green component of the colour image, and blue light from light source bank  606  is provided to image generating means  628  to generate the blue component of the colour image. The image components are combined by a cross prism  630  and output to a projection lens (not shown). 
         [0040]    It should be noted that it is advantageous if the area of the image generating means  624 ,  626 ,  628  facing the cross prism  630  is smaller than the corresponding areas of the cross prism  630 . This will avoid that light hits the borders of the cross prism  630 , which will degrade image quality. To ensure that no light hits the border areas of the cross prism  630 , masks (not shown) can be inserted between the image generating means  624 ,  626 ,  628  and the cross prism  630 . 
         [0041]    The embodiment is shown by example only with three colours, but can be used for any number of colours. 
         [0042]    Approximately half of the light is lost from each LED device by reflecting the s-polarized part of the unpolarized light in the PBSs, such that it does not reach the outout of the light source.  FIG. 7  shows a structure, called a polarisation conversion system (PCS), for directing all light in one direction, with a uniform polarization. The structure  700  comprises a LED device  702 , a first PBS  704 , a second PBS  706 , and a retarder  708 . The LED device  702  emits unpolarized light to the first PBS  704 , which transmits p-polarized light to an output and reflects s-polarized light to the second PBS  706 . The second PBS  706  reflects the s-polarized light to the retarder  708 , which converts the light to p-state. Thus, all light is output as p-polarized light. 
         [0043]      FIG. 8  shows a similar structure as  FIG. 7  for directing all light in one direction, with a uniform polarization. The structure  800  comprises a LED device  802 , a first PBS  804 , a second PBS  806 , and a retarder  808 . The LED device  802  emits unpolarized light to the first PBS  804 , which transmits p-polarized light to the retarder  808  which converts the light to s-state before output, and reflects s-polarized light to the second PBS  806 . The second PBS  806  reflects the s-polarized light to the output. Thus, all light is output as s-polarized light. 
         [0044]    The effect of the polarization conversion system structure can be used in the present invention by modifying the light source structures shown in  FIGS. 2 and 4  to  6 .  FIG. 9  shows an embodiment of a light source  900  according to the present invention, where the effect of the polarization conversion system is used. The light source  900  comprises a plurality of LED devices  902 ,  904 ,  906 ,  908 . The LED devices  902 ,  904 ,  906 ,  908  are arranged to alternately emit light to enable an improved emission. A plurality of polarizing beam splitters (PBSs)  910 ,  912 ,  914 ,  916 ,  918 ,  920  are arranged to direct polarized light from the LED devices  902 ,  904 ,  906 ,  908  towards a light output of the light source. A plurality of switchable retarders  922 ,  924 ,  926 ,  928 ,  930 ,  932 ,  934 ,  936 ,  938  are provided to achieve uniformly polarized light at the output of the light source  900 . 
         [0045]    When the first LED device  902  is active, it emits unpolarized light to the first PBS  910 , which reflects s-polarized light through the first switchable retarder  922 , which is in off-state, to the second PBS  912 , and transmits the p-polarized light all way to the output through the PBSs  914 ,  918  and the switchable retarders  924 ,  930 ,  936 , which are in off-state. The s-polarized light is reflected in the second PBS  912  to the third switchable retarder  926 , which is in on-state. Thus, the light is converted to p-state and is thus transmitted to the output through PBSs  916 ,  920  and switchable retarders  932 ,  938 , which are in off-state. 
         [0046]    When the second LED device  904  is active, it emits unpolarized light to the first PBS  910 , which reflects s-polarized light through the second switchable retarder  924 , which is in on-state and thus converts the light to p-state, to the third PBS  914 , and transmits the p-polarized light all way to the output through the PBS  918  and the switchable retarders  930 ,  936 , which are in off-state. The p-polarized light is converted to s-state in the first switchable retarder  922 , and is then reflected in the second PBS  912  to the third switchable retarder  926 , which is in on-state. Thus, the light is converted to p-state and is thus transmitted to the output through PBSs  916 ,  920  and switchable retarders  932 ,  938 , which are in off-state. 
         [0047]    When the third LED device  906  is active, it emits unpolarized light to the third PBS  914 , which reflects s-polarized light through the fifth switchable retarder  930 , which is in on-state and thus converts the light to p-state, to the fifth PBS  918 , which transmits the p-polarized light to the output through the switchable retarder  936 , which is in off-state. The p-polarized light is converted to s-state in the fourth switchable retarder  928 , and is then reflected in the fourth PBS  916  to the sixth switchable retarder  932 , which is in on-state. Thus, the light is converted to p-state and is thus transmitted to the output through PBS  920  and switchable retarder  938 , which is in off-state. 
         [0048]    When the fourth LED device  908  is active, it emits unpolarized light to the fifth PBS  918 , which reflects s-polarized light through the eighth switchable retarder  936 , which is in on-state and thus converts the light to p-state, to the output. The p-polarized light is converted to s-state in the seventh switchable retarder  934 , and is then reflected in the sixth PBS  920  to the ninth switchable retarder  938 , which is in on-state. Thus, the light is converted to p-state and is thus transmitted to the output. 
         [0049]    Similar structure can be used for the multi-colour systems described in connection to  FIGS. 5 and 6 , with one structure  900  for each colour. The embodiment can be used for any number of colours. 
         [0050]      FIG. 10  shows a light source  1000  according to a further embodiment of the present invention. The light source  1000  comprises a plurality of LED devices  1002 ,  1004 ,  1006 ,  1008 ,  1010 ,  1012 , arranged to alternately emit light to enable an improved emission, a plurality of prisms  1014 ,  1016 ,  1018 ,  1020 ,  1022 ,  1024  for coupling light from the LED devices  1002 ,  1004 ,  1006 ,  1008 ,  1010 ,  1012  to a light guide  1026  reaching along the LED devices  1002 ,  1004 ,  1006 ,  1008 ,  1010 ,  1012  with their prisms  1014 ,  1016 ,  1018 ,  1020 ,  1022 ,  1024 . For small in-coupling angles, the conditions for total internal reflection may not be fulfilled. To overcome this, a reflective layer  1028  is provided on the light guide  1026  on the opposite side to the LED devices  1002 ,  1004 ,  1006 ,  1008 ,  1010 ,  1012  with their prisms  1014 ,  1016 ,  1018 ,  1020 ,  1022 ,  1024 . Between the LED devices  1002 ,  1004 ,  1006 ,  1008 ,  1010 ,  1012  with their prisms  1014 ,  1016 ,  1018 ,  1020 ,  1022 ,  1024  and the light guide there is provided a reflective polarizer  1030  having the properties that it will transmit one polarizing component of light and reflect the perpendicular polarizing component. Between the reflective polarizer  1030  and the light guide is a switchable retarder  1032  provided. The switchable retarder  1032  is segmented such that for each LED device it has an independently switchable region. When operating, the region of the switchable retarder  1032  that correspond to an active LED device is in on-state, and others are in off state. Thus, of unpolarized light  1033  from the active LED device, e.g. LED device  1004  as depicted in  FIG. 10 , that reaches the reflective polarizer  1030 , only light  1035  with a certain polarization, e.g. s-polarization, will pass the reflective polarizer  1030 . The region  1034  of the switchable retarder  1032  is in on-state, and will convert the s-polarized light  1035  to p-polarized light  1037 . The light is then reflected by the reflective layer  1028 , or by total internal reflection, in the light guide  1026 . The light may transmit through another region  1036 , which is in off-state, of the switchable retarder  1032  and be reflected back into the light guide  1026  by the reflective polarizer  1030 , since the light is p-polarized and the reflective polarizer  1030  in this example is arranged to reflect p-polarized light. Finally, eventually after further reflections, the light, which maintains its polarization, will reach an output prism  1038  of the light source  1000  and be outputted. 
         [0051]    The embodiment can be used for any number of colours by arranging one structure  1000  for each colour. An advantageous feature of this embodiment is that a large, flat switchable retarder with the independently switchable regions arranged in a matrix can be used. This will enable easier production and lower costs.