Abstract:
The present disclosure relates to a backlight for illuminating back lit displays (e.g. LC displays of LCD televisions). In order to enable a thin design of the backlight and a high uniformity of the light emitted by the backlight, an apparatus may be provided with transparent and diffusing masking elements that may mask individual light sources and may diffuse light back into a light guide. Absorbing elements and/or retro-reflective elements may be arranged proximate the light sources in order to avoid generation of bright spots or rings around the light sources.

Description:
TECHNICAL FIELD 
     The present disclosure relates to a lighting device. More particularly, the present disclosure relates to light devices for pixelated displays such as for liquid crystal displays. 
     BACKGROUND 
     LCD televisions which, may be very thin, are attractive among consumers. Accordingly, there is a desire to make televisions as thin as possible. Generally, an LCD television comprises a LC display, which is illuminated by a backlight. Accordingly, a thickness of an LCD television may be reduced by reducing the thickness of the backlight. However, reducing the thickness of the LCD backlight may negatively affect the capability of the backlight to illuminate the LC display with a uniform light intensity. 
     To improve the contrast of the displayed picture and to reduce the power consumption of the display backlights, individually controlled segments within a backlight are preferred. By dimming the brightness of a segment of the backlight, the black level of the dimmed part of the displayed picture can be improved, and the power consumption will reduce. Backlights capable of controlling segments individually are generally thicker than backlights without this capability 
     To limit the increase of the thickness, such backlights generally use a high number of small LED&#39;s, resulting in high cost. To reduce the thickness further, light guide concepts may be applied which use expensive materials and which are hard to produce. 
     Accordingly, there is a need for improving such backlights for LCD televisions and for other pixel based displays. 
     SUMMARY 
     In general, the present disclosure seeks to mitigate, alleviate or eliminate one or more of the above mentioned disadvantages relating to thickness singly or in any combination. In particular, the present disclosure provides a lighting device that may solve at least some of the above mentioned problems, or other problems, of the prior art. 
     A lighting device is provided that includes a planar light guide with a back face for receiving light and a front face for emitting light, a light source arranged to transmit light into the back face of the light guide, a semi-transparent masking element having a light diffusing surface arranged to partially diffuse light back into the light guide and partially transmit light through the masking element so as to mask the light source, and a light absorber arranged adjacent to the light source to absorb at least a fraction of light diffused from the diffusing surface of the masking element, or a retro-reflector arranged adjacent to the light source to reflect at least a fraction of light diffused from the diffusing surface of the masking element back to the adjacent masking element. 
     Some of the light which is partially diffused back into the light guide by the diffusing surface of the semi-transparent hits an area surrounding the light source and propagate trough this area. When a reflecting or diffusing surface (e.g. an associated printed circuit board or diffuse reflector for re-using light) is arranged behind the back face, the light which is transmitted through this area will illuminate the reflecting or diffusing surface. This illumination may be visible from the front side as a bright illuminated area such as a bright ring surrounding the light source. By arranging a light absorber, alternatively a retro-reflector, as defined, the brightness of the undesired bright ring can be controlled or eliminated. Thus, the combination of a light source, a semi-transparent masking element, and light absorbers may advantageously provide a lighting device capable of emitting light so that a bright ring around the light source is avoided. 
     The lighting device may be used as a backlight for displays, however, in the configuration with only one light source or possibly a few light sources, such as between 2 and 20 light sources, the lighting device may be particularly advantageous as a general lighting source. 
     A lighting device includes a planar light guide with a back face for receiving light and a front face for emitting light, first and second light sources arranged to transmit light into the back face of the light guide, first and second semi-transparent masking elements having light diffusing surfaces, where the masking elements are arranged to partially diffuse light back into the light guide and partially transmit light through the masking elements so as to mask the respective first and second light sources, and first and second absorbers arranged adjacent to the respective first and second light sources to absorb at least a fraction of light diffused from the diffusing surfaces of the respective first and second masking elements, or first and second retro-reflectors arranged adjacent to the respective first and second light sources to reflect at least a fraction of light diffused from the diffusing surfaces of the respective first and second masking elements back to the respective first and second masking elements. 
     The lighting device which has a larger number of light sources, e.g. more than 20 light sources, or more than 200 or 2000 light sources may be used as a back light for displays, e.g. television or computer displays. The lighting device could also be used as a lighting source. Particularly, the lighting device could be configured with individually controllable light sources so as to provide a light emitting module on which patterns can be shown. Thus, by adjusting the intensity of the individual light sources or switching on or off the individual light sources various light patterns can be formed on or via the front face of the light guide. 
     In another embodiment, a lighting device is particularly suited as a back light for displays since the configuration of the lighting device may enable use of relatively thin lighting devices which as still capable of generating a uniform intensity distribution over the area of the light guide. The light sources of the lighting device may be individually dimmable for improving contrast of the display and for reducing energy consumption. 
     Alternatively the first or second light sources are optically connected to the back face. The optical connection may be achieved by an interface layer between the light source(s) and the back face, such as an interface layer of transparent adhesive. The optical connection could also be achieved by pushing a soft top surface of the light sources against the back face, when the light sources has such a soft top surface, such as a soft silicon surface. The use of an interface layer may advantageously improve in-coupling of light when the refractive index of the interface is selected according to particular criteria. 
     A lighting device may further include out coupling features arranged at the surface of the front face, or the surface of the back face of the light guide, or inside the light guide for coupling light out through the front face at non-masked locations where the masking element is not present. The out coupling features may further improve distribution of light within the light guide so as to provide a lighting device capable of emitting light with a uniform intensity distribution over the area of the plane light guide. 
     A light source may be mechanically connected to the light absorber, alternatively the retro-reflector, and the absorber alternatively the retro-reflector and/or the light source is fixed to the back face. 
     First and second light sources may be mechanically connected to the respective first and second light absorbers alternatively the retro-reflectors and the absorbers, alternatively the retro-reflectors, and/or the light sources are fixed to the back face. 
     A light absorber may provide a convenient way for fixing the light sources to the back of the light guide. The fixation of the light source may further be improved where the light sources are fixed to the back face by a transparent adhesive. 
     Where each of the absorber, alternatively the retro-reflector, and the light source is fixed to the back face by a transparent adhesive, the refractive index of the adhesive for the light source is greater than the refractive index of the adhesive for the absorber. 
     Where the absorbers, alternatively the retro-reflectors, and the light sources are fixed to the back face by a transparent adhesive, the refractive index of the adhesive for the light sources is greater than the refractive index of the adhesive for the absorbers. A transparent adhesive may additionally improve in-coupling of light from the light sources when the refractive index of the adhesive is selected according to particular criteria. 
     A lighting device may further include a light diffusing or reflective surface facing the back face of the light guide which is arranged next to the light absorber, alternatively the retro-reflector, for diffusing or reflecting light transmitted through the back face back into the light guide. The light diffusing or reflective surface may advantageously improve energy efficiency since light is reflected back into the light guide. 
     A light out coupling capability of the out coupling features and a transparency of the masking element(s) may be configured to equalize the light intensity of light coupled out by the out coupling features and the light transmitted by the masking element. 
     In a further embodiment, a display for showing images includes a matrix of image forming transparent pixels and a lighting device so that the front face of the lighting device faces a face of the matrix of image forming transparent pixels. 
     In yet a further embodiment, a method for illuminating a matrix of image forming transparent pixels of a display includes providing a lighting device having a planar light guide with a back face for receiving light and a front face for emitting light, first and second light sources arranged to transmit light into the back face of the light guide, first and second semi-transparent masking elements having light diffusing surfaces, where the masking elements are arranged to partially diffuse light back into the light guide and partially transmit light through the masking elements so as to mask the respective first and second light sources, and first and second absorbers arranged adjacent to the respective first and second light sources to absorb at least a fraction of light diffused from the diffusing surfaces of the respective first and second masking elements, or first and second retro-reflectors arranged adjacent to the respective first and second light sources to reflect at least a fraction of light diffused from the diffusing surfaces of the respective first and second masking elements back to the respective first and second masking elements. The method further includes arranging the matrix of image forming transparent pixels in front of the plate shaped light guide. 
     In summary, the present disclosure relates to a lighting device for use as a lighting source or for a backlight for illuminating a display (e.g. LC displays of LCD televisions). In order to enable a thin design of the backlight and a high uniformity of the light emitted by the backlight, a backlight is provided where transparent and diffusing masking elements masks the individual light sources and diffuses light back into the light guide. Absorbing elements or retro-reflective elements may be arranged so that they surround the light sources in order to avoid generation of bright spots or rings around the light sources. 
     In general the various aspects may be combined and coupled in any way possible within the scope of the present disclosure. These and other aspects, features and/or advantages of the backlight and associated displays will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which 
         FIG. 1  shows a backlight  100  for a display  199 , and 
         FIG. 2  illustrates the improvements in-coupling capabilities of a LED light source  110  achieved by connecting the light source to the light guide  101  by use of an interface layer  210 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows a display  199 , such as a television or computer monitor, for showing images. The display  199  may include a matrix of image forming transparent pixels such as a liquid crystal display  198 . The pixel matrix  198  may be configured to be illuminated by a lighting device  100  arranged so that the front face of the lighting device  100  faces a face of the pixel matrix  198 . The light source  100  may be referred to as a back light. A diffuser plate  197  may be inserted between the pixel matrix  198  and the lighting device  100  for further diffusion of the light emitted by the light source  100 . Generally, the diffuser plate may be combined with other optical foils like brightness enhancement foils. These foils and the diffuser foil or plate constitute an optical stack which may form part of the lighting device  100  for improving uniformity of the light. 
     An embodiment relates to the lighting device  100 . The lighting device  100  may include a planar light guide  101 , e.g. a plate shaped light guide  101  with a back face  102  for receiving light and a front face  103  for emitting light. The plate shaped light guide may be flat, i.e. have the same thickness throughout the plate. The light guide  101  may be made from glass or plastic material such as PMMA. 
     Light is injected into the light guide  101  by light sources  110  such as first and second light sources  111 ,  112  arranged to transmit light into the back face  102  of the light guide  101 . The light sources may be light emitting diodes (LED) and may have a plane light emitting surface from which light is transmitted through. Rays, such as ray  182 , which have angles (relative to the normal) which are not large enough to establish total internal reflection are substantially transmitted through the front face. 
     The size of the out-coupling area on the front face  103  where rays are refracted out through the light guide  101  can be determined by determining the offset d between the boundary of area where the light enters the back face  102 , i.e. the in-coupling area, and the border of the out-coupling area (in case the light source is placed close to back face  102  the boundary of area where the light enters the back face corresponds to the edge of the light source  110 ):
 
 d=t ×tan( ca ),
         where t is the thickness of the light guide  101 , ca is the critical angle for achieving total internal reflection=arcsin(n0/n1) where n0 is the refractive index of the material or air facing the back face  102 , and n1 is the refractive index of the light guide material.       

     For example, with a light guide  101  of 1 mm thick PMMA (with n1=1.5), placed in air (with n0=1), d will be 0.9 mm. So a circular in-coupling area (area of light emitting surface of light source  110 ) with diameter of 1 mm will lead to a circular out-coupling area with diameter 1+2*0.9=2.8 mm. 
     It may be desirable to be able to block or at least partially block rays  182  within the out-coupling area so that transmittance of the rays towards the optical stack or diffuser plate  197  is prohibited or reduced, e.g. so that intensity of light transmitted out towards the optical stack or diffuser plate  197  is reduced. This can be achieved by locating masking elements  120  such as first and second masking elements  121 , 122  at the front side  103 . Thus, the masking elements, which may be in optical contact with the light guide, may be configured to partially transmit light or rays. 
     In order to redirect at least some of the light intensity of the rays  182  back into the light guide  101  the masking element  122  could be configured with a reflective and diffusing surface so as to diffuse incident rays into different directions. Accordingly, at least part of the light intensity, i.e. light power, of the ray  182  is diffused by the diffusing property of the masking element  122  back into the light guide  101  where at least some of the diffused rays are totally internally reflected like ray  184 . The diffusing surface may face and make optical contact with the front face  103 , the diffusing surface may be located on a possible opposite surface of the masking element, or the diffusing surface may be embodied by diffusing features embedded into the masking element. 
     Thus, the capability of masking the light sources  110 , by blocking or partially transmitting rays  182  and diffusing non-transmitted rays may be achieved by arranging diffusing surfaces of the first and second masking elements  121 , 122  so that they faces the front face  103  of the light guide  101  and masks the respective first and second light sources  111 , 112 . Thereby, rays  182  can be partially diffused back into the light guide and partially transmitted through the masking elements. 
     The area of a diffusing element may be larger than the area of a light emitting surface of the light source according to the off-set d. It is understood that the reference to first and second light sources  111 , 112  and first and second masking elements  121 , 122  is only used to make it clear that generally a light source  110  may have an associated masking element  120  for masking light of this associated light source. Thus, individual masking elements  120  of at least a selection of the plurality masking elements may be arranged opposite to respective individual light sources  110  of at least a selection of the plurality of light sources  110 . That is, masking elements  120  may be arranged at the front face  103  oppositely to the light sources  110  arranged at the back face  102 . 
     Some of the rays diffused from the diffusing surfaces of the masking elements  120 , such as ray  183 , may leave the light guide at an area around the location of a light source  110 . Other rays diffused from the diffusing surfaces of the masking elements  120 , such as ray  184 , may hit the back face  102  with an angle which is sufficiently large so that the rays are total internally reflected. The rays  183  which are transmitted through the back face  102  may illuminate a surface located opposite to the back face  102 . This illuminated surface will be visible as a bright ring from the front face  103 . The surface at the back side  102  may be the surface of a printed circuit board to which the light sources  110  are connected to or other surface. Particularly, the surface of a white diffuse reflector placed opposite to the back face  102  for re-using light that would otherwise leave the light guide  101  via the back face would generate an undesired visible bright ring or area around the light sources  110 . 
     Such undesired bright areas around the light sources can be eliminated or reduced by placing light absorbers  130  at the back side which have light absorbing surfaces facing the back face  102 . The light absorbing surfaces may completely or substantial completely absorb the diffused light from the masking elements  120  so that no bright rings or areas are created around the light sources  110 . The light absorbing surfaces may absorb more than 90%, more than 95% or even more than 99% of the incoming diffused light. In alternative embodiments, the light absorbing surfaces may be configured to reflect or diffuse a fraction of the incoming light, e.g. so that more than 10% of the incoming light is diffused back into the light guide  101  and so that less than 90% of the incoming light is absorbed. 
     The absorbers  130  may have a size relative to the masking elements  120  so that the absorbers only extend to a point where rays, such as ray  183 , are transmitted through the back face, but does not extend to a point where rays, such as rays  184  are internally reflected. This extension can be determined by determining the off-set d from d=t×tan(ca) as explained above and as shown in  FIG. 1 . Thus, the area of the absorbing element  110  may be larger than the area of a masking element  120  according to the off-set d. 
     Thus, in order to absorb diffused light from the plurality of masking elements  120 , a plurality of light absorbers  130  comprising first and second absorbers  131 , 132  may be arranged adjacent to the respective first and second light sources  111 , 112  to absorb at least a fraction of light diffused from the diffusing surfaces of the respective adjacent first and second masking elements  121 , 122 . For example, the first and second light absorbers  131 , 132  may be arranged to at least partially surround the respective first and second light sources  111 , 112 , e.g. the absorbers may be planar rings which surrounds the light sources. Thus, the first and second light absorbers  131 ,  132  may be arranged at the back side of the light guide oppositely to the respective first and second masking elements  121 ,  122  arranged at the front side of the light guide. For example, the light absorbers may be formed as a ring of a material which has good light absorbing properties such as black mat plastic foil, or a black matt printing on a part present in this area. 
     In order to reduce loss of light energy retro-reflectors may be used instead of absorbers  130 . Retro-reflectors reflect light back along the same path (or substantially the same path) as it was received from. Thus, light may propagate back and forth as illustrated by rays  185 , e.g. until light is diffused in other directions and possibly total internally reflected at the back face  102  or absorbed by other means. Since rays that are received from the masking elements  120  and reflected from the retro-reflectors generally will not be directly transmitted through the front face  103  no bright areas or rings will be visible through the front face  103 . Accordingly, the retro-reflectors address the same problem as the absorbers  130  but enable a higher energy efficiency of the lighting device  100 . The retro-reflectors are not illustrated in  FIG. 1  but should have the same size as the absorbers  130  according to the off-set d. Accordingly, alternatively or additionally to using absorbers  130  a plurality of retro-reflectors comprising first and second retro-reflectors may be used so that the first and second retro-reflectors at least partially surround the respective first and second light sources  131 , 132  to reflect at least a fraction of light diffused from the diffusing surfaces of the respective opposite first and second masking elements  121 , 122  back to the respective opposite first and second masking elements  121 , 122 . 
     In order to further improve the energy efficiency of the lighting device  100  light diffusing or reflective surfaces  140  which faces the back face  102  may be arranged next to the light retro-reflectors or absorbers  130  for diffusing or reflecting light transmitted through the back face back into the light guide. Accordingly, rays, such as ray  186 , that are transmitted through the back face  102  may be reflected or diffused back into the light guide  101 . 
     Any applied absorbers  130 , retro-reflectors and diffusing or reflective surfaces  140  (except for reflective surfaces  140  which are fully specular reflective) should not be optically connected to the back face  102  in order not to affect the capability of the light guide  101  to provide total internal reflection of rays hitting the internal surfaces with an angle larger than the critical angle ca. Accordingly, any applied absorbers  130 , retro-reflectors and diffusing or non-specular reflective surfaces  140  should be separated from the back face  102  by air or other transparent material with a low refractive index such as materials with a refractive index which is lower than 1.35, preferably lower than 1.1. 
     In order to couple light out of the light guide out coupling features  150  may be arranged at the surface of front face  103 , and/or at the surface of the back face  102 , or inside the light guide  101  for coupling light out through the front face  103  at non-masked locations where masking elements  120  are not present. The out coupling features may be made of discrete features like indents or printed dots on the front and/or back faces and/or particles made of transparent material with a different refractive index than the light guide material which are embedded in the light guide material. The out coupling features should scatter or diffuse the light without absorbing any significant amount of the light. 
     The light emitting faces of the light sources may be optically connected to the back face by arranging an interface layer  210  (see  FIG. 2 ) of optical interface material between the light source and the back face. The optical interface layer is not air, but may be an adhesive having a refractive index larger than the refractive index of air, i.e. larger than one, preferably larger than 1.35. For example, the refractive index of the interface between the light sources and the back face  102  may be around 1.5 or close to the refractive index of the light guide material or close to the refractive index of the encapsulating material of the light source  110 . Alternatively, the optical connection may be achieved by pressing a soft silicon encapsulation of the light sources  110  against the light guide  101 . The optical connection enables a part of the light to enter the light guide at such angles (relative to the normal) which are large enough to establish total internal reflection without reflection by the masking elements, and so improving the percentage of the amount of light from the light sources which is coupled into the light guide.  FIG. 2  illustrates why the optical connection improves the in-coupling of light. In the upper illustration in  FIG. 2  rays  201  and  202  are transmitted through the light guide without total internal reflection and can only enter the light guide again via diffusing surfaces of the masking elements. The ray  203  is total internal reflected within the encapsulation of the light source and may therefore not be transmitted into the light guide  101 . In the lower illustration an interface layer  210  of a solid material such as transparent adhesive is provided between the light emitting surface of the light source  110  and the back face of the light guide. The rays  201   a - 203   a  are equivalent to the rays  201 - 203  with respect to the initial propagation direction within the light source encapsulation. Since the refractive index of the interface layer  210  is comparable to the refractive index of the encapsulation of the light source and the light guide, the ray  203   a  may not be internally reflected within the light source but may enter the light guide  101  and become internally reflected within light guide, or transmitted through the light guide. Accordingly, the interface layer  210  may improve the in-coupling of light from the light sources  110 . 
     The masking elements may be made in various ways. For example, the masking elements may be made by printing dots of ink onto the front and/or back surfaces of transparent foils (which can be applied to the front face  102 ) or directly on the front face  103  of the light guide and covering the printed dots with a metal sheet provided with a hole pattern to establish a certain transparency of the metal film. The mask could be made of a white reflective plastic foil, connected to the front face  103  and with a metal layer on top of it. Holes in the metal layer should preferably not penetrate the white reflective plastic foil. 
     The masking elements  120  could be printed with a dot/hole pattern in one layer using a diffuse reflective and transparent material to provide diffusion and transmitting capability in a single layer. The masking elements  120  could also be realized by a printed layer or multiple layers so that a thickness gradient is formed that is matched with the required translucency. 
     The reflection of the mask can be adapted in such a way that less light is reflected to the absorber, and more light is coupled into the light guide for becoming internally reflected. This may be achieved by replacing the white diffusing layer by a mirror with a Fresnel structure, nanostructures and/or holographic structures. 
     In order to obtain a uniform intensity of light emitted from the lighting device  101  towards the matrix of image forming transparent pixels  198  the light out coupling capability of the out coupling features  150  and a transparency of the masking elements  120  should be adjusted so as to equalize the light intensity of light coupled out by the out coupling features and the light transmitted by the masking elements. This adjustment may be achieved by determining the number, size, or density of out coupling features  150  and the transparency of the masking elements  120 . With this adjustment, the effect of the optical stack has to be taken into account. 
     The first and second light sources  111 , 112  may be mechanically connected with the respective first and second light absorbers  131 , 132  (e.g. by gluing) and faces of the absorbers may be fixed to the back face  102  by a transparent adhesive. The fixation of the light sources to the absorbers may provide an advantageous way of fixing the light sources to the light guide by gluing the absorbers onto the light guide. In order not so severely affect the total internal reflection capabilities of the light guide  101 , the absorbers should be glued to the back face  102  by use of an adhesive having a low refractive index, preferably lower than 1.35. 
     To provide a stronger fixation of the assembly of light sources and absorbers, also the light sources may be fixed to the back face by a transparent adhesive. The part of the fixation of the light source that has the function of interface layer  210  and which has to transmit light should have a relative high refractive index, preferably higher than 1.35, in order to provide good in-coupling of the light from the light sources  110 . Accordingly, the refractive index of the adhesive that has the function of interface layer  210  should preferably be greater than the refractive index of the adhesive for fixation of the absorbers. 
     According to the design of the lighting device as described here, the light guide  101  may be formed as a plate with uniform thickness and smooth surfaces. No holes are required for the light sources  110  to be inserted into. Possibly, small irregularities such as small indents may be made on one or more surfaces for providing out-coupling capabilities. 
     Whereas a lighting device  100  for use in displays  199  has been described, the lighting device  100  may also be used for other applications including general lighting applications. Accordingly, the light source  100  included a plurality of light sources  110  and corresponding pluralities of masking elements  120  and absorbers  130  may be used for lighting applications. However, for lighting applications the lighting device  100  could also be configured with only one light source  110 , only one masking element  120  arranged opposite to the light source, and only one absorber  130  or retro-reflector arranged so that it at least partially surrounds the light source  110 . Whether the lighting device is configured with a plurality of light sources  110  or a single light source  110  the function of the lighting device is the same. 
     While various embodiments have been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; a light device is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A unit may fulfill the functions of several items recited in the claims. 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. Any reference signs in the claims should not be construed as limiting the scope.