Patent Publication Number: US-11656398-B2

Title: Illumination and display apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates to illumination apparatuses and display apparatuses. 
     BACKGROUND 
     Illumination apparatuses in which micro-LEDs are used, e.g. for displays such as laptop or TV displays, are becoming increasingly commercially relevant. In addition, displays with privacy functions are desirable to prevent content being shown on the screens of displays from being seen by unwanted people. 
     BRIEF SUMMARY 
     According to a first aspect of the present disclosure, there is provided an illumination apparatus, comprising a first substrate, an optical structure, an array of light emitting elements disposed on the first substrate and between the first substrate and the optical structure, and a mask comprising a plurality of apertures therein, wherein the optical structure is configured to receive light emitted by the array of light emitting elements, direct the received light into a direction away from the first substrate, direct at least some of the light which has been directed away from the first substrate back towards the first substrate, and direct at least some of the light which has been directed back towards the first substrate through the plurality of apertures of the mask. 
     Advantageously an illumination apparatus may be provided with high output efficiency. Low reflectivity of the illumination apparatus to ambient illumination may be achieved. Low cross talk may be provided between different the apertures. A thin structure may be achieved that may be flexible and have free form shapes. Non-Lambertian output illumination may be provided to achieve increased on-axis efficiency. The display may be incorporated with a mask that matches the texture and appearance of the decor, for example in automotive interiors. When the display is unused it may be hidden to users. 
     The optical structure may comprise a plurality of curved reflectors configured to reflect at least some of the light emitted by the light emitting elements back past the light emitting elements towards the mask. The array of light emitting elements may be disposed between the optical structure and the mask. Achromatic imaging of the light emitting element to the aperture may be provided. High optical power may be provided in thin packages and total thickness may advantageously be reduced. 
     Each aperture may be aligned with a respective light emitting element. Advantageously light may be efficiently directed into each aperture from the respective aligned light emitting element. 
     The optical structure may comprise a plurality of waveguides configured to guide light emitted by the light emitting elements towards the mask, a first reflector configured to reflect at least some of the light guided by the waveguides towards the first substrate, and a second reflector configured to reflect at least some of the light reflected by the first reflector back towards the mask. Each waveguide may be aligned with a respective light emitting element. Each waveguide may comprise a cylindrical structure, and for each waveguide: the cylindrical structure may comprise an open end and a closed end opposite to the open end, and the cylindrical structure may be connected to the second reflector at the open end. Each waveguide may comprise a conical frustrum structure and an annular wall extending around the conical frustrum structure, and for each waveguide: the conical frustrum structure may be connected to the second reflector via the annular wall. 
     The optical structure may be disposed between the array of light emitting elements and the mask. Advantageously the light emitting elements may be arranged on an opaque substrate. The area of opaque electronic components between the light emitting elements may be increased. Increased complexity of driving of the light emitting elements may be achieved. 
     One or more of the apertures of the mask may comprise a light diffuser disposed therein. Advantageously the cone angle of the light output may be increased. The far field illumination profile may have increased solid angle. The illumination apparatus may be visible from a wider range of viewing angles. The uniformity of the illumination apparatus may be increased. 
     One or more of the apertures of the mask may be circular. One or more of the apertures of the mask may be annular. Advantageously a mask suitable for use with an opaque substrate may be provided. Increased complexity of electrical driving of the light emitting elements may be achieved. 
     At least one of the light emitting elements may comprise a first light source and a separate second light source, wherein the first and second light sources are separately controllable. Advantageously at least two different angular profiles may be provided from the illumination apparatus. 
     The illumination apparatus may further comprise a controller configured to control the first and second light sources to switch between a first state in which the first light source is on and the second light source is off, and a second state in which the first light source is off and the second light source is on. The first light source may be configured to provide light with a first cone angle and the second light source is configured to provide light with a second cone angle, wherein the first cone angle is greater than the second cone angle. A switchable cone angle ambient illuminance may be provided. 
     The illumination apparatus may further comprise an array of passive optical nanostructures, each passive optical nanostructure being disposed on a respective one of the light emitting elements, and each passive optical nanostructure comprising an air gap disposed between its respective light emitting element and the optical structure. The first substrate and the optical structure may be bonded to advantageously achieve increased mechanical and thermal stability. 
     The light emitting elements may be micro-LEDs. Advantageously high resolution illumination apparatus may be achieved in a thin structure. 
     According to a second aspect of the disclosure, there is provided a display apparatus comprising the illumination apparatus of the first aspect. Advantageously high image contrast may be achieved in high ambient illumination. For a given illuminance, display brightness may be reduced to achieve desirable contrast such that power consumption is reduced. The display apparatus may have high resolution, high efficiency and may comprise high density addressing electronics that are not visible to the display user. Multiple displays may be arranged behind a mask to achieve seamless stitching over large areas. The display may be flexible and may have free form shapes. 
     The display apparatus may be a privacy display apparatus. In privacy mode of operation increased image security for off-axis snoopers may be achieved. In a share mode of operation, increased luminance and increased display uniformity for off-axis users may be achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    and  FIG.  2    respectively illustrate cross-sectional and perspective views of an embodiment of an illumination apparatus; 
         FIG.  3 A  illustrates a cross-sectional view of an illumination apparatus showing how an optical structure manipulates light that it receives in a structure comprising passive optical nanostructures; 
         FIG.  3 B  illustrates a cross-sectional view of an illumination apparatus showing how an optical structure manipulates light that it receives in a structure comprising no passive optical nanostructures; 
         FIG.  4    illustrates a perspective view of a type of passive optical nanostructure arranged on a light emitting element; 
         FIG.  5    illustrates a cross-sectional view of a further embodiment of the illumination apparatus; 
         FIG.  6    illustrates a plan view of light emitting elements according to the embodiment of  FIG.  5    described above; 
         FIG.  7    shows a plan view of a mask according to the embodiment of  FIG.  5    described above; 
         FIG.  8    illustrates a perspective close-up view of the structure of an annular diffuser; 
         FIG.  9 A  and  FIG.  9 B  illustrate graphs showing the cone angles of the light output from the apertures; 
         FIG.  10    illustrates a cross-sectional view of an embodiment of a tiled display apparatus comprising the illumination apparatus of the embodiment of  FIG.  1   ; 
         FIG.  11    illustrates a cross-sectional view of a further embodiment of the illumination apparatus; 
         FIG.  12 A  illustrates a cross-sectional view of a further embodiment of the illumination apparatus; 
         FIG.  12 B  illustrates a plan view of a mask of the embodiment of  FIG.  12 A ; 
         FIG.  13    illustrates a plan view of an embodiment of a mask of the illumination apparatus; 
         FIG.  14    illustrates a perspective view of the mask and tiling of the display apparatuses of  FIGS.  10   ; and 
         FIG.  15    illustrates a composite display apparatus comprising a plurality of the display apparatuses. 
     
    
    
     DETAILED DESCRIPTION 
     In this specification, (except when qualified by the term “packaged”), “LED” or “micro-LED” refers to an unpackaged LED die chip extracted directly from a monolithic wafer, i.e. a semiconductor element. Micro-LEDs may be formed by array extraction methods in which multiple LEDs are removed from a monolithic epitaxial wafer in parallel and may be arranged with positional tolerances that are less than 5 micrometres. This is different from packaged LEDs. Packaged LEDs typically have a lead-frame and plastic or ceramic package with solder terminals suitable for standard surface-mount PCB (printed circuit board) assembly. The size of the packaged LEDs and limits of PCB assembly techniques means that displays formed from packaged LEDs are difficult to assemble with pixel pitches below about 1 mm. The accuracy of components placed by such assembly machines is typically about plus or minus 30 micrometres. Such sizes and tolerances prevent application to very high-resolution displays. 
     The structure and operation of various directional display devices will now be described. In this description, common elements have common reference numerals. It is noted that the disclosure relating to any element applies to each device in which the same or corresponding element is provided. Accordingly, for brevity such disclosure is not repeated. 
       FIG.  1    and  FIG.  2    respectively illustrate cross-sectional and perspective views of an embodiment of an illumination apparatus  100 . The illumination apparatus  100  comprises an array of light emitting elements  110 , an array of reflective masks  35 , a substrate  200 , an array of passive optical nanostructures  130 , an optical structure  220 , an output mask  150  comprising an array of light transmitting apertures  152 , an adhesive layer  206 , and electronics  520 . 
     The array of light emitting elements  110  is disposed on the substrate  200  that is for example a glass or polymer substrate. Each light reflecting element  110  has a reflective mask  35  arranged between the light emitting element and the substrate  200 . The reflective mask  35  is arranged to recirculate light that is passing from the light emitting element  110  towards the substrate  200  and redirect the light towards the optical structure  220 . Advantageously light from the light emitting element  110  is efficiently directed towards the apertures  152  as will be described below. 
     The light emitting elements  110  are further arranged between the optical structure  220  and the output mask  150 . The light emitting elements  110  are micro-LEDs (i.e. LEDS with a maximum dimension or size of less than 300 micrometres, preferably less than 200 micrometres and more preferably less than 100 micrometres). 
     In this embodiment, the light emitting elements  110  are uniformly distributed on the substrate  200  in the x and y directions, as shown in  FIG.  1    and  FIG.  2   . The light emitting elements may be arranged in a hexagonal arrangement as illustrated, in a square grid, a rectangular grid arrangements, in stripe arrangements or in other packing arrangements. 
     The light emitting elements  110  may be arranged to provide white light output, or may produce separate colour output such as red, green and blue, yellow, white and other know colour pixel arrangements. Advantageously an addressable colour display function may be provided. 
     Each passive optical nanostructure  130  of the array of passive optical nanostructures  130  is disposed on a respective one of the light emitting elements  110  and arranged to receive light from the respective aligned light emitting element  100 . 
     The optical structure  220  is a reflective optical structure configured to reflect at least some of the light it receives back towards the substrate  200 . The optical structure  220  comprises a plurality of concave curved reflectors  222 A,  222 B, each aligned with a respective light emitting element  110  and configured to reflect at least some of the light received from that light emitting element  110 . The curved reflectors  222 A,  222 B comprise a reflective material  252  arranged on a curved surface of the respective optical structure. Thus the optical structure  252  comprises the reflective material  252 . In other figures below in which the reflective material  252  of the optical structure  220  is not illustrated, said curved reflectors  222 A,  222 B may be considered as comprising a reflective surface such as provided by a reflective coating  252 . 
     Specifically, the centre of each curved reflector  222 A is aligned with a respective light emitting element  110 . The optical structure  220  may be formed from a transparent main body and a reflective material disposed thereon constituting the curved reflectors  222 A,  222 B. The body of the optical structure  220  may be a glass or polymer material. The surface relief structure of the curved reflectors may be provided by a moulding or casting process material in a polymer material for example. The reflective layer  252  formed on the curved reflectors  222 A,  222 B, may comprise for example a deposited metal coating that may comprise silver or aluminium materials as well as surface adhesion promoters and protective layers. 
     The output mask  150  is disposed on an opposite side of the substrate  200  to the array of light emitting elements  110 , and comprises a plurality of apertures  152  therein and opaque region  151 . The output mask  150  opaque region  151  may be formed from any appropriate opaque material, for example by printing a black material onto the substrate  200 . The opaque region  151  may alternatively or additionally comprise a nanostructured black absorber, a “nanoblack” or other similar material such as that marketed by Acktar, (Kiryat-Gat, Israel). Advantageously very low reflectivity may be achieved from the front of the illumination apparatus. 
     The output mask  150  is configured to block light from passing therethrough, except for through its apertures  152 . Each of the apertures  152  of the output mask  150  comprises a diffuser  156  disposed therein, the diffusers  156  being configured to scatter light that travels through the apertures  152 . 
     The adhesive layer  206  adheres the optical structure  220  to the substrate  200 , and fills the space between the optical structure  220  and the substrate  200  around the light emitting elements  110  and passive optical nanostructure  130 . 
     In operation, the optical structure  220  is configured to receive light emitted by the array of light emitting elements  110 , direct the received light into a direction away from the substrate  200 , direct at least some of the light which has been directed away from the substrate  200  back towards the substrate  200 , and direct at least some of the light which has been directed back towards the substrate  200  through the plurality of apertures  152  of the output mask  150 . For example, as shown in  FIG.  1   , in sequence, a light ray  180  emitted by a light emitting element  110  travels through the passive optical nanostructure  130  on that light emitting element  110 , travels through the adhesive layer  206 , enters the optical structure  220 , travels within the optical structure  220  to a curved reflector  222 A, is reflected by the curved reflector  222 A back towards the substrate  200 , travels within the optical structure  220  back to the adhesive layer  206 , travels through the adhesive layer  206 , travels through the substrate  200 , and travels through an aperture  152 . In this way, the optical structure  220  focuses the light that it receives towards the apertures  152  within the output mask  150 , as will be explained in more detail below. 
     The diffusers  156  are configured to scatter light that travels through the apertures  152  to provide output light rays  164 . The diffuser in the aperture  152  provides rays  164  that may have a wide angular spread. Advantageously the illumination apparatus may provide illumination over wide illumination angles and in display applications the illumination apparatus may be visible from wide viewing angles. 
     The electronics  520  is disposed on the substrate  500  between adjacent light emitting elements  110  in the x direction, and is for driving one or more of the light emitting elements  110 . Advantageously the present embodiments achieve hiding of the electronics  520  from an observer of the illumination apparatus. The electronics are arranged in the region under the cusps of the curved reflective elements such that light rays are not incident on the electronics, and loss is minimised. Addressing electronics may be provided near to the light emitting elements  110  for advantageously achieving efficient driving and control. 
       FIG.  3 A  illustrates a cross-sectional view of the illumination apparatus  100  showing how the optical structure  220  manipulates light that it receives. Features of the embodiment of  FIG.  3 A  not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features. 
     As shown, the way each curved reflector  222 A of the optical structure  220  reflects light is based on various parameters of the curved reflector  222 A. Said parameters include, for example, the refractive index of the optical structure  220 , the radius r of curvature of the curved reflector  222 A, a distance t in the z direction between the centre of the curved reflector  222 A and the light emitting element  110 , and a distance s in the x direction between the centre and the edge of the curved reflector  222 A. As will be described further below, the light output from the light emitting element and passed by the passive optical nanostructure  130  is within an angle θc of the optical axis  199  that is normal to the input surface  221  of the optical structure  220 . Advantageously light rays do not pass from a light emitting element to adjacent curved reflector  222 B. 
     These parameters are set such that each curved reflector  222 A directs substantially all the light that it reflects towards the aperture  150  that it is aligned with. This helps to focus the light emitted by the light emitting elements  110  towards the apertures  152 . 
       FIG.  3 B  illustrates a cross-sectional view of the illumination apparatus showing how an optical structure manipulates light that it receives in a structure comprising no passive optical nanostructures. Features of the embodiment of  FIG.  3 B  not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features. 
     In comparison to the arrangement of  FIG.  3 A , the passive optical nanostructure  130  and adhesive layer  206  is omitted and an air gap  207  is provided between the light emitting element  110  and optical substrate  220 . Supporting members  208  such as adhesive points may be provided to achieve alignment between the substrate  200  and optical structure  220  and to increase mechanical and thermal stability. The supporting members  208  may be provided in regions between the light emitting elements so are not visible to an external observer and do not provide undesirable light loss. 
     As for  FIG.  3 A , light rays  180  from the light emitting element  110  are confined in the optical structure  220  to within the critical angle θc so that light does not propagate between adjacent curved reflector  222 B from a light emitting element  110  aligned with curved reflector  220 A. Advantageously cross talk and stray light is minimised. 
     The structure and operation of the passive optical nanostructure  130  of  FIG.  3 A  will now be further described. 
       FIG.  4    illustrates a perspective view of a type of passive optical nanostructure  130  that may be mounted on the light emitting elements  110 . Features of the embodiment of  FIG.  4    not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features. 
     The passive optical nanostructure  130  comprises a base layer  131 , a plurality of spacers  132 , and an air gap  133 . In this embodiment, the plurality of spacers  132  are pillars or columns. The plurality of spacers  132  extend perpendicularly from the base layer  132 . The plurality of spacers  132  are uniformly distributed on the base layer  131  such that the distance p between adjacent spacers  132  (also known as the pitch of the spacers  132 ) is substantially the same for each pair of adjacent spacers  132 . Each of the plurality of spacers  132  has substantially the same height h as each of the other spacers  132 . The air gap  133  is defined by the spacers  132  and the base layer  131 . More specifically, the air gap  133  comprises air which fills the space between the spacers  132  from the base layer  131  up to the height of the spacers  132 . 
     The spacers  132  each have a height h greater than the wavelength λ of the light travelling through the air gap  133 , and a width w less than the wavelength λ of the light travelling through the air gap  133 . The width w is sufficiently small so that the light rays effectively experience the refractive index of the air gap due to the sub-wavelength phase structure of the spacers  132 . 
     Desirable dimensional properties of the passive optical nanostructure  130  for a nominal wavelength of 550 nm will now be described. The spacers  132  each have a height h greater than the wavelength λ of the light travelling through the air gap  133 . The width w and pitch p of the spacers  132  is arranged to minimise diffractive light scatter from the spacers  132  of the light travelling through the air gap  133 , and to minimise guiding of light within the spacers  132 . 
     The pitch p may be less than 2λ, preferably less than λ, more preferably less than λ/2 and most preferably less than λ/5. The ratio w/p may be less than 0.5, and preferably less than 0.3 and more preferably less than 0.1. Such elements may provide high angle diffraction or zero order diffraction. Advantageously diffractive scatter from the spacers and gaps between the spacers may be reduced, minimising light scatter between adjacent curved reflectors  222 A,  222 B. Such elements may be provided by lithographic fabrication techniques on a monolithic wafer. The elements may be transferred from the monolithic wafer or may be arranged to provide a replication tool as described elsewhere herein. 
     The passive optical nanostructure  130  has an effective refractive index n 1  given by the equation: 
                     n   1     =     1   +         w   2       p   2       ⁢     (     n   -   1     )                 eqn   .         1               
where n 1  is the effective refractive index, n is the refractive index of the spacers  132 , p is the pitch of the spacers  132  and w is the width of each of the spacers  132 . w, p and n have values such that n i  is a value which causes total internal reflection of at least some of the light reaching the passive optical nanostructure  130 . The critical angle θc of light within the optical structure  220  is then given by the equation:
 
θ c=sin   −1 ( n     1   / n     2   )  eqn. 2
 
where n 2  is the refractive index of the material of the optical structure  220 .
 
     The base layer  131  is, for example, formed from an inorganic material such as silicon dioxide or may be a polymeric material. The spacers  132  are, for example, formed from a patterned inorganic material such as silicon dioxide or may be a polymeric material. 
     The adhesive layer  206  bonds the passive optical nanostructure  130  to the optical structure  220 . More specifically, the adhesive layer  206  is bonded to the top of the spacers  132  and a bottom surface of the optical structure  220 . The adhesive layer  206  may be formed from any appropriate adhesive, for example an optically clear adhesive (OCA) or a pressure sensitive adhesive (PSA). 
     When this type of passive optical nanostructure is used, in operation, light from the light emitting element  110  reaching the interface between the base layer  131  and the air gap  133  is totally internally reflected if it hits the interface at an angle of incidence greater than the critical angle. Thus, only light which hits the interface at an angle of incidence less than the critical angle passes through the interface. The critical angle θc is set so that substantially all the light output from the passive optical nanostructure  130  is at an angle which means that it will be received and reflected by the curved reflector  222 A that is aligned with the light emitting element  110 . This prevents light emitted by a light emitting element  110  from reaching a curved reflector  222 A which is not aligned with the light emitting element  110 . This is due to the passive optical nanostructure  130  on each light emitting element  110  totally internally reflecting light which is emitted at wide angles from the top of its respective light emitting element  110 . 
     It may be desirable to provide an illumination apparatus with switchable angular luminance profiles. 
       FIG.  5    illustrates a cross-sectional view of a further embodiment of the illumination apparatus  100 . Features of the embodiment of  FIG.  5    not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features. 
     In this embodiment, each light emitting element  110  comprises a central circular light source  110 A and an annular light source  110 B around the central circular light source  110 A. Also, in this embodiment, the apertures  152  in the output mask  150  each comprise an annular diffuser  156  around a clear central hole  158 . In this embodiment, the optical structure  220  is configured to direct the light received from the central circular light source  110 A substantially only to the clear central hole  158 , and to direct light received from the annular light source  110 B substantially only to the annular diffuser  156 . 
     The central circular light source  110 A and annular light source  110 B of each light emitting element  110  are each individually controllable, i.e. one may be controlled to be on while the other is controlled to be off and vice versa. When only the central circular light source  110 A of a light emitting element  110  is on, the light output from the aperture  152  aligned with that light emitting element  110  has a first (relatively narrow) cone angle  157 . When only the annular light source  110 B of the light emitting element  110  is on, the light output from the aperture  152  aligned with that light emitting element  110  has a second (relatively wide) cone angle  159  which is higher than the first cone angle  157 , due to the light being scattered by the annular diffuser  156 . The illumination apparatus  100  is operable in a first state in which only the central light sources  110 A of its array of light emitting elements  110  are on and a second state in which only the annular light sources  110 B of its array of light emitting elements  110  are on. 
     When the illumination apparatus is provided as a display the first state may be said to correspond to a privacy mode, since the overall light output by the illumination apparatus in this state has a relatively narrow cone angle and the display has low image visibility for off-axis snoopers. The second state may be said to correspond to a share mode of operation, since the overall light output by the illumination apparatus in this state has a relatively wide cone angle and the displayed image has high image visibility for off-axis users. 
     The illumination apparatus  100  comprises a controller (not shown) configured to perform the control described above. As a result of this functionality, the illumination apparatus  100  may be used to provide the lighting in a privacy display 
       FIG.  6    illustrates a plan view of the light emitting elements according to the embodiment of  FIG.  5    described above. Substrate  200  comprises an array of light emitting elements  110  with central light sources  110 A and annular light sources  110 B. Electronics  520  may be arranged between the light sources in regions in which rays  180  that have been reflected by the curved reflectors  222 A,  222 B are not incident. 
       FIG.  7    shows a plan view of the output mask  150  according to the embodiment of  FIG.  5    described above; and  FIG.  8    illustrates a perspective close-up view of the structure of one of the annular diffusers  156 . As shown, the annular diffuser  156  comprises a plurality of concentric ridges which act to scatter light which passes through the annular diffuser  156 . Annular diffuser  156  around a clear central hole  158  are illustrated, with opaque regions  151  arranged between respective annular diffusers  156 . In operation light from central light sources  110 A is directed to clear central holes  158  and light from annular light sources  110 B is directed to annular diffuser  156 . Advantageously a switchable privacy display may be provided. 
       FIG.  9 A  and  FIG.  9 B  illustrate graphs showing the cone angles of the light output from each of the apertures  152  in the first and second states described above with reference to  FIG.  5   .  FIG.  9 A  illustrates the relatively narrow first cone angle of the light output by the aperture  152  in the first state, for example for use in a privacy mode of operation of a privacy display.  FIG.  9 B  illustrates the relatively wide second cone angle of the light output by each of the apertures  152  in the second state, for example for use in a share mode of operation of a switchable privacy display. The numbered contours in these graphs each represent points in the x-y plane with the same relative luminance, i.e. the relative luminance value indicated on the contour. 
     A privacy display may be provided advantageously with low luminance to off-axis snoopers. A switchable privacy display may be provided with a high share mode luminance to off-axis users. 
     It may be desirable to provide a tiled display with low visibility of seams between the tiles. 
       FIG.  10    illustrates a cross-sectional view of an embodiment of a tiled display apparatus  500  comprising the illumination apparatus  100  of the embodiment of  FIG.  1   . The tiled display apparatus  500  may be, for example, a television. Features of the embodiment of  FIG.  10    not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features. 
     The display apparatus  500  comprises a screen  1220  and controller  1200  configured to control various functions of the display apparatus  500 , e.g. an audio controller  1202  and a display controller  1204 . 
     In this embodiment, the illumination apparatuses are tiled such that there are a plurality of separately controllable tiles  100 A,  100 B, each tile  100 A,  100 B comprising its own group of components of the illumination apparatus  100 . In this embodiment, the output mask  150  is part of the screen  1220 , and the screen  1220  is positioned such that each aperture  152  of the output mask  150  is aligned with a respective light emitting element  110 . 
     In comparison to the arrangement of  FIG.  1   , large display area may be achieved at low cost. Seam regions  1240  between the tiles  100 A,  100 B are not visible in ambient illumination  161  from ambient light sources  160  and are advantageously hidden to external observers. Acoustic apertures  1242  may be provided to advantageously achieve propagation of sound through the screen  1220  across the area of the display apparatus  500 . Localised sound may advantageously be achieved, for example for use in a cinema. 
     It may be desirable to provide a high contrast illumination apparatus comprising an opaque support substrate. 
       FIG.  11    illustrates a cross-sectional view of a further embodiment of the illumination apparatus  100 . Features of the embodiment of  FIG.  11    not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features. 
     In this embodiment, the optical structure  220  is disposed between the array of light emitting elements  110  and the output mask  150 . In operation, the optical structure  220  is configured to receive light rays  182  emitted by the array of light emitting elements  110 , direct the received light into a direction away from the substrate  200 , direct at least some of the light which has been directed away from the substrate  200  back towards the substrate  200 , and direct at least some of the light which has been directed back towards the substrate  200  through the plurality of apertures  152  of the output mask  150 . The optical structure  220  further comprises a waveguide  250 . 
     In comparison to the above embodiments, in this embodiment, the light output from the apertures  152  does not pass through the substrate  200 . Accordingly, in this embodiment, the substrate  200  is an opaque substrate. Increased density of electronics  520  may be advantageously provided in comparison to the embodiments above without attenuating the light output. Increased complexity of the driving architecture of the light emitting elements  110  may be provided and higher resolution may be achieved over large substrate areas. 
     In this embodiment, the optical structure  220  is a catadioptric optical structure that operates by means of refraction and reflection. Light emitting elements  110  are arranged in air and refraction at interface  251  to waveguide  250  provides guiding light rays  182  by total internal reflection within the waveguide  250 . 
     Specifically, the optical structure  220  comprises a plurality of waveguides  250  configured to guide light rays  182  emitted by the light emitting elements  110  towards the output mask  150 . The optical structure  220  comprises the first reflector  252  that is configured to reflect at least some of the light guided by the waveguides  250  towards the first substrate  200 . The optical structure  220  further comprises a second reflector  254  configured to reflect at least some of the light rays  182  reflected by the first reflector  252  back towards the apertures  152  of the output mask  150  that is provided in alignment with the first reflector  252  of the optical structure  220 . 
     Each waveguide  250  comprises a conical frustrum (or frustroconical) structure  251 ,  253  and an annular wall  255  extending around the conical frustrum structure  251 ,  253  although any shape and material which achieves the above function would be appropriate. For each waveguide  250  the conical frustrum structure  251 ,  253  is connected to the second reflector  254  via the annular wall  255 . The conical frustrum structure  251 ,  253  comprises an open end and a closed end opposite to (or facing) the open end. The open end corresponds to the larger circle of the conical frustrum shape and the closed end corresponds to the smaller circle of the conical frustrum shape. The open and closed ends are connected by a frustroconical wall extending between the open and closed ends. The open end corresponds to the top of the frustroconical structure and the closed end corresponds to the bottom of the frustroconical structure. The top of the conical frustum structure  250  is connected to the annular wall  255  at the top of the annular wall  255 , and the second reflector  254  is connected to the annular wall  255  at the bottom of the annular wall  255 . 
     Each waveguide  250  is aligned with a respective light emitting element  110 . The optical structure may be provided in a polymer material formed by moulding for example. 
     In this embodiment, each aperture  152  of the output mask  150  is circular, for example as illustrated in  FIG.  2   . In other embodiments (not shown) the aperture  152  shape may not be circular, such as rectangular to efficiently couple light from the light emitting elements  110  that may also be rectangular. 
     Each aperture  152  is aligned with a respective light emitting element  110 . The first reflector  252  is disposed between the output mask  150  and the second reflector  254 , and comprises circular apertures aligned with the circular apertures  152  of the output mask  150 . The first reflector  252  has the same shape and is aligned with the opaque mask  151 , which may be a black material as described herein. 
     In this embodiment, the second reflector  254  comprises a plurality of curved reflecting elements. Each curved reflecting element is disposed between two adjacent waveguides  250  in the x direction. Each waveguide  250  is disposed between two adjacent curved reflecting elements in the x direction. 
     In operation, in sequence, a light ray  182  emitted by a light emitting element  110  is refracted into the respective waveguide  250  aligned with the light emitting element  110 , is guided by the waveguide (by a combination of refraction when entering the waveguide and reflection on an internal face of the waveguide) towards the output mask  150 , is reflected by the first reflector  252  back towards the substrate  200 , is reflected by the second reflector  254  back towards the mask  150  again, and travels through an aperture  152  in the output mask  150 . 
     Another structure for an opaque substrate will now be described. 
       FIG.  12 A  illustrates a cross-sectional view of a further embodiment of the illumination apparatus  100 . Features of the embodiment of  FIG.  12 A  not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features. 
     In this embodiment, the optical structure  220  is disposed between the array of light emitting elements  110  and the output mask  150 . In operation, the optical structure  220  is configured to receive light rays  180  emitted by the array of light emitting elements  110 , direct the received light into a direction away from the substrate  200 , direct at least some of the light which has been directed away from the substrate  200  back towards the substrate  200 , and direct at least some of the light which has been directed back towards the substrate  200  through the plurality of apertures  152  of the output mask  150 . 
     As with  FIG.  11   , in this embodiment, the light output from the apertures  152  does not pass through the substrate  200 . Accordingly, in this embodiment, the substrate  200  is an opaque substrate. As with  FIG.  11   , increased density of electronics  520  may be advantageously be provided without attenuating the light output in comparison to the other embodiments above. Increased complexity of the driving architecture of the light emitting elements  110  may be provided and higher resolution may be achieved over large substrate areas. 
     In this embodiment, the optical structure  220  is a catadioptric optical structure that operates by means of refraction and reflection. Light emitting element  110  is arranged in air and refraction at interface  251  to waveguide  250  provides guiding light rays  180  by total internal reflection within the waveguide  250 . 
     Specifically, the optical structure  220  comprises a plurality of waveguides  250  configured to guide light rays  180  emitted by the light emitting elements  110  towards the output mask  150 . The optical structure  220  comprises the first reflector  252  that is configured to reflect at least some of the light guided by the waveguides  250  towards the first substrate  200 . The optical structure  220  further comprises a second reflector  254  configured to reflect at least some of the light reflected by the first reflector  252  back towards the apertures  152  of the output mask  150  that is provided in alignment with the first reflector  252  of the optical structure  220 . 
     Each waveguide  250  is aligned with a respective light emitting element  110 . 
     In this embodiment, each waveguide  250  comprises a cylindrical structure, although any shape and material which achieves the above function would be appropriate. 
     For each waveguide  250 : the cylindrical structure comprises an open end  257  and a closed end  251  opposite to (or facing) the open end  257 . The open and closed ends  257 ,  251  are connected by a cylindrical wall  253  extending in the z direction between the open and closed ends  257 ,  251 . 
     The cylindrical structure is connected to the second reflector  254  at the open end  257 . The open end  257  corresponds to the top of the cylindrical structure and the closed end  251  corresponds to the bottom of the cylindrical structure. 
     The optical structure may be provided in a polymer material formed by moulding for example. 
     In this embodiment, each aperture  152  of the output mask  150  is annular and aligned with a respective light emitting element  110 . The first reflector  252  is disposed between the output mask  150  and the second reflector  254 , and comprises annular apertures aligned with the annular apertures  152  of the output mask  150 . The first reflector  252  has the same shape and is aligned with the opaque mask  151  which may be a black material as described herein. 
     In this embodiment, the second reflector  254  comprises a plurality of curved reflecting elements. Each curved reflecting element is disposed between two adjacent waveguides  250  in the x direction, and extends in the x direction substantially in line with the tops of the waveguides  250 . Each waveguide  250  is disposed between two adjacent curved reflecting elements in the x direction. 
     In operation, in sequence, a light ray  180  emitted by a light emitting element  110  is refracted into the respective waveguide  250  aligned with the light emitting element  110 , is guided by the waveguide (by a combination of refraction when entering the waveguide and reflection on an internal face of the waveguide) towards the output mask  150 , is reflected by the first reflector  252  back towards the substrate  200 , is reflected by the second reflector  254  back towards the mask  150  again, and travels through an aperture  152  in the output mask  150 . 
       FIG.  12 B  illustrates a plan view of the output mask  150  of the embodiment of  FIG.  12 A . In this embodiment the apertures  152  are annular. The first reflector  252  and mask  150  may be formed on the optical structure  220  for example by lithography or printing. Advantageously low reflectivity may be provided in ambient illumination. Contrast of a display apparatus may be increased and power consumption may be reduced for a desirable contrast when the display is operated in high ambient illuminance. 
     Arrangements of masks  150  will now be further described. 
       FIG.  13    illustrates a plan view of an embodiment of the output mask  150  described above. The output mask  150  has a light absorbing structure  316  that comprises a corner cube retro-absorber microstructure. Light absorbing structure  316  has a dark or black surface that may be physically shaped such as a plurality of corner cube absorbers each comprising facet surface  331 A,  331 B,  331 C. In operation ambient light rays  326  are incident on the facet surfaces  331 A,  331 B,  331 C of the corner cube absorber. At each surface reflection, light is absorbed and scattered such that the overall luminance of light rays  328  is substantially reduced in comparison to a planar black surface. The light absorbing structure  316  may comprise a surface relief comb structure with high surface area for absorption of incident radiation. Advantageously front of screen reflections are reduced and image contrast enhanced or power consumption reduced for a desirable achieved contrast ratio. Further, known microstructured surface tooling and replication methods can be provided to form the structure of the corner cubes or other micro-structured surface to achieve low cost and high uniformity. The feature size of the micro-structure may be less than the pitch of the aperture  152 . For example, the pitch of the apertures  152  may be 300 micrometres and the microstructure may have a pitch of 100 micrometres. The features of the micro-structure may be arranged to minimise diffractive spreading of reflected light rays  328 , for example by minimising the number of narrow features of less than 20 micrometres size at the cusps of the microstructure. Advantageously a very high contrast display is produced which is not affected by high levels of ambient lighting. The light absorbing structure  316  may be used indoors, where the black material and shape of light absorbing structure  316  help attenuate reflections from light reflected off the faces and clothing of the one or many observers, which would otherwise reduce the perceived contrast of the displayed images, for example when the ambient illumination is absent or very low. The light absorbing structure  316  may further be coated with Teflon or other similar material which resists staining or wetting and is resistant to degradation by dust and/or liquids. 
       FIG.  14    illustrates a perspective view of the output mask  150  and tiling of the display apparatus  100 A,  100 B of  FIG.  10   . Advantageously a tiled display with high contrast in ambient illumination may be provided. The seams between the tiles may be invisible to display users. Very large area displays which may be tiled may be provided. 
       FIG.  15    illustrates a composite display apparatus  600  comprising a plurality of the display apparatuses  500  according to the embodiment of  FIG.  10    tiled together to create a larger display. Advantageously a tiled display with high contrast in ambient illumination may be provided. A very small number of seams may be provided so that seam visibility may be low. Extremely large area displays which may be tiled may be provided, for example for stadium or cinema displays with low power consumption while achieving desirable image contrast. 
     As may be used herein, the terms “substantially” and “approximately” provide an industry-accepted tolerance for its corresponding term and/or relativity between items. Such an industry-accepted tolerance ranges from zero percent to ten percent and corresponds to, but is not limited to, component values, angles, et cetera. Such relativity between items ranges between approximately zero percent to ten percent. Furthermore, in this specification, the term “annular” encompasses both circular and elliptical annular shapes. 
     While various embodiments in accordance with the principles disclosed herein have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of this disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with any claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages. 
     Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the embodiment(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” the claims should not be limited by the language chosen under this heading to describe the so-called field. Further, a description of a technology in the “Background” is not to be construed as an admission that certain technology is prior art to any embodiment(s) in this disclosure. Neither is the “Summary” to be considered as a characterization of the embodiment(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple embodiments may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the embodiment(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.