Illumination apparatus

A directional illumination apparatus comprises an array of micro-LEDs that may be organic LEDs (OLEDs) or inorganic LEDs and an aligned solid catadioptric micro-optic array arranged to provide a water vapour and oxygen barrier for the micro-LEDs as well as reduced sensitivity to thermal and pressure variations. The shape of the interfaces of the solid catadioptric micro-optic array is arranged to provide total internal reflection for light from the aligned micro-LEDs using known transparent materials. A thin and efficient illumination apparatus may be used for collimated illumination in environmental lighting, display backlighting or direct display.

TECHNICAL FIELD

The present disclosure relates to an apparatus comprising a plurality of addressable light-emitting elements aligned to a plurality of optical elements arranged as a solid layer. Such an apparatus may be used for switchable environmental lighting, for switchable indoor or outdoor electronic display screens, or for a switchable backlight to an LCD display.

BACKGROUND

Displays with wide directional light output distributions are typically used to achieve comfortable display viewing from many different viewing angles. Such displays are desirable for multiple users to share image content, and for displays where the viewing position is not substantially fixed in relation to the display centreline.

By way of comparison displays with narrow directional light output distributions are typically used to provide image data for the eyes of users over reduced viewing angles. Such displays are typically used to achieve privacy display (where images that may be seen by snoopers are suppressed), night time display (where ambient illumination is suppressed—for example to reduce reflections from windscreens or to reduce unsociable stray light), low power viewing (where illumination is not supplied to regions away from the eyes of users) and outdoors viewing (where high luminance is provided to a narrow range of viewing positions for no or small increases in backlight power).

In a known method, narrow directional light output distributions can be achieved by the addition of a micro louvered film. Such films can be permanently fixed on display such as for ATM cash machines for privacy viewing or automotive displays for night time operation. Alternatively, such films may be manually placed on the surface of a conventional wide directional light output distribution display by the user for private display use and removed and stored to restore a normal wide-angle viewing. Micro louver films are inefficient because they work by absorbing light from the backlight in the unwanted display angular directions. As a side effect of construction, they also significantly attenuate of the light in the wanted direction.

The viewing angle of a transmissive spatial light modulator such as an LCD (liquid crystal display) is controlled by the output light distribution of a backlight and the angular transmission properties of the LCD panel used. Typically, the backlight incorporates a light guide plate (LGP) that accepts light from sources such as LEDs (light emitting diodes) arranged at an input edge of the LGP. A structured pattern on the LGP output face provides a defined leakage of light across its face as the light propagates through the LGP.

Other known backlights incorporate an array of light emitting diodes (LEDs) in a matrix behind the LCD. The light from the LEDs is strongly diffused to create a largely uniform backlight illumination. The directional light output distribution of light from the backlight, or directional light output distribution, can be altered by the addition of fixed layers such as prismatic films and diffusers within the backlight assembly. The backlight and therefore the display angular light directional light output distribution is fixed by design at the time of manufacture.

Illumination systems for environmental lighting such as automobile headlights, architectural, commercial or domestic lighting may provide a narrow directional light output distribution, for example by means of focusing optics to provide spotlighting effects, or can achieve a wide directional light output distribution for example by means of diffusing optics to achieve broad area illumination effects.

Inorganic LEDs formed using semiconductor growth onto monolithic wafers demonstrate high levels of luminous efficiency (1 m/W) and high luminous emittance (1 m/mm2). In cooperation with light conversion layers, LEDs may provide acceptable CIE Colour Rendering Indices (CRI) or colour space coverage.

Organic light-emitting diodes (OLEDs) can be formed on arbitrarily large substrates; however luminous emittance may be more than 1000 times lower than may be achieved by inorganic LEDs.

In this specification LED refers to (i) an unpackaged inorganic LED die chip extracted directly from a monolithic wafer, i.e. a semiconductor element —this is different from packaged LEDs which have been attached to a lead frame in order to provide electrodes and may be assembled into a plastic package to facilitate subsequent assembly; or (ii) OLED elements that are formed by patterned deposition methods including ink jet printing, contact printing, evaporation through fine metal mask or vertical plane sources and may comprise quantum dot materials.

Packaged LEDs are typically of dimension greater than 1 mm, and more typically of dimension greater than 3 mm and can be assembled by conventional Printed Circuit Board assembly techniques including pick and place methods. The accuracy of components placed by such assembly machines may typically be about plus or minus 30 microns. Such sizes and tolerances prevent application to very high-resolution displays.

Micro-LEDs may be formed by array extraction methods in which multiple LEDs are removed from a monolithic wafer in parallel and may be arranged with positional tolerances that are less than 5 microns. Micro-LEDs may also or alternatively comprise OLED elements.

White LED lighting sources can be comprised of separate spectral bands such as red, green, blue and yellow, each created by a separate LED element. Such sources enable users to resolve the separate colours, and as a result of the separation of the sources in the lamp, can create coloured illumination patches. It would be desirable if the sources were homogenized so that their separation was less than the visual resolution limit.

BRIEF SUMMARY

Directional LED elements can use reflective optics (including total internal reflective optics) or more typically catadioptric optic type reflectors, as described for example in U.S. Pat. No. 6,547,423. Catadioptric elements employ both refraction and reflection, which may be total internal reflection or reflection from metallised surfaces.

It would be desirable to provide a directional display comprising an array of catadioptric optical elements and an array of Micro-LEDs that is resistant to gas such as oxygen and water vapour ingress, thermal variations and changes in external pressure while providing illumination quality suitable for directional applications including directional displays such as privacy displays.

According to a first aspect of the present disclosure there is provided an illumination apparatus, comprising: a plurality of LEDs, the plurality of LEDs being arranged in an LED array, wherein the LEDs of the plurality of LEDs are micro-LEDs; a catadioptric optical structure aligned with the LEDs of the plurality of LEDs to provide a directional light output distribution, the directional light output distribution being of light output from the LEDs of the plurality of LEDs; wherein the catadioptric optical structure comprises a plurality of catadioptric optical elements arranged in a catadioptric optical element array, each of the catadioptric optical elements of the plurality of catadioptric optical elements aligned in correspondence with a respective one or more of the LEDs of the plurality of LEDs, each of the LEDs of the plurality of LEDs being aligned with only a respective one of the catadioptric optical elements of the catadioptric optical structure; wherein each of the plurality of catadioptric optical elements comprises in at least one catadioptric cross-sectional plane through its optical axis: a first cross-sectional outer interface and a second cross-sectional outer interface facing the first cross-sectional outer interface; wherein the first and second cross-sectional outer interfaces each comprise curved interfaces comprising first and second outer interface regions; wherein the first and second cross-sectional outer interfaces extend from a first end of the catadioptric optical element to a second end of the catadioptric optical element, the second end of the catadioptric optical element facing the first end of the catadioptric element; wherein the distance between the first and second cross-sectional outer interfaces at the first end of the catadioptric optical element is less than the distance between the first and second cross-sectional outer interfaces at the second end of the catadioptric optical element; and at least one transparent inner interface arranged between the first and second ends and between the first and second outer interfaces; wherein the catadioptric optical structure comprises: (i) a first transparent non-gaseous material with a first refractive index arranged between the first and second cross-sectional outer interfaces and the at least one transparent inner interface and between the first and second end of each of the catadioptric optical elements; (ii) a second transparent non-gaseous material with a second refractive index lower than the first refractive index arranged between a respective aligned LED and the transparent inner interface of each of the catadioptric optical elements; (iii) a third transparent non-gaseous material with a third refractive index lower than the first refractive index arranged between the first cross-sectional outer interface of a first catadioptric optical element and the second cross-sectional outer interface of an adjacent catadioptric optical element of the plurality of catadioptric optical elements and between the first and second end of each of the catadioptric optical elements.

The tilt angle with respect to the optical axis of the interface normal of each of the first and second cross-sectional outer interfaces may vary continuously with the distance from the first end towards the second end. The derivative of the tilt angle with respect to distance from the optical axis may have a discontinuity at the boundary between the respective first and second outer interface regions of the first and second cross-sectional outer interfaces.

Advantageously a directional illumination apparatus may be provided that can provide a restricted range of illumination directions. Privacy display, power savings, reduced stray light for night time operation and efficient high luminance operation may be achieved. Further oxygen and moisture ingress may be reduced and LED lifetime increased. Uniformity degradation due to thermal expansion differences minimised Misalignments due to environmental pressure changes may be reduced. Further, low cost materials may be provided.

The height from the first end of the first and second outer interfaces may increase monotonically between the first and second end of the catadioptric optical element; and the tilt angle with respect to the optical axis of the interface normal of each of the first and second cross-sectional outer interfaces may increase monotonically between the first and second end of each catadioptric optical element. Advantageously uniform angular optical beam profiles may be provided.

Principal light output rays from the respective aligned LEDs may be provided at the first end and at the optical axis of the respective catadioptric optical element, and may be transmitted through an inner interface, and may be incident on a cross-sectional outer interface, each principal light ray having an angle of incidence at the curved cross-sectional outer interface; wherein the derivative of the difference between the angle of incidence of each principal light ray and the critical angle at the first and second outer interfaces with respect to distance from the optical axis has a discontinuity at the boundary between the first and second outer interface regions. Advantageously a directional illumination apparatus may be provided that can provide a restricted range of illumination directions.

The difference between the angle of incidence of each principal light ray and the critical angle may be a constant across the first outer interface region and the difference between the angle of incidence of each principal light ray and the critical angle monotonically may increase across the second outer interface region. Advantageously the angular width of the optical profile from the catadioptric optical element may be minimised.

The first outer interface region is arranged to reflect principal light output rays in off-axis directions and the second outer interface region is arranged to reflect principal light output rays in on-axis directions. Advantageously the angular width of the optical profile from the catadioptric optical element may be minimised Privacy appearance to snoopers may be reduced and stray light may be minimised.

The principal rays may be reflected by total internal reflection at the cross-sectional outer interfaces between the first and second end of each catadioptric optical element. Advantageously efficiency may be optimised in comparison to arrangements using coated optical elements. Further manufacturing yield may be increased and cost reduced.

In the first outer interface region, reflected principal light rays may be output through the second end in directions different to the optical axis direction and in the second outer interface region reflected principal light rays may be output through the second end substantially parallel to the optical axis. The first outer interface region may be arranged between the first end and the second outer interface region and the second outer interface region is arranged between the second end and the first outer interface region. Advantageously the angular width of the optical profile from the catadioptric optical element may be minimised.

The third transparent material may be arranged to fill the region between the first cross-sectional outer interface of a first catadioptric optical element and the second cross-sectional outer interface of an adjacent catadioptric optical element of the plurality of catadioptric optical elements and between the first and second end of the respective catadioptric optical elements. The third transparent material may be formed as a layer on the first and second cross-sectional outer interfaces of the plurality of catadioptric optical elements. A filler material with a fourth refractive index different to the third refractive index may be arranged to fill the region between the third transparent material formed as a layer on the first and second cross-sectional outer interfaces. Advantageously the volume of very low refractive index materials may be reduced and cost reduced; and the angular width of the optical profile from the catadioptric optical element may be minimised.

Gas and/or water vapour barrier layers are formed between the plurality of LEDs and outer surfaces of the illumination apparatus. Advantageously oxygen and moisture ingress may be reduced and LED lifetime increased.

The micro-LEDs may be organic LEDs. Advantageously device thickness and complexity are reduced in comparison to backlit LCDs.

The LEDs may be from a monolithic wafer arranged in an array with their original monolithic wafer positions and orientations relative to each other preserved; and wherein in at least one direction, for at least one pair of the plurality of LEDs in the at least one direction, for each respective pair there was at least one respective LED in the monolithic wafer that was positioned in the monolithic wafer between the pair of LEDs in the at least one direction and that is not positioned between them in the array of LEDs. Advantageously very high luminance illumination apparatuses may be provided.

The LEDs of the plurality of LEDs are micro-LEDs of width or diameter may be less than 200 microns, preferably less than 100 microns and more preferably less than 50 microns. Advantageously a very high-resolution display may be provided.

In the at least one catadioptric cross-sectional plane the distance between the first and second outer interfaces at the second end of the catadioptric optical element may be less than less than 600 microns, preferably less than 400 microns and more preferably less than 200 microns. Advantageously very low thickness may be provided.

The first refractive index may be greater than 1.49, preferably greater than 1.55 and most preferably greater than 1.58 and the third refractive index may be less than 1.42, preferably less than 1.40 and most preferably less than 1.35. Advantageously known and low cost materials may be used to form the catadioptric optical elements.

In the at least one catadioptric cross-sectional plane at least one of the transparent inner interfaces may have positive optical power. Advantageously light may be efficiently directed from the LED and therefore from the illumination apparatus.

According to a second aspect of the present disclosure a direct display apparatus comprises a switchable illumination apparatus of the first aspect and a control circuit comprising means to drive the plurality of LEDs with image pixel data. Advantageously thin directional displays may be provided that are resistant to thermal, oxygen, water vapour and environmental pressure changes. Further such displays may be conveniently formed on flexible and curved substrates.

According to a third aspect of the present disclosure a backlit display apparatus comprises the illumination apparatus of the first aspect and a spatial light modulator. Advantageously thin directional LCDs may be provided that are resistant to thermal, oxygen, water vapour and environmental pressure changes. Further such displays may be conveniently formed on flexible and curved substrates.

Such an apparatus may be used for domestic or professional lighting and for display.

These and other features and advantages of the present disclosure will become apparent to those of ordinary skill in the art upon reading this disclosure in its entirety.

DETAILED DESCRIPTION

It would be desirable to provide an illumination apparatus for display, display backlighting or for domestic or professional environmental lighting that provides light output directionality over a restricted solid angle, that is, non-Lambertian illumination. Such an illumination apparatus in display applications may provide privacy functionality in which an off-axis snooper may be unable to resolve image content while an on-axis viewer may have a conventional display appearance. Further such a directional display may provide reduced power consumption as power is not required to illuminate off-axis observers. Further such a display may provide reduced stray light in night time operation, for example in automotive applications. Further such a display may provide very high luminance levels without increasing the power consumption in comparison to a wide-angle display at conventional luminance levels.

Environmental lighting may include illumination of a room, office, building, scene, street, equipment, or other illumination environment. Such an illumination apparatus may provide narrow angle lighting of the illuminated environment, such as spot lighting.

In the present disclosure display backlighting means an illumination apparatus arranged to illuminate a transmissive spatial light modulator such as a liquid crystal display. The backlight may provide uniform luminance across the spatial light modulator and pixel data is provided by the spatial light modulator. The micro-LEDs of a display backlight may further be provided with some pixel information, for example in high dynamic range operation.

Direct display means an illumination apparatus wherein the micro-LEDs are arranged to provide pixel image information, and no spatial light modulator is arranged between the illumination apparatus and observer.

It would be desirable to provide directional displays that achieve encapsulation of light emitting elements to provide enhanced environmental ruggedness in comparison to catadioptric optical elements that provide refractive and reflective surfaces in air.

The structure of an illumination apparatus comprising a solid catadioptric structure will now be described.

FIG. 1Ais a schematic diagram illustrating in top view a directional display apparatus100comprising an array of solid catadioptric optical elements and an array of LEDs3; andFIG. 1Bis a schematic diagram illustrating in top view separated components ofFIG. 1A. Features of the arrangements ofFIG. 1Bnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals inFIG. 1A, including any potential variations in the features.

An illumination apparatus100comprises a plurality of LEDs3, the plurality of LEDs3being arranged in an LED array, wherein the LEDs of the plurality of LEDs are micro-LEDs.

The catadioptric optical structure91comprises a plurality of catadioptric optical elements38a,38barranged in a catadioptric optical element array, each of the catadioptric optical elements38of the plurality of catadioptric optical elements38aligned in correspondence with a respective one or more of the LEDs3of the plurality of LEDs, each of the LEDs3of the plurality of LEDs3being aligned with only a respective one of the catadioptric optical elements38of the catadioptric optical structure91.

Each of the plurality of catadioptric optical elements38comprises in at least one catadioptric cross-sectional plane through its optical axis711, a first cross-sectional outer interface46aand a second cross-sectional outer interface46bfacing the first cross-sectional outer interface46a; wherein the first and second cross-sectional outer interfaces46a,46beach comprise curved interfaces comprising first and second outer interface regions52,54. The outer interfaces46a,46bare between different solid materials32,34.

The first and second cross-sectional outer interfaces46a,46bextend from a first end707of the catadioptric optical element38to a second end708of the catadioptric optical element38, the second end708of the catadioptric optical element facing the first end707of the catadioptric element38.

The distance712between the first and second cross-sectional outer interfaces46a,46bat the first end of the catadioptric optical element38is less than the distance714between the first and second cross-sectional outer interfaces46a,46bat the second end of the catadioptric optical element38. As will be described below, reduced cone angle can be used for the output directional distribution120.

At least one transparent inner interface42,44a,44bis arranged between the first and second ends712,714and between the first and second outer interfaces46a,46b.

The catadioptric optical structure91comprises: (i) a first transparent non-gaseous material32with a first refractive index arranged between the first and second cross-sectional outer interfaces46a,46band the at least one transparent inner interface42,44a,44band between the first and second end712,714of each of the catadioptric optical elements38; (ii) a second transparent non-gaseous material30with a second refractive index lower than the first refractive index arranged between a respective aligned LED3and the transparent inner interface42,44a,44bof each of the catadioptric optical elements38; (iii) a third transparent non-gaseous material34with a third refractive index lower than the first refractive index arranged between the first cross-sectional outer interface46bof a first catadioptric optical element38aand the second cross-sectional outer interface46aof an adjacent catadioptric optical element38bof the plurality of catadioptric optical elements38and between the first and second end707,708of each of the catadioptric optical elements38.

For the purposes of the present disclosure, the materials30,32,34,28are solid, which may also include gels.

The first refractive index may be greater than 1.49, preferably greater than 1.55 and most preferably greater than 1.58. The third refractive index may be less than 1.42, preferably less than 1.40 and most preferably less than 1.35, as will be described in further detail below for illustrative embodiments. Example materials may include but are not limited to polymers such as acrylates for the first material32; and fluorinated materials and/or silicone materials for the second and third materials30,34.

Alternatively, some of the materials30,32,34,28may be liquid, for example the material28may comprise a liquid material. Suitable low index liquid materials include silicone liquids.

As illustrated inFIGS. 1A and 1Bthe third transparent material34may be formed as a layer on the first and second cross-sectional outer interfaces46a,46bof the plurality of catadioptric optical elements38and may not fill the region between the interfaces46a,46b.

A filler material28with a fourth refractive index different to the third refractive index may further be arranged to fill the region between the third transparent material34formed as a layer on the first and second cross-sectional outer interfaces46a,46b. Low index materials34such as those using fluorinated materials may be expensive, and further the optical operation to provide total internal reflection within the interfaces46a,46bmay be achieved by thin layers. Advantageously desirable optical operation may be achieved with reduced cost in comparison to arrangements in which the material34fills the region between the outer interfaces46a,46b.

Such material28may be transparent so that light from a second plurality of LEDs5that are arranged between the first plurality of LEDs3may be transmitted to advantageously provide a wide-angle mode of operation as will be described below. Alternatively, the material28may be absorptive to achieve reduced stray light between adjacent pixels of a display apparatus.

It would be desirable to provide a directional display apparatus with low thickness and high resolution. In illustrative embodiments, the at least one catadioptric cross-sectional plane the distance714between the first and second outer interfaces46a, at the second end of the catadioptric optical element38may be less than less than 600 microns, preferably less than 400 microns and more preferably less than 200 microns. The LEDs3of the plurality of LEDs may be micro-LEDs of width or diameter is less than 200 microns, preferably less than 100 microns and more preferably less than 50 microns.

As illustrated inFIG. 1A, the transparent inner interface42may have positive optical power such that light rays300,302are directed in substantially on-axis directions from the location of the micro-LED3that intersects the optical axis711. As will be described below, the inner interfaces44a,44bmay be linear and be illuminated by rays304,306.

Further elements related to the construction of the illumination apparatus ofFIG. 1Awill now be described. Illumination apparatus100may be provided by backplane substrate93and catadioptric array92comprising catadioptric optical structure91and catadioptric substrate95.

Backplane substrate93may comprise a support substrate102, a barrier layer104, LED support substrate106, LED layer108and adhesive layer110. Catadioptric substrate95may comprise barrier layer112, support layer114, anti-reflection layer116that may comprise components such as retarders and polarisers arranged to reduce reflections of ambient light from the display and optical elements. An outer optical diffusing layer118may be provided to achieve increased uniformity of directional output.

Materials used in substrates93,95may be flexible to advantageously achieve a flexible display.

Gas and/or water vapour barrier layers104,112may be formed between the plurality of LEDs3and outer surfaces101,103of the illumination apparatus100. Barrier layers104,112may be arranged to reduce ingress of water vapour202and oxygen200that may degrade optical output and lifetime of the LEDs3, in particular in embodiments in which the LEDs3comprise organic LED materials.

In operation as a direct display, a control circuit106comprising means to drive the plurality of LEDs3with image pixel data may be provided.

FIG. 1Cis a schematic diagram illustrating in top view illumination by the directional display100ofFIG. 1Aof a display user129and an off-axis snooper131. Desirably the snooper131cannot see an image on the display100when the display operates in privacy mode of operation.

As illustrated inFIGS. 1A-B, catadioptric optical structure91is aligned with the LEDs3of the plurality of LEDs3to provide a directional light output distribution120as illustrated inFIG. 1C, the directional light output distribution120being of light output from the LEDs3of the plurality of LEDs.

As illustrated inFIG. 1C, the light rays300,302,304illustrated inFIG. 1Amay be directed towards the centre of the illumination region120such that observer129can observe an illuminated display. Similarly, off-axis rays306are directed from the edge of the LED3towards the illumination region120.

However, the luminance of high angle rays309is substantially lower so that the off-axis snooper131cannot see the display100. Advantageously the display100may operate as a privacy display, or a power saving display for the user129.

Returning toFIG. 1A, further LEDs5that may be micro-LEDs may be provided outside a first end of the catadioptric optical structure91to achieve switching to a wider viewing angle of operation as will be described below. In operation, such LEDs5provide luminance in the direction of light ray309such that an off-axis user can observe an image on the display100. Control of the LEDs5may advantageously provide a switchable privacy display100.

FIG. 1Dis a schematic diagram illustrating in top view a detail of the arrangement of an organic LED and aligned solid catadioptric optical element38. Barrier layer104may comprise two layers104a,104b, for example an organic material and an inorganic material, arranged to minimise water vapour202and oxygen200ingress through substrate102. Transistor4with electrodes7may be used to provide addressing control of light emitting pixel3and layer108provides encapsulation and planarization of the OLED emitter LED3. Adhesive110is arranged to provide attachment to the catadioptric optical element38such that light rays304are directed into the catadioptric optical element38. Barrier layer112may also comprise multiple layers of different materials that may be a combination of organic and inorganic materials.

Advantageously the present embodiments achieve encapsulation of the OLED emitter by means of the solid catadioptric optical element38that has interfaces46a,46barranged to achieve directional illumination with low levels of cross talk to snoopers as will be described below.

FIGS. 1E-Gare schematic diagrams illustrating in top view details of the arrangement of further structures for solid catadioptric optical elements38. In these embodiments the third transparent material34is arranged to fill the region between the first cross-sectional outer interface46aof a first catadioptric optical element and the second cross-sectional outer interface46bof an adjacent catadioptric optical element38bof the plurality of catadioptric optical elements38and between the first and second ends707,708of the respective catadioptric optical elements38. Features of the arrangements ofFIGS. 1E-Gnot 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.

FIG. 1Eillustrates that barrier layer112may be provided on the substrate114. Conveniently the substrate may be provided prior to fabrication of the catadioptric optical structure91.

FIG. 1Fillustrates that a further barrier layer111may be formed on the solid catadioptric optical structure91after fabrication. Advantageously multiple barrier layers, for example double barrier layers may be provided to further reduce water vapour202and oxygen200ingress.

FIG. 1Gillustrates a further embodiment wherein a barrier layer115is formed on the material28of the catadioptric optical structure91prior to addition of the low index materials30,34. Advantageously the material properties and resistance to high temperature processing that may be desirable for barrier layers such as inorganic material evaporation may be improved and increased yield achieved while providing increased resistance to water vapour202and oxygen200ingress. Barrier layers111,112may also be provided as barriers to other gases or vapours.

The design of cross-sectional outer interfaces46of solid catadioptric optical elements38to achieve directional output with low off axis stray light will now be described by considering the propagation of principal light rays.

FIG. 2Ais a schematic diagram illustrating in side view propagation of principal light rays from a light source comprising LED3at the optical axis711of a solid catadioptric optical element38. Features of the arrangements ofFIG. 2Anot 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.

Principal light output rays304a,304b,306a,306bfrom the respective aligned LEDs3are provided at the first end707and at the optical axis711of the respective catadioptric optical element38, are transmitted through an inner interface44a, and are incident on a cross-sectional outer interface46a, each principal light ray304a,304b,306a,306bhaving an angle of incidence164at the curved cross-sectional outer interface46awith respect to the surface normal154that has a tilt angle174with respect to the direction of the optical axis711. The principal rays are reflected by total internal reflection at the cross-sectional outer interfaces46a,46bbetween the first and second end707,708of each catadioptric optical element38.

The first outer interface region52is arranged between the first end707and the second outer interface region54and the second outer interface region54is arranged between the second end708and the first outer interface region52. Outer interface regions52and54are regions of the outer interface46. For illustrative purposes regions52,54have widths52x,54xrespectively in the plane comprising the optical axis711; and regions52,54have heights52z,54zrespectively in the plane comprising the optical axis711. The regions52,54of the outer interface46meet at boundary50that has lateral location50xfrom the optical axis and height that may be the same as the height52zof the region52.

In the first outer interface region52, reflected principal light rays306a,306bfrom the LED3at the optical axis711are output through the second end708in directions different to the optical axis direction711and in the second outer interface region54reflected principal light rays304a,304bare output through the second end708substantially parallel to the optical axis711.

In the first outer interface region52, the surface normal tilt angle174is arranged to reflect principal light output rays306a,306bin off-axis directions with angles192that are not substantially parallel to the optical axis711direction. Thus, the rays306a,306bare reflected by means of total internal reflection, that is the angle of incidence is greater than the critical angle, illustrated by cone150for each incidence location at the cross-sectional outer interface46a.

The second outer interface region may be arranged to reflect principal light output ray directions that are on-axis or close to on-axis for principal rays304a,304b, that is parallel to the optical axis711direction. The rays304a,304bmay further be arranged with small variations from the on-axis direction, for example within 10 degrees of the on-axis direction. Advantageously a comfortable viewing freedom and display uniformity may be achieved across the display100for observer129illustrated inFIG. 1C.

By way of comparison at least some of the rays306a,306bmay have angles192that are at greater angles than 10 degrees with known and low-cost materials top provide first, second and third refractive indices.

Thus the angular difference156a,156bbetween the angle of incidence164and critical angle for principal rays306a,306bhas a different relationship in the first outer interface region52than in the second outer interface region54as will be further described below.

Further arrangements of catadioptric optical elements38will now be described.

FIG. 2Bis a schematic diagram illustrating in side view propagation of principal light rays from a light source3at the optical axis711of one of a pair of solid catadioptric optical elements38A,38B comprising a low index material34between the cross-sectional outer interfaces. In comparison to the arrangement ofFIG. 2A, the coating layer of low index material34on the outer interface46is replaced by a continuous region of low index material between the outer interfaces46of adjacent catadioptric optical elements38. Advantageously manufacturing complexity may be reduced.

FIG. 2Cis a schematic diagram illustrating in top view of a circular catadioptric optical element38. First outer interface region52is arranged on the inner side of boundary50and second outer interface region54is arranged between the boundary50and the outer edge of the catadioptric optical element38.

FIG. 2Dis a schematic diagram illustrating in top view of a hexagonal catadioptric optical element38. Such elements may be used to increase uniformity of output of an array of catadioptric optical elements. The second outer interface region54is arranged between the boundary50and the outer edge of the catadioptric optical element38.

FIG. 2Eis a schematic diagram illustrating in top view of a linear catadioptric optical element38. In comparison toFIGS. 2C and 2D, an elongate optical output may be provided. Advantageously complexity of tooling is reduced and alignment tolerances are relaxed, reducing cost.

Illustrative embodiments will now be provided for the shape of the outer interface46a,46bin the outer interface regions52,54, as illustrated in TABLE 1, where combinations 1 and 2 are illustrative embodiments of the present disclosure.

Shapes of the outer interface46of catadioptric optical elements38will now be described.

FIGS. 3A-3Bare schematic graphs illustrating the profile180of the difference156a,156b,166a,166bbetween the angle of incidence164at the first and second cross-sectional outer interfaces46a,46band critical angle of principal rays306a,306b,304a,304bagainst distance172in the lateral direction (x-axis) from the optical axis711of a catadioptric optical element38for first, second and third combinations of refractive indices respectively.

Features of the arrangements ofFIGS. 3A-Bnot 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.

FIGS. 3A and 3Billustrate that the (mathematical) derivative of the profile180of the difference156,166between the angle of incidence164of each principal light ray306a,306band the critical angle at the first and second outer interfaces46a,46bwith respect to lateral location (i.e. location in the x or y plane) has a discontinuity at the boundary50between the first and second outer interface regions52,54.

Considering again the rays inFIG. 2A, in the embodiments ofFIGS. 3A and 3Bthe difference156a,156bbetween the angle of incidence164of each principal light ray306a,306band the critical angle is a constant across the first outer interface region52and the difference,166a,166bbetween the angle of incidence164of each principal light ray304a,304band the critical angle monotonically increases across the second outer interface region54. Thus, in operation principal light rays306a,306bare incident on the cross-sectional outer interface46ain the first outer interface region42such that the difference between the angle of incidence is substantially the same. In the illustrative example, the difference in the angle of incidence to the critical angle is +2 degrees, that is greater than the critical angle. The difference may be set to provide a tolerance for tooling and replication errors of the catadioptric optical element, as well as providing total internal reflection for rays from the edge of the LED3as well as for the principal rays that are typically from the centre of the LED3.

Thus using known and low cost materials for a solid catadioptric element38, light rays304aare reflected by total internal reflection at the outer interface46aand directed towards the output end708with angle192athat is not parallel to the optical axis711, but provides a fan of output directions192a,192bthat depend on the initial direction of the respective principal ray from the LED3.

At the location of the boundary50between the outer interface regions52,54, the rays are directed parallel to the optical axis711when the difference between the angle of incidence164and the critical angle is 2 degrees. At positions in the above the interface50, the tilt angle174of the surface normal is adjusted to achieve alignment of output principal rays304a,304bthat are parallel to the optical axis711.

Advantageously most of the principal rays are directed in a direction that is parallel to the optical axis by total internal reflection, providing high levels of collimation for light rays reflected from the outer surfaces46a,46b. Light rays that are incident on the region52are directed in directions that are close to but not identical to the collimation direction for known low cost materials in a solid catadioptric optical element38.

Further shapes for the outer interface46will now be described.

FIG. 3Afurther illustrates an alternative shape for the outer interface comprising profile181of difference156in a first outer interface region52and of difference161in a second outer interface region54, with boundary51between the two regions.FIG. 4Aillustrates profile183that describes the shape of the outer interface46, andFIG. 5Aillustrates profile185that describes the variation of tilt angle174with position172.

In comparison to the profile180, the profile181in the first outer interface region52is not constant but increases with distance from the optical axis711. The embodiment of profile180provides total internal reflection for principal light rays from the centre of the LED3, however some light rays from one edge of the LED3may be incident on the surface at angles less than the critical angle. Such light rays are partially transmitted through the cross-sectional outer interface46and contribute to stray light and degraded privacy performance.

Advantageously, the profile181in the first cross-sectional outer interface region52can provide improved illumination output directionality for finite size LEDs3.

Further profiles181,183,185are illustrated as having a different profile in the second cross-sectional outer interface region54for locations outside the boundary51. Reduced collimation may be provided in comparison to the arrangement of profile180. Advantageously a smoother roll-off in display luminance with viewing angle may be provided, increasing display uniformity in a privacy display application as will be described below.

The shape of the outer reflective surface will now be described.

FIGS. 4A-4Bare schematic graphs illustrating the profile182of height of a cross-sectional outer interface46of a catadioptric optical element38against distance172from the optical axis711for first, second and third combinations of refractive indices respectively.FIGS. 4A and 4Billustrate that the height170from the first end707of the first and second outer interfaces46a,46bincreases monotonically between the first and second end707,708of the catadioptric optical element38. In other words, the shape of the surface may increase monotonically without kinks or discontinuities. Features of the arrangements ofFIGS. 4A-Bnot 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 profile of surface tilt174of the surface profiles ofFIGS. 4A-4Bwill now be described.

FIGS. 5A-5Bare schematic graphs illustrating the profile184of cross-sectional outer interface tilt174of a catadioptric optical element38against distance172from the optical axis711for first, second and third combinations of refractive indices respectively.FIGS. 5A and 5Billustrate that the tilt angle174with respect to the optical axis711of the interface normal154of each of the first and second cross-sectional outer interfaces46a,46bincreases monotonically between the first and second end707,708of each catadioptric optical element38.

At boundary50, the rate of change of tilt changes due to the discontinuity illustrates inFIGS. 3A-3B. Thus, the derivative of the surface tilt angle174with respect to distance172from optical axis711has a discontinuity at the boundary50between the respective first and second outer interface regions52,54of the first and second cross-sectional outer interfaces46a,46b.

A smooth surface illustrated inFIGS. 4A-4Bmay be achieved. Advantageously such surfaces may be provided with high accuracy by known tooling and replication methods. Further such a surface may provide low levels of stray light in comparison to discontinuous surface height profiles.

Features of the arrangements ofFIGS. 5A-Bnot 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 respective profiles of a collimating catadioptric element38for operation in air will now be described for purposes of comparison with the present embodiments.

FIGS. 3C, 4C and 5Cillustrate the structure of catadioptric optical elements38in air by way of comparison with the present solid catadioptric optical elements38, that is air is arranged between the outer interfaces of adjacent catadioptric optical elements38a,38b.

As illustrated in TABLE 1, the critical angle in the catadioptric optical element38in air is less than 45 degrees so that light rays that are parallel to the LED substrate in the x-direction may be directed by an inclined surface at 45 degrees by internal reflection in a direction parallel to the optical axis711. Thus, no boundary50between first and second outer interface regions is provided in order to achieve collimated light. Features of the arrangements ofFIGS. 3C, 4C and 5Cnot 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 simulated angular distribution120of luminance from catadioptric optical elements38with the properties described in TABLE 1 and illustrated inFIGS. 3A-3C, 4A-C and5A-C will now be illustrated.

FIGS. 6A-6Care schematic graphs illustrating the profile186of output luminance190with viewing angle192for first, second and third combinations of refractive indices respectively.

As illustrated inFIG. 6C, a narrow luminance distribution of maximum angular output direction of less than 10 degrees may be provided.

FIG. 6Aillustrates schematically profile186of output luminance190with viewing angle192for the profiles180,182,184ofFIGS. 3A, 4A and 5Aalso illustrates schematically profile187of output luminance190with viewing angle192for the profiles181,183,185. Thus the profile187has a broader angular profile close to the optical axis711. Advantageously display luminance roll-off with viewing angle and display uniformity may be improved in a privacy mode of operation. The profile187may further have reduced stray light losses, and thus reduced image cross talk for large sized LEDs3.

By way of comparison the present embodiments have low levels of luminance above 30 degrees for the first combination of refractive indices and low levels of luminance above 40 degrees for the second combination of refractive indices. Such illumination is not as collimated as for elements in air, however as will be described below provide suitable illumination levels for directional display operation while achieving benefits of solid catadioptric optical elements.

FIG. 6Bfurther illustrates region189that is illuminated by light rays that are directed by the refractive inner interface42. As the difference in first and second refractive indices are reduced in comparison toFIG. 6Athen the relative amount of light in region189increases. In region179, illumination is provided by light rays from the outer interface46in the first outer interface region52.

The operation of the present embodiments in a privacy display will now be further described.

FIG. 6Dis a schematic diagram illustrating in top view operation of a privacy display comprising solid catadioptric optical elements comprising first and second cross-sectional outer interface regions. Ray304that is incident on the second interface region54between the boundary50and the second end708of the catadioptric optical elements38is directed in an on-axis direction towards the observer129located in a head-on in directional distribution120. By way of comparison light rays306are directed in of-axis directions towards the edge of the directional distribution120. Snooper131is located outside the directional distribution provided by the display100and thus is not able to perceive an image on the display100with high luminance levels.

Returning toFIG. 6Bthe privacy performance of the display may be dominated by the amount of light in region179. Advantageously, the present embodiments achieve reduced luminance in region179in a solid catadioptric optical element. Features of the arrangements ofFIG. 6Dnot 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.

Advantageously a solid catadioptric optical element38may be arranged to provide privacy display operation with low image visibility for an off-axis snooper.

The operation of the cross-sectional outer interfaces46a,46bwill now be described further using forwards raytracing.

FIG. 7Ais a schematic diagram illustrating in top view propagation of principal light rays in a solid catadioptric optical element that is provided with interfaces of the types illustrated inFIGS. 3A, 4A and 5Awith two cross-sectional outer interface regions52,54. Example output principal rays304,306are illustrated for the profiles180,182,184. Light rays304are directed parallel to the optical axis711by outer interface region54while rays306from outer interface region52is directed towards an off-axis direction. Features of the arrangements ofFIG. 7Anot 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.

Collimation of light is provided by at least some parts of the outer interface46, and other parts provide light rays at small angles to the optical axis711. Advantageously luminance at angles close to snooper131locations are minimised.

FIG. 7Bis a schematic diagram illustrating in top view propagation of principal light rays in a solid catadioptric optical element that is provided with interfaces of the types illustrated inFIGS. 3B, 4B and 5Bwith two cross-sectional outer interface regions52,54and material indices as shown in TABLE 1. Features of the arrangements ofFIG. 7Bnot 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.

Output principal rays304,306for the profiles181,183,185. In the first cross-sectional outer interface region52, principal light rays306are directed at a higher far field angle192than inFIG. 7A. As illustrated inFIG. 6B, a wider field of view is provided than forFIGS. 6A and 7A, however a lower cost material system may advantageously be provided while maintaining a degree of privacy performance. To compensate for the increase in field angle the size of LEDs3may be reduced to achieve similar cone angles of output distribution120. Advantageously desirable privacy performance may be provided.

By way of comparison with the present embodiments, the operation of outer interfaces designed for use in catadioptric optical elements operating in air will now be described.

FIG. 7Cis a schematic diagram illustrating in side view propagation of principal light rays in a catadioptric optical element in air that is provided with interfaces of the shape illustrated inFIGS. 3C, 4C and 5Cfor use in air and without two outer interface regions. All output rays304may be provided parallel to optical axis711arising from the high index step between the material32and air. As illustrated inFIG. 6C, such a design can achieve a narrow distribution120, with size determined by LED3size.

By way of comparison with the present embodiments, the operation of the outer interface46ofFIGS. 3C,4C,5C and 7Cwhen used in a solid catadioptric optical element will now be described.

FIG. 7Dis a schematic diagram illustrating in top view propagation of principal light rays in a solid catadioptric optical element that is provided with interfaces of the shape illustrated inFIGS. 3C, 4C and 5C(that are designed for use in air) and without two outer interface regions. By way of comparison with present embodiments, stray light rays303may be transmitted through the cross-sectional outer interfaces46. Such stray light may undesirably provide image cross talk and high angle illumination. The profile ofFIGS. 3C, 4C and 5Cthus achieve undesirable illumination output.

The operation of the cross-sectional outer interfaces46a,46bwill now be described further using reverse raytracing, that is raytracing from a source at infinity towards the LED3location.

FIG. 7Eis a schematic diagram illustrating in side view propagation of on axis light rays towards the source in a solid catadioptric optical element38with interfaces of the types illustrated inFIGS. 3A, 4A and 5Awith two cross-sectional outer interface regions52,54; andFIG. 7Fis a schematic diagram illustrating in side view propagation of on axis light rays towards the source in a solid catadioptric optical element38with interfaces of the types illustrated inFIGS. 3B, 4B and 5Bwith two cross-sectional outer interface regions52,54.

By way of comparison with the present embodiments,FIG. 7Gis a schematic diagram illustrating propagation of on axis light rays towards the source in a catadioptric optical element operating in air and without two outer interface regions52,54. Features of the arrangements ofFIGS. 7E-Gnot 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 operation of the interfaces can be further represented by examining the paths of rays195for an on-axis light source (not shown) that is directed through the second end708of the catadioptric optical element38. Such reverse raytraces are for illustrative purpose and do not fully describe the propagation of light from the LED3, for example stray light rays191are not present in light directed by the LED3.

FIG. 7Gillustrates that an element in air is capable of collimating light for all principal rays, and first and second cross-sectional outer interface regions are not provided to achieve a suitable illumination for privacy display operation.

By way of comparison in the embodiments ofFIGS. 7E-F, light rays that are in the second outer interface regions54are reflected by total internal reflection towards the LED3. However, light rays incident on the first outer interface region52have angles of incidence that are less than the critical angle and are transmitted through the outer interfaces, thus not reaching the location of the LED3. Light rays from regions52are not directed in a collimated direction substantially parallel to the optical axis by total internal reflection, but are directed in off-axis directions as described above.

Such transmitted light rays illustrate that the solid catadioptric optical elements38do not provide on-axis collimation for principal rays.

The advantages of the solid catadioptric optical elements38with cross-sectional outer regions52,54will now be further described.

As illustrated inFIGS. 6A and 6B, far field luminance profiles may provide illumination structures from solid catadioptric optical elements38and respective aligned LEDs3such that privacy display users129may be provided with high levels of image luminance while off-axis snoopers131at typical angles greater than 45 degrees may have minimal image visibility.

As illustrated inFIGS. 1D-1G, solid catadioptric optical elements38can be used to provide barrier layers to prevent water vapour202and oxygen200ingress and advantageously improve the performance of LEDs, particularly organic LEDs. By way of comparison, non-solid catadioptric optical elements could be provided with oxygen-free gas or a vacuum in the region between the respective cross-sectional outer interfaces; however preserving the oxygen-free gas or vacuum during the lifetime of the device may be expensive and impractical.

Thus cross-sectional outer surface46shapes may be provided for solid catadioptric optical elements38that prevent ingress of moisture or oxygen to the LEDs of the array while achieving privacy display.

The effect of atmospheric pressure changes and thermal variations for non-solid catadioptric optical elements will now be described.

FIG. 8is a schematic diagram illustrating in side view effect of reduced atmospheric pressure on alignment of a non-solid catadioptric optical element array. In environments with reduced atmospheric pressure, gas pressure within the illumination apparatus may provide forces197from pressure differentials that separate the optical components, particularly if the substrates are flexible. In other words the gas present within the illumination apparatus is subject to expansion forces197when atmospheric pressure is reduced, for example when at altitude. Similarly the gas may be compressed, distorting the display the when the display is placed under high pressure for example when placed or operated under water. The illumination apparatus100comprising solid catadioptric optical elements38bonded to the LED backplane of the present disclosure advantageously minimise misalignment from atmospheric pressure changes. Further such apparatus advantageously enables flexible substrates to be used for curved and bendable displays.

Thermal variations for example due to heating from LEDs on the backplane93and material selection for the respective substrates92,93may create differential expansion forces199a,199bbetween the backplane92and optical substrate92, causing misalignment between the two substrates and degrading the quality of the optical output. The bonded solid catadioptric optical elements38of the present embodiments advantageously minimise non-uniformities due to thermal effects.

Applied pressure to the display, for example from user pressing the display, for example a touch screen function, may also undesirably move optical elements in air spaced optical components. Advantageously the present embodiments have reduced sensitivity to applied pressure.

Features of the arrangements ofFIG. 8not 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.

By way of comparison with the present embodiments, a solid catadioptric optical element in which the difference between the angle of incidence and the critical angle is constant across the cross-sectional outer surface will be described.

FIG. 9Ais a schematic graph illustrating the variation of the difference between the angle of incidence and critical angle of principal rays against distance from the optical axis for a first combination of refractive indices respectively for a fixed difference;FIG. 9Bis a schematic diagram illustrating in top view propagation of principal light rays in a solid catadioptric optical element that is provided with interface profile illustrated inFIG. 9AandFIG. 9Cis a schematic graph illustrating the variation of output luminance with viewing angle for the arrangement ofFIG. 9A. The first refractive index combination of TABLE 1 is used for illustrative purposes.

Thus, no discontinuity of the derivative of the difference156with respect to distance from optical axis is provided, that is all principal rays307are incident on the cross-sectional outer surface46at an offset of 2 degrees from the critical angle. In comparison to the embodiments ofFIGS. 6A and 6B, the luminance from the outer interface46in the region179is substantially increased for an observer close to 45 degrees, and thus such a privacy display would have limited privacy performance.

Features of the arrangements ofFIGS. 9A-Cnot 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.

Further structures and operation of illumination apparatuses with solid catadioptric optical elements38will now be described further.

FIG. 10is a schematic diagram illustrating in top view a switchable display apparatus100comprising a solid catadioptric optical element and arranged to switch between narrow and wide directional distributions in a wide-angle mode of operation, as described in GB1705364.6 and incorporated herein in its entirety by reference.

In comparison toFIG. 1A, further LEDs5are arranged on the backplane93in the regions between the second cross-sectional outer interface46bof a first catadioptric optical element38aand the first cross-sectional outer interface46aof a second adjacent catadioptric optical element38b. During a privacy mode of operation, only LEDs3are driven, whereas in the wide-angle mode of operation, LEDs3and LEDs5are illuminated to provide light rays305that are transmitted through the outer interfaces46of the catadioptric optical elements38to directional distributions122so that the image on the display100may be seen by adjacent observers131.

Features of the arrangements ofFIG. 10not 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.

FIG. 11Ais a schematic diagram illustrating in perspective front view a display apparatus comprising an extended array of solid catadioptric optical elements. Such an array may be used to provide a directional distribution with increased angular profile orthogonal to the y-axis in comparison to the directional distribution in the x-axis direction. Such a display may advantageously have wide viewing freedom for rotation about the x-axis direction. Further, tooling and replication of the linear catadioptric optical elements may be more conveniently provided, increasing manufacturing yield and reducing manufacturing cost.

FIGS. 11B-11Care schematic diagrams illustrating in top view a plurality of LEDs for the switchable directional display ofFIG. 10in relation to the position of the interface42of the aligned catadioptric optical elements38. Backplane93may comprise first array of micro-LEDs3and second array of LEDs5. InFIG. 11B, the LEDs5are provided as a single LED between each column of micro-LEDs3. Such elements are arranged to substantially fill the area between the columns of micro-LEDs3.FIG. 11Cillustrates another embodiment wherein the LEDs5are arranged as staggered columns of pixels. Advantageously increased control of output illumination may be achieved.

FIG. 11Dis a schematic diagram illustrating in side perspective view a directional display apparatus comprising a backlight98with a two dimensional array of solid catadioptric optical elements38comprising first and second cross-sectional outer interface regions arranged to illuminate a spatial light modulator300. LEDs3are aligned with the catadioptric optical elements38respectively. Advantageously a flexible backlight for a display such as a liquid crystal display can be made with high tolerance to water vapour and oxygen ingress, thermal expansion and environmental pressure changes.

Features of the arrangements ofFIGS. 11A-Dnot 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.

FIGS. 12A-Dare schematic diagrams illustrating in side views a method to form a plurality of light guides.

In a first step as illustrated inFIG. 12A, a plurality of catadioptric surfaces is formed on a substrate114in a first transparent material32with first refractive index by moulding, UV casting, embossing or other known replication methods from a tool400that is formed with surfaces442,444,446with appropriate shapes including border450between first and second outer regions of surfaces446to form the solid catadioptric optical element38interfaces as described elsewhere herein.

In a second step as illustrated inFIG. 12B, inner interfaces42,44a,44bare formed by filling with transparent material30with a second refractive index, for example by inkjet droplet application.

In a third step as illustrated inFIG. 12C, an outer coating material with material34is applied, for example by means of evaporation, dip coating or other known coating methods. Alternatively, the material34may be arranged to fill the region between the material32.

In a fourth step as illustrated inFIG. 12Da final planarization layer28may be provided to fill the region between the material34.

Features of the arrangements ofFIGS. 12A-Dnot 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 structure and operation of catadioptric optical elements will now be further described.

FIG. 13is a schematic diagram illustrating in perspective view a light source700with first area Ain and first solid angle Qin of light cone702for input into an unspecified optical structure (not shown); andFIG. 14is a schematic diagram illustrating in perspective view the output interface704of area Aout and cone703of solid angle Ωout for output light after light rays from the light source ofFIG. 13has been directed by the optical structure. Conservation of brightness, or étendue, means that
Aout*Ωout<=Ain*Ωin  eqn. 1

FIG. 15is a schematic diagram illustrating in perspective view a catadioptric optical element38with at a first end707a micro-LED3with an input area Ain and input solid angle Ωin in cone706. Second end708of the catadioptric optical element38has area Aout and transmitted light cone710has solid angle Ωout. Equation 1 teaches that Aout is thus greater than Ain, thus in at least one dimension the output width of the catadioptric optical element is greater than the input width to provide the reduction in cone solid angle Ωout. Thus the smaller solid angle of cone710is achieved by increasing the output area Aout of second end708in comparison to the area of the micro-LED3. The catadioptric optical element may be extended; then the width of the micro-LED3may be less than the width of the second end708.

FIG. 15further illustrates the optical axis711of a rotationally symmetric catadioptric optical element38. In this embodiment, the optical axis711is a line along which there is rotational symmetry and is a line passing through centres of curvature of the refractive interface42and outer reflective interface46of the catadioptric optical element38.

In embodiments in which the catadioptric optical element38is arranged to operate on-axis, the output luminance may be arranged to be provided in a direction normal to the output interface, for example normal to the transparent support substrate47. In such embodiments, the optical axis711may be the axis of reflective symmetry of the refractive interface42and outer reflective interfaces46a,46b.

Features of the arrangements ofFIGS. 13-15not 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 arrangement and operation of catadioptric optical elements38will now be further described.

FIG. 16is a schematic diagram illustrating in side view the input width712and output width714of a catadioptric optical element38in at least one cross-sectional plane through its optical axis177. Thus the cross-sectional plane is the x-z plane and the optical axis711is in the cross-sectional plane.

Each of the catadioptric optical elements38of the plurality of catadioptric optical elements comprises, in at least one cross-sectional plane through its optical axis711a first outer interface46aand a second outer interface46bfacing the first outer interface46a. The first and second outer interfaces46a,46bextend from a first end707of the catadioptric optical element38to a second end708of the catadioptric optical element38, the second end708of the catadioptric optical element708facing the first end707of the catadioptric element.

The distance712between the first and second outer interfaces46a,46bat the first end of the catadioptric optical element is less than the distance714between the first and second outer interfaces46a,46bat the second end708of the catadioptric optical element38. At least one transparent inner interface42,44is arranged between the first and second ends707,708and between the first and second outer interfaces46a,46b.

End708may be provided by an output interface of the catadioptric optical element38, or may be for example arranged in a layer of a moulded optical component, for example on transparent support substrate92ofFIG. 1A.

Each of the catadioptric optical elements38of the plurality of catadioptric optical elements is aligned in correspondence with a respective one or more of the LEDs3of the first plurality of LEDs, each of the LEDs of the first plurality of LEDs being aligned with only a respective one of the catadioptric optical elements of the plurality of catadioptric optical elements. The alignment in correspondence between a catadioptric optical element38of the plurality of catadioptric elements and its respective one or more of the LEDs3of the first plurality of LEDs comprising the respective one or more of the LEDs3of the first plurality of LEDs is by being positioned at the first end707of the catadioptric optical element38and aligned with the catadioptric optical element38.

The LEDs3may be positioned at the first end707of the catadioptric optical element38and between the at least one transparent inner interface42,44of the catadioptric optical element38and aligned with the catadioptric optical element. For example, in the cross-sectional plane the centre of the micro-LED3may be aligned with the optical axis711of the catadioptric optical element. In the present disclosure the terminology “at the first end of” the catadioptric optical element includes, for example, the micro-LED being a small amount under the first end707, in the same plane as the end707of the catadioptric optical element38, or in the vicinity of the end707, or in the proximity of the end707or adjacent the end. In each case this may include aligned with the optical axis of the catadioptric optical element. The above description can be applied to all the embodiments.

A catadioptric optical structure uses both reflection and refraction of light. Further, a catadioptric optical structure is one where refraction and reflection are combined in an optical structure, usually via lenses (dioptrics) and curved mirrors (catoptrics). Catadioptric optical elements are sometimes referred to as RXI optical elements. An RXI optical element produces ray deflections by refraction (R), reflection from metals (X), and total internal reflection (I).

The first and second outer interfaces46a,46beach comprise curved interfaces that extend from a first end707of the catadioptric optical element to the second end708of the catadioptric optical element38, the second end708of the catadioptric optical element facing the first end707of the catadioptric optical element38. Further the transparent inner interface42,44comprises at least one curved interface42. The exterior angle715between the first end707and the first outer interface46aat the first end707may be less than the exterior angle717between the first end707and the first outer interface46aat the second end708. Further the exterior angle between the first end707and the second outer interface46bat the first end707is less than the exterior angle between the first end707and the second outer interface46bat the second end708.

Advantageously collimated light may be provided with a directional light output distribution that has a narrow cone angle.

The catadioptric optical element38may be arranged to provide substantially collimated output light from the micro-LED3for light that is incident on the curved outer interfaces46a,46band the at least one of the transparent inner interface44which may have positive optical power. Further at least one of the transparent inner interfaces42,44may have zero optical power. Advantageously interfaces44may be conveniently provided during tooling and moulding steps of manufacture. Further, such interfaces may cooperate to provide collimated light for all light rays from LED3over a high output solid angle, as will be described below with reference toFIG. 19in comparison toFIGS. 4A and 4B.

Thus some of the light output illustrated by ray718of LEDs3of the first plurality of LEDs is transmitted by the at least one transparent inner interface44before it is reflected at the first or second outer interfaces46a,46band directed into the first directional light output distribution120; and some of the light output illustrated by ray716of LEDs3of the first plurality of LEDs is transmitted by the at least one transparent inner interface42and directed into the first directional light output distribution120without reflection at the first or second outer interfaces46a,46b.

FIG. 16further illustrates that a refractive optical element706may be provided between the micro-LEDs3of the first plurality of LEDs and the at least one transparent inner interface42,44. The refractive optical element706may a hemispherical lens that is arranged to achieve increased efficiency of light output coupling from the high index materials that are typically used for inorganic micro-LEDs3. The hemispherical lens706increases the effective area Ain of the source9comprising the LED and hemispherical lens706, so the light from the micro-LED3is distributed over a larger cone angle than would be provided by the micro-LED3alone.

Advantageously, higher efficiency output coupling may be provided.

In at least one cross-sectional plane, the present embodiments provide a reduction in the width of the output directional light output distribution to provide directionality with a directional light output distribution (as described by solid angle Ωout) that is smaller than the input directional light output distribution (as described by solid angle Ωin) by the catadioptric optical element.

Features of the arrangements ofFIG. 16not 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.

It may be desirable to provide an off-axis illumination from the catadioptric optical elements.

By way of comparison known display backlights may use large area edge input light guides and optical films such as BEF from 3M Corporation and rear reflectors. Such backlights may typically have a thickness less than 8 mm, and more typically around 4 mm. The micro-optics of the present embodiments may provide reduced thickness backlights compared to conventional backlights using area light guides. Further direct displays may be provided with low thickness compared to backlit LCDs and similar to the thickness that can be achieved by OLED displays.

FIG. 17is a schematic diagram illustrating in perspective view illumination by a plurality of refractive optical elements740,741providing a background glow744and central spot beams,742,743. Background glow744may be provided by light that propagates outside the refractive optical elements740, and may have a directional light output distribution that is similar to the input light source that may be for example a micro-LED3. The glow744may disadvantageously provide stray light that degrades the quality of illumination, for example increasing background privacy level for unauthorised viewers, and degrading privacy performance. Further additional spot beams743with high luminance may undesirably be provided.

FIG. 18is a schematic diagram illustrating in perspective view illumination by a plurality of reflective optical elements providing an outer halo746and a central spot beam742. In comparison to the arrangement ofFIG. 17, the additional spot beam743may not be present, however undesirably the halo746distributes light over a wider area and degrades background illuminance level, for example reducing privacy performance. The size of the halo746may be reduced by increasing the length749of the reflective optic, however such increases device thickness.

FIG. 19is a schematic diagram illustrating in perspective view illumination by a plurality of catadioptric optical elements providing a central spot beam. In comparison to the arrangements ofFIGS. 17,18, the background glow744or halo746are not present. Advantageously, low stray light can be provided in a thin package.

Features of the arrangements ofFIGS. 17-19not 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.

A method to form an illumination apparatus will now be further described.

FIGS. 20A-Dare schematic diagrams illustrating in perspective views a method to form an illumination apparatus110comprising a plurality of micro-LEDs3and a plurality of catadioptric optical elements38as described in US patent application U.S. Pat. No. 8,985,810 and incorporated by reference herein in its entirety.

As illustrated inFIG. 20A, the monolithic wafer2that may be gallium nitride for example and may be formed on a substrate4that may be sapphire for example.

As illustrated inFIG. 20B, a non-monolithic array of micro-LEDs3may be extracted from the monolithic wafer2to provide micro-LEDs3a,3bwith separation s1.

As illustrated inFIG. 20C, micro-LEDs3a,3bmay be arranged on substrate93in alignment with electrodes and other optical elements (not shown).

As illustrated inFIG. 20D, the substrate93may be aligned with a plurality of catadioptric optical elements38with separations s4to provide an illumination apparatus, such that separation s4may be the same as separation s1. Advantageously large numbers of elements may be formed over large areas using small numbers of extraction steps, while preserving alignment to a respective array of optical elements.

Thus the LEDs may be from a monolithic wafer2arranged in an array with their original monolithic wafer positions and orientations relative to each other preserved; and wherein in at least one direction, for at least one pair of the plurality of LEDs in the at least one direction, for each respective pair there was at least one respective LED in the monolithic wafer that was positioned in the monolithic wafer between the pair of LEDs in the at least one direction and that is not positioned between them in the array of LEDs.

FIG. 20Eis a schematic diagram illustrating in perspective view singulation of an illumination apparatus.FIG. 20Eillustrates that illumination apparatuses with desirable directional light output distribution characteristics can be singulated from large area substrates93,47, for example to provide different size elements600,602or different shape elements604. Further device seal lines601may be provided at the edge of each element to provide hermetic sealing of the optical elements, and reduce dust and other material ingress into the optical elements during use.

Features of the arrangements ofFIGS. 20A-Enot 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 words “substantially” and “approximately”, as may be used in this disclosure provide a tolerance which is accepted in the industry for its corresponding word and/or relativity between items. Such an industry-accepted tolerance ranges from zero to ten percent and corresponds to, but is not limited to, lengths, positions, angles, etc. Such relativity between items ranges between approximately zero to ten percent.

Embodiments of the present disclosure may be used in a variety of optical structures. The embodiment may include or work with a variety of lighting, backlighting, optical components, displays, tablets and smart phones for example. Aspects of the present disclosure may be used with practically any apparatus related to displays, environmental lighting, optical devices, optical systems, or any apparatus that may contain any type of optical system. Accordingly, embodiments of the present disclosure may be employed in displays, environmental lighting, optical systems and/or devices used in a number of consumer professional or industrial environments.

It should be understood that the disclosure is not limited in its application or creation to the details of particular arrangements illustrated, because the disclosure is capable of other embodiments. Moreover, aspects of the disclosure may be set forth in different combinations and arrangements to define embodiments unique in their own right. Also, the terminology used in this disclosure is for the purpose of description and not of limitation.

While embodiments in accordance with the principles that are disclosed herein have been described, it should be understood that they have been presented by way of example only, and not limitation. Therefore, the breadth and scope of this disclosure should not be limited by any of the exemplary embodiments described, but should be defined only in accordance with any claims and their equivalents issuing from this disclosure. In addition, 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.

The section headings herein are included to provide organizational cues. These headings shall not limit or characterise the embodiments set out in any claims that may issue from this disclosure. To take a specific 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 field. Further, a description of technology in the “Background” is not to be construed as an admission that certain technology is prior art to any embodiment in this disclosure. Neither is the “Summary” to be considered as a characterization of the embodiments in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is merely one 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 define the embodiments, and their equivalents, that are protected by them. In all instances, the scope of claims shall be considered on their own merits in the light of this disclosure, and should not be constrained by the headings used in this disclosure.