Hologram manufacturing process

Abstract

Herein is described a method and apparatus for testing and / or manufacturing holograms. In particular, there is provided a method of manufacturing and / or testing holograms on a flat or a substantially flat substrate or on a cylindrical drum the method comprising the steps of: generating a target holographic optical element (HOE), which is based on input geometries and input system parameters; generating digital master printer control files and testing pass / fail criteria; printing a master hologram; testing the master hologram, and if the master hologram passes the testing criteria: generating a first witness replica from the master hologram; testing the replica hologram, and if the replica hologram passes the testing criteria: using the master and / or replica hologram to mass manufacture replica copies onto a curved substrate.

Description

[0001] Hologram Manufacturing process

[0002] Technical Field

[0003] The present invention relates to apparatus and methods for manufacturing and / or testing and / or characterising the performance of holograms. More specifically, the present invention relates to the manufacturing and / or measurement and / or testing of reflective or transmission holograms configured, for example, as either reflective diffusers or as a generic reflective holographic optical element (HOE).

[0004] Background of the Invention

[0005] There exists a need in the art to provide a universal process for high volume manufacturing of holograms such as reflective holograms to be incorporated into curved glass substrates such as automotive windscreens. By unwrapping the curved hologram surface of the final application onto a flat substrate, each point of the hologram can be treated separately. The entire manufacturing process relies upon a point-by-point digital mastering, replication and testing of holograms completed upon flat substrates. The flexibility of this approach allows each new curved holographic design to be produced with the same flat substrate manufacturing toolset.

[0006] There also exists a need in the art to provide a test apparatus to characterise holograms such as reflective holograms. The purpose of the test apparatus described in the present disclosure is to capture all of the relevant diffraction properties of a Transparent Display (TD) product. The test apparatus can be used for process control of reflective hologram manufacturing steps, from digital mastering through to subsequent replica holograms.

[0007] One example application of the Transparent Display (TD) component involves the TD applied to the inner surface or embedded within the glass of a vehicle windscreen, to provide a display in the windscreen itself. The main advantage of such a display is in driver safety improvement; since it can be viewed on the windscreen surface, the driver can see both the display and the outside road view simultaneously without the need to look down at a traditional instrument cluster which distracts the driver from the road view. Another potential area where TDs can be used is in displays for passenger infotainment. An advantage of using TDs in this area is that the passenger experiences less motion sickness. This greatly enhances safety and is an improvement to road safety in general.

[0008] The application of the TD is not limited to vehicle windscreens and could easily be implemented in other situations such as but not limited to traditional building window glazing, as well as other vehicle windows such as side windows and rear windows. Other applications include indicator lights, stop-lights, logo projectors, indeed any situation where a well-defined diffracted angular light beam is required from a hologram.

[0009] The operation of the TD requires a “projector” to create an image on the surface of the TD, the TD re-directs (diffracts) the light from the image into a pre-defined “eye-box” for the driver to view the display image. This eye-box represents an angular space that can be modified by a digital holographic printer approach. This eye-box shape can be designed to be any shape of polygon or elliptical shape as required by the final application.

[0010] The projector light source can be based on either Light Emitting Diodes (LEDs) or Laser sources. In addition, the TD technology is agnostic to the type of core micro-display in the projector such as DLP (digital light processing), LCOS (Liquid crystal on silicon), laser-based MEMs (Micro Electro-Mechanical systems) scanners, TFT (thin-film-transistor), and microLED displays for example.

[0011] By feeding relevant information to the projector video signal, the TD can be used to display diverse content, e.g. cinematic films, video calls, infotainment, driver information (speed, direction, etc.), and so on. The TD therefore combines the benefits of hardware, application, and installation flexibility through the holographic technology, and display flexibility through the use of standard and readily available projection technology.

[0012] A key requirement of the present invention is that there is a need to produce large format displays, bright and uniform in luminance and colour to be acceptable to the user. The requirements necessitate that micro-details of holograms must be understood, which has driven the present applicant to develop a novel tester for measuring performance of our parts.

[0013] The following abbreviations will be used throughout the application:

[0014] AOI, Angle Of Incidence;

[0015] CD, Compact Disk

[0016] DLP, Digital Light Projector;

[0017] DUT, Device under test;

[0018] FWHM, Full Width Half Maximum;

[0019] HOE, Holographic Optical Element;

[0020] HUD, Head Up Display;

[0021] LCOS, Liquid Crystal On Silicon;

[0022] LD, Laser diode; LED, Light Emitting Diode;

[0023] MEMs, Micro Electro-Mechanical systems;

[0024] TD, Transparent display; and

[0025] TFT, Thin Film Transistor liquid crystal display.

[0026] US 7,839,547 B2 (Miura et al.) describes a system for ensuring holographic printed media displays images as expected. This application shows that illumination is done at a predetermined angle, whereas the present application allows for adjustable angles. Furthermore, this patent does not describe analysing diffraction efficiency, nor does it measure angular response.

[0027] US 9,036,153 B1 (Gupta et al.) describes a device and method to measure the reflectivity of materials, and does not consider or discuss holograms. However, the device under test in this patent is mounted on a rotating carriage, and comprises a spherical surface. The integrating sphere in this patent is in reflection, whereas in the present application the sphere is in transmission. Furthermore, the sample scale, scheme and mounting details in this patent are completely different to the present application.

[0028] There is therefore a need to overcome the above-mentioned problems with the art.

[0029] Summary of the Invention

[0030] According to a first aspect there is provided a method of manufacturing and / or testing holograms on a flat or a substantially flat substrate or on a cylindrical drum the method comprising the steps of: generating a target holographic optical element (HOE), which is based on input geometries and input system parameters; generating digital printer control files and testing pass / fail criteria; printing a master hologram; testing the master hologram, and if the master hologram passes the testing criteria: generating a first witness replica from the master hologram; testing the replica hologram, and if the replica hologram passes the testing criteria: using the master and / or replica hologram to mass manufacture replica copies onto a curved substrate. Using the master and / or replica hologram mass manufacture of replica copies onto a curved substrate may occur once or there may be more than one or multiple or a plurality of generations of this process.

[0031] The method may also include generating replicator control files and testing pass / fail criteria.

[0032] The method may also comprise generating control files and testing pass / fail criteria.

[0033] To provide a functional and acceptable HOE in the final windscreen, with high yield it has been found that the following is important: the manufacturing and test systems are completed on flat or substantially flat (or equivalent cylindrical drum) substrates and that the mastering and testing is achieved on a point by point (e.g. pixel by pixel) basis.

[0034] The curvature of the final application is used to create control files of both the mastering, replication and test systems. These control files adjust the recording laser beam angles and tests angles of the processed HOE on a point by point (pixel by pixel) basis.

[0035] The flexibility of the HOE test system is critical to providing performance feedback at each stage of the HOE manufacturing process, supplying pass / fail criteria for each master and replication stage before committing to large volume production. At the beginning of the design process digital equipment control files are created for both the digital printer, replicator and also the tester, controlling the mastering, replication and test angles and wavelengths for both master and copy holograms, including pass / fail criteria controlling diffraction efficiency; diffraction angles; diffraction eye-box sizes and all tailored for flat test substrates.

[0036] The flat or substantially flat substrate may be a flat or substantially flat planar structure.

[0037] The cylindrical drum may be rotatable about a mechanical axis wherein the unexposed photopolymer layer is reeled onto the cylindrical drum, the surface of the drum is effectively flat over the extent of a thin copy laser beam line.

[0038] The curved substrate onto which the copies are mastered and / or replicated may be in a concave or convex form.

[0039] The input geometries may be at least one of or any combination of: windscreen curvature; projector position; transparent display size eye-box position and size; and / or windscreen composition. The hologram may consist of a patterned diffuser in the shape of an image, the image can be viewed in the eye-box when illuminated by a simple LED light source.

[0040] The testing may be done on a point-by-point basis or a pixel-by-pixel basis.

[0041] The substrate curvature of the ultimate replica copies may be used to create at least one or a plurality of control files, which may be used to adjust the recording of beam angles and test angles of the processed HOE.

[0042] The input system parameters may be at least one of or any combination of: display luminance target; projector lumen output; and / or projector light source spectrum.

[0043] The holograms may be either reflective or transmission holograms.

[0044] In the event the master hologram fails the testing criteria, the master hologram may be reprinted and the test is repeated. The test may include feedback into creation of mastering or replication.

[0045] In the event the replica hologram fails the testing criteria, the replica hologram may be regenerated from the master hologram and the test is repeated.

[0046] According to a further aspect there is provided an apparatus for manufacturing and / or testing holograms, the apparatus comprising: a substrate comprising a holographic optical element (HOE) to be tested; a light source for providing an illumination area on the holographic optical element (HOE) to be tested, wherein the light source is located apart from the substrate and is movable relative to the substrate to control the position and angle of incidence of the illumination area on the holographic optical element (HOE) to be tested; collection optics to capture images of the illumination area of the holographic optical element (HOE) to be tested, and to capture the angular extent and / or shape of a diffracted eye-box, wherein the collection optics can be re-positioned to various angular positions to represent a range of final eye-box locations; and an optical integrating device which comprises an inlet aperture wherein the illumination area and a spectrometer are connected to the optical integrating device to measure the transmitted spectrum of the holographic optical element (HOE) to be tested. The angular extent may be defined as the angular size of an object, and in this case, may be the size of the eye-box.

[0047] The inlet aperture may be located adjacent to the illumination area on the holographic optical element (HOE), to allow light into the optical integrating device.

[0048] The light source may be a collimated light source. The light source may be from a laser light source, LED or a halogen bulb. The light source may emit white light, or any other colour or wavelength of light. There is no restriction on wavelength range, i.e. the light source may also be infra-red, or near infra-red. The light source may be any wavelength of light.

[0049] The holographic optical element (HOE) may be a transparent and / or transmissive display or a reflective holographic optical element (HOE).

[0050] The light source may be attached to a multi-axis robotic arm.

[0051] The robotic arm may be a 6-axis robotic arm.

[0052] The collection optics may comprise a detection means which may comprise a plurality of cameras.

[0053] The substrate may be translatable in the horizontal and / or vertical planes to allow the illumination area to move relative to the holographic optical element (HOE) to be tested. Alternatively, the test equipment and light source may translate and the HOE may remain stationary, resulting in the same relative movement between the illumination area and the HOE.

[0054] The illumination area may be an illumination spot.

[0055] The substrate may be a substantially flat glass sheet, which a holographic optical element (HOE) can be laminated onto. Alternatively, the HOE may be an inherent part of the substrate (in the same way music is an inherent part of a CD).

[0056] The collection optics can be re-positioned to various angular positions to represent a range of final eye box locations.

[0057] The optical integrating device may be an optical integrating sphere. In another aspect there is provided a method of manufacturing holograms, the method comprising the steps of: generating a target holographic optical element (HOE), which is based on input geometries and input system parameters; generating digital printer control files and testing pass / fail criteria; printing a master hologram; testing the master hologram, and in the event the master hologram passes the testing criteria: generating replication equipment control files and testing pass / fail criteria; generating a first witness replica from the master hologram; testing the replica hologram, and in the event the replica hologram passes the testing criteria: using the master hologram to mass manufacture replica copies.

[0058] It should be noted that there may be multiple generations of replicas produced by the master hologram in the above method.

[0059] The input geometries may be at least one of or any combination of: windscreen curvature; projector position; transparent display size and / or eye box position and size. The input geometries may be any other parameter associated with the geometry of the device under test. For example, the input geometries may include windscreen composition data, such as type of glass - clear, light green, mid green, dark green etc.; the glass thickness; any glass coatings; additional interlayers.

[0060] The input system parameters may be at least one of or any combination of: display luminance target; projector lumen output; and / or projector light source spectrum. There may be other input system parameters, which would be apparent to the skilled person in the art.

[0061] The holograms may be either reflective and / or transmissive holograms. Any type of hologram may be tested using the described apparatus and related methods.

[0062] In the event the master hologram fails the testing criteria, the master hologram may be reprinted and the test may be repeated. This step may be repeated until a suitable master is created. The test may include feedback into creation of mastering or replication. In the event the replica hologram fails the testing criteria, the replica hologram may be regenerated from the master hologram and the test may be repeated. This step may be repeated until a suitable replica hologram is created. The testing may include feedback into creation of mastering or replication.

[0063] Testing the master hologram may be performed via the apparatus according to the previously described apparatus.

[0064] In another aspect, there is provided a method for manufacturing and / or testing and / or characterising the performance of a holographic optical element (HOE) using the apparatus according to any apparatus previously described, wherein the method comprises: moving the illumination area to unique locations of the hologram to be tested in a point- by-point manner such that the whole hologram is analysed.

[0065] The above method may also be applied to a subset of the full measurement sweep, i.e. to measure a specific region of interest. This may be useful when only a portion of the HOE is to be analysed.

[0066] The hologram may either be either reflective or transmissive.

[0067] The collection optics may be rotated each time the illumination area is moved, to accurately capture images of the holographic optical element (HOE).

[0068] The illumination area may be moved relative to the holographic optical element (HOE) by moving the substrate which comprises the holographic optical element (HOE) in the horizontal and vertical planes. Alternatively, the illumination area may only be moved in the vertical plane, in any direction.

[0069] The present invention relates to reflection holograms configured as either a diffuser element (TD) or as a more generic Holographic element (HOE). The present invention also relates to an apparatus configured to characterise the performance of such holograms.

[0070] The advantages of the test apparatus include simultaneous capture of all relevant TD characteristics for manufacturing process control, including: Red, green and blue diffraction efficiencies and their corresponding peak wavelength; Full Width half maximum (FWHM) spectral bandwidths of the diffracted light; Capture of the eye-box shape and angular distribution of the diffracted light; and Near-field pixel “fill factor” images of the individual colour holograms pixels generated by the digital holographic printer.

[0071] The apparatus may examine one illuminated area (e.g. illuminated spot) on the hologram at a time, with the ability to scan across the surface of the element to build up, for example, a two- dimensional picture of the hologram performance.

[0072] Some final applications require the Transparent Display to be applied to compound curved surfaces. Often manufacturing processes use flat substrates, i.e., the mastering, replication, and testing processes.

[0073] Since testing is achieved by scanning across the hologram surface, it can then provide a local prescription file that accounts for any curvature in the final application. Hence, it is possible to print, replicate and test the hologram all on flat substrates prior to applying the hologram to the final curved surface required by the application.

[0074] The tester may also be reconfigurable to allow testing of a wide range of geometries represented by the projector position and eye-box position relative to the TD component. Flexible robot positioning of an illumination source allows it to represent a projector AOI (Angle of Incidence) ranging from 0 to around 80° relative to the hologram surface normal. In addition, the eye-box collection module optics can also be repositioned to cover a wide variety of system geometry requirements.

[0075] In another aspect, the holographic test apparatus may comprise: a collimated broad band “white” light source generated from a “halogen” bulb for instance;

[0076] The collimated light source may be attached to a 6-axis robot arm allowing complete control of the position and AOI of the illumination spot on the hologram; a reflective holographic element may be under test, either a transparent display (TD) or a reflective HOE; a set of stages to move the hologram under test to enable a point-by-point examination of the hologram may be carried out; a separate set of collection optics and cameras to capture images of the digitally printed holographic pixel and the angular extent and shape of the eye-box diffracted by the holographic element; the collection optics can also be re-positioned to various angular positions to represent the wide range of final application eye-box locations; and / or a combination of an optical integrating sphere and a spectrometer can be used to measure the transmitted spectrum of the hologram. The transmitted spectrum can then be converted into diffracted spectrum present at the eye-box.

[0077] Various novel features in the present invention are discussed below, which may apply to any of the features discussed in the present application.

[0078] The robotic positioning of illumination may allow for 6-axis positioning of collimated light source. This means the robot may be free to move in the three spatial coordinates and three angular coordinates. This allows the system to provide light from any point, at any angle, onto the hologram.

[0079] The robotic positioning may also allow adjustment of the illumination dynamically across the extents of the hologram, i.e. spatially varying illumination during the measurement. As there are 6 axes of freedom on the robotic arm, the device is not constrained to operate in any way, such as in one particular plane, around a spatial axis, at a point, etc.

[0080] For large holograms, it is necessary to ensure that the illumination source retains a set, calibrated, distance from the plane of the DUT (the “source distance”). It is also necessary to ensure that angles are calibrated. The applicant has developed necessary calibration techniques to ensure that the source distance is maintained, and the angles are calibrated, ensuring that the correct illumination for all parts of the hologram is provided.

[0081] The illumination area (e.g. illumination spot) may typically be much smaller than the full hologram. This allows for the whole DUT to be measured in a piecewise fashion, according to a prescription. This is a feature which is not present in the art, but allows the system to scan various sizes of DUTs accurately and efficiently.

[0082] The DUT may be a reflection diffuser hologram for Transparent Display applications. These may be in a very large format. Every holographic pixel is uniquely programmed, to optimally redirect light from projector to the whole eye-box. The hologram is not “attached” to a medium, it is an inherent part of the structure of the photopolymer film, in the same way that music is an inherent part of a CD.

[0083] The DUT is measured as a planar stack up and typically it will be applied / used in a curved geometry. In simple terms, it’s flat when measured, and curved when used. This needs to be accounted for in use. Similarly, it may be a different generation hologram from the final application usage, e.g. measuring a master, when application usage is a replica from a master. Again, this needs accounted for when determining pass / fail criteria for the part.

[0084] The DUT may be a transparent photopolymer, on transparent substrate adhered to transparent carrier (e.g. glass), held in a test frame. Entire DUT may be a rigid, planar stack.

[0085] Reference marks may be provided on the DUT. Alignment of measurements relative to real world application usage are therefore enabled. This can be automated to account for any part- to-part or measurement-to-measurement variation.

[0086] The invention may also measure diffraction efficiency in transmission. This may be done by using an integrating sphere with a large angular extent to gather the transmitted light (i.e.it is widely tolerant to angle of illumination), which means that the transmission detector can be fixed in location. This mechanically simplifies the system. The effect is that a hologram is measured by seeing how much of the transmitted source light is “missing” in transmission due to diffraction. These measurements can be compared to a reference specification for pass / fail criteria.

[0087] Measuring the eye-box (reflection diffracted light), centre angle and angular extent, from a recorded image. This can be compared to a reference specification for pass / fail criteria.

[0088] The near field behaviour of the hologram pixel is reported as a pixel "fill-factor", for all Red / Green / Blue components, in the vertical and / or horizontal directions of the print. This is relevant to the final diffraction efficiencies. This can be compared to a reference specification for pass / fail criteria.

[0089] Other benefits and advantages of the invention are listed below. The present invention’s hologram can be integral within a photopolymer e.g. a Bayfol (registered trade mark) Photopolymer. Illumination can be carried out at any adjustable angle. The present invention can use a single light source. The diffraction efficiency is measured and qualified in the present invention, as is angular response.

[0090] The embodiments described herein will be better understood with reference to the following description. Illustrative examples are appended in the following pages.

[0091] Brief Description of the Drawings

[0092] Embodiments of the present invention will now be described, by way of example only, with reference to the following figures:

[0093] Fig. 1 illustrates an overview of an example system according to an embodiment of the present the invention;

[0094] Fig. 2 illustrates a flowchart of a method according to an embodiment of the present the present invention;

[0095] Fig. 3 illustrates the concept of holographic curvature compensation during the manufacture and testing steps of a method according to an embodiment of the present the present invention;

[0096] Fig. 4 represents an example range of motion and angles of various subsystems contained within the test system according to an embodiment of the present the present invention;

[0097] Fig. 5 illustrates the components of the holographic tester according to an embodiment of the present invention;

[0098] Figs. 6, 7 and 10 illustrate an example hologram dataset gathered from an example testing device according to an embodiment of the present the invention;

[0099] Fig. 8 represents another type of holographic element HOE (603) configured in a Head Up Display (HUD) application that can also be tested by an embodiment of the present the present invention;

[0100] Fig. 9 illustrates another advantage of an embodiment of the present invention;

[0101] Fig. 11 illustrates an example of a prior art method;

[0102] Fig. 12 illustrates the compensation of a non-uniform intensity or colour variation from an image projection unit; Fig. 13 illustrates a large laser beam analogue recording of a reflection hologram directly into an unexposed laser of photopolymer;

[0103] Fig. 14 illustrates an example of a digital printed pixel;

[0104] Fig. 15 describes the overall manufacturing process;

[0105] Fig. 16 illustrates a holographic replication method using a rotating drum; and

[0106] Fig. 17 illustrates the equivalent flat-bed holographic replication process.

[0107] Detailed Description of the Drawings

[0108] The present invention relates to the measurement of the performance of a holographic element during the various stages of its manufacturing process such as digital mastering and replication, using a test system.

[0109] The test system is used to validate each step of the manufacturing process. The manufacturing process flow is shown in Fig. 2 in more detail.

[0110] Fig. 1 represents one of the applications of transparent display (TD) holograms and identifies the various parts of the system. It shows a projection light source (101) illuminating the TD hologram (103) and forming an image (104) on its surface.

[0111] An exemplar ray (102) lands on a unique position on the surface of the TD (103), each position of the TD scatters / diffracts light into a multiple set of rays (107) to form a unique eye-box shape (105) as defined by the specification of the final application (204). The eye-box (105) represents a unique solid angle scattering cone emanating from each position of the TD (103). An observer (106) can view the image (104) if they position their eyes (or at least one eye) within the eye-box (105) solid angle. Moving outside the eye-box (105) of the user (106), the image (104) is not visible.

[0112] Figure 2 shows the manufacturing process of hologram digital printing, replication, and testing at each stage, and indicates that the input geometry (202) of the application and other system performance parameters (203) are required at the start of the process (201) to generate an application specific HOE specification (204). The input geometry (202) may comprise data such as the windscreen curvature, the projector position / location, the TD display size, and the eye-box position and size. From the specification (204) all of the digital printer, replicator and tester machine control files (205) can be generated for a “flat” substrate manufacturing set of processes. Since the digital print process is based on pixel process where each pixel design is unique, then any curvature of the final application design substrate can be compensated for at this stage for printing on a flat substrate.

[0113] The next stage is a pixel-by-pixel printing of a master hologram (213) in which the angles of any recording laser beams are manipulated as the hologram is recorded into a photopolymer material (for instance Covestro’s Bayfol (Registered Trade Mark)).

[0114] The master hologram is then scanned and tested (206) across its surface to validate that it meets the PASS / FAIL criteria (205). Once passed the master hologram (213) is used to produce a witness hologram copy (207) using replication equipment, this test equipment is once again used to validate (209) that the holographic replication equipment is set correctly prior to initiating mass manufacturing volume replication copies (211).

[0115] To reiterate, the test apparatus is critical to the successful manufacture of high-volume holographic components.

[0116] Fig. 3 represents the concept of holographic curvature compensation during the manufacture and testing steps. The advantage of the test system is that it can measure on a flat substrate, on a point-by-point basis both the digitally printed master hologram (213) and any witness replica (207) or subsequent generation of replica.

[0117] Each point, (308) for example, on the hologram has a unique illumination ray (306) from the projector and diffracted ray (307) directed to the centre of the eye-box (105). The surface normal (305) at this point (308) is defined by the curvature of the substrate (304) such as a vehicle windscreen. The illumination AOI (301) and diffracted angle (302) are unique to that point on the TD (308). These angles (301 ,302) are reproduced in the digital printing, replication, and test apparatus but on a flat glass substrate (303) and on a point-by-point across its surface. In effect the curved surface and each pixel surface normal are unwrapped onto a flat substrate for hologram manufacture. Note, that although the diagrams are represented as 2-dimensional in Fig. 3, it will be obvious that the angles can be extended to a 3-dimensional space. Fig.4 represents the range of motion and angles of various subsystems contained within the test system. The holographic element (409) is laminated onto a flat glass substrate (408). In order to scan on a point-by-point basis the glass substrate (408) is mounted into a stage that can move in both horizontal and vertical directions.

[0118] The illuminating light source is attached to a robot such as a multi-axis robotic arm e.g. a 6- axis robotic arm (402) allowing the illumination of a point at the AOI (301) required by the final application. The robot (402) adjusts the AOI as the HOE (409) is scanned across its surface. The final range of motion required in the test system is that of the collection optics (413) that image the light diffracted by the hologram.

[0119] The collection optics (413) have a limited optical aperture and need to be rotated to allow capture of the diffracted beam (307) at its diffraction angle (302). The collection optics comprise a beam splitter (412) which allows the light to be transmitted to camera (415) and reflected to diffracted eye-box via camera (406). The aperture of the collection optics (307) is large enough to capture the entire range of angles represented by a particular eye-box (105) design and generated across the entire surface of the HOE (409) as is scanned on a point- by-point basis.

[0120] In Fig. 5 details of the components of the holographic tester of this application are shown. A point (410) of the hologram (409) is illuminated. The illumination consists of a broad band light source (401) such as a halogen bulb or multi-wavelength LED system. The light is coupled into a flexible multimode fibre optic cable (407) that transports the light to the mounting head of a multi-axis robot arm (402).

[0121] The output aperture of the fibre optic cable is then re-imaged (405) onto the holographic spot under test (410). The light from the light source (401) therefore travels from the fibre optic cable (407) which is in the bottom left of the image, to the top right, where it meets the holographic spot under test (410).

[0122] As the tester substrate (408) is moved to scan across the surface of the hologram (409) the AOI of the illuminating beam (405) is adjusted by the robot arm (402) to follow the projector beam AOI (301) as required by the application specification (204) and tester control files (205).

[0123] Note, that although the diagrams are represented as 2-dimensional in Fig. 5, it will be obvious that the angles can be extended to a 3-dimensional space. A further sub-system of the tester apparatus is represented by the collection optics (413). The collection optics image the digitally printed pixel at the point under test (410) using camera (415) and additionally image the solid angle of the diffracted eye-box via camera (406).

[0124] The final subsystem of the tester is shown as a combination of integrating sphere (402) with input aperture (414), spectrometer (403) and fibre-optic cable (404) providing a sample of the integrating sphere light to the spectrometer (403). The spectrometer system allows the transmission spectrum of the hologram (409) to be measured across the bandwidth of the broad-band light source (401).

[0125] A more detailed description of the data captured by the test apparatus is shown in Fig. 6, Fig. 7, and Fig. 10.

[0126] At each point under test (410) a colour image of the digitally printed pixels (801) is captured by camera (405). A pixel “fill-factor” for each colour of the reflection hologram, typically red, green and blue, is recorded. An objective may be to maximise the holographic pixel “fill-factor” and any pixels below a manufacturing quality pass / fail threshold will be recorded and used to optimise the digital printer set-up.

[0127] The collection optics (413) also record a colour image (802) of the eye-box using camera (406). The eye-box image (802) is calibrated in angle to measure the angular extents and shape of the generated eye-box (105) which is in turn compared to the design pass / fail criteria (205) of the hologram under test.

[0128] Additional test information is available from the spectrometer (403), the light transmitted through the HOE (409) at the point (410) is compared to the spectrum of the illuminating light (405). The spectrum diffracted by the HOE can then be calculated (707) and once again compared to the design pass / fail criteria (205). This diffracted spectra (707) typically consists of multiple spectral peaks, typically red (706), green (705) and blue (704) and are matched to the projector (101) light source spectra of the final application.

[0129] A hologram (409) may only use be designed to produce 1 diffraction peak for a monochrome element, however there may be 2, 3 or more separate peaks in any particular design. The test apparatus determines the peak diffraction efficiency (702), centre wavelength (703) and the full width have maximum (701) of each individual spectral component of the HOE. Fig. 8 represents another type of holographic element HOE (603) configured in a Head Up Display (HUD) application that can also be tested by the present invention. Fig. 8 shows a picture generation unit (PGU) (601) where an image (603) is created at its output surface. In this configuration an image is not formed at the surface of the HOE (603) element, the HOE functions as an optical mirror that both reflects the light from the PGU and creates a magnified virtual image (602) that the observer (106) sees at some distance from the HOE surface. In essence, each point of the HOE (603) diffracts a unique ray from the PGU (601). This differs from the previous TD component (103) description in Fig.1 where each point of the TD scatters a set of rays to form its eye-box (105).

[0130] The invention described in this application can measure and evaluate both types of holographic elements, the TD (103) and HOE (603). The TD is a specific exemplar of a generic HOE.

[0131] It will be apparent to the skilled person that the virtual image (602) set up can be used in various applications, such as in road vehicles. The HOE (603) in this case may be the vehicle windscreen, and therefore the virtual image (602) may be projected into the driver’s field of view appearing in front of the vehicle. This can be highly beneficial from a safety perspective as the driver can see important information without moving their field of view from the road ahead.

[0132] A further advantage of the test system is outlined in Fig. 9. The test system must be flexible enough to measure holograms (103) designs that have a wide range of projector (101) positions and eye-box (105) locations relative to the hologram.

[0133] The extreme AOI (902) of the illumination beam from the projector (102) can range from around 0 deg to 80 deg. Hence the robot arm (402) of the tester must be able to access this range of AOI for the illumination beam (405). In addition, the extreme diffraction angle (901) of the eyebox (105) as created by ray (107), the diffraction angle (901) can also range from 0 deg to around 80 deg. Hence the angular position of the collection optics module (413) must be adjustable as required for each hologram geometric condition provided in the HOE specification (205).

[0134] An example of the prior art recording of a holographic element into a laminated automotive can be found in US20210373492A1 “Laminated Holographic Display and Manufacturing Thereof”. In particular paragraphs 0016 to 0019 and Figure 1 are of note. Figures 1 and 2 have been merged into Figure 11 in this application. This document describes a manufacturing process where unrecorded photopolymer film is arranged between the two glass shells of an automotive windscreen. The subsequent hologram is then recorded using the traditional analogue recording interference of two laser beams. Since the size of the recoding beams are large, covering the full extent of the hologram, there are the following limitations to this approach:

[0135] 1. There will be a natural gaussian intensity drop-off from the centre of the two recording beams that result in a non-uniformity of the recoded hologram diffraction efficiency profile. Manifesting itself in the final application as a roll-off in the luminance of the viewed image towards the edges of the hologram (Figure 13 of the current application). In simpler terms, non-uniform recording beams give non-uniform display products.

[0136] 2. The complete assembly and lamination of the windscreen must be done in dark lighting conditions to avoid pre-exposure of the photopolymer layer. This is difficult to achieve in a windscreen manufacturing site.

[0137] 3. The hologram recording process requires a minimum laser fluence (Joules / cm2) to initiate the fringe recording process. By expanding the laser beams to cover the hologram area, for example 30cm by 15cm, the illuminance (Watts / cm2) naturally drops. As a result, the hologram exposure time will be anywhere from ~10sec to a few minutes to achieve the target laser fluence required. It is extremely difficult to maintain laser fringe stability between the two interfering beams during this period, once again resulting in degraded hologram diffraction efficiencies. Any windscreen manufacturing site would need to have a station where any vibrations are damped to a minimum in an attempt to stabilise the laser fringes during the hologram recording. During manufacture this would be a low yield process resulting in significant increase in cost.

[0138] Compare this to the digital hologram pixel recording used by the applicant where the entire laser beam is concentrated into an area of around 0.25mm square and the illuminance concomitantly higher, the resulting exposure time required to form a pixel hologram drops to below 1 millisecond (see FIGURE 14). The fringe stability and hologram recording are then relatively easy to control when the exposure times are below 1 millisecond. In addition to this the applicant’s approach is to manufacture and yield holograms prior to integration into any windscreen, where only good quality holograms are used for subsequent windscreen lamination. 4. This full HOE footprint hologram recording system requires a custom set of optics to switch from one vehicle / HOE combination design to another. Each new design requires expensive custom large area optics (similar to the size of the HOE) to record the HOE. This is very slow, costly and impractical.

[0139] Some advantages of the system and method according to the present invention are now given.

[0140] The prior art approach above is an attempt to record holograms directly into a windscreen that has a curved surface without any loss in image quality. But as outlined above this is a low- yield and high-cost approach.

[0141] The digital holographic printing, replication and manufacturing process followed by the applicant is a much more robust approach to producing high volume and high yield holograms into curved windscreens.

[0142] The entire manufacturing process is based on recording, replication and testing of holograms using flat glass substrate carriers (Figure 15). The “fixed” and stable hologram is only introduced to the curved windscreen lamination process as a final step. The windscreen lamination factory does not require dark lighting conditions and a vibration free analogue recording set-up since the hologram is already manufactured and provided as a fixed photopolymer layer on a stable and flexible carrier substrate.

[0143] Other benefits of the digital printing process include:

[0144] 1. The whole process is software controlled, each new hologram design for a vehicle uses the same printer toolset, the only item that changes is a printer control file that adjusts the input angles of the two interfering beams recorded at each pixel location of the hologram.

[0145] 2. The maximum size of the hologram is scalable, only limited by the size of the 2D stages travel range used to move the flat glass substrate to the extents of the hologram.

[0146] 3. There are no large area custom optics required which can be costly and take a long time to manufacture. The digital printing process can be quickly reconfigured by adjusting the software and hence pixel printing control file, i.e. a completely digital process from the vehicle design CAD file defining the HOE in the car to the digital printing control file. 4. Most vehicle windscreens produced today consist of a compound curvature which may change across the surface of the windscreen glass. The analogue approach described in the prior art, attempts to compensate for this curvature or freeform shape by direct analogue recording of the HOE into the laminated and unexposed photopolymer within the curved windscreen. The flexible angle control of the digital printing process allows the hologram to be printed on a flat glass substrate, keeping the angle relationship between the two hologram recording beams and the normal of the glass to that required by the final curved windscreen application.

[0147] 5. The diffraction efficiency (or greyscale) of each pixel can be controlled locally, e.g. by adjusting the fluence, either by changing the laser powers at the pixel or by adjusting the pixel exposure time. In this way it is possible to compensate for deficiencies in the final projector used to illuminate the HOE in the vehicle, such as an intensity drop-off or variations in colour red / green / blue across the projected image. Control of the relative red, green and blue HOE diffraction efficiencies at each pixel can be used to compensate for such deficiencies. In this way a final image viewed by the driver can be both uniform in luminance and colour across the HOE surface.

[0148] 6. Multiple HOE masters can be recorded in one sheet of photopolymer film. Each HOE represents a transparent display (TD) that is illuminated with its own independent projector image. Separate areas may also be printed with fixed image symbols and scenes that only require a simple illuminating light source to reveal the image to the driver. The digital printing process allows this mosaic of different element HOEs in the same photopolymer film.

[0149] As mentioned previously, a master hologram printed on a flat glass substrate is then used to produce large numbers of replicated HOEs. An unexposed layer is laminated onto the master either on a flat substrate or a roll-to-roll drum. The master can be replicated in multiple ways. For example, a laser line can be swept across the surface of the laminated (master / fresh photopolymer) to create a replicated copy of the HOE in the fresh film. Since a laser line copy is aligned along the axis and on the crest of any drum copying technique it is equivalent to a flat substrate copy method.

[0150] The key to producing a functional and acceptable HOE in the final windscreen, with high yield relies on: the manufacturing and test systems are completed on flat (or equivalent cylindrical drum) substrates and that the mastering and testing is achieved on a point by point (pixel by pixel) basis. The curvature of the final application is used to create control files of both the mastering, replication and test systems. These control files adjust the recording laser beam angles and tests angles of the processed HOE on a point by point (pixel by pixel) basis.

[0151] The flexibility of the HOE test system is critical to providing performance feedback at each stage of the HOE manufacturing process, supplying pass / fail criteria for each master and copy stage before committing to large volume production. At the beginning of the design process digital control files are created for both the digital printer and also the tester, controlling the test angles and wavelengths for both master and copy holograms, including PASS / FAIL criteria controlling diffraction efficiency; diffraction angles; diffraction eye-box sizes and all tailored for flat test substrates.

[0152] A simplified description of the manufacturing process using the holographic digital mastering and replication equipment is shown in Figure 14 and Figure 15. In these figures the angle of incidence of the two recording beams (1405 & 1406) are shown as unchanging throughout the manufacturing processes at a particular pixel location (1402). However, it is well known in the holography field that these angles can change during the manufacturing process and depend upon parameters such as: the laser wavelengths used to master and replicate the hologram. the peak wavelength of the final copy hologram created which is expected to match the source wavelengths of the projection unit creating the primary image on the holographic surface. any shrinkage of the photopolymer laser during holographic recording.

[0153] The key item is that the angles of incidence 1406 from the projector and into the diffracted eye-box 1405 match the design requirements when integrated into the application curved windscreen. These values are unique for each printed pixel location on the hologram. In addition, the central peak of the diffracted spectrum must coincide with the projector primary wavelengths, be they LED, laser or some other light source.

[0154] Fig. 11 illustrates an example of a prior art method of direct laser holographic recording of a HOE display element into a previously unexposed photopolymer layer that is laminated into a windscreen.

[0155] Fig. 12 illustrates the compensation of a non-uniform intensity or colour variation 1201 from an image projection unit 1202. The diffraction efficiency of the multiplexed red, green and blue holograms 1203 can be adjusted at each pixel location 1402. The resulting improved display luminance uniformity 1204 viewed by a driver is a combination of the projector intensity and compensating hologram efficiency.

[0156] Fig. 13 illustrates a large laser beam analogue recording of a reflection hologram directly into an unexposed laser of photopolymer 1302. The photopolymer 1302 is laminated between two glass shells of an automotive windscreen 1301 and 1303. The large variation of the two laser beam intensities across the surface of the hologram element are represented by 1304 and 1305 highlighting the difficulty in achieving a uniform diffraction efficiency across the large area of the hologram.

[0157] Fig. 14 illustrates an example of a digital printed pixel 1402. The size of the pixel 1402 may range anywhere from 0.1mm to 10mm in diameter. The unexposed photopolymer layer 1302 is laminated onto a flat glass substrate 1401. The two recording laser beams (1404 and 1403) are concentrated into the small pixel area 1402, their corresponding angle of incidences (1405 and 1406) are controlled by the printer opto-mechanics to match the required application design.

[0158] As the printing process pixel moves across the surface of the HOE the intensity AND / OR the exposure time of the two recording beams 1404,1403 can be adjusted to compensate for any HOE or final projector imperfections, i.e. adjusting the local diffraction efficiency of any multiplexed red, green and blue holograms.

[0159] Fig. 15 describes the overall manufacturing process according to an example of the present invention.

[0160] Step 1 (1501): Design of the hologram: Each pixel 1502 across the surface of the curved hologram surface 1503 is designed and digital printing, replication and testing control files are created for their respective equipment . The angle of incidence at each pixel is determined by the beam from the projector 1504 and the angle to the eye-box 1505 relative to the local windscreen normal direction 1508.

[0161] Step 2 (1507): The original curved surface 1503 is unwrapped onto a flat surface 1506 ready for the digital printing process. At each pixel location the design AOI (1405, 1406) is maintained relative to the surface normal. Step 3 (1509): The master hologram is now tested with the HOE test system. The tester checks the two AOI’s (1405, 1406) as well as the diffraction efficiency spectral profile and peak efficiency across the surface of the hologram, on a pixel by pixel basis 1502. Once again this is performed on flat glass substrate 1506.

[0162] Step 4 (1510): The digitally printed master hologram is used to generate multiple copies using raw unexposed photopolymer film laminated onto the master HOE surface. Once again the replication is achieved using a flat or drum surface 1506.

[0163] Step 5 (1511): The performance of the replicated HOE is now evaluated using the HOE test system, once again using a flat substrate 1506.

[0164] Step 6 (1512): The final step is to laminate the already exposed HOE replica between the curved glass shells (1301 , 1303) of the windscreen, assuming the curved profile of the original design 1503.

[0165] Fig. 16 illustrates a holographic replication method using a rotating drum 1601. This allows creation of multiple copies of the master hologram using a roll-to-roll process. The digitally printed master hologram generated 1602 using process 1507 is wrapped around the outside of the drum 1601.

[0166] A fresh layer of unexposed holographic photopolymer material 1603 is then laminated on top of this master hologram 1602. A linear laser line 1604 is generated using an input laser beam 1608 and a scanning mirror system 1605 and illuminates both the master 1602 and unexposed photopolymer layer 1603. The master hologram 1602 diffracts the input laser line 1604 generating the beam 1606 that creates the eye-box 1607. A copy of the master hologram is then recorded by the two interfering laser beams 1604 & 1606 into the photopolymer layer 1603.

[0167] An important point to emphasis is that the illuminating laser line 1604 is substantially parallel to the mechanical axis 1609 of the drum. In this way the line illumination of the master hologram 1603 and copy 1603 is equivalent to a flatbed copy process (Figure 17), i.e. the laser scan line appears to be illuminating a flat substrate.

[0168] Fig. 17 illustrates the equivalent flat-bed holographic replication process. Both the master 1602 and replicated 1603 holograms are laminated onto a flat substrate 1701. Once again, the laser scan line 1604 traverses across the hologram stack by linear translation of the flat substrate 1701 and is equivalent to the rotating drum 1601 replication process.

[0169] Any of the features in any of the embodiments may be combined in any manner with any of the embodiments shown.

[0170] Whilst specific embodiments of the present invention have been described above, it will be appreciated that departures from the described embodiments may still fall within the scope of the present invention.

Claims

CLAIMS1 . A method of manufacturing and / or testing holograms on a flat or a substantially flat substrate or on a cylindrical drum the method comprising the steps of: generating a target holographic optical element (HOE), which is based on input geometries and input system parameters; generating digital printer control files and testing pass / fail criteria; printing a master hologram; testing the master hologram, and if the master hologram passes the testing criteria: generating a first witness replica from the master hologram; testing the replica hologram, and if the replica hologram passes the testing criteria: using the master and / or replica hologram to mass manufacture replica copies onto a curved substrate.

2. The method according to claim 1 , wherein the method also comprises generating replicator control files and testing pass / fail criteria.

3. The method according to claim 1 , wherein the flat or substantially flat substrate is a flat or substantially flat planar structure.

4. The method according to claim 1 , wherein the cylindrical drum is rotatable about a mechanical axis wherein the substrate is reeled onto the cylindrical drum.

5. The method according to any preceding, wherein the curved substrate onto which the copies are mastered and / or replicated is in a concave or convex form.

6. The method according to any preceding claim, wherein the input geometries are at least one of or any combination of: windscreen curvature; projector position; transparent display size eye-box position and size; and / or windscreen composition.

7. The method according to any preceding claim, wherein the testing is done on a point- by-point basis or a pixel-by-pixel basis.

8. The method according to any preceding claims, wherein the curvature of the replica copies of the substrate is used to create at least one or a plurality of control files, which is used to adjust the recording of beam angles and test angles of the processed HOE.

9. The method according to any preceding claim, wherein the input system parameters are at least one of or any combination of: display luminance target; projector lumen output; and / or projector light source spectrum.

10. The method according to any preceding claim, wherein the holograms are either reflection or transmission holograms.

11. The method according to any preceding claim, wherein in the event the master hologram fails the testing criteria, the master hologram is re-printed and the test is repeated.

12. The method according to any preceding claim, wherein in the event the replica hologram fails the testing criteria, the replica hologram is regenerated from the master hologram and the test is repeated.

13. An apparatus for manufacturing and / or testing holograms, the apparatus comprising: a substrate comprising a holographic optical element (HOE) to be tested; a light source for providing an illumination area on the holographic optical element (HOE) to be tested, wherein the light source is located apart from the substrate and is movable relative to the substrate to control the position and angle of incidence of the illumination area on the holographic optical element (HOE) to be tested; collection optics to capture images of the illumination area of the holographic optical element (HOE) to be tested, and to capture the angular extent and / or shape of a diffracted eye-box, wherein the collection optics can be re-positioned to various angular positions to represent a range of final eye-box locations; and an optical integrating device which comprises an inlet aperture wherein the illumination area and a spectrometer are connected to the optical integrating device to measure the transmitted spectrum of the holographic optical element (HOE) to be tested.

14. An apparatus according to claims 13, wherein the substrate is a flat or a substantially flat substrate or a cylindrical drum.

15. The apparatus of claims 13 or 14, wherein light source is a collimated light source.

16. The apparatus of any of claims 13 to 15, wherein the holographic optical element (HOE) is a transparent and / or transmissive display or a reflective holographic optical element (HOE).

17. The apparatus of any of claims 12 to 15, wherein the light source is attached to a multi-axis positioning system such as a multi-axis robotic arm.

18. The apparatus of claim 17, wherein the robotic arm is a 6-axis multi-axis positioning system such as a 6-axis robotic arm.

19. The apparatus according to any of claims 13 to 18, wherein the collection optics comprise a plurality of detection means such as cameras.

20. The apparatus of any of claims 13 to 19, wherein the substrate is translatable in the vertical plane to allow the illumination area (e.g. illuminated spot) to move relative to the holographic optical element (HOE) to be tested.

21. The apparatus of any of claims 13 to 20, wherein the substrate is a flat or substantially flat glass sheet or a sheet reeled on a cylindrical drum, onto which a holographic optical element (HOE) can be laminated.

22. The apparatus of any of claims 13 to 21 , wherein the collection optics can be repositioned to various angular positions to represent a range of final eye-box locations.

23. The apparatus of any of claims 13 to 22, wherein the optical integrating device is an optical integrating sphere.

24. A method for characterising the performance of a holographic optical element (HOE) using the apparatus according to any of claims 13 to 23, wherein the method comprises: moving the illumination area (e.g. illumination spot) to unique locations of the hologram to be tested in a point-by-point manner such that the whole hologram is analysed.

25. A method according to claim 23, wherein the hologram is either reflective or transmissive.

26. A method according to claim 24, wherein the collection optics are rotated each time the illumination area is moved, to accurately capture images of the holographic optical element (HOE).

27. A method according to claims 23 to 25, wherein the illumination area is moved relative to the holographic optical element (HOE) by moving the substrate which comprises the holographic optical element (HOE) in the horizontal and / or vertical planes.

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