Facet-resolved inspection of sliceable block intermediates for lightguide manufacture

Abstract

An intermediate work product (stack or slice) block (30, 50) with mutually parallel, partially reflective internal surfaces (32) is inspected using multichromatic, substantially collimated illumination from an illumination source (60) that injects a beam (6) along an injection geometry that traverses the surfaces (32) at angles within or near coating design ranges. Partial reflections from the surfaces form surface-specific deflected beams (7) that exit an output surface (51). Coupling prisms (52, 53) preferably provide orthogonal external faces for entry and exit of the beams. An imaging sensor (61) with optics (8) images an image sampling plane (66) to generate a color-quantifying output image set. A processing system (80) segments facet-indexed regions and assesses one or more uniformity conditions, including color and optionally luminance, and outputs a manufacturing decision, such as accepting / rejecting the product or feedforward parameter updates. A variant for verifying parallelism of the internal surfaces is also provided.

Description

[0001] Facet-Resolved Inspection of Sliceable Block Intermediates for Lightguide Manufacture

[0002] FIELD AND BACKGROUND OF THE INVENTION

[0003] The present invention relates to optical metrology for manufacturing optical lightguide assemblies. More particularly, it concerns inspection of an intermediate work product comprising a block of transparent material with mutually parallel, partially reflective internal surfaces, using multichromatic, substantially collimated illumination and imaging to evaluate inter-facet color and luminance characteristics for manufacturing control.

[0004] Optical lightguide assemblies used in near-eye displays and other compact imaging systems often implement two-dimensional aperture expansion using a set of internal, mutually parallel, partially reflective surfaces. These surfaces are typically realized by multilayer dielectric coatings engineered to provide partial reflection over a defined range of incident angles. Production is a multi-stage process typically including bonding coated plates to form a stack, slicing the stack along an interface plane to generate a precursor block, integrating the precursor block with other precursor components to form a precursor assembly, and slicing the precursor assembly along parallel planes to generate multiple compound lightguides ready for integration into an end product.

[0005] Achieving consistent image quality requires tight control of both color and luminance behavior associated with each internal surface. Variations in coating deposition, bonding conditions, or surface figure can introduce facet-to-facet differences that manifest as color balance non-uniformity and intensity non-uniformity in the end product. End-of-line measurements on a finished device can assess intra-image uniformity across a displayed field, but it would be very valuable to detect defects at an early stage of manufacture, and preferably before integration of a defective subcomponent precursor with other non-defective subcomponents, thereby minimizing wastage of materials and process time.

[0006] SUMMARY OF THE INVENTION

[0007] The present invention is a method and system for inspection of an intermediate work product.

[0008] According to the teachings of an embodiment of the present invention there is provided, a method for inspecting an intermediate work product comprising a block of transparent material containing a set of mutually-parallel internal surfaces each having a coating configured to provide partial reflection within a range of incident angles, the block being sliceable along parallel planes to generate a plurality of lightguide subcomponents each containing a part of each of the internal surfaces, the method comprising: (a) optically coupling to the block a multi-chromatic illumination arrangement configured to generate, sequentially or simultaneously, light of a plurality of colors in a substantially collimated beam directed so as to traverse a plurality of the internal surfaces at an incident angle within the range of incident angles for at least one of the internal surfaces, the beam being partially deflected by partial reflection at each of the internal surfaces to generate a surface- specific deflected beam propagating in a deflected direction so as to exit the block from an output surface; (b) employing an imaging sensor deployed to sample an image of a plane that traverses the surface-specific deflected beams from the plurality of the internal surfaces to generate a color-quantifying output image set including one or more images selected from the group consisting of: a colorimetric image, and a set of images including an image for each color of the plurality of colors; (c) identifying a plurality of regions in the color-quantifying output image set, each of the regions corresponding to the surface- specific deflected beam for a corresponding one of the internal surfaces; (d) assessing whether the plurality of regions satisfy at least one uniformity condition selected from a color-uniformity condition and a luminance-uniformity condition; and (e) outputting at least one manufacturing decision contingent on whether the at least one uniformity condition is satisfied, the manufacturing decision being selected from the group consisting of: accepting or rejecting the intermediate work product; modifying a production parameter of a manufacturing process for producing a lightguide product from the intermediate work product; and modifying a production parameter for a subsequent intermediate work product.

[0009] According to a further feature of an embodiment of the present invention, the output surface of the block is oblique to the deflected direction, the method further comprising deploying a wedge output prism with a first surface of the wedge output prism in facing relation to the output surface of the block and a second surface of the wedge output prism orthogonal to the deflected direction.

[0010] According to a further feature of an embodiment of the present invention, an incident angle at which the deflected beams reach the output surface of the block is greater than a critical angle at a block-air interface, the method further comprising introducing an optical liquid between the first surface of the wedge output prism and the output surface of the block.

[0011] According to a further feature of an embodiment of the present invention, the illumination arrangement is optically coupled to the block via a wedge input prism having a first input prism surface deployed in facing relation to an input surface of the block and a second input prism surface orthogonal to the substantially collimated beam output from the multi-chromatic illumination arrangement.

[0012] According to a further feature of an embodiment of the present invention, a wedge angle of the wedge input prism is such that the collimated beam reaches the first input prism surface at an angle of incidence greater than a critical angle at a prism-air interface, the method further comprising introducing an optical liquid between the first input prism surface and the input surface of the block. According to a further feature of an embodiment of the present invention, the output surface of the block is parallel to the internal surfaces of the block.

[0013] According to a further feature of an embodiment of the present invention, the output surface of the block is an interface plane that is subsequently bonded to a second intermediate work product prior to slicing to form multiple lightguides.

[0014] According to a further feature of an embodiment of the present invention, a width of the substantially collimated beam from the illumination arrangement is sufficiently small to prevent overlap of the plurality of regions.

[0015] According to a further feature of an embodiment of the present invention, the block is displaced relative to the illumination arrangement and the imaging sensor so as to sample images at multiple positions along the internal surfaces.

[0016] According to a further feature of an embodiment of the present invention, facet parallelism is verified by: (a) reconfiguring the imaging sensor to be focused at infinity; (b) scanning a mask across an output plane so that illumination reaching the imaging sensor derives from partial reflection at a currently selected internal surface; and (c) monitoring for variation in a spot position at the imaging sensor.

[0017] There is also provided according to an embodiment of the present invention, an inspection apparatus for an intermediate work product comprising a block of transparent material containing a set of mutually-parallel internal surfaces each having a coating configured to provide partial reflection within a range of incident angles, the block being sliceable along parallel planes to generate a plurality of lightguide subcomponents each containing a part of each of the internal surfaces, the apparatus comprising: (a) a multi-chromatic illumination arrangement configured to generate, sequentially or simultaneously, light of a plurality of colors in a substantially collimated beam and to inject the beam into the block along an injection geometry directed so as to traverse a plurality of the internal surfaces at an incident angle within the range of incident angles for at least one of the internal surfaces, the beam being, when the apparatus is operated, partially deflected by partial reflection at each of the internal surfaces to generate a plurality of surfacespecific deflected beams propagating in respective deflected directions so as to exit the block from an output surface; (b) an imaging sensor deployed to sample an image of a plane that traverses the surface- specific deflected beams and configured to generate a color-quantifying output image set including one or more images selected from the group consisting of: a colorimetric image, and a set of images including an image for each color of the plurality of colors; and (c) a processing module configured to: (i) identify in the color-quantifying output image set a plurality of regions each corresponding to a surface-specific deflected beam, (ii) assess whether the plurality of regions satisfy at least one uniformity condition selected from a color-uniformity condition and a luminance-uniformity condition, and (iii) output decision data indicative of at least one manufacturing decision selected from accepting or rejecting the intermediate work product, modifying a production parameter of a manufacturing process for producing a lightguide product from the intermediate work product, and modifying a production parameter for a subsequent intermediate work product.

[0018] According to a further feature of an embodiment of the present invention, there is also provided a wedge output prism having a first surface in facing relation to the output surface of the block and a second surface orthogonal to a corresponding one of the deflected directions.

[0019] According to a further feature of an embodiment of the present invention, an incident angle at which the deflected beams reach the output surface of the block is greater than a critical angle at a block-air interface, the apparatus further comprising an optical liquid deployed between the first surface of the wedge output prism and the output surface of the block.

[0020] According to a further feature of an embodiment of the present invention, there is also provided a wedge input prism having a first input prism surface in facing relation to an input surface of the block and a second input prism surface orthogonal to the substantially collimated beam output from the multi-chromatic illumination arrangement so as to avoid chromatic aberration associated with non-orthogonal injection.

[0021] According to a further feature of an embodiment of the present invention, a wedge angle of the wedge input prism is such that the collimated beam reaches the first input prism surface at an angle of incidence greater than a critical angle at a prism-air interface, the wherein an optical liquid is deployed between the first input prism surface and the input surface of the block.

[0022] According to a further feature of an embodiment of the present invention, the output surface of the block is parallel to the internal surfaces of the block.

[0023] According to a further feature of an embodiment of the present invention, the output surface of the block is an interface plane configured to be subsequently bonded to a second intermediate work product prior to slicing to form multiple lightguides.

[0024] According to a further feature of an embodiment of the present invention, a beam- shaping element configured to set a width of the substantially collimated beam sufficiently small to prevent overlap of the plurality of regions on the imaging sensor.

[0025] According to a further feature of an embodiment of the present invention, there is also provided a positioning stage configured to displace the block relative to the illumination arrangement and the imaging sensor so as to enable sampling of images at multiple positions along the internal surfaces.

[0026] According to a further feature of an embodiment of the present invention, there is also provided: a mask deployed at an output plane traversed by the surface- specific deflected beams; a scanning mechanism configured to scan the mask so that illumination reaching the imaging sensor derives from partial reflection at a currently selected internal surface; and a focus control configured to set the imaging sensor to focus at infinity, the processing module being further configured to monitor for variation in a spot position at the imaging sensor.

[0027] BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

[0029] FIGS. 1 A and IB are schematic isometric views of an optical system including two displays each implemented using a lightguide optical element (LOE), constructed and operative according to the teachings of an embodiment of the present invention, illustrating a top-down and a sideinjection configuration, respectively.

[0030] FIG. 2 is a schematic isometric view of the LOE showing an image-ray path propagating by internal reflection, reflecting at an internal partially reflective surface (or “facet”) in a first region, passing an interface between regions, and entering a second region for out-coupling.

[0031] FIGS. 3A-3D are schematic isometric views illustrating stages in an example production sequence related to the lightguide of FIG. 2. FIG. 3A shows a bonded stack. FIG. 3B shows a modified stack and a dashed outline of a slice to be cut. FIG. 3C shows a slice. FIG. 3D shows the slice combined with second-region precursors for subsequent slicing to yield multiple compound lightguides.

[0032] FIG. 4 is a schematic isometric view of a stack with an overhanging top plate to facilitate stack-stage inspection configuration.

[0033] FIG. 5 is a schematic side view of a stack-stage inspection configuration, constructed and operative according to an embodiment of the present invention, for inspecting the stack of FIG. 4, the configuration including an illumination arrangement, coupling elements, and an imaging sensor.

[0034] FIG. 6 is a schematic side view of a slice-stage inspection configuration, constructed and operative according to an embodiment of the present invention, for inspecting an obliquely sliced intermediate work product, the configuration including an illumination arrangement, coupling elements, and an imaging sensor.

[0035] FIG. 7 is a schematic side view of a slice-stage inspection configuration, constructed and operative according to a further aspect of the present invention, the inspection configuration being generally similar to FIG. 6 but modified for inspection of parallelism between internal surfaces of the slice.

[0036] FIG. 8 is a block diagram showing modules of a processing system from the inspection configurations of FIGS. 5-7. FIG. 9 is a flow diagram of a method for inspection of an intermediate work product according to an aspect of the present invention.

[0037] DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] The present application provides methods and systems for inspection of an intermediate work product at one or more stages during manufacture of a lightguide optical element, typically for use in a near-eye display.

[0039] According to certain embodiments, an intermediate work product in the form of a sliceable block of transparent material includes a set of mutually parallel, partially reflective internal surfaces. A multichromatic illumination arrangement generates, sequentially or simultaneously, a substantially collimated beam directed along a selected injection geometry so as to traverse multiple internal surfaces at an incident angle within a coating design range for at least one surface and within a tolerance of that range for the remainder. Each internal surface partially reflects the traversing beam to form a corresponding surface- specific deflected beam that exits via an output surface.

[0040] An imaging sensor samples an image of a plane intersected by the surface-specific deflected beams and generates a color-quantifying output image set, which may be a single colorimetric image or a sequential set of monochrome images acquired under different spectral illuminations. From the image set, facet-indexed regions are identified, and one or more uniformity conditions, selected from color-uniformity and luminance-uniformity conditions, are assessed across the regions. Based on the assessment, a manufacturing decision is generated, including accepting or rejecting the intermediate work product and / or computing feedforward updates to one or more production parameters for further processing of this intermediate work product and / or for production of subsequent intermediates. These outputs may be provided via a human-machine interface (HMI) for implementation by an operator or, in certain implementations, may be directly implemented by automated processes.

[0041] The same inspection concept applies at different manufacturing stages. In a bonded, unsliced stack, access is via a large-area surface and the illumination and return paths to distal internal surfaces traverse multiple intervening surfaces. In an obliquely sliced work product, the geometry and output plane correspond more closely to the usage scenario in the final lightguide assembly. The inspection method and system of the present invention may be used to advantage at either of these stages, or at both.

[0042] Definitions of Terms

[0043] It will be useful to define various terms as they are used throughout this document and in the appended claims. • Intermediate work product / block: a bonded optical structure that contains a plurality of mutually parallel, partially reflective internal surfaces and is sliceable along parallel planes to generate multiple subcomponents of a final work product. Subcategories of intermediate work products addressed in this document are: o Stack: a bonded optical structure that contains a plurality of mutually parallel, partially reflective internal surfaces resulting directly from bonding multiple parallel-faced plates one above the other, prior to any cutting process. o Slice: a component derived by cutting a stack along at least one plane oblique to the internal surfaces and intersecting at least some of those internal surfaces along a plane that will correspond to an interface (boundary) of the subcomponents derived from the slice in a final lightguide assembly. The stack is typically cut along additional planes to generate the slice so as to remove additional material not required in the final product.

[0044] • Internal surfaces; partially reflective internal surfaces: interfaces within the block that provide partial reflection over a coating- specific operational design range of incident angles, thereby deflecting incident light within the block.

[0045] • Surface-specific deflected beam: a beam resulting from partial reflection at a corresponding internal surface, propagating along a deflected direction toward an output surface.

[0046] • Multichromatic illumination arrangement: a source or combination of sources providing two or more spectral bands, applied sequentially or simultaneously, delivering a substantially collimated beam along a selected injection geometry. The illumination polarization may be set to emulate the polarization expected at the internal surfaces in end use.

[0047] • Substantially collimated beam: a beam of light which is collimated so that the rays within the beam are parallel, or is sufficiently close to collimated over the length of the light path through the inspection system to allow differentiation of deflected beams from different internal surfaces as described herein.

[0048] • Injection geometry: a deployment of an illumination arrangement, any in-coupling prism and surfaces of the intermediate work product block that determine the light path through the block, and hence an angle of incidence of the illumination beam at the internal surfaces. The injection geometry preferably ensures that the beam crosses an entrance surface that is orthogonal to the beam direction to mitigate chromatic aberration. In certain cases, a desired injection geometry may require introduction of an optical liquid that is index matched or sufficiently reduces an index mismatch at a prism-block interface where the desired beam direction would otherwise result in total internal reflection at an air interface.

[0049] • Color- quantifying output image set: one or more images that, taken together, provide color-quantifying information. This includes a single image from an imaging colorimeter or a set of monochrome images acquired under spectrally distinct illuminations.

[0050] • Region: a segmented portion of the image set corresponding to a surface- specific deflected beam for a particular internal surface.

[0051] • Uniformity condition; color-uniformity condition; luminance-uniformity condition: a criterion applied to at least one metric computed by statistical methods across the set of regions indicative of a degree of uniformity or lack thereof in the color balance of multichromatic illumination deflected by reflection by different internal surfaces and / or in the intensity of the deflected beams. The criterion may be a binary pass / fail quality control (QC) condition, or may be a criterion which generates a qualitative or quantitative indication of a production defect. In the latter case, the criterion is preferably used to derive an actionable output defining modification of one or more process parameters which may modify a production parameter of a subsequent manufacturing process for producing a lightguide product from the intermediate work product and / or may modify a production parameter to correct the defect in subsequently produced intermediate work products. Color differences may be expressed in device-independent spaces, for example Au'v' or AE_{00}. Luminance statistics may include percentile spreads, coefficient of variation, or normalized min-max ranges. Metrics may be computed per region or via co-processing across multiple regions without deriving distinct scalar values per region.

[0052] • Output surface: a surface of the intermediate work product block from which the surfacespecific deflected beams exit the block. In a slice- stage configuration, the output surface corresponds to the interface through which the image light exits the first lightguide region in the final product.

[0053] • Image sampling plane: a focal plane of the imaging sensor crossing, and preferably perpendicular to, the surface-specific deflected beams.

[0054] • Feedforward: computation and application of updated production parameters, for example coating or bonding parameters, to subsequent intermediate work products based on inspection results.

[0055] To provide an illustrative but non-limiting context to better understand the present invention, FIGS. 1A and IB illustrate a near-eye display apparatus 10 that includes a lightguide optical element 12 having opposed major surfaces 12a, 12b that guide image light by total internal reflection. An image projector assembly 14 is optically coupled to the lightguide so as to introduce image light corresponding to a collimated image that propagates within the lightguide in a first guided propagation direction by internal reflection at the major surfaces. A support structure 20 positions the lightguide relative to the user’s head so that a viewer’s eye 100 is located within an eye-motion box 102 defined spatially in relation to the lightguide. In the examples of FIGS. 1A and IB, the optical system includes two displays, each implemented using a lightguide optical element (LOE), illustrating a top-down configuration and a side-injection configuration, respectively.

[0056] In one non-limiting set of implementations, the light injected into LOE 12 by image projector assembly 14 impinges on a set of partially reflecting surfaces that are parallel to each other and inclined obliquely to a direction of propagation of the image light. Each successive surface deflects a proportion of the image light into a deflected direction that remains guided by internal reflection within the substrate. These surfaces, which are not illustrated individually in FIGS. 1A and IB, are located in a first region 16. This partial reflection at successive surfaces progressively redirects the direction of guided propagation while expanding the optical aperture in a first dimension. The redirected image illumination then passes into a second region 18, which may be implemented as an adjacent distinct substrate or as a continuation of a single substrate, in which a coupling-out arrangement, either a further set of partially reflective surfaces or a diffractive optical element, progressively couples out a proportion of the image illumination toward an eye 100 located within an eye-motion box 102, thereby achieving a second dimension of optical aperture expansion.

[0057] The overall device may be implemented separately for each eye, and is preferably supported relative to the head of a user with each LOE 12 facing a corresponding eye 100. A support arrangement 20 may be implemented as an eyeglass frame with sides for supporting the device relative to the user’s ears. Other forms of support arrangement may also be used, including head bands, visors, or devices suspended from helmets.

[0058] Reference is made to an X axis which extends in the general extensional direction of the first region of the LOE, and to a Y axis which extends perpendicular thereto. In FIG. 1A the X axis is horizontal and the Y axis is vertical. In FIG. IB the X axis is vertical and the Y axis is horizontal. In approximate terms, the first region 16 may be considered to achieve aperture expansion in the X direction while the second region 18 achieves aperture expansion in the Y direction. For simplicity of presentation in subsequent drawings, the invention will be exemplified in the context of an LOE with orientation similar to FIG. IB, with the understanding that the same principles apply to aperture-expansion lightguides with other orientations or designs. The image injected into the lightguides is preferably a collimated image, in which light of each image pixel is a parallel beam, collimated to infinity, with an angular direction corresponding to pixel position so that the image illumination spans an angular field of view in two dimensions. The near-eye display 10 may include additional components such as a controller 22 for actuating image projector 14, powered by an onboard battery or another suitable power source. The projector technology and coupling approach are not limiting, and additional options are described later in the detailed description.

[0059] FIG. 2 is a schematic illustration of LOE 12 corresponding to the side-injection arrangement of FIG. IB illustrating a typical ray path 31 followed by image light. Image light from projector 14 enters the LOE 12, is guided by internal reflection between major surfaces 12a, 12b, and impinges on an internal partially reflective surface in the first region 16 (illustrated as 26) which progressively redirects the guided image light from a first guided propagation direction to a second guided propagation direction. The redirected image light passes through an interface 25 and into the second region 18, where an out-coupling arrangement, for example reflective out- coupling surfaces 28 or a diffractive out-coupler, directs the image light toward the viewer’ s eye 100 located within an eye motion box (EMB) designated by rectangle 102. The polarization state of the image light at various locations along the light path is typically controlled by the projector design and / or may be modified by polarization modifying components located, for example, at the coupling-in region of the lightguide. In certain cases, an optical element may be deployed at interface 25, for example, to modify a polarization of light passing from the first region to the second region.

[0060] A preferred but non-limiting exemplary production sequence for the LOE 12 is illustrated in FIGS. 3A-3D.

[0061] FIG. 3A shows a bonded stack 30 formed by assembling and bonding parallel-faced plates to create an intermediate work product with mutually parallel, partially reflective internal surfaces 32. These plates are pre-coated with appropriate coatings, typically multi-layer dielectric coatings, according to coating design parameters that produce the required partially reflective properties for each internal surface and for the polarization state and range of incident angles with which that internal surface is designed to be used in the final product. The coatings may be provided on one face of each plate used to form the stack or, in certain cases, on both sides of alternate plates. In the latter case, the double-sided coated plates are formed into a stack by bonding with alternate uncoated plates.

[0062] FIG. 3B shows a modified stack including an oblique side surface formed by cutting the bonded stack and, in certain implementations, bonding a clear block 34 to that oblique surface to provide a facet-free subregion, where such a subregion is desired in the final product. A dashed rectangle 36 on a side surface delineates the outline of a slice to be cut from the modified stack.

[0063] FIG. 3C shows a slice 50 cut from the modified stack according to the profile indicated by 36 in FIG. 3B. The slice provides an output surface 51 that corresponds to interface 25 between the subcomponents of the final product.

[0064] FIG. 3D shows slice 50 in the process of being attached to bonded precursors of a second lightguide region 57 to assemble a compound precursor block assembly 58. Subsequent slicing of the combined precursor block assembly along parallel planes yields multiple compound lightguides that are ready for profiling of their exterior contour and for integration into a display product, such as product 10 of FIGS. 1 A or IB.

[0065] The inspection system and method of the present invention may advantageously be implemented at various stages of the production process, and in particular, when applied to the bonded-stack stage of FIG. 3A and / or at the slice stage of FIG. 3C. In each case, inspection at a relatively early stage of the production process, prior to integration with other intermediate work products and prior to final slicing and polishing steps, allows early identification of manufacturing defects, with consequent savings of materials, machine hours and man hours that would otherwise be wasted on subsequent production steps for a defective product, and allows accelerated optimization of the early processing steps of the production process.

[0066] Turning now to a first implementation of the present invention, FIG. 4 is a schematic isometric view of a stack 30 with an overhanging top plate 38 configured to facilitate a stack-stage inspection configuration. The overhang provides physical clearance for coupling elements positioned adjacent to stack 30 during inspection, as described with reference to FIG. 5.

[0067] FIG. 5 is a schematic side view of a stack-stage inspection configuration, constructed and operative according to an embodiment of the present invention, for inspecting stack 30. A multichromatic illumination arrangement 60 generates, sequentially or simultaneously, light of a plurality of colors in a substantially collimated beam 6. The beam is injected into stack 30 via an input coupling prism 52 having an external surface 52a orthogonal to the incident beam 6 to mitigate chromatic aberration associated with non-orthogonal injection. The injection geometry is selected so that the beam traverses a plurality of the internal partially reflective surfaces 32 at an incident angle within a coating design range for at least one surface and within a tolerance of that range for the remainder. Where an inspection parameter set would otherwise result in total internal reflection at an air interface between an output surface 52b of prism 52 and the surface of top plate 38, an optical liquid may be introduced at the prism-to-stack interface to provide full or partial index matching so as to avoid TIR conditions. Beam 6 is partially reflected at each internal surface 32 to generate a corresponding surface- specific deflected beam 7 that exits stack 30 via an output surface of the stack, which in this case is again the upper surface of top plate 38, which is parallel to the internal surfaces. Surface- specific deflected beams 7 are coupled through an output coupling prism 53 whose external surface 53a is preferably orthogonal to the deflected direction. Here too, the surface 53b of the output coupling prism 53 that is in facing relation to the output plane 38 may employ an optical liquid to avoid TIR conditions.

[0068] An image sampling plane 66 is located to intersect the surface-specific deflected beams, and an imaging sensor 61 includes a focusing arrangement, illustrated schematically as a lens 8, that focuses an image of image sampling plane 66 onto the sensor. Imaging sensor 61 is configured to generate a color-quantifying output image set. In preferred implementations, the illumination polarization is set to emulate the polarization expected at the internal surfaces in end use, and for which the partially reflecting coatings are designed. In certain preferred implementations, a positioning stage 70 supports displacement of stack 30 relative to illumination arrangement 60 and imaging sensor 61 for sampling at multiple positions along the internal surfaces in the “Z direction” (see FIG. 4). The Z direction is typically roughly perpendicular to the slicing planes which will subsequently be used to subdivide the intermediate work product into portions of separate lightguides. Scanning along this direction may therefore useful to ensure uniformity between regions of the block which will end us in different end products. A processing system 80 with a controller module preferably operates the illumination arrangement 60, actuates the positioning stage 70, acquires the image set from the imaging sensor 61, identifies facet-indexed regions, computes one or more uniformity metrics, and generates the manufacturing decision and feedforward parameter updates, all discussed further below.

[0069] FIG. 6 is a schematic side view of a slice-stage inspection configuration, constructed and operative according to an embodiment of the present invention, for inspecting an obliquely sliced intermediate work product such as slice 50 of FIG. 3C. Inspection at this stage of the production sequence is conceptually similar to the stack inspection described above but allows the use of light paths that are more closely analogous to the scenario in which the partially reflecting internal surfaces are used in the final product.

[0070] Here too, a multichromatic illumination arrangement 60 generates, sequentially or simultaneously, light of a plurality of colors in a substantially collimated beam 6. Beam 6 is injected into slice 50 via an input coupling prism 52 having an external surface 52a orthogonal to the incident beam 6 to mitigate chromatic aberration associated with non-orthogonal injection. Inspection at the “slice” stage typically provides better accessibility for injecting the illumination beam, for example, from an end of the slice, thereby typically avoiding the need for special steps to avoid TIR at an interface between an output surface 52b of prism 52 and a facing surface of slice 50. In some cases, an edge surface of slice 50 may be oriented so that prism 52 is unnecessary. In other cases, an edge surface of slice 50 may be used for internal reflection of the injection geometry, and an input coupling prism may be deployed to provide a correctly oriented (orthogonal) external surface for injection of the illumination beam from below the slice (in the orientation illustrated here).

[0071] The injection geometry is selected so that beam 6 traverses a plurality, and preferably all, of internal partially reflective surfaces 32 at an incident angle within a coating design range for at least one surface and within a tolerance of that range for the remainder. Beam 6 is partially reflected at each internal surface 32 to generate a corresponding surface- specific deflected beam 7 that exits slice 50 via output surface 51. Because output surface 51 corresponds to the interface through which image light exits the first lightguide region in the final product, the inspection ray paths provide a good approximation to the usage scenario of this subcomponent, including the impact of any adjacent internal surfaces encountered prior to exit. Surface-specific deflected beams 7 are coupled through an output coupling prism 53 whose external surface 53a is preferably orthogonal to deflected beams 7. In order to present an external surface 53a orthogonal to beams 7, the wedge angle of prism 53, i.e., the angle between external surface 53a and the slice-facing surface 53b, should correspond to the incident angle at which the deflected beams 7 reach output surface 51. In this geometry, that incident angle (and the prism wedge angle) is preferably smaller than the critical angle for the slice material in air such that TIR conditions are not encountered. This avoids the need for application of any index matching liquid at the interface, simplifying the inspection procedure and subsequent clean-up of the block. Input coupling prism 52 and output coupling prism 53 are preferably made of optically clear material compatible with the block or slice (i.e., with similar refractive indices) to limit chromatic effects.

[0072] An imaging sensor 61 is configured and focused (optics represented schematically by lens 8) to sample an image of an image sampling plane 66 located to intersect the surface-specific deflected beams 7 and generate a color-quantifying output image set. In certain preferred implementations, the illumination polarization is set to emulate the polarization state of image light expected to interact with internal surfaces 32 according to the end product design. A positioning stage 70 may support displacement of slice 50 relative to illumination arrangement 60 and imaging sensor 61 to enable sampling at multiple positions along the internal surfaces. A processing system 80 with a controller module may operate illumination arrangement 60, actuate positioning stage 70, acquire the image set from imaging sensor 61, identify facet-indexed regions, compute one or more uniformity metrics, and generates the manufacturing decision and feedforward parameter updates, as described elsewhere herein.

[0073] FIG. 7 is a schematic side view of a slice-stage inspection configuration, constructed and operative according to a further aspect of the present invention, generally similar to FIG. 6 but modified to facilitate inspection of parallelism between internal surfaces 32 of slice 50. In this configuration, an output-plane mask 67 is configured to allow transmission of only a selected surface- specific deflected beam 7 and is scanned across the deflected beams by an actuator 68 so that light reaching imaging sensor 61 derives only from partial reflection at a currently selected internal surface 32. The optics (schematically represented by lens 8) of imaging sensor 61 is reconfigured to be focused to infinity so that all collimated rays reflected from a set of parallel internal surfaces are focused to a single spot within measurement tolerance at the imaging sensor focal plane array. Any displacement of this spot from the common focal point (or of its subpixel center of mass where it falls on multiple pixels) is an indication of non-parallelism of an internal surface. During the scan, processing system 80 monitors spot position at imaging sensor 61 as a function of the selected internal surface. Substantial invariance of spot position for the different internal surfaces indicates that the internal surfaces are parallel to each other within a predefined tolerance. The remaining elements of the configuration, including the illumination arrangement 60, input coupling prism 52 and output coupling prism 53, are as described with reference to FIG. 6.

[0074] FIG. 8 is a block diagram of a processing system 80 from the inspection configurations of FIGS. 5-7. Processing system 80 includes a controller module 81 configured to operate any and all of: illumination arrangement 60, positioning stage 70 and stage actuator, mask actuator 68, and focus-mode switching of imaging sensor 61 for infinity-focus operation. Processing system 80 may also include a preprocessing module 82 to perform preprocessing on sampled images, for example, performing registration across sequential images sampled with different color illumination and / or combining them into a desired image format for subsequent processing. A region identification or segmentation module 83 is typically provided to identify regions in the image set corresponding to the surface- specific deflected beams for respective internal surfaces. This segmentation may be based on predetermined geometrical determination of the light paths or on image processing. Processing system 80 also includes a metric computation module 84 to compute color-uniformity and, where implemented, luminance-uniformity statistics, according to any suitable absolute or relative measure of each, such as those discussed below. A decision logic module 85 is configured to assess one or more uniformity conditions, and hence to determine a manufacturing decision, and / or compute feedforward parameter updates for further processing of a product based on this intermediate work product and / or for production of subsequent intermediate work products. Preferably, a data store 86 records inspection results, and a feedforward or MES interface 87 outputs parameter updates to manufacturing equipment. A human-machine interface (HMI) 88 preferably supports system operation, parameter entry and review of inspection results. A communications I / O 89 preferably provides external connectivity to support functionality of the aforementioned modules.

[0075] FIG. 9 is a flow diagram of a method for inspection of an intermediate work product according to an aspect of the present invention. The flow includes: 90 receive intermediate work product block; 92 optically couple a multichromatic illumination arrangement to the block; 94 direct a substantially collimated beam through the internal surfaces along a selected injection geometry; 98 sample an image of the surface- specific deflected beams to generate a colorquantifying output image set; 102 identify, in the image set, a region corresponding to a surfacespecific deflected beam for each internal surface; 106 assess whether the regions satisfy one or more uniformity conditions selected from a color-uniformity condition and a luminanceuniformity condition; and 110 output at least one manufacturing decision contingent on whether the one or more uniformity conditions are satisfied, including accepting or rejecting the intermediate work product and modifying one or more production parameters for subsequent intermediate work products.

[0076] Processing system 80 and its various modules may be implemented as an embedded controller, an industrial PC, or a distributed system. Controller module 81 generates synchronized triggers for 60, 61, 68, and the stage actuator of 70, and selects sensor focus mode for the FIG. 7 configuration.

[0077] The illumination polarization is preferably set to emulate the polarization state delivered to the internal surfaces in end use. The multichromatic illumination arrangement 60 may provide multiple spectral bands sequentially or simultaneously. When sequential, the color-quantifying output image set comprises multiple monochrome images, in which case imaging sensor 61 may advantageously be implemented as a monochrome imaging sensor. Although a sequential color illumination implementation may complicate determining absolute color for any given internal surface, such an implementation is nevertheless effective for determining non-uniformity of color response between surfaces, which is typically the factor that is important for evaluation of the intermediate work product. If the color performance is uniform, any required global white balance correction can be performed elsewhere in the system, such as by digital color correction in the projected image. When simultaneous illumination by a multichromatic source is used, imaging sensor is preferably an imaging colorimeter that directly generates an “image set” containing a single colorimetric image for further processing. In certain cases, a color camera may provide a lower-cost implementation of the inspection system sufficient for certain applications.

[0078] Each multilayer coating is typically optimized over a design range of incident angles that may differ among internal surfaces. In order to simultaneously inspect and compare the color performance of multiple internal surfaces, a single beam at the same incident angle to each internal surface is preferably used. In some cases, the design range of incident angles for all surfaces overlap such that a suitable inspection beam angle can be found lying within the design range for all of the internal surfaces. Where this is not the case, an incident angle should be chosen that is sufficiently close to the design range for all surfaces that it is likely to be representative of the relevant properties. Practically, a single compromise injection geometry is typically selected so that, internally, the beam is within the coating design range for at least one internal surface and within a tolerance of that range for others. In cases where the ranges of angles differ widely, such as for very wide fields of view, the inspection may be performed for a subset of the internal surfaces with a first incident beam angle and for a second set after rearrangement so as to provide a second incident beam angle. This may require swapping the coupling prisms. Preferably, some of the internal surfaces are included in both subsets to allow normalization of the metrics between the two subsets, to be recorded in data store 86 together with the recipe and angle settings, and thus assessment of overall performance that is not limited to each subset.

[0079] In slice- stage inspection, the output surface of the slice preferably corresponds to the interface through which image light exits the first lightguide region in the final product (interface 25 in FIG. 2). The inspection ray paths hence approximate the end-use scenario, including interactions with any adjacent internal surfaces encountered prior to exit. External faces of coupling prisms 52 and 53 are preferably orthogonal to the respective incident and deflected directions to mitigate chromatic aberration introduced by non-orthogonal refraction.

[0080] Beam width is preferably chosen as a balance between two considerations. Narrower beams simplify segmentation by reducing and preferably eliminating overlap between regions at the image sampling plane 66. On the other hand, broader beams sample larger areas of each internal surface. Slight overlap between adjacent regions can be tolerated and excluded during segmentation by rejecting a locally elevated-intensity strip at the overlap boundary.

[0081] Regions in the color-quantifying output image set are determined to correspond to surfacespecific deflected beams from respective internal surfaces, either by determining the geometrical region corresponding to each facet- specific deflected beam, by image processing of the sampled images, or by any other suitable calibration process. A short calibration step may be used to initialize a mapping between internal surfaces and regions at the image sampling plane 66 prior to performing measurements, after which the mapping is maintained by geometry or periodic verification, or may be determined during the processing of each inspection image.

[0082] Assessment of uniformity is carried out across the set of facet-indexed regions. Color differences may be computed in device-independent color spaces, for example Au'v' or AE_{00}, referenced to a mean, median, or designated reference region. In some cases, non-uniformity of color response may be assessed using a measure of covariance or correlation between the different color components across the relevant regions without determining absolute color values. Luminance statistics may include coefficient of variation, percentile spreads, or normalized min- max ranges. Robust estimators may be used. Metrics may be computed per region and aggregated, or co-processed across multiple regions without deriving distinct scalar representatives per region. The disclosure is not limited to these examples.

[0083] For the configuration of FIG. 7, an output-plane mask 67 is scanned by actuator 68 so that illumination reaching sensor 61 derives from a selected internal surface. Output-plane mask 67 defines an aperture that selects light predominately from one surface- specific deflected beam at a time. Actuator 68 scans mask 67 across the locus of the deflected beams, optionally at the region corresponding to image sampling plane 66 in FIG. 6, under control of controller module 81. Scan position, dwell time, and synchronization with sensor 61 are recorded to associate each measurement with the corresponding internal surface. Imaging sensor 61 is set to infinity-focus mode, either by switching one or more component of the optics 8 or by focus adjustment. Substantial invariance of spot position across internal surfaces indicates that the internal surfaces are parallel within a tolerance.

[0084] Positioning stage 70 may provide translation along the direction of the internal surfaces to sample multiple positions, with optional fine adjustment orthogonal to that direction to maintain alignment with the injection geometry. Stage motion may be manual or motorized, and is coordinated by controller module 81 to hold position during image acquisition. Position feedback, where provided, may be recorded in data store 86 with inspection results for traceability.

[0085] In certain implementations, the beam may be injected through a sliced surface of the slice and directed by internal reflection at an inclined end surface to achieve a desired internal path. This option provides geometric flexibility while maintaining orthogonal external faces where appropriate.

[0086] As mentioned, in various implementations, illumination arrangement 60 provides multichromatic light either simultaneously or sequentially. A number of options may be chosen for implementing the illumination arrangement. In implementations intended to support objective colorimetry, illumination arrangement 60 may comprise one or more of: a broadband “white” source such as a halogen or xenon arc lamp with a stabilized power supply, a white LED module, or a laser-pumped phosphor source. Spectral band selection may be provided by interchangeable or wheel-mounted interference filters, an acousto-optic tunable filter, or a liquid-crystal tunable filter. Where objective colorimetry is desired, the spectral power distribution of illumination arrangement 60 is preferably characterized against a reference and, when used with an imaging colorimeter as 61, the combined source-sensor system is profiled to yield device-independent color values at image sampling plane 66. While objective colorimetry may provide useful information, effective inspection of color uniformity to ensure acceptable display output quality may be achieved using light that emulates the end-use display conditions. In this case, illumination arrangement 60 may comprise RGB (or multi-primary) sources substantially matching the spectral bands of the product projector (for example, discrete RGB LEDs, RGB laser diodes with speckle mitigation, or LED+filter combinations). Bands may be driven simultaneously to form a composite multichromatic beam, or sequentially to acquire a color-quantifying image set as multiple monochrome frames. When sequential operation is used, imaging sensor 61 may be a monochrome sensor; when simultaneous operation is used, 61 may be an imaging colorimeter or a color camera. Where exact spectral matching is not practical, the relative weights of the primaries may be adjusted so that measured color differences between regions correlate with those expected under the product spectra.

[0087] To emulate the polarization delivered to the partially reflective internal surfaces in end use, illumination arrangement 60 may include a linear polarizer and, where circular or elliptical states are desired, a quarter-wave plate. Polarization is selected according to the coating design of the internal surfaces and the end-use optical train.

[0088] For sources driven by pulse- width modulation or frame-sequential primaries, illumination arrangement 60 is synchronized to the exposure timing of 61 via the controller module to ensure repeatable integration per frame. Source intensity is preferably stabilized and monitored.

[0089] Where laser or narrowband sources are employed, speckle may be reduced by one or more of: rotating or vibrating diffusers, multimode fiber agitation, or path diversity. Spatial uniformity may be improved with an integrating sphere, light pipe, or microlens-array homogenizer, before beam collimation.

[0090] The illumination arrangement 60 is configured to generate a collimated or substantially collimated beam directed along the selected injection geometry. Collimation may be provided by standard optical arrangement, chosen according to the properties of the light source used, all as is known in the art. Examples include, but are not limited to: a fiber-collimator pair, a Kohler-type condenser with field stop, and a telecentric collimator lens assembly. In one implementation, light from illumination arrangement 60 is injected into a single-mode or multimode fiber and delivered to a fiber collimator that sets the beam diameter and divergence. In another implementation, a lamp or LED array is imaged onto the aperture of a collimating lens to produce a spatially uniform, substantially collimated output.

[0091] Beam width is defined to balance region separation at the sensor and sampling area per internal surface. Suitable implementations include:

[0092] • an adjustable circular iris placed at a pupil conjugate of the collimating optics,

[0093] • a rectangular or slit aperture to form a stripe or rectangular beam footprint, • a telescope / beam expander to set diameter followed by an aperture stop to clip edges,

[0094] • a top-hat beam shaper or light-pipe homogenizer to flatten the spatial profile.

[0095] As explained above, a narrower beam simplifies segmentation by reducing or eliminating overlap between facet-indexed regions at image sampling plane 66, while a broader beam increases the sampled area per internal surface. Slight overlap is tolerable and may be excluded during segmentation by rejecting elevated-intensity overlap zones.

[0096] It will be appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the scope of the present invention as defined in the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method for inspecting an intermediate work product comprising a block of transparent material containing a set of mutually-parallel internal surfaces each having a coating configured to provide partial reflection within a range of incident angles, the block being sliceable along parallel planes to generate a plurality of lightguide subcomponents each containing a part of each of said internal surfaces, the method comprising:(a) optically coupling to said block a multi-chromatic illumination arrangement configured to generate, sequentially or simultaneously, light of a plurality of colors in a substantially collimated beam directed so as to traverse a plurality of said internal surfaces at an incident angle within the range of incident angles for at least one of said internal surfaces, the beam being partially deflected by partial reflection at each of said internal surfaces to generate a surface- specific deflected beam propagating in a deflected direction so as to exit said block from an output surface;(b) employing an imaging sensor deployed to sample an image of a plane that traverses the surface-specific deflected beams from said plurality of said internal surfaces to generate a color-quantifying output image set including one or more images selected from the group consisting of: a colorimetric image, and a set of images including an image for each color of the plurality of colors;(c) identifying a plurality of regions in said color-quantifying output image set, each of said regions corresponding to the surface-specific deflected beam for a corresponding one of said internal surfaces;(d) assessing whether said plurality of regions satisfy at least one uniformity condition selected from a color-uniformity condition and a luminance-uniformity condition; and(e) outputting at least one manufacturing decision contingent on whether said at least one uniformity condition is satisfied, the manufacturing decision being selected from the group consisting of: accepting or rejecting the intermediate work product; modifying a production parameter of a manufacturing process for producing a lightguide product from the intermediate work product; and modifying a production parameter for a subsequent intermediate work product.

2. The method of claim 1, wherein said output surface of said block is oblique to the deflected direction, the method further comprising deploying a wedge output prism with a first surface of said wedge output prism in facing relation to said output surface of said block and a second surface of said wedge output prism orthogonal to the deflected direction.

3. The method of claim 2, wherein an incident angle at which said deflected beams reach said output surface of said block is greater than a critical angle at a block-air interface, the method further comprising introducing an optical liquid between said first surface of said wedge output prism and said output surface of said block.

4. The method of claim 2, wherein said illumination arrangement is optically coupled to said block via a wedge input prism having a first input prism surface deployed in facing relation to an input surface of said block and a second input prism surface orthogonal to the substantially collimated beam output from the multi-chromatic illumination arrangement.

5. The method of claim 4, wherein a wedge angle of said wedge input prism is such that said collimated beam reaches said first input prism surface at an angle of incidence greater than a critical angle at a prism-air interface, the method further comprising introducing an optical liquid between said first input prism surface and said input surface of said block.

6. The method of claim 2, wherein said output surface of said block is parallel to the internal surfaces of said block.

7. The method of claim 2, wherein said output surface of said block is an interface plane that is subsequently bonded to a second intermediate work product prior to slicing to form multiple lightguides.

8. The method of claim 1, wherein a width of the substantially collimated beam from said illumination arrangement is sufficiently small to prevent overlap of said plurality of regions.

9. The method of claim 1, further comprising displacing said block relative to said illumination arrangement and said imaging sensor so as to sample images at multiple positions along the internal surfaces.

10. The method of claim 1, further comprising verifying facet parallelism by:(a) reconfiguring said imaging sensor to be focused at infinity;(b) scanning a mask across an output plane so that illumination reaching said imaging sensor derives from partial reflection at a currently selected internal surface; and(c) monitoring for variation in a spot position at said imaging sensor.

11. An inspection apparatus for an intermediate work product comprising a block of transparent material containing a set of mutually-parallel internal surfaces each having a coating configured to provide partial reflection within a range of incident angles, the block being sliceable along parallel planes to generate a plurality of lightguide subcomponents each containing a part of each of said internal surfaces, the apparatus comprising:(a) a multi-chromatic illumination arrangement configured to generate, sequentially or simultaneously, light of a plurality of colors in a substantially collimated beam and to inject the beam into the block along an injection geometry directed so as to traverse a plurality of said internal surfaces at an incident angle within said range of incident angles for at least one of said internal surfaces, the beam being, when the apparatus is operated, partially deflected by partial reflection at each of said internal surfaces to generate a plurality of surface-specific deflected beams propagating in respective deflected directions so as to exit the block from an output surface;(b) an imaging sensor deployed to sample an image of a plane that traverses the surface- specific deflected beams and configured to generate a color-quantifying output image set including one or more images selected from the group consisting of: a colorimetric image, and a set of images including an image for each color of the plurality of colors; and(c) a processing module configured to:(i) identify in the color-quantifying output image set a plurality of regions each corresponding to a surface- specific deflected beam,(ii) assess whether said plurality of regions satisfy at least one uniformity condition selected from a color-uniformity condition and a luminance- uniformity condition, and(iii) output decision data indicative of at least one manufacturing decision selected from accepting or rejecting the intermediate work product, modifying a production parameter of a manufacturing process for producing a lightguide product from the intermediate work product, and modifying a production parameter for a subsequent intermediate work product.

12. The apparatus of claim 11, further comprising a wedge output prism having a first surface in facing relation to the output surface of the block and a second surface orthogonal to a corresponding one of the deflected directions.

13. The apparatus of claim 12, wherein an incident angle at which said deflected beams reach said output surface of said block is greater than a critical angle at a block-air interface, the apparatus further comprising an optical liquid deployed between said first surface of said wedge output prism and said output surface of said block.

14. The apparatus of claim 11, further comprising a wedge input prism having a first input prism surface in facing relation to an input surface of the block and a second input prism surfaceorthogonal to the substantially collimated beam output from the multi-chromatic illumination arrangement so as to avoid chromatic aberration associated with non-orthogonal injection.

15. The apparatus of claim 14, wherein a wedge angle of said wedge input prism is such that said collimated beam reaches said first input prism surface at an angle of incidence greater than a critical angle at a prism-air interface, the wherein an optical liquid is deployed between said first input prism surface and said input surface of said block.

16. The apparatus of claim 12, wherein the output surface of the block is parallel to the internal surfaces of the block.

17. The apparatus of claim 12, wherein the output surface of the block is an interface plane configured to be subsequently bonded to a second intermediate work product prior to slicing to form multiple lightguides.

18. The apparatus of claim 11, further comprising a beam-shaping element configured to set a width of the substantially collimated beam sufficiently small to prevent overlap of the plurality of regions on the imaging sensor.

19. The apparatus of claim 11, further comprising a positioning stage configured to displace the block relative to the illumination arrangement and the imaging sensor so as to enable sampling of images at multiple positions along the internal surfaces.

20. The apparatus of claim 11, further comprising: a mask deployed at an output plane traversed by the surface- specific deflected beams; a scanning mechanism configured to scan the mask so that illumination reaching the imaging sensor derives from partial reflection at a currently selected internal surface; and a focus control configured to set the imaging sensor to focus at infinity, the processing module being further configured to monitor for variation in a spot position at the imaging sensor.

Images