New techniques for inspection of light guide optical elements

Abstract

Inspecting a light guide optical element (LOE) may include optically placing a first slit between a projector configured to emit light and a first major surface of the LOE, and optically placing a second slit between a second major surface of the LOE and a detector. The facet parallelism between the two facets may be inferred based on an offset of an image reflected from the first facet to the second facet relative to light transmitted perpendicular to the first and second major surfaces through a portion of the substrate that does not include the facets. The facet refractive index uniformity or deviation may be inferred based on light transmitted through the facet relative to light transmitted perpendicular to the first and second major surfaces through a portion of the substrate that does not include the facet.

Description

Technical Field

[0001] The present disclosure relates to the field of near-eye display systems, such as head-mounted displays. More specifically, the present disclosure relates to techniques for inspecting facets of a light guide optical element (LOE) of a near-eye display system. Background Art

[0002] Consumer demand for improved human-machine interfaces has led to growing interest in high-quality image head-mounted displays (HMDs) or near-eye displays (NEDs), often referred to as smart glasses. These devices can provide virtual reality (VR) or augmented reality (AR) experiences, enhancing the way users interact with digital content and the user's surrounding environment.

[0003] Consumers are looking for better image quality, immersive experience, and greater comfort when using HMDs. Consumers expect displays to have high resolution, vivid colors, and minimal distortion to produce a realistic and interesting viewing experience.

[0004] A key component in a NED system is the waveguide, which guides light from the system image projector to the user's eyes. The waveguide works based on total internal reflection along the major surfaces of the waveguide to propagate the light, and uses reflection from facets placed along the waveguide to guide the light to the user's eyes. Achieving optimal waveguide performance requires precise design and manufacturing to prevent defects that could degrade the user's visual experience.

[0005] Evaluating the optical performance of waveguides before integrating them into NED systems can help reduce production costs. Key factors in achieving optimal waveguide performance include ensuring the facets are parallel and have a uniform refractive index. Typically, testing these properties is time-consuming and expensive, which limits the availability and use of NED systems.

[0006] Therefore, innovative techniques are needed to efficiently inspect waveguides. Summary of the invention

[0007] The present disclosure introduces innovative techniques for measuring the optical performance of LOEs. Previous methods for measuring the parallelism of facets in LOEs use coupling prisms, such as those disclosed in U.S. Patent No. 11,226,261 and PCT International Application Publication WO2023 / 007491. In contrast, the techniques disclosed herein do not require coupling prisms. In one embodiment, LOE is measured by coupling light into a waveguide through one or more major surfaces to be reflected or transmitted through one or more facets. This eliminates the need for coupling prisms, simplifies the measurement system structure and improves test efficiency.

[0008] The accompanying drawings that are incorporated in the specification and form a part of the specification illustrate various example systems, methods, etc., which illustrate various example implementations of various aspects of the present invention. It will be understood that the element boundaries (e.g., boxes, box groups, or other shapes) shown in the figures represent an example of boundaries. It will be understood by those of ordinary skill in the art that an element can be designed as multiple elements, or multiple elements can be designed as one element. An element shown as an internal component of another element can be implemented as an external component, and an element shown as an external component of another element can be implemented as an internal component. In addition, elements may not be drawn to scale. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 An exemplary implementation of a near-eye display device is shown.

[0010] Figure 2 A schematic diagram of an exemplary system for measuring the parallelism of partially reflecting facets of a LOE is shown.

[0011] Figure 3 A schematic diagram of an exemplary system for measuring the parallelism of partially reflecting facets of a LOE is shown.

[0012] Figure 4 A schematic diagram of an exemplary system for measuring the parallelism of partially reflecting facets of a LOE is shown.

[0013] Figure 5 A schematic diagram of an exemplary system for measuring the parallelism of partially reflecting facets of a LOE is shown.

[0014] Figure 6 A schematic diagram of an exemplary system for measuring the parallelism of partially reflecting facets of a LOE is shown.

[0015] Figure 7 A schematic diagram of an exemplary system for measuring the uniformity of the refractive index of a facet of a LOE is shown.

[0016] Figure 8 A flow chart illustrating an exemplary process for measuring the parallelism of partially reflective facets of a LOE.

[0017] Fig. 9 A flow chart illustrating an exemplary process for measuring the uniformity of the refractive index of a facet of a LOE. DETAILED DESCRIPTION

[0018] For purposes such as head mounted displays (HMDs) commonly referred to as smart glasses or near eye displays (which may be virtual reality or augmented reality displays), certain embodiments of the present invention provide light projection systems and optical systems for achieving optical aperture expansion. Consumer demand for better and more comfortable human-machine interfaces has stimulated demand for better image quality and smaller devices.

[0019] Figure 1 An exemplary implementation of the near-eye display device 1 is shown. The near-eye display device 1 is disclosed herein only as an example, and the inventive technology disclosed herein is not limited to such a device.

[0020] exist Figure 1 In the illustrated embodiment, the near-eye display 1 employs an optically coupled compact image projector or projection unit 3 to project an image into a light-guiding optical element (LOE) 10. Optical aperture expansion of light from the projection unit 3 can be achieved within the LOE 10 by one or more arrangements employing a set of partially reflective surfaces (interchangeably referred to as "facets") for progressively redirecting image illumination, the set of partially reflective surfaces being ideally parallel to each other and inclined to the direction of propagation of the image light, wherein each successive facet deflects a portion of the image light into the deflection direction. The partially reflective facets can also be used as an out-coupling arrangement that progressively couples a portion of the image illumination toward the eye of an observer, who is located within a region defined as an Eye-Motion Box (EMB).

[0021] Preferably, the entire device 1 is supported relative to the user's head, wherein each projection unit 3 and LOE 10 serves a respective eye of the user. In one particularly preferred option as shown here, the support arrangement is implemented as a face-mounted set of lenses (e.g., Rx lenses, sunglasses, etc., colloquially referred to herein as "glasses" or "smart glasses") having a lens 5 and a frame, to which the projection unit 3 and LOE 10 are optically connected, the frame having sides 7 that support the device relative to the user's ears. Other forms of support arrangements may also be used, including but not limited to headbands, masks, or devices suspended from helmets.

[0022] The near-eye display 1 may include various additional components, typically including a controller 9 for actuating the projection unit 3, which controller 9 typically uses power from a small on-board battery (not shown) or some other suitable power source. The controller 9 may include all necessary electronic components for driving the image projector 3, such as a processing unit or processing circuit.

[0023] Figure 2A schematic diagram of a system 50 for measuring the parallelism of partially reflective facets 101-104 of a LOE 10 is shown. The system 50 may include: a display calibrator (whose image is imaged at infinity) portion 20 including an imaging calibrator 21 and a calibration lens 22; two devices 131, 132 having corresponding slits formed thereon; and a calibration image acquisition portion 30 including a lens 31 and a detector 32. The system 50 may also include a processing unit 100, which may control the display calibrator portion 20, the image acquisition portion 30, and one or more of the devices 131, 132. The processing unit 100 may also control a machine 40 for moving the inspected LOE 10 relative to other elements of the system 50 (i.e., controlling the lateral positioning of the inspected LOE 10).

[0024] The processing unit 100 may include one or more processors (e.g., microprocessors, microcontrollers, etc.), memory, etc., which are programmed (e.g., software, firmware, etc.) to control various elements of the system disclosed herein, and to calculate / infer the parallelism and uniformity of the refractive index of the LOE facets.

[0025] like Figure 2 As shown in , the devices 131, 132 can be placed so that the respective slits of the devices 131, 132 are arranged opposite to each other and the LOE 10 is located therebetween, so that the image projected by the display calibrator portion 20 reaches the image acquisition portion 30, in particular the detector 32, without any angular offset caused by the LOE facets 101-104. This can be used to calibrate the system 50 as a reference for the image position of an ideal pair of parallel facets.

[0026] Figure 3 A schematic diagram of a system 50 is shown, wherein both the device 132 and the LOE 10 are Figure 2 . The slits of device 131 and device 132 are placed so that the direct image projected by display calibrator portion 20 reaches facet 101. Any light transmitted by facet 101 is prevented from reaching image acquisition portion 30. Light reflected by facet 101 reaches facet 102, which reflects some light to image acquisition portion 30. The light reaching detector 32 corresponds to light that is reflected twice, the first time by facet 101 and the second time by facet 102. By measuring the Figure 3 The detection image captured at the detector 32 in the arrangement of Figure 2 By offsetting the images captured in the arrangement shown, system 50 can infer the parallelism between surface 101 and surface 102.

[0027] Figure 4A schematic diagram of a system 50 is shown, wherein the LOE 10 is Figure 3 Its position as shown is offset to measure the parallelism between surface 102 and surface 103 in this arrangement. The slits of device 131 and device 132 are placed so that the direct image projected by display calibrator portion 20 reaches facet 102. Any light transmitted by facet 102 is prevented from reaching image acquisition portion 30. Light reflected by facet 102 reaches facet 103, which reflects some light to image acquisition portion 30. The light reaching detector 32 corresponds to light that is reflected twice, the first time by facet 102 and the second time by facet 103. By measuring the Figure 4 The detection image captured at the detector 32 in the arrangement of Figure 2 By offsetting the images captured in the arrangement shown, system 50 can infer the parallelism between surface 102 and surface 103. Figure 2 , Figure 3 as well as Figure 4 Based on the information, system 50 can also infer the parallelism between facet 101 and facet 103 through simple mathematical calculations.

[0028] For example, assuming that in an arrangement where two slits face each other (e.g. Figure 2 The detected image is centered at zero position. Figure 3 The arrangement shown has the image center offset 20" to the left, while for Figure 4 In the case shown, the image is positioned 20". to the right. According to the equation α = θ / 2n, where α is the mechanical offset angle between the two facets (α = 0 indicates perfect parallelism), θ is the observed angular image shift, and n is the refractive index of the LOE, if the material of the LOE has a refractive index of 1.5, then based on these measurements, system 50 can infer that facet 101 is parallel to facet 102 and that facet 101 is rotated 6.66". clockwise.

[0029] By continuing to deflect the entire LOE 10 so that all of the facets of the LOE 10 are rotated in front of the slit of the device 132, the system 50 can infer the parallelism of the entire facet structure.

[0030] Can be used Figure 5A similar technique is implemented in the system 60 of FIG. 60. In the system 60, the display calibrator portion 20 includes an automatic calibrator, and the imaging calibrator 30 in the previous figures is replaced by the reflector 14. The slit of the device 132 is optically arranged between the second major surface 13 and the reflector 14, so that the light from the projector 20 travels through the slit of the device 131 to the first major surface 12 and to the first facet 101, and the light reflected by the first facet 101 and the second facet 102 travels from the second facet 102 to the second major surface 13 and reaches the reflector 14 through the slit of the second device 132. The light reflected by the reflector 14 travels through the second slit of the device 132 to the second major surface 13 and reaches the second facet 102, and the light reflected by the second facet 102 and the first facet 101 travels from the first facet 101 to the first major surface 12 and reaches the detector 32 through the first slit of the device 131. Light reaching the detector 32 will have traveled through the two relevant facets twice, and therefore the intensity of the light received at the detector 32 is significantly reduced compared to the light originally projected.

[0031] Any type of structure that measures the deviation of the reflection of light striking at least one pair of facets in the array can be considered. For example, instead of measuring the reflection of light between the first facets i and i+1, the reflection of light can be measured between the first facets i and i+2, or even between the first facets i and i+n, where n is an offset integer between 1 and N-1, where N is the total number of facets. Figure 6 An exemplary system 70 is shown in FIG. 1 , where the slit of device 132 is located in front of facet 103 and may be offset to facet 104 .

[0032] Figure 7 A system 80 is shown for measuring the uniformity of the refractive index of facets 101 to 104 of a LOE 10. The LOE 10 can be fabricated from individually coated plates or slices 1000 to 1004 that are glued together. The interfaces between adjacent plates form the facets. For example, the interface between adjacent plates 1000 and 1001 forms facet 101. The quality of the LOE 10 is controlled not only by the degree of parallelism of the facets 101 to 104, but also by the uniformity of the refractive index of the plates 1000 to 1004 at the interfaces that form the facets 101 to 104.

[0033] In order to measure the uniformity of the refractive index of the facets 101 to 104, the devices 131, 132 can be placed so that the slits of the devices 131, 132 face each other, as shown in FIG. Figure 1 and Figure 7 As shown. Figure 1The system is calibrated in the arrangement shown, wherein the LOE 10 is placed relative to the devices 131, 132 so that the projected light propagates through the LOE 10 through a single slice 1000 without passing through different plates along the way. That is, in calibration, the light from the imaging unit 20 is transmitted through the portion 1000 of the LOE 10 that does not include the facets, perpendicular to the first major surface 12 and the second major surface 13.

[0034] To measure the uniformity of the refractive index of the facet 101, the devices 131, 132 are kept placed so that the slits of the devices 131, 132 face each other. Figure 7 As shown, LOE 10 has been shifted to the left so that the projected light enters LOE 10 through first major surface 12 at slice or plate 1001 and then passes through the interface between plate 1001 and plate 1000 (i.e., facet 101). If the refractive index of plate 1001 and plate 1000 is different at interface 101, then according to Snell's law, there will be a deflection of the light as it passes through the interface between the two surfaces, which can be explained using Figure 7 The arrangement is measured.

[0035] By shifting LOE 10 further to the left, the interfaces between plates 1002 and 1001, plates 1002 and 1003, etc. can be examined so that differences in the refractive index of facet 102, facet 103, etc. can be detected. Assuming that light enters the major surface 12 of LOE 10 perpendicularly and exits at an angle β, we can infer that where θ is the tilt angle of the facet (θ=0 means the facet is parallel to the major surface of the LOE 10, and θ is equal to 38 degrees in the figure).

[0036] method

[0037] Reference Figure 8 and Fig. 9 The exemplary method can be better understood by the flowchart of the embodiment. Although the method shown is shown and described as a series of blocks for the purpose of simplifying the description, it is to be understood that these methods are not limited by the order of these blocks, because some blocks can appear in different orders, or can appear simultaneously with other blocks shown and described. In addition, it may be necessary to implement the exemplary method with fewer blocks than all the blocks shown. In addition, other methods, alternative methods or both can adopt other blocks not shown.

[0038] In the flowchart, the block represents a "processing block" that can be implemented with logic. The processing block can represent a method step or an equipment element for executing the method step. The flowchart does not depict the syntax of any specific programming language, method or style (e.g., procedural, object-oriented). Instead, these flowcharts show functional information that can be used by those skilled in the art to develop logic to perform the processing shown. It will be understood that in some examples, program elements such as temporary variables, routine loops, etc. are not shown. It will also be understood that electronic applications and software applications can involve dynamic and flexible processes, so that the blocks shown can be executed in other orders different from the order shown, or the blocks can be combined or separated into multiple components. It will be understood that various programming methods (such as machine language, procedural, object-oriented or artificial intelligence techniques) can be used to implement these processes.

[0039] Figure 8 A flow chart of an exemplary method 800 for measuring the parallelism of facets of a LOE is shown. Figure 8 As shown, method 800 may include calibration by the following steps: at 810, projecting light perpendicularly to a major surface of the LOE; at 820, offsetting one or more slits to face each other on a portion of the LOE that does not include a small facet; and at 830, measuring the projected light to measure the unbiased parallelism.

[0040] At 840, method 800 may include offsetting at least one slit and / or LOE so that light from the projector travels through the first slit to the first major surface and to the first facet (facet i), and so that the second slit blocks light transmitted by the first facet, and so that light reflected from the first facet to the second facet travels from the second facet to the second major surface and through the second slit to the detector. At 850, method 800 may include measuring, based on an image detected by the detector, a deviated reflection relative to light transmitted perpendicular to the first and second major surfaces through a portion of the substrate that does not include the facets. At 860, method 800 may include inferring parallelism between the first facet and the second facet based on the measured deviation. The method may then be repeated for other facets under the LOE.

[0041] Fig. 9 A flow chart of an exemplary method 900 for measuring the uniformity of the refractive index of a facet of a LOE is shown. Fig. 9 As shown, method 900 may include calibration by the following steps: at 910, projecting light perpendicularly onto a major surface of the LOE; at 920, offsetting one or more slits to face each other on a portion of the LOE that does not include a facet; and at 930, measuring the projected light on the portion of the LOE that does not include a facet.

[0042] At 940, method 900 may include offsetting at least one slit and / or LOE so that light from the projector travels through the first slit to the first major surface and to a first of the facets, and light transmitted through the first facet travels from the first facet to the second major surface and through the second slit to the detector. At 950, method 900 may include measuring the light transmission deviated from the first facet. At 960, method 900 may include inferring the uniformity of the slice based on the measured deviated light transmission relative to light transmitted through the portion of the LOE that does not include the facets perpendicular to the first and second major surfaces.

[0043] Although the drawings show various actions occurring in series, it should be understood that the various actions shown can occur substantially in parallel, and although actions occurring in parallel can be shown, it should be understood that these actions can occur substantially in series. Although multiple processes are described relative to the method shown, it should be understood that a greater or lesser number of processes can be used, and lightweight processes, conventional processes, threads, and other methods can be used. It should be understood that in some cases, other exemplary methods may also include actions occurring substantially in parallel. The exemplary methods shown and other embodiments can operate in real time, faster than real time in a software implementation or a hardware implementation or a hybrid software / hardware implementation, or slower than real time in a software implementation or a hardware implementation or a hybrid software / hardware implementation.

[0044] definition

[0045] The following includes definitions of selected terms used herein. These definitions include various examples or forms of components that fall within the scope of the terms and can be used for implementation. These examples are not intended to be limiting. Both the singular and plural forms of the terms may be within the definitions.

[0046] An "operable connection" or a connection through which entities are "operably connected" is a connection through which signals, physical communications, or logical communications can be sent or received. Typically, an operable connection includes a physical interface, an electrical interface, or a data interface, but it is noted that an operable connection may include different combinations of these or other types of connections sufficient to allow operable control. For example, two entities may be operably connected by being able to pass signals directly to each other or through one or more intermediate entities (such as a processor, operating system, logic, software, or other entity). Logical or physical communication channels may be used to create an operable connection.

[0047] To the extent that the term "include" or "including" is employed in the detailed description or claims, it is intended to be inclusive in a manner similar to the term "comprising" in that the term "comprising" is interpreted as a transitional word in the claims when used. Furthermore, to the extent that the term "or" is employed in the detailed description or claims (e.g., A or B), it is intended to mean "A or B or both." When applicants intend to mean "only A or B but not both," the term "only A or B but not both" is employed. Thus, the use of the term "or" herein is an inclusive use, not an exclusive use. See Bryan A. Garner, Dictionary of Modern Legalese 624 (2nd ed. 1995).

[0048] Although example systems, methods, etc. have been described by describing examples, and although examples have been described in considerable detail, it is not the intention of the applicant to limit or in any way restrict the scope to such details. Of course, in order to describe the systems, methods, etc. described herein, it is impossible to describe every conceivable combination of components or methods. Additional advantages and modifications will be apparent to those skilled in the art. Therefore, the present invention is not limited to the specific details, representative devices, and illustrative examples shown and described. Therefore, the present application is intended to include changes, modifications, and variations that fall within the scope of the appended claims. In addition, the foregoing description is not meant to limit the scope of the present invention. On the contrary, the scope of the present invention will be determined by the appended claims and their equivalents.

Claims

1. A system for measuring the parallelism and uniformity of the refractive index of a facet of a light-guiding optical element (LOE), the LOE comprising a light-transmissive substrate having a first major surface and a second major surface, the first major surface and the second major surface being parallel to each other, such that light coupled into the light-transmissive substrate is trapped between the first major surface and the second major surface by total internal reflection, and the facet is configured to couple light out of the substrate, the system comprising: one or more devices having a first slit and a second slit formed thereon; a processing unit configured to control at least one of the first slit and the second slit so that the first slit is optically disposed between a projector configured to emit light corresponding to an image and the first major surface, the first slit is disposed so that light from the projector travels through the first slit to the first major surface and to a first facet of the facets, and so that the second slit is optically disposed between the second major surface and a detector, the second slit is disposed so as to block light transmitted by the first facet, and so that light reflected by the first and second facets of the facets travels from the second facet to the second major surface and through the second slit to the detector; The processing unit is configured to infer parallelism between the first facet and the second facet based on a shift of the image detected by the detector relative to light transmitted perpendicularly to the first and second major surfaces through a portion of the substrate not including the facets; The processing unit is configured to control at least one of the first slit and the second slit so that the second slit is optically disposed between the second major surface and the detector so that light from the projector travels through the first slit to the first major surface and to the first facet, and light transmitted through the first facet travels from the first facet to the second major surface and through the second slit to the detector; and The processing unit is configured to determine a uniformity or deviation in refractive index between surfaces of the first facet based on light transmitted through the first facet detected by the detector relative to light transmitted perpendicularly to the first and second major surfaces through a portion of the substrate that does not include the facet.

2. The system of claim 1 , wherein the processing unit is configured to control lateral movement of the LOE relative to the first slit and the second slit, or to control lateral movement of the first slit and the second slit relative to the LOE, so as to measure parallelism and uniformity of the refractive index of another small facet of the LOE.

3. The system of claim 1, the processing unit being configured to perform calibration by capturing an image emitted by the projector at the detector, the image traveling through a portion of the substrate that does not include a facet.

4. The system according to claim 1, comprising: A reflector, wherein the second slit is optically arranged between the second major surface and the reflector so that light from the projector travels through the first slit to the first major surface and to the first facet, light reflected by the first facet and the second facet travels from the second facet to the second major surface and through the second slit to the reflector, light reflected by the reflector travels through the second slit to the second major surface and to the second facet, and light reflected by the second facet and the first facet travels from the first facet to the first major surface and through the first slit to the detector.

5. The system according to claim 1, wherein The projector comprises a calibrator and the detector comprises a camera, or The projector and the detector are parts of an automatic calibrator.

6. A system for measuring the parallelism of a facet of a light-guiding optical element (LOE), the LOE comprising a light-transmissive substrate having a first major surface and a second major surface, the first major surface and the second major surface being parallel to each other, such that light coupled into the light-transmissive substrate is trapped between the first major surface and the second major surface by total internal reflection, and the facet is configured to couple light out of the substrate, the system comprising: one or more devices having a first slit and a second slit formed thereon; a processing unit configured to control at least one of the first slit and the second slit so that the first slit is optically disposed between a projector configured to emit light corresponding to an image and the first major surface, the first slit is disposed so that light from the projector travels through the first slit to the first major surface and to a first facet of the facets, and so that the second slit is optically disposed between the second major surface and a detector, the second slit is disposed so as to block light transmitted by the first facet, and so that light reflected by the first and second facets of the facets travels from the second facet to the second major surface and through the second slit to the detector; and The processing unit is configured to infer parallelism between the first facet and the second facet based on an offset of the image detected by the detector relative to light transmitted perpendicularly to the first and second major surfaces through a portion of the substrate that does not include the facets.

7. The system of claim 6, the processing unit being configured to move the LOE laterally relative to the first slit and the second slit, or to move the first slit and the second slit laterally relative to the LOE, to measure parallelism of additional facets of the LOE.

8. The system of claim 6, the processing unit being configured to perform calibration by capturing an image emitted by the projector at the detector, the image traveling through a portion of the substrate that does not include a facet.

9. A system for measuring the uniformity of the refractive index of a facet of a light-guiding optical element (LOE), the LOE comprising a light-transmissive substrate having a first major surface and a second major surface, the first major surface and the second major surface being parallel to each other, such that light coupled into the light-transmissive substrate is trapped between the first major surface and the second major surface by total internal reflection, and the facet is configured to couple light out of the substrate, the system comprising: one or more devices having a first slit and a second slit formed thereon; The processing unit is configured to control at least one of the first slit and the second slit so that the second slit is optically disposed between the second major surface and the detector so that light from the projector travels through the first slit to the first major surface and to the first facet, and light transmitted through the first facet travels from the first facet to the second major surface and through the second slit to the detector; and The processing unit is configured to determine a uniformity or deviation in refractive index between surfaces of the first facet based on light transmitted through the first facet detected by the detector relative to light transmitted perpendicularly to the first and second major surfaces through a portion of the substrate that does not include the facet.

10. The system of claim 9, wherein the processing unit is configured to move the LOE laterally relative to the first slit and the second slit, or to move the first slit and the second slit laterally relative to the LOE, to measure the uniformity of the refractive index of additional facets of the LOE.

11. The system according to claim 9, comprising: reflector, the processing unit is configured to optically place the second slit between the second major surface and the reflector, so that light from the projector travels through the first slit to the first major surface and to the first small facet, light reflected by the first small facet and the second small facet travels from the second small facet to the second major surface and through the second slit to the reflector, light reflected by the reflector travels through the second slit to the second major surface and to the second small facet, and light reflected by the second small facet and the first small facet travels from the first small facet to the first major surface and through the first slit to the detector.

12. A method for measuring the parallelism and uniformity of the refractive index of a facet of a light-guiding optical element (LOE), the LOE comprising a light-transmissive substrate having a first major surface and a second major surface, the first major surface and the second major surface being parallel to each other, such that light coupled into the light-transmissive substrate is trapped between the first major surface and the second major surface by total internal reflection, and the facet is configured to couple light out of the substrate, the method comprising: placing a first slit optically between a projector configured to emit light corresponding to an image and the first major surface, the first slit being positioned such that light from the projector travels through the first slit to the first major surface and to a first of the facets; placing a second slit optically between the second major surface and a detector, the second slit being positioned to block light transmitted by the first facet and to allow light reflected by the first and second ones of the facets to travel from the second facet to the second major surface and through the second slit to the detector; inferring parallelism between the first facet and the second facet based on a shift of the image detected by the detector relative to light transmitted perpendicularly to the first and second major surfaces through a portion of the substrate that does not include the facets; placing the second slit optically between the second major surface and the detector so that light from the projector travels through the first slit to the first major surface and to the first facet, and light transmitted through the first facet travels from the first facet to the second major surface and through the second slit to the detector; as well as The uniformity or deviation of the refractive index between the surfaces of the first facet is determined based on light transmitted through the first facet detected by the detector relative to light transmitted perpendicular to the first and second major surfaces through a portion of the substrate that does not include the facet.

13. The method of claim 12, comprising moving the LOE laterally relative to the first and second slits, or moving the first and second slits laterally relative to the LOE, to measure the parallelism and uniformity of the refractive index of additional facets of the LOE.

14. The method according to claim 12, comprising: Calibration is performed by capturing at the detector an image emitted by the projector, the image travelling through the portion of the substrate that does not include facets.

15. A method for measuring the parallelism of a facet of a light-guiding optical element (LOE), the LOE comprising a light-transmissive substrate having a first major surface and a second major surface, the first major surface and the second major surface being parallel to each other, such that light coupled into the light-transmissive substrate is trapped between the first major surface and the second major surface by total internal reflection, and the facet is configured to couple light out of the substrate, the method comprising: placing a first slit between a projector configured to emit light corresponding to an image and the first major surface, the first slit being positioned such that light from the projector travels through the first slit to the first major surface and to a first of the facets; placing a second slit between a detector and the second major surface, the second slit being positioned to block light transmitted by the first facet and to allow light reflected by the first and second ones of the facets to travel from the second facet to the second major surface and through the second slit to the detector; as well as Parallelism between the first facet and the second facet is inferred based on an offset of the image detected by the detector relative to light transmitted perpendicular to the first and second major surfaces through a portion of the substrate that does not include the facets.

16. The method according to claim 15, comprising: The LOE is moved laterally relative to the first and second slits, or the first and second slits are moved laterally relative to the LOE, to measure the parallelism of additional facets of the LOE.

17. The method according to claim 15, comprising: Calibration is performed by capturing at the detector an image emitted by the projector, the image travelling through the portion of the substrate that does not include facets.

18. A method for measuring the uniformity of a refractive index of a facet of a light-guiding optical element (LOE), the LOE comprising a light-transmissive substrate having a first major surface and a second major surface, the first major surface and the second major surface being parallel to each other, such that light coupled into the light-transmissive substrate is trapped between the first major surface and the second major surface by total internal reflection, and the facet is configured to couple light out of the substrate, the method comprising: placing a first slit between a projector configured to emit light corresponding to an image and the first major surface, the first slit being positioned such that light from the projector travels through the first slit to the first major surface and to a first of the facets; placing a second slit between the second major surface and a detector so that light from the projector travels through the first slit to the first major surface and to the first facet, and light transmitted through the first facet travels from the first facet to the second major surface and through the second slit to the detector; as well as The uniformity or deviation of the refractive index between the surfaces of the first facet is determined based on light transmitted through the first facet detected by the detector relative to light transmitted perpendicular to the first and second major surfaces through a portion of the substrate that does not include the facet.

19. The method according to claim 18, comprising: The LOE is moved laterally relative to the first and second slits, or the first and second slits are moved laterally relative to the LOE, to measure the uniformity of the refractive index of additional facets of the LOE.

20. The method according to claim 18, comprising: Calibration is performed by capturing at the detector an image emitted by the projector, the image travelling through the portion of the substrate that does not include facets.

Images