New inspection technology for optical elements

Abstract

For the inspection of a light optical element (LOE), it may include optically arranging a first slit between a projector configured to emit light and a first main surface of the LOE, and optically arranging a second slit between a second main surface of the LOE and a detector. The facet parallelism between two facets can be estimated based on the shift of the image reflected from the first facet to the second facet with respect to the light transmitted perpendicularly to the first and second main surfaces through a part of the substrate without facets. The facet refractive index homogeneity or deviation can be estimated based on the light passing through the facets with respect to the light transmitted perpendicularly to the first and second main surfaces through a part of the substrate without facets.

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-computer interfaces has increased interest in high-quality image head-mounted displays (HMDs) and near-eye displays (NEDs). These devices, commonly known as smart glasses, provide virtual reality (VR) and augmented reality (AR) experiences and can improve the way users interact with digital content and their surrounding environment.

[0003] Consumers seek a higher quality and more immersive experience when using HMDs, as well as a more comfortable user experience. They expect high-resolution, vivid colors, and a display with little distortion to create a realistic and enjoyable viewing experience.

[0004] An important component in NED systems is the waveguide that guides light from the system image projector to the user's eyes. Waveguides function based on total internal reflection to propagate light along their major surfaces and direct the light reflected by surfaces disposed along the waveguide to the user's eyes. To optimize the performance of waveguides, precise design and manufacturing are required to prevent imperfections that could degrade the user's visual experience.

[0005] Evaluating the optical performance of waveguides before integrating them into NED systems can reduce manufacturing costs. The main factors for optimizing the performance of waveguides include having parallel facets and a uniform refractive index. Conventionally, testing these characteristics has been time-consuming and costly, limiting the availability and adoption of NED systems.

[0006] Therefore, innovative technologies for efficiently inspecting waveguides are in demand.

Summary of the Invention

[0007] This disclosure introduces innovative technologies for measuring the optical performance of LOE. In the conventional method for measuring the parallelism of the facets of LOE, a coupling prism was used, as disclosed, for example, in U.S. Patent No. 11,226,261 and PCT International Application Publication No. WO2023 / 007491. In contrast, in the technology disclosed herein, a coupling prism is not required. In one embodiment, LOE is measured by coupling light into a waveguide through one or more of its main surfaces and reflecting or transmitting it through one or more facets. This eliminates the need for a coupling prism, simplifies the mechanism of the measurement system, and improves test efficiency.

[0008] The accompanying drawings are incorporated herein and constitute a part thereof, and illustrate various exemplary systems, methods, etc. related to various aspects of the present invention, showing various exemplary embodiments. It will be understood that the boundaries of the elements shown in the figures (e.g., boxes, groups of boxes, or other shapes) represent an example of a boundary. One of ordinary skill in the art will understand that an element may be designed as multiple elements in some cases, or multiple elements may be designed as one element in some cases. An element shown as an internal component of another element may be implemented as an external component in some cases, and vice versa. Furthermore, the elements may not be drawn to scale.

Brief Description of the Drawings

[0009]

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[0010] Certain embodiments of the present invention provide a projection system and an optical system for realizing optical aperture expansion for virtual reality and augmented reality displays, such as head-mounted displays (HMDs), near-eye displays, and generally smart glasses. Consumer demands for better and more comfortable human-computer interfaces are driving the demand for higher-quality images and smaller devices.

[0011] FIG. 1 shows an exemplary implementation of a near-eye display device 1. The near-eye display device 1 is disclosed here merely as an example, and the inventive techniques disclosed herein are not limited to such a device.

[0012] In the illustrated embodiment of FIG. 1, the near-eye display 1 uses a compact image projector or an optically coupled projection unit 3 to project an image onto a light optical element (LOE) 10. The optical aperture expansion of the light from the projection unit 3 is realized within the LOE 10 by one or more arrangements that stepwise redirect the image illumination using a set of partially reflective surfaces (also called "facets") that are ideally parallel to each other and inclined obliquely to the propagation direction of the image light. Each successive facet deflects a proportion of the image light in the deflection direction. The partially reflective facets also function as a coupling-out configuration that gradually couples a proportion of the image illumination towards the pupil of the observer located within a region defined as the eye-motion box (EMB).

[0013] The overall device 1 is preferably supported relative to the user's head such that each projection unit 3 and LOE 10 serves the corresponding eye of the user. In one particularly preferred option illustrated here, the support configuration is implemented as a face-mounted lens set (e.g., Rx lenses, sunglasses, etc., colloquially referred to here as "glasses" or "smart glasses") comprising a lens 5 to which the projection unit 3 and LOE 10 are optically connected, and a frame is provided with side portions 7 for supporting the device relative to the user's ears. Other forms of support structures can also be used. For example, a headband, a visor, a device suspended from a helmet, etc.

[0014] The near-eye display 1 includes various additional components and typically includes a controller 9 for operating the projection unit 3 that is powered from a small on-board battery (not shown) or other suitable power source. The controller 9 may include all the necessary electronic components such as a processing unit or processing circuitry for driving the image projector 3.

[0015] Figure 2 shows a schematic diagram of a system 50 for measuring the parallelism of the partial reflection facets 101-104 of the LOE 10. The system 50 may include a display collimator (imaging its image at infinity) section 20 including an imaging collimator 21 and a collimating lens 22, two devices 131, 132 each having a respective slit formed thereon, and a collimated image acquisition section 30 including a lens 31 and a detector 32. The system 50 may also include a processing unit 100 that controls one or more of the display collimator section 20, the image acquisition section 30, and the devices 131, 132. The processing unit 100 can also control a machine 40 for moving the LOE 10 to be inspected (i.e., controlling the lateral positioning) with respect to the other elements of the system 50.

[0016] The processing unit 100 may include one or more processors (e.g., microprocessors, microcontrollers, etc.), memory, etc., programmed (e.g., software, firmware, etc.) to perform various controls of the elements of the systems disclosed herein and to calculate / deduce the parallelism and homogeneity of the refractive index of the LOE surface.

[0017] As shown in Figure 2, the devices 131, 132 are arranged such that their respective slits face each other with the LOE 10 in between, and the image projected by the display collimator section 20 may reach the image acquisition section 30, specifically the detector 32, without an angular deviation caused by the surfaces 101-104 of the LOE. This can be used to calibrate the system 50 as a reference for the position of the image for an ideal set of parallel facets.

[0018] Figure 3 shows a schematic diagram of the system 50 in which the device 132 and the LOE 10 have moved from the positions shown in Figure 2. The slits of the devices 131 and 132 are arranged such that the direct image projected by the display collimator unit 20 reaches the facet 101. The light transmitted by the facet 101 is blocked from reaching the image acquisition unit 30. The light reflected by the facet 101 reaches the facet 102, and a part of it is reflected to the image acquisition unit 30. The light reaching the detector 32 corresponds to the light reflected first by the facet 101 and then by the facet 102. By measuring the shift of the detection image captured by the detector 32 in the arrangement of Figure 3 and comparing it with the image captured in the arrangement shown in Figure 2, the system 50 can infer the parallelism between the surfaces 101 and 102.

[0019] Figure 4 shows a schematic diagram of the system 50 in which the LOE 10 has been moved from the position shown in Figure 3 to measure the parallelism between the surfaces 102 and 103. The slits of the devices 131 and 132 are arranged such that the direct image projected by the display collimator unit 20 reaches the facet 102. The light transmitted by the facet 102 is blocked from reaching the image acquisition unit 30. The light reflected by the facet 102 reaches the facet 103, and a part of it is reflected to the image acquisition unit 30. The light reaching the detector 32 corresponds to the light reflected first by the facet 102 and then by the facet 103. By measuring the shift of the detection image captured by the detector 32 in the arrangement of Figure 4 and comparing it with the image captured in the arrangement shown in Figure 2, the system 50 can infer the parallelism between the surfaces 102 and 103. From the information in Figures 2, 3, and 4, the system 50 can also estimate the parallelism between the facets 101 and 103 by simple mathematical calculations.

[0020] For example, assuming that the image detected in the arrangement where two slits face each other (Figure 2) has its center placed around the zero position, in the arrangement shown in Figure 3, the center of the image is shifted 20″ to the left, and in the situation shown in Figure 4, the image is positioned 20″ to the right. From these measurements, the system 50 can infer that the facets 101 and 102 are parallel and that the facet 101 has rotated 6.66 times clockwise. When the refractive index of the LOE is 1.5 and follows the formula α = θ / 2n, when the material of the LOE has a refractive index of 1.5, the mechanical refractive angle between the two facets (α = 0 means perfect parallelism), θ is the shift of the observed angular image, and n is the refractive index of the LOE.

[0021] By continuously shifting the entire LOE 10 so that all of its facets are lined up in front of the slits of the device 132 in sequence, the system 50 can infer the parallelism of the entire facet structure.

[0022] A similar technique may be implemented using the system 60 of Figure 5. In the system 60, the display collimator unit 20 includes an autocollimator and replaces the imaging collimator 30 of the previous figure with a mirror 14. The slit of the device 132 is optically arranged between the second main surface 13 and the mirror 14 such that the light from the projector 20 travels through the slit of the device 131 to the first main surface 12, the light reflected by the first facet 101 travels from the first facet 101 to the second facet 102, the light from the second facet 102 travels to the second main surface 13, and then through the slit of the second device 132 to the mirror 14. The light reflected by the mirror 14 passes through the second slit of the device 132 and travels to the second main surface 13 and the second facet 102. The light reflected by the second facet 102 and the first facet 101 passes through the first main surface 12 from the first facet 101, through the first slit of the device 131, and to the detector 32. Since the light reaching the detector 32 passes through twice via two related facets, the intensity of the light received by the detector 32 is significantly reduced from the originally projected light.

[0023] Any kind of structure for measuring the reflection deviation of light corresponding to at least one pair of facets in the array is conceivable. For example, instead of i+1, the reflection of light between the first facet i and i+2 can be measured, or, when N is the total number of facets, as an integer n that shifts between 1 and N-1, the reflection of light between the first facet i and i+n can also be measured. An exemplary system 70 is shown in FIG. 6. In this system, the slit of the device 132 is arranged in front of the facet 103 and can be shifted to the facet 104.

[0024] FIG. 7 shows a system 80 for measuring the refractive index homogeneity of the facets 101-104 of the LOE10. The LOE10 can be manufactured by bonding individual coated plates or slices 1000-1004. The interfaces between adjacent plates form the facets. For example, the interface between adjacent plates 1000 and 1001 forms the facet 101. The quality of the LOE10 depends not only on the parallelism of the facets 101-104 but also on the refractive index homogeneity of the plates 1000-1004 at the interfaces forming the facets 101-104.

[0025] To measure the refractive index homogeneity of the surfaces 101-104, the devices 131, 132 can be arranged such that the slits face each other as shown in FIGS. 1 and 7. The calibration of the system is performed in the arrangement shown in FIG. 1, and the LOE10 is arranged with respect to the devices 131, 132 such that the projected light propagates through the LOE10 through a single slice 1000 without crossing different plates. That is, when performing the calibration, the light from the imaging unit 20 is transmitted perpendicularly to the first main surface 12 and the second main surface 13 through a part 1000 that does not include the facets in a part of the LOE10.

[0026] To measure the refractive index uniformity of surface 101, devices 131 and 132 remain arranged such that the slits face each other. However, as shown in FIG. 7, LOE 10 is shifted to the left, and light incident on LOE 10 through the first major surface 12 in slice or plate 1001 crosses the interface between plate 1001 and plate 1000 (i.e., facet 101). If the refractive indices of plates 1001 and 1000 are not the same at interface 101, according to Snell's law, the light ray refracts when the light crosses the interface between the two surfaces, and this refraction can be measured using the arrangement of FIG. 7.

[0027] By further shifting LOE 10 to the left, interfaces between plates 1002 and 1001, 1002 and 1003, etc. can be inspected, and differences in refractive index of facets 102, 103, etc. can be detected. Assuming that light normally enters the major surface 12 of LOE 10 and exits at an angle β,

[0028] [Equation] is derived. Here, θ is the tilt angle of the facet (θ = 0 means that the facet is parallel to the major surface of LOE 10, and in the figure, θ is equal to 38 degrees). [Method]

[0029] Referring to the flowcharts of FIGS. 8 and 9, the excellent method may be more understandable. To simplify the explanation, the illustrated methodology is shown and described as a series of blocks, but the methodology is not limited by the order of the blocks. Some blocks may occur in a different order than shown and described, or may occur simultaneously with other blocks. Further, not all of the illustrated blocks may be required to implement the illustrated methodology. Additionally, in further methodology, alternative methodology, or both, additional blocks not shown may be used.

[0030] Blocks in a flow diagram represent "processing blocks" that can be implemented in logic. A processing block may represent a method step or an apparatus element for executing a method step. A flow diagram does not depict the syntax of a particular programming language, methodology, or style (e.g., procedural, object-oriented). Rather, a flow diagram exemplifies functional information that a person of ordinary skill in the art may adopt to develop the logic for executing the illustrated processing. In some examples, it will be understood that program elements such as temporary variables, routine loops, etc. are not shown. Further, it will be understood that electronic and software applications may include dynamic and flexible processes such that the illustrated blocks are executed in other orders different from those shown, or the blocks are combined or divided into multiple components. It will be understood that the process may be implemented using various programming approaches such as machine language, procedural, object-oriented, or artificial intelligence techniques.

[0031] FIG. 8 shows a flow diagram of an exemplary method 800 for measuring the parallelism of the facets of an LOE. As shown in FIG. 8, method 800 may include projecting light onto the main surface of the LOE as normal at 810, moving one or more of the slits to face each other across a portion of the LOE that does not include the facets at 820, and measuring the projected light at 830 to perform a calibration for measuring non-deviated parallelism.

[0032] At 840, method 800 may include moving at least one slit and / or LOE such that light from a projector passes through a first slit to reach a first major surface and a first facet (facet i), a second slit blocks the light transmitted by the first facet, light reflected from the first facet to a second facet reaches a second major surface from the second facet, and passes through the second slit to reach a detector. At 850, method 800 may include measuring a deviated reflection from an image detected by a detector with respect to light transmitted perpendicularly to the first and second major surfaces through a portion of a substrate without facets. At 860, method 800 may include inferring a parallelism between a first facet and a second facet based on the measured deviation. Thereafter, the method may be repeated further with respect to the facet along the LOE.

[0033] FIG. 9 shows a flowchart of an exemplary method 900 for measuring the homogeneity of the refractive index of the surface of an LOE. As shown in FIG. 9, method 900 may include calibration by normally projecting light onto the major surface of the LOE at 910, shifting one or more of the slits to face each other across a portion of the LOE without facets at 920, and measuring the light projected across the portion of the LOE without facets at 930.

[0034] At 940, method 900 may include shifting at least one slit and / or LOE such that light from a projector passes through a first slit to reach a first major surface and a first facet of the facet, passes from the first facet through the first facet to reach a second major surface, and passes through a second slit to reach a detector. At 950, method 900 may include measuring a light transmission deviated by the first facet. At 960, method 900 may include inferring the homogeneity of a slice based on the measured deviated light transmission with respect to light transmitted perpendicularly to the first and second major surfaces through a portion of the LOE without facets.

[0035] The figures show various operations occurring sequentially, but it should be understood that the various operations shown may occur substantially in parallel. Also, although the operations are shown as occurring in parallel, it should be understood that these operations may occur substantially sequentially. Several processes are described in connection with the illustrated method, but it should be understood that a greater or lesser number of processes may be employed, and that lightweight processes, standard processes, threads, and other approaches may be employed. It should also be understood that other exemplary methods may, in some cases, include operations that occur substantially in parallel. Exemplary illustrative methods and other embodiments may operate in real time, or faster than real time, or slower than real time, in a software or hardware or software / hardware hybrid implementation.

[0036] [Definitions]

[0037] The following describes the definitions of the terms used in this document. The definitions include various examples or forms of components that fall within the scope of the terms and may be used in an implementation. These examples are not intended to be limiting. Both the singular and plural forms of the terms may be included in the definitions.

[0038] "Operable connection", or "operable connections", refers to a connection through which signals, physical communication, or logical communication can be transmitted and received. Usually, operable connections include physical interfaces, electrical interfaces, or data interfaces, but it should be noted that operable connections may include these different combinations, or other types of connections, sufficient to enable operable control. For example, two entities can be functionally connected by being able to communicate signals directly with each other, or through one or more intermediate entities such as processors, operating systems, logic, software, or other entities. Functional connections can be established using logical or physical communication channels.

[0039] In the detailed description or claims, when the term "comprising" or "comprises" is used, it is used in an inclusive sense, similar to the term "consisting of" when used as a transitional term in the claims. Further, when the term "or" is used in the detailed description or claims (e.g., A or B), it means "A or B or both". If the applicant intends to indicate "only A or B, but not both", the expression "only A or B, but not both" is used. Accordingly, the use of the term "or" in this specification is inclusive and not exclusive. See Bryan A. Garner, A Dictionary of Modern Legal Usage, page 624 (2d ed. 1995).

[0040] The illustrative systems, methods, etc. are described by way of example, and the examples are described in considerable detail, but the applicant's intention is not to limit the scope to such details or to restrict it in any way. Of course, it is impossible to describe all possible combinations of components and methodologies for explaining the systems, methods, etc. described herein. Those skilled in the art will readily envision additional advantages and modifications. Accordingly, the present invention is not limited to the specific details, representative devices, and exemplary embodiments disclosed and described. Therefore, this application is intended to cover changes, modifications, and variations included within the scope of the appended claims. Further, the foregoing description is not intended to limit the scope of the invention. Rather, the scope of the invention should be determined by the appended claims and their equivalents.

Claims

1. A system for measuring the parallelism and refractive index homogeneity of facets of a light guiding optical element (LOE), wherein the LOE has a light transmissive substrate having first and second major surfaces parallel to each other, light incident on the light transmissive substrate is confined between the first and second major surfaces by total reflection, and the facets are configured to emit the light from the substrate, The system includes one or more devices having a first slit and a second slit formed therein, a processing unit configured to control at least one of the first slit and the second slit such that the first slit is optically disposed between a projector configured to emit light corresponding to an image and the first major surface, wherein light from the projector passes through the first slit to the first major surface and then to a first facet, and the second slit is optically disposed between the second major surface and a detector, the second slit being arranged to block light transmitted by the first facet, and light reflected by the first and second facets from the facet travels from the second facet to the second major surface and then through the second slit to the detector, The processing unit is configured to estimate the parallelism between the first facet and the second facet based on a shift of the image detected by the detector with respect to light transmitted perpendicularly to the first and second major surfaces through a facet-free portion of the substrate from the facet, The processing unit is configured to control at least one of the first slit and the second slit such that the second slit is optically disposed between the second major surface and the detector, light from the projector passes through the first slit to the first major surface and then to the first facet, and light transmitted through the first facet travels from the first facet to the second major surface and then through the second slit to the detector, The processing unit is configured to determine the homogeneity or deviation of the refractive index between the surfaces of the first facet based on the light transmitted through the first facet, compared with the light transmitted perpendicularly to the first and second main surfaces through the portion of the substrate without the facet from the facet detected by the detector. A system.

2. The system according to claim 1, wherein the processing unit is configured to control the lateral movement of the LOE with respect to the first and second slits, or the reverse movement thereof, to measure the parallelism and the homogeneity of the refractive index of additional facets of the LOE.

3. The system according to claim 1, wherein the processing unit is configured to perform calibration by capturing, with the detector, an image projected from the projector and passing through the portion of the substrate without the facet.

4. A mirror and a second slit optically disposed between the second main surface and the mirror, wherein light from the projector passes through the first slit to the first main surface and then to the first facet, and light reflected by the first and second facets travels from the second facet to the second main surface, then through the second slit to the mirror, light reflected by the mirror passes through the second slit, to the second main surface, and then to the second facet, and light reflected by the second and first facets travels from the first facet to the first main surface and then through the first slit to the detector. The system according to claim 1.

5. The projector includes a collimator, the detector includes a camera, or The projector and the detector are part of an autocollimator, The system according to claim 1.

6. A system for measuring the parallelism of facets of a light guiding optical element (LOE), the LOE including a light transmissive substrate having first and second main surfaces parallel to each other, light incident on the light transmissive substrate being confined between the first and second main surfaces by total reflection, and the facets being configured to emit the light from the substrate. The system is One or more devices having a first slit and a second slit formed therein, A processing unit configured to control at least one of the first slit and the second slit, and having, The first slit is optically disposed between a projector configured to emit light corresponding to an image and a first main surface, and the first slit is configured such that light from the projector passes through the first slit to the first main surface and then to the first facet of the facet. And the second slit is optically disposed between the second main surface and the detector, and the second slit is disposed so as to block the light transmitted by the first facet, and the light reflected by the first facet and the second facet is arranged to travel from the facet, from the second facet to the second main surface, and through the second slit to the detector, The processing unit is configured to estimate the parallelism between the first facet and the second facet based on a shift of the image detected by the detector with respect to light sent perpendicularly to the first and second main surfaces through a portion of the substrate not including the facet from the facet. , System.

7. The system according to claim 6, wherein the processing unit is configured to move the LOE in a horizontal direction or the reverse direction with respect to the first and second slits to measure the parallelism of an additional surface of the LOE.

8. The system according to claim 6, wherein calibration of an image output by the projector that travels through a portion of the substrate not including the facet is performed by capturing with the detector.

9. A system for measuring the homogeneity of the refractive index of the facets of a light guiding optical element (LOE), the LOE having a light transmissive substrate having first and second main surfaces parallel to each other, and light incident on the light transmissive substrate being totally reflected. Confined between the first and second main surfaces, and the facet is configured to emit the light from the substrate, The system is, One or more devices having a first slit and a second slit formed therein, A processing unit configured to control at least one of the first slit and the second slit, wherein the second slit is arranged optically between the second main surface and the detector such that light from a projector passes through the first slit to the first main surface, then to the first facet, and then light transmitted through the first facet travels from the first facet to the second main surface and then through the second slit to the detector. The system, wherein the processing unit is configured to determine the homogeneity or bias of the refractive index between the surfaces of the first facet based on the light transmitted through the first facet, which is detected by the detector for light transmitted perpendicularly to the first and second main surfaces through a part of the substrate excluding the facet from the facet. Claim 10 The system according to claim 9, wherein the processing unit is configured to move the LOE horizontally or in the reverse direction with respect to the first and second slits to measure the homogeneity of the refractive index of additional facets of the LOE. Claim 11 The system according to claim 9, comprising a mirror and the processing unit configured such that the second slit is optically arranged between the second main surface and the mirror, wherein light from the projector passes through the first slit to the first main surface and the first facet, and light reflected by the first and second facets travels from the second facet to the second main surface, then through the second slit to the mirror, and light reflected by the mirror passes through the second slit to the second main surface and the second facet, and light reflected by the second and first facets travels from the first facet to the first main surface and then through the first slit to the detector. Claim 12 A method for measuring the parallelism and the homogeneity of the refractive index of facets of a light guiding optical element (LOE), the LOE having a light transmissive substrate with first and second main surfaces parallel to each other, light incident on the light transmissive substrate being confined between the first and second main surfaces by total reflection, and the facets being configured to emit the light from the substrate. The method comprises An optical first slit is disposed between a projector configured to emit light corresponding to an image and a first main surface such that light from the projector passes through the first slit and travels toward the first main surface and a first facet of the facet. An optical second slit is disposed between the second main surface and a detector such that the second slit blocks light transmitted by the first facet, and light reflected by the first facet and a second facet travels from the facet, to the second main surface from the second facet, and then to the detector through the second slit. Based on a shift of the image detected by the detector with respect to light sent perpendicularly to the first and second main surfaces through a part of the substrate that does not include the facet from the facet, the parallelism between the first facet and the second facet is estimated. The second slit is optically disposed between the second main surface and the detector such that light from the projector passes through the first slit, travels to the first main surface, and then to the first facet, and the light transmitted through the first facet travels from the first facet to the second main surface and then to the detector through the second slit. Based on the light transmitted through the first facet such that the light is detected by the detector with respect to light sent perpendicularly to the first and second main surfaces through a part of the substrate that does not include the facet from the facet, the homogeneity or bias of the refractive index between the surfaces of the first facet is determined. A method comprising the above.

13. Moving the LOE horizontally with respect to the first and second slits or vice versa to measure the parallelism of additional facets of the LOE and the uniformity of the refractive index. The method according to claim 12.

14. Calibration is performed by capturing, with the detector, an image output by the projector and transmitted through the part of the substrate that does not include the facet. The method according to claim 12, comprising the above.

15. A method for measuring the parallelism of facets of a light guiding optical element (LOE), wherein the LOE has a light transmissive substrate having first and second main surfaces parallel to each other, light incident on the light transmissive substrate is confined between the first and second main surfaces by total reflection, and the facets are configured to emit the light from the substrate, the method comprising: placing a first slit between a projector configured to emit light corresponding to an image and the first main surface, the first slit being arranged such that light from the projector passes through the first slit to the first main surface and then to a first facet of the facets, placing a second slit between a detector and the second main surface, the second slit being arranged to block light transmitted by the first facet, and light reflected by the first and second facets traveling from the facets to the second main surface from the second facet and then through the second slit to the detector, estimating the parallelism between the first and second facets based on a shift of the image such that the detector detects light sent perpendicular to the first and second main surfaces through a part of the substrate that does not include the facets from the facets, a method including this.

16. The method according to claim 15, wherein the LOE is moved horizontally with respect to the first and second slits or in the reverse direction to measure the parallelism of additional facets of the LOE.

17. The method according to claim 15, wherein calibration is performed by capturing, with the detector, an image output by the projector and passing through a part of the substrate that does not include the facets.

18. A method for measuring the refractive index homogeneity of facets of a light guiding optical element (LOE), wherein the LOE has a light transmissive substrate having first and second main surfaces parallel to each other, light incident on the light transmissive substrate is confined between the first and second main surfaces by total reflection, and the facets are configured to emit the light from the substrate, the method comprising: A first slit is disposed between a projector configured to emit light corresponding to an image and the first main surface, and the first slit is disposed such that light from the projector passes through the first slit to the first main surface and then to the first facet of the facet. A second slit is disposed between the second main surface and the detector such that light from the projector passes through the first slit to the first main surface and then to the first facet, and the light transmitted through the first facet proceeds from the first facet to the second main surface and then through the second slit to the detector. A method of determining the homogeneity or bias of the refractive index between the surfaces of the first facet based on the light transmitted through the first facet, as detected by the detector, compared to the light sent perpendicularly to the first and second main surfaces through a part of the substrate that does not include the facet from the facet. **Claim 19** Measuring the homogeneity of the refractive index of additional surfaces of the LOE by moving the LOE horizontally with respect to the first and second slits or vice versa. The method according to claim 18, comprising the above. **Claim 20** Performing calibration by capturing, with the detector, an image output by the projector and transmitted through a part of the substrate that does not include the facet. The method according to claim 18, comprising the above.

Images