Technique for measuring refractive index inhomogeneity between plates of light guide optical element (LOE)

Abstract

Systems and methods for measuring refractive index non-uniformities between plates of lightguide optical elements (LOEs) using innovative measurement techniques based on shear interference techniques typically used for observation interference and collimation of test beams. Another feature of the current implementation is an innovative approach for analyzing features of the generated interferograms to characterize differences between adjacent plates in the LOE.

Description

[0001] This application is a divisional application of an invention patent application with an application date of June 20, 2019, application number 201980041785.8 (PCT / IB2019 / 055207), and invention name “Measurement technology of refractive index inhomogeneity between plates of light-guiding optical element (LOE)”. Technical Field

[0002] The present invention relates generally to optical testing and, more particularly, to a non-contact measurement technique for refractive index differences. Background Art

[0003] One of the manufacturing challenges in producing light guide optical elements (LOEs) is achieving uniformity in the refractive index. Non-uniformity leads to degradation in image quality. If the refractive index of the individual plates is not well matched, angular deviations will be introduced when light passes through the facets, especially at large angles relative to the normal to the facet. Summary of the invention

[0004] According to the teachings of the present embodiment, a method for measuring refractive index inhomogeneity is provided, comprising the following steps: projecting projection light onto the front surface of a light guide, the light guide comprising: first outer surfaces parallel to each other, the outer surfaces comprising the front surface and the back surface, and a group of coated plates, the group of coated plates being: parallel to each other, between the first outer surfaces, and at non-parallel angles relative to the first outer surfaces and each plate having a corresponding refractive index; and capturing an interference image of an interference pattern to measure the refractive index inhomogeneity between the plates, the interference pattern being between refracted / reflected light and external reflected light, the refracted / reflected light being the result of the projection light passing through the light guide, reflecting inside the light guide, and then leaving the light guide via the front surface, and the external reflected light being the result of the projection light reflecting from the front surface.

[0005] In an alternative embodiment, the projection is at an oblique angle relative to the front surface. In another alternative embodiment, the projection is perpendicular relative to the front surface. In another alternative embodiment, the interference image corresponds to a refractive index inhomogeneity between the refractive indices of the plates.

[0006] In another optional embodiment, it also includes: determining the deviation between one or more parts of the stripes and another one or more parts of the stripes, each part corresponding to one or more of the plates, and the deviation corresponds to the refractive index difference (inhomogeneity) between the refractive indices of the plates.

[0007] According to the teachings of the present embodiment, a method for determining refractive index inhomogeneity is provided, comprising the following steps: providing an interference pattern, the interference pattern comprising one or more fringes, the interference pattern being generated by the following operations: projecting projection light onto the front surface of a light guide, the light guide comprising: first pairs of outer surfaces parallel to each other, the outer surfaces comprising a front surface and a rear surface, and a set of plates, the set of plates being: parallel to each other, between the first pairs of outer surfaces, and at non-parallel angles relative to the first pairs of outer surfaces and each plate having a corresponding refractive index; and capturing an interference image of an interference pattern to measure the refractive index inhomogeneity between the plates, the interference pattern being between refracted / reflected light and external reflected light, the refracted / reflected light being the result of the projection light passing through the light guide, reflecting inside the light guide, and then leaving the light guide via the front surface, and the external reflected light being the result of the projection light reflecting from the front surface; and determining the deviation between one or more portions of the fringes and another one or more portions of the fringes, each portion corresponding to one or more of the plates, the deviation corresponding to the refractive index inhomogeneity between the refractive indices of the plates.

[0008] In an alternative embodiment, each portion of the stripes corresponds to one of the plates. In another alternative embodiment, the deviation is between adjacent plates. In another alternative embodiment, the deviation is across multiple plates. In another alternative embodiment, the deviation is across all plates.

[0009] In another alternative embodiment, determining the deviation includes determining how much fringes jump in the interference pattern.

[0010] In another optional embodiment, the step of calculating a pass / fail metric based on the deviation is also included.

[0011] In another optional embodiment, the deviation is determined using at least one technique selected from the group consisting of: analyzing the interference pattern to determine the axis of the interference pattern, rotating and / or translating the interference pattern, dividing the interference pattern into N discrete signal arrays orthogonal to the fringes, where N is significantly greater than the number of facets in the LOE, and calculating the phase of the signal array n=1 to N by using a phase extraction algorithm selected from the group consisting of: a three-bucket method or a four-bucket method, and a wavelet transform.

[0012] In another alternative embodiment, a pass / fail metric is derived from the deviation determined across the entire interferogram, and other pass / fail metrics are then derived from the deviation between adjacent plates.

[0013] In another optional embodiment, the deviation is determined by the following steps: dividing the interference pattern into N discrete signal arrays orthogonal to the fringes, calculating the phase of the signal arrays n=1 to N by using a phase extraction algorithm, and plotting the phase according to n, and determining the maximum phase difference between the overall maximum and minimum values ​​in the phase diagram.

[0014] In another alternative embodiment, the deviation is determined by using a portion of one of the fringes, performing a best fit extrapolation on the portion of one of the fringes, and comparing the extrapolation to another portion of the fringes.

[0015] In another alternative embodiment, the deviation is determined by extrapolating one of the fringes to generate an ideal fringe and calculating the deviation of the actual fringes in the fringes from the ideal fringe.

[0016] In another alternative embodiment, the projected light is slightly defocused collimated light. In another alternative embodiment, the projected light is monochromatic light. In another alternative embodiment, the projected light is in the visible spectrum.

[0017] According to the teachings of the present embodiment, a system for measuring refractive index inhomogeneity is provided, the system comprising: a display source, a collimating optical device, a light guide and a capture device, the light guide comprising: first outer surfaces parallel to each other, the outer surfaces comprising a front surface and a rear surface; and a group of plates, the group of plates being: parallel to each other, between the first outer surfaces, and at a non-parallel angle relative to the first outer surfaces and each plate having a corresponding refractive index, the capture device being arranged to capture an interference image of an interference pattern to measure the refractive index inhomogeneity between the plates, the interference pattern being between refracted / reflected light and externally reflected light: the refracted / reflected light is the result of projected light passing through the light guide, reflecting inside the light guide, and then leaving the light guide via the front surface, and the externally reflected light is the result of projected light reflecting from the front surface.

[0018] In an optional embodiment, the system also includes a processing system comprising one or more processors, the processing system configured to determine a deviation between one or more portions of the fringes and another one or more portions of the fringes, each portion corresponding to one or more of the plates, the deviation corresponding to a refractive index inhomogeneity between the refractive indices of the plates.

[0019] According to the teachings of the present embodiment, a system for determining refractive index inhomogeneity is provided, the system comprising a processing system including one or more processors, the processing system being configured to: process an interference pattern comprising one or more fringes, the interference pattern being generated by: projecting projection light onto a front surface of a light guide comprising: first outer surfaces parallel to each other, the outer surfaces comprising a front surface and a rear surface, and a set of plates, the set of plates being: parallel to each other, between the first outer surfaces, and at a non-parallel angle relative to the first outer surfaces, and each plate having a corresponding refractive index; and capture an interference image of an interference pattern to measure refractive index inhomogeneity between the plates, the interference pattern being between refracted / reflected light and external reflected light: the refracted / reflected light is the result of the projection light passing through the light guide, reflecting inside the light guide, and then leaving the light guide via the front surface, and the external reflected light is the result of reflecting the projection light from the front surface; and determine a deviation between one or more portions of the fringes and another one or more portions of the fringes, each portion corresponding to one or more of the plates, the deviation corresponding to the refractive index inhomogeneity between the refractive indices of the plates.

[0020] According to the teaching of this embodiment, a non-transitory computer-readable storage medium is provided, on which a computer-readable code for measuring refractive index inhomogeneity is embedded, and the computer-readable code includes a program code for executing any of the above methods.

[0021] According to the teachings of the present embodiment, a non-transitory computer readable storage medium having embedded thereon computer readable code for determining refractive index inhomogeneity, the computer readable code including program code for executing any of the above methods.

[0022] According to the teaching of this embodiment, there is provided a computer program which can be loaded onto a server connected to a client computer via a network, so that the server running the computer program constitutes a processing system in a system according to any of the above disclosures. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Embodiments are described, by way of example only, with reference to the accompanying drawings, in which:

[0024] Figure 1 shows a conventional prior art folded optics arrangement.

[0025] Figure 2 is a side view of an exemplary light guide optical element (LOE).

[0026] Figure 3A and Figure 3B The desired reflection behavior of a selectively reflecting surface is shown.

[0027] Figure 4is a detailed cross-sectional view of an array of selectively reflective surfaces that couple light into a substrate and then out into the viewer's eye.

[0028] Figure 5 An example of a LOE embedded in a standard eyeglass frame is shown.

[0029] Figure 6 A method for fabricating an array of selectively reflecting surfaces is shown.

[0030] Figure 7 are further details of a method for fabricating an array of selectively reflecting surfaces.

[0031] Figure 8 is an overview of an exemplary portion of a LOE test.

[0032] Fig. 9 is a high-level side-view overview of an exemplary test setup.

[0033] Fig. 10A A side-view overview of how the stripes are formed.

[0034] Fig. 10B is a detailed overview of the interference.

[0035] Fig.11 is an overview of the overall pattern of regions that produce LOE.

[0036] Fig.12 It is a photo of a good stripe pattern.

[0037] Fig.13 It is a photograph of a poor stripe pattern.

[0038] Fig.14 It is a photo of a very poor stripe pattern.

[0039] Fig.15 is a high-level partial block diagram of an exemplary system configured to implement the processing modules of the present invention.

[0040] Abbreviations and definitions

[0041] For ease of reference, this section contains a brief list of abbreviations, acronyms, and short definitions used in this document. This section should not be considered limiting. A more comprehensive description can be found below and in the applicable standards.

[0042] 1D - One Dimension

[0043] 2D - Two Dimensions

[0044] CCD - Charge Coupled Device

[0045] CRT - cathode ray tube

[0046] DMD - Digital Micromirror Device

[0047] EMB-Eye Tracking Box

[0048] FOV - Field of View

[0049] HMD-Head Mounted Display

[0050] HUD - Heads Up Display

[0051] LCD-Liquid Crystal Display

[0052] LcoS - Liquid Crystal on Silicon

[0053] LED - Light Emitting Diode

[0054] LOE - Light Guide Optical Element

[0055] OLED - Organic Light Emitting Diode Array

[0056] OPL - Optical Path Length

[0057] SLM - Spatial Light Modulator

[0058] TIR - Total Internal Reflection DETAILED DESCRIPTION

[0059] The principles and operation of the system according to the present embodiment may be better understood with reference to the drawings and accompanying description. The present invention is a system and method for measuring refractive index inhomogeneity between plates of a light-guiding optical element (LOE). The new measurement technique is based on the known shearing interferometry technique commonly used to observe interference and collimation of the test beam. Another feature of the current implementation is an innovative method for analyzing the characteristics of the generated interference pattern to characterize the differences between adjacent plates in the LOE.

[0060] One of the manufacturing challenges in producing LOE is achieving uniformity of refractive index. Non-uniformity leads to degradation of image quality. If the refractive index of the individual plates is not well matched, angular deviations will be introduced when light passes through the facets, especially at large angles relative to the normal of the facet.

[0061] Basic technology - Figure 1 to Figure 7

[0062] FIG1 shows a conventional prior art folded optical arrangement in which a substrate 2 receives light from a display source 4. The display is collimated by collimating optics 6 (e.g., a lens). Light from the display source 4 is coupled into the substrate 2 via a first reflective surface 8 such that the chief ray 11 is parallel to the plane of the substrate. A second reflective surface 12 couples the light out of the substrate and into the eye of a viewer 14. Although compact, this configuration has significant disadvantages. In particular, only very limited FOVs and eye movement boxes (EMBs) can be achieved.

[0063] Reference now Figure 2 , Figure 2 is a side view of an exemplary light guide optical element (LOE). To alleviate the above limitations, an array of selectively reflective surfaces fabricated within the light guide optical element (LOE) may be used. A first reflective surface 16 is illuminated by collimated display light (incident light beam) 18 emitted from a light source (not shown) located behind the device. To simplify the present figure, only one ray, incident light beam 18 (also referred to as a "light beam" or "incident light"), is generally depicted. Other incident light rays, such as beams 18A and 18B, may be used to specify the edges of an incident light pupil, for example, the left and right edges of the incident light pupil. In general, where an image is represented by a beam herein, it should be noted that the beam is a sample beam of the image, which is typically formed by multiple beams at slightly different angles, each corresponding to a point or pixel of the image. The beam shown is typically the centroid of the image, except where specifically referred to as the end of the image.

[0064] The reflective surface 16 is typically 100% reflective (full mirror) and reflects incident light from a source such that the light is trapped within the light guide 20 by total internal reflection. The light guide 20 is also referred to as a "planar substrate" and a "light-transmitting substrate." The light guide 20 includes at least two (major) surfaces parallel to each other, shown in the current figure as a first (rear, major) surface 26 and a second (front, major) surface 26A. Note that the "first" and "second" designations with respect to the major surfaces (26, 26A) are for ease of reference. Coupling into the light guide can be done from a variety of surfaces (e.g., the front edge, the rear edge, the side edge, or any other desired coupling-in geometry).

[0065] An incident ray 18 enters the substrate at its proximal end (right side of the figure). The light propagates through the light guide and one or more facets (usually at least a plurality of facets and typically several, more than 3 or 4 facets) toward the distal end of the light guide (left side of the figure). The light propagates through the light guide in both an initial reflected direction of propagation 28 and another direction of propagation 30.

[0066] The captured light propagates through the light guide, reflects from the interior surface of substrate 20 and passes through an array of selectively reflective surfaces 22. Selectively reflective surfaces 22 are typically partially reflective outcoupling facets, each of which couples a portion of the light out of the substrate into the eye 24 of a viewer.

[0067] Internal partially reflective surfaces, such as the selectively reflective surface 22, are generally referred to as "facets" in the context of this document. For augmented reality applications, the facets are partially reflective, allowing light from the real world to enter via the front surface 26A, pass through the substrate including the facets, and exit the substrate via the rear surface 26 to the viewer's eye 24. Partial reflection from the facets also allows a portion of the propagating light to continue to propagate to subsequent facets. The internal partially reflective surface 22 typically at least partially passes through the light guide 20 at an oblique angle to the elongated direction of the light guide 20 (i.e., neither parallel nor perpendicular). Partial reflection can be achieved by a variety of techniques, including but not limited to transmitting a certain percentage of the light and / or using polarization.

[0068] Figure 3A and Figure 3B shows the expected reflectivity behavior of a selectively reflecting surface. Figure 3A In FIG. 3 , light 32 is partially reflected from facet 34 and coupled out 38 of substrate 20. Figure 3B In FIG. 3 , light 36 passes through facet 34 without any significant reflection.

[0069] Figure 4 is a detailed cross-sectional view of an array of selectively reflective surfaces that couple light into the substrate and then out to the viewer's eye. It can be seen that light 18 from the light source 4 impinges on the first reflective surface 16 and the reflected light 42 is coupled into the substrate by total internal reflection. The captured light is gradually coupled out of the substrate at point 44 through the other two exemplary partially reflective surfaces 22. The coating properties of the first reflective surface 16 are not necessarily similar to the coating properties of the other reflective surfaces 22, 46. Such a coating can be completely reflective or a metallic beam splitter, a dichroic beam splitter, or a hybrid metallic dichroic beam splitter.

[0070] Figure 5An example of a LOE 20a / 20a' and a LOE 20b embedded in a standard eyeglass frame 107 is shown. The display source 4 and the folding and collimating optical device 6 are assembled in the arm 112 of the eyeglass frame adjacent to the LOE 20a / 20a', and the LOE 20a / 20a' is located at the edge of the second LOE 20b. For the case where the display source is an electronic component (e.g., a small CRT, a liquid crystal display (LCD), a liquid crystal on silicon (LCoS), a digital micromirror device (DMD), a micro LED array, or an OLED), the drive electronics 114 for the display source can be assembled in the rear of the arm 112. The power supply and data interface 116 can be connected to the arm 112 by leads 118 or other communication means including radio or optical transmission. Alternatively, the battery and micro data link electronics can be integrated into the eyeglass frame. The current figure is an example, and other possible head-mounted display arrangements can be constructed, including assemblies in which the display source is mounted parallel to the LOE plane or in front of the LOE.

[0071] Further details of this basic technique can be found in US Pat. No. 7,643,214.

[0072] Figure 6 A method of manufacturing a partially reflective surface array is shown. The surfaces of a plurality of transparent flat plates (422R, shown as five exemplary plates S1 to S5) are coated with a desired coating 440, and the plates (S1 to S5) are then joined together to form a stacked form 442. Segments 444 are then cut from the stacked form by cutting, grinding, and polishing to produce the desired reflective surface array 446, which can then be assembled with other elements to achieve the entire LOE. More than one array 446 can be manufactured from each segment 444, depending on the actual size of the coated plates (S1 to S5) and the desired size of the LOE.

[0073] Figure 7 are further details of a method for manufacturing an array of selectively reflective surfaces. The sliced ​​light guide 446 is cut along the imaginary line 420A at the proximal end that will become the light guide and along the imaginary line 420B at the distal end that will become the light guide (e.g., cut vertically along both sides). This produces the light guide 420. Optionally, the light guide 424 and the light guide 426 are attached to the proximal end and the distal end of the light guide 420, respectively, and the combination (of the light guide 420, the light guide 424, and the light guide 426) is polished to produce the light guide (LOE) 429. Alternatively, the end plates (S1 and S5) can be thicker than the middle plates (S2, S3, S4), resulting in the sliced ​​light guide 446 having a structure similar to the light guide 429, wherein the regions of the attached light guide 424 and the light guide 426 are from the end plates (S1 and S5).

[0074] Typically, each of the original transparent flat plates 422R produces a set of coated plates 422. Each plate 422 has three opposite faces, each parallel to the outer surface of the plate. One pair of opposite faces (shown as the left and right sides in the current figure) is coated and joined to an adjacent plate to form the main axis of the light guide (LOE). The other opposite faces ("ends", shown in the current figure as at the top and bottom) are then ground and polished separately to form the main outer surfaces (including the front surface 26A and the rear surface 26). The set of coated plates includes facets produced where the plates meet, so the set of plates includes a set of facets. The set of facets are parallel to each other, between the pair of main outer surfaces (26A, 26), and at non-parallel angles relative to the main outer surfaces (26A, 26).

[0075] Although the current figures show a LOE with non-overlapping facets, this is not limiting and implementations of the present invention may also be applied to overlapping facets, e.g., double overlap and triple overlap (not shown, see PCT / IL2018 / 050025 filed on January 8, 2018, the entire contents of which are incorporated herein by reference).

[0076] Detailed description of the first embodiment ( Figures 8 to 15 )

[0077] Reference now Figure 8 , Figure 8 is an overview of an exemplary portion of a test of a LOE light guide 20. The current exemplary implementation uses the Figure 2 LOE as described. In operation, the incident light 18 can enter the LOE 20 at the proximal end 54. In the current test setup, as is known in the art, the display source 804 outputs light to the collimating optical device 806 to produce light for testing (as shown by the test light of projected light R). The test light (projected light R) can be collimated, but is typically not strictly collimated, but is slightly defocused to produce the projected light R for testing. The test is preferably performed using monochromatic light (i.e., light of a single wavelength), but can also be performed with any wavelength (typically in the visible spectrum). This implementation is not limiting and other wavelengths, such as infrared, can be used. For orientation, in the current figure, the front surface 26A is oriented toward the incident light R, typically at an angle as shown in the following figure ( Figures 9 to 10B ). In this case, the incident light rays exit the page at a downward angle to the front surface 26A of the LOE 20. The projection light R is typically projected toward the front surface 26A at an oblique angle relative to the front surface 26A as shown. This exemplary implementation is not limiting, and the projection light R may alternatively be projected perpendicularly toward the front surface 26A relative to the front surface 26A.

[0078] Refracted / reflected ray R' and externally reflected ray Re' travel out of the page from front surface 26A at an upward angle. Fig. 9This can be better understood with reference to test setup 900 in (described below).

[0079] Test rays R are shown as exemplary test rays R1 to R7, which pass through and exit the LOE as corresponding exemplary refracted / reflected rays R' (shown as R1' to R7'). LOE light guide 20 includes facets 22, which are shown as double lines between the major surfaces (front surface 26A and back surface 26). Note that the paths of refracted / reflected rays R1' to R7' are simplified for clarity. As described below, the optical path length of each refracted / reflected ray R' at facet 22 will vary due to the mismatch (non-uniformity) between the plates on either side of the facet.

[0080] The test light R will also be reflected by the outer surface (front surface 26A) as external reflected light Re'. For clarity, only one exemplary external reflected light R7e' is shown in the current figure (for clarity, it is obviously not at the correct reflection angle).

[0081] Shown in the present figure is an exemplary plate 422 comprising a first refractive index n 1 The plate 1S1 has a second refractive index n 2 The plate 2S2 has a third refractive index n 3 The plate 3S3 has a fourth refractive index n 4 Plate 4S4, having a fifth refractive index n 5 The plate 5S5 has a sixth refractive index n 6 Also shown are facets F12 between plate 1S1 and plate 2S2, facet F23 between plate 2S2 and plate 3S3, facet F34 between plate 3S3 and plate 4S4, facet F45 between plate 4S4 and plate 5S5, and facet F56 between plate 5S5 and plate 6S6. Alternatively, the plates may be described as being between facets, e.g., plate 2S2 being between facets F12 and facet F23.

[0082] Ideally, the index of refraction of each plate should match, i.e., the index should be uniform: n 1 =n 2 =n 3 =n 4 =n 5 =n 6 However, in practice there are usually some differences between one or more indices. In the exemplary case, the first group (n 1 、n 2 、n 4 、n 6 ) have different indices, and the second group (n 3 、n 5) have the same index different from that of the first group. In the present case, the test ray R1 will pass through 20% of the LOE via the plate 2S2 having a refractive index of n2, refract at the facet F23 to pass through 80% (the remaining width) via the plate 3S3 having a refractive index of n3, pass from the back surface 26 as a refracted / reflected ray R1' via the plate 3S3 having a refractive index of n3 to pass through the remaining 20% ​​of the width of the LOE via the plate 2S2 having a refractive index of n2, and exit the LOE via the front surface 26A.

[0083] Similarly, test rays R2 to R7 will pass through the LOE as respective reflected rays R2' to R7' as follows (only one direction is described for the corresponding refractive index in brackets for each plate):

[0084] R2 as 100% via R2' of plate 3S3(n3),

[0085] R3 as R3' with 80% via plate 3 S3 (n3) and 20% via plate 4 S4 (n4),

[0086] R4 as R4' 50% via plate 3 S3 (n3) and 50% via plate 4 S4 (n4),

[0087] R5 as 20% via plate 3S3(n3) and 80% via plate 4S4(n4) R5',

[0088] R6 as 100% via R6' of plate 4S4(n4), and

[0089] R7 is taken as 100% via R7' of plate 5S5(n5).

[0090] In the current situation, due to n 3 =n 5 , so the effective optical path lengths of test light R2 and test light R7 (and the corresponding refracted / reflected light R2' and refracted / reflected light R7'), and the phase differences between the portions of light reflected from front surface 26A and rear surface 26 should be similar. However, as the ratio of the non-uniform substrate (plate) changes, the optical path lengths of test light R2 to test light R6 and the resulting phase differences should change.

[0091] Reference now Fig. 9 , Fig. 9is a high-level side view schematic diagram of an exemplary test setup. Test setup 900 shows an exemplary configuration for testing. For clarity, the external reflected light Re' is shown using a double dot-dash line in the figure, and the refracted / reflected light R' is shown using a dot-dash line. The display source 804 outputs light to the collimating optical device 806 to produce the test light R. The refracted / reflected light R' is output from the LOE 20 and received by the capture device 1002. The external reflected light Re' is reflected from the outer surface of the LOE 20 and is received by the capture device 1002. The capture device 1002 captures an image of the resulting interference signal between the refracted / reflected light R' and the external reflected light Re'. For convenience, the interference image generated by the refracted / reflected light R' and captured by the capture device 1002 will be referred to as an interference pattern in this specification, that is, a pattern formed by wave interference, especially a pattern represented in a photograph or figure, as described below.

[0092] The interference pattern is used to measure the refractive index non-uniformity between the plates 422. The interference pattern is between the refracted / reflected light (R') and the external reflected light (Re'). The refracted / reflected light (R') is the result of the projected light R passing through the light guide 20, reflecting inside the light guide 20, and then exiting the light guide 20 via the front surface 26A. The external reflected light (Re') is the result of reflecting the projected light R from the front surface 26A.

[0093] The capture device may include various implementations (such as are convenient and sufficient for the particular desired test). For example, the interference signal (light rays R', Re') may be projected onto a screen, and a handheld camera may be used to take a picture of the resulting interference pattern. In another example, a CCD (charge coupled device) may be positioned to directly capture the interference of the light rays (R', Re'). The interference pattern may optionally be stored and processed by the processing module 1004.

[0094] Reference now Fig. 10A , Fig. 10A1 is a side view of how fringes are formed. Fringes are formed when a reference ray (projected test ray R) reflected from the front surface 26A of the first LOE 20 (externally reflected light Re') meets a corresponding refracted / reflected ray R' (which enters the LOE 20, reflects from the rear surface 26), and then passes from the LOE 20 to the screen (capture device 1002). In the current figure, exemplary portions of (reference test ray R) are shown as a first portion Ra and a second portion Rb. The first portion Ra passes through, reflects, refracts, and exits the LOE 20 as a reflected ray Ra'. The second portion Rb is reflected from the external front surface 26a of the LOE 20 as an externally reflected ray Rb'. Fringes are observed when the outgoing beam (test ray R) from the (automatic) collimator system is slightly defocused. The defocus of the collimator 806 does not affect the relative change in the fringe deviation. A greater defocus of the collimator 806 will result in more fringes that are closer together than a lesser defocus of the collimator 806 will result in relatively fewer fringes that are further apart. If the collimator is not defocused, the resulting generated interference pattern will be a single fringe, i.e., all one shadow. Typically, the collimator 806 is adjusted (defocused) enough to observe enough fringes to identify fringe jumps, but not adjusted (defocused) too much, as the fringes will increase in number and become closer together and changes in the shape of the fringes (the form of the fringes) will be discernible. Typically, the outer surfaces of the light guide 20 (front surface 26A and rear surface 26) are machined to a high degree of precision and do not contribute to fringes.

[0095] For reference, Figure 8 The middle lines R1 to R7 are "stacked" on Fig. 10A In the page shown in , the current Fig. 10A The view of LOE 20 in Figure 8 The left side of LOE 20.

[0096] Reference now Fig. 10B , Fig. 10B is a detailed overview of interference. The appearance of the signal (captured image) on the screen indicates the optical path difference between the external reflected ray Re' (dash-dotted line) and the reflected / refracted ray R' (dash-dotted line) of the current figure. The current figure is an example of refraction at the interface between air and S-TIM8 glass (n=1.5955), so the refraction angle inside LOE 20 is 26.3°.

[0097] Reference now Fig.11 , Fig.11is an overview of the overall pattern of the various regions that produce the LOE. Since the fringe pattern includes interference between light that does not pass through the LOE 20 (externally reflected light Re') and light that passes through the LOE 20 (refracted / reflected light R'), the overall pattern shows comparative information about the light that passes through the various regions (i.e., different plates) of the LOE 20. In the current figure, the plates S11, S12, S13 have respective refractive indices (n 1 、n 2 、n 1 ). This is shown in the current figure, where ray path 1101 and ray path 1102 pass through LOE 20 through different plates with potentially different refractive indices. The current figure shows these rays (1101, 1102). Fig. 10B In the orientation of , the ray path 1101 and the ray path 1102 overlap and are shown as reflected / refracted ray R'. The resulting interference pattern 1200 shows the optical path difference between the external reflected ray Re' and the reflected / refracted 1101 / 1102 ray as a band B or "stripes" between black areas or dark areas D. Three exemplary bands B1, B2, B3 are shown between four exemplary dark areas D1, D2, D3, D4. In the case of a helium-neon red laser source, the band B will be red between the black areas D. Alternatively, a grayscale image or black / white image can be used, for example, the current black areas D and white stripes B of the current figure.

[0098] The fringe (band B) is the area extending from the center of the dark area of ​​the interference pattern through the bright area and back to the center of the next dark area. For example, fringe B1 is the bright area between dark area D1 and dark area D2. The bright area appears when light rays from two different paths coincide and are in phase with each other. That is, the optical path lengths of the light rays are separated by a whole wavelength, and constructive interference occurs. Therefore, when light rays from two different paths coincide and are out of phase with each other, that is, the optical path lengths are separated by (n+1 / 2) wavelengths, destructive interference occurs and the area will be dark. Therefore, the resulting fringe pattern is a direct indication of the relative path lengths in the LOE, which in turn is an indication of the refractive index change.

[0099] In the present figure, the fringes are generally horizontal bands corresponding to the alignment along the major axis of LOE 20. Two slopes are shown: slope 1130, which corresponds to the area of ​​LOE 20 for plates S11 and S12, and slope 1132, which corresponds to the area of ​​LOE 20 for plates S12 and S13. The slopes in interference pattern 1200 represent the ray paths (reflected / refracted rays R', as shown in FIG. Figure 8 The area gradually separated between the two plates (n in the current figure) 1 and n 2 The line labeled 1211 defines the ratio of n1 Characteristics (plate S11 has a refractive index n 1 , where 100% of the light passes through plate S11 (i.e., 100% n 1 ) passes through LOE). Similarly, the line labeled 1212 defines n 2 Characteristics (plate S12 has a refractive index n 2 , where 100% of the refracted / reflected light passes through plate S12 / 100% n 2 through LOE).

[0100] If there is a difference between the refractive indices of the plates (S11, S12, S13), then the 1101 / 1102 rays will have different optical path lengths (OPLs) and the change in phase will be reflected in the deviation of the fringe pattern. This difference between the features of the (usually adjacent) plates is called a "fringe jump". The fringe jump can be seen in the interference pattern 1200 by the vertical difference between a portion of the fringe at a first horizontal position and another portion of the fringe at another horizontal position. As a non-limiting example, it can be seen from the exemplary fringe B2 that there is a fringe jump 1114. In general, the fringe (fringe size, fringe width) will be the same size as shown by the fringe size 1116. In general, the fringe size and fringe jump can be measured from any point in the fringe, for example, from the midpoint of the dark area D or the midpoint of the fringe B. In the current example, the fringe jump 1114 is measured from the first portion having the first point 1111 at line 1212 to the second portion having the second point 1112 at line 1212. In this case, the difference in optical path length from plate S11 to plate S12 results in a difference between fringes of approximately one fringe (fringe jump 1114 of one fringe 1116).

[0101] As an example calculation, with 1×10 -4 How will the unevenness of the stripes on adjacent plates differ?

[0102] The refraction angle inside the material is calculated by the following formula:

[0103]

[0104] where θ 1 Yes 1 The refraction angle at the plate, and θ 2 Yes 2 The refraction angle at the plate.

[0105] Then the difference in optical path length between the paths in these adjacent plates will be:

[0106]

[0107] where d is the slice thickness (width of LOE 20), m is the number of resulting fringe jumps, and λ is the wavelength of the test light R.

[0108] For parameter n 1 =1.5955 n 2 =1.5956 d=1.5mm Δn=1·10 -4

[0109] λ=632.8.10 -9

[0110] →m=0.40

[0111] Therefore, 1×10 -4 Refractive index inhomogeneities in the plate will appear as fringe jumps of 0.40 fringe.

[0112] Reference now Fig.12 , Fig.12 is a photograph of a good fringe pattern. In the current figure, the interference pattern shows that the maximum fringe jump is less than 0.3 of the fringe. This means that the inhomogeneity is less than 1×10 -4 .

[0113] Reference now Fig.13 , Fig.13 is a photograph of a poor fringe pattern. In this example, a “poor” fringe pattern indicates that the inhomogeneity is greater than the maximum allowable inhomogeneity for the specific application of the device under test (LOE 20). On the left side of the interferogram in the current figure, fringe jumping can be seen almost across the fringe. This is consistent with a 2×10 -4 The non-uniformity corresponds to .

[0114] Reference now Fig.14 , Fig.14 is a photograph of a very poor fringe pattern. This significant non-uniformity results in clearly visible jumps in the pattern viewed using LOE 20 for this test.

[0115] The vertical lines 1400 drawn on the current plot represent facet boundaries. The fringe separations represented by points 1402 represent the maximum inhomogeneity. In the current interferogram, each of the six plates has a significant refractive index difference. In the worst case, the fringe jump is two full fringes (fringe jump 1406 between horizontal lines 1404). This is consistent with approximately 5×10 -4 The non-uniformity corresponds to .

[0116] Determining Stripe Jumps

[0117] Determining the deviation between one or more portions of the fringes and another or more portions of the fringes corresponds to a refractive index inhomogeneity between the refractive indices of the plates. Various methods can be used to calculate the amount (number) of fringe jumps in the interference pattern and the resulting deviation calculations (inhomogeneities). As is known in the field of image processing, it is generally not possible for a person to perform the interpretation or calculations, and a processor (e.g., processor 602) must be utilized to complete the task, e.g., the amount, accuracy, and / or complexity of the processing calculations. The use of software for interferogram analysis can facilitate these steps, and is generally required to implement methods for analyzing the interferograms to characterize the differences between adjacent plates in the LOE.

[0118] Current implementations include non-contact measurement techniques for determining the non-uniformity of a plate within a light-guiding optical element (LOE), methods for qualitative evaluation of the interferogram to determine the non-uniformity. Deviations of the fringes can be used to calculate a pass / fail metric for the non-uniformity of the LOE.

[0119] The resulting interferogram shows fringes or "jitter" resulting from the wave interference pattern of the input test light.

[0120] The interference image can be analyzed to extract a pass / fail metric, which in the context of this document is referred to as the "Quantitative Facet Index Delta" (AFID) metric. The metric can be between adjacent plates, and is typically a delta across all plates.

[0121] One exemplary implementation of analyzing the interference pattern and determining the amount of fringe jump between fringes is to extrapolate a portion of the fringes, best fit the fringes, and then compare the extrapolation to another portion of the interference pattern.

[0122] Another exemplary implementation is to extrapolate the fringes to construct (generate) "ideal" fringes. Then, the deviation (error, jump) of the actual fringe(s) from the interferogram compared to the "ideal" fringes is calculated. The measurement of the deviation can be made in sections of the fringes. The measurement of the deviation can then be used to calculate the non-uniformity of the LOE 20.

[0123] By way of example, the image (interference pattern) is analyzed in a sequence that may include all or a subset of the following steps:

[0124] 1. Analyze the interference pattern to determine the axis of the interference pattern, ie, the direction of the uniform path difference for a perfect LOE.

[0125] 2. Image rotation transformation (this step can be completed through image processing or through optical devices).

[0126] 3. Divide the interference pattern into N discrete signal arrays orthogonal to the fringes, where N is significantly larger than the number of facets in the LOE, for example, an order of magnitude larger.

[0127] 4. Calculate the phase of the signal array n=1 to N by using a known phase extraction algorithm such as

[0128] a. 3-bucket method or 4-bucket method,

[0129] b. Wavelet transform, or

[0130] c. Other known methods.

[0131] 5. Plot the phase as a function of n and determine the maximum phase difference between the overall maximum and minimum values ​​in the phase plot, ie, calculate the AFID metric.

[0132] In another embodiment, the AFID metric is calculated according to steps 1 to 4 above, and then calculated pairwise for each pair of adjacent tiles in the LOE.

[0133] Although the present embodiment is described with respect to a LOE light guide 20, it is contemplated, based on this description, that the disclosed techniques may be used to test other devices, such as devices that include multiple plate configurations and / or refractive combinations.

[0134] Fig.15 600 is a high-level partial block diagram of an exemplary system 600 configured to implement the processing module 1004 of the present embodiment. The system (processing system) 600 includes a processor 602 (one or more) and four exemplary memory devices: a random access memory (RAM) 604, a boot read-only memory (ROM) 606, a mass storage device (hard disk) 608, and a flash memory 610, all communicating via a common bus 612. As is known in the art, the processing and memory may include any computer-readable medium storing software and / or firmware and / or any hardware element (s), including but not limited to field programmable logic array (FPGA) elements (s), hardwired logic elements (s), field programmable gate array (FPGA) elements (s), and application-specific integrated circuit (ASIC) elements (s). Any instruction set architecture may be used in the processor 602, including but not limited to a reduced instruction set computer (RISC) architecture and / or a complex instruction set computer (CISC) architecture. The module (processing means) 614 is shown on the mass storage device 608, but it is apparent to one skilled in the art that the module (processing means) 614 may be located on any memory device.

[0135] The mass storage device 608 is a non-limiting example of a non-transitory computer-readable storage medium carrying computer-readable code for implementing the measurement technique methods described herein. Other examples of such computer-readable storage media include read-only memories such as CDs carrying such code.

[0136] The system 600 may have an operating system stored on a storage device, the ROM may include boot code for the system, and the processor may be configured to: execute the boot code to load the operating system to the RAM 604; execute the operating system to copy computer readable code to the RAM 604 and execute the code.

[0137] The network connection 620 provides communication to and from the system 600. Typically, a single network connection provides one or more links, including virtual connections, to other devices on a local and / or remote network. Alternatively, the system 600 may include more than one network connection (not shown), each network connection providing one or more links to other devices and / or networks.

[0138] System 600 may be implemented as a server or a client connected to the client or server, respectively, via a network.

[0139] Note that, depending on the application, various implementations of modules and processes can be used. The modules are preferably implemented in software, but can also be implemented in hardware and firmware, on a single processor or distributed processors, at one or more locations. The above module functions can be combined and implemented as fewer modules or divided into sub-functions and implemented as a greater number of modules. Based on the above description, those skilled in the art will be able to design implementations for specific applications.

[0140] Note that the above examples, numbers used, and exemplary calculations are used to aid in the description of the present embodiment. Inadvertent typographical errors, mathematical errors, and / or the use of simplified calculations do not detract from the utility and essential advantages of the present invention.

[0141] To the extent that the appended claims are drafted without multiple references, this is solely to accommodate formal requirements in jurisdictions that do not permit such multiple references. Note that all possible combinations of features implied by implementation of such multiple references are expressly contemplated and are considered part of the present invention.

[0142] It should be understood that the above description is intended to be exemplary only and that many other implementations are possible within the scope of the invention as defined in the appended claims.

Claims

1. A method for measuring refractive index inhomogeneity, comprising the following steps: (a) projecting projection light (R) onto a front surface (26A) of a light guide (20), the light guide (20) comprising: (i) a first pair of outer surfaces parallel to each other, said outer surfaces comprising said front surface (26A) and rear surface (26), and (ii) a set of coated plates (422), the set of plates (422): (A) parallel to each other, (B) between the first pair of outer surfaces, and (C) is at a non-parallel angle relative to the first pair of outer surfaces, and (iii) each of the plates (422) has a corresponding refractive index (n), and (b) capturing an interference pattern image (1200) of the interference pattern between the refracted / reflected light (R') and the external reflected light (Re') to measure the refractive index inhomogeneity between the plates (422), (i) the refracted / reflected light (R') is the result of the projected light (R) passing through the light guide (20), reflecting within the light guide (20), and then exiting the light guide (20) via the front surface (26A), and (ii) The external reflected light (Re') is a result of the projection light (R) being reflected from the front surface (26A).

2. The method according to claim 1, wherein: The projection is selected from the group consisting of: (a) is at an oblique angle relative to the front surface (26A), and (b) perpendicular to the front surface (26A).

3. The method according to claim 1, wherein: The interference pattern image (1200) corresponds to a refractive index inhomogeneity between the refractive indices (n) of the plates (422).

4. The method according to claim 1, further comprising: A deviation is determined between one or more portions of the fringes and another one or more portions of the fringes, each of the portions corresponding to one or more of the plates, the deviation corresponding to a refractive index difference between the refractive indices of the plates.

5. A method for determining refractive index inhomogeneity, comprising the steps of: (a) providing an interference pattern, wherein the interference pattern comprises one or more fringes, and the interference pattern is generated by the following operations: (i) projecting projection light (R) onto a front surface (26A) of a light guide (20), the light guide (20) comprising: (A) a first pair of mutually parallel outer surfaces, said outer surfaces comprising said front surface (26A) and rear surface (26), and (B) a set of plates (422), wherein the set of plates (422): (I) parallel to each other, (II) between the first pair of outer surfaces, and (III) is at a non-parallel angle relative to the first pair of outer surfaces, and (C) each of the plates (422) has a corresponding refractive index (n), and (ii) capturing an interference pattern image (1200) of an interference pattern between the refracted / reflected light (R') and the external reflected light (Re') to measure the refractive index inhomogeneity between the plates (422), (A) the refracted / reflected light (R') is the result of the projected light (R) passing through the light guide (20), reflecting within the light guide (20), and then exiting the light guide (20) via the front surface (26A), and (B) the external reflected light (Re') is the result of the projection light (R) being reflected from the front surface (26A), and (b) determining a deviation between one or more portions of the fringes and another one or more portions of the fringes, each of the portions corresponding to one or more of the plates, the deviation corresponding to a refractive index inhomogeneity between the refractive indices of the plates.

6. The method according to claim 5, wherein: Each of the portions of the stripes corresponds to one of the plates.

7. The method according to claim 5, wherein: The deviation is between adjacent ones of the plates.

8. The method according to claim 5, wherein: The deviation is across a plurality of the plates.

9. The method according to claim 5, wherein: The deviation is across all of the panels.

10. The method according to claim 5, wherein: Determining the deviation includes determining how much fringes jump in the interference pattern.

11. The method of claim 5, further comprising the step of calculating a pass / fail metric based on the deviation.

12. The method according to claim 5, wherein: The deviation is determined using at least one technique selected from the group consisting of: (a) analyzing the interference pattern to determine the axis of the interference pattern, (b) rotating and / or translating the interference pattern, (c) dividing the interference pattern into N discrete signal arrays orthogonal to the fringes, where N is significantly larger than the number of facets in the LOE, and (d) calculating the phase of the signal array n=1 to N by using a phase extraction algorithm selected from the group consisting of: (i) the three-bucket method or the four-bucket method, and (ii) Wavelet transform.

13. The method according to claim 5, wherein: A pass / fail metric is derived from the deviations determined across the interference pattern, and further pass / fail metrics are then derived from the deviations between adjacent plates.

14. The method according to claim 5, wherein: The deviation is determined by: (a) dividing the interference pattern into N discrete signal arrays orthogonal to the fringes, (b) calculating the phase of the signal array n=1 to N by using a phase extraction algorithm, and (c) Plotting the phase as a function of n and determining the maximum phase difference between the overall maximum and minimum values ​​in the phase plot.

15. The method according to claim 5, wherein: The deviation is determined by: (a) using a portion of one of said stripes, (b) performing a best fit extrapolation on said portion of one of said fringes, and (c) comparing the extrapolation to another portion of the fringes.

16. The method according to claim 5, wherein: The deviation is determined by: (a) extrapolating one of the fringes to generate an ideal fringe, (b) Calculating the deviation of actual stripes among the stripes from the ideal stripes.

17. A method according to any one of the preceding claims, wherein: The projected light is slightly defocused collimated light.

18. A method according to any one of the preceding claims, wherein: The projected light is monochromatic light.

19. A method according to any one of the preceding claims, wherein: The projected light is in the visible spectrum.

20. A system for measuring refractive index inhomogeneity, the system comprising: (a) display source (804), (b) collimating optics (806), (c) a light guide (20), the light guide (20) comprising: (i) a first pair of outer surfaces parallel to each other, said outer surfaces comprising said front surface (26A) and rear surface (26), and (ii) a set of plates (422), the set of plates (422): (A) parallel to each other, (B) between the first pair of outer surfaces, and (C) is at a non-parallel angle relative to the first pair of outer surfaces, and Each of the plates (422) has a corresponding refractive index (n), and (d) a capture device (1002) arranged to capture an interference pattern image (1200) of an interference pattern between the refracted / reflected light (R') and the external reflected light (Re') to measure the refractive index inhomogeneity between the plates (422), (i) the refracted / reflected light (R') is the result of the projected light (R) passing through the light guide (20), reflecting inside the light guide (20), and then exiting the light guide (20) via the front surface (26A), (ii) The external reflected light (Re') is a result of the projection light (R) being reflected from the front surface (26A).

21. The system according to claim 20 further includes a processing system (600) comprising one or more processors, wherein the processing system (600) is configured to determine a deviation between one or more portions of the stripes and another one or more portions of the stripes, each of the portions corresponding to one or more of the plates, the deviation corresponding to a refractive index inhomogeneity between the refractive indices of the plates.

22. A system for determining refractive index inhomogeneity, the system comprising a processing system (600) comprising one or more processors, the processing system being configured to: (a) processing an interference pattern, the interference pattern comprising one or more fringes, the interference pattern being generated by: (i) projecting projection light (R) onto a front surface (26A) of a light guide (20), the light guide (20) comprising: (A) a first pair of mutually parallel outer surfaces, said outer surfaces comprising said front surface (26A) and rear surface (26), and (B) a set of plates (422), wherein the set of plates (422): (I) parallel to each other, (II) between the first pair of outer surfaces, and (III) is at a non-parallel angle relative to the first pair of outer surfaces, and (C) each of the plates (422) has a corresponding refractive index (n), and (ii) capturing an interference image (1200) of an interference pattern between the refracted / reflected light (R') and the external reflected light (Re') to measure the refractive index inhomogeneity between the plates (422), (A) the refracted / reflected light (R') is the result of the projected light (R) passing through the light guide (20), reflecting within the light guide (20), and then exiting the light guide (20) via the front surface (26A), and (B) the external reflected light (Re') is the result of the projection light (R) being reflected from the front surface (26A), and (b) determining a deviation between one or more portions of the fringes and another one or more portions of the fringes, each of the portions corresponding to one or more of the plates, the deviation corresponding to a refractive index inhomogeneity between the refractive indices of the plates.

Images