Optical coatings to eliminate ghosts in optical metrology tools

Abstract

The disclosure relates to metrology measurement systems and related methods. In one or more embodiments, a measurement system is provided. The measurement system includes a table operable to hold an object, and a light engine positioned above the table. The light engine includes a light source directed toward the object, a first lens operable to collimate or focus light of the light source, a grating tray located between the light source and the first lens, and a grating connected with the grating tray. The grating includes a pattern and an anti-reflective coating on the grating. And the coating is aligned with the pattern.

Description

Technical Field

[0001] Embodiments of the present disclosure generally relate to optical elements for augmented reality, virtual reality, and mixed reality. More specifically, the embodiments described herein provide metrology methods and systems. Background Art

[0002] Virtual reality is generally considered a computer-generated simulation environment in which a user has an apparent physical presence. A virtual reality experience can be generated in 3D and viewed through a head-mounted display (HMD), such as glasses or other wearable display devices having a near-eye display panel as a lens, to display a virtual reality environment that replaces the actual environment.

[0003] However, augmented reality enables a user to see virtual object images generated for display while viewing the surrounding environment through the display lens of glasses or other HMD devices, and these images appear as part of the environment. Augmented reality can include any type of input, such as audio and tactile inputs, as well as virtual images, graphics, and videos that enhance or extend the user experience environment. As an emerging technology, augmented reality faces many challenges and design limitations.

[0004] One of the challenges is to measure the image quality standards of optical elements. To ensure compliance with the image quality standards, metrology metrics of the manufactured optical elements must be obtained. However, existing measurement systems lack the required field of view and are plagued by ghosting, which is also commonly referred to as "ghost images". Therefore, in the prior art, there is a need for measurement systems with improved field of view and reduced occurrence of ghosting, and methods of using the same. Summary of the Invention

[0005] The present disclosure relates to metrology measurement systems and related methods. In one or more embodiments, a measurement system is provided. The measurement system includes a workbench operable to hold an object, and an optical engine located above the workbench. The optical engine includes a light source pointing at the object, a first lens operable to collimate or focus the light of the light source, a grating tray located between the light source and the first lens, and a grating connected to the grating tray. The grating includes a pattern and an anti-reflection coating located on the grating. The coating is aligned with the pattern.

[0006] In one or more embodiments, a grating is provided. The grating includes a pattern and an anti-reflection coating located on the pattern, and the coating is opaque.

[0007] In one or more embodiments, a method is provided. The method includes projecting a light beam from a light engine to an optical element. The light engine is located in a measurement system. The method further includes directing the light beam through a grating to the optical element, where the light beam undergoes total internal reflection within the optical element. The method also includes using a coating located on the grating pattern to absorb the reflected light, detecting an image of one or more light beams when the light beam is coupled to a detector, and processing the image to extract metrology metrics. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] To enable a detailed understanding of the manner in which the above-recited features of the present disclosure can be obtained, reference may be made to certain embodiments, which are illustrated in the accompanying drawings. It should be noted, however, that the drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of its scope, and may include other equally effective embodiments.

[0009] Figure 1A is a perspective front view of a substrate according to an embodiment described herein.

[0010] Figure 1B is a perspective front view of an optical element according to an embodiment described herein.

[0011] Figure 2 is a schematic cross-sectional view of a measurement system according to an embodiment described herein.

[0012] Figure 3 is according to an embodiment described herein Figure 2 schematic diagram of the light engine and detector configuration within the measurement system body.

[0013] Figure 4 is a schematic diagram of a grating tray of a measurement system according to an embodiment described herein.

[0014] Figure 5 is a flowchart of an optical element metrology method according to an embodiment described herein.

[0015] For ease of understanding, the same reference numerals have been used, where possible, to indicate the same elements that are common in the figures. It is contemplated that elements and features of one embodiment can be beneficially incorporated into other embodiments without further recitation. DETAILED DESCRIPTION

[0016] Embodiments of the present disclosure generally relate to optical elements for augmented reality, virtual reality, and mixed reality. More specifically, the embodiments described herein provide metrology methods and systems. The metrology methods and systems are shown and described herein.

[0017] The described technology includes using an optical engine to project a pattern with light rays from the optical engine. The projected pattern is received by an optical element and undergoes total internal reflection through the optical element, and then is coupled to a sensor within a reflection detector. Images of one or more patterns are detected by the reflection detector. The technology also includes processing the images to extract metrology metrics.

[0018] As described above, one challenge encountered in measuring an optical element to meet image quality standards is the presence of ghosting. One source of ghosting is light reflected from the material used to form the pattern on the grating. For example, gratings typically use patterns formed of reflective materials such as metallic materials (e.g., steel). However, when these gratings are used to perform metrology, light may accidentally reflect from the object being measured (e.g., a waveguide or other optical element) back to the grating. These reflected light rays may then reflect from the pattern on the grating and return to the object being measured again, causing the object to receive a ghost image of the pattern on the grating.

[0019] Accordingly, in various embodiments, a layer of coating can be disposed on the grating, for example, by aligning and / or placing the coating on the pattern, to reduce the amount of light reflected by the pattern. These embodiments will be described in further detail in Figure 1A-5 which will be described in further detail.

[0020] Figure 1A is a perspective front view of a substrate 101 according to an embodiment described herein. The substrate includes a plurality of optical elements 100 located on a surface 103 of the substrate 101. In some embodiments that can be combined with other embodiments described herein, the optical elements 100 are waveguide combiners for virtual, augmented, or mixed reality. In some embodiments that can be combined with other embodiments described herein, these optical elements 100 are planar optical elements, such as metasurfaces.

[0021] The substrate 101 can be any substrate used in the art and can be transparent or opaque to the selected laser wavelength, depending on the use of the substrate 101. The substrate 101 includes, but is not limited to, silicon (Si), silicon dioxide (SiO2), fused quartz, quartz, silicon carbide (SiC), germanium (Ge), silicon germanium (SiGe), indium phosphide (InP), gallium arsenide (GaAs), gallium nitride (GaN), silicon nitride (SiN), or sapphire-containing materials. Additionally, the substrate 101 can have different shapes, thicknesses, and diameters. For example, the diameter of the substrate 101 may be from about 150 mm to about 300 mm. The substrate 101 can be circular, rectangular, or square in shape. The thickness of the substrate 101 may be between about 300 μm and about 1 mm. Although only nine optical elements 100 are shown on the substrate 101, any number of optical elements 100 can be placed on the surface 103 of the substrate 101.

[0022] Figure 1B is a front perspective view of the optical element 100. It should be understood that the optical element 100 described herein is an exemplary optical element, and other optical elements may be used or modified to implement various aspects of the present disclosure. The optical element 100 includes a plurality of optical element structures 102 located on the surface 103 of the substrate 101. The optical element structures 102 may be nanostructures having sub-micron dimensions, such as nanoscale dimensions. The regions of the optical element structures 102 correspond to one or more gratings 104, such as a first grating 104a, a second grating 104b, and a third grating 104c. In one embodiment that may be combined with other embodiments described herein, the optical element 100 includes at least a first grating 104a corresponding to an input coupling grating and a third grating 104c corresponding to an output coupling grating. In another embodiment that may be combined with other embodiments described herein, the optical element 100 further includes a second grating 104b corresponding to an intermediate grating. The optical element structures 102 may be tilted or binary. The optical element structures 102 may have other cross-sections, including but not limited to circular, triangular, elliptical, regular polygon, irregular polygon, and / or irregularly shaped cross-sections.

[0023] During operation, the first grating 104a receives incident light beams from a light engine, and these light beams have a certain intensity. In one embodiment that may be combined with other embodiments described herein, the light engine is a microdisplay. The incident light beams are split into T1 light beams by the optical element structures 102, and these T1 light beams have the full intensity of the incident light beams in order to direct a virtual image to the intermediate grating (if used) or the third grating 104c. In one embodiment that may be combined with other embodiments described herein, the T1 light beams pass through the optical element 100 and undergo total internal reflection (TIR) until the T1 light beams contact the optical element structures 102 of the intermediate grating. The optical element structures 102 of the intermediate grating diffract the T1 light beams into T-1 light beams, and these T-1 light beams pass through the optical element 100 and undergo TIR, and finally enter the optical element structures 102 of the third grating 104c. The optical element structures 102 of the third grating 104c couple the T1 light beams to the user's eyes. The T1 light beams coupled to the user's eyes display a virtual image from the light engine from the user's perspective and further enhance the viewing angle of the user to view the virtual image. In another embodiment that may be combined with other embodiments described herein, the T1 light beams pass through the optical element 100 and undergo total internal reflection (TIR) until the T1 light beams contact the optical element structures 102 of the third grating 104c and are coupled to display a virtual image from the light engine.

[0024] To ensure that the optical element 100 meets the image quality standards, it is necessary to obtain the metrology metrics of the manufactured optical element 100. The metrology metrics of each optical element 100 are tested to ensure that the predetermined values are achieved. The embodiments of the measurement system 200 described in this specification provide the ability to obtain multiple metrology metrics with increased throughput. The metrology metrics include one or more of the following: angular uniformity metric, contrast metric, efficiency metric, color uniformity metric, modulation transfer function (MTF) metric, field of view (FOV) metric, ghosting metric, and eye box metric.

[0025] Figure 2 FIG. is a schematic cross-sectional view of the measurement system 200 described in this specification. The measurement system 200 includes a main body 201 having a first opening 203 and a second opening 205 to allow the workbench 207 to pass through. The workbench 207 is movable in the main body 201 of the measurement system 200 in the X, Y, and Z directions. The workbench 207 includes a tray 209 operable to hold the optical element 100 (as shown herein) or one or more substrates 101, and the optical element 100 is located on the substrate 101.

[0026] The measurement system 200 is operable to obtain one or more metrology metrics, including one or more of the following: angular uniformity metric, contrast metric, efficiency metric, color uniformity metric, MTF metric, FOV metric, ghosting metric, or eye box metric. The workbench 207 and the tray 209 may be transparent so that the metrology metrics obtained by the measurement system 200 are not affected by the translucency of the workbench 207 or the tray 209. The measurement system 200 communicates with a controller 220. The controller 220 is operable to facilitate the operation of the measurement system 200.

[0027] The measurement system 200 includes an upper part 204 facing the top side 222 of the optical element 100 and a lower part 206 facing the bottom side 224 of the optical element 100. The upper part 204 of the measurement system 200 includes an alignment camera 208, a light engine 210, and a reflection detector 212. The alignment camera 208 is operable to determine the position of the workbench 207. The alignment camera 208 is also operable to determine the position of the optical element 100 located on the workbench 207. The alignment camera 208 includes an alignment camera body 211. The light engine 210 is operable to project light. For example, the light engine 210 is operable to illuminate the first grating 104a of the optical element 100. The light engine 210 includes a light engine body 213. In an embodiment that can be combined with other embodiments described herein, the light engine 210 projects a pattern onto the first grating 104a. The reflection detector 212 detects the coupled light beams projected from the third grating 104c of the optical element 100. These coupled light beams can emanate from either the top side 222 or the bottom side 224 of the optical element 100. These coupled light beams may correspond to the pattern of the light engine 210. The reflection detector 212 detects the image of one or more patterns. The image of the one or more patterns can be processed through the controller 220 to extract each metrology metric.

[0028] The lower part 206 of the measurement system 200 includes a code reader 214 and a transmission detector 216. The code reader 214 and the transmission detector are located opposite the alignment camera 208, the light engine 210, and the reflection detector 212, on the other side of the workbench 207. The code reader 214 is operable to read the code of the optical element 100, such as the quick response (QR) code or bar code of the optical element 100. The code read by the code reader 214 may include identification information and / or an indication for obtaining one or more metrology metrics of the optical element 100. The transmission detector 216 detects the coupled light beams projected through the bottom side 224 of the optical element 100 from the third grating 104c. In an embodiment that can be combined with other embodiments described herein, the transmission detector 216 is connected to a transmission detector workbench 226. The transmission detector workbench 226 is operable to move the transmission detector 216 in the X direction, Y direction, and Z direction. The transmission detector workbench 226 is operable to adjust the position of the transmission detector 216 to enhance the detection of the coupled light beams projected from the third grating 104c.

[0029] In operation, metrology metrics are obtained by illuminating a first grating 104a of the optical element 100 with the light engine 210. The light engine 210 projects a pattern onto one or more optical elements 100. The incoming light undergoes TIR until the light is coupled (e.g., reflected or transmitted) out of the optical element 100. The pattern is captured by the reflection detector 212 as one or more images. The one or more images may correspond to red, green, and blue channels. The one or more images may also correspond to one or more different metrology metrics. In various embodiments, the one or more images are full-field images.

[0030] Figure 3 An embodiment described herein shows a schematic diagram of the configuration 300 of the light engine 210 and the reflection detector 212 in the body 201 of the measurement system 200( Figure 2 ). The light engine 210 includes a light source 302, a first lens 306, and a grating tray 400. The reflection detector 212 includes a second lens 310 and a sensor 312. The light source 302, the first lens 306, the grating tray 400, and the second lens 310 are all located within the body 201.

[0031] The light source 302 is operable to project a first light beam 341. The first light beam 341 may be white light corresponding to a wavelength range. In one or more embodiments that may be combined with other embodiments described herein, the light source 302 is an LED. In another embodiment that may be combined with other embodiments described herein, the wavelength range is 390 nm to 750 nm, corresponding to white light.

[0032] The grating tray 400 is operable to move in one or more of the X, Y, and Z directions. Thus, the grating tray 400 can be adjusted so that light is projected through the grating 322. The grating tray 400 is adjusted in the Z direction to improve the quality of the pattern to be projected. For example, adjusting the grating tray 400 in the Z direction may change the angle and intensity of the light incident on the grating 322. The grating tray 400 is located between the object 350 and the light source 302. The object 350 may be the optical element 100 and / or the optical element substrate.

[0033] The first lens 306 is located between the grating tray 400 and the object 350. The first lens 306 collimates or focuses the first light beam 341 onto the object 350. In one embodiment that may be combined with other embodiments described herein, the first lens 306 is an eyepiece lens.

[0034] The optical element 100 is located on the tray 209. The optical element 100 includes a first grating 104a and a third grating 104c. The first grating 104a corresponds to the input coupling grating of the optical element 100. The third grating 104c corresponds to the output coupling grating of the optical element 100.

[0035] The second lens 310 is located between the third grating 104c and the sensor 312 of the reflection detector 212. The second lens 310 focuses light onto the sensor 312. The sensor 312 is used to measure the properties of the object 350.

[0036] The grating tray 400 includes a grating 322. The grating tray 400 may include one or more gratings 322. The grating 322 includes one or more patterns (e.g., Figure 4 the pattern 410 shown), as will be described in further detail below. In one or more embodiments, the grating 322 is a transparent substrate with an opaque pattern 410. In certain embodiments, the pattern 410 of the grating 322 comprises a metallic material such as chromium, aluminum, or silver. A coating 330 is also attached to the grating 322.

[0037] In various embodiments, the coating 330 is an anti-reflection coating. The coating 330 is located on one or more patterns 410 of the grating 322. In certain embodiments, the coating 330 is located on a non-full range of the grating, e.g., aligning the coating 330 with the pattern 410. Additionally, in certain embodiments, the coating 330 may be located on top of the pattern 410.

[0038] In one or more embodiments, the coating 330 is a multilayer coating. For example, the coating 330 may include a first layer 303 and a second layer 305. Although the coating 330 is shown as having a first layer 303 and a second layer 305, other embodiments are also contemplated. In different embodiments, the coating 330 includes two or more layers. For example, the coating 330 may include a first layer 303, a second layer 305, and a third layer. The thickness of the coating 330 is between about 10 nanometers and about 10 micrometers, such as between about 50 nanometers and about 5 micrometers, between about 100 nanometers and about 1 micrometer. In different embodiments, the first layer 303 and / or the second layer 305 may comprise a metallic material. The metal may include one or more of gold (Au), platinum (Pt), aluminum (Al), silver (Ag), chromium (Cr), and / or titanium (Ti). In different embodiments, the first layer 303 and / or the second layer 305 is a dielectric layer. The dielectric layer may comprise one or more of silicon oxide (SiOx), titanium oxide (TiOx), niobium oxide (NbOx), zirconium oxide (ZrOx), tantalum oxide (TaOx), silicon nitride (SiN), magnesium fluoride (MgF2), and / or silicon. In different embodiments, the coating 330 may be composed of alternating metal layers and dielectric layers as the first layer 303 and the second layer 305. In certain embodiments, the coating 330 includes three or more layers that alternate between metal layers and dielectric layers, such as four or more layers (e.g., two metal layers interleaved with two dielectric layers), such as six or more layers (e.g., three metal layers interleaved with three dielectric layers).

[0039] In different embodiments, the refractive index of the coating 330 is less than 6. In certain embodiments, the refractive index of the coating 330 is between about 1 and about 4.2, such as the refractive index is between about 1.2 and about 4.

[0040] In various embodiments, the coating 330 reduces the amount of reflected light 343 that is reflected back to the optical element 100 from the pattern 410 of the grating 322. For example, the coating 330 may include an opaque material that absorbs light without reflecting it back to the optical element 100. In certain embodiments, the material of the coating 330 is selected based on the type of material used to form the pattern 410. For example, if the pattern 410 is formed by depositing a metallic material on the grating 322, the coating 330 may be an oxide of that metallic material. In a specific embodiment, when the pattern 410 includes chromium, the first layer 303 and / or the second layer 305 of the coating 330 may include chromium oxide.

[0041] The material of the coating 330 is selected such that when the reflected light 343 reaches the pattern 410 through the coating 330, the reflected light 343 is absorbed by the coating 330 and reduced to 20% or less. For example, if the reflected light 343 enters the coating 330 with an intensity of 100%, the intensity of the reflected light 343 reflected back from the pattern 410 to the coating 330 is about 20% or less, such as about 15% or less, such as about 5% or less. By implementing alternating layers (such as the alternation of metal layers and dielectric layers), the reflected light 343 is more effectively absorbed by the coating 330 rather than being reflected back to the optical element 100.

[0042] In some embodiments that can be combined with other embodiments described herein, the first layer 303 includes the same metal material as the pattern 410, while the second layer 305 is a dielectric layer.

[0043] As described above, when the reflected light 343 is received by the optical element 100 and coupled out from the third grating 104c, the reflected light 343 causes a ghost image (e.g., an image of the pattern 410) to be received by the sensor 312. As described in more detail below, implementing the coating 330 on the grating 322 can reduce the occurrence of ghost images.

[0044] In operation, the light source 302 projects a first light beam 341 through the grating 322 of the grating tray 400 to generate a projected pattern 342 corresponding to the pattern 410 ( Figure 4 ). The projected pattern 342 is received by the first lens 306, which is operable to collimate or focus the projected pattern 342 onto the object 350. In one or more embodiments, the object 350 is the optical element 100 in FIG. 1. For example, the first lens 306 may collimate the projected pattern 342, and then the pattern is received by the first grating 104a. The projected pattern 342 then undergoes TIR within the optical element 100 and is coupled out as the coupled-out light 345 from the third grating 104c.

[0045] The coupled-out light 345 is transmitted to the second lens 310. The second lens 310 focuses the coupled-out light 345 onto the sensor 312 of the reflection detector 212.

[0046] In various embodiments, a portion of the first light beam 341 is reflected back from the optical element 100 to form the reflected light 343. The reflected light 343 passes back through the first lens 306 from the optical element 100 and is absorbed by the coating 330, preventing the light from being reflected back to the first grating 104a. Thus, the coating 330 reduces the occurrence of ghost images, thereby improving the measurement accuracy.

[0047] Figure 4Schematic diagram of the grating tray 400 of the measurement system 200 according to an embodiment of the present disclosure. The grating tray 400 includes one or more grating holes 409. These one or more gratings 322 are located in the grating holes 409. The grating 322 may include one or more patterns 410 to be projected onto the first grating 104a of the optical element 100. The grating 322 includes a transparent region 430 in which there is no coating 330( Figure 3 ) and / or pattern 410.

[0048] As described above in connection with Figure 3 the pattern 410 may include one or more rectangular patterns 410a, linear patterns 410b, pie patterns 410c, or any other shape or combination thereof. The pattern 410 provides a reference of known size and shape so that any change in the pattern 410 can be detected when the projected pattern 342( Figure 3 ) passes through the grating of the optical element 100.

[0049] In some embodiments, each pattern 410 of the grating 322 may correspond to a different metrology metric to be determined by the measurement system 200. For example, one or more patterns 410 may be implemented to measure different types of geometric distortions.

[0050] In some embodiments that may be combined with other embodiments described herein, a single pattern may be used to measure multiple metrology metrics. In some embodiments that may be combined with other embodiments described herein, the metrology metrics may require the use of more than one pattern. Thus, one or more gratings 322 may be used to obtain different metrology metrics of the optical element 100. The grating tray 400 is not limited to one grating 322. In some embodiments, the grating tray 400 is operable to hold multiple gratings 322, such as three or more gratings 322. For example, an array of gratings 322 may be arranged on the grating tray 400.

[0051] In some embodiments, the coating 330 is aligned with the pattern 410 such that the coating 330 is located between the pattern 410 and the object to be measured. By aligning the coating 330 with the pattern 410 such that the regions of the grating 322 not covered by the pattern 410 remain transparent, the first beam 341 can pass through the grating 322 to form a projection pattern, but the reflected light 343 will either pass through the transparent region 430 of the grating 322 or be absorbed by the coating 330, thereby reducing the occurrence of reflections and ghosts.

[0052] Figure 5is a flowchart of an optical element metrology method 500 according to the embodiments described herein. The method 500 can be used to project a pattern onto the first grating 104a of the optical element 100. The method 500 can be used in conjunction with the configuration 300 of the light engine 210. In one embodiment, which can be combined with other embodiments described herein, the light engine 210 is operable to be disposed on a rotating worktable such that the light engine 210 can be rotated and / or tilted as needed during the method 500.

[0053] In operation 501, a pattern is projected. The pattern is projected by the light engine 210. As Figure 3 shown, a first light beam 341 can be projected by the light source 302. The first light beam 341 can be directed towards the grating 322. The first light beam 341 passes from the light source 302 through the grating 322 into the first lens 306 to collimate the light. The first light beam 341 corresponds to a wavelength or a wavelength range.

[0054] In some embodiments, which can be combined with other embodiments described herein, as Figure 3 shown, the grating 322 is selected according to one or more metrology metrics to be determined. A pattern corresponding to the pattern 410 is projected onto the first grating 104a of the optical element 100. The patterns 410a, 410b, 410c can be directed by the first lens 306 towards the first grating 104a.

[0055] In operation 502, one or more pattern images are detected. The one or more pattern images are captured by the sensor 312. The pattern undergoes TIR until it is coupled out (e.g., reflected or transmitted) and captured by the reflection detector 212 as one or more images. The one or more images are processed to extract metrology metrics. In various embodiments, these images are full-field images. The one or more images can be processed by the controller 220 (as Figure 2 shown). The controller 220 can be a remote controller 220 operable to receive the one or more images. The controller 220 may include a central processing unit (CPU) configured to process computer-executable instructions stored in memory. The computer-executable instructions may include an algorithm configured to extract metrology metrics. For example, the controller 220 is configured to execute the embodiments of the method 500 described herein, such as processing the one or more images to determine the values of the metrology metrics corresponding to the respective patterns captured in the one or more images. Those skilled in the art will understand that one or more elements of the controller 220 may be located remotely and accessed via a network.

[0056] In operation 503, operations 501 and 502 are repeatedly applied to subsequent gratings 322 and / or patterns 410 located thereon.

[0057] Advantages of the present disclosure include reducing detected ghosting by applying a coating to the reflective surface of the grating 322.

[0058] While the foregoing is directed to embodiments of the present disclosure, other embodiments of the disclosure may be devised without departing from its basic scope, which is determined by the claims that follow.

Claims

1. A measurement system, the measurement system comprising: A workbench, the workbench being operable to hold an object; And An optical engine, the optical engine being located above the workbench, the optical engine comprising: A light source, the light source being directed towards the object; A first lens, the first lens being operable to collimate or focus light from the light source; A grating tray, the grating tray being located between the light source and the first lens; and A grating, the grating being connected to the grating tray, the grating comprising: A pattern; and An anti-reflection coating, the anti-reflection coating being located on the grating, the anti-reflection coating being aligned with the pattern.

2. The measurement system according to claim 1, wherein the coating comprises a multi-layer coating.

3. The measurement system according to claim 1, wherein the coating is configured to absorb light reflected from the object.

4. The measurement system according to claim 1, wherein the coating is located between the pattern and the object.

5. The measurement system according to claim 1, wherein the coating is opaque.

6. The measurement system according to claim 1, wherein the object comprises one or more optical elements located on a substrate.

7. The measurement system according to claim 6, wherein the one or more optical elements comprise waveguides.

8. The measurement system according to claim 1, wherein the pattern comprises metal.

9. The measurement system according to claim 8, wherein the coating comprises an oxide of the metal.

10. A grating, the grating comprising: A pattern; And An anti-reflection coating, the anti-reflection coating being located on the pattern, the coating being opaque.

11. The grating according to claim 10, wherein the pattern comprises metal.

12. The grating according to claim 11, wherein the anti-reflection coating comprises an oxide of the metal.

13. The grating according to claim 10, wherein the anti-reflection coating comprises a multi-layer coating.

14. The grating according to claim 10, wherein the pattern and the anti-reflection coating are located on a first side of the grating.

15. A method, the method comprising: Projecting a light beam from an optical engine towards an optical element, the optical engine being located in a measurement system; Passing the light beam through a grating towards the optical element, the light beam undergoing total internal reflection within the optical element; Using a coating located on the pattern of the grating to absorb the reflected light; Detecting one or more images of the light beam when the light beam is coupled to a detector; and Processing the images to extract metrological metrics.

16. The method according to claim 15, wherein the optical engine comprises: A body; A light source, the light source being located within the body; A first lens, the first lens being operable to collimate or focus light from the light source; And A grating tray, the grating tray being located between the light source and the first lens, the grating tray containing the grating.

17. The method according to claim 15, wherein the coating is opaque.

18. The method according to claim 15, wherein the coating is a multi-layer coating.

19. The method according to claim 15, wherein the pattern comprises metal.

20. The method according to claim 15, wherein the coating comprises an oxide.

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