Modulation transfer function measurement device and method for optical element

Abstract

Embodiments herein provide a measurement system for determining an optical element modulation transfer function (MTF). The measurement system includes a stage operable to hold an optical element or an optical element substrate on which at least one optical element is disposed. A light engine is disposed over the platform. The light engine includes a light source operable to project light of a wavelength range to the optical element. A primary photomask is operable to form a pattern from the light projected from the light source. A first lens is operable to collimate the light from the light source toward the optical element or optical element substrate. A near field detector is operable to detect the light from the optical element or optical element substrate. The far field detector is operable to detect the light from the optical elements of the optical element substrate.

Description

Technical Field

[0001] Embodiments of the present disclosure generally relate to optical components. More specifically, embodiments described herein provide a method and apparatus for determining the modulation transfer function (MTF) of an optical component. Background Art

[0002] Virtual reality is generally considered a computer-generated simulated environment in which the user has the apparent physical presence. The VR experience can be produced in 3D and viewed with a head-mounted display (HMD), such as glasses or other wearable display elements that have near-eye display panels as lenses to display the VR environment in place of the real environment.

[0003] However, augmented reality allows the user to still view their surroundings through the display lenses of glasses or other HMD components, while also being able to see images of virtual objects that are generated for display and appear as part of the environment. Augmented reality can include any type of input, such as audio and tactile input, as well as virtual images, graphics, and video of the environment that enhance or augment the user's experience. As an emerging technology, augmented reality presents many challenges and design constraints.

[0004] One such challenge is determining the optical resolution of an optical component to ensure that image quality standards are met. Current measurement systems for optical components generally have low sampling rates and low throughput over large fields of view and cannot adequately compensate for image quality imperfections introduced by cameras and image projectors within the measurement system. Furthermore, the measurement system can be cumbersome and susceptible to imperfections associated with the measurement system's image projector. Thus, it would be desirable to have a system and method for optical resolution measurement that would not be affected by imperfections associated with image projectors or cameras and would have improved throughput. Thus, there is a need in the art for a method for determining the MTF of an optical component. Summary of the Invention

[0005] In one embodiment, a measurement system is provided. The measurement system includes a platform operable to hold an optical element or an optical element substrate having at least one optical element disposed thereon, a light engine, a near-field detector, and a far-field detector. The light engine is disposed on the platform and includes a light source, a primary light mask operable to form a pattern based on light projected from the light source, and a first lens operable to collimate light from the light source toward the optical element or the optical element substrate. The light source is operable to project light of a range of wavelengths onto the optical element. The first lens is operable to collimate light from the light source toward the optical element or the optical element substrate. The near-field detector is operable to detect light from the optical element or the optical element substrate. The far-field detector is operable to detect light from the optical element of the optical element substrate.

[0006] In another embodiment, a method is disclosed. The method includes: projecting an image of a pattern, the image projected from a light engine of a measurement system; capturing a near-field image at a near-field detector; capturing a far-field image at a far-field detector; analyzing the near-field image and the far-field image to locate a first plurality of points on the near-field image and the far-field image; converting the first plurality of points into a near-field function and a far-field function; obtaining a near-field fast Fourier transform (FFT) of the near-field function and a far-field FFT of the far-field function; and determining an optical element modulation transfer function (MTF) of one or more optical elements, the optical element MTF being determined by comparing a near-field FFT corresponding to the image with a far-field FFT. The measurement system includes a platform disposed below the light engine, the platform being operable to dispose one or more optical elements thereon. The light engine is disposed above the platform and projects a baseline image onto the one or more optical elements. The near-field detector is operable to detect light from the optical elements. The far-field detector is operable to detect light from the optical elements.

[0007] In another embodiment, a controller for a measurement system is disclosed. The controller stores instructions that, when executed by a processor, cause the measurement system to measure optical properties of an optical element of an optical element substrate disposed in the measurement system. The optical properties of the optical element or optical element substrate include a baseline image of a projected pattern. The baseline image is projected from a light engine of the measurement system. A near-field image is captured at a near-field detector of the measurement system. A far-field image is captured at a far-field detector of the measurement system. The near-field image and the far-field image are analyzed to locate a first plurality of points on the near-field image and the far-field image. The first plurality of points are converted into a near-field function and a far-field function. A near-field fast Fourier transform (FFT) of the near-field function and a far-field FFT of the far-field function are obtained. An optical element modulation transfer function (MTF) is determined for one or more optical elements. The optical element MTF is determined by comparing the near-field FFT corresponding to the baseline image with the far-field FFT. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] In order to enable a detailed understanding of the manner in which the above-described features of the present disclosure are employed, a more particular description of the present disclosure, briefly summarized above, may be made with reference to its embodiments, some of which are illustrated in the accompanying drawings. It will be noted, however, that the drawings illustrate only exemplary embodiments and are not to be considered limiting of its scope, as other equally effective embodiments may be admitted.

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

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

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

[0012] Figures 3A to 3E is a schematic illustration of a measurement system for determining the modulation transfer function (MTF) of an optical element according to embodiments described herein.

[0013] Figure 4 is a flow chart of a method for determining a modulation transfer function (MTF) of an optical element according to embodiments described herein.

[0014] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. DETAILED DESCRIPTION

[0015] Embodiments of the present disclosure generally relate to optical components. More specifically, embodiments described herein provide a method for determining the modulation transfer function (MTF) of an optical component.

[0016] Figure 1A FIG2 is a perspective front view of a substrate 101 according to an embodiment described herein. The substrate 101 includes a plurality of optical elements 100 disposed on a surface 103 of the substrate 101. The optical elements 100 are waveguide combiners for virtual, augmented, or mixed reality. In some embodiments, which may be combined with other embodiments described herein, the optical elements 100 are flat optical elements, such as metasurfaces.

[0017] The substrate 101 can be any substrate used in the art and can be opaque or transparent to the selected laser wavelength, depending on the application of the substrate 101. In addition, the substrate 101 can have a variety of shapes, thicknesses, and diameters. For example, the substrate 101 can have a diameter of about 150 mm to about 300 mm. The substrate 101 can have a circular, rectangular, or square shape. The substrate 101 can have a thickness 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 disposed on the surface 103.

[0018] Figure 1B is a perspective front view of an optical element 100. It will be understood that the optical element 100 described herein is an exemplary optical element, and that other optical elements may be used with or modified to implement aspects of the present disclosure. The optical element 100 includes a plurality of optical element structures 102 disposed on a surface 103 of a substrate 101. The optical element structures 102 may be nanostructures having submicron dimensions (e.g., nanometer-sized dimensions). 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 some embodiments, which 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 some embodiments, which may be combined with other embodiments described herein, the optical element 100 also includes a second grating 104b corresponding to an intermediate grating. The optical element structures 102 may be angled or binary. The optical element structure 102 may have other shapes, including but not limited to a circle, a triangle, an ellipse, a regular polygon, an irregular polygon, and / or an irregular cross-section.

[0019] In operation, the first grating 104a receives an incident light beam (virtual image) having an intensity from a light source. The incident light beam is split by the optical element structure 102 into a T1 beam having all the intensity of the incident light beam, thereby directing the virtual image to the intermediate grating (if used) or the third grating 104c. In some embodiments, which may be combined with other embodiments described herein, the T1 beam undergoes total internal reflection (TIR) ​​through the optical element 100 until it contacts the optical element structure 102 of the intermediate grating. The optical element structure 102 of the intermediate grating diffracts the T1 beam into a T-1 beam, which then undergoes TIR through the optical element 100 to the optical element structure 102 of the third grating 104c. The optical element structure 102 of the third grating 104c couples the T-1 beam output to the user's eye, thereby modulating the field of view of the virtual image generated by the light source from the user's perspective and further increasing the viewing angle of the virtual image that the user can view. In some embodiments, which may be combined with other embodiments described herein, the T1 beam undergoes TIR through the optical element 100 until it contacts the optical element structure 102 of the third grating 104c and is output coupled to modulate the field of view of the virtual image generated from the light source.

[0020] In order to ensure that the optical element 100 meets image quality standards, the optical element MTF of the optical element 100 is obtained. In some embodiments, the optical element MTF provides image quality information related to image resolution and image contrast. The embodiments of the measurement system 200 described herein provide the ability to obtain the optical element MTF with increased throughput and better quality control. In addition, the embodiments of the measurement system 200 described herein provide the ability to obtain the optical element MTF so that the measured optical element MTF is not strongly affected by imperfections of the image projector and / or camera, such as distortion and astigmatism. The embodiments described herein allow for the separation of image quality between the optical element 100 and the measurement system 200, which may include imperfections attributable solely to the camera or projector. The MTF is a metric used to determine the ability of the optical element 100 to transfer contrast from an object to an image at a specific resolution.

[0021] Figure 2 2 is a schematic cross-sectional view of a measurement system 200 according to an embodiment described herein. The measurement system 200 includes a body 201 having a first opening 203 and a second opening 205 to allow a platform 207 to move therethrough. The platform 207 is operable to move in the X, Y, and Z directions within the body 201 of the measurement system 200. The platform 207 includes a tray 209 that is operable to hold an optical element 100 (as shown herein) or one or more substrates 101.

[0022] The measurement system 200 is operable to project an image so that the MTF of the optical element 100 can be determined. The platform 207 and the tray 209 can be transparent so that the MTF obtained using the measurement system 200 is not affected by the translucency of the platform 207 or the tray 209. The measurement system 200 is in communication with a controller 220 that is operable to control the operation of the measurement system 200 and the method 400 described herein.

[0023] Measurement system 200 includes an upper portion 204 oriented toward the top side of optical element 100 and a lower portion 206 oriented toward the bottom side of optical element 100. Upper portion 204 of measurement system 200 includes an alignment camera 208, a light engine 210, and a reflection detector 212. Reflection detector 212 detects an outcoupled beam projected from the top side of optical element 100 from third grating 104c. Reflection detector 212 may include a near-field detector, a far-field detector, a wave sensor, or a combination thereof. Reflection detectors 212 of measurement systems 200a and 200c include a near-field detector 320. Near-field detector 320 is operable to capture an image of the outcoupled beam at the pupil plane of third grating 104c in measurement system 200a. By capturing an image of the outcoupled beam at the pupil plane of third grating 104c, measurement system 200a can image the outcoupled beam within third grating 104c. The reflection detector 212 of measurement system 200c further includes a lens 322 that focuses the outcoupled beam toward the near-field detector 320 when the near-field detector 320 is not in the pupil plane. The reflection detector 212 of measurement systems 200b and 200e includes a far-field detector 310. The far-field detector 310 is operable to capture an image of the outcoupled beam at the location of the display onto which the image will be projected. The reflection detector 212 of measurement system 200d includes a wavefront sensor 325. The wavefront sensor 325 is operable to capture an image of the outcoupled beam at the pupil plane of the third grating 104c in measurement system 200d. By capturing the image of the outcoupled beam at the pupil plane of the third grating 104c, measurement system 200d can image the outcoupled beam before it experiences distortion and dispersion as it propagates toward the display.

[0024] The transmission detector 216 detects the outcoupled beam projected from the third grating 104c through the bottom side of the optical element 100. The transmission detector 216 includes a near-field detector, a far-field detector, a wavefront sensor, or a combination thereof. The transmission detectors 216 of measurement systems 200a and 200c include a far-field detector 310. The far-field detector 310 is operable to capture an image of the outcoupled beam at the location of the display onto which the image will be projected. The transmission detector 216 of measurement system 200b includes a near-field detector 320. The near-field detector 320 is operable to capture an image of the outcoupled beam at the pupil plane of the third grating 104c in measurement system 200b. By capturing the image of the outcoupled beam at the pupil plane of the third grating 104c, measurement system 200b can image the outcoupled beam before it experiences distortion and dispersion as it propagates toward the display. The transmission detector 216 of measurement system 200e includes a wavefront sensor 325. The wavefront sensor 325 is operable to capture an image of the outcoupled beam at the pupil plane of the third grating 104 c in the measurement system 200 e. By capturing the image of the outcoupled beam at the pupil plane of the third grating 104 c, the measurement system 200 e can image the outcoupled beam before it experiences distortion and dispersion as it propagates toward the display.

[0025] The alignment camera 208 is operable to determine the position of the platform 207 and the optical element 100. The light engine 210 is operable to illuminate the first grating 104a. In some embodiments, which may be combined with other embodiments described herein, the light engine 210 projects an image of a pattern onto the first grating 104a.

[0026] The lower portion 206 of the first subsystem 202 includes a code reader 214 and a transmission detector 216. The code reader 214 is operable to read a code of an optical element, such as a quick response (QR) code or a barcode of the optical element 100. The code read by the code reader 214 may include instructions for obtaining the optical element MTF of various optical elements 100.

[0027] Controller 220 is coupled to measurement system 200. Controller 220 includes a processor 252, memory 254, and support circuitry 256 coupled to one another. Controller 220 is electrically coupled to measurement system 200 via wiring 258. In some embodiments, wiring 258 may also represent a wireless connection. Processor 252 may be any form of general-purpose microprocessor or a general-purpose central processing unit (CPU), each of which may be used in an industrial setting, such as a programmable logic controller (PLC), a supervisory control and data acquisition (SCADA) system, a general-purpose graphics processing unit (GPU), or other suitable industrial controller. Memory 254 is non-transitory and may be one or more readily available memories, such as random access memory (RAM), read-only memory (ROM), or any other form of digital storage (local or remote). Memory 254 contains instructions that, when executed by processor 252, facilitate execution of method 400. The instructions in memory 254 are in the form of a program product, such as a program that implements the methods of the present disclosure. The program code of the program product may conform to any of several different programming languages. Illustrative computer-readable storage media include, but are not limited to: (i) non-writable storage media on which information is permanently stored (e.g., a read-only memory element within a computer, such as a CD-ROM disk readable by a CD-ROM drive, flash memory, ROM chip, or any type of solid-state non-volatile semiconductor memory); and (ii) writable storage media on which variable information is stored (e.g., a floppy disk or hard disk drive within a disk drive or any type of solid-state random-access semiconductor memory). When carrying computer-readable instructions that direct the functions of the methods described herein, such computer-readable storage media are embodiments of the present disclosure.

[0028] Figures 3A to 3Eis a schematic diagram of measurement systems 200a-200e. Measurement systems 200a-200e may be used in method 400 for determining the MTF of an optical element. Measurement system 200 includes a light engine 210, a transmission detector 216, and a reflection detector 212. Light engine 210 includes a light source 302, a primary light mask 304, and a first lens 306. Light engine 210 may further include at least one of a quarter-wave plate or a linear polarizer. In some embodiments, which may be combined with other embodiments described herein, light source 302 is configured to project red light (wavelength of approximately 610 nm-660 nm), green light (wavelength of approximately 510 nm-560 nm), and blue light (wavelength of approximately 420 nm-470 nm). Primary light mask 304 may be a display.

[0029] In one embodiment, Figure 3A As shown in measurement system 200a of FIG. 1 , the transmission detector 216 includes a far-field detector 310. The far-field detector 310 is operable to detect an outcoupled beam projected from the third grating 104c through the bottom side of the optical element 100 to form a far-field image. The second lens 308 is operable to focus the outcoupled image toward the far-field detector 310. The far-field detector 310 captures a far-field image that may have lost some optical data due to MTF failure of the optical element 100. The reflection detector 212 includes a near-field detector 320. The near-field detector 320 is operable to detect an outcoupled beam projected from the third grating 104c through the top side of the optical element 100 to form a near-field image. In some embodiments, the near-field detector 320 is positioned less than approximately 3 mm from the third grating 104c of the optical element 100, such as less than approximately 1 mm from the third grating 104c of the optical element 100. The near-field detector 320 is positioned as close as possible to the third grating 104c of the optical element 100 in order to minimize the amount of optical data lost due to MTF failure of the optical element 100. The near-field image captured at the near-field detector 320 and the far-field image captured at the far-field detector 310 are operable to be compared to determine whether an MTF failure has occurred.

[0030] In another embodiment, Figure 3BAs shown in measurement system 200b of FIG. 1 , the reflection detector 212 includes a far-field detector 310. The far-field detector 310 is operable to detect the outcoupled light beam projected from the third grating 104c through the top side of the optical element 100 to form a far-field image. The second lens 308 is operable to focus the outcoupled image toward the far-field detector 310. The far-field detector 310 captures a far-field image that may have lost some optical data due to MTF failure of the optical element 100. The transmission detector 216 includes a near-field detector 320. The near-field detector 320 is operable to detect the outcoupled light beam projected from the third grating 104c through the bottom side of the optical element 100 to form a near-field image. In some embodiments, the near-field detector 320 is positioned less than approximately 3 mm from the third grating 104c of the optical element 100, such as less than approximately 1 mm from the third grating 104c of the optical element 100. The near-field detector 320 is positioned as close as possible to the third grating 104c of the optical element 100 in order to minimize the amount of optical data lost due to MTF failure of the optical element 100. The near-field image captured at the near-field detector 320 and the far-field image captured at the far-field detector 310 are operable to be compared to determine whether an MTF failure has occurred.

[0031] In another embodiment, Figure 3C As shown in measurement system 200c of FIG. 1 , the transmission detector 216 includes a far-field detector 310. The far-field detector 310 is operable to detect an outcoupled beam projected from the third grating 104c through the bottom side of the optical element 100 to form a far-field image. The second lens 308 is operable to focus the outcoupled image toward the far-field detector 310. The far-field detector 310 captures a far-field image that may have lost some optical data due to MTF failure of the optical element 100. The reflection detector 212 includes a near-field detector 320. The near-field detector 320 is operable to detect an outcoupled beam projected from the third grating 104c through the top side of the optical element 100 to form a near-field image. A third lens 322 is disposed between the third gratings 104c. The third lens is positioned less than approximately 3 mm from the third grating 104c of the optical element 100, such as less than approximately 1 mm from the third grating 104c of the optical element 100. The third lens 322 is positioned as close as possible to the third grating 104c of the optical element 100 in order to minimize the amount of optical data lost due to MTF failure of the optical element 100. The third lens 322 is operable to focus the near-field image toward the near-field detector 320. The near-field detector 320 is positioned approximately 10 mm to approximately 50 mm from the third grating 104c. The near-field image captured at the near-field detector 320 and the far-field image captured at the far-field detector 310 are operable to be compared to determine whether an MTF failure has occurred.

[0032] In another embodiment, Figure 3D As shown in measurement system 200d of FIG. 1 , the transmission detector 216 includes a far-field detector 310. The far-field detector 310 is operable to detect the outcoupled beam projected from the third grating 104c through the bottom side of the optical element 100 to form a far-field image. The second lens 408 is operable to focus the outcoupled image toward the far-field detector 310. The far-field detector 310 captures an image that may have lost some optical data due to MTF failure of the optical element 100. The reflection detector 212 is a wavefront sensor 325. The wavefront sensor 325 is operable to detect the outcoupled beam projected from the third grating 104c through the top side of the optical element 100 to form a near-field image. In some embodiments, the wavefront sensor 325 is positioned less than approximately 3 mm from the third grating 104c of the optical element 100, such as less than approximately 1 mm from the third grating 104c of the optical element 100. The wavefront sensor 325 is positioned as close as possible to the third grating 104c of the optical element 100 to minimize the amount of optical data lost due to MTF failure of the optical element 100. The near-field image captured at the wavefront sensor 325 and the far-field image captured at the far-field detector 310 can be compared to determine whether MTF failure has occurred. The wavefront sensor 325 is further capable of measuring the phase (e.g., direction) of the light of the image.

[0033] In another embodiment, Figure 3E As shown in the measurement system 200e of FIG. 1 , the reflection detector 212 includes a far-field detector 310. The far-field detector 310 is operable to detect the outcoupled light beam projected from the third grating 104c through the top side of the optical element 100 to form a far-field image. The second lens 308 is operable to focus the outcoupled image toward the far-field detector 310. The far-field detector 310 captures a far-field image that may have lost some optical data due to MTF failure of the optical element 100. The transmission detector 216 is a wavefront sensor 325. The wavefront sensor 325 detects the outcoupled light beam projected from the third grating 104c through the bottom side of the optical element 100 to form a near-field image. In some embodiments, the wavefront sensor 325 is positioned less than approximately 3 mm from the third grating 104c of the optical element 100, such as less than approximately 1 mm from the third grating 104c of the optical element 100. The wavefront sensor 325 is positioned as close as possible to the third grating 104c of the optical element 100 to minimize the amount of optical data lost due to MTF failure of the optical element 100. The near-field image captured at the wavefront sensor 325 and the far-field image captured at the far-field detector 310 can be compared to determine whether MTF failure has occurred. The wavefront sensor 325 is further capable of measuring the phase (e.g., direction) of the light of the image.

[0034] Figure 4is a flow chart of a method 400 for determining the MTF of an optical element. Method 400 is operable to be performed in other measurement systems not described herein. Method 400, described herein, includes illuminating the first grating 104a of the optical element 100 using the light engine 210, wherein the in-coupled light undergoes TIR until it is out-coupled (e.g., reflected or transmitted) from the third grating 104c as an image captured by the transmission detector 216 and the reflection detector 212. The image may correspond to the red, green, and blue channels from the light engine 210. As described in method 400, the image is processed to extract the MTF of the optical element 100.

[0035] At operation 401, a baseline image is projected toward the optical element 100 using the light engine 210 of the measurement system 200. The baseline image is projected after the light source 302 projects red, green, or blue light through the master reticle 304 to form a pattern. The baseline image has a pattern. In some embodiments that may be combined with other described embodiments, the light engine 210 is a high-resolution image projector having a field of view (FOV) of approximately 10 degrees to approximately 120 degrees. The FOV of the light engine 210 is fixed or adjustable. The pattern is determined by the master reticle 304. The master reticle 304 may have one of a checkerboard pattern, a line pair pattern, or a dot matrix pattern. In some embodiments that may be combined with other described embodiments, the master reticle 304 is a high-resolution patterned mask. The pattern of the master reticle 304 may be formed by electron beam, ion beam, or photolithography. In other embodiments that may be combined with other described embodiments, the light engine 210 is one of an LCOS, a CLP, a micro-LED, a laser beam scanner, or an organic light emitting diode (OLED) microdisplay.

[0036] The pattern is projected onto the first grating 104a through the first lens 306. The first lens 306 focuses the pattern toward the first grating 104a. The pattern undergoes TIR through the optical element 100 until the pattern is output from the third grating 104c. In some embodiments that may be combined with other embodiments described herein, the optical element 100 may include a surface relief grating-based waveguide combiner, a volume hologram-based waveguide combiner, a birdbath waveguide combiner, a partially reflective mirror array combiner, or a combiner without optical components. The pattern is determined by the master mask 304. The master mask 304 may have one of a checkerboard pattern, a line pair pattern, or a dot matrix pattern. In some embodiments that may be combined with other embodiments described herein, the master mask 304 is a high-resolution patterned mask.

[0037] At operation 402, an image is captured at the reflection detector 212 and the transmission detector 216. The image has a pattern formed by the master reticle 304. In some embodiments that can be combined with other embodiments described herein, the transmission detector 216 includes a far-field detector 310, a near-field detector 320, a wavefront sensor 325, or a combination thereof. In some embodiments, the reflection detector 212 includes a far-field detector 310, a near-field detector 320, a wavefront sensor 325, or a combination thereof. The near-field detector 320 or the wavefront sensor 325 detects a near-field image of the pattern formed by the master reticle. The near-field image is captured at or near the pupil plane of the third grating 104c, thereby minimizing optical data loss due to any potential MTF failure within the optical element 100. The far-field detector 310 detects the far-field image at a distance approximately equal to the display distance of the optical element 100 (e.g., the distance at which the optical element will project the image for viewing by a user of the optical element). Any potential MTF failure can cause distortion / degradation of the far-field image.

[0038] In operation 403, the near-field image and the far-field image are analyzed. In some embodiments that may be combined with other embodiments described herein, the near-field image and the far-field image are analyzed to locate a plurality of points on the near-field image and the far-field image. Each of the plurality of points may correspond to a different field of view (FOV) on the near-field image and the far-field image. For example, each of the plurality of points may correspond to an edge of an adjacent square in a checkerboard pattern. Depending on the pattern formed, the plurality of points are converted into near-field and far-field functions. For example, the functions may be a point spread function, a line spread function, or an edge spread function.

[0039] In operation 404, the near-field function MTF of the near-field function and the far-field function MTF of the far-field function are obtained. To correctly obtain the MTF corresponding to the baseline image, it is necessary to minimize the light intensity variation across the image. The light intensity variation across the image can be reduced by adjusting the exposure time of the image. The exposure time can be adjusted for each of the multiple points of the near-field function and the far-field function.

[0040] At operation 405 , the optical element MTF is obtained. By correlating near-field and far-field images using multiple points and functions, MTF performance can be measured and faults can be detected. Furthermore, pupil function degradation along the propagation path in the third grating 104 c can be detected. Thus, the cause of the MTF fault can be detected. The optical element MTF obtained using method 400 is less susceptible to defects in the light engine 210.

[0041] In summary, a method and apparatus for determining the MTF of an optical element are described herein. The apparatus includes a measurement system comprising a platform operable to hold an optical element or an optical element substrate having at least one optical element disposed thereon. A light engine is disposed on the platform. The light engine includes a light source operable to project light of a range of wavelengths onto the optical element. A primary light mask is operable to form a pattern based on the light projected from the light source. A near-field detector is operable to detect light from the optical element or the optical element substrate at a pupil plane of the optical element. A far-field detector is operable to detect light from the optical element of the optical element substrate at a display distance of the optical element. Images of the light detected at the far-field detector and the near-field detector are compared to determine the MTF of the optical element substrate or the optical element. The cause of the MTF failure can be further detected by the measurement system.

[0042] While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the following claims.

Claims

1. A measurement system, comprising: a platform operable to hold an optical element or an optical element substrate having at least one optical element disposed thereon; A light engine is disposed on the platform, and includes: a light source operable to project light of a range of wavelengths onto the optical element; a main light mask operable to form a pattern according to the light projected from the light source; and a first lens operable to collimate the light from the light source toward the optical element or optical element substrate; a near field detector operable to detect the light from the optical element or optical element substrate; and A far field detector is operable to detect the light from the optical element of the optical element substrate. 2 . The measurement system of claim 1 , wherein the near-field detector is positioned at a top side of the optical element or optical element substrate, and the far-field detector is positioned at a bottom side of the optical element or optical element substrate. 3 . The measurement system of claim 1 , wherein the near-field detector is positioned at a bottom side of the optical element or optical element substrate, and the far-field detector is positioned at a top side of the optical element or optical element substrate. The measurement system of claim 1 , wherein the near-field detector is a wavefront sensor. The measurement system of claim 1 , wherein the platform is operable to move in an X direction, a Y direction, and a Z direction. The measurement system of claim 1 , wherein the platform is transparent.

7. The measurement system of claim 1, further comprising an alignment camera operable to determine the position of the stage and the optical element or the optical element substrate.

8. A method comprising: a baseline image of a projected pattern, the baseline image projected from a light engine of a measurement system having: a platform disposed below the light engine, the platform being operable to have one or more optical elements disposed thereon, wherein the light engine disposed above the platform projects the baseline image onto the one or more optical elements; as well as a near-field detector operable to detect the light from the optical element; as well as a far-field detector operable to detect the light from the optical element; capturing a near-field image at the near-field detector; capturing a far-field image at the far-field detector; analyzing the near-field image and the far-field image to locate a first plurality of points on the near-field image and the far-field image, wherein the first plurality of points are converted into a near-field function and a far-field function; Obtaining a near-field fast Fourier transform (FFT) of the near-field function and a far-field FFT of the far-field function; as well as An optical element modulation transfer function (MTF) of the one or more optical elements is determined, the optical element MTF being determined by comparing the near-field FFT corresponding to the baseline image to the far-field FFT.

9. The method of claim 8, wherein the near-field detector is positioned at a top side of the optical element or optical element substrate and the far-field detector is positioned at a bottom side of the optical element or optical element substrate.

10. The method of claim 8, wherein the near-field detector is positioned at a bottom side of the optical element or optical element substrate and the far-field detector is positioned at a top side of the optical element or optical element substrate.

11. The method of claim 8, wherein the near-field detector is a wavefront sensor.

12. The method of claim 8, wherein the baseline image is projected through a reticle to form the pattern.

13. The method of claim 8, wherein the light engine has a field of view of about 10 degrees to about 120 degrees.

14. The method of claim 8, wherein the platform is operable to move in an X direction, a Y direction, and a Z direction.

15. A controller for a measurement system storing instructions, which, when executed by a processor, cause the measurement system to measure an optical property of an optical element of an optical element substrate disposed in the measurement system, wherein the optical property of the optical element or optical element substrate comprises: a baseline image of the projected pattern, the baseline image projected from a light engine of the measurement system; capturing a near-field image at a near-field detector of the measurement system; capturing a far-field image at a far-field detector of the measurement system; analyzing the near-field image and the far-field image to locate a first plurality of points on the near-field image and the far-field image, wherein the first plurality of points are converted into a near-field function and a far-field function; Obtaining a near-field fast Fourier transform (FFT) of the near-field function and a far-field FFT of the far-field function; as well as An optical element modulation transfer function (MTF) of the one or more optical elements is determined, the optical element MTF being determined by comparing the near-field FFT corresponding to the baseline image to the far-field FFT.

16. The controller of claim 15, wherein the baseline image is projected through a reticle to form the pattern.

17. The controller of claim 15, wherein the near-field detector is positioned at a top side of the optical element or optical element substrate, and the far-field detector is positioned at a bottom side of the optical element or optical element substrate.

18. The controller of claim 15, wherein the near-field detector is positioned at a bottom side of the optical element or optical element substrate, and the far-field detector is positioned at a top side of the optical element or optical element substrate.

19. The controller of claim 15, wherein the near-field detector is a wavefront sensor.

20. The controller of claim 15, wherein the light engine has a field of view of about 10 degrees to about 120 degrees.

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