Optical inspection systems and methods including consideration of optical path length variations within a sample

Abstract

The present application relates to optical inspection systems and methods including consideration of changes in optical path length within a sample. An illuminator / collector assembly may deliver incident light to a sample and collect return light returned from the sample. A sensor may measure light intensity based on the light position and light angle of the collected return light. A light selector may select a first subset of light that meets a first selection criterion from the collected return light at the sensor. In some examples, the light selector may aggregate the light intensity into bins, each bin corresponding to light in the collected return light that passes through an estimated optical path length within a corresponding optical path length range within the sample. A characterizer may determine physical properties of the sample such as absorptivity based on the light intensity, light position, and light angle of the first subset of light. Considering changes in the optical path length passed through within the sample may improve accuracy.

Description

[0001] This application is a divisional application of an invention patent application with an application date of December 22, 2015, application number 201580065101.X, and name “Optical inspection system and method including consideration of changes in optical path length within a sample”.

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] This application claims the benefit of U.S. Provisional Patent Application No. 62 / 096,276, entitled “OPTICAL INSPECTION SYSTEM AND METHOD INCLUDING ACCOUNTING FOR VARIATIONS OF OPTICAL PATH LENGTH WITHIN A SAMPLE,” filed on December 23, 2014, which is hereby incorporated by reference in its entirety. Technical Field

[0004] The present disclosure relates to an optical inspection system for optical characterization of a sample that is capable of accounting for variations in optical path length within the sample. Background Art

[0005] Many optical inspection systems deliver light to a sample, collect light reflected or scattered from the sample, and use the collected light to analyze a portion of the sample. It may be desirable to improve these optical inspection systems. Summary of the invention

[0006] The illuminator / collector assembly may deliver incident light to the sample and collect return light from the sample. A sensor such as a Shack-Hartmann sensor may measure light intensity based on the light position and light angle of the collected return light. A light selector may select a first subset of light that meets a first selection criterion from the collected return light at the sensor. In some examples, the light selector may aggregate the light into binned signals, each binned signal corresponding to light in the collected return light that passes through an estimated optical path length within a corresponding optical path length range within the sample. The characterizer may determine physical properties of the sample such as absorptivity based on the light intensity, light position, and light angle of the first subset of light. Accounting for changes in the optical path length passed through within the sample may improve accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] In the accompanying drawings, the same reference numerals may describe similar components in different views, wherein the drawings are not necessarily drawn to scale. The same reference numerals with different letter suffixes may represent different examples of similar components. The accompanying drawings generally illustrate various examples discussed in this document by way of example and not by way of limitation.

[0008] Figure 1 An example of an optical inspection system for optically characterizing a sample is shown, according to some embodiments.

[0009] Figure 2 A portion of an incident optical path in an example of an optical inspection system according to some embodiments is shown.

[0010] Figure 3 Shown is a portion of a return optical path in an example of an optical inspection system according to some embodiments.

[0011] Figure 4A , 4B and 4C according to some embodiments Figure 3 Individual views of the three light rays in the return optical path.

[0012] Figure 5 Illustration of tracking from an illuminator / collector assembly to a microlens array to a detector according to some embodiments Figure 4A , 4B and 4C light.

[0013] Figure 6 According to some embodiments, Figure 5 End view of the detector.

[0014] Figure 7 shows the proximity according to some embodiments Figure 6 End view of an example of a mask of a detector setup.

[0015] Figure 8 A flow chart illustrating an example of a method for optically characterizing a sample according to some embodiments. DETAILED DESCRIPTION

[0016] An optical inspection system can deliver light to a sample, collect light reflected or scattered from the sample, and use the collected light to determine a physical property of the sample, such as absorptivity or refractive index. For highly absorbing or scattering samples, the high absorption or scattering can reduce the intensity of light propagating through a portion of the sample. This reduction in intensity can significantly depend on the length of the optical path traversed within the sample.

[0017] For example, in some configurations where light is delivered to a sample and light reflected from the sample is collected, illumination light is delivered as a light cone that converges toward a specific location at or below the sample surface. Similarly, collected light is retrieved from the light cone that diverges away from a specific location in the sample. For these light cones, different portions of the cone may traverse different optical path lengths within the sample. For example, light rays at the center of the cone may traverse a shorter optical path within the sample than light rays at the edge of the cone.

[0018] The optical inspection system can account for differences in optical path lengths traversed within the sample based on the position and angle of the light rays within the collected light. For some samples, it is possible that the incident light ray is redirected by a single scattering event within the sample, such as reflection or redirection from a specific particle within the sample or from an interface between two internal structures within the sample.

[0019] For these samples, the optical inspection system can use the geometry of single scattering events to determine regions on the detector where the optical path length traversed within the sample can be determined relatively accurately. For these regions, light rays striking the region may have a known optical path length within the sample, or may have a relatively compact distribution of optical path lengths within the sample. For example, these regions may describe the location on the detector of light rays returning from the sample that are generated as a result of a single scattering event within the sample. For some examples, these detector regions may be relatively strongly weighted in determining physical properties of the sample. These detector regions may be selected based on the sample type, to best use the available dynamic range of the detector pixels, and so on.

[0020] Similarly, the detector may have regions where the optical path length traversed within the sample cannot be accurately determined. For these regions, light rays striking the region may have one of many optical path lengths within the sample, or may have a relatively broad distribution of optical path lengths within the sample. For example, these regions may describe the locations on the detector of light rays returning from the sample that are the result of multiple consecutive scattering events within the sample. For these multiple scattered light rays, there may be many possible paths within the sample. For some examples, these detector regions may be relatively weakly weighted or excluded when determining the physical properties of the sample.

[0021] Figure 1 An example of an optical inspection system 100 for optically characterizing a sample 102 is shown, according to some embodiments. The sample 102 is not part of the optical inspection system 100. Figure 1 The optical inspection system 100 is only one example; other suitable optical inspection systems may also be used.

[0022] The optical inspection system 100 includes an illuminator / collector assembly 104. In some examples, the illuminator / collector assembly 104 is a single objective lens. In other examples, the illuminator / collector assembly 104 includes separate illumination optics and collection optics.

[0023] The illuminator / collector assembly 104 delivers incident light 106 to the sample 102. The incident light 106 may have a certain propagation angle range at the sample 102. In some examples, the range may have an angular width of less than or equal to 20 degrees. In other examples, the range may have an angular width of less than or equal to 10 degrees. In some of these examples, the incident light 106 may be collimated, where the range has an angular width of substantially zero. In examples where refraction is exhibited at the sampling interface, the angular width at the sampling interface may be defined in a medium having a larger angular range due to the difference in refractive index. In some examples, the incident light 106 includes more than one wavelength at the same time, such as multiple discrete wavelengths or a relatively wide spectrum. In some examples, the incident light 106 includes one wavelength at a time, but the wavelength may be selectively shifted over time. In still other examples, the incident light 106 includes a relatively wide spectrum that is sequentially shifted over time. In still other examples, the incident light 106 includes multiple relatively wide non-overlapping spectral regions that all shift together over time. In some examples, the incident light 106 may be pulsed or modulated at one or more specified frequencies. In some examples, the incident light 106 may include multiple spectral regions, each of which is pulsed or modulated at its own unique frequency. In some examples, the illuminator / collector assembly 104 may include one or more light sources, such as a single semiconductor laser, multiple semiconductor lasers having the same wavelength, multiple semiconductor lasers having different wavelengths, a single light emitting diode, multiple light emitting diodes having the same wavelength, multiple light emitting diodes having different wavelengths, one or more quantum cascade lasers, one or more superluminescent light sources, one or more sources of amplified spontaneous radiation, any combination of the above, or other suitable light sources. In some examples, the illuminator / collector assembly 104 may also include one or more collimating and / or focusing optical devices, such as lenses, that can collimate and / or focus the light generated by the one or more light sources. In some examples, the illuminator / collector assembly 104 may also include one or more beam steering elements, such as beam splitters, that can reflect an incident beam and transmit a collected beam or can transmit an incident beam and reflect a collected beam.

[0024] The illuminator / collector assembly 104 can deliver incident light 106 to the sample 102 through the sampling interface 108. Figure 1In a specific example of the sampling interface 108, the sampling interface 108 is a surface of the objective lens that faces the sample 102. In some examples, the sampling interface 108 can be an outwardly facing surface of the device that is placed in contact with the sample 102 during operation. In some examples, the sampling interface 108 can be a cover glass that is placed in contact with the sample 102 during operation. In some examples, the sampling interface 108 can be a lens surface or an optical surface that is spaced apart from the sample 102 in air or another incident medium. In some examples, the sampling interface 108 can include independent first and second surfaces, wherein the incident light passes through the first surface and the returning light passes through the second surface.

[0025] The illuminator / collector assembly 104 can collect return light 110 from the sample 102 through the sampling interface 108 to form collected return light 112. In some examples, the incident light 106 and the return light 110 pass through the same sampling interface 108.

[0026] The sensor 114 may receive the collected return light 112. Unlike conventional multi-pixel detectors that are largely insensitive to light angles, the sensor 114 may measure light intensity based on the light position and light angle of the collected return light 112. Figure 1 In the example of FIG. 1 , the sensor 114 is a Shack-Hartmann sensor including a microlens array 116 and a multi-pixel detector 118 located at or near the focal plane of the microlens array 116. Another example of a suitable sensor 114 is a pinhole array, where the multi-pixel detector is disposed after the pinhole array in the optical path. These are just two examples; other suitable sensors may also be used.

[0027] Sensor 114 can generate a plurality of signals 120 corresponding to the measured light intensity. In some examples, signal 120 is electrical. In other examples, signal 120 is optical, such as having optical intensity in an optical fiber. In some examples, each pixel from multi-pixel detector 118 can generate its own signal 120. In other examples, one or more groups of pixels can be optically or electrically coupled together to form signal 120.

[0028] Computer 122 may receive the plurality of signals 120 from sensor 114. In some examples, computer 122 may aggregate at least some of the received signals into a plurality of binned signals. Each binned signal may be formed by a corresponding light ray striking sensor 114. Each of the corresponding light rays traverses a corresponding optical path length within sample 102. Figure 1In the example of , the optical path length traversed by rays of incident light 106 within sample 102 is represented by quantity A, and the optical path length traversed by rays of return light 110 within the sample is represented by quantity B, such that the total optical path length traversed within the sample is represented by quantity A+B. In some examples, it may be advantageous for quantity A+B to be equal to or approximately the inverse of the expected absorption coefficient of sample 102; this is discussed in detail in the Appendix. For these binned signals, each binned signal may correspond to rays of collected return light 112 that traversed an estimated optical path length within a corresponding optical path length range within sample 102. In some examples, the optical path length ranges may be contiguous.

[0029] Computer 122 may include a characterizer configured to determine a physical property 124 of sample 102 based on the light intensities, light positions, and light angles of the first subset of light rays. Signal 120 may supply the light intensities, light positions, and light angles to computer 122. In some examples, the characterizer may perform a first set of operations using the first subset of light rays and a second set of operations using the second subset of light rays in determining the physical property 124 of sample 102. The subsets are determined by a light selector (discussed below).

[0030] In some examples, the characterizer may fit one or more of the signals to a Beer's law calculation. In some of these examples, the characterizer may determine the absorptivity of the sample 102 based on the Beer's law calculation. In some of these examples, the characterizer may apply different weights to the signals when determining the absorptivity of the sample 102. Beer's law describes how much light is transmitted through an absorptive or scattering sample. One example of a configuration for Beer's law is T(λ)=exp(-L×c×a(λ)), where λ is the wavelength of the light, L is the optical path length that the light travels through the sample, a(λ) is the wavelength-dependent absorptivity of the substance within the sample, c is the concentration of the substance within the sample, and T(λ) is the fraction of light that leaves the sample. Other suitable configurations of Beer's law may also be used. In some examples, the absorptivity is an output quantity from the optical inspection system 100. In other examples, the output quantity may be functionally equivalent to the absorptivity, such as transmittance or complex refractive index. In other examples, absorbance may be an intermediate quantity used internally by optical inspection system 100 to calculate one or more other physical properties of sample 102 , such as analyte concentration or other suitable physical properties.

[0031] The optical inspection system may include a light selector. The light selector may select a first subset of light rays from the collected return light 112 at the sensor 114 that meets a first selection criterion. In some examples, the first selection criterion may include a first estimated path length range such as a path length traversed within the sample 102, or a path length distribution such as a distribution of path lengths traversed within the sample 102. In other examples, the first selection criterion may include a first estimated light penetration depth range within the sample 102. In some examples, the light selector 116 may additionally select a second subset of light rays from the collected return light 112 that meets a second selection criterion. For these examples, the first selection criterion and the second selection criterion may include a first estimated path length range or a first path length distribution range and a second estimated path length range or a second path length distribution range. In other examples, the light selector 116 may select more than two subsets of light rays, each with a corresponding selection criterion. Such subsets of light rays may be referred to as bins or binned light rays.

[0032] In some examples, the light selector may be operated entirely in software. For example, the computer 122 may include a lookup table that matches each pixel at the multi-pixel detector 118 to a corresponding optical path traversed within the sample. The corresponding optical path within the sample may include a light position and a light angle at a suitable longitudinal position at or near the sample 102. Such a lookup table may be prepared by tracing light rays that pass through the geometry of the illuminator / collector assembly 104 and through any additional elements between the sample 102 and the sensor 114. Such ray tracing may be performed once, with the results stored in the lookup table and accessible by the computer 122. Based on the lookup table, the computer 122 may group the detector pixels into bins, where each bin corresponds to a specified range of optical path lengths traversed within the sample 102. In some examples, the light selector may receive signals from corresponding pixels of the detector 118, average a first subset of these signals to form a first binned signal, average a second subset of these signals to form a second binned signal, and average additional subsets of these signals to form additional binned signals. In some examples, computer 122 is configured to aggregate signals from multi-pixel detector 118 into binned signals, each binned signal corresponding to light rays in the collected return light that traversed an estimated optical path length within a corresponding optical path length range within the sample. In some of these examples, the optical path length ranges may be contiguous to each other.

[0033] In other examples, the light selector may be operated at least partially in hardware. For these examples, at least some of the binning may occur at the hardware level. For example, instead of using a pixel grid at the detector, the detector area may be divided into shapes corresponding to the boxes discussed above. In this example, each detector area may receive light that passes through an optical path length within a specified optical path length range within the sample. In some of these examples, each detector area may generate a corresponding signal 120 that is directed to the computer 122. In some examples, the light selector may include a first pixel and a second pixel of a sensor, wherein the shape and size of the first pixel and the second pixel are set to measure the light intensity of the light within the first subset of light and the second subset of light. For these examples, the first pixel and the second pixel may output a first binning signal and a second binning signal, respectively. For another example, the detector may use a pixel grid, and each pixel may generate its own signal, but these signals are combined in hardware and then delivered at the computer 122.

[0034] In still other examples, the ray selector may operate in a combination of software and hardware, with some binning occurring at the sensor 114 and / or at the signal level, and some binning occurring at the computer 122 .

[0035] For each position at the detector 118, it is possible to calculate the corresponding estimated optical path traversed within the sample. However, the reliability of the calculation may vary at different positions at the detector. For some geometries, such as those associated with a single scattering event (discussed in detail below), the calculation may be relatively reliable.

[0036] In some examples, the optical inspection system 100 may set a specified threshold for reliability. Signals from detector positions exceeding the threshold may be used for subsequent calculations, and signals from detector positions below the threshold may be excluded from use in subsequent calculations. For some of these examples, the optical inspection system 100 may include an optional mask 126 positioned between the microlens array 116 and the multi-pixel detector 118. The mask 126 may block light rays that the optical inspection system 100 cannot reliably determine the optical path length passing through the sample. In some examples, the mask 126 may be directly adjacent to the multi-pixel detector 118, in contact with the multi-pixel detector 118, or deposited on the multi-pixel detector 118. The mask 126 may include at least one blocking portion that blocks a specified light ray in the collected return light 112. The mask 126 may block specific pixels on the multi-pixel detector 124. In some examples, the mask 126 is stationary over time. For example, mask 126 may be formed as a coating on an optical surface, wherein the coating includes a reflective portion, a transmissive portion, and / or an absorptive portion that does not change over time. In other examples, mask 126 may be reconfigurable over time. For example, mask 126 may be formed as a reconfigurable panel, such as a micro-mirror array, which may selectively direct portions of the collected return light 112 toward or away from detector 118, and which portions may be directed toward or away from detector 118 as desired. Alternatively, threshold processing may be performed in software in combination with or entirely by computer 122.

[0037] In some examples, the optical inspection system 100 may assign relative weights to the binned signals instead of using a single threshold. Each weight may represent a confidence level that the optical path length within the sample 102 is the same or falls within a specified range for all rays within the corresponding bin. In some examples, the relative weight varies inversely with the width of the estimated distribution of the corresponding optical path length of the binned signal. In these examples, the term "inversely" is intended to represent a monotonic relationship between the relative weight and the width of the distribution without requiring a mathematical proportional relationship. For example, as the width of the estimated distribution decreases, the relative weight may increase. As another example, as the width of the estimated distribution increases, the relative weight may decrease. In some examples, each of the corresponding rays is separated from the corresponding incident ray within the incident light by a corresponding distance when traced back through the illuminator / collector assembly 104, and the relative weight varies inversely with the corresponding distance. In some examples, the weight of each binned signal corresponds to the probability that the light that generated the binned signal originated from a single scattering event in the sample. Figures 2 to 7 and the accompanying discussion below discuss the geometry of a single scattering event.

[0038] Computer 122 may be included in a computer system comprising hardware, firmware, and software. Binning electrical signals, assigning relative weights to binned electrical signals, determining physical properties, and some intermediate computational tasks such as tracing rays and characterization may be performed in software or in digital electronic hardware, where ray tracing and characterization are hardwired into the hardware; for this document, such hardwired digital electronic hardware is considered to execute ray tracing software and / or characterizers. Examples may also be implemented as instructions stored on a computer-readable storage device that can be read and executed by at least one processor to perform the operations described herein. A computer-readable storage device may include any non-transient mechanism for storing information in a form readable by a machine (e.g., a computer). For example, a computer-readable storage device may include a read-only memory (ROM), a random access memory (RAM), a disk storage medium, an optical storage medium, a flash memory device, and other storage devices and media. In some examples, a computer system may include one or more processors that are optionally connected to a network and may be configured with instructions stored on a computer-readable storage device.

[0039] Figures 2 to 7 The specific geometry associated with a single scattering event in a sample is discussed. For light that satisfies this geometry, the optical path traversed within the sample can be determined with relatively high confidence. It is instructive to explore the geometry of a single scattering event in detail.

[0040] Figure 2 A portion of an incident optical path in an example of an optical inspection system 200 according to some embodiments is shown. Incident light 208 from a light source 204, optionally redirected by a mirror, strikes a central portion of a pupil of an illuminator / collector assembly 206 and propagates within a sample 202 substantially coincident with a central axis 210 of the illuminator / collector assembly 206. Figure 2 208 passes through three scattering events 212A, 212B, 212C. In practice, each of the scattering events 212A, 212B, 212C may redirect a portion of the incident light 208 into a corresponding redirected light ray, with the remaining portion continuing to propagate along the central axis 210 away from the illuminator / collector assembly 206. It should be understood that Figure 2 The configuration of and the use of three discrete scattering events is only one example, and other configurations may also be used. It should also be understood that the incident optical path does not necessarily have to coincide with the optical axis.

[0041] Figure 3 Shown is a portion of a return optical path in an example of an optical inspection system according to some embodiments. Figure 3 The components numbered 300-312 are similar in structure and function to Figure 2Like numbered elements 200-212 in FIG. 1 are identical. Light rays 314A, 314B, 314C are generated by corresponding scattered events 312A, 312B, 312C. Illuminator / collector 306 collects and redirects light rays 314A, 314B, 314C to form return light rays 316A, 316B, 316C.

[0042] Figure 4A , 4B and 4C according to some embodiments Figure 3 Because light rays 416A, 416B, 416C originate from points along the central axis 410 of the illuminator / collector assembly 406, light rays 416A, 416B, 416C propagate within respective planes 418A, 418B, 418C, all of which include the central axis 410 of the illuminator / collector 406.

[0043] Figure 5 5. A schematic diagram showing tracking from the illuminator / collector assembly 506 to the array 520 of microlenses 522 to the detector 528 according to some embodiments. Figure 4A , 4B Detector 528 may include a plurality of zones 530 in a one-to-one correspondence with array 520 of microlenses 522. Each zone 530 may have a center 532 corresponding to center 524 of a corresponding microlens 522. Array 520 of microlenses 522 redirects light rays 516A, 516B, 516C to form light rays 526A, 526B, 526C that strike detector 528 at locations 534A, 534B, 534C.

[0044] Figure 6 According to some embodiments, Figure 5 628. Each zone 630 may include a corresponding region 636 where light propagating from the sample to the Shack-Hartmann sensor propagates in a plane that includes the central axis of the illuminator / collector assembly. The region 636 may be linear, may be radially oriented relative to the central axis of the illuminator / collector assembly or the center 638 of the detector 628, and may include the center 632 of the corresponding zone 630. Light rays 526A, 526B, 526C ( Figure 5 ) hits detector 628 at corresponding locations 634A, 634B, 634C. When the Shack-Hartmann sensor is positioned so that the microlens array is at the Fourier plane or front focal plane of the illuminator / collector, the distance away from the zone center 632 can be the same as the distance of light 516, 516B, 516C ( Figure 5) is proportional to (or approximately proportional to, or related in a 1:1 correspondence with) the propagation angle of the optical inspection system. Other possible arrangements include input light propagating in a direction that does not coincide with the central axis of the optical inspection system.

[0045] In alternative configurations, where the incident light has a slightly larger footprint and extends through a region that may include the central axis, the light rays may be traced back through the illuminator / collector assembly and may intersect corresponding incident light rays within the incident light. For some of these configurations, the incident light may have a propagation angular range at the sampling interface, where the range may have an angular width of less than or equal to 20 degrees, or preferably less than or equal to 10 degrees. As the angular range increases, it becomes more difficult to accurately determine the optical path length traversed within the sample.

[0046] For light rays that reside in a plane that includes the central axis of the illuminator / collector assembly (or are close enough), the light path can be used to calculate the corresponding optical path length traversed within the sample. The light intensity can be correlated to the optical path length. The correlation can be used to determine physical properties of the sample, such as absorptivity, transmittance, refractive index, and other physical properties. For other light rays that do not reside in the plane (or are close enough), the optical path length cannot be easily calculated.

[0047] The specific light rays such as 310( Figure 3 ) or 410 (FIG. 4) can determine whether the particular ray resides in a plane that includes the central axis (or alternatively, the direction of the incident optical path). If the propagation angle and position of the ray show that the ray resides in one of the planes, the contribution from the ray can be relatively heavily weighted for downstream calculations of physical properties of the sample. If the propagation angle and position of the ray show that the ray does not reside in one of the planes, the contribution from the particular ray can be relatively lightly weighted for downstream calculations of physical properties of the sample.

[0048] One way to determine whether a particular ray resides in a plane that includes the central axis is to trace the ray back through the illuminator / collector assembly to the sample. There will be a particular longitudinal position where the traced ray has a minimum distance from the central axis. This minimum distance for each traced ray can be used to determine a weight, such as a relative weight, which indicates how much confidence can be determined that the length of the optical path that the traced ray traversed in the sample. If the traced ray intersects the central axis, the minimum distance is zero and the weighting factor can be its maximum value. If the traced ray becomes relatively close to the central axis, the weighting factor can be relatively high. If the traced ray is relatively far away from the central axis, the weighting factor can be relatively low. In some examples, the weighting factor used for the traced ray may vary inversely with the minimum distance between the traced ray and the central axis.

[0049] In some examples, it may be advantageous to identify one or more detector regions where it may be difficult to reliably determine an estimated optical path length traversed within the sample for light rays striking the identified regions. For example, the identified one or more regions may correspond to one or more detector regions where a confidence level for the estimated optical path length falls below a threshold. Other suitable means for determining the identified detector regions may also be used.

[0050] In some examples, the light in the one or more identified detector regions may be weighted more lightly than light outside the identified detector regions when determining the physical properties of the sample. For example, each detector pixel or a suitable group of detector pixels may be assigned a relative weight. In some examples, the relative weight may represent a confidence level that the optical path lengths within the sample are the same. In some examples, the relative weight may represent a confidence level that the optical path lengths are similar or nearly the same or span a range of widths that are within a specified percentage of the average optical path lengths traversed by all light rays included in the corresponding bin within the sample. In other examples, the relative weight may vary inversely with the width of the estimated distribution of the corresponding optical path lengths within the sample. In other examples, each light ray in the collected light is a corresponding distance away from the corresponding incident light ray in the incident light when traced back through the illuminator / collector assembly, and the relative weight may vary inversely with the corresponding distance. In other examples, the relative weight may correspond to the probability that the corresponding light ray originates from a single scattering event in the sample. Other weighting schemes are also possible.

[0051] In some examples, the light in the one or more identified detector regions can be completely excluded from determining the physical properties of the sample. This exclusion can be viewed as extreme weighting, where the excluded light is assigned a zero weight. In some examples, this exclusion can be performed by software, such as by a computer such as 122 ( Figure 1 ) is performed. In other examples, the exclusion may be performed entirely in hardware, such as using a mask such as 126 ( Figure 1 ) to execute.

[0052] Figure 7 FIG. 1 shows a schematic diagram of a Shack-Hartmann sensor according to some embodiments. Figure 6 An end view of an example of a mask 700 for a detector arrangement of FIG. Mask 700 blocks portion 1002 and transmits portion 1004 to the detector. Figure 7In the example of , mask 700 passes only light rays that pass within a threshold distance of the central axis of the illuminator / collector assembly when traced back through the illuminator / collector assembly. For this example, there are two bins. One bin corresponding to light rays falling within the transmitted portion 704 is used to calculate the physical properties of the sample. This bin may have a 100% relative weight or another suitable measure. The other bin corresponding to light rays falling within the blocked portion 702 is not used to calculate the physical properties of the sample. This bin may have a 0% relative weight or another suitable measure.

[0053] exist Figure 7 In the example of , the detector may be a multi-pixel detector, where the pixels are arranged in a rectangular pattern, and each pixel generates a corresponding electrical signal. Alternatively, the detector may be a multi-element detector, where the regions have any suitable size and shape, and each region generates a corresponding electrical signal. As another alternative, the detector may be a single-element detector that generates a single electrical signal.

[0054] Figure 8 800 for optically characterizing a sample according to some embodiments. The method 800 may be performed in an optical inspection system such as 100 ( Figure 1 ) is performed on a sample. Method 800 is only one example; other suitable methods for optically inspecting a sample may also be used.

[0055] At 802, method 800 illuminates a sample with incident light. At 804, method 800 collects return light from the sample to form collected return light. At 806, method 800 measures the light intensity based on the light position and light angle of the collected return light. At 808, method 800 aggregates at least some of the electrical signals into a plurality of binned signals. Aggregation into bins may be performed in software only, in hardware only, or in a combination of software and hardware. At 810, method 800 determines a physical property of the sample based on the binned signals. In some examples, each binned signal may correspond to a light ray in the collected return light that passes through an estimated optical path length within a corresponding optical path length range within the sample. In some examples, method 800 may also include fitting the binned signals to a Beer's law calculation; and determining the absorbance of the sample based on the Beer's law calculation.

[0056] appendix

[0057] The optical properties of a particular type of sample, such as human tissue, may vary from sample to sample, but often fall within a well-defined range of values. For example, the scattering coefficient of a particular sample typically falls within a particular scattering coefficient range, where the range may represent a distribution of values ​​of the scattering coefficient for a group of samples of the same type as the particular sample. The range may be centered on a so-called expected value, such as an expected scattering coefficient. In some examples, the expected value may be used when designing the geometry of an optical inspection system, where it is expected that most actual measured values ​​will be relatively close to, but different from, the expected value.

[0058] In an optical inspection system designed to inspect a particular type of opaque sample such that light propagating through the sample is scattered and / or absorbed by the sample as it propagates, the amount of scattering and / or absorption by the sample can affect the amount of light that reaches a detector in the optical inspection system. In other words, variations in the scattering and / or absorption coefficients between samples can cause variations in the optical power that reaches the detector between samples. Such sample-to-sample variations can be undesirable and may not fully utilize the full dynamic range of the detector. It is possible to design the optical inspection system to reduce the sensitivity of the detected optical power to sample-to-sample variations in the scattering coefficient.

[0059] The analytical model of the optical inspection system may assume that light enters the sample at an input point, is scattered once at a scattering location, changes direction only once at the scattering location, and leaves the sample at an output point. The input beam is attenuated by a factor exp[-A(μ s +μ a )], where the quantity μ s and μ a are the scattering coefficient and absorption coefficient of the sample, respectively, and the quantity A is the optical path length between the input point and the scattering location. At the scattering location, the portion of the remaining input beam, γμ s is scattered toward the output, where the factor γ describes the scattering phase function. The light scattered toward the output position is further attenuated by some amount exp[-B(μ s +μ a )], where the quantity B is the optical path length between the scattering location and the output point. The fraction of the optical power leaving the sample at the output location divided by the optical power entering the sample at the input location is given by the quantity γμ s exp[-L(μ s +μ a )], where quantity L is equal to quantity A+B and is the total optical path length traversed within the sample.

[0060] The portion of the optical power that leaves the sample is equal to 1 / μ when its derivative is equal to zero, for example when the total optical path length L traversed within the sample is equal to the inverse of the sample's scattering coefficient. sWhen L = 1 / μ s The optical power reaching the detector is maximized, which is advantageous, and is relatively insensitive to sample-to-sample variations in the amount of scatter, which is also advantageous.

[0061] To take advantage of this relative insensitivity, the optical inspection system can be designed so that the total optical path length traversed within the sample can be equal to or relatively close to the inverse of the expected scattering coefficient of the sample. For example, the total optical path length traversed within the sample can be within 0.1%, 1%, 10%, or 50% of the inverse of the expected scattering coefficient of the sample. Other suitable values ​​can also be used. The expected scattering coefficient can represent a distribution of values ​​of the scattering coefficient of a group of samples of the same type as a particular sample, such as human tissue.

[0062] The above analysis assumes a single scattering event within the sample. The above analysis also applies to multiple scattering events and limited position and angular resolution, for geometries where the detector elements receive most of the light affected by a single high-angle scattering event. The above analysis can also be applied to confocal detection with narrow collimated input and angle-limited output.

[0063] The above specific embodiments and appendices are intended to be exemplary rather than restrictive. For example, the above examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments may be used by those skilled in the art after reviewing the above description. An abstract of the invention is provided to allow the reader to quickly determine the nature of the technical disclosure. It is not considered that it will be used to interpret or limit the scope or meaning of the claims. In addition, in the above specific embodiments, various features may be grouped together to streamline the present disclosure. This should not be interpreted as meaning that unclaimed disclosed features are essential to any claim. Rather, the subject matter of the invention may be less than all the features of a particular disclosed embodiment. Therefore, the following claims are hereby incorporated into the specific embodiments as examples or embodiments, wherein each claim is independently a separate embodiment, and it is hoped that such embodiments can be combined with each other in various combinations or arrangements. The scope of the present invention should be determined by reference to the attached claims together with the full scope of equivalents that such claims have.

Claims

1. An optical inspection system for optically characterizing a sample, the optical inspection system comprising: one or more illumination sources configured to deliver incident light to the sampling interface; a collector configured to collect light returned from the sample to form collected return light; a sensor comprising a plurality of regions and measuring the collected return light, wherein the sensor generates a plurality of signals indicative of a measured light intensity, a light position, and a light angle of the collected return light, wherein the light position is a position of the collected return light at the sensor, and the light angle is an angle of the collected return light at the sensor; and A computer, the computer comprising a characterizer, the characterizer being configured to: assigning weights to the plurality of signals, wherein the assigning includes assigning different weights to signals corresponding to different regions of the sensor; and A physical property of the sample is determined based on at least some of the weighted signals.

2. The optical inspection system of claim 1 , further comprising a mask, the mask comprising: a blocking portion for blocking one or more light rays in the collected return light, and A transmissive portion for allowing one or more rays of collected return light to pass therethrough.

3. The optical inspection system of claim 1 , wherein the computer is further configured to: including a first signal of the plurality of signals in determining the physical property, wherein the first signal is associated with a first weight, and A second signal of the plurality of signals is excluded in determining the physical property, wherein the second signal is associated with a second weight, the second weight having a value of zero.

4. The optical inspection system of claim 3, wherein the first signal is associated with a first optical path length and the second signal is associated with a second optical path length.

5. The optical inspection system of claim 1, wherein: The assigned weight varies inversely with the width of the distribution of the optical path lengths of the collected return light; and The assigned weight decreases as the width of the distribution of the optical path lengths of the collected return light increases.

6. A method for optically characterizing a sample, comprising: delivering a set of incident light rays having incident light intensity to the sampling interface using one or more illumination sources of the optical assembly; collecting a set of collected light rays having a collected light intensity returned from the sampling interface using a collector of the optical assembly; determining, using a processor, incident ray paths of the set of incident light rays and collected ray paths of the set of collected light rays through an illumination source and a collector of the optical assembly; determining, using the processor, a range of propagation angles at a sampling interface based on tracking the set of incident rays and the set of collected rays; determining, using the processor, an optical path traversed within the sample; as well as The contributions are weighted based on the propagation angles of the set of collected light rays and the collected light paths.

7. The method according to claim 6, wherein: The set of incident light rays extends over an area including a central axis of the optical component; Each incident light ray in the set of incident light rays has a propagation angle range having an angular width less than twenty degrees at the sampling interface; and The intensity of the collected light rays in the set of collected light rays is related to the optical path.

8. The method according to claim 6, further comprising: A determination is made as to whether a collected light ray within the set of collected light rays lies within a plane that includes a central axis of the optical assembly.

9. The method according to claim 8, wherein: If the collected light rays are determined to be within the plane or sufficiently close to the plane, the intensity contribution of the collected light rays is used to calculate a physical property of the sample.

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