Retroreflective marker with evenly distributed microelements

Abstract

A device with a retroreflective section. The retroreflective section comprises an outer surface, an inner surface, and several retroreflective microelements. One or more of the several retroreflective microelements comprise multiple reflective surfaces. Each of the several retroreflective microelements extends from the inner surface at a location distinct from the locations of the other retroreflective microelements, and each of the several retroreflective microelements comprises a central axis extending along a direction corresponding to a surface normal of the outer surface.

Description

CLAIMING PRIORITY

[0001] This application claims priority under 35 USC § 119(e) to U.S. Patent Application No. 63 / 525,382, filed July 7, 2023, the entire contents of which are hereby incorporated by reference. TECHNICAL FIELD

[0002] This disclosure relates to retroreflective markers or markings, in particular to retroreflective markers having uniformly distributed retroreflective microelements. BACKGROUND

[0003] Tracking systems (e.g. optical tracking systems) can rely on objects on which one or more markers are attached.

[0004] An optical tracking system typically comprises multiple retroreflective markers mounted at various positions with appropriate orientations to achieve retroreflection. The retroreflective markers typically contain multiple microelements that are significantly smaller in characteristic size than the retroreflective markers or the tools or equipment to which the markers are attached. These microelements are typically retroreflective, allowing incoming optical signals traveling along an incoming path to be reflected along a path substantially opposite the incoming path.

[0005] Optical tracking systems with retroreflective markers can have a variety of applications. For example, optical tracking systems can capture the movements of one or more objects, which can be used for animations, sports analytics, medical research, surgical procedures, etc. Optical tracking systems can also be used in the fields of augmented reality (AR) and virtual reality (VR), biomechanics, robotics, and computer vision. SUMMARY

[0006] Described herein is a device used as retroreflective markers, or at least as part of a retroreflective marker, for a tracking system configured to determine the position of a tracked object in an environment by estimating the location(s) or pose(s) (e.g., positions and orientations) of one or more markers attached to the tracked object. Manufacturing techniques that produce three-dimensional (3D) markers (e.g., with a curved surface) from two-dimensional (2D) retroreflective markers (e.g., 2D sheets containing many markers) can result in 3D markers having an uneven distribution of small reflective elements (e.g., microelements) and, in some cases, deforming the microelements.Accordingly, the brightness of the retroreflected optical signals generated by these markers with a curved surface becomes less uniform for incoming optical signals at different angles of incidence, thereby reducing the tracking accuracy of the tracking system.

[0007] In stark contrast, the device described here can be manufactured as a single structure, e.g., by injection molding or other suitable techniques, providing a desired distribution (e.g., a uniform distribution) of retroreflective microelements. The desired distribution can be predetermined depending on the various retroreflective tasks and achieved through one or more manufacturing processes (e.g., injection molding), etc. In addition to the desired distribution, unwanted changes in the desired distribution are reduced, undesirable deformations are reduced, etc.

[0008] More specifically, the device described herein includes a retroreflective portion for reflecting incoming optical signals in directions substantially parallel to their respective input directions (or at corresponding angles of incidence). The retroreflective portion may include one or more coupling mechanisms for coupling the retroreflective portion to one or more other retroreflective portions of corresponding devices to form an assembly. The device, or an assembly of one or more devices, may be used as retroreflective markers for a tracking system. The retroreflective portion includes an outer surface and a corresponding inner surface. The outer surface is generally curved and smooth and defines a surface normal with different directions.The inner surface can have a corresponding topology (and optionally a corresponding surface normal) as the outer surface.

[0009] The retroreflective portion also contains multiple retroreflective microelements that are generally small relative to the size of the retroreflective portion or marker. For example, a retroreflective microelement may have a characteristic length of one or more millimeters, less than one millimeter, etc. The retroreflective microelements may have one or more shapes, such as microbeads or microspheres, microcorner cubes, microprisms, etc. Each retroreflective microelement extends from the inner surface at a different location than the other retroreflective microelements. Each retroreflective microelement has a central axis extending in a direction corresponding to the surface normal of the outer surface. The alignment between the central axis and the surface normal improves the consistency of the retroreflective brightness.Because the surface normal has corresponding directions at different locations and the central axes of various retroreflective microelements are substantially aligned with the surface normal, the central axes at different locations extend in corresponding directions that correspond to the surface normal. In some cases, the central axes may extend in directions that deviate from the surface normal by a predetermined offset angle.

[0010] The retroreflective portion may further include a reflective coating applied to the surfaces of the plurality of retroreflective microelements to enhance the retroreflective effect. For example, a reflective coating may provide a wider viewing angle for retroreflection with slightly reduced brightness. Whether a reflective coating should be applied to the retroreflective portion depends on the retroreflectivity (or retroreflection) requirements. For applications where wider retroreflectivity is not required, the retroreflective portion may, for example, have retroreflectivity without a reflective coating. The reflective coating may be composed of one or more materials, such as silver, aluminum, or another similar reflective material.Furthermore, one or more of the plurality of retroreflective microelements may have multiple reflective surfaces to reflect or retroreflect incoming visual signals. For example, a first retroreflective microelement may have two reflective surfaces to reflect or retroreflect incoming light.

[0011] In one general aspect, this document describes a device having a retroreflective portion. The retroreflective portion includes an outer surface, an inner surface, and a plurality of retroreflective microelements. Each of the plurality of retroreflective microelements extends from the inner surface at a location different from the locations of the other retroreflective microelements. Each of the plurality of retroreflective microelements includes a central axis extending along a direction corresponding to a surface normal of the outer surface. The device further includes a reflective coating applied to the surfaces of the plurality of retroreflective microelements.

[0012] The embodiments may include one or more of the following features.

[0013] One of the plurality of retroreflective microelements may be a microprism. The retroreflective portion may further include a coupling mechanism for connecting the retroreflective portion to one or more other retroreflective portions.

[0014] In some embodiments, the retroreflective portion may form at least a portion of a retroreflective marker having a hollow spherical shape, a hollow conical shape, or a hollow cylindrical shape. In some cases, the retroreflective portion may form one or more retroreflective markers.

[0015] The geometry and arrangement of the plurality of retroreflective microelements can be predetermined for the manufacture of the retroreflective portion. In some embodiments, the retroreflective portion can be manufactured by injection molding.

[0016] Each edge of one of the plurality of retroreflective microelements may have a length of less than 1 mm. Each of the plurality of retroreflective microelements may define a microprism center, and two adjacent retroreflective microelements of the plurality of retroreflective microelements may be arranged at a distance of less than 1 mm between the respective microprism centers.

[0017] In some embodiments, the central axis may extend in a direction parallel to the surface normal of the outer surface by an offset. The offset may be determined by applying a randomization algorithm to the plurality of retroreflective microelements.

[0018] In some embodiments, the retroreflective portion may include a polymer that is transmissive in the near-infrared (NIR) range. The reflective coating may be applied to the surfaces of the plurality of retroreflective microelements by vapor deposition. The reflective coating may include a metallic material.

[0019] Additionally, the retroreflective portion may form part of a body of an optical tracking tool. Alternatively, the retroreflective portion may form part of a marker configured to be attached to an optical tracking tool. In some cases, the retroreflective portion may include a first retroreflective portion and a second retroreflective portion. The first retroreflective portion may form part of a body of an optical tracking tool, and the second retroreflective portion may form part of a marker configured to be attached to the tool.

[0020] Another aspect is that a marker comprises a first portion including one or more interconnected retroreflective portions. One of the one or more retroreflective portions includes an outer surface, an inner surface, and a plurality of retroreflective microelements. Each of the plurality of retroreflective microelements extends from the inner surface at a location different from the locations of the other retroreflective microelements, and each of the plurality of retroreflective microelements includes a central axis extending along a direction corresponding to a surface normal of the outer surface. The one retroreflective portion further includes a reflective coating applied to the surfaces of the plurality of retroreflective microelements.

[0021] The marker also includes a second part having a connection mechanism configured to connect the first part to a device.

[0022] Another aspect is that an apparatus includes an optical tracking device and a retroreflective marker. The retroreflective marker is attached to the device by a retroreflective marker connection mechanism. The retroreflective marker includes one or more retroreflective portions coupled together. One of the one or more retroreflective portions includes an outer surface, an inner surface, and a plurality of retroreflective microelements. Each of the plurality of retroreflective microelements extends from the inner surface at a location different from the locations of the other retroreflective microelements, and each of the plurality of retroreflective microelements includes a central axis extending along a direction corresponding to a surface normal of the outer surface.The one retroreflective portion further comprises a reflective coating applied to the surfaces of the plurality of retroreflective microelements.

[0023] The embodiments described herein may provide various technical advantages. For example, the described device and markers (including an assembly of multiple devices) may improve the uniformity of the retroreflected optical signals along various angles, further improving the tracking accuracy of a tracking system employing the described device and markers. Because the described device is manufactured using techniques such as injection molding, which is controlled by predetermined geometries and parameters, the distribution and shape of the retroreflective microelements are substantially the same as those controlled for the injection molding process. Thus, the brightness of the reflected optical signals generated by the described device and the corresponding markers is substantially uniform and controllable.

[0024] Furthermore, the described techniques for fabricating a marker are robust and efficient for various tracking tasks. Because the described device is manufactured by injection molding and includes one or more coupling mechanisms configured to couple with one or more other devices, the geometries (e.g., size, shape, or both) of the described device, the geometries of the assembled marker using one or more described devices, and the arrangement (e.g., size, shape, distribution, etc.) of the retroreflective microelements on the described device or marker can be predetermined or adapted, or both, according to the different requirements for different tracking tasks.In addition, the use of injection molding processes for the retroreflective sections and the ability to design different assemblies from multiple retroreflective sections can reduce the time and cost required to research and develop new configurations of retroreflective markers.

[0025] The details of one or more embodiments are illustrated in the accompanying drawings and the following description. Further features and advantages will become apparent from the description and drawings, as well as from the claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 shows an example of a retroreflective section in a device. Fig. 2A, Fig. 2B and Fig. 2C are bottom views of examples of retroreflective sections. Fig. 3 shows an example of a building block with cube angle variation. Fig. Figure 4A is a perspective view of a retroreflective building block formed on a flat surface and having a rectangular geometry with varying cube angles. Fig. Figure 4B shows an example of a retroreflective building block formed on a flat surface with different cube angles. Fig. Figure 4C shows another example of a retroreflective building block positioned on a flat surface with different cube angles. Fig. 5A, Fig. 5B and Fig. 5C are perspective views of a retroreflective building block forming on a curved surface with different cube angles. Fig. 6A and Fig. 6B show example markers with a retroreflective array, each connected to a base. Fig. Figure 7 is a perspective view of a tracking system that uses three markers attached to an object to optically track the object. Fig. Figure 8 shows an example of a tracking system that is able to use markers, e.g. Fig. 1, 2A-C, 3, 4A-C, 5A-C, 6A, 6B, and 7. Fig. Figure 9 shows a diagram of an example computer system.

[0026] Like reference numbers and designations in the different drawings indicate like elements. DETAILED DESCRIPTION

[0027] Described herein is an apparatus that can be used by a tracking system to optically track various objects in an environment (e.g., to optically track tools used in an operating room). In some embodiments, the apparatus can be used to generate a marker (e.g., a retroreflective marker) that can be used by the tracking system. A tracking system is generally configured to determine the location (e.g., the positions and corresponding orientation) of a tracked object in an environment. A tracked object generally includes one or more retroreflective markers that may be similar or different (e.g., similar structure, different capabilities, etc.). These retroreflective markers are configured to reflect incoming signal beams (e.g., light beams) along one or more directions (e.g.,from a signal source) and provide reflected signals to the tracking system. One or more sensors of the tracking system are located near the signal source and are configured to receive the signals reflected from the retroreflective markers. Once received by the sensor(s), the reflected signals can be processed (e.g., based on the light paths of the incoming and reflected signals and the reflected points) to determine the attitude information associated with the tracked object, and in some cases, this attitude information can be further processed to obtain even more information about the tracked object. In one example, the tracking system is configured to estimate the position (or location) of the markers in the environment based on the location where the reflected signal is detected.The position of the tracked object is then determined based on a predetermined relationship between the position of each of the markers.

[0028] In some embodiments, a tracking system may be an optical tracking system that operates in the optical part of the electromagnetic spectrum, and the signal source may emit optical signals (e.g., light rays) at different wavelengths (e.g., visible light rays, invisible light rays such as infrared, ultraviolet, etc.). The retroreflective properties of the markers allow an optical signal to be reflected back to the source along a parallel path (e.g., a reflected optical signal propagates along a path parallel to the path of the incoming optical signal). The system may employ various devices, techniques, etc. to detect the reflected optical signals. For example, one or more sensors (e.g., similar or different sensors), such as one or more cameras, may be configured to detect retroreflection (e.g.,a glitter) from one or more markers.

[0029] The described device and the corresponding markers can be used in various fields, such as medicine, industrial metrology, navigation, areas where medical devices are used, etc. For example, a marker described here can be connected to a medical device to track the position of the medical device during a procedure (e.g., a surgical intervention).

[0030] Retroreflective markers can include 2D markers with a predominantly two-dimensional shape (e.g., a flat 2D sheet with retroreflective properties), 3D markers (e.g., markers and associated structures with a 3D geometry such as a spherical shape, a conical shape, a cylindrical shape, etc.), a combination of 2D and 3D markers, etc.

[0031] 2D and 3D markers have retroreflective microelements located on at least one side (or surface). A microelement generally refers to a small component or unit included in a retroreflective marker, a part of a marker, etc., to provide reflective properties. For example, the average edge size of a microelement can be less than 1 millimeter. Microelements are typically designed to redirect incident optical signals back to the source along essentially the same path. Microelements can take various forms, such as microprisms (also called micro-corner cubes), microbeads (also called micro-cat's-eye elements or microspheres), etc., and are often arranged in specific patterns or configurations to maximize the retroreflective effect.In general, a 2D marker can have multiple microelements attached to one surface of the 2D marker. Similarly, a 3D marker can have multiple microelements attached to one or more surfaces (e.g., an inner surface, an outer surface, or both) of the 3D structure.

[0032] In some manufacturing processes, a 3D marker is produced by forming a 3D structure from a corresponding 2D marker film. For example, a 2D marker with several retroreflective microelements is heated (e.g., using a heating system) and placed on a mold with the desired geometry. The 2D marker film is formed into the desired geometry using external pressure. For example, one or more vacuums can be applied (vacuum forming) to form the 2D marker film into a 3D marker. However, such a process can alter individual microelements, the distribution of the microelements, etc., as well as other aspects of the 2D marker film. The resulting 3D marker may have an undesirable distribution of microelements, e.g., an uneven distribution of retroreflective microelements. Furthermore, one or more microelements of the 2D marker film may be lost during production (e.g.,During a manufacturing process, the microelements can be deformed from their desired shape. Changes in the distribution of microelements, the shape of the 2D marker film, and the shape of the microelements in the 2D marker can reduce the tracking accuracy of a tracking system used by the 3D marker fabricated using the conventional technique. Another conventional technique involves fabricating a marker containing a 3D structure with one or more retroreflective microelements affixed to the structure (e.g., to at least one side or surface of the structure). However, the affixing process can be inefficient and costly, and the uniformity of the microelement distribution is difficult to control.

[0033] One or more methods may be used to manufacture a 3D retroreflective marker comprising one or more injection-molded retroreflective sections. Through the use of injection molding, the described retroreflective section may include multiple retroreflective microelements evenly distributed and precisely aligned as specified, resulting in more uniform brightness of the retroreflective optical signals (e.g., light rays) generated by the retroreflective sections from incident lights at different angles of incidence and from different directions. The uniform brightness at different angles of incidence improves the tracking accuracy of a tracking system employing the described device or one or more corresponding markers.In some embodiments, 2D markers can also be manufactured using the injection molding technique described here.

[0034] Generally, injection molding involves injecting molten molding compound into a mold. The mold is formed with a desired 3D structure featuring a predetermined arrangement of microelements (e.g., shape, size, orientation, and distribution of the microelements). In some configurations, the mold is then cooled to solidify the molding compound. Once the molding compound has fully solidified, the retroreflective portion is removed from the mold, and the retroreflective microelements located on the retroreflective portion should have the desired distribution and shape as designed by the mold. The molding material can contain various types of materials, such as polymers (acrylic, silicone, etc.) that are transparent in the near-infrared (NIR) range. The molding compound can attenuate optical signals in the visible spectrum (e.g., 400–740 nm).In this way, the molding compound can filter visible light and be compatible with optical sources in the NIR spectral range. However, the molding compound can be selected to filter optical signals in different spectral ranges, as long as the molding compound is compatible with the optical source for object tracking.

[0035] Fig. 1 shows an example of a retroreflective portion 105 in device 100. In this example, retroreflective portion 105 has the geometry of a spherical body segment; however, it should be noted that the geometry and shape of device 100 are examples for the purposes and convenience of illustration. Other geometries and shapes of device 100 may also be used.

[0036] The retroreflective portion 105 may be manufactured by injection molding, as described above. The retroreflective portion 105 may be part of a 3D marker with a specific geometry. For example, the retroreflective portion 105 may form part of a 3D marker with a hollow spherical shape (e.g., a spherical marker 600 as shown in Fig. 6A). One or more other geometries may also be used, e.g., the retroreflective portion 105 may form part of a 3D marker having a cylindrical shape (e.g., a cylindrical marker 650 in Fig. 6B).

[0037] Retroreflective sections can be assembled into a 3D marker using one or more coupling mechanisms provided on the retroreflective sections (e.g., markers 600 and 650 in Fig. 6A and 6B, respectively). In general, a coupling mechanism refers to a mechanism or structure that enables the attachment or connection of one or more retroreflective sections. Common examples of a coupling mechanism include adhesives, heat seals, glues, mechanical connections such as hinges, hooks and loops, etc. Using the coupling mechanism, multiple retroreflective markers can be coupled together to form a more complex and larger marker structure. A corresponding 3D marker can be formed by joining more than one retroreflective section, e.g., two, five, ten, or any other suitable number of retroreflective sections, using one or more coupling mechanisms provided by each of the retroreflective sections.

[0038] For example, the retroreflective marker 105 of the device 100 may have a coupling mechanism 170 on at least one side. The coupling mechanism 170 may, for example, include a connection for locking a corresponding connection of another retroreflective portion. The connection may include a dovetail connection (not shown) having a trapezoidal groove, and the corresponding dovetail connection of a corresponding retroreflective portion may include a corresponding protrusion or profile configured to fit within the trapezoidal groove to form a locking connection.In some cases, each retroreflective section may have a groove on each of one or more sides for receiving corresponding projections from other retroreflective sections and a projection on each of one or more other sides for being received by other retroreflective sections. Furthermore, the dovetail coupling mechanism ensures a reliable and robust connection between different retroreflective sections and allows for quick assembly and disassembly as needed, facilitating the manufacture of versatile retroreflective markers with adjustable configurations for different tracking tasks and requirements. In some embodiments, two or more retroreflective sections that are heat-deformable (e.g., thermoplastic materials) may be joined or sealed together by heat sealing.Heat sealing generally refers to techniques that bond two or more materials together using heat. For example, welding techniques such as friction welding or ultrasonic welding may be used to generate frictional heat between the retroreflective portions to soften and bond them. Alternatively or additionally, one or more laser sources may be used to heat the retroreflective portions. In some embodiments, one or more materials may be used as the bonding or interface material for welding retroreflective portions.

[0039] The retroreflective portion 105 further includes a retroreflective portion 110. The retroreflective portion 110 includes an outer surface 115, an inner surface 120, and a plurality of retroreflective microelements 130. The outer surface 115 defines a surface normal in a direction extending away from and perpendicular to a local tangent of the outer surface 115. Except for a planar surface, the surface normal of the outer surface 115 generally varies at different locations on the outer surface 115. For example, the surface normal 150 of the outer surface 115 may change orientation at different locations on the outer surface 115. Similarly, an inner surface 120 defines a surface normal in a direction extending away from and perpendicular to a local tangent of the inner surface 115.The surface normal of the inner surface 120 may correspond to the surface normal of the outer surface 115 if the inner surface 120 and the outer surface 115 share a common curvature profile. In implementations where the outer surface 115 and the inner surface 120 do not share a common curvature profile, the surface normals of the corresponding surfaces generally do not align.

[0040] The outer surface 115 and the inner surface 120 are generally smooth to improve retroreflectivity. However, in some embodiments, a portion of the outer surface 115, the inner surface 120, or both may be non-smooth (e.g., have a rough surface). The degree of non-smoothness, the location of the non-smoothness, and the size of the non-smooth areas, etc., are determined according to the various retroreflective task requirements.

[0041] In this example, the retroreflective portion 105 includes a plurality of retroreflective microelements 130 extending from the inner surface 120 at various locations. Each of the plurality of retroreflective microelements 130 defines a central axis 140 extending along a direction corresponding to the surface normal 150 of the outer surface 115. Retroreflective microelements, each having a central axis aligned with the surface normal 150, improve the retroreflection of the portion 105 (e.g., the uniformity of retroreflective brightness). Generally, a central axis of a retroreflective microelement is represented by a vector passing through the geometric center of the retroreflective microelement. In cases where a retroreflective microelement is not symmetrical (e.g., different edge sizes, non-perpendicular interior angles, skewed, etc.),), a central axis of the retroreflective microelement is defined as an axis passing through the geometric center and forming the same angle with all edges formed by two adjacent surfaces of the retroreflective microelement. Accordingly, the central axis has the same angle with respect to the normals of all surfaces of the retroreflective microelement. In cases where a retroreflective microelement is a corner cube with equal-length edges, a central axis of the retroreflective microelement may be an axis passing through the geometric center and perpendicular to a base 155 of the retroreflective microelement.

[0042] In some embodiments, the microelements 130 may be configured such that their central axes 140 extend parallel to the surface normal of the inner surface 120. By aligning the central axes 140 and the surface normal 150, the retroreflective portion 105 may improve the brightness consistency of the generated retroreflected optical signals (represented by graphic arrow 165) that propagate in a direction parallel to (but opposite to) the direction of propagation of an incoming optical signal (represented by graphic arrow 160).

[0043] In addition, the retroreflective portion 105 includes one or more coatings, e.g., a reflective coating 180, applied to the surfaces of the plurality of retroreflective microelements 130. The reflective coating 180 can enhance the retroreflection of the microelements 130. The reflective coating 180 can include one or more materials (such as silver, aluminum, etc.), which can be applied individually or in combination. The reflective coating 180 can be applied to the surfaces of the microelements using various techniques. One example described here is vapor deposition. In general, vapor deposition is a method for applying a thin layer of a reflective coating using a vacuum-based deposition technique. For application, at least a portion of the retroreflective portion 105 (e.g.,The plurality of microelements 130 are placed in a vacuum-sealed coating chamber. The coating material is evaporated by heat and fed into the coating chamber. Once the evaporated coating material hits the target surface (e.g., the surfaces of the microelements 130), the evaporated coating material quickly condenses and forms a thin layer of reflective material on the target surface. No air gap remains between the reflective coating and the surfaces of the microelements.

[0044] In some cases, the reflective coating can be applied to the retroreflective surfaces using other techniques, such as chemical deposition, spray coating with spray guns or atomizers, etc.

[0045] The thickness of the reflective layer can be controlled by adjusting parameters such as deposition time, vapor temperature, distance between the coating source and the surface, etc. In general, the thickness of the reflective layer can be selected to achieve a reflectance of at least 85% or more at all points on the outer surface of the retroreflective section. Since the surface normals of the microelements vary across the retroreflective section, the thickness of the reflective coating changes accordingly to achieve the desired retroreflection. The thickness can be, for example, 10 µm, 20 µm, 50 µm, 100 µm, 200 µm, 500 µm, 1 mm, or other suitable values.In some cases, the thickness of the reflective coating in the valleys between the microelements may be higher than on the surfaces of the microelements, so that a sufficient reflective coating can be applied to the surfaces of the microelements to achieve a desired level of retroreflectivity.

[0046] As described above, whether a reflective coating is applied to the retroreflective layer depends on the retroreflection requirements. For example, a retroreflective section without a reflective coating may form a "brighter" area under the incoming optical signals, and a retroreflective section with a reflective coating may provide a wider viewing angle for incoming optical signals. In cases where a wider viewing angle for retroreflection is not required or desired, the retroreflective section 105 may provide retroreflection through multiple reflective surfaces of the microelements without applying a reflective coating to the reflective surfaces (in Fig. 1 (not shown). In these cases, the reflective surfaces are the surfaces of the plurality of microelements. For example, a microelement may have two surfaces that serve as reflective surfaces to reflect or retroreflect incoming visual signals. In this way, the retroreflective portion 105 can be designed with lower manufacturing costs and a slightly smaller viewing angle for retroreflection.

[0047] In addition, one or more retroreflective microelements may extend from the outer surface 115 at various locations on the outer surface 115, rather than extending from the inner surface 120 (in Fig. 1 not shown). In these cases, the retroreflective microelements must be additionally covered by a reflective coating applied to the surfaces of the retroreflective microelements 130 to ensure or enhance retroreflection. More specifically, a surface without a reflective coating generally does not reflect an incoming optical signal traveling from a medium with a lower refractive index to a higher refractive index, e.g., from air to a polymer. Furthermore, in cases where the retroreflective microelements extend from the outer surface 115, a reflective coating applied to the surfaces of the retroreflective microelements 130 may provide a wider viewing angle for retroreflection, as described above.

[0048] Because these retroreflective microelements 130 are located on the outer surface 115, liquid and dust from the environment can become trapped in the spaces between them and impair the retroreflection of the marker 105. Therefore, the marker 105 can be additionally protected by a transparent or translucent cover to protect the retroreflective microelements from contamination by liquid or dust in the environment.

[0049] In some embodiments, the retroreflective portion 105 forms a fully 3D marker (e.g., a hollow spherical, conical, or cylindrical shape). In these embodiments, the retroreflective portion 150 may not include a coupling mechanism 170.

[0050] Fig. 2A, Fig. 2B and Fig. 2C each show a bottom view of various examples of retroreflective sections used separately or together in a device (e.g., device 100 of Fig. 1) can be used. The device described herein may include multiple retroreflective microelements extending from an interior surface of the device. In general, the retroreflective microelements may have different geometries and shapes. For example, a retroreflective microelement may be a microbead (also called a microsphere or cat's eye shape) or a microprism (also called a corner cube). Fig. 2A, Fig. 2B and Fig. 2C each show a bottom view of different microprism geometries in the respective retroreflective section.

[0051] As in Fig. 2A, a retroreflective portion 205 (e.g., of a device) includes a two-dimensional array (referred to as array 210) of retroreflective microprisms 210a, b, c, d, e, f, g, h, i, j, k, l, m, n, p extending from the inner surface 207 of the retroreflective portion 205. In this configuration, each microprism has an upper vertex 220 and three lower vertices 225. Each microprism 210a-p includes a plurality of edges 230 (e.g., three side edges and three bottom edges). Each of the three side edges connects the upper vertex 220 and one of the three lower vertices 225, and each of the three lower edges connects two adjacent lower vertices 225. Because the retroreflective portion 205 may have a curvature, the three lower edges 225 are not necessarily straight lines. For example, the three lower edges 225 may be curved edges.The lower edges generally form a triangular shape. The spatial angles between adjacent side faces 235 of a microprism can be substantially 90 degrees to form a cube corner. A cube corner (also called a corner reflector) generally consists of three flat surfaces that intersect at right angles to each other. It is configured to reflect optical signals back toward the optical source in a direction parallel to the incident directions, but with a positional shift. The three intersecting faces of the cube corner reflector are typically square in shape. As with the microprism, the spatial angles between the connecting faces can include suitable angles other than 90 degrees, depending on the various retroreflecting tasks.For example, the solid angles can be between 75 degrees and 105 degrees to account for situations where one or more optical sources are offset from one or more optical sensors.

[0052] Several parameters related to the retroreflective microelements can be adjusted to control the brightness and uniformity of the retroreflective signal. These parameters can include the total number of microelements, the size of the individual microelements, the interior angles of the microelements, etc. In some cases, the microprisms 210a-p are designed so that each edge 230 is less than 1 millimeter and the distance D (in Fig. 2A) between two adjacent upper vertices 220 is less than 1 millimeter.

[0053] Achieving a precise and uniform surface shape in a microprism structure is critical for optimizing retroreflective performance. Irregularities or imperfections in the surface shape can lead to lower retroreflective efficiency or scattering of optical signals. To control the brightness of the retroreflected optical signals over differences of up to 15 degrees, the surface shape of each microprism 210a-p is formed to a surface finish of less than 1 micrometer. The term "surface shape" generally refers to the extent of deviations from the desired or intended shape of a retroreflective microelement. For example, the surface shape can indicate a defect or deviation in a radius, curvature, etc., in a retroreflective microelement surface.Another example: The surface shape can also indicate a deviation of the angle of adjacent side surfaces from the desired angles. The term "surface finish" generally refers to the quality and precision level of a retroreflective microelement's surface. Surface finish can be represented by parameters such as roughness, waviness, location, etc. of a microelement's surface. While surface shape generally refers to deviations from the desired shape or from specified tolerances, surface finish refers to the texture, quality, and properties of the outermost layer of a surface. Defects in surface shape are also referred to as "low-frequency deviations," and defects in surface finish are referred to as "high-frequency deviations."

[0054] To improve the overall consistency of the brightness of the retroreflective light and optionally increase the range of angles of incidence that enable retroreflection, the microprism array 210 can be designed such that the directions of the central axes are offset. More specifically, one or more microprisms can be selected such that the directions of their respective central axes are perturbed (or altered) such that these central axes deviate from their corresponding surface normals by a specified offset value. The process of offsetting the directions of one or more central axes can be performed using a randomization algorithm configured to, once executed, sample central axes according to a predetermined distribution and perturb (or alter) directions with values ​​within a predetermined range.Although the overall brightness of the retroreflected optical signals may decrease due to the offset values, the overall viewing angle increases. Because the sampling and perturbation are controlled by a randomization algorithm, the described techniques ensure a uniform brightness loss and prevent a strong brightness gradient in different local areas of the retroreflective section, resulting in improved brightness consistency at different orientations. Details of the randomization algorithm and corresponding examples are described with reference to [referring to]. Fig. 3, Fig. 4A, Fig. 4B, Fig. 4C, Fig. 5A, Fig. 5B and Fig. 5C.

[0055] In addition, other parameters of the microprism 210a-p can be modified to improve the functionality of a retroreflective section. In cases where the optical source is not coaxial with an optical sensor (e.g., offset parallel), a similar randomization algorithm can be applied to perturb the spatial angles of a retroreflective microelement to further improve the overall brightness consistency for retroreflective sections at different locations between the retroreflective section and a corresponding optical sensor.Using the microprism 210a-p as an example, the spatial angles between the side surfaces 235 and the overall sizes of the various microprisms 210a-p can be sampled and perturbed within a predetermined range to increase the overall brightness consistency of the retroreflective portion at the cost of a slight decrease in the overall brightness of the retroreflected optical signals. Fig. Figure 2B shows another example of a microprism geometry. Similarly, the retroreflective portion 245 includes a two-dimensional array of microprisms. Each of the microprisms has an upper vertex 240 and six lower vertices. Each microprism has a plurality of edges 250 (e.g., three side edges and six bottom edges). Because the retroreflective portion 245 generally has a curvature, the bottom edges are not necessarily straight lines; they may be curved edges. The bottom edges generally form a hexagonal shape. The spatial angles between adjacent side faces of each microprism may be substantially 90 degrees to form a cube corner. In some embodiments, the spatial angles between the connecting faces may also be other suitable angles according to the various retroreflective tasks.To achieve the desired brightness level, the length of each edge is less than 1 millimeter, and the distance D between two adjacent upper vertices 240 is less than 1 millimeter. Furthermore, the geometries of one or more microprisms can be . Fig. 2B in a similar manner as in Fig. 2A described and disturbed.

[0056] Fig. Figure 2C shows another example of the geometry of the microprisms. Similarly, the retroreflective portion 265 includes a two-dimensional array of microprisms. Each of the microprisms includes an upper vertex 270 and five lower vertices. Each microprism has a plurality of edges 280 (e.g., three side edges and six bottom edges). Because the retroreflective portion 265 generally has a curvature, the bottom edges are not necessarily straight lines; they may be curved edges. The bottom edges generally form a rectangular shape. The spatial angles between adjacent side faces of each microprism may be substantially 90 degrees to form a cube corner. In some embodiments, the spatial angles between the connecting faces may also include other suitable angles according to the various retroreflective tasks.To achieve the desired brightness level, the length of each edge is less than 1 millimeter, and the distance D between two adjacent upper vertices 270 is less than 1 millimeter. Furthermore, the geometries of one or more microprisms can be . Fig. 2C in a similar manner as in Fig. 2A described and disturbed.

[0057] In some embodiments, more or fewer bottom vertices may be included (e.g., four, seven, ten, or any other suitable number of vertices). The bottom edges may form any suitable polygons, even a circle. Although the bottom surfaces of bodies 205, 245, and 265 are shown as rectangular for illustrative purposes, the shape and size of a retroreflective section may be any suitable shape. The arrangement, number, and size of the microprisms may vary according to different retroreflective tasks.

[0058] The randomization algorithm described here for perturbing the solid angles of a retroreflective microelement can be implemented across a building block of a retroreflective section. In general, a building block can comprise at least a subset of the retroreflective microelements in the retroreflective section. For simplicity, the following description assumes that the solid angles in a retroreflective microelement are identical, which is referred to as the cube angle of the retroreflective microelement.

[0059] The randomization algorithm, once executed, can perturb the cube angle of each retroreflective microelement in the device. The cube angle perturbation pattern (hereinafter also referred to as cube angle variation) generated by the randomization algorithm for the device can be repeatedly copied to perturb the cube angles of other retroreflective microelements in the retroreflective section (or in the retroreflective marker composed of multiple retroreflective sections). In this way, the retroreflective marker or section can contain one or more building blocks with the same cube angle variation. These building blocks can substantially cover the inner surface of the retroreflective marker or section. In some embodiments, these building blocks can be arranged side by side without overlap or gap. Alternatively, these building blocks can be arranged side by side with a gap.For example, the gap can be between 0 micrometers and a size comparable to the size of a building block.

[0060] Fig. Figure 3 shows an example of a building block 300 with cube angle variation. For illustration and simplification, Fig. 3 a plan view of a building block 700 in the coordinate system 380. Each microelement in the building block 300 has a hexagonal shape, similar to that in connection with Fig. 2B. Note that the device 300 and the corresponding cube angle variation can be applied to a flat or curved surface. Examples of application on a flat surface are described with reference to Fig. 4A, Fig. 4B and Fig. 4C, and applications on a curved surface are described with reference to Fig. 5A, Fig. 5B and Fig. 5C.

[0061] As in Fig. As shown in Figure 3, the device 300 has an N by M array of retroreflective microelements. Here, the device 300 has an array of 8 x 8 retroreflective microelements. Note that the device 300 may also contain other suitable numbers of microelements for edge 360 ​​or 370, e.g., three, four, five, nine, eleven, or other suitable numbers of microelements.

[0062] Each microelement defines a cube angle to be perturbed by the randomization algorithm. For example, microelement 310 defines three intersecting planes or faces A, B, and C. In a magnified view 315 of microelement 310, numerical identifier 320 refers to face A in microelement 310, numerical identifier 330 refers to face B in microelement 310, and numerical identifier 340 refers to face C in microelement 340. Furthermore, two different surfaces define a corresponding planar angle at the corresponding intersection point. As shown in the magnified view 315 of microelement 310, surfaces 320 and 330 define a planar angle of 325, surfaces 330 and 340 define a planar surface of 335, and surfaces 320 and 340 define a planar surface of 345.For simplicity, this description assumes that the plane angles 325, 335, and 345 are identical, which, as described above, are also referred to as cube angles. Thus, when the randomization algorithm described here is executed, it perturbs the cube angle for each microelement in the device, so that each microelement has a corresponding perturbed cube angle.

[0063] The randomization algorithm for perturbing the cube angles of microelements in the building block can be implemented using random seeds (or starting values) and bins (or containers, fields, or ranges). More specifically, a system (e.g., a computer system with a processor) implementing the randomization algorithm can receive data from a user for the perturbation. The data can include a base cube angle, a range of angular perturbation, a distribution for the perturbation, random seeds for reproducibility, and a number of bins to control the granularity of the perturbation within the range. For example, the angle of the base cube can be 90 degrees, and the range of angular perturbation can be one degree, two degrees, four degrees, or other suitable degrees. Another example is the distribution for generating random seeds, e.g.,a Gaussian distribution, a Poisson distribution, or other suitable distributions or combinations of distributions. Another example is that the random seeds can be based on current machine time or other suitable mechanisms.

[0064] After receiving the user data, the system may first divide the range of angular disturbance into a number of bins, with each bin representing a corresponding portion of the disturbance range. For example, a disturbance ranging from zero to two degrees may be divided into eight bins, i.e., [0, 0.25], [0.25, 0.5], [0.5, 0.75], [0.75, 1], [1, 1.25], [1.25, 1.5], [1.5, 1.75], and [1.75, 1] ​​degrees. Alternatively, the disturbance may also include negative degrees, e.g., a variation between a negative degree and a positive degree. In these cases, the system may set the bins to start with the minimum disturbance value (e.g., negative one degree) and the maximum value (e.g., positive one degree).

[0065] For each of the multiple bins, the system generates a random number from a specific distribution using one of the received random seeds. The random number can be a real number between zero and one. The system then scales the random number to fit the range of variation allowed for the bin. For example, if the random number is 0.3 for the bin [0.5, 0.75], the output perturbation is 0.575 degrees. For example, if the random number is 0.7 for the bin [1.25, 1.5], the output perturbation is 1.425 degrees. The system then applies the output perturbation values ​​to the angle value of the base cube (e.g., 90 degrees as described above) to determine the perturbed angle value for the microelement. The perturbed cube angle values ​​can be, for example, 90.575 degrees, 91.425 degrees, 89.425 degrees, 88.575 degrees, or other suitable degrees.

[0066] The system repeatedly performs the procedure described above for each microelement in the device to determine all angular perturbations. Once the angular perturbation has been determined for all microelements in the device, the system can copy the cube angle variation into the device to cover the surface of the retroreflective section or the entire retroreflective marker.

[0067] As described above, the cube angle variation generated by the randomization algorithm can be applied to a flat or a curved surface. Fig. Figure 4A is a perspective view of a retroreflective building block formed on a flat surface and having a rectangular geometry with varying cube angles. The building block 410 is shown according to the coordinate system 430. As shown in Fig. As shown in Figure 4A, the device 410 has an eight by eight array of retroreflective microelements, and each edge 415 or 420 has eight arrays of microelements along the corresponding edge direction.

[0068] Although Fig. 4A shows an eight-by-eight array of the package 410 for clarity, but other sized arrays may be used for a package according to different cube angle variation requirements. For example, a package may include a five-by-five array of microelements, a six-by-six array of microelements, a ten-by-ten array of microelements, or other suitable sizes. In some embodiments, a package may have a rectangular array, where the package may have more microelements along a first edge than along a second edge. For example, a package may include a four-by-six array of microelements, a seven-by-three array of microelements, or other suitable sizes.

[0069] Fig. Figure 4B shows an example of a retroreflective building block formed on a flat surface with different cube angles. The two building blocks 440 and 450 are Fig. 4B in a top view. Both devices 440 and 450 include retroreflective microelements extending from the corresponding inner surface (or bottom surface) and a corresponding reflective coating on the corresponding outer surface (or top surface). Device 450 has a darker appearance than device 440 because device 450 is arranged in a different orientation to the ambient illumination. Device 440 has a different arrangement of retroreflective microelements than device 450. For example, device 440 may have a different number of microelements, microelements with different cube angles, different spacing between adjacent microelements, microelements of different sizes, etc.

[0070] Fig. Figure 4C shows another example of a retroreflective building block arranged on a flat surface with different cube angles. Building blocks 460 and 470 correspond to building blocks 440 and 450, respectively, in Fig. 4B, except that both devices 460 and 470 lack a reflective coating. Device 460 has the same pattern of microelements as device 440, except for a reflective coating. Similarly, device 470 has the same pattern of microelements as device 450, except for a reflective coating.

[0071] Cube angle perturbation can be applied to curved building blocks. Fig. 5A, Fig. 5B and Fig. 5C, for example, show perspective views of a retroreflective device forming on a curved surface with different cube angles. Here, devices 510, 540, and 570 are different perspective views of the same curved device. The retroreflective microelements extend from the inner surface of the device. Each microelement (e.g., microelement 520, 550, or 580) in the device has a corresponding perturbed cube angle determined by the randomization algorithm described above.

[0072] Furthermore, the tracking system can use the randomization algorithm described here to perturb other parameters. For example, the system can use the randomization algorithm to perturb the distance between two adjacent microelements. As another example, the tracking system can also use the randomization algorithm to perturb the position, size, shape, or other parameters of each microelement in a building block and propagate such perturbations using building blocks for the retroreflective section or the marker.

[0073] Here are some examples of the perturbation described above. Microelements can have a characteristic size (e.g., edge length) between 20 µm and 30 µm and can be perturbed by up to 1 µm. Microelements can be rotated between -20 degrees and +20 degrees, and the rotation can be perturbed by up to 1 degree. The distances between microelements can vary so that each microelement is 10–50 µm from its nearest neighbor and can be perturbed by up to 1 µm.

[0074] Fig. 6A and Fig. 6B show examples of markers 600 and 650, each including a retroreflective assembly connected to a base.

[0075] As in Fig. 6A, the marker 600 includes a first part 610 (also referred to as a retroreflective assembly) having retroreflective portions (e.g., portions 615, 620, 625, and 630 shown in Fig. 6A). In this example, one or more retroreflective sections 615, 620, 625, and 630 may be interconnected by coupling mechanisms of the respective sections. An exemplary coupling mechanism may include a connection with a groove or corresponding protrusion. The assembled retroreflective sections 615, 620, 625, and 630 of the first part 610 generally form a hollow spherical marker.

[0076] The marker 600 also includes a retroreflective portion 635 with a connection mechanism for connecting the first portion 610 of the marker 600 to the retroreflective tracking device 645. For example, the connection mechanism is configured to connect the first portion 610 of the marker 600 to the base 640. The connection mechanism may include threads, grooves, etc., that engage corresponding threads, guides, and channels on the base 640 of the marker. In some embodiments, the second portion 635 may include a screw with threads on both ends to connect corresponding threads in the first portion 610 and the base 640.

[0077] The base 640 is attached to the device 645 using various techniques. For example, the base 640 can be connected to the device 646 via a hinge, so that the first part 610 of the marker 600 can be rotated about the hinge formed between the base 640 and the device 645.

[0078] In contrast to the hollow spherical shape in Fig. 6A shows Fig. 6B shows a marker 650 having a hollow cylindrical shape. Similarly, the marker 650 includes a first part 660 (also referred to as a retroreflective assembly) having one or more retroreflective sections 665, 670, 675, and 680. As described above, one or more retroreflective sections 665, 670, 675, and 680 may be interconnected by coupling mechanisms of the respective sections. An exemplary coupling mechanism may include a connection with a groove or corresponding protrusion.

[0079] The marker 650 also includes a second portion 685 with a connection mechanism for connecting the first portion 660 of the marker 650 to a retroreflective tracking device 695. For example, the connection mechanism is configured to connect the first portion 660 of the marker 650 to the base 690. The connection mechanism may include threads, grooves, etc., that engage corresponding threads, guides, and channels on the base 690 of the marker. In some embodiments, the second portion 685 may include a screw with threads on both ends to connect corresponding threads in the first portion 660 and the base 690.

[0080] Similar to above in connection with Fig. 6A, the base 690 can be attached to the device 695 by means of a hinge so that the first part 660 of the marker 650 can rotate about the hinge formed between the base 690 and the device 695.

[0081] Although Fig. 6A and Fig. 6B show markers with a spherical or cylindrical shape for clarity, respectively. A marker may have other custom shapes (or multiple shapes) that differ from or deviate from a spherical or cylindrical shape to meet different retroreflectivity requirements. For example, a marker may have a hollow conical shape (not shown). As another example, a marker may have a shape that differs slightly from a standard sphere or cylinder, such as an ellipsoid or other suitable shapes.

[0082] Fig. 7 is a perspective view of an exemplary tracking system 700 that utilizes three markers 702, 704, and 706 attached to an object for optical tracking. Markers 702, 704, and 706 correspond to or are similar to markers 600 and 650 in Fig. 6A and Fig. 6B. The markers 702, 704 and 706 can be identified by the retroreflective portions 105 of Fig. 1 and the device 200 of Fig. 2A-C. For clarity, markers 702, 704, and 706 are depicted in a hollow cylindrical shape. However, other shapes and geometries are possible for various retroreflecting tasks. In some embodiments, the optical tracking object may include a reference tool placed near a patient during a surgical procedure and establishing a fixed or reference coordinate for optical tracking. Furthermore, the optical tracking object may include a positioning tool used during a surgical procedure, and the markers may be used to determine the position and orientation of the positioning tool according to the reference coordinate.

[0083] As in Fig. 7, the exemplary tracking system 700 includes a tracked object 716, markers 702, 704, and 706, an optical sensor 708 (e.g., a camera), an optical signal source 718, and a computer system 720. The tracking system 700 is configured to determine the position of the tracked object 716 in an environment by determining the location of each marker 702, 704, and 706. The tracked object 716, as shown in Fig. 4, comprises a cross-beam structure in which one end of a first beam is connected to a middle portion of a second beam. Marker 704 is located at the free end of the first beam and is spaced from the middle portion of the second beam by a distance L2. Markers 702 and 706 are located at two free ends of the second beam and are spaced by a distance L1.

[0084] The optical signal source 718 is located near the optical sensor 708 and may include light-emitting devices such as light-emitting diodes (LEDs), an LED array, or other such source in the NIR spectrum. The optical sensor 708 may include a sensing device such as a CCD or CMOS optical sensor and is configured to detect reflected electromagnetic waves (e.g., optical signals) from the markers 702, 704, and 706 attached to the tracked object 716. Because the markers 702, 704, and 706 are highly retroreflective, they appear as bright spots (the optical signatures) in the captured images at certain wavelengths of the electromagnetic waves. The captured sensor data is provided to the computer system for further processing.

[0085] The computer system 720 may determine the locations of the tracked object 716 using the received sensor data, predetermined parameters (e.g., positions or locations) of the optical sensor 708, and initial positions and locations of the markers 702, 704, and 706. In some embodiments, the computer system 720 is configured to analyze predetermined relationships between the markers 702, 704, and 706 and their respective retroreflective centers to determine the locations of the tracked object 716. In some embodiments, the computer system 720 may estimate offset values ​​for retroreflective centers based on the geometries of various markers.

[0086] The tracked object 716 may be any object tracked by the tracking system 700, but generally includes rigid or semi-rigid objects such that the markers 702, 704, and 706 do not move relative to each other in space during tracking. The tracked object 716 is generally configured to carry multiple markers. Although in Fig. 7, for better illustration, three markers 702, 704 and 706 are connected to the tracked object 716, a different number of markers can be connected to the tracked object 716.

[0087] In some embodiments, the tracked object 716 comprises a tool for surgical tasks. Additionally or alternatively, the tracked object may also be used for other applications and industries, such as industrial metrology or navigation. For example, the tracked object may comprise a digitizing probe for, e.g., full-field and / or tactile measurements, an optically trackable scanner, a scale for photogrammetry and laser tracking applications, or other suitable devices. Furthermore, the markers described herein may be attached to a device used in other industries that also utilize medical devices and other devices. In some cases, the markers described herein may be attached to one or more medical devices or other types of devices.

[0088] In addition, a tool used in various applications (e.g., surgical tracking, industrial metrology or navigation, etc.) can be manufactured such that a portion of the body is retroreflective (e.g., at least a portion of the body is protected by the retroreflective portion 105 in Fig. 1). In this way, the optical tracking tool does not need to be attached to a marker formed by one or more of the retroreflective sections (e.g., retroreflective section 105 in Fig. 1). However, in some embodiments, an optical tracking tool may be manufactured such that, in addition to a retroreflective portion forming at least a portion of the tool body, the tool may be attached to a marker formed by one or more of the retroreflective portions to enhance retroreflection.

[0089] Fig. Figure 8 shows an example of a tracking system 800 (e.g., an optical tracking system) capable of using markers, e.g., from Fig. 1, 2A-C, 3, 4A-C, 5A-C, 6A, 6B and 7. The markers can, for example, be applied to the retroreflective section 105 of Fig. 1, the retroreflective Marker 600 from Fig. 6A, the retroreflective marker 650 from Fig. 6B and the retroreflective markers 702, 704 and 706 of Fig. 7. The tracking system 800 includes an illumination / image capture unit 802 in which a marker capture device (e.g., a camera, an array of cameras 804a-b, etc.) and one or more marker illumination devices 818a-b (e.g., a source of electromagnetic waves) are rigidly mounted. In this example, the illumination devices 818a-b emit electromagnetic waves, e.g., visible light, infrared light, etc. The electromagnetic waves are directed toward an area containing one or more retroreflective markers (e.g., the markers 600 in Fig. 6A or the marker 650 in Fig. 6B) attached to a medical device (or object). Fig. 1, the medical device (or an object) may be a tool 808 (e.g., a surgical tool, a medical device for treating a patient, etc.). The object is sometimes referred to as a tracked object. The retroreflective markers are configured to exhibit retroreflection to reflect incoming electromagnetic waves parallel and in the opposite direction to the input direction. The cameras 804a-b capture one or more images of the illuminated retroreflective markers. Due to the high retroreflectivity of the retroreflective markers, each marker appears as a relatively bright point in the captured images, and the system can determine the spatial coordinates (e.g., Cartesian, spherical, cylindrical, etc.) and an intensity value representing, for example, the brightness of each corresponding point.Because the retroreflective markers have a wider range of angles of incidence, the marker can remain visible in various locations or positions relative to the optical sources. This data is provided to a computing device (e.g., a processor) of a computer system 810. The computing device is configured to determine where in the region or environment the retroreflective markers (and the corresponding objects to which they are attached) are located relative to the cameras 804a-b.

[0090] Generally, the computing device is part of the computer system 810, which is connected to the array of cameras 804a-b via communication links 812 (e.g., wired or wireless communication links). In some examples, the computing device is located within the camera mounting unit 802. The computer system 810 may include one or more of various forms of digital computers, such as laptops, desktops, workstations, personal digital assistants, servers, blade servers, mainframes, and other suitable computers. The computer system 810 may include one or more of various forms of mobile devices, including, for example, personal digital assistants, tablet computers, mobile phones, smartphones, and other similar computing devices.The components shown herein, their connections and relationships, and their functions are intended only as examples and are not intended to limit embodiments of the techniques described and / or claimed herein.

[0091] Given the known positions of the cameras 804a-b included in the array and the positions of the retroreflective markers (e.g., marker 600 of Fig. 6A or Marker 650 from Fig. 6B), the computing device may calculate a position and / or orientation of the object 808. Further, based on the known relationship between the position of each of the retroreflective markers and the position of the tool tip 820 of the object 808 in the work volume (e.g., a tool coordinate system), the computing device may calculate the coordinates of the tool tip 820 in space. In cases where the tool 808 is being handled by a user (e.g., a surgeon 814) and the tool tip 820 is being pressed against or otherwise in contact with a surface (e.g., a patient's body 816), the coordinates of the tool tip 820 correspond to the coordinates of the point at which the tool tip 820 contacts the surface.

[0092] A number of embodiments of the subject matter have been described. However, it should be understood that various changes may be made without affecting the spirit and scope of the subject matter disclosed herein. For example, the position of retroreflective markers and tracked objects may change over time. The computing device may be configured to automatically detect which bright spots in an image are identified as returns provided by the retroreflective markers at a first time and at first locations, and which bright spots in the image are identified as returns provided by the retroreflective markers at a second time and at second locations. Accordingly, other embodiments are also within the scope of the following claims.

[0093] Some embodiments of the subject matter and acts described in this specification may be implemented in digital electronic circuits, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more thereof. For example, in some embodiments, tracking system 800 and computer system 810 may be implemented using digital electronic circuits, or in computer software, firmware, or hardware, or in combinations of one or more thereof.

[0094] Some of the embodiments described in this specification may be implemented as one or more groups or modules of digital electronic circuitry, computer software, firmware, or hardware, or in combinations of one or more thereof. Although various modules may be used, not each module needs to be separate, and multiple modules may be implemented on the same digital electronic circuitry, computer software, firmware, or hardware, or a combination thereof.

[0095] Some of the embodiments described in this specification may be implemented as one or more computer programs, that is, one or more modules of computer program instructions encoded on a computer storage medium for execution by or to control the operation of a data processing device. For example, a computer system (such as computer system 810) may be used in the tracking system to control the operation of the emitter and process the images captured by the image sensor. A computer storage medium may be or be included in a computer-readable storage device, a computer-readable storage substrate, a random or serial access storage array or device, or a combination of one or more of these media.Although a computer storage medium is not a transmitted signal, a computer storage medium can be a source or destination of computer program instructions encoded in a synthetically generated transmitted signal. The computer storage medium can also consist of or be contained within one or more separate physical components or media (e.g., multiple CDs, floppy disks, or other storage devices).

[0096] The term "computing device" encompasses all types of devices, apparatus, and machines for processing data, including, for example, a programmable processor, a computer, a system on a chip, or multiple or combinations of the foregoing devices. In some embodiments, the computing system 810 comprises a computing device as described herein. The device may include specialized logic circuitry, such as a Field Programmable Gate Array (FPGA) or an Application Specific Integrated Circuit (ASIC). In addition to hardware, the device may also include code that provides an execution environment for the computer program in question, such as code representing processor firmware, a protocol stack, a database management system, an operating system, a cross-platform runtime environment, a virtual machine, or a combination of one or more of these elements.The device and execution environment can implement various computer model infrastructures, such as web services, distributed computing, and grid computing infrastructures.

[0097] A computer program (also called a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, and declarative or procedural languages. A computer program may, but does not have to, correspond to a file in a file system. A program may be stored in a portion of a file that also contains other programs or data (for example, one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (for example, files storing one or more modules, subroutines, or portions of code).A computer program may be provided for execution on one computer or on multiple computers located at one site or distributed across multiple sites and interconnected by a communications network.

[0098] Some of the processes and logic flows described in this specification may be performed by one or more programmable processors executing one or more computer programs to perform actions by responding to input data and generating output. The processes and logic flows may also be performed by special-purpose logic circuits, such as an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit), and the device may be implemented as such.

[0099] Processors suitable for executing a computer program include, for example, both general-purpose and special-purpose microprocessors, as well as processors of all types of digital computers. Generally, a processor receives instructions and data from read-only memory or random-access memory, or both. A computer includes a processor for performing actions in accordance with instructions and one or more memories for storing instructions and data. A computer may also include one or more mass storage devices for storing data, such as magnetic disks, magneto-optical disks, or optical disks, or be operatively coupled to receive data from or transfer data to them, or both. However, a computer is not required to have such devices.Devices suitable for storing computer program instructions and data include all forms of non-volatile memory, media, and storage devices, including, for example, semiconductor memory devices (e.g., EPROM, EEPROM, flash memory devices, and others), magnetic disks (e.g., internal hard disks, removable disks, and others), magneto-optical disks, and CD-ROM and DVD-ROM disks. The processor and memory may be supplemented by or integrated with dedicated logic circuitry.

[0100] To enable interaction with the user, operations may be performed on a computer that has a display device (such as a monitor or other type of display device) for presenting information to the user, and a keyboard and pointing device (such as a mouse, trackball, tablet, touch-sensitive screen, or other type of pointing device) for the user to provide input to the computer. Other types of devices may also be used to interact with the user; for example, feedback to the user may be provided in any form of sensory feedback, such as visual, auditory, or tactile, and input from the user may be received in any form, including auditory, verbal, or tactile input.In addition, a computer can interact with a user by sending and receiving documents to and from a device used by the user, for example, by sending web pages to a web browser on a client device of the user in response to requests received from the web browser.

[0101] A computer system may comprise a single computing device or multiple computers operating near or generally remote from each other, typically communicating over a communications network. Examples of communications networks include a local area network ("LAN") and a wide area network ("WAN"), an internetwork (such as the Internet), a network with a satellite connection, and peer-to-peer networks (such as ad hoc peer-to-peer networks). A client-server relationship may arise because computer programs run on the respective computers and have a client-server relationship with each other.

[0102] Fig. 9 shows an example computer system 900 (e.g., similar to or including the computer system 810 of Fig. 8), which includes a processor 910, a memory 920, a storage device 930, and an input / output device 940. Each of the components 910, 920, 930, and 940 may be interconnected, for example, via a system bus 950. The processor 910 is capable of processing instructions for execution within the system 900. In some embodiments, the processor 910 is a single-threaded processor, a multi-threaded processor, or another type of processor. The processor 910 is capable of processing instructions stored in the memory 920 or on the storage device 930. The memory 920 and the storage device 930 may store information within the system 900.

[0103] The input / output device 940 enables input / output operations for the system 900. In some embodiments, the input / output device 940 may include one or more network interface devices, e.g., an Ethernet card, a serial communication device, e.g., an RS-232 port, and / or a wireless interface device, e.g., an 802.11 card, a 3G wireless modem, a 4G wireless modem, an 8G wireless modem, etc. In some embodiments, the input / output device may include driver devices configured to receive input data and send output data to other input / output devices, e.g., keyboard, printer, and display devices 960. In some embodiments, mobile computing devices, mobile communication devices, and other devices may be used.

[0104] This specification contains numerous details, which, however, should not be construed as limitations on the scope of the claims, but rather as descriptions of features specific to particular examples. Certain features described in this specification in connection with separate embodiments may also be combined. Conversely, various features described in connection with a single embodiment may also be implemented in multiple embodiments separately or in any suitable subcombination.

[0105] Thus, specific embodiments of the optical tracking system and retroreflective markers, as well as methods of using the optical tracking system to track retroreflective markers, have been disclosed. However, it should be apparent to those skilled in the art that many modifications beyond those already described are possible without departing from the inventive concepts contained herein. Therefore, the scope of the invention is not intended to be limited except as defined in the disclosure. Furthermore, in interpreting the disclosure, all terms should be interpreted in accordance with the context as much as possible.In particular, the terms “comprises” and “comprising” should be interpreted to refer to elements, components or steps in a non-exclusive manner, meaning that the mentioned elements, components or steps may be present or used or combined with other elements, components or steps not expressly mentioned.

[0106] If a range of values ​​is specified, every intermediate value, to the nearest tenth of the unit of the lower limit, between the upper and lower limits of that range and any other specified or intermediate value within that specified range is deemed to be encompassed by the subject matter disclosed herein, unless the context clearly dictates otherwise. The fact that the upper and lower limits of these smaller ranges may independently be included within the smaller ranges is also encompassed by the subject matter disclosed herein, subject to any expressly excluded limit in the specified range. If the specified range includes one or both limits, ranges excluding one or both of those limits are also encompassed by the subject matter disclosed herein.

[0107] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this subject matter belongs. While any methods and materials similar or equivalent to those described herein may be used in the practice or testing of the subject matter, a limited number of exemplary methods and materials are described herein.

Claims

[1] Device comprising: a retroreflective section with: an outer surface, an inner surface, and a plurality of retroreflective microelements comprising a plurality of reflective surfaces, wherein each of the plurality of retroreflective microelements extends from the inner surface at a location different from the locations of the other retroreflective microelements, and each of the plurality of retroreflective microelements includes a central axis extending along a direction corresponding to a surface normal of the outer surface. [2] The device of claim 1, wherein one or more of the plurality of reflective surfaces are formed by applying a reflective coating to surfaces of the plurality of retroreflective microelements. [3] The device of claim 2, wherein the reflective coating is applied to one or more of the plurality of reflective surfaces of the plurality of retroreflective microelements by vapor deposition. [4] The device of claim 2, wherein the reflective coating contains a metal material. [5] The device of claim 1, wherein one or more of the plurality of reflective surfaces are formed without a reflective coating applied to the surfaces of the plurality of retroreflective microelements. [6] The device of claim 1, wherein one of the plurality of retroreflective microelements is a microprism. [7] The device of claim 1, wherein the retroreflective portion further comprises a connecting mechanism for connecting the retroreflective portion to one or more other retroreflective portions. [8] The device of claim 1, wherein the retroreflective portion forms at least a portion of a retroreflective marker having a hollow spherical shape. [9] The device of claim 1, wherein the retroreflective portion forms at least a portion of a retroreflective marker having a hollow cylindrical shape. [10] The device of claim 1, wherein the retroreflective portion forms at least a portion of a retroreflective marker having a hollow conical shape. [11] The device of claim 1, wherein the retroreflective portion forms a retroreflective marker. [12] The device of claim 1, wherein a geometry and an arrangement of the plurality of retroreflective microelements are predetermined for the manufacture of the retroreflective portion, wherein the retroreflective portion is manufactured by injection molding. [13] The device of claim 1, wherein each edge of one of the plurality of retroreflective microelements has a length of less than 1 mm. [14] The device of claim 1, wherein each of the plurality of retroreflective microelements defines a microprism center and two adjacent retroreflective microelements of the plurality of retroreflective microelements are arranged at a distance of less than 1 mm between the respective microprism centers. [15] The device of claim 1, wherein the central axis extends in the direction parallel to the surface normal of the outer surface with an offset. [16] The apparatus of claim 15, wherein the offset is determined by applying a randomization algorithm to the plurality of retroreflective microelements. [17] The device of claim 1, wherein the retroreflective portion comprises a polymer that is transmissive in the near infrared (NIR) spectral band. [18] The apparatus of claim 1, wherein the retroreflective portion forms part of a body of an optical tracking tool. [19] The apparatus of claim 1, wherein the retroreflective portion forms part of a marker configured to be attached to an optical tracking tool. [20] The apparatus of claim 1, wherein the retroreflective portion comprises a first retroreflective portion and a second retroreflective portion, the first retroreflective portion forming part of a body of an optical tracking tool and the second retroreflective portion forming part of a marker configured to be attached to the tool. [21] Marker that includes: a first part comprising one or more interconnected retroreflective sections, wherein one of the one or more retroreflective sections comprises: an outer surface, an inner surface and a plurality of retroreflective microelements comprising a plurality of reflective surfaces, wherein each of the plurality of retroreflective microelements extends from the inner surface at a location different from locations of the other retroreflective microelements, and each of the plurality of retroreflective microelements includes a central axis extending along a direction corresponding to a surface normal of the outer surface; and a second part comprising a connection mechanism configured to connect the first part to a device. [22] The marker of claim 21, wherein one or more of the plurality of reflective surfaces are formed by applying a reflective coating to surfaces of the plurality of retroreflective microelements. [23] The marker of claim 22, wherein the reflective coating is deposited on the one or more of the plurality of reflective surfaces of the plurality of retroreflective microelements by vapor deposition. [24] The marker of claim 21, wherein the marker has a hollow spherical shape, a hollow cylindrical shape, or a hollow conical shape. [25] The marker of claim 21, wherein one of the plurality of retroreflective microelements is a microprism and each edge of the microprism has a length of less than 1 mm. [26] Device comprising: an optical tracking device, and a retroreflective marker attached to the device by a retroreflective marker connection mechanism, the retroreflective marker comprising one or more retroreflective portions coupled together, one of the one or more retroreflective portions comprising: an outer surface, an inner surface and a plurality of retroreflective microelements comprising a plurality of reflective surfaces, wherein each of the plurality of retroreflective microelements extends from the inner surface at a location different from the locations of the other retroreflective microelements, and each of the plurality of retroreflective microelements includes a central axis extending along a direction corresponding to a surface normal of the outer surface. [27] The device of claim 26, wherein one or more of the plurality of reflective surfaces are formed by applying a reflective coating to surfaces of the plurality of retroreflective microelements. [28] The device of claim 27, wherein the reflective coating is deposited on the one or more of the plurality of reflective surfaces of the plurality of retroreflective microelements by vapor deposition. [29] The device of claim 26, wherein the retroreflective marker has a hollow spherical shape, a hollow cylindrical shape, or a hollow conical shape. [30] The device of claim 26, wherein one of the plurality of retroreflective microelements is a microprism and each edge of the microprism has a length of less than 1 mm.

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