Optical coherence tomography scanning of gemstones

Abstract

A method of measuring one or more features in a polished gemstone comprises providing a 3D surface model of the gemstone; carrying out a first scan of an interior of the gemstone using optical coherence tomography, OCT; carrying out a second scan of a selected surface of the gemstone using OCT; determining, from the second scan, a plane in which the selected surface lies; registering an output of the first scan with the 3D surface model using the determined plane of the selected surface; and mapping a location of one or more features within the interior of the gemstone based upon the registering.

Description

[0001] OPTICAL COHERENCE TOMOGRAPHY SCANNING OF GEMSTONES

[0002] Technical field

[0003] The invention relates to measuring properties of polished gemstones, in particular diamonds.

[0004] Background

[0005] An impurity inclusion, or internal crack, is more common in natural diamonds than in synthetic diamonds, and reduces the value of the diamond because it can be directly visible due to absorption or reflection of the light by the impurity, inclusion, or other interruption of the crystal structure. The interruption of the pure crystal lattice hinders the propagation of a light ray within the lattice. The polished surfaces of a cut diamond are created specifically to provide an intricate pattern of refraction, internal reflections, and reflection back to the user. This pattern will be disturbed by any interruption of the crystal lattice. The position of the interruption within the diamond also determines the amount of disruption. For example, a scattering or absorption centre near a tip of the diamond will scatter or absorb a larger number of light rays than a centre underneath the main surface, even though a centre underneath the main surface may be more visible. Once determined, the size and location of an interruption (such as an inclusion) within the diamond may be used for grading or otherwise assessing the quality or value of the diamond.

[0006] Statement of invention

[0007] In a first aspect of the present invention there is provided a method of measuring one or more features in a polished gemstone. The method comprises providing a 3D surface model of the gemstone; carrying out a first scan of an interior of the gemstone using optical coherence tomography, OCT ; carrying out a second scan of a selected surface of the gemstone using OCT; determining, from the second scan, a plane in which the selected surface lies; registering an output of the first scan with the 3D surface model using the determined plane of the selected surface; and mapping a location of one or more features within the interior of the gemstone based upon the registering. The location of the one or more features may be determined within a reference frame of an OCT measurement apparatus.

[0008] The selected surface may a table facet of the gemstone. The first scan may be carried out through the table facet.

[0009] The first scan and / or the second scan may be carried out after depositing a fluid film on the selected surface of the gemstone, said fluid film preferably comprising droplets of a volatile liquid, optionally water or acetone. The first scan may be carried out in the absence of the fluid film.

[0010] The second scan may comprise: one or more 2D scans, or a 3D scan.

[0011] The method may comprise carrying out a third scan of the interior of the gemstone using OCT, wherein the third scan is carried out at a different OCT reference arm position to the first scan. An output of the third scan and the output of the first scan may be combined by selecting only data points present in both scan outputs to remove artefacts, said artefacts not comprising features.

[0012] The first scan may be carried out before the second scan, or the second scan may be carried out before the first scan.

[0013] The method may comprise capturing one or more 2D or 3D images of the gemstone under one or more of diffuse, dark field or visible lighting conditions.

[0014] The method may comprise projecting rays vertically to data points in the output of the first 3D scan; determining which facet of the gemstone the features lie beneath; tracing rays to these data points; and refracting rays to real locations in 3D space, based upon a refractive index of the gemstone.

[0015] The providing of a 3D surface model of the gemstone may comprise providing a dataset of coordinate data representing an exterior surface of the gemstone.

[0016] The providing a 3D surface model of the gemstone may comprise obtaining, at a first location, a plurality of 2D silhouette images of the surface of the gemstone at one or more rotational positions of the gemstone, and combining the plurality of 2D silhouette images to generate a 3D surface model of the gemstone.

[0017] The method may comprise moving the gemstone from the first location to a second location, said second location being within optical range of an OCT scanning apparatus configured to carry out the first and the second OCT scans. An orientation of the gemstone may be preserved between the first and the second locations.

[0018] In a further aspect there is provided a method of grading a polished gemstone, the method comprising measuring one or more features within the gemstone in accordance with the first aspect above.

[0019] In another aspect there is provided an apparatus for measuring one or more features in a polished gemstone. The apparatus comprises an OCT scanning device configured to carry out a scan of an interior of the gemstone; carry out a second scan of a selected surface of the gemstone. The apparatus further comprises a processor configured to determine, from the second scan, a plane in which the selected surface lies; register an output of the first scan with a 3D surface model of the polished gemstone, using the determined plane of the selected surface; and map a location of one or more features within the interior of the polished gemstone based upon the registering.

[0020] The apparatus may further comprise a humidifier, configured to deposit a fluid film on the selected surface of the gemstone, said fluid film preferably comprising droplets of a volatile liquid, optionally water or acetone.

[0021] The apparatus may be further configured to carry out a third scan of the interior of the gemstone, wherein the third scan is carried out at a different position of a reference arm of the OCT scanning device to the first scan.

[0022] In another aspect there is provided a system for measuring one or more features in a polished gemstone, comprising: the apparatus of the aspect above, and an apparatus configured to provide a 3D surface model of the gemstone by obtaining, at a first location, a plurality of 2D silhouette images of the surface of the gemstone at one or more rotational positions of the gemstone, and combining the plurality of 2D silhouette images to generate a 3D surface model of the gemstone. The system may comprise a vacuum chuck configured to move laterally between the first location and a second location, said second position within range of the OCT scanning device.

[0023] The system may comprise a transfer rail, wherein said vacuum chuck is configured to move laterally along said transfer rail to transport the gemstone from the first location to the second location.

[0024] The vacuum chuck may be configured to preserve an orientation of the gemstone between the first and the second locations.

[0025] Brief Description of Figures

[0026] Figure 1 is a flow chart illustrating a method of identifying one or more features within a polished gemstone;

[0027] Figure 2 illustrates 3D plots of OCT data, shown superimposed on a 3D model of a stone, with the table plane of the stone highlighted and inclusions marked;

[0028] Figure 3a illustrates a projection of the top (table) of the stone of Figure 2, and Figure 3b illustrates a projection of the bottom (pavilion) of the stone of Figure 2, with inclusions marked;

[0029] Figure 4 illustrates a dark field image of a 0.41 ct polished round cut diamond, with inclusions obtained from OCT data marked, and a wireframe model of the stone together with a computed Score and Grade from a grading method shown in the box below; and Figure 5 is a schematic illustration of an example system for identifying one or more features within a polished gemstone.

[0030] Description

[0031] Optical coherence tomography (OCT) is an imaging technique used for medical imaging. OCT is based on interferometry, whereby low coherence light or broadband light is projected onto tissue, such as the eye or skin, in a first arm of the interferometer. The output of a second arm, the scanning arm, is combined with reflected light in the first arm (the reference arm) to create an interference signal that is imaged onto a detector. Only the reflected light that has travelled the same distance as the length of the scanning arm interferes constructively, so only a small portion of the tissue, typically of the order of a few micrometers, is detected at one time. OCT works well for medical applications such as soft tissue and has been used widely for that purpose, but does not work so well for other applications. The inventors have realised that OCT can be adapted such that it can also be used for applications that pose particular challenges to standard OCT.

[0032] Standard OCT does not work well for detecting features, such as inclusions, in polished diamond or other polished gemstones because of the large number of reflecting surfaces of the polished stone. While in the detection of biological tissues the reference arm will select only a single area of tissue, the many possible optical pathways caused by internal reflection within a diamond create a multitude of optical signals that will interfere constructively with the reference arm as long as they will have travelled the same distance. This can lead to distortion of features such as inclusions; for example, the features may appear stretched out, at the wrong scale, and in the wrong place.

[0033] For standard OCT, the internal and external reflecting surfaces of a polished stone therefore pose two related challenges: the internal reflections from the surfaces obscure any inclusion and the external surfaces themselves are difficult to detect when the scanning beam travels through the stone to reach the external surface. When the precise location and orientation of the surfaces is uncertain, or unknown, the location of an inclusion relative to the rest of the stone is difficult to determine, even if the reflection from an inclusion can be detected. As discussed above, the relative location of a feature such as an inclusion can be important in determining the quality of the stone.

[0034] The inventors have devised a method that overcomes the challenges of using standard OCT with polished gemstones, by combining a volumetric dataset of a stone, obtained using OCT, with a dataset representing a three-dimensional (3D) model of the stone’s surface, thereby enabling accurate mapping of measured features, such as inclusions.

[0035] Although outer surfaces are difficult to detect when the scanning beam propagates through the stone, it is more feasible to detect an outer surface when that surface faces the incoming scanning beam because in that arrangement the light which has travelled past the surface and through the stone, possibly after multiple internal reflections, will not constructively interfere with the reference arm. In an alternative example, the stone is rotated such that the OCT apparatus can scan all the outer surfaces, while the surfaces face the incoming beam in turn. The degree of rotation is recorded and combined with the measurements to construct a data set of the outer surface of the stone. The measured features such as inclusions can then be combined with the surface measurements to accurately position the inclusions within the stone. However, in the embodiments described below, the surface measurements, or surface model, are obtained by different means than the OCT apparatus. A scan of at least one surface of the stone is still taken with OCT in order to relate the surface measurements, or model, to the measurements collected by the OCT apparatus.

[0036] As illustrated in Figure 1 , the method comprises providing a 3D surface model of the gemstone; carrying out a first (3D) scan of an interior of the object gemstone using OCT ; carrying out a second scan of a selected surface of the gemstone using OCT; determining, from the second scan, a plane in which the selected surface lies; mapping (registering) an output of the first scan to (with) the 3D surface model using the determined plane of the selected surface; and determining (mapping) a location of one or more features within the interior of the stone based upon this registration.

[0037] Thus, although OCT is not as well suited for detecting the surfaces of the stone for scanning beams that traverse the stone, and the position of the surfaces in 3D space, data from an OCT scan of a stone’s interior (a volumetric scan) can be related to or fitted to coordinate data defining a 3D surface model of the stone (the 3D surface model obtained by techniques not including OCT) by OCT scanning both the volume (interior) of the stone and a selected surface of the stone with the stone in the same position in 3D space. Once a position of the selected surface with reference to the OCT scanning apparatus has been determined, it can be “matched” to a position of the same selected surface within the 3D surface model, and the 3D surface model itself can be registered with, or related to, the OCT volumetric data.

[0038] The selected surface is preferably an outside surface facing the incoming scanning light, because in that arrangement the scanning light does not need to travel through any interior part of the stone before being reflected, thereby avoiding the constructive interference of multiple internal reflections.

[0039] Said another way, a 3D model of the exterior of the stone, obtained for example by silhouette imaging, can be accurately fitted in terms of scale and orientation to a 3D model of the interior of the stone, obtained by OCT scanning. The advantages of OCT scanning can therefore be beneficially used in the field of polished gemstones, providing a highly accurate 3D map of the location and size of features, such as inclusions, within the stone and their precise spatial relationship to the stone’s external facets.

[0040] The selected facet for determination by OCT scanning is preferably the table facet, since ideally the OCT volumetric scan (also referred to as the first scan) is carried out through the table facet, it being easier to detect inclusions from this direction. The volumetric scan may be a 3D scan or may comprise a series of 2D scans which can be “stacked” to form a 3D volumetric image of the interior of the stone.

[0041] Figure 2 illustrates an example of a 3D mapping of features within a cut and polished gemstone, shown with reference to spatial axes x, y and z. As described above, this mapping combines OCT data from at least two OCT scans with a 3D surface model 16 of the exterior shape of the stone 10. This OCT data may be thresholded to remove noise. The 3D surface model, which in this example is a wireframe model, can be seen in greatest detail towards the pavilion 14 of the stone 10. In this example, the selected surface of the stone 10 is the table facet 12. The data output from the volumetric OCT scan can be seen to have identified several features - in this case, inclusions - one of which is identified by reference numeral 18. As a result of the mapping, the spatial position and size of the features can be seen relative to the facets of the stone. Points which appear outside the envelope or boundary of the stone may include signals from the mount, apparent inclusion positions, and artefacts.

[0042] Figure 3a illustrates a projection of the table 12 of the stone 10 of Figure 2, and Figure 3b illustrates a projection of the bottom 14 (pavilion) of the stone 10 of Figure 2, with inclusions 18 marked.

[0043] In order to detect the table or other selected facet using OCT (also referred to as the second scan), two example methods will now be described. However, the invention is not limited thereto.

[0044] The first example method of detecting the selected facet of the stone using OCT comprises depositing a fluid film on the selected surface, then scanning the interior of the stone using OCT. This surface scan may be a 2D or a 3D scan. In one example, the table facet is set at a fixed position, 0.4mm from the DC level (zero frequency) 0mm z position of the scan. The inventors have realised that the surface effects on the detection of inclusions can be mitigated by covering at least part of the stone’s surface with a fluid during the OCT scan. The fluid can be provided to improve the signal to noise ratio for any of the OCT measurements. In the absence of a fluid, the stone is typically surrounded by air and the relatively large change in refractive index between air and the stone causes reflection and refraction. The refractive index of diamond is between 2.417 and 2.419, the refractive index of sapphire is between 1.67 and 1.77, and for emerald the refractive index is between 1.57 and 1.58, and for regular glass it is 1.52. The refractive index of dry air is close to 1. It is the high refractive index of diamond, and the relatively high step change to air which creates the attractive ‘sparkling’ appearance due to the many internal reflections, while a glass object of the same shape is much duller in its appearance. The refractive index of a fluid or liquid is closer to that of a stone when compared to air, and the change of refractive index is much smaller when compared to the change of refractive index between air and a stone. The refractive index of water is 1.33 and that of acetone is 1.36. The signal from the surfaces is significantly reduced by coating the surfaces with a liquid because the critical angle for total internal reflection is reduced, as well as the amount of refraction. The reduction in the step change of refractive index reduces reflections and refraction of the OCT light beam at the diamond surface, thereby improving the signal to noise ratio of the detection. There will still be a step change of the refractive index, so there will still be diffraction, refraction and internal reflection at the surface, but reduced when compared to a stone to air interface. Thus, covering the selected surface of the stone with a fluid during the OCT scan produces a strong signal from the surface, enabling detection of the selected surface using OCT. This signal can be used to calculate or otherwise determine an equation for the geometric plane in which the selected surface lies, in the reference frame of the OCT scanning apparatus.

[0045] In one example, the fluid film comprises fine droplets or mist of a volatile liquid that forms a thin layer on the surface of polished gemstone, such as the table facet. The liquid is selected so as to quickly evaporate from the stone’s surface, and may be distilled water, for example. In one example, a humidifier with tightly coiled piping is used to produce fine water droplets that form a thin vapour of volatile liquid coating onto the stone and prevent large droplets from condensing onto it. Large droplets in the piping coalesce and drop under gravity back into the water reservoir of the humidifier. Such an arrangement is illustrated in Figure 5, described in greater detail below. Such an apparatus can be incorporated into an OCT set up so that the surface of the stone can be misted with the volatile liquid while in position for the OCT scan(s) to take place and the stone does not need to be moved between OCT scans. Fast evaporation of the fluid from the surface of the stone ensures that where the OCT volumetric scan is carried out after the detection of the selected facet, the fluid has completely evaporated prior to commencement of the volumetric scan, and so the signal from the stone’s surface does not obscure the signals from any inclusions within the volume of the stone. However, the invention is not limited thereto and the OCT scans may be carried out in any order.

[0046] The second example method of detecting the selected facet of the stone using OCT comprises reducing an angle between a normal of the stone and an optical axis of the OCT scanning apparatus before scanning the selected surface of the stone using OCT. This surface scan may be a 2D or a 3D scan.

[0047] As described above, a volumetric scan of the interior of a polished gemstone is obtained using an OCT scanning apparatus. For this volumetric scan, it is preferable to arrange the stone and the OCT scanning apparatus such that the table facet of the stone lies outside the range of the aperture of the lens that collects the OCT signal, thereby minimising surface reflections which can interfere with detection of inclusions within the stone.

[0048] The second example method therefore comprises moving one or both of the stone and the OCT lens aperture so that an angle between the normal of the stone and the optical axis of the OCT scanning apparatus is reduced to the point at which reflections from the selected surface of the stone are detected by the OCT scanning apparatus. The normal of the stone can be defined as a line perpendicular to a plane in which the table facet of the stone lies.

[0049] Once a signal from the selected surface has been detected via OCT, this signal can be used to calculate or otherwise determine an equation for the plane in which the selected surface lies, in the reference frame of the OCT scanning apparatus. However, in order to carry out the volumetric scan, either the stone or the OCT scanning apparatus, or both, is moved to minimise surface reflections which can interfere with detection of inclusions within the stone. The location and orientation (poise) of the stone with respect to the OCT scanning apparatus is then re-determined. As discussed above, the OCT scans may be carried out in any order.

[0050] Once the 3D position of a selected surface of the cut stone, e.g. the table facet, has been located with reference to the OCT scanning apparatus (for example, with reference to the OCT scan axis) it can be easier to identify the locations of the corners and vertices of the stone, as these scatter the OCT beam so are visible in the OCT volumetric data. Said another way, the facet edges that are aligned with the fast axis of the scan scatter back more than tilted facet edges that are perpendicular to it.

[0051] The OCT data obtained from the volumetric scan of the stone’s interior may include noise and other artefacts, including but not limited to saturation artefacts, horizontal and vertical line artefacts, and background noise. In order to remove these, in one example, an additional OCT volumetric scan (also referred to as a third scan) is carried out, with the stone and the scanning arm of the OCT apparatus in the same position in each scan, but with the reference arm of the OCT apparatus in a different position.

[0052] Said another way, the reference arm may move - for example, by around 15 pixels - between the first and the third OCT scans. The data from each of the first and third scans can be accurately combined as neither the stone nor the scanning arm move between scans.

[0053] Any quality features, such as inclusions, will appear in the same position in the first and third volumetric OCT scans. However, any artefacts will no longer be in the same position. Therefore, by processing the 3D OCT volumetric datasets generated by the first and third scans using a logical AND gate, for example, any features which do not appear in both datasets are removed, eliminating any potential coherent noise artefacts.

[0054] Further filtering and processing of one or both of the 3D OCT datasets (the scan outputs) may be carried out using known methods, for example, thresholding, improving contrast, Weiner filtering, wavelet transforms, and side lobe removal methods such as deconvolution. However, the invention is not limited to these methods. Further image filtering and processing may be carried out on the dataset produced by the second OCT scan, which is carried out to determine the location of the selected facet, optionally before calculating or otherwise determining the plane equation of the selected facet.

[0055] Optimum focus of the OCT scanning apparatus may be set to gather scattering signals from inclusions in the stone. In one example, the centre of a 0.4ct stone is initially used and the signal received from the crystalline structure at the core of the 0.4ct stone is maximised, thereby optimising the focus.

[0056] As discussed above, the determination of a location of a feature such as an inclusion with respect to the facets of a cut stone can be important to the grading or valuation of a stone. In some methods, the relative area of the inclusions relative to the size of the stone is also important.

[0057] An additional feature used in some grading and / or valuation methods is contrast, which can be described as how visible inclusions are against their background and how they appear for example under overhead, bright or dark field lighting. Contrast information can be used in some grading methods for borderline cases, which may include stones which fall between two grades or other quality descriptors.

[0058] In general, it is not possible to obtain contrast information using OCT scanning. However, the inventors have realised that integrating additional imaging techniques into an OCT scan head can augment data produced from OCT volumetric scans, providing valuable contrast data which may optimise grading methods. Such imaging techniques may require additional illumination means, but can use the same imaging lens, beam splitter, CMS camera etc. already present in the OCT set up.

[0059] The additional imaging techniques used to obtain contrast information use one or more of diffuse lighting, dark field lighting or other visible lighting under which to capture one or more 2D (or even 3D) images of the gemstone. Dark field light can be defined as capturing an image of an object off-axis, for example, positioning a light source at around 45° - 90° from the optical axis of the image capture device. Figure 4 illustrates an example of a 3D dark field image obtained of the table 12 of a cut and polished gemstone. In this example, the gemstone is a 0.41 ct polished round cut diamond 10, with inclusions 18 obtained from OCT data marked, combined with a wireframe model 16 of the stone. A computed Score and Grade from a grading method, discussed in more detail below, is shown in the box below.

[0060] As described above, the location of the scattering centre or inclusion can be determined using an improved OCT method, and the location is determined with respect to the reference frame of the OCT apparatus. In addition, a set of measurements of the stone’s surface and orientation can be used to determine the location of the scattering centre with respect to the reference frame of the cut stone itself.

[0061] The reference frame of the stone can be determined by measuring the outer surfaces of the stone to produce a dataset representing a 3D model of the stone’s surface, in conjunction with carrying out the various OCT scans discussed above. Alternatively or additionally, a set of earlier measurements can be obtained in combination with relating those earlier measurements to the orientation of the stone during the OCT measurements. The 3D model and / or the earlier measurements may be provided in the form of a wireframe model to indicate the vertices and corners of a polished diamond, or a larger set of measurements can be provided. As previously described, although OCT may be used for measuring the polished surfaces of a cut gemstone, it is preferable to use another measurement method more suitable for determining the presence and shape of the outer surfaces.

[0062] One example of a measurement method for determining the presence and shape of outer surfaces of a cut stone is a silhouette imaging method. In this method, the stone is mounted on a dop, pillar or other support in a table down orientation. The mount is rotatable through 360 degrees. One or more telecentric light sources are used to illuminate the stone as it is rotated, and a plurality of 2D silhouette images of the stone is obtained. These silhouette images are combined using known methods to produce a 3D surface model of the stone.

[0063] One known apparatus suitable for obtaining silhouette images and for generating a 3D surface model is produced by Sarine Technologies Ltd. In one example, a set of measurements of the outer surfaces of the stone that was obtained at an earlier stage is obtained, and a small set of new measurements to relate the earlier measurements to the current orientation of the stone is taken, so that the location of the scattering centre can be related to the reference system of the stone itself. The earlier set of measurements can be in the form of a wireframe model to indicate the vertices and corners of the polished diamond, or it can be a larger set of measurements including more information about the shape and colour characteristics of the stone. A small set of reference measurements is collected once the stone is in place, such as the determination of one or more of the main surfaces, such that the earlier measurements can be related to the current orientation of the stone.

[0064] The examples of obtaining a set of measurements of the external surfaces of the stone are based on measurement collected by different devices than the OCT device. It will be appreciated that if the stone remains in the same place between the surface measurement and the OCT measurement, the reference frames of the two devices can be related to each other by making a calibration measurement. However, if the stone is moved from the surface measurement device to the OCT device, the movement should take place along a predetermined path to avoid losing the information obtained about the stone’s orientation.

[0065] In one non-limiting example, a 3D surface model of a cut gemstone is produced using an apparatus configured to obtain a plurality of silhouette images of the stone at a range of rotational positions while the stone is mounted on a rotatable mount in a table-down orientation (this may also be referred to as the stone’s first position). In this example, the 3D surface model apparatus is co-located with the OCT scanning apparatus and the silhouette images and resulting 3D surface model are obtained more or less simultaneously to the OCT scans. For example, the 3D surface model apparatus and the OCT scanning apparatus may be located in the same room such that a cut gemstone may be transferred between the two. However, as discussed above, the invention is not limited thereto, and a 3D surface model or other set of measurements may have been previously obtained elsewhere.

[0066] Once the silhouette imaging of the stone is complete, the stone is removed from the first position by a vacuum wand, or other arrangement configured to hold the stone using negative pressure. The wand is configured to hold the stone by its pavilion and to move the stone under vacuum to a position within range of the OCT scanning apparatus (which may also be referred to as the stone’s second position). Moving under vacuum may comprise a suction nozzle keeping the stone in a fixed position. In this example, the OCT apparatus is inverted, such that an OCT scan may be obtained via the table facet of the stone while the pavilion of the stone is held by the vacuum wand. In this way, the orientation of the stone between the first and second positions of the stone does not change, and registration of the 3D surface model with the OCT scan data is facilitated.

[0067] Subsequently, the first, second and third OCT scans are carried out as described above. The order in which the OCT scans are carried out may be varied. The 3D surface model is registered with the OCT volumetric data, using the OCT data for the selected surface as a “key”.

[0068] One example of a system comprising an apparatus 120 configured to generate a 3D surface model of a stone and an OCT scanning apparatus 150 is illustrated in Figure 5. In this illustrated example, a diamond 100 is mounted at first position A, table facet down on a rotatable first vacuum chuck 110, which forms part of a silhouetting machine 120. The silhouetting machine 120 is configured to obtain a plurality of 2D silhouette images of the diamond 100 at incremental rotational positions. Such silhouetting machines are known to the skilled person and further discussion of the machine’s operation and the method of producing the 3D surface model from the 2D silhouette images will be omitted here.

[0069] Once the imaging process is completed by the silhouetting machine 120, the diamond 100 is collected by a spring loaded second vacuum chuck 130, and the negative pressure applied to the stone 100 by the first vacuum chuck 110 is released. The second vacuum chuck 130 is configured to move laterally along a substantially horizontal transfer rail 140 in order to translate I transport the diamond 100 from first position A to a second position B, within an optical range of an OCT scanning device 150 (in this example, a swept source OCT, having a wavelength centred at around ~1 pm that sweeps across a narrow band of wavelengths).

[0070] It will be appreciated that at position B, the diamond 100 is held in place by its pavilion, such that a volumetric OCT scan may be carried out via the table facet. To facilitate the OCT scan, the OCT laser scan head 160 is substantially inverted. A focus drive of the scan head is moveable along an axis perpendicular to a normal of the diamond 100 in order to provide optimum focus.

[0071] It will further be appreciated that the orientation of the diamond 100 (e.g. the poise) is substantially preserved between position A and position B, even if the precise position in 3D space is not.

[0072] At position B, the first, second and third OCT scans are carried out as described above by the OCT scanning apparatus 150. During each of these scans, the diamond remains stationary at position B.

[0073] As illustrated in Figure 5, during the second scan the table facet of the diamond 100 is misted with droplets of water produced by a humidifier 170. Where the second scan is carried out before the first scan, the scans are configured so that the water droplets evaporate from the table facet prior to the first scan being carried out.

[0074] Of course, additional scans and filtering I image processing may be carried out, but for simplicity of understanding these are not shown in Figure 5.

[0075] In one example, ray tracing is used determine the correct location of features such as an inclusion in 3D space. Rays are projected vertically to data points in the 3D-OCT volumetric data in order to determine which facet (from the fitted 3D surface model) the inclusions lie beneath. Rays are then traced to these points and refracted to real locations in 3D space, by understanding the effects of the refractive index of the gemstone on apparent depth locations. Standard ray-tracing software may be used to determine the path of light rays within the stone.

[0076] In one example, all scattering data points in proximity to the 3D model surface are removed, leaving only the inclusions. This step may be carried out after the data points are traced to their true position and checked relative to the stone model position.

[0077] As outlined above, accurate mapping of the size and relative location of features within a cut gemstone may be used to grade the stone, as shown in Figure 4. However, the inventive concept is not limited thereto, and may have other applications, such as reidentification of a cut gemstone, determination of point of origin, and so on. For example, mapping of the size and relative location of features within a gemstone, using the methods and apparatus described herein, may be used to create a “fingerprint” or identifier of the gemstone. This “fingerprint” can be stored as data in a database, a Blockchain, or the like, together with metadata, such as ownership history, mine of origin, grading, location history and so on. Subsequently, the stored “fingerprint” may be used to re-identify the gemstone as the same stone from which the “fingerprint” was created. Thus, metadata associated with the stored “fingerprint” can be reliably associated with the re-identified stone.

[0078] Similarly, a rough gemstone may be mapped to identify the size and relative locations of features within, whether using OCT or otherwise. This can then also be used as a fingerprint as described above. By subsequently mapping the features in a polished stone it may be possible to match it with an earlier fingerprint from a rough stone.

[0079] Polished Clarity Grading ranges from Internally flawless (IF) where there are no inclusions to I included. In between these we have VVS (very very slightly included), VS (very slightly included) and SI (slightly included). These grades are then usually subdivided into 2 or 3 subgrades i.e. VVS1 to VVS2, VS1 to VS2, SI1 to SI2 and 11 to I3. Granularity is increased further with each of these split into a low, middle and high subgrade with high being the least included of the range.

[0080] In one example, a clarity grade for the polished gemstone from the data generated is obtained through a modified, objective ‘Cowing’ like grading process. In the known Cowing method, the total area of the inclusions from the OCT images of the table and pavilion is normalised to the stone area (provided by the 3D surface model) and provides a Score, linked to a grade, as shown in the equation below:

[0081] Score ocr) = ( 1 / 2 * log(Area) — log (10))) / log (2) + Baseline

[0082] The Baseline is used to translate boundaries in order to overlap the Score obtained from the OCT data to better fit grades obtained from human expert graders. The lower the Score, the higher the grade and thus the fewer inclusions are observed.

[0083] Additional grade modifiers are applied for position of the inclusion and contrast with the surrounding area of the stone. For example, the accurately mapped inclusion data can be used as a mask to overlay with conventional imaging. Unlike conventional methods of grading, OCT can give an accurate measure of depth of the inclusion, which can also be used as a modifier of the grade as deeper inclusions usually impact the appearance more.

[0084] The position modifier is relative to zones demarked by rings, which start from just outside the table facet and progress in larger diameters outwards to the girdle of the stone. An inclusion of a certain size thus has a better grade (negative score modifier) if in, or touching, and outer ring than if in, or touching, an inner ring.

[0085] The contrast modifier, i.e. , how visible inclusions are against their background and how they appear for example under overhead, bright or dark field lighting, can be particularly important near grade boundaries. Contrast information may be obtained using dark field lighting, as described above.

[0086] Once the size and relative position of all internal scatterers (such as inclusions) has been accurately determined, and the relevant modifiers have been applied, the resulting information can be used to form a clarity grade for the stone.

[0087] The following provides one example of the sequence of steps in a method of measuring one or more features in a polished gemstone and mapping a location of the one or more features within the interior of the stone. This example is not intended to be limiting and the described steps may be carried out in a different order, omitted, augmented, substituted and combined without limitation.

[0088] Data Capture Process:

[0089] • Acquire a model independently using known silhouetting techniques and processing (e.g., Sarine) using model acquisition equipment;

[0090] • Ideally preserve orientation of the stone by translating the stone under vacuum from said model acquisition equipment to an inverted 3D-OCT system;

[0091] • Set optimum focus to gather signal captured from inclusions (depends on stone size), for example, using the middle of a 0.4ct stone and maximising the signal from the crystalline structure at its core; • Acquire a reference 3D-OCT scan prior to the main data set, i.e. at an offset reference arm position in the interferometer;

[0092] • Conduct an 3D-OCT scan to record volumetric scattering data in combination with dark field imagery with the OCT capture equipment set at an angle outside the normal of the scan lens to reduce strong surface scatter;

[0093] • Use a humidifier with tightly coiled piping to produce fine water droplets that form a thin vapour of volatile liquid coating onto the stone and prevents large droplets from condensing onto it. Large droplets in the piping coalesce and drop under gravity back into the water reservoir of the humidifier;

[0094] • Conduct a third synchronised 2D or 3D OCT scan, which includes the step of coating the polished gemstone with fine droplets of volatile liquid that form a thin layer on the polished gemstone. The scans are synchronized to occur between the vapour dispersing, and evaporating from the surface of the gemstone.

[0095] Data Processing:

[0096] • Read in both 3D-OCT data sets and threshold them, to remove noise and saturation artefacts;

[0097] • Filter and process the OCT data to improve contrast and reduce horizontal and vertical line artefacts and background noise, including Wiener, Wavelet transforms and side lobe removal methods such as deconvolution;

[0098] • Process both 3D-OCT data sets with for example a logical AND gate, wherein features that do not appear in both sets are removed, eliminating any potential coherent noise artefacts that do not track with stone position;

[0099] • Read in surface model data;

[0100] • Read in selected surface / table plane data;

[0101] • Clean up selected surface I table plane data with image filters for processing, to then be able to determine the plane equation of the surface I table and to re-orientate the surface model so as to fit to the 3D-OCT data;

[0102] • Project rays vertically to data points in the 3D-OCT stack and determine which facet (from the fitted model) the inclusions lie beneath;

[0103] • Trace rays to these points and refract rays to real locations in 3D space, understanding the effects of the refractive index of diamond on apparent depth locations;

[0104] • Remove all scattering data points in proximity to the model surface, leaving only the inclusions. It will be appreciated that in one example, each of the OCT scanning apparatus and 3D surface model generating apparatus and associated components may be controlled by a separate processor. Alternatively or additionally, a single processor may control both apparatus. The modelling, filtering, image processing and calculation / determination steps described herein may be performed by one or more processors directly associated with one or both of the described apparatus. Alternatively or additionally, these steps may be carried out by one or more remote processors, communicating over a wired and / or a wireless network.

[0105] One example of the OCT scanning apparatus (set up) described herein is a Thorlabs Vega ™ system.

[0106] One example of the silhouetting machine described herein is a Sarine Technologies Ltd DiaMension HD, but the invention is not limited thereto.

[0107] While the above description makes particular reference to diamond, it will be appreciated that the methods and apparatus described herein may also be used for other types of gemstone.

Claims

CLAIMS:

1. A method of measuring one or more features in a polished gemstone, the method comprising: providing a 3D surface model of the gemstone; carrying out a first scan of an interior of the gemstone using optical coherence tomography, OCT; carrying out a second scan of a selected surface of the gemstone using OCT ; determining, from the second scan, a plane in which the selected surface lies; registering an output of the first scan with the 3D surface model using the determined plane of the selected surface; and mapping a location of one or more features within the interior of the gemstone based upon the registering.

2. The method of claim 1, wherein the location of the one or more features is determined within a reference frame of an OCT measurement apparatus.

3. The method of claim 1 or 2, wherein the selected surface is a table facet of the gemstone.

4. The method of claim 3, wherein the first scan is carried out through the table facet.

5. The method of any preceding claim, wherein the first scan and / or the second scan is carried out after depositing a fluid film on the selected surface of the gemstone, said fluid film preferably comprising droplets of a volatile liquid, optionally water or acetone.

6. The method of claim 5, wherein the first scan is carried out in the absence of the fluid film.

7. The method of any preceding claim, wherein the second scan comprises: one or more 2D scans, or a 3D scan.

8. The method of any preceding claim, comprising carrying out a third scan of the interior of the gemstone using OCT, wherein the third scan is carried out at a different OCT reference arm position to the first scan.

9. The method of claim 8, comprising combining an output of the third scan and the output of the first scan by selecting only data points present in both scan outputs to remove artefacts, said artefacts not comprising features.

10. The method of any preceding claim, wherein the first scan is carried out before the second scan, or wherein the second scan is carried out before the first scan.11 . The method of any preceding claim, comprising capturing one or more 2D or 3D images of the gemstone under one or more of diffuse, dark field or visible lighting conditions.

12. The method of any preceding claim, comprising projecting rays vertically to data points in the output of the first 3D scan; determining which facet of the gemstone the features lie beneath; tracing rays to these data points; and refracting rays to real locations in 3D space, based upon a refractive index of the gemstone.

13. The method of any preceding claim, wherein the providing a 3D surface model of the gemstone comprises providing a dataset of coordinate data representing an exterior surface of the gemstone.

14. The method of any preceding claim, wherein the providing a 3D surface model of the gemstone comprises obtaining, at a first location, a plurality of 2D silhouette images of the surface of the gemstone at one or more rotational positions of the gemstone, and combining the plurality of 2D silhouette images to generate a 3D surface model of the gemstone.

15. The method of claim 14, comprising moving the gemstone from the first location to a second location, said second location being within optical range of an OCT scanning apparatus configured to carry out the first and the second OCT scans.

16. The method of claim 15, comprising preserving an orientation of the gemstone between the first and the second locations.

17. A method of grading a polished gemstone, the method comprising measuring one or more features within the gemstone in accordance with any of claims 1-16.

18. An apparatus for measuring one or more features in a polished gemstone, the apparatus comprising: an OCT scanning device configured to; carry out a scan of an interior of the gemstone; carry out a second scan of a selected surface of the gemstone; a processor configured to: determine, from the second scan, a plane in which the selected surface lies; register an output of the first scan with a 3D surface model of the polished gemstone, using the determined plane of the selected surface; and map a location of one or more features within the interior of the polished gemstone based upon the registering.

19. The apparatus of claim 18, further comprising a humidifier, configured to deposit a fluid film on the selected surface of the gemstone, said fluid film preferably comprising droplets of a volatile liquid, optionally water or acetone.

20. The apparatus of claim 18 or 19, the apparatus further configured to carry out a third scan of the interior of the gemstone, wherein the third scan is carried out at a different position of a reference arm of the OCT scanning device to the first scan.

21. A system for measuring one or more features in a polished gemstone, comprising: the apparatus of claims 18-20; and an apparatus configured to provide a 3D surface model of the gemstone by obtaining, at a first location, a plurality of 2D silhouette images of the surface of the gemstone at one or more rotational positions of the gemstone, and combining the plurality of 2D silhouette images to generate a 3D surface model of the gemstone.

22. The system of claim 21 , comprising a vacuum chuck configured to move laterally between the first location and a second location, said second position within range of the OCT scanning device.

23. The system of claim 22, comprising a transfer rail, wherein said vacuum chuck is configured to move laterally along said transfer rail to transport the gemstone from the first location to the second location.

24. The system of claim 23, wherein said vacuum chuck is configured to preserve an orientation of the gemstone between the first and the second locations.

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