A method of and apparatus for screening gemstones

Abstract

A method of screening gemstones as natural or non-natural includes illuminating a gemstone with pulses of UV light and capturing a luminescence image of the gemstone; illuminating the gemstone with pu

Description

Technical Field The present invention relates to the field of screening gemstones. In particular, the present invention relates to the screening of diamonds. Background Synthetic or man-made diamonds, manufactured by HPHT (high pressure high temperature), CVD (chemical vapour deposition) or other industrial, non-geological processes, have a wide variety of industrial applications. Being man-made, they do not attract the high values associated with natural diamonds of similar colour and quality and it is desirable from a consumer perspective to provide reliable means of identifying and separating synthetic diamonds from natural ones. There are numerous characteristics that can be used to distinguish between a diamond from nature and one produced from an industrial process (which may be called a synthetic) but the inherent variability in the natural diamond and of the synthetic processes makes such a task difficult and onerous. One characteristic that has proven to be of utility is the emission of luminescence when a diamond is illuminated (or excited) by a source of energy, most commonly but not exclusively, electromagnetic radiation. An ultraviolet lamp, emitting radiation with a wavelength of 365nm or 254nm (nanometres), might lead to the observation of what would be called fluorescence. Fluorescence is a type of luminescence characterised as being produced only when the ultraviolet excitation is on. Phosphorescence, which may also be observed following UV excitation, is a type of luminescence that remains once the excitation is removed but which gradually decays away. Through interpretation of any such luminescence present, taking into account the observable temporal characteristics, colours and spatial distribution, inferences on the nature of the diamond under test may be drawn. For example, fluorescence colour and pattern can be utilised to determine whether growth-related features are characteristic of synthetic or natural diamond. Long-lived phosphorescence, which may persist for several seconds or more after an excitation pulse has ended, can be used to provide an indication as to whether a diamond is natural or synthetic, being rare in natural diamond and much more common in synthetic diamond. It can therefore be used as a simple, although not definitive, method to distinguish a large proportion of synthetic diamonds from the vast majority of natural diamonds. An exception to the above is natural Type lib diamonds, which contain significant boron impurities. However, type lib diamonds account for perhaps only 0.1% of all natural diamonds, so are fairly uncommon. It has also been noted that some synthetic diamonds grown by CVD (chemical vapour deposition) do not exhibit the type of long-lived phosphorescence described above. Detection of long-lived phosphorescence in isolation could therefore not be used to distinguish CVD stones from natural diamonds and other methods would need to be employed to definitively identify these synthetics. It has been discovered that it is possible to distinguish between natural and synthetic diamond through measurement of much shorter-lived and weaker phosphorescence. This type of phosphorescence occurs for less than 100 milliseconds after removal of the UV source and therefore cannot be measured using the known methods described above, as the short-lived phosphorescence is typically masked or swamped by other forms of luminescence. Typically, this short-lived phosphorescence exhibits a broad emission with a peak at 455 nm and a decay time of less than 80ms. Where analysis of an image of the diamond confirms the presence of blue phosphorescence in a time window starting at or after the end of an excitation pulse (around 80ps) and finishing at around 80ms after the excitation pulse, this is an indicator that the diamond sample being tested is a natural diamond and can be used to indicate both weak Type la and type Ila natural diamonds as well as the majority type laAB diamonds. The above “blue fast phosphorescence”, with a decay time of around 8.8 ms and a peak wavelength at 455 nm, has been found to be typical of >99.9% of colourless natural diamonds. A method of using “blue fast phosphorescence”, in addition to other highly specific “markers” characterised by luminescence decay time and luminescence wavelength is disclosed in WO 2017 / 001835 A1. This method is capable of identifying or referring for further analysis all synthetic diamonds under test (i.e. gives no false positives) while referring only a very low percentage of natural diamonds (0.7% for the core sample, 1.9% for small stones). It therefore represents a highly sophisticated and reliable method of definitively distinguishing between natural diamonds and a range of synthetics. An instrument configured to carry out this method is also described in WO 2017 / 001835 A1. This instrument is expected to be used at laboratories or operations with higher volume screening activities, and may be unsuitable to deploy at retail level due to its price, size and the need for significant user input to optimize image collection parameters. An alternative instrument, which is smaller, less expensive and suitable for operation in retail outlets is therefore desirable. Summary of Invention In one aspect, there is provided a method of screening gemstones. The method comprises: illuminating a gemstone with one or more pulses of ultraviolet, UV, light and capturing at least one luminescence image of the gemstone during and / or after each UV pulse; illuminating the gemstone with one or more pulses of visible light and capturing at least one visible image of the gemstone during each visible light pulse; analysing the at least one luminescence image to identify pixels, associated with the gemstone, in which markers characterised by luminescence decay time and luminescence wavelength appear; classifying the gemstone pixels as natural or non-natural based upon the identified markers; modifying pixels of the at least one visible image based upon the classification, such that pixels classified as natural appear in a first colour, and pixels classified as non-natural appear in a different colour; and displaying the modified at least one visible image via a screen. The analysing, classifying, generating and modifying steps are carried out prior to displaying the modified image on the screen. A gemstone to be screened may be mounted in a jewellery piece. The steps of: illuminating the gemstone with one or more pulses of ultraviolet light and capturing at least one image of the gemstone during and / or after each UV pulse; illuminating the gemstone with one or more pulses of visible light and capturing at least one image of the gemstone during each visible light pulse; analysing the at least one luminescence image to identify pixels, associated with the gemstone, in which markers characterised by luminescence decay time and luminescence wavelength appear; classifying the gemstone pixels as natural or non-natural based upon the identified markers; and modifying pixels of the at least one visible image based upon the classification, such that pixels classified as natural appear in a first colour, and pixels classified as non-natural appear in a different colour; may be repeated at different rotational and / or translational positions of the gemstone. A modified image may be generated at different rotational and / or translational positions of the gemstone. Modified images generated at different rotational and / or translational positions of the gemstone may be combined to generate a video, or the like. The analysing, classifying, modifying and displaying steps may be carried out prior to illuminating the gemstone with a second pulse of UV light and a second pulse of visible light. A gemstone may be further illuminated with UV light from an angle offset from an image capture axis. The visible light may be diffuse light. A gemstone may be located within a substantially hemispherical dome and illuminated with visible light from within said dome. A substantially hemispherical dome may be provided with a substantially matte black interior having sufficient reflectivity to reflect the visible light onto the gemstones. The UV pulses and the visible light pulses may be controlled not to overlap in time. One of the markers may comprise short-lived phosphorescence having an emission peak at around 455 nm and a decay time of less than 80ms, said phosphorescence visible in a time window starting at or after the end of the first UV pulse. The step of classifying the gemstone pixels as natural or non-natural based upon the identified markers may be carried out using an algorithm, preferably a look-up table, LUT, algorithm or a support vector machine, SVM, algorithm. Classifying the gemstone pixels as non-natural may include one of: identifying the gemstone pixels as synthetic diamond; identifying the gemstone pixels as non-diamond; or identifying the gemstone pixels as requiring further testing. In another aspect there is provided a method of identifying a gemstone as natural or non-natural. The method may comprise illuminating the gemstone with ultraviolet, UV, light; capturing luminescence emitted by the gemstone; analysing the captured luminescence to determine whether the gemstone is natural or non-natural; illuminating the gemstone with visible light; capturing an image of the gemstone under visible illumination; modifying pixels of the image based upon the analysis such that pixels relating to a natural appear in a first colour and pixels relating to a non-natural appear in a second, different colour; and displaying the modified image. In a further aspect there is provided an apparatus for screening gemstones. The apparatus comprises a source of ultraviolet, UV, light; a source of diffuse white light; an image capture device configured to capture one or more images of the gemstone under UV illumination and under diffuse white light illumination; a processor configured to analyse the one or images of the gemstone under UV illumination to identify pixels, associated with the gemstone, in which markers characterised by luminescence decay time and luminescence wavelength appear. The processor is further configured to: classify the gemstone pixels as natural or non-natural based upon the identified markers; modify pixels of the at least one image of the gemstone under diffuse white light illumination based upon the classification, such that pixels classified as natural appear in a first colour, and pixels classified as non-natural appear in a different colour; and output the modified image via a screen. The processor is configured to carry out the steps of analysing, classifying, and modifying prior to outputting the modified result on the screen. The apparatus may further comprise a diffuser assembly comprising the source of diffuse white light. The diffuser assembly may comprise a substantially hemispherical dome and the source of diffuse white light may be arranged within the dome. The dome may be provided with a matt black interior. The apparatus may comprise a moveable drawer configured to support the gemstone during illumination. The apparatus may comprise a mount for supporting a jewellery piece including the gemstone. The mount may be actuated by a motor to rotate and / or move along a predetermined path. The source of UV light may be adjustable along an axis and / or at a pivot point. The processor may be configured to control the UV source and the diffuse white light source such that there is no temporal overlap therebetween. The apparatus may further comprises a screen, display or the like. Brief Description of Figures Figure 1 illustrates screening results overlaid on high quality visible images of gemstones; Figure 2 illustrates example components of a screening apparatus; Figures 3a and 3b illustrate examples of a UV strobe and UV assembly, and a diffuser assembly; Figure 4a illustrates further detail of the UV assembly of Figures 3a and 3b; Figure 4b illustrates an alternative assembly; Figure 5 illustrates an example diffuser apparatus; Figure 6 illustrates additional detail of the diffuser apparatus of Figure 5; Figure 7 illustrates visible images of a jewellery piece overlaid with classification results; Figure 9 illustrates the production of a sequence of images; and Figure 9 illustrates a method of screening gemstones. Detailed Description Described herein is a method of screening gemstones. The method comprises the steps of illuminating a gemstone with one or more pulses of ultraviolet, UV, light and capturing at least one luminescence image of the gemstone during and / or after each UV pulse. The method further comprises illuminating the gemstone with one or more pulses of visible (white) light and capturing at least one visible image of the gemstone during each visible light pulse. The method further comprises analysing the at least one luminescence image (i.e. the image of luminescence) to identify pixels associated with the gemstone, in which markers characterised by luminescence decay time and luminescence wavelength appear. Subsequently, the method comprises classifying the gemstone pixels as natural or non-natural based upon the identified markers, and modifying pixels of the at least one visible image based upon the classification, such that pixels classified as natural appear in a first colour, and pixels classified as non-natural appear in a different colour. The modified at least one visible image is then displayed via a screen. The analysing, classifying, generating and modifying steps are carried out prior to displaying the modified image on the screen. It will be appreciated that gemstone pixels classified as natural are pixels corresponding to a natural gemstone, and that gemstone pixels classified as non-natural are pixels corresponding to a non-natural gemstone. The step of classifying the pixels above may be considered to include the generation of a colour map or overlay, used to modify, merge with or modulate specific pixels of the visible image. While the description herein predominantly refers to diamonds, the skilled person will appreciate that the inventive features can also be applied to other gemstones. The inventors have developed an apparatus and a method that provides screening capabilities that classify stones (e.g. distinguish natural from synthetic (laboratory grown) diamonds, and / or from non-diamond material). In particular, this screening capability can be used for diamonds mounted in jewellery pieces, such as rings, bracelets, earrings, studs, pendants, watches and so forth. The invention is not limited to jewellery, however, and may be used to screen loose stones, such as melee, individual unmounted stones, stones in other settings than jewellery, and the like. The apparatus described herein is configured to have a relatively small footprint, to be less costly and to have a high level of automation, such as to be operable by an untrained operator with little or no specialist knowledge, such as a jeweller, for example. In this respect, the inventors have devised an apparatus and method that allows both visible images, and images of luminescence, to be acquired of sufficient quality to allow automated generation of classification results, in terms of whether a specific stone is natural or synthetic. Further, the generated classification results are overlaid onto the visible images of the stone (for example, mounted within a jewellery piece) which are displayed to a user on a screen, to enhance and simplify detection capability. As will be appreciated, small stones mounted within jewellery pieces may be particularly challenging to screen. For example, acquiring a set of luminescence images at a range of collection parameters, such that all gemstones within the field of view can be analysed to determine which are natural and which are not, may be particularly challenging for jewellery pieces such as rings. The present invention provides the necessary image processing and classification for robust screening of stones commonly found at retail, while maintaining a compact configuration and low price-point. Further, the inventors have developed an apparatus and method which enables mounted stones, such as jewellery pieces, to be rotated and re-positioned during the screening process. As illustrated in Figure 1, the screening results are overlaid on high quality visible images of the stones, such that an operator may view the classification of each stone in the piece via a display screen, optionally in real time. In this example, although all stones within the jewellery piece 10 appear colourless to the eye, within the displayed overlay, stones classified as natural 12 appear in blue, while stones classified as nonnatural 14 appear in red. Other stones (such as simulants) may be classified as non emitters of relevant luminescence and in this example, these stones appear as colourless, or grey 16. It will of course be appreciated that the colours applied to each stone within the colour map or overlay relate to the classification result, and therefore other colours may be used as long as a visual indication of the classification results is provided to the operator. For example, natural stones may appear green, synthetics may appear yellow and simulants may appear red. Said another way, naturals may appear using a first colour, and nonnaturals (e.g. synthetics, simulants) may appear in one or more different colours. In one example, where a stone is not associated with a particular colour, or appears grey, that stone may be considered as requiring further testing to determine its composition (referral). In one non-limiting example, a moveable drawer is provided to support loose stones during screening. In another non-limiting example, the drawer may be replaced or supplemented by one or more mounts of different types. In the specific example of Figure 1, this mount 18 is configured to support a ring 10. The mount may be optionally associated with an indexed wheel (not shown) which enables manual rotation of the mount through 360 degrees, at 30 degree increments. This is of course a non-limiting example, and mounts of different shapes with alternative rotational and / or translational mechanisms and capabilities may be provided. Alternatively or additionally, a simple platform can be provided on which one or more stones or jewellery pieces may be supported. Alternatively or additionally, a jewellery piece, such as a ring containing multiple stones, can be mounted within the apparatus, and the apparatus configured to automatically rotate and / or translate the mounted piece along a predetermined route. In one example, a single visible image and associated overlay (colour map) derived from luminescence data and classification is obtained at a number of predetermined positions, e.g. rotational positions, along this predetermined route. Subsequently, each of the visible images plus the associated overlay is “stitched” together or otherwise combined to form a sequence, or video. In a further example, the piece is viewable in real-time, with visible images and associated overlay obtained “on the fly”. Where a jewellery piece is under screening, movement of the piece, whether manually or automatically, enables visualisation of all aspects of the piece, even where the piece exceeds the field of view (which in one non-limiting example is approximately 20 mm x 20mm, preferably 17 mm x 17 mm). However, it will be appreciated that screening may be alternatively or additionally carried out on a stationary jewellery piece or on stationary loose stones. It will also be appreciated that while viewing a jewellery piece in real-time, or as a sequence of images and associated overlay obtained at predetermined positions, may be advantageous for stones mounted in jewellery pieces, such as rings, for loose stones this type of view may be unnecessary. Therefore, as an alternative to the above real-time display, or sequence of images at predetermined positions along a route, a visible image and “superimposed” overlay may be generated and displayed via the screen after a single pulse of UV light and single visible image capture. Said another way, a static image which does not change over time may be produced and displayed to the user on the screen. The inventors envisage one example of the inventive apparatus providing one or more of the following three imaging scenarios: (i) real-time (live) display; (ii) single image display (e.g. for loose stones); and (iii) a series of single images taken at several rotational positions stitched together to form a short sequence. Of course, where a single image or sequence are produced, or where the stone is viewed in real-time, an image, sequence or video could optionally be saved for display later. While the term “superimposed” is used herein to describe displaying a visible colour image plus a colour map or overlay, as discussed below, in one example, pixels of the white light (visible) image are modified, so as to be (for example) blue or red if the pixels map to pixels within the signal ranges set by the luminescence images. The intensity / level of saturation of each colour is modified by the intensity levels of the white light images. Said another way, the colour map is modulated by the intensity of the diffuse lighting image. Therefore, in some examples “superimposed” may refer to a merging of images rather than separate image layers per se. Alternatively or additionally, data obtained from analysis of luminescence produced under and / or following UV excitation is used to modify or modulate specific pixels of one or more white light (visible) images. An example configuration of the main components of the apparatus will now be discussed with reference to Figure 2. It is to be understood that this example configuration is not to be considered limiting, and that individual components may be omitted, present in singular form, present in multiple form, and combined without limitation. Moreover, the specific circuitry, relative location, assembly format, design and so forth as shown in Figure 2 is provided as an example only. The apparatus of Figure 2 principally comprises an imaging device 20 (camera); an ultraviolet (UV) strobe and optics assembly 22, a processor 24, a diffuser assembly 26, a colour display (not shown in Figure 2) and a housing (not shown in Figure 2). The housing is arranged so as to exclude stray light during imaging and to protect the user from exposure to UV light. In one example, the housing interior may form part of the diffuser assembly. One, non-limiting example of a suitable imaging device is a DFM37UX287-ML camera containing a 10 bit image sensor and pixel size of 7pm. The ImagingSource camera API records 16bit per channel images. A fixed exposure and 2db Gain with simple gamma correction to the image to enhance low signals and reduce bright signals can be employed with this camera. The arrangement of this example is telecentric in object space, to keep objects the same size through focus and to reduce any potential confusion from out-of-focus asymmetric luminescence blurring possible from a camera perspective. In the example of Figure 2, a single camera 20 is used to capture both visible light and any luminescence emitted. It will of course be understood that the above details are provided as examples only and that any suitable form of imaging device may be provided and configured according to specific requirements. The present invention provides both good illumination of the stone(s) under excitation from the UV strobe, whilst maintaining high quality imaging of both the visible light image and the phosphorescence image. The UV optics assembly 22, diffuser assembly 26 (described below) and the associated imaging device 20 (described above) are configured to provide high definition images of a jewellery piece, loose stones and so forth, including modified illumination which reduces scatter and reflections. The apparatus of Figure 2 is further discussed with reference to Figure 3a and the schematic of Figure 3b. The UV strobe comprised in the UV optics assembly 22 in the example shown in Figures 3a and 3b is configured to provide low angle, pulsed illumination, which avoids the need for a beamsplitter (although it is envisaged that one or more beamsplitters could be used, on-axis, as an alternative). As illustrated, the UV optics assembly 22 and UV strobe are provided in an off-axis configuration. In one nonlimiting example, the angle between imaging and illumination arms (e.g. between the imaging device 20 axis and the UV strobe axis) is configured to be around 20 degrees, see Figure 3b. In this non-limiting example, the UV optics assembly 22 comprises all UV-fused silica optics. The example arrangement of Figures 3a and 3b is illustrated in more detail in Figure 4a, which includes the UV source (strobe) and asphere on the left. In order of position from the UV strobe, the assembly illustrated in Figure 4a includes: a UV asphere 50, a double-convex lens 52 and field lens 54 to project the field iris (collection limiting aperture) onto the sample plane. This is to produce uniform Kohler illumination on the sample (the stone(s) being screened), in order to provide a level signal to noise ratio across the field of view. In the examples described with reference to Figures 3a, 3b and 4a, a point at which a strobe axis and an imaging device (camera) axis intersect is fixed. In a further example, illustrated in Figure 4b, the strobe may be adjustable along its axis, and may additionally be adjustable at a pivot point. Alternatively or additionally, the camera focal point may have a vertical adjustment. Therefore, in the example of Figure 4b, the intersection point (i.e. the location of the gemstones under test) between the strobe axis and the camera (CAM) axis can be adjusted. This allows for much greater flexibility in the size and positioning of the gemstones or jewellery that can be illuminated and positioned in optimum focus. As illustrated in Figure 4b, a distance X can be adjusted (AX) via adjustment of the strobe along the strobe axis, as well as a pivot point. It will of course be understood that the above details are provided as examples only and that any suitable form of UV strobe and associated optical assembly may be provided and configured according to specific requirements. As discussed above, the present invention provides both good illumination of the stone(s) under screening from the UV strobe, whilst maintaining high quality imaging of both the visible light image and the emitted luminescence (e.g. phosphorescence). To this end, as shown in the non-limiting example of Figure 5, the diffuser assembly comprises a substantially hemispherical dome, the interior of which is matt black. In this specific example, the radius of the dome is around 38 mm and the stone(s) or piece under screening is located substantially at the centre of the dome’s radius of curvature. As previously mentioned, the UV strobe is located above the dome at an offset angle. Of course, the apparatus shown in Figure 5 is one example of a diffuser assembly, and various other options may be provided. For example, an alternative arrangement around the stone(s) or jewellery piece to be imaged may be non-hemispherical in design. For ease of reference, the substantially hemispherical dome will be referred to hereinafter, but it will be appreciated that this is an example only. The matt black interior of the dome shown in Figure 5 reduces photoluminescence scattering from the dome surface back onto the stone(s). For example, a white interior would act to reflect luminescence back onto the stone(s), and would potentially interfere with luminescence detection. As shown in the Figure 5 example, the diffuser assembly comprises a ring of white LEDs 28 is provided at the base of and inside the dome, such that diffuse light from the LEDs illuminates the dome interior and scatters back onto the sample under screening. As illustrated in the example of Figure 3b, one or more baffles may be used to direct light from the LEDs towards the dome interior. The one or more baffles may also reduce stray light entering the dome via ports and the like. In one example, the LED ring may be provided on a board, see Figure 6. In one example, twenty-four white light LEDs are provided on a board. The LED ring is pulsed, such that visible light and any luminescence are detected by the same imaging device (camera). Visible (white) light is generally considered to include light of wavelengths between around 390 nm and around 700 nm. The dome has enough reflectivity that light emitted from the LED ring is reflected onto the stones with sufficient intensity for the imaging device to obtain a visual image of the stones. Additional detail of the LED ring 28 is illustrated in Figure 6, which shows four strings each of six LEDs. Each string can be separately controlled to provide quadrant lighting if required. Of course, the above description is not intended to be limiting, and different numbers and arrangement of LEDs, or other suitable forms of diffuse lighting, may be used. For example, the white light sources may be arranged in a square, rectangle or other symmetrical or asymmetric, planar or non-planar configuration. The above-described dome is one example of a diffuser assembly, comprising a dome and a white light LED ring, which acts to reduce pleochroism, dispersion and intense sparkle effects, and to produce a “flatter” image to serve as the base image for the classification pixel-by-pixel overlay or colour map. An alternative to the above-described dome is a diffuser assembly which, instead of a substantially hemispherical dome, essentially comprises the apparatus housing having a matt black interior (again with a reflectivity sufficient to reflect a small amount of visible light). Alternatively or additionally, the diffuser assembly may comprise any chamber of any shape in which it is possible to provide enough visible light to produce crisp and clear visible images, while still detecting faint luminescence. As shown in the examples described above, the stone(s) or jewellery piece under test is illuminated generally from above by a UV source (e.g. strobe), and further illuminated from a generally circular arrangement of one or more white light sources (e.g. LEDs). The white light sources are arranged to be substantially co-planar or in a plane slightly above or below the stone(s). Preferably, images of the stone(s) or jewellery piece under both white light illumination and UV illumination are captured from above by a single image detector (e.g. camera). The platform, mount or drawer on which the stone(s) or jewellery piece is supported is not particularly limited in nature and need not be in any way transparent, since in the examples described above all illumination and imaging is carried out from above (or in the same plane as) the stone(s) or jewellery piece. Referring back to Figure 2, the processor is configured to control the triggering of the UV strobe, diffuse illumination and image capture. In this example, the UV strobe and the diffuse illumination are controlled to pulse at different times, such that there is no temporal overlap between the UV light and white light pulses. Avoiding overlap of the white light and UV illumination prevents weak phosphorescence emitted after UV excitation from being obscured by white light illumination. Image capture may be delayed from the UV pulses in order to create time windows (e.g. a start image capture time and an end image capture time) such that the presence or absence of specific luminescence “markers” characterised by luminescence decay time and luminescence wavelength is detected by the image capture device following UV pulses. Examples of such “markers” are referred to in WO 2017 / 001835 A1, described earlier. Thus, in one example, the image capture device may be controlled to delay for 100 ps from the start of the UV pulse before beginning capture of any phosphorescence. Of course, the start of the UV pulse, while light pulse and image capture can be temporally controlled as required in order to create specific time windows in which the presence or absence of certain of these “markers” may be determined. An example sequence is provided as follows: a single frame of white light image followed by an eight-frame average short phosphorescence image. This example sequence may be used for real-time imaging and for imaging at predetermined positions along a path, as described above. It may also be used to provide a single, static image. The parameters of this (non-limiting) example sequence may take the following form: White Light parameters Autogain - on Gain 2 Delay 0 Exposure 200 pSec 1 frame All 4 white LED quadrants on UV Strobe off Balance ratio: R1,58:G1 :B1.88. Fluorescence parameters Autogain - off Gain 12db Exposure delay 0 pSec Exposure 22 pSec 8 frame average White LEDs off UV Strobe triggered Balance ratio R 1.58:G1:B1.88. Short Phosphorescence parameters Autogain - off Gain 12db Exposure delay 100 pSec Exposure 25000 pSec 8 frame average White LEDs off UV Strobe triggered Balance ratio R1,58:G1 :B1.88. It will of course be understood that the above details are provided strictly as examples only and that any suitable sequence of control of the UV strobe, diffuse illumination and image capture device may be provided according to specific requirements. For example, the gain may be set to 5 db. As discussed above, the classification of each stone within the field of view of the image capture device optionally takes place in real time. Therefore, decision making by the processor related to classification, using one or more algorithms (as further discussed below) and based upon the captured luminescence images under UV strobe light, takes place before the next set of UV and visible light pulses happens, where real-time imaging takes place. For example, the apparatus can be configured to illuminate a stone or stones with a diffuse, white light pulse and capture one or more images of the stone(s); illuminate the same area with a UV light pulse, and capture one or more luminescence images of any emitted luminescence; process the captured visible white light and luminescence images; classify each stone within the field of view of the image capture device according to a decision, present the visible white light image and colour map or overlay on a display screen, then continue by repeating the pulse sequence. For real-time imaging, this method repeats until classification is stopped (for example, when a user has completed a desired screening of a stone(s) or jewellery piece). In respect of classification, the processor is configured to carry out image processing and analysis necessary to determine which of the individual stones within the field of view are (for example) natural diamond, which are (for example) synthetic diamond, and so on. The processor is also configured to display the results of this analysis on a display screen which in one example is integrated with or associated with the apparatus. In one example, classification is carried out on the basis of the presence or absence of the above-described “blue fast phosphorescence” marker, also referred to herein as “short phosphorescence”, having a decay time of around 8.8 ms and a peak wavelength at 455 nm. Where analysis of an image of a diamond confirms the presence of blue phosphorescence in a time window starting at or after the end of an excitation pulse (around 80ps) and finishing at around 80ms after the excitation pulse, this is an indicator that the diamond sample being tested is a natural diamond and can be used to indicate both weak Type la and type Ila natural diamonds as well as the majority type laAB diamonds. In one example, a single white light image, and an 8-frame average “short phosphorescence” image are used to provide a good white light image with visually good white balance, and a timely “short phosphorescence” image suitable for classification. In this example, frame rates of around ~100 Hz white light image alone, or 4 Hz when collecting and classifying a white light and short phosphorescence image, are achieved. Traditional methods of classification have used segmentation to segment stones from the background in white light images, so that classification is only conducted in the segmented stone area, rather than everywhere in the field of view, including the support on which the stones rest. This reduces the complexity of the classification process. However, these segmentation methods can have high computational overheads. The present invention uses an alternative classification approach, which avoids the need to segment, but which relies on classifying each pixel of the image instead. This alternative approach is computationally simpler and faster, using (in one example) signal to noise on each pixel to determine whether it should be classified or not. Two classification algorithms were investigated by the inventors and the apparatus may be configured to use either algorithm. Alternatively or additionally, the apparatus may employ different algorithms, or combinations of algorithms. A. Look Up Table (LUT) Algorithm In one non-limiting example, the LUT algorithm proceeds as follows: 1. Image pre-processing - Remove single pixel noise in short phosphorescence image (smallest median blur to retain sharpness); 2. Remove most background. Mask off bright areas (stones or ring) on black background or ring holder using thresholding and native C++ OpenCV connected components algorithm; 3. Identify presence of gold (where stones are jewellery-mounted) to further mask off metal; 4. Lookup pixel classification from a pre-prepared RG-BG lookup table (fast); 5. Identify presence of natural diamond, synthetic diamond or common simulant; 6. Adjust pixel classification according to presence of non-diamond near overlap regions; 7. Colour white light image according to white light intensity (this refers to the masking that the white light images provide, this is to prevent classification of objects that appear very dark under diffuse lighting); 8. Remove extraneous colour “blobs”. In one example, the pre-prepared RG-BG lookup table is derived from plots of pixel colour values for “short or fast phosphorescence” of Red-Green ratio vs Blue-Green ratio of a set of -174000 pixels of natural diamonds, synthetic diamonds, and common simulant stones (CZ, Sapphire, Spinel, GGG, Yag). Software then classifies pixels according to the RG-BG regions. This refers to the spectral distribution of short or fast phosphorescence as captured by a colour camera. On a plot of R / G v B / G the pixels of a natural stone form a distribution, and any pixel under this distribution has a high probability of coming from a natural stone. In the above example, the simple lookup table of Red-Green ratio (RG) vs Blue-Green ratio (BG) is sufficient to allow pixel by pixel classification by “colouring” the white light image according to the RG-BG region the pixel occupies. Optionally, critical overlap regions bounding the natural zone are designated unknown and left uncoloured in the classification result. The algorithm can be further optimised by recognising the absence pixels falling outside an overlap region and so designating pixels falling within the overlap region as known, and hence classifying them. A series of hand drawn masks are used to delineate the regions and coded to different pixel colours according to the region as a diagnostic, and subsequently to colour as blue natural, red refer, and clear unknown. Smoothing image data in the image pre-processing is avoided to prevent overcolouring and being able to gauge the classification. An exclusion region which prevents pixel colouring is used to fine tune classifications. B. Support Vector Machine classification algorithm In one example, this classification algorithm provides a model allowing incorporation of more features such as intensity and texture which can be used to linearly separate the regions corresponding to individual stones. The Support Vector Machine (SVM) uses the same image data as the lookup table (LUT) classifier for training. In one example, a SVM was trained using five features (Blue / Green ratio, Red / Green ratio, Blue intensity, Green intensity, Red intensity) on normalised and reduced pixel data. Normalisation and data Reduction was included in the training process. The images were pre-processed (smoothing) before feature detection. A morphology based image processing method was used in post-processing to remove artefacts. The trained classifier comprised 5 features and ~200 support vectors. It will be appreciated that alternative or additional methods of image processing may be used to add colouring representing the classification result of stones within the field of view. Regardless of the specific algorithm or algorithms used, the inventive concept avoids ambiguous assignments in which a stone is coloured with - for example - both red and blue pixels. As previously discussed, and as illustrated in the example of Figure 7, classification results based upon luminescence are used to “colour” pixels of the visible image captured using white light, via an overlay or colour map, which are displayed on a colour screen (display), thereby enabling an operator or user to clearly see which part of the visible image the classification applies to. As discussed above, in one example, pixels of the white light image are modified, so as to be coloured blue or red if they map to pixels within the signal ranges set by the luminescence images. The intensity / level of saturation of each colour however is modified by the intensity levels of the white light images. Said another way, the colour map is modulated by the intensity of the diffuse lighting image. Note that the captured images of luminescence themselves are not displayed to the operator or user via the display screen. Instead, the luminescence images are processed to create a classification result, and it is this classification result which is displayed in combination with the captured visible images to the operator. In the example of Figure 7, image pixels (of the visible image) of the stones classified as natural are coloured blue, while image pixels of the stones identified as synthetic (e.g. CVD or HPHT) or non-diamond (e.g. CZ), or requiring referral and further testing, are coloured red. As previously noted, stones classified as non-emitters such as topaz and sapphire may remain uncoloured or may be coloured grey. The colouration provided by the classification overlay or colour map is artificial, since all stones in this example appear colourless under visible light. Therefore, different colours or other means (e.g. shading, texture, indicia etc.) of distinguishing natural from non-natural stones may be envisaged. The specific example of Figure 7 illustrates classification results provided by the abovedescribed Look Up Table algorithm (LUT), left, and Support Vector Machine algorithm (SVM), right. All four images are of the same jewellery piece, in this case a ring having three rows of gemstones. The diagram on the left serves to illustrate the classification results, and shows that one stone in the top row has been identified as a non-natural (e.g. synthetic) diamond gemstone; one stone in the centre row has been identified as a non-natural (e.g. synthetic) diamond gemstone and one stone in the centre row has been identified as non-diamond material (e.g. cubic zirconia); and one stone in the bottom row has been identified as a non-diamond material (e.g. cubic zirconia). A further stone in the bottom row (not shown in the diagram) appears grey in colour and may require referral or further testing. The images of the jewellery piece illustrated in Figure 7 could be still images taken from a real-time classification of the piece, as discussed above, or could be still images taken from a number of images obtained of the piece at predetermined positions and subsequently stitched together to form a short sequence (see Figure 8). The images could also be single or static images obtained of the piece at one position only. One example of obtaining multiple images of a stone or jewellery piece at a sequence of rotational and / or translational positions, optionally along a predetermined route, is illustrated in Figure 8. The rotation and / or translational in this example is provided by an automated mount, actuated by a motor. Here, four positions (Positions 1-4) are present, and at each of the four positions the process of UV excitation, capturing emitted luminescence, illumination and imaging under visible white light, identification of pixels in the visible images corresponding to “markers” in the emitted luminescence, and modification of these pixels to provide a colour map or overlay that is displayed with the visible image to a user or operator, is carried out. The modified single images from each of the four positions are then stitched together or otherwise sequentially grouped in order to produce a short sequence, video, animated gif or the like. This sequence may optionally be saved for later viewing, perhaps as part of a bundle of data which corresponds to the specific stone, stones or jewellery piece in question. In one example, the display is integrated into the screening apparatus. However, a separate display or multiple displays may be provided. It will be appreciated that the term display is not necessarily limited to any particular type of screen but could include virtual displays and the like. As a result of the image processing and classification methods applied, classification results can be provided in real-time, and in one example the apparatus operates at a frame rate of 4 Hz. Optionally, the apparatus includes a user interface providing display options, for example, to show or omit the classification results (the overlay or colour map). The user interface may also provide one or more of: set up options, viewing options, options to start and stop real-time (live) display, options to start and stop real-time classification; options to zoom, pan and so forth. Optionally, the user interface may show a segment outline as an overlay on screen and label it with the classification result. A method of screening gemstones is illustrated in Figure 9. The method comprises the steps of: • illuminating a gemstone with one or more pulses of ultraviolet, UV, light and capturing at least one luminescence image of the gemstone during and / or after each UV pulse; • illuminating the gemstone with one or more pulses of visible light and capturing at least one visible image of the gemstone during each visible light pulse; • analysing the at least one luminescence image to identify pixels associated with the gemstone, in which markers characterised by luminescence decay time and luminescence wavelength appear; • classifying the gemstone pixels as natural or non-natural based upon the identified markers; • modifying pixels of the at least one visible image based upon the classification, such that pixels classified as natural appear in a first colour, and pixels classified as nonnatural appear in a different colour; and • displaying the modified at least one visible image via a screen; • wherein the analysing, classifying, generating and modifying steps are carried out prior to displaying the modified image on the screen. Generating classification results in real time allows jewellery or other pieces to be repositioned whilst being classified. Whereas still images may extend beyond the field of view of the apparatus, therefore giving the impression that there are more coloured or uncoloured stones in areas of the jewellery piece than is actually the case, a real-time (live) overlay or “superimposed” colour map used to modify or modulate the pixels of the visible white light image allows more stones to be correctly identified as natural or synthetic by providing the ability to rotate and / or translate the stones across the field of view, while monitoring the pixel coloration. As used herein, classification refers to the determination of stones as natural, synthetic, simulant / non-diamond and so on. A gemstone can comprise a natural, which is a natural diamond, or a non-natural, which comprises synthetic (e.g. lab-grown) diamond or any other gemstone material being tested. A gemstone may be classified as natural (e.g. consistent with luminescence from natural diamond) or non-natural (e.g. inconsistent with luminescence from natural diamond). As used herein, real-time refers to continuous and near simultaneous generation of visible image frames and “superimposed” colour map (overlay) based on classification results from UV detection and imaging. Said another way, an operator or user may manually or automatically cause a stones, stones or jewellery piece to move in relation to the imaging device, and is able to view on the display the classification (e.g. the modification or modulation of the visible image pixels based on the classification) in correspondence with the movement. It is important to note that the captured luminescence images themselves are not displayed to the operator. Instead, captured luminescence images are processed to produce classification result data, said data being used to generate an overlaid I merged colour map which presents specific pixels of the visible image frames in artificially-added (false) colour or colours. It is this colour map which is displayed to the operator via the display screen together with the visible “white light” images.

Claims

1. A method of screening gemstones, the method comprising:illuminating a gemstone with one or more pulses of ultraviolet, UV, light and capturing at least one luminescence image of the gemstone during and / or after each UV pulse;illuminating the gemstone with one or more pulses of visible light and capturing at least one visible image of the gemstone during each visible light pulse;analysing the at least one luminescence image to identify pixels, associated with the gemstone, in which markers characterised by luminescence decay time and luminescence wavelength appear;classifying the gemstone pixels as natural or non-natural based upon the identified markers;modifying pixels of the at least one visible image based upon the classification, such that pixels classified as natural appear in a first colour, and pixels classified as nonnatural appear in a different colour; anddisplaying the modified at least one visible image via a screen;wherein the analysing, classifying, generating and modifying steps are carried out prior to displaying the modified image on the screen.

2. The method according to claim 1, wherein the gemstone is mounted in a jewellery piece.

3. The method according to claim 1 or 2, wherein the steps of:illuminating the gemstone with one or more pulses of ultraviolet light and capturing at least one image of the gemstone during and / or after each UV pulse;illuminating the gemstone with one or more pulses of visible light and capturing at least one image of the gemstone during each visible light pulse;analysing the at least one luminescence image to identify pixels, associated with the gemstone, in which markers characterised by luminescence decay time and luminescence wavelength appear;classifying the gemstone pixels as natural or non-natural based upon the identified markers; andmodifying pixels of the at least one visible image based upon the classification, such that pixels classified as natural appear in a first colour, and pixels classified as non-natural appear in a different colour;are repeated at different rotational and / or translational positions of the gemstone.

4. The method according to claim 3, further comprising generating a modified image at different rotational and / or translational positions of the gemstone.

5. The method according to claim 4, further comprising combining the modified images at different rotational and / or translational positions of the gemstone to generate a video.

6. The method according to claim 1, wherein the analysing, classifying, modifying and displaying steps are carried out prior to illuminating the gemstone with a second pulse of UV light and a second pulse of visible light.

7. The method according to any preceding claim, further comprising illuminating the gemstone with UV light from an angle offset from an image capture axis.

8. The method according to any preceding claim, wherein the visible light is diffuse light.

9. The method according to any preceding claim, wherein the gemstone is located within a substantially hemispherical dome and illuminated with visible light from within said dome.

10. The method according to claim 9, wherein the substantially hemispherical dome is provided with a substantially matte black interior having sufficient reflectivity to reflect the visible light onto the gemstones.

11. The method according to any preceding claim, wherein the UV pulses and the visible light pulses are controlled not to overlap in time.

12. The method according to any preceding claim, wherein one of the markers comprises short-lived phosphorescence having an emission peak at around 455 nm anda decay time of less than 80ms, said phosphorescence visible in a time window starting at or after the end of the first UV pulse.

13. The method according to any preceding claim, wherein the step of classifying the gemstone pixels as natural or non-natural based upon the identified markers is carried out using an algorithm, preferably a look-up table, LUT, algorithm or a support vector machine, SVM, algorithm.

14. The method according to any preceding claim, wherein classifying the gemstone pixels as non-natural includes one of: identifying the gemstone pixels as synthetic diamond; identifying the gemstone pixels as non-diamond; or identifying the gemstone pixels as requiring further testing.

15. A method of identifying a gemstone as natural or non-natural, the method comprising:illuminating the gemstone with ultraviolet, UV, light;capturing luminescence emitted by the gemstone;analysing the captured luminescence to determine whether the gemstone is natural or non-natural;illuminating the gemstone with visible light;capturing an image of the gemstone under visible illumination;modifying pixels of the image based upon the analysis such that pixels relating to a natural appear in a first colour and pixels relating to a non-natural appear in a second, different colour; anddisplaying the modified image.

16. An apparatus for screening gemstones, the apparatus comprising:a source of ultraviolet, UV, light;a source of diffuse white light;an image capture device configured to capture one or more images of the gemstone under UV illumination and under diffuse white light illumination;a processor configured to analyse the one or images of the gemstone under UV illumination to identify pixels, associated with the gemstone, in which markers characterised by luminescence decay time and luminescence wavelength appear;the processor further configured to:classify the gemstone pixels as natural or non-natural based upon the identified markers;modify pixels of the at least one image of the gemstone under diffuse white light illumination based upon the classification, such that pixels classified as natural appear in a first colour, and pixels classified as non-natural appear in a different colour; andoutput the modified image via a screen;wherein the processor is configured to carry out the steps of analysing, classifying, and modifying prior to outputting the modified result on the screen.

17. The apparatus according to claim 16, wherein the apparatus further comprises a diffuser assembly comprising the source of diffuse white light.

18. The apparatus according to claim 17, wherein the diffuser assembly comprises a substantially hemispherical dome and the source of diffuse white light is arranged within the dome.

19. The apparatus according to claim 18, wherein the dome is provided with a matt black interior.

20. The apparatus according to any of claims 16 to 19, further comprising a moveable drawer configured to support the gemstone during illumination.

21. The apparatus according to any of claims 16 to 20, further comprising a mount for supporting a jewellery piece including the gemstone.

22. The apparatus according to claim 21, wherein the mount is actuated by a motor to rotate and / or move along a predetermined path.

23. The apparatus according to any of claims 16 to 22, wherein the source of UV light is adjustable along an axis and / or at a pivot point.

24. The apparatus according to any of claims 16 to 23, wherein the processor is configured to control the UV source and the diffuse white light source such that there is no temporal overlap therebetween.

25. The apparatus according to any of claims 16 to 24, further comprising the screen.

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