Technical Field The present invention relates to an instrument for viewing gemstones. In particular, although not exclusively, the invention relates to an instrument for viewing the phosphorescence of a parcel of gemstones to assist in sorting such gemstones.

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, but currently form only a small percentage of the gemstone industry. Being man-made, they do not attract the high values associated with natural diamonds of similar colour and quality and it is clearly desirable from a consumer perspective to provide reliable means of identifying and separating synthetic and natural diamonds.

One characteristic of diamond and other gemstones that has proven to be of utility for characterising a stone is the emission of luminescence when a stone is illuminated (or excited) by a source of energy, most commonly but not exclusively, electromagnetic radiation. A gemmologist would normally have an ultraviolet lamp, perhaps emitting radiation with a wavelength of 365 nm or 254 nm, these being common lines in the emission of the low pressure mercury lamp, and might observe fluorescence, produced when the ultraviolet excitation is on. Phosphorescence, which may also be observed, may be viewed once the excitation is removed. Through interpretation of any such luminescence present, taking into account their observable temporal characteristics, colours, and spatial distribution, inferences on the task at hand may be drawn as is known in the art.

In particular, it is known that phosphorescence is a useful tool for identifying diamonds made by the HPHT process. It has been shown that all near-colourless HPHT synthetic diamonds exhibit phosphorescence, which cannot be removed by changes to the synthesis process. The presence of phosphorescence can then be a useful marker of HPHT synthetic stones. In addition to screening larger, individual stones, it is also necessary to screen large numbers of smaller diamonds, including stones sometimes known as melee. Melee is a term of the trade that does not have a well-defined size range, but can be considered in practice to refer to stones smaller than about 0.2 carats (20 points), and usually (but not necessarily) larger than about 0.01 or 0.02 carats. Due to their small size, melee stones are typically sold in parcels or lots. Since one parcel may contain hundreds of stones, it is possible for synthetic diamonds to be mixed in with natural stones. It would be desirable for an operator to be able to view a large number of stones simultaneously in such a way that he or she can identify stones exhibiting phosphorescence and sort phosphorescing stones from non-phosphorescing stones.

Summary In accordance with one aspect of the present invention there is provided an instrument for viewing gemstones. The instrument comprises a chamber and a removable tray assembly for supporting the gemstones to be viewed, the tray assembly configured to be inserted into the chamber with the gemstones supported thereon or therein. A UV source is provided for illuminating gemstones in or on the tray assembly with UV light. An imaging device is directed towards the tray assembly. A control unit is operatively connected to the UV source and the imaging device and is configured to cause the UV source to emit pulses of UV light, and to cause the imaging device to obtain phosphorescence images of the gemstones between the pulses of UV light. A display is configured to display images obtained by the imaging device in real time (e.g. as a video stream) so that a user can see gemstones exhibiting phosphorescence in the images.

This enables a user to see images of a parcel of gemstones on a viewer, and enables ready identification of those gemstones which are phosphorescent.

The instrument may include a visible light source for illuminating gemstones in or on the tray assembly with visible light. In order to assist the user in tracking where all of the stones are, the control unit may be operable to cause the visible light source to emit visible light, cause the imaging device to obtain a visibly illuminated image of the gemstones while illuminated by the visible light, save the visibly illuminated image, cause the visible light source to cease emission of visible light, cause the UV light source to begin emitting pulses of UV light and the imaging device to begin to obtain phosphorescence images, and cause the display to overlay the visibly illuminated image with the real time phosphorescence images.

The instrument may be further designed to provide a visible display to the user while stones are inserted. This may be achieved by making the control unit operable to cause the imaging device to obtain a plurality of visibly illuminated images of the gemstones while illuminated by the visible light, to cause the display to display the visibly illuminated images in real time and, when the visible light source ceases emission of visible light, to save a last of the plurality of visibly illuminated images to be overlaid with subsequent real time phosphorescence images.

In order to provide a further check of the gemstones following sorting, the control unit may be operable to cause the UV light source to cease emitting pulses of UV light, save a last of the phosphorescence images of the gemstones, cause the visible light source to emit visible light, cause the imaging device to obtain visibly illuminated images of the gemstones while illuminated by the visible light, and cause the display to display the visibly illuminated images of the gemstones in real time overlaid with the last of the phosphorescence images of the gemstones.

In accordance with another aspect of the present invention there is provided an instrument for viewing gemstones. The instrument comprises a chamber for accommodating the gemstones to be viewed and a removable tray assembly for supporting the gemstones to be viewed, the tray assembly configured to be inserted into the chamber with the gemstones supported thereon or therein. The instrument further comprises a UV source for illuminating gemstones in or on the tray assembly with UV light, a visible light source for illuminating in or on the tray assembly with visible light, an imaging device directed towards the tray assembly, a display configured to display images obtained by the imaging device in real time, and a control unit operatively connected to the UV source, the visible light source, the imaging device and the display. The control device is configured to cause the UV source to emit a pulse of UV light, cause the imaging device to obtain a phosphorescence image of the gemstones a predefined period after the pulse of UV light, save the phosphorescence image of the gemstones, cause the visible light source to emit visible light, cause the imaging device to obtain visibly illuminated images of the gemstones while illuminated by the visible light; and cause the display to display the visibly illuminated images in real time overlaid with the phosphorescence image. The chamber may be encompassed by a light-proof housing having an aperture therethrough providing access for a user to manipulate the stones in response to images viewed on the display. For example, a user may insert a manipulation tool into the chamber to move the phosphorescent gemstones to a separate region of the tray assembly. The manipulation tool may be coated with phosphorescent material so that it is visible to the user in the images on the display.

The aperture may include a resilient membrane having an access slit therein to allow insertion of the manipulation tool and inhibit egress of light from the chamber. The resilient membrane may be supported by a slideable frame in the aperture to allow movement of the access slit relative to the tray assembly in the chamber. A brush may be provided in the chamber behind the aperture to allow insertion of the manipulation tool and further inhibit egress of light from the chamber.

Alternatively, the aperture may comprise a ball joint and a manipulation tool passing through the ball joint into the chamber, such that an outside end of the manipulation device is controllable by the user externally of the instrument so as to cause an inside end of the manipulation device to contact gemstones in or on the tray assembly and move gemstones between regions of the tray assembly. The ball joint may be provided in a plate inserted into the aperture, the plate moveable horizontally and/or vertically relative to the aperture.

The tray assembly may include a sample region and a reject region so that a user can move gemstones exhibiting phosphorescence from the sample region to the reject region. The reject region and optionally the sample region may be outlined by phosphorescent material so that the outline is visible to a user viewing images on the display. The sample region and reject region may be provided by different trays.

The tray assembly may comprise a jewellery mount for enabling a ring having one or more gemstone mounted thereon to be inserted into the chamber. The jewellery mount may comprise a conical spindle for insertion into the ring. The spindle may be rotatable about an axis thereof. This axis may be inclined to the horizontal such that a surface of the spindle is in a focal plane of the imaging device when the tray assembly is in the chamber. In order to allow the use of simple, low-cost components, the control unit may cause the imaging device to operate a rolling shutter. In order to overcome difficulties with synchronising a rolling shutter imaging device and a pulsed UV source, the control unit may be configured to initiate an image request to the imaging device and then, after a predetermined time delay, to trigger a pulse from the UV source. After a further delay (determined by the communication between the imaging device and control unit) the imaging device may acquire an image and send it to the display. The control unit may be configured to cause the display to crop the images displayed to suppress strobe artefacts. In one embodiment of the invention there is provided a kit of parts for sorting gemstones exhibiting phosphorescence out of a parcel of gemstones, comprising the instrument of any preceding claim and a manipulation tool for grasping or moving a gemstone, the manipulation tool coated in phosphorescent material so that it is visible to a user viewing the display.

In accordance with another aspect of the present invention there is provided a method of viewing gemstones. The method comprises placing one or more gemstones into or onto a tray assembly, inserting the tray assembly into a chamber, illuminating the tray assembly by a series of pulses of UV light, operating an imaging device in between the pulses of UV light to obtain images of phosphorescent gemstones in or on the tray assembly, and displaying the images to a user on a display in real time (e.g. as a video stream).

Before the tray assembly is illuminated by the series of pulses of UV light, the tray assembly may be illuminated with visible light. The imaging device may be operated to obtain a visibly illuminated image of the gemstones while illuminated by the visible light. The visibly illuminated image may be saved, and emission of visible light ceased. The UV pulses may then be started and the visibly illuminated image overlaid with the video stream of phosphorescence images displayed to the user. While the tray assembly is illuminated with visible light, the imaging device may be operated to obtain a plurality of visibly illuminated images, which may be displayed in real time on the display as a video stream, so as to assist the user in monitoring insertion of the tray assembly. When the visible light emission ceases, a last of the plurality of visibly illuminated images may be saved and then overlaid with subsequent real time phosphorescence images on the display.

The emission of pulses of UV light may be ceased. When this happens, a last of the phosphorescence images of the gemstones may be saved. The gemstones may subsequently be illuminated with visible light, and the imaging device operated to obtain visibly illuminated images of the gemstones while illuminated by the visible light. The visibly illuminated images of the gemstones may be displayed on the display in real time overlaid with the last of the phosphorescence images of the gemstones. In order to sort the gemstones, a user may use a manipulation tool inserted into the chamber to move the phosphorescent gemstones to a separate region of the tray assembly while viewing the images on the display. The manipulation tool may be coated in phosphorescent material so that it is visible in the images on the display. Brief Description of the Drawings

Some preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a schematic cross section view of an instrument for viewing and sorting gemstones;

Figure 2 is a perspective view of a removable tray assembly for supporting gemstones in the instrument of Figure 1 ;

Figure 3 is a cross section through the instrument of Figure 1 ;

Figure 4 is a perspective view of a sliding aperture mechanism for enabling access to the interior of the instrument of Figure 1 ; Figure 5 is a view of an exemplary shown by a display of the instrument of Figure 1 in use.

Figure 6 is a perspective view of the assembly of Figure 2 incorporating a jewellery mount;

Figure 7 is a schematic cross section view of the instrument of Figure 1 with the jewellery mount in place; Figure 8 is a schematic cross section view of an alternative instrument for viewing and sorting gemstones;

Figure 9 is a cross section through the instrument of Figure 8; Figure 10 is a perspective view of a sliding ball joint mechanism for enabling access to the interior of the instrument of Figure 8; and

Figure 1 1 is a schematic diagram illustrating a suitable timing scheme for synchronising camera and strobe operation for a rolling shutter camera.

Detailed Description

Figure 1 is a schematic cross section view of an instrument 100 for viewing and sorting gemstones such as diamond. The instrument comprises a casing 101 , enclosing a chamber 102. A removable tray assembly 103, shown in perspective view in Figure 2, is located at the bottom of the chamber. The tray assembly includes one or more trays 104, 105 for supporting gemstones 106, 107. In this example, the tray assembly comprises a sample tray 104 and a reject tray 105, such that a user can place stones in either tray. In other arrangements, a single tray may be used rather than two separate trays. Stones may be moved to different regions of the same tray.

In the exemplary tray assembly 103 shown in Figure 2A, the sample tray 104 and reject tray 105 are designed as interlocking units which can be removed from the sample assembly for replacement by other arrangements of trays or jewellery mounts if desired. The tray assembly 103 also includes a handle 201 to assist with inserting and removing the tray assembly 103 from beneath the chamber 102 of the instrument 100.

Figure 2B illustrates an alternative tray 204 having separate sample and reject regions 204a, 205. This tray can be inserted into the tray assembly 103 in place of the pair of interlocking trays 104, 105. The reject region 205 may be separated from the sample region 204a by a small ridge 206, and/or may have a base lower than the sample region to facilitate stones being pushed into the reject region from the sample region. The instrument 100 also comprises a UV source 108 for illuminating gemstones 106, 107 in the trays 104, 105 with UV light. UV light is generally considered to be light with a wavelength between about 10 nm and about 380 nm. In practice, for the present purposes a source emitting light with a wavelength between about 180 nm and about 380 nm is likely to be suitable, and optionally between 180 nm and 250 nm. The source is operatively connected to a control unit 109 which causes the source to operate as a pulsed or strobe source.

An imaging device 1 10 such as a camera extends into the chamber 102, such that the trays 104, 105 are in a field of view and focal plane thereof. The camera 1 10 is connected to a display unit 1 1 1 for displaying images obtained by the camera in real time for viewing by a user of the instrument. The camera is also operatively connected to the control unit 109, which causes the camera to obtain discrete images between the pulses of light from the UV source 106. Images of the gemstones 106, 107 in the trays 104, 105 are thus obtained when the stones are not illuminated by UV light. Stones which exhibit phosphorescence in the visible spectrum are therefore visible in the images obtained by the camera 1 10 and displayed by the display unit 1 1 1 , whereas stones which do not phosphoresce are invisible, or appear very dark, in these images.

Figure 1 also illustrates an access aperture 1 12 through which a user can insert a manipulation tool such as a probe or a pair of tweezers to manipulate stones 106, 107 within the chamber 102 on the tray 204 or trays 104, 105. The access aperture does not form part of the cross section of the rest of Figure 1 , and it will be understood that it is in practice located in front of the section represented by the rest of the figure. The aperture 1 12 is shown in more detail in Figure 3, and includes a sliding frame 301 , which slides longitudinally within the aperture. The sliding frame 301 encloses a membrane 302 having an access slit 303 therein through which tweezers or probes can be passed. The membrane is held in place by a support structure 304. The aperture 1 12 is designed to avoid light passing into or out of the chamber, but enables access to both trays 104, 105 (or the whole of the single tray 204) by enabling the access slit 303 within the sliding frame 301 to move along the wall of the chamber 102.

Figure 4 is a cross section through the instrument 100 along the line IV-IV shown in Figure 1 . It will be appreciated that Figure 1 is a section along the line l-l shown in Figure 4. Figure 4 also shows cross sections through the apertures 112 (optionally one on each side of the casing). It can be seen that the membranes 302 are supported in the frame 301 by the support structure 304. The frame 301 includes tongues 401 which cooperate with a grooves 402 mounted on the outside of the housing 101 , enabling the frame to be slid longitudinally relative to the housing 101. Behind each aperture 1 12 is a soft brush 403 for inhibiting UV light from reaching the membrane 302 and access slit 303. The combination of the membrane and slit and the brushes inside prevents light from escaping, even when tweezers 404 are inserted into the instrument for manipulating the gemstones, as shown in Figure 4. In use, gemstones for analysis or sorting are placed into the sample tray 104 of the removable tray assembly 103, which is then slid into position under the chamber 102 of the instrument. Phosphorescent material may be painted around the outside of the sample tray 104 and reject tray 105 or around the edge of the single tray 204, or around the edge of the sample region 204a and reject region 205 so that the extents of the trays or regions can be identified by the user when no visible light is applied. The instrument is then activated in a "phosphorescence imaging" mode so that the UV source 108 operates as a strobe and emits a series of pulses of UV light. The camera 1 10 is triggered to obtain an image between each pulse, and the images are displayed in real time on the display 1 1 1 as a video stream. The user can thus see a real time, dynamic view of those gemstones which exhibit phosphorescence.

An example image 501 that can be seen by a user on the display is shown in Figure 5. Stones which phosphoresce are visible to the user in the image, whereas those that do not are invisible. The edges of the trays 104, 105 are also visible because they have been painted with phosphorescent material. It is desirable that the manipulation tool (e.g. tweezers or probe) 404 are also painted with phosphorescent material so that they are also visible to the user. It will be understood that it is not critical for the outline of the sample tray 104 or sample region 204a to be painted with phosphorescent paint: it is more important that the outline of the reject tray 105 or reject region 105a is phosphorescent so that the user can see where stones to be transferred must be placed.

The user inserts the manipulation tool 404 into the instrument through the access slit 303 and brush 403 into the chamber 102. Watching the stream of images on the display, he uses the manipulation tool to pick up or manipulate the stones 106 he can see (i.e. those which are phosphorescing) and move them from the sample tray 104 into the reject tray 105 (or from the sample region 204a to the reject region 205 of a single tray). Stones which do not phosphoresce therefore remain in the sample tray 104 or sample region 104a.

The instrument is then deactivated, and the tray assembly 103 removed. The stones remaining in the sample tray 104 or sample region 204a are then known not to exhibit phosphorescence and are therefore not HPHT synthetic. Further tests may be required on these stones to determine if they have been otherwise treated, but it will be appreciated that the instrument enables a large number of stones to be sorted in a very short space of time. In practice it is possible to empty a parcel of five hundred stones into the sample tray or sample region and extract all of the stones which are phosphorescing and move them into the reject tray or reject region. In an alternative mode of operation, a pulse of UV light may be provided by the UV source 108, and an image in the visible spectrum obtained after a short period of time. In this image only those stones exhibiting phosphorescence will be visible. The chamber 102 is then illuminated by light in the visible spectrum, e.g. by the use of an LED (not shown in the drawings). The visible spectrum is usually considered to include light with wavelengths between about 390 nm and about 700 nm. The display 1 1 1 is configured to display a live video stream of the stones illuminated by visible light, overlaid with the image obtained immediately after the UV pulse which shows the phosphorescent stones. The phosphorescent (and therefore potentially HPHT synthetic) stones are thus highlighted and can be sorted as required under visible light. As sorting continues, the overlay image can be refreshed as necessary by disabling the visible light source, activating the UV light source, and obtaining a new phosphorescence image. This is likely to be required after a number of stones have been moved. This mode of operation requires simpler camera triggering and has the advantage the sorting takes place under visible illumination, but the user cannot follow the progress of phosphorescing stones in real time.

In a further mode of operation, the stones are loaded into the instrument (using the tray assembly 103) with the chamber illuminated by a visible LED source. The display screen 1 1 1 presents a live view of this to the user to assist in the loading operation. Once the tray assembly is loaded, the user may select phosphorescence imaging as described above so that he can see on the display 1 11 the stones exhibiting phosphorescence. When the phosphorescence imaging mode is activated, the last frame which was seen with the visible source is saved. After that, when in phosphorescence imaging mode, the user can request that this saved visible image is overlaid on the live view of the phosphorescing stones. This is very useful in showing where non-phosphorescent stones are located prior to moving any stones under phosphorescent imaging. Once some of the stones are moved by the manipulation tool this image becomes obsolete and can be refreshed by exiting and re-entering the phosphorescence imaging mode so as to obtain a fresh visible image.

In a yet further mode of operation, when exiting the phosphorescence imaging mode the last frame which was seen under UV excitation is saved and the chamber then illuminated by the visible LED source. The user would now wish to check that any stones which are in the reject tray 105 or reject region 205 are indeed all phosphorescent. By overlaying the last phosphorescent image on top of this live visible image it is very easy to see if any of the stones in the reject region or reject tray are not phosphorescent, or indeed whether a phosphorescent stone is still in the sample region 204a or tray 104. It will be appreciated that any of the above modes can be used independently or in combination with each other.

Figure 6A is a perspective view of the tray assembly 103 with the tray or trays 104, 105, 204 removed from the assembly. An optional jewellery mount 601 is located in the assembly below an opening 602 for receiving the trays 104, 105 (or single tray 204) when present. The jewellery mount may be a cylinder for insertion into a ring, but it is preferred that the surface is slightly tapered to form a section of a cone so as that it can be inserted into rings of a variety of sizes. The mount 601 is rotatable about its axis so that a ring mounted thereon can be rotated to bring a stone (or stones) into the field of view of the imaging device 1 10 of the instrument in use. The mount may therefore be mounted on a rotatable shaft 701 (not shown in Figure 6) connected to the tray handle 201 , which is itself rotatable. This enables the mount to be rotated while the instrument is in use.

Figure 7 is a schematic illustration of the instrument 100 in use with the tray(s) 104, 105, 204 removed and a ring 702 mounted on the jewellery mount. The ring 702 has a gemstone 703 mounted thereon. It can be seen that the rotation axis of the jewellery mount 601 is inclined to the horizontal such that the top of the jewellery mount is horizontal and a stone 703 mounted on a ring 702 on the mount will always be in the same plane relative to the imaging device 1 10, whatever the size of the ring.

The jewellery mount enables an assessment of stones to be made without the need to remove them from the jewellery by identifying whether such stones fluoresce. This can be beneficial particularly where many stones are mounted on a single ring: it is possible to rotate the ring and view an image in real time to identify which (if any) of the stones are likely to be HPHT synthetics.

In one embodiment an interlock may be provided to prevent the UV source 108 from being activated unless the tray assembly 103 is in place beneath the chamber 102. The tray assembly 103 may itself incorporate a mechanical interlock 603 such that, if two trays are to be used, the sample tray 104 cannot be slid onto the assembly unless the reject tray 105 is already in place. An alternative arrangement is shown in Figure 6B, in which an alternative tray assembly 103a is specifically designed for use only with a single tray 204, and the mechanical interlock 603 is not present.

Figure 8 illustrates an alternative exemplary instrument 800. The instrument is similar to the instrument 100 shown in Figures 1 , 4 and 7, and the same reference numerals have been used to indicate the same features. The instrument differs from that shown in Figure 1 in that a different frame 801 is inserted into the access aperture 1 12. Figure 9 illustrates a cross section of the instrument along the line IX-IX shown in Figure 8, and Figure 10 shows the frame 801 in the aperture 1 12 in more detail.

As shown in Figures 8 and 10, in this embodiment the aperture 1 12 includes a sliding frame 801 which is slideable longitudinally within the aperture: tongues 802 cooperate with grooves 402 mounted to the housing 101 . Instead of the membrane 302 and access slit 303, a plate 804 is mounted within slots 805 in the sliding frame 801 , enabling vertical movement of the plate 801 relative to the sliding frame, and thus to the housing 101. The plate has a ball joint 806 mounted therein, and a shaft 807 of a manipulation tool 808 passes through the ball joint 806 and into the chamber 102. Mounted to the end of the shaft 807, inside the chamber, is a probe 809. The ball joint 806 allows the shaft 807 to be moved horizontally and vertically relative to the housing 101 , and also allows the angle of the shaft 807 to be changed relative to the housing. The shaft can also be rotated about its axis.

The probe 809 extending from the end of the shaft may extend on in the same direction as the shaft or may (as shown in Figure 9) include one or more bends along its length. The ball joint provides enough degrees of freedom to the shaft to enable the manipulation tool to be used to manipulate stones in the tray 204 from the sample region 204a into the reject region 205. The manipulation tool can be slid in and out through the ball socket and thus (from the point of view of the user) each manipulation tool has a full range of movement: back/forth, up/down, in/out, pivotable on the ball socket joint and rotatable on its own axis. It will be appreciated that the probes are not removable from the chamber using this ball joint mechanism, and this is very effective at preventing egress of UV light.

It will also be appreciated that, although two manipulation tools are shown in Figure 9, an instrument with a single manipulation tool will serve the same function. Two probes are advantageous because they increase ease of use for left and right handed operators.

The probes may have tips designed for moving gemstones. It is desirable for an instrument of this type to be available to jewellers and users such that an assessment of stones can be made without the need to take the stones to a laboratory. It is therefore desirable for the instrument to be manufactured at a low cost without the need for sophisticated imaging devices and unnecessary processing power. This a particular challenge given the need to synchronise image capture with a stroboscopic UV source.

One way of reducing the cost of the components is to use a simple imaging/computing/display platform. A popular and widely available low cost computing platform is the Raspberry Pi ®. This is offered with a number of similarly low cost accessories such as touch-screen displays and camera modules. In particular the camera modules feature a direct connection via a special connector to the Raspberry Pi PCB which allows good data transfer rates. In combination, this would appear to allow the requirements for a low cost imaging/computing/display platform to be easily satisfied.

In stroboscopic applications which require phosphorescence to be excited a typical solution is to trigger a flash and at some time afterwards grab an image from a camera which is viewing the scene. Normally in such applications the camera would be specified to have a 'global' shutter in which the entire scene is captured instantaneously on the chip. The data describing this scene then read out afterwards. In contrast, lower cost cameras often implement a 'rolling' shutter in which the entire scene is not captured instantaneously but for which the image data is simply read out line by line. The artefacts of this may sometimes be observed as a 'tearing' effect when a fast moving scene is being viewed. Consequently a rolling shutter tends not to be suitable for stroboscopic applications owing to the need for a precise relationship between the image illumination and acquisition.

The Raspberry Pi ® camera chip features both a rolling and global shutter but the proprietary firmware incorporated into the Raspberry Pi implements only the rolling shutter mode.

A further complication is caused by the proprietary nature of the software which implements the camera control functionality in the Raspberry Pi. (In general the Raspberry Pi operating system software is open source; this is an exception). The effective presence of an 'abstraction layer' means that, when user software requests that an image be acquired, the actual acquisition will take place at some point in the future rather than at the instant the request was made. In practice, once a request to grab an image is made there will be an exchange with the camera in which various settings are communicated. The user-programmer has very little control over this. Also, since camera data is sent and received via serial links a further delay is therefore present. These aspects tend to mean that, in a system whereby a strobe is fired and an image immediately requested, any phosphorescence will have decayed by the time that the image is taken.

Experiments have showed, however, that the actual camera timings are relatively predictable when fixed exposure times are used. This allows a different timing scheme to be employed, as shown in Figure 1 1 :

1 . An image is requested from the camera 1 101 and at the same time a time delay 1102 of fixed duration is started.

2. When the time delay 1 102 expires the strobe light is triggered 1 103.

3. After a further period of time an image is acquired 1 104 by the camera.

4. The image is received from the camera and displayed.

The sequence returns to step 1 .

By careful adjustment of the time delay and with the use of predictable, fixed exposure it is possible to identify a value at which strobe artefacts are not present in the image (and are therefore roughly synchronised to the period between frames when control and status information is being exchanged between the camera and imaging sensor). Such timing may be implemented by means of software or hardware either on the Raspberry Pi platform or external to it. Since the Raspberry Pi runs a Linux operating system precise timing cannot be guaranteed using software timing techniques since Linux is a non-deterministic operating system but in practice a useful compromise can be achieved.

A further tolerance may be added by cropping the height of the video window about its centre and then displaying only the cropped image. This has the useful effect of increasing the time window available during which the strobe may be triggered without being imaged. The cropping provides a greater margin for error on the strobe timings which could otherwise occasionally cause white bands from the strobe illumination to creep into either the top or the bottom of the image caused by timing jitters related to the operating system. Cropping the image for height about its centre removes a band of pixels at the top and bottom of the image where strobe artefacts might appear. In one suitable arrangement, the exposure time of the camera may be 50 ms and the delay 802 from image request to strobe trigger 69 ms, providing a frame rate of 13 frames per second. This approach can be operated using a Raspberry Pi ® system where the camera is an Omnivision 5647 device, mounted in an Arducam PiCam_RevC module with an M12 lens. It will be appreciated that many other configurations of cameras are available which use the OV5647 chip.