Patent Publication Number: US-8116552-B2

Title: Apparatus for generating data for determining a property of a gemstone and methods and computer programs for determining a property of a gemstone

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 10/921,441 of the same title, filed on Aug. 19, 2004, now U.S. Pat. No. 7,239,739, which is a divisional of U.S. patent application Ser. No. 10/107,585 of the same title, filed on Mar. 25, 2002, now U.S. Pat. No. 6,813,007. The disclosures of the above-described filed applications are incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The present invention relates to electronic apparatus for generating data for determining properties of gemstones, such as cut diamonds, as well as methods of and computer programs for determining properties of gemstones using the generated data. In particular, but not exclusively, it relates to electronic apparatus for generating data for determining properties as well as methods of and computer programs for determining properties of gemstones in which a plurality of images of the gemstone are captured for analysis under different lighting conditions. 
     BACKGROUND OF THE INVENTION 
     The beauty of a gemstone, such as a cut diamond, derives from its light handling ability. What attracts the eye is the “game of light” played by a well-cut diamond as incident light is reflected and refracted off its many facets. Diamonds and other gemstones may be cut according to many different standardised cut patterns such as the standard round brilliant cut, oval, pear, marquise, radiant, princess, heart, emerald cut etc. The most popular cut is the standard round brilliant (SRB) cut as shown in  FIG. 1 . Diamond cutting and polishing is a highly skilled art and a well-cut diamond, having superior optical performance, will command a significant price premium over a poorly cut diamond having inferior optical performance. 
     When showing a cut diamond to an untrained observer, one frequently refers to the four C&#39;s of a gemstone, being its carat weight, its clarity, its cut and its colour. Carat weight, clarity and colour can be relatively easily measured objectively and are therefore generally useful. Cut may also be specified or measured in terms of the geometry of the various facets. However, what really matters is the light handling ability of the gemstone and reference is often made to more subjective parameters of a cut diamond, such as its brilliance (the intensity of light returned), scintillation (fast and local fluctuations in the light returned as the diamond moves relative to the lighting conditions), fire (the dispersion of white light into spectral colours) and symmetry (the symmetry of light patterns such as the so-called “hearts and arrows”). It can be difficult, particularly for an untrained observer, to make a personal appraisal of these subjective parameters. It can also be difficult to compare the light handling abilities of two diamonds with the same carat weight, clarity and colour, and therefore to appreciate why one is more valuable than the other. 
     It is recognised that properties such as brilliance, fire and symmetry should be derived, whether by experimental observation or theoretical computer modelling, under a variety of lighting conditions. Moreover, with the property of scintillation it is clearly essential to observe or model the diamond under a variety of lighting conditions. Electronic apparatus for capturing images of gemstones under varying lighting conditions for analysis are known. 
     DESCRIPTION OF THE RELATED TECHNOLOGY 
     International Patent Publication number WO 96/23207 describes a device which captures colour images of a gemstone placed in an analysis chamber and illuminated by a uniform annular light which may be moved along an axis such that the gemstone may be illuminated from a plurality of different angles. The device performs a spectral analysis of the captured images using a tuneable optical band pass filter to determine the colour of the gemstone. Digital images of the gemstone may also be stored, displayed or transmitted over a data network. 
     The website (www.gemex.com) of GemEx Systems, Inc, a US company, describes a device called the BrillianceScope Analyser which is described as an imaging spectrophotometer. Colour images of a diamond are captured in a controlled lighting environment consisting of six lighting angles, five of which provide reflected light and one of which provides diffuse lighting. These images may then be analysed to generate a report on the diamond. The BrillianceScope Analyser device operates on the same principle as the device described in International Patent Publication number WO 96/23207 referred to above, in that the gemstone is placed in an analysis chamber and illuminated by a uniform annular light which may be moved along an axis such that the gemstone is illuminated from different angles. The images may be analysed by a computer, and the properties of “white light”, “coloured light” and “scintillation” for a diamond are determined and displayed on three line chart scales from ‘low’, to ‘medium’ to ‘high’. Captured images may also be shown in a repeating sequence in one display area, giving the effect of light movement. 
     International Patent Publication number WO 99/61890 describes a system for the standardised grading of gemstones. A gemstone is subject to a plurality of incident light sources and images are captured for analysis. Images of the gemstone, such as a SRB cut diamond, may be captured from various viewpoints such as from the pavilion, from the crown and side-on. The gemstone is supported by a rotatable platform which is rotated when images are being captured from a side-on viewpoint to obtain profile and colour images from a variety of rotational positions and to detect internal flaws and inclusions. When capturing images from above and below the gemstone, the platform is moved along an axis from a level position to a down and an up position respectively. The fixed focal length camera is also moved along an axis to focus on the gemstone when the platform is moved between the up, down and level positions. A captured image may be analysed by a processor to obtain colour measurements and measures of the brilliance and scintillation of the gemstone. 
     Some aspects provide an electronic apparatus, method and computer program for generating data for improved determination of properties of a gemstone, such as brilliance, scintillation, fire and symmetry. 
     Some aspects provide an electronic apparatus for generating data for determining properties of gemstones which is more compact, lightweight, mechanically simpler, and therefore less expensive to manufacture, than conventional apparatus, making it more suitable for use in retail premises, such as jewellers shops. 
     Some aspects provide an electronic apparatus, method and computer program enabling a observer to more easily compare two diamonds and their properties, in particular the properties of brilliance, scintillation, fire and symmetry. 
     Some aspects provide an electronic apparatus, method and computer program for generating data for determining properties of a cut gemstone which is adapted to and takes into account attributes of the shape and/or symmetry of the cut pattern of a particular gemstone. 
     Some aspects provide an electronic apparatus, method and computer program for generating data for determining properties of a gemstone in which the gemstone is illuminated under more realistic lighting conditions. 
     Some aspects provide an electronic apparatus, method and computer program for measuring the symmetry of a gemstone in terms of the light returned. 
     SUMMARY OF CERTAIN INVENTIVE ASPECTS 
     One aspect is a method of determining a property of gemstone using image data corresponding to two or more electronic images of the gemstone captured under different lighting conditions, the method includes selecting a first image portion from a first electronic image of the gemstone which was captured under a first lighting condition, the first image portion including one or more pixels corresponding to a region of the gemstone. The method also includes selecting a second image portion from a second electronic image of the gemstone which was captured under a second lighting condition different from the first lighting condition, the second image portion including one or more pixels corresponding to the same region of the gemstone as the first image portion, and comparing the first image portion with the second image portion. 
     Another aspect is a computer readable medium including a computer readable instructions, which, when executed, cause a computing device to perform a method of determining a property of gemstone using image data corresponding to two or more electronic images of the gemstone captured under different lighting conditions. The method includes selecting a first image portion from a first electronic image of the gemstone which was captured under a first lighting condition, the first image portion including one or more pixels corresponding to a region of the gemstone. The method also includes selecting a second image portion from a second electronic image of the gemstone which was captured under a second lighting condition different from the first lighting condition, the second image portion including one or more pixels corresponding to the same region of the gemstone as the first image portion, and comparing the first image portion with the second image portion. 
     Another aspect is an apparatus for determining a property of gemstone using image data corresponding to two or more electronic images of the gemstone captured under different lighting conditions. The apparatus includes a support structure configured to support a gemstone at an observation position, an illuminator configured to illuminate the gemstone with different lighting conditions, a camera configured to capture an electronic image of light from the gemstone and to output the electronic image as image data, and a data processing device. The data processing device is configured to select a first image portion from a first electronic image of the gemstone which was captured under a first lighting condition, the first image portion including one or more pixels corresponding to a region of the gemstone, select a second image portion from a second electronic image of the gemstone which was captured under a second lighting condition different from the first lighting condition, the second image portion including one or more pixels corresponding to the same region of the gemstone as the first image portion, and compare the first image portion with the second image portion. 
     According to a first aspect of the present invention there is provided an apparatus for generating data for use in determining a property of a gemstone, such as a cut diamond. The apparatus comprises a support structure for supporting a gemstone placed at an observation position, the support structure being arranged such that, if the gemstone has an axis of symmetry, the gemstone is supportable such that the axis of symmetry is parallel to an axis X passing through the observation position. The apparatus also comprises an illuminator arranged to illuminate a gemstone so placed with a spatially varied light pattern, a rotator arranged to cause relative rotation between the light pattern and the support structure generally about the axis X; and a camera arranged to capture, at each of a plurality of rotational positions, an image of light returned by the gemstone and to output said images as image data. 
     By rotating the spatially varied light pattern relative to the supported gemstone about the axis of symmetry of the gemstone, and capturing images at a plurality of rotational positions, the apparatus is particularly adapted to determining properties of cut gemstones having radially symmetric cut patterns, such as SRB cut diamonds. The intensity levels, and colour component proportions, of light returned at any particular spatial region of the gemstone are sensitive to rotational movement. Thus, brilliance, symmetry, fire and scintillation are more easily, accurately and objectively measurable. Furthermore, unlike the prior art referred to above, which are generally mechanically complex, and have laterally moving parts for varying the lighting conditions, the apparatus of the present invention advantageously employs rotationally moving parts for varying the lighting conditions, thus enabling the apparatus to be manufactured in a relatively compact and lightweight form, and relatively inexpensively. 
     In some embodiments, the light pattern comprises at least one relatively light region and at least one relatively dark region, the regions being arranged radially about the axis X. Preferably, the light pattern is generally symmetrical about the axis X. Also preferably, the light pattern has an n-fold symmetry, such as a 4-fold or 8-fold symmetry, where n is a whole number multiple or factor of s, a standardised gemstone cut having an s-fold symmetry. 
     Thus, the light pattern illuminating the gemstone, corresponds closely to particular gemstone cut patterns, such as the SRB cut, which have a particular shape and a particular arrangement of radially symmetric facets, resulting in the images captured having greater contrast levels between different spatial regions of the gemstone and the intensity levels, and colour component proportions, of light returned at any particular spatial region of the gemstone having greater sensitivity to rotational movement. Thus, brilliance, symmetry, fire and scintillation are more easily, accurately and objectively measurable. Furthermore, the light pattern illuminating the gemstone is a more realistic simulation of the light that would illuminate a gemstone in normal, everyday use—for instance when mounted in an item of jewellery such as a ring. Light is not normally uniform around the axis of symmetry of the diamond and incident only at a narrow range of angles off the axis, as in prior art approaches. Instead, light is normally incident at a wide range of angles off the axis of symmetry of the diamond and not uniform around the axis as in the present invention. 
     In other embodiments, the illuminator comprises a reflector having a concave surface arranged to reflect light generally towards the gemstone, the concave surface having at least one relatively reflective region and at least one relatively unreflective region, thereby creating said light pattern. Preferably, the reflector is rotatably mounted in said apparatus generally about the axis X, thereby enabling relative rotation between the light pattern and the support structure generally about the axis X. Alternatively, the platform is rotatably mounted in said apparatus generally about the axis X, thereby enabling relative rotation between the light pattern and the support structure generally about the axis X. Preferably, the illuminator comprises an annular light source arranged to emit light towards the concave surface and an annular baffle arranged to prevent direct light reaching the gemstone. 
     Thus, the apparatus may require only a single stationary light source and a rotatable reflector or rotatable support structure to enable rotation of the light pattern relative to the supported gemstone, resulting in improved mechanical simplicity, as well as reduced size, weight and cost of manufacture. 
     In another embodiment, the apparatus comprises a data processing device arranged to determine a property of the gemstone by comparing, using image data corresponding to one or more of said electronic images, the intensity of light at a first and a second image portion, each portion comprising one or more pixels, said first and second image portions corresponding to a first and a second region of the gemstone respectively, said first and second regions being related to each other in accordance with a symmetrical property of a standardised gemstone cut. 
     Thus, an objective measurement of the symmetry of a gemstone is enabled in terms of the actual light returned from the gemstone and the expected symmetry of the gemstone, such as the 8-fold symmetry of an SRB cut diamond, rather than based on potentially unrealistic or inaccurate models of the geometry of the cut pattern of the gemstone. 
     In another embodiment, the apparatus comprises a data processing device arranged to determine a property of the gemstone by comparing, using image data corresponding to two of said electronic images, the intensity of light at one or more pixels of a first image, captured at a first rotational position, with the intensity of light at one or more pixels of a second image, captured at a second rotational position different to said first rotational position, said one or more pixels of the first image corresponding to the same region or regions of the gemstone as said one or more pixels of the second image. 
     Thus, an improved objective measurement of scintillation is enabled, using data from a plurality of images obtained at different rotational positions of the spatially varied light pattern. With cut gemstones having cut patterns with many radial facets, such as the SRB cut with 32 differently angled facets in the crown and 24 differently angled facets in the pavilion, scintillation may be measured more accurately as, in particular, the intensity levels of returned light have a greater sensitivity to rotational movement of the light pattern relative to the gemstone than to variation of the angle of incidence of the light source. 
     In another embodiment, the apparatus comprises a data processing device arranged to determine a property of the gemstone by comparing, using image data corresponding to two of said electronic images, the proportions of colour components at one or more pixels of a first image, captured at a first rotational position, with the proportions of colour components at one or more pixels of a second image, captured at a second rotational position different to said first rotational position, said one or more pixels of the first image corresponding to the same region or regions of the gemstone as said one or more pixels of the second image. 
     Thus, an improved objective measurement of fire is enabled, using data from a plurality of images obtained at different rotational positions of the spatially varied light pattern. With cut gemstones having cut patterns with many radial facets, such as the SRB cut with 32 differently angled facets in the crown and 24 differently angled facets in the pavilion, fire may be measured more accurately as, in particular, the proportions of colour components of returned light have a greater sensitivity to rotational movement of the light pattern relative to the gemstone than to variation of the angle of incidence of the light source. 
     Another aspect is a method of generating data for use in determining a property of a gemstone, such as a cut diamond. The method comprises supporting a gemstone at an observation position, such that, if the gemstone has an axis of symmetry, the gemstone is supportable such that the axis of symmetry is parallel to an axis X passing through the observation position. The method also includes illuminating the gemstone with a spatially varied light pattern, causing relative rotation between the light pattern and the gemstone generally about the axis X, and capturing, at each of a plurality of rotational positions, an image of light returned by the gemstone. 
     Another aspect is a method of determining a property of a gemstone by comparing, using image data corresponding to one or more electronic images of the gemstone, the intensity of light at a first and a second image portion, each portion comprising one or more pixels, said first and second image portions corresponding to a first and a second region of the gemstone respectively, said first and second regions being related to each other in accordance with a symmetrical property of a standardised gemstone cut. 
     Another aspect is a method of determining a property of a gemstone by comparing, using image data corresponding to two or more electronic images of the gemstone captured under different lighting conditions, the intensity of light at one or more pixels of a first image, captured under a first lighting condition, with the intensity of light at one or more pixels of a second image, captured under second different lighting condition, said one or more pixels of the first image corresponding to the same region or regions of the gemstone as said one or more pixels of the second image. 
     Another aspect is a method of determining a property of the gemstone by comparing, using image data corresponding to two or more electronic images of the gemstone captured under different lighting conditions, the proportions of colour components at one or more pixels of a first image, captured under a first lighting condition, with the proportions of colour components at one or more pixels of a second image, captured under a second different lighting condition, said one or more pixels of the first image corresponding to the same region or regions of the gemstone as said one or more pixels of the second image. 
     Another aspect is a computer-implemented method of comparing properties of gemstones, such as cut diamonds. The method comprises receiving first image data in respect of a first gemstone and second image data in respect of a second gemstone, said first and second image data each comprising one or more images of said first and second gemstones, respectively. The method also includes determining a property of each of said first and second gemstones in dependence on said first and second image data respectively, and simultaneously displaying one or more images of each of said first and second gemstones together with a representation of the determined property for each of said first and second gemstones. 
     Thus, an observer, such as a potential purchaser untrained in evaluating properties of gemstones, may compare two gemstones by means of a side by side graphical representation on a computer screen and by means of objectively determined properties, such as brilliance, scintillation, fire and symmetry. 
     Another aspect is an apparatus for comparing properties of gemstones, such as cut diamonds. The apparatus comprises a support structure for supporting a gemstone placed at an observation position, an illuminator arranged to illuminate a gemstone so placed, a camera arranged to capture an image of light returned by a gemstone and to output said images as image data, and a data processing device. The data processing device is arranged to receive first image data in respect of a first gemstone and second image data in respect of a second gemstone, said first and second image data each comprising one or more images of said first and second gemstones, respectively. The data processing device is also configured to determine a property of each of said first and second gemstones in dependence on said first and second image data respectively, and to simultaneously display one or more images of each of said first and second gemstones together with a representation of the determined property for each of said first and second gemstones. 
     Due to the compact, lightweight and inexpensive format of the apparatus, the gemstone analysis and comparison may be performed in retail premises, such as in a jeweller shop, with the potential purchaser present. 
     Another aspect is an apparatus for generating data for use in determining a property of a gemstone, such as a cut diamond. The apparatus comprises a support structure for supporting a gemstone placed at an observation position, the support structure being arranged such that, if the gemstone has an axis of symmetry, the gemstone is supportable such that the axis of symmetry is parallel to an axis X passing through the observation position. The apparatus also comprises an illuminator arranged to illuminate a gemstone so placed with a spatially varied light pattern, a rotator arranged to cause relative rotation between the light pattern and the support structure, and a camera arranged to capture, at each of a plurality of rotational positions, an image of light returned by the gemstone generally along the axis X and to output said images as image data. 
     Another aspect is an apparatus for generating data for use in determining a property of a gemstone, such as a cut diamond. The apparatus comprises a support structure for supporting a gemstone placed at an observation position, an illuminator arranged to illuminate a gemstone so placed with a spatially varied light pattern, a rotator arranged to cause relative rotation between the light pattern and the support structure generally about an axis X passing through the observation position, and a camera arranged to capture, at each of a plurality of rotational positions, an electronic image of light returned by the gemstone generally along the axis X and to output said images as image data. 
     Another aspect is an apparatus for generating data for use in determining a property of a gemstone, such as a cut diamond. The apparatus comprises a support structure for supporting a gemstone placed at an observation position, an illuminator arranged to illuminate a gemstone so placed with a spatially varied light pattern, a rotator arranged to cause relative rotation between the light pattern and the support structure generally about an axis X passing through the observation position, a camera arranged to capture, at each of a plurality of rotational positions, an electronic image of light returned by the gemstone and to output said images as image data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further aspects and features of the present invention are set out in the appended claims, and further advantages will be apparent from the following description, given by way of example only, in which: 
         FIGS. 1   a  and  1   b  show a standard round brilliant cut diamond from an elevated side-on perspective and from a top-down view, respectively; 
         FIG. 2  shows an apparatus for generating data for determining a property of a gemstone according to the present invention; 
         FIGS. 3   a  and  3   b  show the concave surface  26  of the apparatus of  FIG. 2  having exemplary patterns of relatively reflective and relatively unreflective regions; 
         FIG. 4  shows a screen shot of the main screen of a computer program for analysing images of gemstones according to the present invention; 
         FIG. 5  shows four images of a cut diamond captured a different rotational positions. 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS 
       FIGS. 1   a  and  1   b  show the geometry of a standard round brilliant (SRB) diamond.  FIG. 1   a  shows the diamond from an elevated side-on view. The top most domed-shaped portion of the diamond is known as the crown  10 . The bottom most conical portion of the diamond is known as the pavilion  12 . At the top of crown  10  at the centre is a relatively large facet known as the table  14 . The bottom most point of the pavilion  12  is known as the culet  16 .  FIG. 1   b  shows the SRB diamond from a top-down view, looking along an axis from the centre of the table  14  through the culet  16 . There are 32 facets on the crown  10  of the SRB cut diamond, not including table  14 , and  24  facets on the pavilion, not including culet  16 . It can be seen that the radial facets of the SRB cut diamond (56 in total plus one for the table and one for the culet) have an 8-fold symmetry about an axis passing though the centre of table  14  and culet  16 . 
       FIG. 2  shows a cross-sectional view of an apparatus for generating data for determining properties of a gemstone according to the present invention. A gemstone such as a cut diamond  20  is placed on a platform (not shown) at an observation position with its table-side face-down. The platform is an optically clear glass plane of regular thickness arranged within the apparatus so that it is substantially horizontal when the apparatus is in a horizontal position. The platform may be coated with an anti-reflection coating and provided with a small ring underneath to reduce glare. The apparatus is mounted in a housing (not shown) which prevents external light from reaching the diamond  20  and dust from entering the mechanical and optical components. The housing has a access lid above the platform for placing and removing a gemstone to be measured. The inner surface of the housing and lid above the region of the platform is coated with an unreflective material so that substantially no light is reflected back from the lid or housing towards the gemstone or platform. 
     Diamond  20  is illuminated by an annular light  24 , such as a fluorescent tube light or halogen light. Annular light  24  emits visible light of frequency comparable to daylight. A suitable annular light is a Stocker and Yale microscope illuminator with a White 5500HC fluorescent ring light having a colour temperature of 5500° K, which produces a light close to Northern daylight. Light from annular light  24  is prevented from directly reaching diamond  20  by an annular baffle  28  disposed between the annular light  24  and the diamond  20 . However, light from annular light  24  is reflected off a concave surface  26  of a reflector and generally towards diamond  20 . The reflector may be a semi-spherical shell centred on the observation position with the inner surface of the shell being concave surface  26 . 
     The reflector is mounted within the apparatus such that concave surface  26  is rotatable about an axis  22  perpendicular to the platform and such that when diamond  20  is placed at the observation position, the centre of its table and its culet lie approximately along axis  22 . Annular light  24  and annular baffle  28  are stationary and disposed within the apparatus such that they are also perpendicular to and centred around axis  22 . A stepper motor (not shown) is provided for rotating the reflector, and concave surface  26 , about axis  22 . 
     A viewing hole  34  is present at the bottom of the reflector and concave surface  26  where they meet axis  22 . A digital camera having a charged couple device (CCD) sensor array, or a complementary metal-oxide semiconductor (CMOS) sensor array, and capable of being controlled by a personal computer (PC), is positioned within the apparatus such that it can capture an image of diamond  20  along the axis  22 . The camera is a colour camera having a fixed focal length, at least a 640×480 resolution, a memory capable of storing at least one image, and a data communication interface, compatible with standards such as the Universal Serial Bus (USB), RS 422 parallel port or IEEE 1394 “Firewire” standards, for transferring captured image data to an external device, such as a PC. The camera is focussed on the plane made by the topmost surface of the platform on which diamond  20  is placed, and has a suitable depth of field such that sharp images may be captured of gemstones of the largest size reasonably expected to be measured. An optically clear mirror  32  may be disposed within the apparatus so that the light path between camera  30  and diamond  20  need not be a straight line, thereby enabling a more compact format of apparatus. A suitable digital CCD camera is a Unibrain Fire-i Digital CCD colour camera with a resolution of 640×480 or a Unibrain Fire-i400 Industrial version with a similar resolution. A suitable digital CMOS camera is a Silicon Imaging MegaCamera SI-3170 RGB camera, with a maximum resolution of 2056×1560, a 12-bit per pixel colour depth. 
     The apparatus, including the light  24 , baffle  28 , reflector with concave surface  26 , mirror  34 , stepper motor, camera  30 , and housing, but not including the PC, is compact in size (having dimensions of approximately 123 mm×112 mm×200 mm) and lightweight (approximately 3.875 kg). 
     Camera  30  and the stepper motor are connected to and controllable by a PC  36 . PC  36  may be a portable PC such as a laptop or notebook computer having an Intel Pentium III central processing unit (CPU), 128 megabytes of memory, an LCD panel screen and a 10 gigabyte hard disc drive. PC  36  has a USB port, a parallel port and/or IEEE 1394 “Firewire” port for connecting to the camera and stepper motor, and a 2D video processing chipset for frame grabbing. By means of a suitable computer program, as will be described in greater detail below, PC  36  controls the stepper motor to rotate concave surface  26  through a series of predetermined rotational positions. PC  36  also controls camera  30  to capture images of diamond  20  at a suitable frame rate such that an image may me stored at each of the series of rotational positions of concave surface  26 . The image data captured by camera  30  is transferred to PC  36  in the form of a bitmap or other suitable image file format for display and analysis. The image data is transmitted as a continual live image feed to the PC  36 . 
     The range of angles through which concave surface  26  is rotated is dependent upon the symmetry of the light pattern reflecting off concave surface  26 . With a light pattern having a 4-fold symmetry, for example, images are captured at a plurality of rotational positions as concave surface  26  is rotated through a 90° range. Within the range, the number of images captured at different rotational positions for use in analysis depends on the cut pattern of the gemstone being measured, or the cut pattern of the most faceted gemstone likely to be measured. Generally, the number of images should be at least 4 times the number of differently angled facets within the range through which concave surface  26  is rotated. Thus, with a SRB cut diamond having 32 differently angled facets in its crown and pavilion and thus 8 differently angled facets within a 90° range, at least 32 images (4*8) should be captured over the 90° range. For general purpose, it has been found that a generally suitable number of images to be captured is 45. Thus, over a 90° range, concave surface  26  is rotated in steps of 20. It will be understood that higher or lower numbers of images may be used as appropriate to the cut pattern of the gemstone, the accuracy of measurement required and the processing capabilities of the PC  36 . 
       FIGS. 3   a  and  3   b  show the concave surface  26  looking down from the diamond observation position along axis  22 . Concave surface  26  has a plurality of relatively reflective regions  40  and relatively unreflective regions  42  formed by coating the surface with relatively reflective and relatively unreflective materials.  FIG. 3   a  shows one configuration of regions  40  and  42  in which concave surface  26  is divided into eight equal radial sectors, arranged around the axis  22 , which are alternately relatively reflective and relatively unreflective.  FIG. 3   b  shows a further configuration of regions  40 ,  42  in which concave surface  26  is divided into 16 equal sectors, arranged around the axis  22 , of alternate relatively reflective and relatively unreflective regions. It can be seen that the configuration of regions  40  and  42  of  FIG. 3   a  each have a four-fold symmetry about the axis  22  whereas the configuration of regions  40  and  42  of  FIG. 3   b  each have an eight-fold symmetry about axis  22 . Other configurations of relatively reflective regions  40  and relatively unreflective regions  42  are envisaged within the scope of the present invention. Concave surface  26  may have a matt finish. 
     During operation of the apparatus, it can be seen that the light reflecting off concave surface  26  towards the diamond  20  at its observation position has a spatially varied pattern determined by the configuration of relatively reflective regions  40  and relatively unreflective regions  42 . In particular, the light pattern, as observed in the plane of the platform, will have a series of radial peaks and troughs of light intensity corresponding to the configuration. Thus, with the configuration of  FIG. 3   a , the light pattern will have four radial peak lines and four radial trough lines. Similarly, with the configuration of  FIG. 3   b , the light pattern will have 8 radial peaks and 8 radial troughs. Furthermore, with diamond  20  table-side down on the platform, the light will be reflected generally towards the crown at a broad range of angles of incidence relative to axis  22 , as predominantly occurs when diamonds are mounted in rings and other jewellery for everyday use. 
     The selection of a particular configuration of relatively reflective regions  40  and relatively unreflective regions  42  is dependent upon the standardised cut of diamond  20 . For example, a diamond of SRB cut has an eight-fold symmetry as described above, and a suitable configuration of regions  40  and  42  would be that as shown in  FIG. 3   a , in which there are eight sectors in total—four relatively reflective sectors  40  and four relatively unreflective sectors  42 . Thus, the light pattern reflecting of concave surface  26 , having four radial peaks and four radial troughs corresponds to the symmetry of the cut gemstone in that adjacent symmetrical sectors of the gemstone (of 45°) will receive corresponding radial light pattern sectors (of 45°) having adjacent peaks and troughs. As concave surface  26  is rotated through 90°, the intensity of light as observed at any radial line in the plane of the platform and about axis X, will go through a single complete cycle having a single peak and a single trough. 
     PC  36  may run a standard operating system, such as Microsoft Windows XP or the like. PC  36  also executes a computer program arranged to control the stepper motor to rotate concave surface  26  and to control camera  30  to capture and transfer to PC  36  images of diamond  20  at each of the predetermined rotational positions, for example, 45 images taken at rotational steps of 20 over a total range of 90°. Control over the stepper motor is achieved by using a conventional stepper motor control circuit, such as a Motorola MC 3479 stepper motor controller, to interface between PC  36  and the stepper motor and executing corresponding program elements on PC  36  for sending digital control signals to the stepper motor control circuit. Control over camera  30  is achieved using the camera&#39;s inbuilt control interface and executing corresponding program elements on PC  36  for sending digital control signals to camera  30 . 
     The program elements controlling the stepper motor and camera  30  are themselves under control of a main computer program executing on PC  36  and able to receive user instructions via a user interface to cause the series of images of diamond  20  to be captured and transferred from camera  30  to the PC  36 , to analyse the images using various algorithms to obtain measurements of optical properties of diamond  20  and to display the images on the screen of the PC  36 .  FIG. 4  shows a screen shot of the main menu screen of the computer program. On the right and left sides of the main menu screen are images of two different diamonds, captured in separate scanning operations. An image of a diamond on the right or left sides of the main menu screen may be a “live” image as currently being captured by camera  30  or a “video” image as previously captured during a scanning operation and stored in the hard disk drive of the PC  36 . A “video” image may be presented as a moving image with the diamond being shown in consecutive rotational positions. Beneath each image are measurements of the diamond&#39;s optical properties of brilliance, scintillation and symmetry represented in numerical format (0 to 100) and as a graphical bar chart. Algorithms for calculating these measurements will be described below. An average of the three measurements is also provided labelled “Total” giving an overall measure of the three optical properties. Thus, a user, such as prospective buyer can compare two diamonds, scanned in two separate scanning operations, side by side both visually on PC  36  screen and in terms of objective measurements of the optical properties of brilliance, scintillation and symmetry. 
     Five push buttons are presented in the centre of the screen for user control of the computer program and apparatus. A “Scan” push button is provided for causing the computer program to initialise a scan of diamond  20 . Initially, diamond  20  is manually placed table-side down on the platform and centred on axis  22 . This may be assisted by observing the live image of the diamond displayed on PC  36  screen. Then, the stepper motor is controlled to rotate concave surface  26  to a “home” position and then to each of the series of rotational positions, for example 45 positions over a 90° range in steps of 2° Frames grabbed from camera  30  at each of these positions are stored in the hard disc drive of PC  36  for later display and analysis. The results of analysis, ie the measurements of brilliance, scintillation and symmetry, are then displayed. 
     A “Calibrate” push button is provided for calibrating the system to compensate for variations in the intensity of the light produced by annular light  24 . Calibration is performed by rotating concave surface  26  to a predetermined position, and placing an angled mirror on the platform in a predetermined position, such that camera  30  views a known portion of concave surface  26 . The known portion concave surface  26  may comprise relatively reflective and relatively unreflective regions  40 ,  42 . An image captured by camera  30  of the known portion of concave surface  26  is then analysed by integrating the light intensity levels over all pixels to determine a total light intensity level received. The total light intensity level is then used to adjusts gain and brightness settings of camera  30 . Calibration is preferably performed at regular intervals and immediately prior to scanning. 
     A “Camera” push button is provided for altering the default settings of the camera. A “Tools” push button is provided for selecting various options for the computer program such as a) whether the computer program determines the circumference of a gemstone automatically or manually; b) if manually, for providing a user interface to indicate it; c) how fast concave surface  26  is rotated; and d) the frequency with which stored frames, captured at different rotational positions, are sequentially displayed on the screen of PC  36 . An “Exit” push button is also provided for closing exiting the computer program. 
     To calculate the three measures of brilliance, scintillation and symmetry, from the stored images, three separate algorithms are used. In each case, the main computer program first analyses the images to determine the circumference of diamond  20  and its centre point. The circumference is determined by first summing the light intensity levels at each pixel over all the images at different rotational positions, for example 45 images, to obtain a composite image. Then, all pixels of the composite image having a light intensity level above a predetermined threshold (representing a light level slightly above the level of the black background) are selected. Then the smallest circle containing all the selected pixels is determined and this is defined as the circumference of diamond  20 . 
     Once the circumference and centre of diamond  20  is determined, the three algorithms are executed to calculate measurements of the three optical properties only in respect of pixels contained within the circumference and excluding pixels outside the circumference.  FIG. 5  shows four images of a cut diamond captured at different rotational positions together with a circle defining the circumference of the diamond and a cross marking the centre point. It can be seen that various geometrical patterns of light and dark regions are formed and, in different rotational positions, the regions appear either relatively light or relatively dark. 
     To calculate a measure of the brilliance of diamond  20 , the average light intensity level (ie brightness) is determined over each pixel within the circumference of diamond  20  and for each of the stored images at different rotational positions. Thus, if there are n pixels in the circumference of diamond  20 , and 45 images at different rotational positions, the light intensity level is averaged over 45*n pixels in total. This results in an average light intensity level for diamond  20  over all images at different rotational positions, which provides an objective measure of the brilliance of diamond  20 . 
     To calculate a measure of the scintillation of diamond  20 , the difference in light intensity levels (ie brightness) between a pixel from a first image (captured at a first rotational position) and its corresponding pixel (at the same coordinate position) from a second image (captured at a second rotational position, one rotational step after the first rotational position) is determined. This is repeated for all pixels within the circumference of diamond  20  in the first and second images, and for all pairs of first and second images captured at rotational positions which are one rotational step apart. Thus, if there are n pixels in the circumference of diamond  20 , and 45 images at different rotational positions, 44*n differences are calculated. The number of times the absolute difference in light intensity levels is greater than a predetermined threshold is counted for all pixels in the circumference and for all pairs of images which are one rotational step apart. The ratio of this number over the total number of pixel pairs, 44*n, gives an objective measure of the scintillation of diamond  20 . 
     To calculate a measure of the symmetry of diamond  20 , for the composite image, composed of the stored images at each of the different rotational positions, pixels within the circumference of diamond  20  are divided into 8 approximately equal radial sectors about the centre of diamond  20 . The number of sectors is chosen to correspond to the 8-fold symmetry of the SRB cut pattern. Thus, if there are n pixels within the circumference of diamond  20  in an image, each sector has approximately n/8 pixels. Then, the difference in light intensity levels (ie brightness) between each pixel in each of the 8 sectors of an image and its corresponding pixels (ie the corresponding pixels as rotated by i x 45°, where i=1 to 7) in the seven other sectors of the same image is determined. Thus (7+6+5+4+3+2+1)*n/8=7*n/2 differences are calculated. The average of the absolute values of these differences is then calculated to give an objective measure of the symmetry of diamond  20 . 
     In alternative embodiments of the present invention a measure of the fire of diamond  20  may be calculated by using an algorithm similar to that for determining scintillation. However, instead of measuring the difference in light intensity levels, the difference in the relative proportions of colour components (ie red, green, and blue (RGB)) between a pixel from a first image (captured at a first rotational position) and its corresponding pixel (at the same coordinate position) from a second image (captured at a second rotational position, one rotational step after the first rotational position) are determined. This is repeated for all pixels within the circumference of diamond  20  in the first and second images, and for all pairs of first and second images captured at rotational positions which are one rotational step apart. For each pixel pair, the number of times the absolute difference in the relative proportions of any of the three colour components is greater than a predetermined threshold is counted for all pixels in the circumference and for all pairs of images which are one rotational step apart. The ratio of this number over the total number of pixel pairs gives an objective measure of the fire of diamond  20 . 
     It will be appreciated that, with different shapes and/or symmetries of particular gemstone cut patterns, such as square, oval, pear, heart-shaped or irregular shapes, the algorithms used to determine the periphery of the gemstone and the various measurements of optical properties, as described above, may be varied to take into account the shape and symmetry of the particular gemstone cut pattern. 
     It will be appreciated that, with different shapes and/or symmetries of particular gemstone cut patterns, such as square, oval, pear, heart-shaped or irregular shapes, the configuration of relatively reflective regions  40  and relatively unreflective regions of concave surface  26 , may be varied to take into account the shape and symmetry of the particular gemstone cut pattern. It will also be appreciated that the configuration of relatively reflective regions  40  and relatively unreflective regions of concave surface  26  may be varied to take into account a particular property being determined. For instance, when determining a measure of the fire of a gemstone, it is desirable to for relatively reflective regions  40  to be thin radial lines arranged around the axis  22 , such that the light pattern reflected comprises relatively narrow peaks and relatively wide troughs. Thus, spectrally coloured light will be generally less overpowered by white light and more visible. 
     It will be appreciated that in alternative embodiments, concave surface  26  may be held stationary within the apparatus and the platform is rotated instead. In this arrangement, the images captured of diamond  20  rotate and extra processing is required to take that into account when analysing those images. When comparing a first image at a first rotational position with a second image at a second different rotational position (and with subsequent third, fourth . . . images) processing must be performed so that pixels of the first and second (and subsequent) images correspond to the same region or regions of diamond  20 . To achieve this, the second (and subsequent) image may be digitally rotated back about the point corresponding to the centre of rotation of the platform to correct for the rotation of diamond  20  in the images. Alternatively, when comparing selected pixels of a first and second (or subsequent) image, to obtain a measure of scintillation for example, the pixels of the second (or subsequent) image may be selected so as to correspond to a portion of the image rotated back about the point corresponding to the centre of rotation of the platform to correct for the rotation of diamond  20 . However, due to limitations on the resolution of the captured images, accuracy of comparison is reduced in both cases and this arrangement is less preferable than the former arrangement in which the platform is stationary and concave surface  26  rotated. 
     It will be appreciated that in further alternative embodiments, concave surface  26  may be held stationary within the apparatus, and instead the camera  30  and the platform both rotated by a single or separate stepper motors in a coordinated fashion. This arrangement eliminates the need for extra processing to correct for the rotation of the images of diamond  20 , but involves additional mechanical complexity and increased cost of manufacture. 
     While the above embodiment has described an apparatus arranged to i) support a gemstone having an axis of symmetry such that the axis of symmetry is parallel to the axis  22 , ii) rotate the light pattern relative to the platform about the axis  22 , and iii) capture images of the gemstone along the axis  22 , it is important to realise that the present invention is not limited to this particular arrangement of the three axes, although this arrangement is generally preferred. In particular, the axis of relative rotation between the light pattern and the platform need not be co-linear or even parallel to the axis  22  (ie from the axis parallel to an axis of symmetry of a gemstone when supported in the apparatus) and/or the axis along which the images are captured need not be co-linear or even parallel to the axis  22 . Furthermore, the axis of relative rotation between the light pattern and the platform and the axis along which the images are captured need not be co-linear or even parallel between themselves. 
     What is important is that a gemstone having an axis of symmetry may be supported in the apparatus such that the axis of symmetry, the axis of relative rotation between the light pattern and the means of support, and the axis along which the images are captured are coordinated such that i) the apparatus is able to take advantage of the shape and/or symmetry of the cut pattern of the particular gemstone when rotating the light pattern relative to the gemstone, and ii) the apparatus is able to capture images of the gemstone, such as images of the crown of a SRB cut diamond, from which features resulting from the shape and/or symmetry of the gemstone may be observed. For instance, the axis of relative rotation between the light pattern and the means of support may be at an angle of incidence to the axis of symmetry of up to about 30° without serious degradation to the performance of the apparatus. Similarly, the axis along which the images are captured may at an angle of incidence to the axis of symmetry of up to about 45° without serious degradation to the performance of the apparatus.