Abstract:
A gemstone to be optically examined is held by a holder having an internal compartment. A mounting plate supports the gemstone table-side up in an upright position within the internal compartment. A protective lid is mounted on the holder for movement between a closed position in which the lid overlies the mounting plate and the gemstone supported thereon, and an open position in which the lid uncovers the gemstone for optical examination.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/308,062, filed Feb. 25, 2010. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention generally relates to an arrangement for, and a method of, holding gemstones, especially cut diamonds, to be examined by illuminating the gemstone, capturing images of the illuminated gemstone, and analyzing the captured images with repeatability. 
         [0003]    The beauty and price of a gemstone, such as a cut multi-faceted diamond, are based, for example, on its cut, carat weight, clarity and color. Many different geometrical patterns of cuts, such as round brilliant, oval, pear, marquise, radiant, princess, heart, emerald, etc. are now standardized. The cut, the carat weight, the clarity and the color of the gemstone are typically evaluated and/or measured by a human appraiser. Such evaluated and/or measured properties are often objectively presented to a consumer, typically in certificate form, for price valuation. 
         [0004]    Optical performance of the gemstone, that is, how the gemstone “plays with light”, and optical efficiency of the gemstone, that is, how the gemstone “reflects light”, are difficult to subjectively evaluate and measure, even for the experienced human appraiser. Optical performance and efficiency of the gemstone are typically characterized by such properties as its brilliance (the amount and intensity of incident light returned from the gemstone), scintillation (fast and local fluctuations in the incident light returned as the gemstone moves), fire (the dispersion of incident white light into its spectral colors), coverage (the area of the incident light returned compared to the total area of the gemstone table), contrast (the intensity of the incident white light returned compared to the intensity of the non-returned or black light), and symmetry (the balance of the pattern of the incident light returned). A more visually active gemstone is deemed more valuable than a less visually active gemstone, even with the same cut, carat weight, clarity and color. 
         [0005]    To objectively measure such optical properties of gemstones, the art has disclosed various computer-based systems for capturing and analyzing images of gemstones illuminated under varying lighting conditions. For example, U.S. Pat. No. 5,615,005 discloses a gemstone evaluation system that captures images of a gemstone placed table-side face-down on a glass plate in an analysis chamber and illuminated from a plurality of different angles by a movable light source that is moved toward the gemstone during the evaluation. Captured images of the gemstone are analyzed by a computer, and various optical properties of the gemstone are measured and displayed or printed. 
         [0006]    As another example, U.S. Pat. No. 6,813,007 discloses another computer-based system that captures images of a gemstone also placed table-side face-down on a glass plate, but illuminated by light reflected off a rotary reflector that rotates during the evaluation. Captured images of the gemstone are again analyzed by a computer, and various optical properties of the gemstone are measured and displayed or printed. 
         [0007]    However, as advantageous as such computer-based systems have been, they have not proven to be altogether satisfactory in use. A moving system component, such as a rotating reflector or a moving light source, causes mechanical variability and vibrations that can interfere with the measurements, and degrade measurement accuracy and repeatability. The light source also concomitantly generates heat that can interfere with the measurements. Also, the glass plate on which the gemstone is placed creates an interface at which light interference can occur due to light refraction, thereby again degrading measurement accuracy and repeatability. This interference effect is aggravated by frequent contamination with dust, dirt, oil from an operator&#39;s fingers, scratches, or like contaminants on the glass plate. Furthermore, the placement of the gemstone on the glass plate is variable, thereby still further worsening measurement accuracy and repeatability. An objective, accurate and repeatable examination of a gemstone is essential for true price valuation of the gemstone. 
       SUMMARY OF THE INVENTION 
       [0008]    One aspect of this invention is directed to an arrangement for holding a gemstone to be optically examined. The arrangement includes a holder having an internal compartment, a mounting plate for supporting the gemstone table-side up in an upright position within the internal compartment, and a protective lid mounted on the holder for movement between a closed position in which the lid overlies the mounting plate and the gemstone supported thereon, and an open position in which the lid uncovers the gemstone for optical examination. The arrangement is mounted in an apparatus for accurately examining, with repeatability, one or more optical properties, such as coverage and/or symmetry of the gemstone, especially a cut, multi-faceted diamond having a table (i.e., the uppermost, largest facet at the top of the diamond) and a culet (i.e., the pointed or blunted bottom of the diamond). An energizable, stationary light source is spaced away from the gemstone and is operative for directing light rays at different orientations to the table of the gemstone when energized. 
         [0009]    In one embodiment, the stationary light source comprises a plurality of light sources spaced along an axis away from, and facing, the table of the gemstone, for directly directing the light rays at the different orientations to the table of the gemstone. Each light source preferably comprises a multitude of light-emitting elements, such as light emitting diodes, arranged in an annulus around the axis, with each annulus having a different diameter. A support, preferably annular in shape, is advantageously provided for supporting the light sources at different axial distances away from the gemstone. An optional homogenizer may be provided and supported by the support for homogenizing and making more uniform the light from each light source directed to the gemstone. 
         [0010]    In another embodiment, a support is spaced from the light source, for supporting a plurality of light reflectors of different reflectivity, e.g., different colors of the light spectrum, and at different angular orientations and distances along an axis away from the table of the gemstone. Each light reflector is preferably arranged in an annulus around the axis, each annulus having a different diameter. The stationary light source, preferably a plurality of light emitting diodes lying in a plane, emits uniform light for reflection simultaneously from all the light reflectors as the light rays at the different orientations directly to the table of the gemstone. 
         [0011]    An energizable stationary backlight is oppositely positioned from either support to face the culet of the gemstone. The backlight lies in a plane and uniformly illuminates the gemstone when energized. The backlight could also comprise multiple light emitting diodes arranged in a two-dimensional array, preferably overlaid with a light homogenizer. 
         [0012]    A controller or microprocessor, preferably a programmed computer, is operative for energizing the light source to generate return light from the gemstone for each light ray, as well as for energizing the backlight to illuminate the gemstone from behind The controller advantageously controls a power supply that supplies the voltages for energizing the light source and the backlight. In an advantageous embodiment, the backlight is energized first, and then the light source is energized. When a plurality of light sources is employed, they are individually energized, preferably, but not necessarily, in an ordered sequence. 
         [0013]    A solid-state imager is spaced along an axis away from the gemstone, and preferably has a two-dimensional array of cells or photosensors, which correspond to image elements or pixels in a field of view of the imager. An optical focusing lens assembly is provided for capturing the return light from the gemstone and the light in the field of view of the backlit gemstone, and for projecting the captured light onto the imager during an exposure time period. The imager may be a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, together with associated integrated bandpass spectral filters and electronic circuits for producing electrical signals corresponding to a two-dimensional array of pixel information over the field of view, and is similar to that used in a digital camera. The imager is operative for imaging the return light from the light rays as a plurality of frontlit images, and for imaging the light in the field of view of the backlit gemstone as a backlit image. 
         [0014]    The controller is further operative, as described in detail below, for processing at least one of the frontlit images and the backlit image, to determine the coverage and/or symmetry properties of the gemstone. Other optical properties that can be determined are the aforementioned contrast, fire, brilliance and scintillation. 
         [0015]    The apparatus advantageously also comprises one or two position adjusters for axially adjusting a position of the imager and/or the light source relative to each other and the gemstone. A gemstone holder or fixture is operative for holding and automatically positioning the gemstone in a predetermined upright position in which the table of the gemstone is uncovered and directly exposed to each light ray. A support plate, preferably of rigid material, commonly mounts the light source, the backlight, the imager and the gemstone holder. Shock-absorbers on the support plate are employed for resisting shock forces from reaching the light source, the backlight, the imager and the gemstone holder. Such forces can originate from the environment exterior to the apparatus, or from the interior, for example, from cooling fans within the power supply, also mounted on the support plate. 
         [0016]    Still another aspect of this invention is directed to a method of examining at least one optical property, such as coverage and/or symmetry, of the gemstone. The method is performed by directing light rays at different angular orientations to the gemstone by energizing a stationary light source to generate return light from the gemstone for each light ray, imaging the return light as a plurality of frontlit images, and processing at least one of the frontlit images. 
         [0017]    The method is further performed by directing the light rays to an uncovered table of the gemstone, and by positioning an energizable stationary backlight to face a culet of the gemstone. The backlight is energized to illuminate the gemstone from behind The light in the field of view of the backlit gemstone is imaged as a backlit image. The backlit image is processed, preferably together with the at least one frontlit image, to determine the optical property of the gemstone. 
         [0018]    As a preferred initial step, the backlit image is processed to determine a silhouette or outline of the backlit gemstone, and a number of total pixels is counted within an area or region of interest of the gemstone. Symmetry is advantageously determined by processing each frontlit image of the return light, generating virtual images mirror symmetrical to the frontlit images, counting a number of symmetrical pixels common to each frontlit image and its respective virtual image within the area of the gemstone, and analyzing the counted number of symmetrical pixels compared to the number of total pixels. Coverage is advantageously determined by counting a number of coverage pixels having an intensity above a predetermined value in coverage regions of the backlit image within the area of the gemstone, and analyzing the number of coverage pixels compared to the number of total pixels. 
         [0019]    Thus, the apparatus of this invention has no rotating light reflectors or moving light sources, as in the prior art, that could cause mechanical variability and vibrations to interfere with the measurements, and thereby degrade measurement accuracy and repeatability. The light emitting diodes of this invention are stationary and are relatively cool-running devices that do not generate heat, as in the prior art, to interfere with the measurements. Rather than moving the light incident on the gemstone, the spaced-apart light sources in one embodiment are energized, one at a time, or the light reflectors in another embodiment simultaneously reflect light incident thereon, thereby obtaining multiple frontlit images to be processed. The arrangement of this invention does not place the gemstone table-side face-down on a glass plate and thus avoids light interference effects, as in the prior art, due to light refraction from a covered table. The arrangement of this invention holds the gemstone in a predetermined, upright position in which the table is uncovered and directly exposed to the light rays. The lack of a glass plate means that no contamination can occur due to dust, dirt, oil from an operator&#39;s fingers, scratches, or like contaminants. The arrangement of this invention provides an objective, accurate and repeatable examination of the coverage and/or symmetry, as well as other optical properties, of a gemstone essential for true price valuation of the gemstone. 
         [0020]    The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is an exploded, front elevational view of an apparatus for accurately determining at least one optical property, such as symmetry and/or coverage, of gemstones in accordance with the method of this invention; 
           [0022]      FIG. 2  is a side elevational view of part of the apparatus of  FIG. 1 , with a cover removed; 
           [0023]      FIG. 3  is an enlarged, sectional view of one embodiment of an annular support for supporting a plurality of light sources facing a table of a gemstone, as employed in the apparatus of  FIG. 1 ; 
           [0024]      FIG. 4  is a perspective view of a gemstone holder being prepared to receive a gemstone, prior to being placed in the apparatus of  FIG. 1 ; 
           [0025]      FIG. 5  is a perspective view of the gemstone holder of  FIG. 4  during loading of the gemstone; 
           [0026]      FIG. 6  is a perspective view of the gemstone holder of  FIG. 5  after loading of the gemstone; 
           [0027]      FIG. 7  is a perspective view of the gemstone holder of  FIG. 6  ready to be placed in the apparatus of  FIG. 1 ; 
           [0028]      FIG. 8  is a broken-away, perspective view of the gemstone holder of  FIG. 7  placed in an open drawer of the apparatus of  FIG. 1 ; 
           [0029]      FIG. 9  is a broken-away, perspective view of the gemstone holder of  FIG. 7  during closure of the drawer of the apparatus of  FIG. 1 ; 
           [0030]      FIG. 10  is an exploded, perspective view of parts of the gemstone holder of  FIG. 4 ; 
           [0031]      FIG. 11  is an exploded, perspective view of parts of the drawer of  FIG. 8 ; 
           [0032]      FIG. 12  is a backlit image of the gemstone taken in accordance with this invention; 
           [0033]      FIG. 13  is a display setting forth the coverage and symmetry properties of the gemstone examined in accordance with this invention; and 
           [0034]      FIG. 14  is an enlarged, sectional view of another embodiment of an annular support analogous to that shown in  FIG. 3 , but operative for supporting a plurality of light reflectors facing a table of a gemstone, for use in the apparatus of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]    Referring now to  FIG. 1  of the drawings, reference numeral  10  generally identifies an apparatus for accurately determining, with repeatability, one or more optical properties such as coverage and/or symmetry of a gemstone  12 , especially a cut, multi-faceted diamond having, as best shown in  FIG. 3 , a table  14  (i.e., the uppermost, largest facet at the top of the diamond) and a culet  16  (i.e., the pointed or blunted bottom of the diamond). Although  FIG. 3  depicts that the gemstone  12  has a round cut, the apparatus  10  is equally applicable for determining the coverage and symmetry of gemstones having other cuts, such as oval, pear, marquise, radiant, princess, heart, emerald, etc. 
         [0036]    As shown in  FIGS. 1-2 , the apparatus  10  includes a horizontal support plate  18 , preferably of rigid material, on which an upright, vertical standard  20  is mounted. A gemstone holder  22  or fixture, as best shown in  FIGS. 4-11 , is mounted on the support plate  18  behind a drawer front panel  126  and is operative, as described below, for holding and automatically positioning the gemstone  12  in a predetermined upright position. An annular support  24 , as best shown in  FIG. 3 , is mounted on the standard  20  above the gemstone holder  22 . In the embodiment of  FIG. 3 , the annular support  24  is operative, as described below, for supporting a plurality of light sources that face the table  14  of the gemstone  12  in the holder  22 . In an alternate embodiment, as described below in connection with  FIG. 14 , a different annular support is operative for supporting a plurality of light reflectors that face the table  14  of the gemstone  12  in the holder  22 . A backlight  30  is mounted on the support plate  18  and faces the culet  16  of the gemstone  12  in the holder  22 . 
         [0037]    A solid-state imager  26  is also mounted on the standard  20  above the annular support  24  and is operative, as described below, for imaging light returning from the gemstone  12 . An optical focusing lens assembly  28  is also mounted on the standard  20  between the annular support  24  and the imager  26 , and is operative for capturing the light returning from the gemstone, and for projecting the captured light onto the imager  26  during an exposure time period. The imager  26 , the lens assembly  28 , the annular support  24 , the gemstone  12  and the backlight  30  are all aligned along an optical path or axis  32  that is parallel to the elongation of the vertical standard  20 . 
         [0038]    A first position adjuster  34  is operative for axially adjusting a position of the imager  26  and the lens assembly  28  along the optical path  32  relative to the gemstone  12 . A second position adjuster  36  is operative for axially adjusting a position of the annular support  24  along the optical path  32  relative to the gemstone  12 . A power supply  38  is mounted on the support plate  18  and is operative, as described below, for supplying power to the light sources in the annular support  24  and to the backlight  30 . Shock absorbers  40  between the power supply  38  and the support plate  18 , as well as shock absorbers or feet  42  on the bottom of the support plate  18 , are employed for resisting shock forces from reaching the light sources, the backlight  30 , the imager  26 , the lens assembly  28  and the gemstone holder  22 . Such forces can originate from the environment exterior to the apparatus  10 , or from the interior, for example, from cooling fans within the power supply  38 . A cover  44  is removably mounted over the components mounted on the support plate  18  and on the standard  20 . The cover  44  has a cutout  46  to provide access to a handle  48  on the drawer front panel  126 , and clearance for the latter. 
         [0039]    As shown in  FIG. 3 , the annular support  24  has a cylindrical lower housing portion  50  and a frustoconical upper housing portion  52 , together resembling a domed structure symmetrical about the axis  32 . The upper housing portion  52  has an axial passage  56  through which light passes en route to the imager  26 . An optional dome-like homogenizer  54  of light-homogeneous material may be provided within the annular support  24  and has opposite openings  58 ,  60  through which light passes. 
         [0040]    A plurality of energizable, stationary light sources  62 ,  64 ,  66 ,  68 ,  70 ,  72 ,  74  is supported by the annular support  24  and spaced along the axis  32  at different axial distances away from, and facing, the table  14  of the gemstone  12 . Each light source  62 ,  64 ,  66 ,  68 ,  70 ,  72 ,  74  comprises a multitude of light-emitting elements, such as light emitting diodes (LEDs), arranged in an annulus around the axis  32 , with each annulus having a different diameter. In a preferred embodiment, there are seven annular light sources, and each annular light source comprises about fifty to seventy LEDs mounted on respective annular flexible circuit boards  76 . Each LED is operative for emitting light rays to the gemstone when energized. More or fewer than seven annular light sources could be employed. More or fewer than fifty to seventy LEDs may comprise each annular light source. Advantageously, the LEDs are preselected such that their individual output powers are substantially the same. When energized, each annular light source generates a light ray in an annular zone of generally uniform illumination, but at a different angular orientation relative to, as well as a different axial distance from, the gemstone  12 . 
         [0041]    As noted above, the energizable stationary backlight  30  faces the culet  16  of the gemstone  12 , lies in a plane and uniformly illuminates the gemstone from behind when energized. The backlight  30  also comprise multiple LEDs arranged in a two-dimensional array, preferably overlaid with a planar light homogenizer of light-homogeneous material. When energized, the backlight  30  generates a planar zone of homogenized light of generally uniform illumination behind the gemstone  12 . 
         [0042]    A controller  80  (see  FIG. 1 ) or microprocessor, preferably a programmed computer, is operatively connected to the power supply  38 , and is operative for individually energizing the light sources  62 ,  64 ,  66 ,  68 ,  70 ,  72 ,  74  to generate return light from the gemstone  12  for each light source in the annular support, as well as for energizing the backlight  30  to illuminate the gemstone  12  from behind The power supply  38  has an output  82  that supplies the voltages for energizing the light sources, and an output  84  that supplies the voltage for energizing the backlight  30 . The electrical wiring between the power supply  38  the light sources and the backlight  30  are not shown for clarity. In an advantageous embodiment, the backlight  30  is energized first, and then the plurality of light sources  62 ,  64 ,  66 ,  68 ,  70 ,  72 ,  74  is energized, preferably, but not necessarily, in an ordered sequence, e.g., from the closest to the furthest annular light source. 
         [0043]    As noted above, the solid-state imager  26  is spaced along the axis  32  away from the gemstone  12 , and preferably has a two-dimensional array of cells or photosensors, which correspond to image elements or pixels in a field of view of the imager. The optical focusing lens assembly  28  is operative for capturing the return light from the gemstone  12  and the light in the field of view of the backlit gemstone  12 , and for projecting the captured light onto the imager  26  during an exposure time period. The imager  26  may be a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, together with associated integrated spectral filters and electronic circuits for producing electrical signals corresponding to a two-dimensional array of pixel information over the field of view, and is similar to that used in a digital camera. The imager  26  is operative for imaging the return light from the gemstone  12  from each light source through the passage  56  as a plurality of frontlit images, and for imaging the light in the field of view of the backlit gemstone  12  through the passage  56  as a backlit image. The frontlit images represent different images of the gemstone taken at different relative orientations and/or distances between the gemstone and the light source and simulate the effect of a moving light source, but without the above-described drawbacks thereof. 
         [0044]    The controller  80  is further operative, as described in detail below, for processing the backlit and the frontlit images, to determine the symmetry and/or coverage of the gemstone  12 . The controller  80  is located exteriorly of the apparatus outside the cover  44 , but could be incorporated within the cover. Input data can be input to the controller  80  via an input device  86 , e.g., a mouse, keyboard, joystick, etc. Measurement data can be output from the controller  80  via an output device, e.g., a monitor  88 , a printer  92 , an internet connection  90 , etc. 
         [0045]    As noted above, the gemstone holder  22  or fixture is operative for holding and automatically positioning the gemstone  12  in a predetermined upright position in which the table  14  of the gemstone  12  is uncovered (see  FIG. 3 ) and directly exposed to the light from each annular light source  62 ,  64 ,  66 ,  68 ,  70 ,  72 ,  74 . In the predetermined upright position, the light from each LED enters the gemstone  12  slightly below the table  14  to minimize any specular reflections off the table  12 . In the predetermined upright position, the table  14  of the gemstone  12  is preferably slightly above a top surface  128  of the holder  22 . 
         [0046]    As depicted in  FIGS. 4-7 , the gemstone holder  22  includes a protective lid  96  pivotably mounted on the gemstone holder  22  for movement between an open position ( FIG. 5 ) and a closed position ( FIG. 7 ). A stop  130  is mounted on the holder  22  and extends above the top surface  128  to abut against an extension  132  of the lid  96  to define the open position. A shoulder  138  (see  FIG. 10 ) formed in a curved recess  140  on the underside of the lid  96  abuts against a lock  142  that extends above the top surface  128  to define the closed position. A release  122  releases the lock  142  when depressed.  FIG. 10  depicts a stationary gear  100  fixed to the holder  22 . Gear  100  meshes with sector gear  102  that is mounted on a spindle  104  that is jointly movable with the lid  96 . A tensionable spring  106  is connected to the lid  96  via the spindle  104 . The spindle  104  has a curved surface  134  and a flat surface  136 . The spring  106  acts to constantly bias the lid  96  to the open position. The gears  100 ,  102  act to slow the rate at which the lid  96  is moved to the open position by the spring  106 . 
         [0047]    In  FIG. 4 , an operator has already manually depressed the release  122  and released the lock  142  from the shoulder  138 , thereby enabling the tensioned spring  106  to pivot the lid  96  until it is held in the illustrated open position by mutual engagement between the stop  130  and the extension  132 . During this pivoting movement, the lock  142  passes with clearance along the curved recess  140  in the lid  96 . A mounting plate  94  having a hole  98  therein (see  FIG. 5 ) is exposed in the open position.  FIG. 10  depicts a flanged sleeve  108  that holds the mounting plate  94  against the restoring force of a return spring  110 . The mounting plate  94  and the sleeve  108  are jointly movable up-and-down. In  FIG. 4 , the operator pushes the mounting plate  94  down, where it is latched in the down position by mutual frictional engagement between the sleeve  108  and the curved surface  134  of the spindle  104 . 
         [0048]    In  FIG. 5 , the operator loads the gemstone  12  into the hole  98  of the mounting plate  94 . Holes  98  of different sizes and mounting plates  94  of different heights can be used to accommodate differently sized gemstones. Tweezers  112  or gloves are recommended to avoid contamination. In  FIG. 6 , the operator pivots the lid  96  against the force of the spring  106  to the closed position until the lock  142  lockingly engages the shoulder  138 . This action turns the spindle  104  until the flat surface  136  faces the sleeve  108 , thereby unlatching the sleeve  108  and the mounting plate  94 , both of which are released and move up under the force of the spring  110 . In  FIG. 7 , the table  14  of the gemstone  12  is pressed against the underside of the closed lid  96  inside and against an inner wall of another curved recess  144  (see  FIG. 10 ) due to the force of the spring  110 . This is the aforementioned predetermined upright position of the gemstone  12 , which is repeatable for the same gemstone, as well as from one gemstone to the next. The table  14  is positioned in a plane slightly above the top surface  128  of the holder  22 . The gemstone holder  22  with its pre-positioned gemstone  12  is now ready to be placed in the apparatus  12 . 
         [0049]    In  FIG. 8 , a drawer  114  is pulled out in the direction of the arrow A by the handle  48 , and the gemstone holder  22  with its pre-positioned gemstone  12  is placed in the drawer  114  and held in a predetermined position by a magnetic mount  116 .  FIG. 11  depicts that the drawer  114  includes a linear toothed track  118  that meshes with a spring-biased, viscous oil-dampened gear  120 . When the drawer  114  is pulled out, the track  118  rotates the gear  120  and tensions the spring therein. The drawer  114  is then released and moves in the opposite direction of the arrow A due to the restoring force of the spring. This return released movement of the drawer  114  is slowed due to the viscous oil and prevents the gemstone from being jarred from its predetermined upright position. 
         [0050]    During the return movement of the drawer  114 , the gem holder  22  approaches the overhead annular support  24  and eventually is positioned in a loaded position directly thereunder, as shown in  FIG. 3 . Just prior to being positioned in the loaded position, the lid  96  is unlatched and released to its open position, as depicted in  FIG. 9 . This is accomplished by mutual abutment between a release member  146  mounted on a side block  124  and the release  122  on the holder  22 . When the release member  146  abuts the release  122  during the return movement of the drawer  114 , the lock  142  is disengaged from the shoulder  138 , and the lid  96  is automatically opened under the force of the spring  106 . The gemstone  12  is not disturbed from its predetermined upright position during the opening of the lid  96 , since the gemstone passes with clearance along the curved recess  144 . 
         [0051]    As previously mentioned, the controller  80  processes the backlit and the frontlit images, to determine the aforementioned symmetry and/or coverage, as well as other optical properties, of the gemstone  12 . As an initial step, the controller  80  energizes the backlight  30  and processes the backlit image, which resembles a circular dark region of interest for a round cut gemstone, analyzes the silhouette or shape, fills in any bright areas within the dark region of interest, rotates the dark region of interest (only if it is non-circular), determines the perimeter and the area of the dark region of interest, and counts the number of total pixels within the area of the gemstone. 
         [0052]    Coverage is advantageously determined by processing the backlit image, as depicted in  FIG. 12 , and counting the number of total pixels within an area of the gemstone as described above, counting a number of coverage pixels having an intensity above a predetermined value in coverage regions of the backlit image within the area of the gemstone, and analyzing the number of coverage pixels compared to the number of total pixels to obtain a coverage ratio indicative of the percentage of light that is reflected. The higher the coverage ratio, the greater the coverage. 
         [0053]    Optical symmetry, as distinguished from geometrical symmetry, is advantageously determined by processing each frontlit image of the return light, generating virtual images mirror symmetrical to the respective frontlit images, counting a number of symmetrical pixels common to each frontlit image and its respective virtual image within the area of the gemstone, and analyzing the counted number of symmetrical pixels compared to the number of total pixels as previously counted. More specifically, each frontlit image and its respective virtual image are juxtaposed, and the pixels of the juxtaposed images are compared. Intensity values are determined for the compared pixels, and their difference is determined. If each difference lies within a certain range, e.g., between zero and  10 , then the compared pixels are deemed symmetrical. The number of the symmetrical pixels is then compared to the number of total pixels to obtain a symmetrical ratio for each set of frontlit and virtual images. All the symmetrical ratios are averaged to obtain a final symmetrical ratio. The higher the final symmetrical ratio, the greater the optical symmetry. 
         [0054]    Thus, the apparatus of this invention provides an objective, accurate and repeatable measure of symmetry and/or coverage properties of a gemstone essential for true price valuation of the gemstone. Other optical properties, such as contrast, fire, brilliance and scintillation can also be determined.  FIG. 13  depicts an exemplary printout or certificate printed by the local printer  92  and depicting the symmetry and coverage properties in a line graph format, in which such designations as “fair, good, very good and excellent” are employed, rather than numerical percentages, for convenience. Other display formats could be employed. This measurement data can also be printed on a remote printer via the internet  90 , or locally displayed on the monitor  88 , or remotely displayed. 
         [0055]    As previously mentioned,  FIG. 14  depicts an alternate embodiment, analogous to that shown in  FIG. 3 , for illuminating the uncovered table  14  of the gemstone  12 . An annular hemispherical support  150  is spaced from a light source  152 , for supporting a plurality of light reflectors  154 ,  156 ,  158 ,  160 ,  162 ,  164 , and  166  of different reflectivity, e.g., different colors of the light spectrum, i.e., red, orange, yellow, green, blue, indigo and violet, and at different orientations and/or distances along the axis  32  away from the uncovered table  14  of the gemstone  12 . Each light reflector is preferably arranged in an annulus around the axis  32 , each annulus having a different diameter. More or fewer than the seven illustrated reflectors could be employed. The stationary light source  152 , preferably a plurality of light emitting diodes lying in a plane, emits uniform light for reflection simultaneously from all the light reflectors as the light rays at the different orientations directly to the table  14  of the gemstone  12 . A light baffle  168  blocks any light from the light source  152  from entering the gemstone from behind 
         [0056]    When the stationary light source  152  is energized, each annular light reflector reflects a light ray in an annular zone of generally uniform illumination and at a different wavelength, but at a different angular orientation relative to, as well as a different axial distance from, the gemstone  12 . The support  150  has opposite openings  170 ,  172  through which light reflected off the gemstone passes en route to the imager, as described above. In a variant of the embodiment of  FIG. 14 , rather than reflectors, light-transmissive windows of different wavelengths can be employed. 
         [0057]    It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of constructions differing from the types described above. 
         [0058]    While the invention has been illustrated and described as embodied in an arrangement for, and a method of, holding a gemstone, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
         [0059]    Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims. 
         [0060]    What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.