Apparatus and method for the assessment of gemstones

Abstract

The present invention provides an apparatus for determining the optical characteristics of a gem stone, the apparatus comprising: an integrating sphere (202) having an internal volume defined by an internal surface; a mount (216) for receiving a gem stone (100), such that at least one surface of the gem stone is exposed to the internal volume; a light source (208) configured to project light into the gem stone; a light sensor (212) configured to measure light at least one position on the internal surface of the integrating sphere.

Description

Apparatus and method for the assessment of gemstonesTechnical Field

[0001] The present invention is concerned with an apparatus and method for the assessment of gemstones. More specifically, the present invention is concerned with an apparatus and method for objectively determining the brilliance of a precious stone.Background Art

[0002] Gemstones are pieces of crystalline mineral which are shaped and polished to be used as decorations, adornments and in jewellery. Gemstones come in the form of precious stones (diamond, sapphire, ruby and emerald) as well as semiprecious stones such as amethyst, garnet, opal and topaz.

[0003] Precious stones in particular are rare, difficult to extract, and therefore valuable. The value of any particular stone is based on a number of qualities. For example, the four Cs of:• Carat - the mass of the stone (1 carat = 200mg);• Colour - how close the stone is to an ideal colour. For example, for diamonds the ideal stone has no hue.• Clarity - how clear the stone is. For diamonds, this is ideally perfectly transparent.• Cut - what shape the stone has been cut into, and how close the cut is to the ideal shape. In principle, the cut of a diamond is precisely defined by the well-known angles necessary to create total internal reflection of any light entering the gemstone.

[0004] The ideal cut will produce the optimum values for the four sub-measurements of cut which are: brilliance, fire, sparkle and symmetry. Each of these characteristics are traditionally determined by a skilled cartier viewing the stone under incident light.

[0005] The precise geometry of gemstone is regularly altered during manufacture to maximise carat weight, to remove inclusions to improve clarity and / or adjust the colour value. This means that every diamond gemstone is unique and requires individual grading on a subjective scale. Normally this is done by the skilled cartier on behalf of an industry governing body such as the Gemological Institute of America (GIA).

[0006] Each of these four qualities of any given stone is linked to its value. A large, transparent, perfectly cut and clear diamond will be of a significantly higher value than a smaller, coloured, poorly cut and unclear diamond. Therefore, there is a need to be able to objectively measure these properties in a repeatable and verifiable manner.

[0007] Referring to Figure 1 , a brilliant cut gemstone 100 has a crown 102 and a pavilion 104 either side of a girdle 106. Within the crown 102 there is provided the table 108 and a plurality of other crown facets- the star facets 1 10, bezel facets 1 12 and upper girdle facets 1 14. The pavilion 104 has pavilion facets 1 16 extending between a culet 1 18 and the girdle 106, as well as lower girdle facets 120.

[0008] A gemstone's brilliance is assessed by how well light (represented here by dashed line L) passes into the crown, reflects internally from both sides of the pavilion and exits in substantially the opposite direction to entry. The higher the proportion of light entering the crown 102 that exits the crown 102, the higher the perceived brilliance by a user's eyes 10. Evidently, the cut (specifically the angle of the cuts on the pavilion) need to be such that this reflection occurs.

[0009] In the prior art, brilliance is assessed by eye by an individual (the cartier), and as such is highly subjective. Significant variance can also occur from individual to individual.

[0010] The gem's 'fire' is its tendency to refract light to emit light in the wavelengths at the red end of the visible spectrum. The stone's sparkle is given by the changes in the position of the light emitted as the stone moves relative to the observer and / or the light source.

[0011] It is an aim of the present invention to provide an improved way of measuring the brilliance of a gemstone.Summary of Invention

[0012] According to a first aspect of the present invention there is provided an apparatus for determining the optical characteristics of a gem stone, the apparatus comprising: an optical component having an internal volume defined by an internal surface, the internal surface configured to give diffuse reflection; a mount for receiving a gem stone, such that at least one surface of the gem stone is exposed to the internal volume;a light source configured to project light into the gem stone; a light sensor configured to measure light at least one position on the internal surface of the optical component.

[0013] By "diffuse reflection" we mean a surface providing uniform, or substantially uniform, reflectance in all directions. The surface is therefore matte, or Lambertian in nature.

[0014] Advantageously, the use of an optical component as defined provides an accurate and repeatable way of measuring the amount of light existing the at least one surface of the stone. In one specific example, measurement of the quality of the cut of the gemstone can be measured by introducing light into the crown, and measuring the light emitted from the crown (defined as the surfaces or facets above the girdle).

[0015] Preferably the first optical component is an integrating sphere.

[0016] Preferably the internal surface comprises a coating of diffusely scattering, or matte material.

[0017] Preferably the coating is greaterthan 99% diffusely reflective across the wavelengths of light of interest (i.e. those emitted by the emitter of the apparatus). More preferably the material comprises barium sulphate or spectralon (RTM).

[0018] Preferably the apparatus further comprises a second optical component, the second optical component having an internal volume defined by an internal surface, the internal surface configured to give diffuse reflection; wherein the mount is configured such that a further surface of the gem stone is at least 90% of the external surface of the gem stone and is exposed to the internal volume of either the first or the second optical component.

[0019] Preferably the first and second optical components meet at an interface, and wherein the mount is positioned at the interface.

[0020] Preferably the second optical component is an integrating sphere.

[0021] Preferably the internal surface ofthe second optical component comprises a coating of matte material.

[0022] Preferably the coating is greaterthan 99% diffusely reflective across the wavelengths of light of interest (i.e. those emitted by the emitter of the apparatus) comprises barium sulphate or spectralon (RTM).

[0023] According to a second aspect there is provided an apparatus for determining the optical characteristics of a gem stone, the apparatus comprising: a first optical component having an internal volume defined by an internal surface, the internal surface configured to give diffuse reflection; a second optical component having an internal volume defined by an internal surface, the internal surface configured to give diffuse reflection; a mount for receiving a gem stone, such that: a first portion of the surface of the gem stone is exposed to the internal volume of the first optical component; and, a second portion of the surface of the gem stone is exposed to the internal volume of the second optical component; such that the only light transmission path between the first and second optical components is through the gem stone; a light source configured to project light into the gem stone from the first optical component; a first light sensor configured to measure light within the first optical component; a second light sensor configured to measure light within the second optical component. In this manner light passing through the stone from the light source into the second optical component.

[0024] According to a third aspect there is provided a method for determining the optical characteristics of a gem stone, the method comprising the steps of: providing first optical component having an internal volume defined by an internal surface, the internal surface configured to give diffuse reflection;positioning a gem stone such that at least one surface of the gem stone is exposed to the internal volume; projecting light into the gem stone; measuring light at least one position on the internal surface of the optical component.

[0025] Preferably the first optical component is an integrating sphere.

[0026] Preferably the internal surface comprises a coating of matte material.

[0027] Preferably the coating is greaterthan 99% diffusely reflective across the wavelengths of light of interest (i.e. those emitted by the emitter of the apparatus) comprises barium sulphate or spectralon.

[0028] Preferably the method comprises the steps of: providing a second optical component, the second optical component having an internal volume defined by an internal surface, the internal surface configured to give diffuse reflection; positioning a gem stone such that at least a further surface of the gem stone is exposed to the internal volume of the second optical component.

[0029] Preferably at least 90% of the external surface of the gem stone is exposed to the internal volume of either the first or the second optical component.

[0030] Preferably the first and second optical components meet at an interface, and wherein the mount is positioned at the interface.

[0031] Preferably the second optical component is an integrating sphere.

[0032] Preferably the internal surface ofthe second optical component comprises a coating of diffusely scattering material.

[0033] Preferably the coating is greaterthan 99% diffusely reflective across the wavelengths of light of interest (i.e. those emitted by the emitter ofthe apparatus) and comprises barium sulphate or spectralon.

[0034] According to a further aspect there is provided a method for determining the optical characteristics of a gem stone, the method comprising the steps of: providing first optical component having an internal volume defined by an internal surface, the internal surface configured to give diffuse reflection; providing a second optical component having an internal volume defined by an internal surface, the internal surface configured to give diffuse reflection; positioning a gem stone such that: a first portion of the surface of the gem stone is exposed to the internal volume of the first optical component; and, a second portion of the surface of the gem stone is exposed to the internal volume of the second optical component; such that the only light transmission path between the first and second optical components is through the gem stone; projecting light into the gem stone from the first optical component; measuring light at least one position on the internal surface of the first optical component; measuring light at least one position on the internal surface of the second optical component; comparing the amount of light measured in the optical component to the amount of light measured in the second optical component to determine a value for brilliance.Brief Description of Drawings

[0035] An embodiment of the present invention will now be described with reference to the following figure in which:FIGURE 1 is a schematic side view of a brilliant cut gemstone such as diamond;FIGURE 2 is a schematic side view of a first apparatus in accordance with the present invention;FIGURE 3 is a detail view of the area III of Figure 2;FIGURE 4 is a schematic view of the light paths in a part of the apparatus of Figure 2; and,FIGURE 5 is a flow chart of a method according to the invention.Description of the first embodiment

[0036] A measurement apparatus 200 according to an embodiment of the present invention is shown in Figures 2 and 3.Configuration

[0037] The apparatus 200 comprises a first integrating sphere 202 and a second integrating sphere 204. The two spheres meet at an interface 206.

[0038] The apparatus comprises a light source 208 configured to project light into the gem stone, a camera 210 and a first light sensor, ideally a spectrometer 212, all adjacent the first sphere 202. The apparatus may also comprise a second light sensor, such as a spectrometer 214, adjacent the second sphere 204.

[0039] The light source 208 is a variable output device. Preferably, the wavelength of the light is matched to the measurement range of the sensors, for example 300nm to 800nm for spectrometers. It is capable of emitting both white light (across a broad range of frequencies) as well as narrower band coloured light via LEDs. In the present embodiment LEDs are provided (rather than lasers) for the luminescence measurements. LEDs are inexpensive, easily obtainable, straightforward to install and produce a "flood" light effect.

[0040] The camera 210 is a Pi Camera which is inexpensive. A camera upgrade such as the 8IVIP Sony IMX219sensor produced high quality images with good resolution (4K image).

[0041] The spectrometers 212, 214 may include flame spectrometers which produce incredibly accurate colour and intensity measurements. A range from 250nm to 2000nm is preferred, although 300-800nm is valid (matched to the emitter) with a high spectral resolution (<2nm) to determine both total light in each sphere for brilliance, and the colour of light in each sphere for the determination of the luminescence data to work out the inclusions and colour of the diamond.

[0042] The apparatus further comprises a mount 216 (Figure 3) in which a gemstone 100 can be positioned. The mount 216 is optimally positioned at the interface 206, and can beremoved from the spheres 202, 204 in orderto replace the gemstonel 00 within. The mount is configured such that when in an installed position (per Figure 3) the subject gemstone 100 is positioned at the interface between the spheres 200, 204 with the point at which the spheres 202, 204 meet, positioned at the girdle 106. Therefore, ideally all (or most) of the volume of the crown 102 is positioned within the first sphere 202, and all (or most) of the volume of the pavilion 104 is positioned within the second sphere 204.

[0043] Because of the need for a mount, optionally a recess is provided in each sphere to accommodate it. It will be noted that a variety of mounts are provided for a variety of stones. Preferably the mount is constructed from, or coated with, a highly light-absorbent material in order to ensure the light path between spheres 202, 204 is only via the gem positioned therebetween.

[0044] The integrating spheres 202, 204 (also known in the art as Ulbricht spheres) are configured that all light entering is diffused throughout the internal volume. The object is to achieve total even dispersion of light such that the light measured at each point on the sphere is equal. The ideal integrating sphere is a perfect diffuser (although retaining the emitted power).

[0045] In a preferred arrangement, the body of each sphere 202, 204 is constructed from a material such as a solid polymer or metallic material, ideally by 3D printing. The spheres are coated in a high efficiency material such as barium sulphate (coating 203, 205 respectively). In one embodiment, the coating is 99% purity barium sulphate mixed with PVA adhesive.Use

[0046] A schematic of a method of measuring the visual characteristics of a gemstone is shown in Figure 5.

[0047] In this embodiment, at step 300, the gemstone 100 is loaded into the mount 216. At step 302 the mount 216 is inserted into the region of the interface 206 between the spheres 202, 204.

[0048] At step 304 the light source 208 is configured to emit light at a predetermined wavelength / range of wavelengths. As this occurs, referring to Figure 4, light L1 enters the crown 102 of the gemstone 100. The light is refracted and reflected within the gemstone 100 (as described above) depending on the cut. At least some light (L2, L3, L4) exits thegemstone and passes to the internal facing surface of the sphere coating where it is reflected in a highly diffuse manner (LD).

[0049] At step 306 the light sensor, such as a spectrometer 212, measures the frequency and amplitude of the incident light at that point. In this way, the total light that is emitted from the crown of the diamond, as well as an average "colour" and ratio of constituent colours can be determined.

[0050] At step 308, simultaneously with the step 306, the spectrometer 212 measures the frequency and amplitude of the incident light at that point. In this way, the total light that is emitted from the pavilion of the diamond, as well as an average "colour" and ratio of constituent colours can be determined.

[0051] At step 310 the intensity and wavelength of the light detected at the spectrometer 212 is compared to that detected at the spectrometer 214. Because the intensity of light detected at each spectrometer is indicative of the total light emitted into each respective sphere from the gemstone 100, the comparison provides an objective indication of how much incident light is reflected within the gemstone to leave the crown. The higher the percentage of light in the first sphere compared to the second, the higher the brilliance. The indication of the diamond's "brilliance" is an indication of the quality of the cut.

[0052] In addition, the detection of specific wavelengths (and ranges of wavelengths) can provide a further profile of the diamond's characteristics. For example, when "red" light is emitted (whether within white light or solely), the brilliance as measured in the "red" wavelength of light is an indication of what is referred to as the diamond's "fire".

[0053] In terms of manufacture of the present invention, the spheres 202, 204 may be manufactured by an additive process such as 3D printing. Each sphere is printed in two parts (hemispheres) which are connected together to form a hollow spherical volume. In this way, the inside surface of each sphere can be accessed for polishing of the rough surfaces, and the barium sulphate coating can be applied.

[0054] Ports are then manually drilled (or the spheres can be printed with the ports for the camera, light and spectrometer fittings).Description of the second embodiment

[0055] A second embodiment according to the invention utilises a single integrating sphere. The lower half of the diamond (the pavilion) is covered and only the sphere 202 is utilised. Although the ratio of light exiting each part of the diamond cannot be determined directly, if the apparatus is calibrated the amount of light emitted by the crown may be compared to a predetermined level to obtain an indication of brilliance.Variations

[0056] The camera 210 is an optional feature allowing for finer measurement of colour in the diamond.

[0057] In one embodiment, lasers mounted to move in small increments may be used to determine where, for example, impurities are situated. In this embodiment, the laser is configured to emit a highly concentrated, focussed beam of light which can be moved around the surface of the crown. By using the above-described apparatus to measure differences in the light output in the upper and lower spheres, imperfections can be detected.

[0058] The spheres 202, 204 should be at least 90% coated for them to be effective. Because the ports for the light source and detectors are generally very small in comparison to the surface area of the sphere, several such ports can be incorporated.

[0059] For example, in a 25cm diameter sphere, with average port size of 0.8cm in diameter, up to 10 ports may be provided on each sphere without significantly impacting performance.

[0060] Spectrometers have been mentioned above with respect to light sensors. It will be noted that other sensors such as photodiodes may be used.

Claims

Claims1. An apparatus for determining the optical characteristics of a gemstone, the apparatus comprising: a first optical component having an internal volume defined by an internal surface, the internal surface configured to give diffuse reflection; a second optical component having an internal volume defined by an internal surface, the internal surface configured to give diffuse reflection; a removable mount for receiving a gem stone, such that: a first portion of the surface of the gemstone is exposed to the internal volume of the first optical component; and, a second portion of the surface of the gemstone is exposed to the internal volume of the second optical component; such that the only light transmission path between the first and second optical components is through the gemstone; a light source configured to project light into the gemstone from the first optical component; a first light sensor configured to measure light within the first optical component; a second light sensor configured to measure light passing through the stone from the light source into the second optical component.

2. An apparatus according to claim 1 , wherein the first and / or second optical component is an integrating sphere.

3. An apparatus according to claim 1 or 2, wherein the internal surface of each optical component comprises a coating of diffusely scattering material.

4. An apparatus according to claim 3, wherein the coating comprises at least one of: barium sulphate; and, spectralon (RTM).

5. An apparatus according to any preceding claim, wherein the external surface of the gemstone exposed to the internal volume of the first and the second optical component is at least 90% of the surface of the gem stone.

6. An apparatus according to any preceding claim, wherein the first and second optical components meet at an interface, and wherein the mount is removably positioned at the interface.

7. A method for determining the optical characteristics of a gemstone, the method comprising the steps of: providing first optical component having an internal volume defined by an internal surface, the internal surface configured to give diffuse reflection; providing a second optical component having an internal volume defined by an internal surface, the internal surface configured to give diffuse reflection; positioning a gemstone such that: a first portion of the surface of the gemstone is exposed to the internal volume of the first optical component; and, a second portion of the surface of the gemstone is exposed to the internal volume of the second optical component; such that the only light transmission path between the first and second optical components is through the gemstone; projecting light into the gemstone from the first optical component; measuring light at least one position on the internal surface of the first optical component; measuring light at least one position on the internal surface of the second optical component; comparing the amount of light measured in the optical component to the amount of light measured in the second optical component to determine a value for brilliance.

8. A method according to claim 7, wherein the first and / or second optical component is an integrating sphere.

9. A method according to claim 7 or 8, wherein the internal surface comprises a coating of diffusely scattering material.1 0. A method according to claim 9, wherein the coating comprises at least one of: barium sulphate; and, spectralon (RTM).1 1 . A method according to any of claims 5 to 1 0, wherein the external surface of the gemstone exposed to the internal volume of the first and the second optical component is at least 90% of the surface of the gem stone.1 2. A method according to any of claims 5 to 1 1 , wherein the first and second optical components meet at an interface, and wherein the mount is positioned at the interface.

Images