WO2019162666A1

WO2019162666A1 - Uv transparency of gemstones

Info

Publication number: WO2019162666A1
Application number: PCT/GB2019/050468
Authority: WO
Inventors: Peter Lanigan; Simon Craig Lawson
Original assignee: De Beers Uk Limited
Priority date: 2018-02-23
Filing date: 2019-02-20
Publication date: 2019-08-29
Also published as: GB201802965D0

Abstract

A method of determining the ultraviolet (UV) transparency of a gemstone (18, e.g. diamond) is provided. The method comprises irradiating the gemstone with at least one excitation pulse of incident UV light (by UV light source 11) and detecting UV light (by UV detector 12) that is transmitted into, internally reflected within, and returned from the gemstone (18). An image is generated using the internally reflected UV light. A UV transparency or a nitrogen content of the gemstone is determined, based upon the brightness within the image.

Description

UV TRANSPARENCY OF GEMSTONES

Technical Field

The present invention relates to a method of and an apparatus for determining the ultraviolet (UV) transparency of one or more gemstones. Where the gemstones comprise diamond, the invention relates to a method of and an apparatus for determining the nitrogen content of one or more diamonds.

Background

Diamonds are traditionally classified into different types by the presence and nature of chemical impurities at the atomic level. The most common of these impurities is nitrogen. As illustrated schematically in Figures 1 a to 1 d respectively, Type la diamonds (Figure 1 a) contain nitrogen atoms predominantly in pairs (A centres) or clusters of four around a vacancy (B centres) within the carbon lattice; Type lb diamonds (Figure 1 b) contain nitrogen atoms predominantly at single substitutional sites; Type lla diamonds (Figure 1c) contain few or no measurable impurities of any kind, and Type lib diamonds contain significant levels of uncompensated boron.

Synthetic as well as natural diamonds may comprise nitrogen; high pressure high temperature (HPHT) synthetic diamonds often contain high levels of nitrogen impurities, derived from the atmosphere and/or the graphite source. Some chemical vapour deposition (CVD) synthetics are now grown with few or no nitrogen impurities.

All diamonds i.e. Type la and Type lla have an energy bandgap of (5.46eV/225nm), which means that electron transitions between bands (valence to conduction) are possible when excited with photons at 225nm. While Type lla diamonds have no or very little impurities, Type la diamonds usually contain either single substitutional nitrogen impurities or nitrogen aggregates. These typically populate donor states in the energy gap that strongly absorbs in the UV <300nm.

Methods of determining the type ( e.g . Type la) of a cut and polished diamond via its nitrogen content are known. For example, infrared absorption spectroscopy may be used. Since nitrogen impurities also lead to absorption of light in the UV region, the measurement of UV absorption using excitation sources, including mercury (Hg) lamps and UV LEDs, in conjunction with spectrometers or photodiodes is also known.

However, known methods of measuring UV absorption or transparency are limited. Only one stone at a time can be investigated via spectroscopy, and the accuracy of the measurement is reduced by the presence of dust or dirt, or by interference from surface reflection if the stone is mounted rather than loose e.g. in a ring, necklace, bracelet or the like. Further, spectroscopic results must be interpreted by an experienced user.

Summary

In accordance with one aspect of the present invention there is provided a method of determining the ultraviolet (UV) transparency of a gemstone. The method comprises irradiating the gemstone with at least one excitation pulse of incident UV light; detecting UV light that is transmitted into, internally reflected within, and returned from the gemstone; generating an image using the returned UV light; and determining a UV transparency of the gemstone based on brightness within the image.

In accordance with another aspect of the present invention there is provided a method of determining the nitrogen content of a diamond. The method comprises irradiating the diamond with at least one excitation pulse of UV light; detecting UV light that is returned from the diamond following internal reflection; generating an image using the returned UV light; determining a UV transparency of the diamond based on brightness within the image; and determining the nitrogen content of the diamond based on the determined UV transparency.

In accordance with another aspect of the present invention there is provided an apparatus for determining the ultraviolet (UV) transparency of a gemstone. The apparatus comprises a UV source configured to irradiate the gemstone with at least one excitation pulse of incident UV light; a UV detector configured to detect UV light that is transmitted into, internally reflected within, and returned from the gemstone and generate an image of the gemstone based on the detected UV Light; and a display configured to display the image. Alternatively to, or instead of, the display, the apparatus may comprise a processor configured to identify a portion of the image corresponding to the gemstone; measure a brightness of the portion; and determine a UV transparency of the gemstone on the basis of the brightness.

In accordance with another aspect of the present invention there is provided an apparatus for determining the nitrogen content of a diamond. The apparatus comprises a UV source configured to irradiate the diamond with at least one excitation pulse of UV light and a UV detector configured to detect UV light that is internally reflected by the diamond and generate an image of the diamond based on the detected UV light. A processor is configured to identify a portion of the image corresponding to the diamond; measure a brightness of the portion; determine a UV transparency of the diamond on the basis of the brightness; and determine a nitrogen content of the diamond on the basis of the UV transparency.

Further aspects and preferred features are set out in claims 2 et seq.

Brief Description of the Drawings

Figure 1 a illustrates the atomic structure of a Type la diamond;

Figure 1 b illustrates the atomic structure of a Type lb diamond;

Figure 1 c illustrates the atomic structure of a Type lla diamond;

Figure 1 d illustrates the atomic structure of a Type lib diamond;

Figure 2 illustrates an apparatus for imaging UV light returned from a gemstone following internal reflection;

Figures 3a to 3c illustrate different types of diamond under UV illumination;

Figures 4a and 4b illustrate a correlation between UV transparency and nitrogen content of diamonds; and

Figure 5 illustrates a method of determining the UV transparency of one or more gemstones.

Detailed Description

The invention provides a simple process and apparatus for enabling a user to make a quick determination of the UV transparency of a gemstone such as diamond, even if it is in mounted in jewellery, and without the need for a UV absorption spectrometer. This makes it possible to identify the type of diamond with a reasonable degree of certainty.

Embodiments of an apparatus 10 for determining the UV transparency of one or more gemstones, particularly diamonds, will now be described with reference to Figures 2 to 4. The apparatus comprises a source of electromagnetic radiation configured to irradiate at least one gemstone with at least one pulse of electromagnetic radiation in the ultraviolet (UV) band; a UV light detector configured to detect UV light that is internally reflected through the at least one gemstone; a display for displaying a visible image produced using the detected UV light, and a processor for analysing the image.

Figure 2 illustrates an exemplary imaging apparatus 10, which is configured to irradiate the table facet of one or more gemstones 18 with UV light in an energy range below the band gap of diamond, for example 225nm < wavelength < 300nm. In one arrangement light at 250nm with 10nm FWHM, which can be considered to be“about 250nm”, may be used. The apparatus is configured to detect UV light that enters the gemstone, is internally reflected from one or more facets, and then exits the gemstone. The apparatus 10 is further configured to display an image of the irradiated stone(s) 18.

It will be appreciated that, where the gemstone is a diamond, nitrogen impurities within the diamond lattice will absorb light in the UV region, such that the intensity of UV light transmitted into, internally reflected within, and subsequently returned from the diamond is an indicator of the stone’s nitrogen content. The nitrogen A centre absorbs UV light at <300nm, with the peak in the single nitrogen absorption at around 270nm. Therefore, the more nitrogen the stone contains, the higher the level of UV absorption and the less UV light is returned by the stone. As previously discussed, a diamond’s nitrogen content is an indicator of its type.

The apparatus 10 comprises a UV light source (with a range as described above) 1 1 , a UV light detector 12 and a display 17 for displaying an image of the irradiated stone or stones 18. Further elements of the apparatus comprise one or more filters 13, lenses 14, an iris 15 and a stop 16. The UV light detector 12 is capable of detecting light at <300nm. The elements of the apparatus 10 are generally comprised in an illumination arm A and an imaging arm B. An exemplary UV source 1 1 may consist of a pulsed (10 ps) Xenon flash lamp having a typical output ranging from the deep ultraviolet (DUV) to the near infrared (NIR) i.e. from around 190 nm to around 2000 nm. Alternatively, the UV source 1 1 may comprise any UV source emitting electromagnetic radiation of <300nm, such as one or more UV LEDs or filtered mercury lamps.

The UV source 1 1 is synchronised with a UV light detector 12, such that UV light is only detected by the detector 12 during irradiation of the gemstone(s) 18. This synchronisation reduces the effect of background interference. The UV detector 12 described herein is a back-thinned CMOS (complementary metal-oxide semiconductor) camera, providing high quantum efficiency. The camera sends a trigger pulse to the UV source when the camera is in pseudo global start mode.

The configuration for irradiation/illumination of the gemstone(s) 18 may be of the Kohler type, which produces an extremely even illumination of the gemstone(s) 18 and projects a uniform image of a field iris 15 diaphragm onto the table of the irradiated gemstone(s) 18. In the exemplary apparatus of Figure 2, the imaging of the reflected UV is carried out off-axis to negate the need for a beam-splitter. However, in an alternative embodiment (not shown here) a beam-splitter may be used, although this can lead to problems with direct reflection. In one embodiment, the illumination A and imaging B arms are at a steep angle (for example about 30°) relative to one another to ensure there is no direct reflection from the table of the gemstone into the UV detector 12

The imaging can be either ento- or tele-centric, and suitable UV lenses 14 of UV grade material e.g. UV grade fused silica or calcium fluoride are used. Alternatively, some other UV catoptric or catadioptric system is used. Filters 13 are included in both illumination and imaging arms A, B, and are matched to ensure that the UV light returned from the stone(s) 18 and detected by the UV detector 12 is at the same wavelength (i.e. <300nm) and is thus due to absorption within the stone, and not photoluminescence, in which electromagnetic radiation absorbed by the stone is re emitted at different wavelengths.

As illustrated in Figure 2, in the illumination arm A, paired UV lenses 14 are used to collect as much light from the UV light source, or strobe, 1 1 as possible. The iris 15 is used to control the illumination patch size. The filter 13 is an excitation filter used to block out any unwanted light that could swamp the required signal to be captured, and the lens 14 is used to project the image of the iris 15 onto the sample ( i.e . the gemstone 18) in order to produce uniform lighting.

On the imaging arm B, an objective lens 14 is used to collect UV light returned from the sample 18. A filter 13 is used to block out any unwanted light ( e.g . fluorescence) from the sample 18 that could reduce contrast in and/or swamp the signal to be captured. A stop 16 (having an exemplary setting of F16-F32) is used to control depth of field, and a lens 14 is used to focus the light onto the imaging UV light detector 12.

In use, the apparatus 10 illuminates one or more gemstones 18, such as diamonds, with UV light at a wavelength greater than 225 nm, the UV light having passed through one or more UV lenses 14, an iris 15 and a filter 13. Depending upon their chemical composition, the one or more gemstones 18 will absorb a portion of the emitted UV light and return the remainder (following total internal reflection) along path P. The reflected UV light is detected by the UV detector 12 after passing through one or more UV lenses 14, a filter 13 and a stop 16. Detection is synchronised with the UV source 11 so that detection does not occur when the source 1 1 is not switched on. This may be achieved by configuring the UV detector 12 and the UV source 1 1 to switch on substantially simultaneously (e.g. by configuring the detector 12 to send a trigger pulse to the source 1 1 ) and/or configuring the UV detector 12 to cease detection before the UV source 1 1 switches off. For example, in global start mode, the lines of a CMOS camera expose simultaneously for a set exposure time. During this exposure time, the strobe is triggered (i.e. flashes for a short period), and the camera then reads out (i.e. produces an image).

An image (not shown here) of the illuminated stone(s) 18 is generated using the detected UV light and is displayed to a user on an associated display 17. The display 17 comprises any suitable colour or black and white screen which enables a user to view the image in sufficient detail to make a qualitative determination as to the stone’s UV transparency, based upon the extent to which, and/or the brightness of, a sparkle pattern of the UV light reflected from the internal facets of the stone appears in the image (a bright image or a clear pattern of internal facets corresponds to low UV absorption, and a dark image or few discernible internal facets corresponds to high UV absorption). The images may be continuously displayed to the user. Where the gemstones are diamonds, the degree of UV transparency can be correlated with nitrogen content, and used in a determination of the type ( e.g . Type la) of the diamond. The display 17 should be sufficiently large to image a single gemstone or multiple gemstones simultaneously. As previously discussed, the imaged gemstones may be loose or mounted stones. A series of images can be continuously displayed to a user in real time, so that the display effectively shows a video of the“brightness” or visible sparkle pattern of light reflected from the internal facets, and thus the UV transparency of one or many stones. This would enable the user to use the image or images to sort these stones“by eye” into those displaying high UV transparency (and which are thus likely to be Type II stones) and those displaying lower UV transparency (Type I stones).

In this example, the apparatus 10 additionally comprises a processor 19 which analyses the image or images of the illuminated stone(s) in order to determine a mean brightness of the table facet. The processor 19 may further classify the stone into a type on the basis of the mean brightness, as will be described in further detail below.

Figures 3a to 3c illustrate images 30 produced by the apparatus 10, described herein. The imaged stones illustrated here are cut and polished diamonds. The direct image or sparkle pattern 30a shown in Figure 3a is a result of the imaged stone returning a substantial proportion of the UV light emitted by the UV source, indicating that the imaged stone contains low quantities of nitrogen. The image 30a brightness is a function of the nitrogen content of the imaged stone. Thus, a user viewing the image 30a can determine that the stone is likely to be a Type II diamond with little or no nitrogen impurities.

The image 30b shown in Figure 3b is a result of the imaged stone returning less of the UV light emitted by the UV source, indicating that the imaged stone contains higher quantities of nitrogen than the stone of Figure 3a. Further, the UV light is returned by particular sectors of the stone. A user viewing the image 30b can determine that the stone is likely to be a Type I diamond, and possibly a Type la stone, wherein the nitrogen impurities occur as A or B clusters.

The image 30c shown in Figure 3c is a result of the imaged stone absorbing substantially all of the UV light emitted by the UV source, such that substantially no UV light is returned. In other words, the imaged stone is opaque rather than transparent to UV light. This indicates that the imaged stone contains higher quantities of nitrogen than the stone of Figure 3b. A user viewing the image 30c can determine that the stone is likely to be a Type I diamond.

It will be appreciated that using the exemplary apparatus 10 described herein, multiple gemstones within a tray or other support arrangement can be imaged simultaneously. This includes gemstones that are mounted in jewellery, such as rings, necklaces, bracelets etc., watches or in other objects. Background noise is reduced by synchronisation of the UV source 1 1 and UV detector 12. The use of direct imaging enables a user who is unfamiliar with spectroscopic techniques to determine the UV transparency of a gemstone, and where that gemstone comprises diamond, to use the image or images to visually distinguish one type of diamond from another, based upon the presence/brightness of internally reflected UV light. Further, the apparatus 10 is less complex and therefore less costly than the apparatus required by known techniques.

Furthermore, analysis can be carried out on the images produced. In the arrangement shown in Figure 2, the images generated by the UV detector include the table facet of a cut gemstone and can be analysed by the processor to identify the portions therein corresponding to the table facet of the stone (or of each stone, if there is more than one). Therefore, each image portion corresponding to a table facet can be further analysed to identify the brightness of that portion, and a more quantitative estimation of transparency can be made from that brightness.

This can be understood from Figures 4a and 4b. Figure 4a shows twelve images of diamond gemstones having increasing nitrogen content from left to right. The two stones on the left are Type lla stones and the remainder are Type la stones. Figure 4b is a bar chart showing the mean signal intensity ( i.e . brightness) of the portion of each image corresponding to the table facet of the stone in that image. It is apparent that the two Type lla stones on the left in Figure 4a are much brighter than the other stones. Such image analysis could be used to make an automatic classification of each stone, based on the mean brightness of the table region of its image. This may be done using a pre-determined brightness threshold, for example. A method of classifying one or more gemstones (in particular diamonds) using an imaging technique, will now be described with reference to Figure 5. The method comprises irradiating at least one gemstone with at least one excitation pulse of UV light (S1 ); detecting UV light that is internally reflected by the at least one gemstone (S2); and generating an image using the reflected UV light (S3). The image may be displayed to the user (S4) and/or analysed to determine a mean brightness of the, or each, table portion of the gemstone (S5). The brightness perceived by the user and/or calculated from the analysis may then be used to classify the stone on the basis of its nitrogen content (S6). Where present, analysis based upon the image may be carried automatically out by a processor and a classification of the stone into a type ( e.g . Type I la) may be presented to the user.

It will be appreciated by the person skilled in the art that various modifications may be made to the above described embodiments, without departing from the scope of the present invention. For example, although the methods and apparatus herein are described with specific reference to diamonds, they can also be used to determine the type of other gemstones.

The term“natural” is used herein to indicate a stone from nature consisting exclusively of diamond produced by geological processes i.e. a non-synthetic stone.

The term “synthetic” is used herein to indicate a man-made stone consisting exclusively of diamond produced by artificial or industrial processes, such as chemical vapour deposition or high pressure high temperature processes.

The term“treated” is used herein to indicate a natural stone, as described above, which has been modified in order to improve its colour or clarity, for example by chemical or mechanical means, by irradiation or by pressure or heat treatments.

As used herein, “type” is defined using the standard diamond classification system which separates stones based on their physical and chemical properties, e.g. Type la, Type lib etc.

Claims

CLAIMS:

1. A method of determining the ultraviolet (UV) transparency of a gemstone, the method comprising:

irradiating the gemstone with at least one excitation pulse of incident UV light; detecting UV light that is transmitted into, internally reflected within, and returned from the gemstone;

generating an image using the returned UV light; and

determining a UV transparency of the gemstone based on brightness within the image.

2. A method as claimed in claim 1 , further comprising displaying the image to a user.

3. A method as claimed in claim 2, wherein:

the gemstone is irradiated with a series of pulses;

an image is obtained corresponding to each pulse; and

the images are displayed to the user continuously.

4. A method as claimed in claim 1 , 2 or 3, comprising synchronising irradiating and detecting by beginning irradiation and detection substantially simultaneously for the or each pulse.

5. A method as claimed in any preceding claim, comprising applying wavelength filtering to the incident UV light and/or to the returned UV light.

6. A method as claimed in claim 5, wherein the light is filtered to allow passage of the returned UV light and the incident UV light in the same wavelength range.

7. A method as claimed in claim 6, wherein the light is filtered to allow passage of light having a wavelength less than about 300 nm.

8. A method as claimed in any preceding claim, further comprising:

analysing the image to identify a portion of the image corresponding to the gemstone; determining the mean brightness of the image in the at least one portion; and determining a UV transparency of the gemstone on the basis of the mean brightness.

9. The method of claim 8, wherein the step of analysing the image to identify a portion of the image corresponding to the gemstone includes identifying a table portion of the image corresponding to a table facet of the gemstone, and the UV transparency is determined based on the mean brightness of the table portion.

10. A method as claimed in any preceding claim, comprising irradiating a plurality of gemstones with the least one excitation pulse of incident UV light, such that the image includes the plurality of gemstones.

1 1. A method as claimed in any preceding claim, wherein the gemstone is a diamond, and the method further comprises classifying the diamond by nitrogen content based on the determined UV transparency.

12. A method as claimed in any preceding claim, wherein the gemstone is mounted.

13. A method as claimed in any preceding claim, wherein the at least one excitation pulse of UV light comprises electromagnetic radiation at a wavelength of between about 225nm and about 300nm.

14. A method of determining the nitrogen content of a diamond, the method comprising:

irradiating the diamond with at least one excitation pulse of UV light;

detecting UV light that is returned from the diamond following internal reflection; generating an image using the returned UV light;

determining a UV transparency of the diamond based on brightness within the image; and

determining the nitrogen content of the diamond based on the determined UV transparency.

15. An apparatus for determining the ultraviolet (UV) transparency of a gemstone, the apparatus comprising:

a UV source configured to irradiate the gemstone with at least one excitation pulse of incident UV light;

a UV detector configured to detect UV light that is transmitted into, internally reflected within, and returned from the gemstone and generate an image of the gemstone based on the detected UV Light; and

a display configured to display the image.

16. An apparatus for determining the ultraviolet (UV) transparency of a gemstone, the apparatus comprising:

a processor configured to:

identify a portion of the image corresponding to the gemstone;

measure a brightness of the portion; and

determine a UV transparency of the gemstone on the basis of the brightness.

17. An apparatus as claimed in claim 16, wherein the processor is configured to identify a table portion of the image corresponding to a table facet of the gemstone and determine a UV transparency of the gemstone on the basis of the brightness of the table portion.

18. An apparatus as claimed in claim 15, 16 or 17, wherein the UV source and UV detector are synchronised so that detection of returned light takes place only during the at least one pulse.

19. An apparatus as claimed in any of claims 15 to 18, comprising one or more wavelength filters configured to apply wavelength filtering to the incident UV light and/or the returned UV light.

20. An apparatus as claimed in any of claims 15 to 19, wherein the UV source comprises one of: a Xenon flash lamp, one or more UV LEDs, and a filtered mercury lamp.

21 . An apparatus as claimed in any of claims 15 to 20, wherein the UV detector comprises a CMOS camera and the CMOS camera triggers the UV source.

22. An apparatus for determining the nitrogen content of a diamond, the apparatus comprising:

a UV source configured to irradiate the diamond with at least one excitation pulse of UV light;

a UV detector configured to detect UV light that is internally reflected by the diamond and generate an image of the diamond based on the detected UV light; and a processor configured to:

identify a portion of the image corresponding to the diamond;

measure a brightness of the portion;

determine a UV transparency of the diamond on the basis of the brightness; and

determine a nitrogen content of the diamond on the basis of the UV transparency.

23. An apparatus as claimed in claim 22, wherein the processor is further configured to classify the diamond by the determined nitrogen content.