IL108478A - Method and apparatus for examining a gemstone - Google Patents

Description

9 3 4 5 "METHOD AND APPARATUS FOR EXAMINING A GEMSTONE" "in i-W ηρ»τΰί ipnni no*©" THE APPLICANT:-GERSAN ESTABLISHMENT, A LIECHTENSTEIN COMPANY, STAEDTLE 36, 9490 VADUZ, LIECHTENSTEIN.

THE INVENTORS: 1. SMITH, MARTIN PHILLIP 18, HAMILTON ROAD, WARGRAVE, BERKSHIRE RGIO SEE, ENGLAND 2. SMITH, ROBIN WYNCLIFFE 79, OUTRAM ROAD, CROYDON, SURREY CRO 6XJ, ENGLAND 3. WELBOURN, CHRISTOPHER MARK TYTHEBARN COTTAGE, BREADCROFT LANE, LITTLEWICK GREEN, MAIDENHEAD, BERKS, SL6 SOP ENGLAND METHOD AND APPARATUS FOR EXAMINING A GEMSTONE Background of the Invention This is a first division from Patent Application No. 097947.

In general terms, the invention relates to ..examining or classifying a gemstone by detecting the spectral properties of the gemstone. The invention is particularly concerned with identifying gemstones such as diamonds, e. g. distinguishing diamonds from diamond-like simulants WO 86/07457 discloses a method for distinguishing diamond from diamond-like simulant, by visually detecting the Raman signal emitted from a specimen which is irradiated with suitable exciting radiation. The Raman emission has two peaks, one on either side of the wavelength of the exciting radiation, termed the Stokes signal and the anti -Stokes signal. The Stokes signal is much stronger than the anti-Stokes signal, but it is still very weak. One of the problems is that if a diamond-like simulant luminesces, it is very hard to discern the appropriate Raman peak against the luminescent background.

Diamond simulant comprises dense non-diamond material (eg. metal, oxides, particularly zirconium dioxide) which has similar refractive properties to diamond.

The Invention.

The invention provides a method of and apparatus for examining a gemstone as set out in Claims 1, 2, 11, 12, 16 or 17. Preferred and/or optional features are set out in Claims 3 to 10 and 13 to 15.

At least in its preferred forms, the invention enables diamonds and other suitable gemstones to be examined and classified by operators with little scientific or technical training.

According to the invention, the object is irradiated with stimulating radiation and the emission/luminescence of the object is examined.

In order to distinguish diamond from diamond simulant, it is preferable to use a laser that will cause Raman activation in the visible spectrum; a suitable Raman wavelength is about 552.4 nm which can be produced by an argon ion laser operating at 514.5nm, and in general terms the laser wavelength may lie between 450 nm and 1064 nm, but may be outside this range.

As the band passed by the narrow band filter is altered, the signal detected will be markedly different, depending on whether the gemstone is say a diamond, or a diamond simulant which does not exhibit Raman emission at the correct wavelength, and does not luminesce, or a diamond simulant which luminesces. This is explained later with reference to the drawings.

Though it may be possible to use other methods, a simple method of altering the band passed is by tilting the filter about an axis normal to its optical axis. With narrow band pass filters, the cut-offs are most clearly defined when the filter is correc ly orientated with its optical axis; as the filter is tilted, the centre of the band passed changes, and the band widens - this widening is not essential to the preferred embodiment of the invention, but is an incidental effect.

The apparatus may be very simple to use and construct, as it only has a small number of components. The whole apparatus may only occupy a space of about 25 x 10 x 15 cm, being suitable for use on a bench top. The method does not require any great skill on the part of the operator and is suitable for producing an answer very quickly.

The Drawings.

The invention will be further described by way of example with reference to the accompanying drawings, in which: - Figure 1 is a schematic side view of apparatus accordance with the invention; Figure 2 illustrates the bands passed by the two filters in Figure 1; Figures 3a to 3 illustrate what occurs in figure 1 when a diamond with no luminescence is examined, showing the signal position, the no signal position, and the result of rocking; Figures 4a to 4c correspond, but show the situation for a diamond with luminescence; Figures 5a to 5c correspond, but show the situation with a diamond simulant without luminescence; and Figures 6a and 6c correspond, but show the situation with a diamond simulant with luminescence.

Figure 1 shows a stone 1 on a dop 2, the stone 1 being illuminated with a laser 3 which may be an argon ion laser operating a 514.5nm (shown as 9 in Figure 2), thereby stimulating luminescence of the stone 1 if the stone 1 is capable of it at the stimulating wavelength. The stone 1 is viewed by eye through a laser blocking filter 4 which rejects light at laser wavelength (for example a Schott OG 530 rejects the laser light at 514.5nm) and a narrow band pass filter 5, for example a lnm F HM 4 cavity design centred at 552.4 nm, manufactured by the Andover Corporation in the USA. The narrow band pass filter 5 can be tilted about an axis 6 normal to its optical axis, and a drive 7 is shown for rocking or oscillating the filter 5.

The band passes of the narrow band pass filter 5 are shown in Figure 2, which shows graphs of transmissivity τ against wavelength λ for incident light parallel to and for incident light inclined to the optical axis of the filter 5. The continuous lines indicate the bands passed when the filter is set up properly, normal to the optical or viewing axis. The filter 5 passes a narrow band of about 1 nm (measured at the height of half maximum transmissivity), centered on the Raman emission (Stokes) at 552.4 nm. It can be seen that with this set-up a narrow band of detectable radiation wavelengths can pass - in this case, detectable radiation is radiation that can be detected by the eye.

As the filter 5 is tilted about the axis 6, its optical characteristics alter in that its cut-on moves to lower wavelengths and the band passed widens - this is illustrated in the dashed lines in Figure 2.

The filter 5 can be set-up so that it is rocked from a first setting represented by the continuous line in Figure 2 to a second setting represented by the dashed line in Figure 2. This rocking can be done by hand, or, as indicated in Figure 1, by a motor drive 7.

Figures 3a-3c illustrate what occurs when the stone 1 is a non-luminescing diamond. Figures 3a and 3b are graphs of τ against λ, while Figure 3 is a graph of intensity i against time t. The Stokes emission (illustrated as the peak in the emission 8) is passed when the filter 5 is in its first setting, and thus there is a signal (Figure 3a). When the filter 5 is in its second setting, the Stokes emission 8 is blocked and there is no signal (Figure 3b). As the filter 5 is rocked between the first and second settings, there is a an approximately square-wave signal with no dc shift, i.e. the troughs are at zero intensity (Figure 3c).

If the stone 1 is a diamond with luminescence, there will be a signal when the filter 5 is at normal incidence (Figure 4a) comprising the Raman and luminescence background. When the filter 5 is tilted (Figure 4b) a signal corresponding to luminescence background is produced . As the filter 5 is rocked between the first and second settings, there is an approximately square-wave signal with the troughs at non-zero intensity (Figure 4c).

If the stone 1 is a diamond simulant with no' luminescence, there is no signal at all (Figures 5a to 5c).

If the stone 1 is a diamond simulant with luminescence, in the first setting of the filter 5 there will be a signal. However, as the filter 5 is gradually moved from the first setting to the second setting, its band pass width slightly increases whilst its peak transmission drops slightly; in many cases, to a good first approximation the total integrated light transmission is unchanged and the total signal from the background luminescence spectrum is approximately constant. Thus, the signal is as in Figure 6c.

Instead of the human eye, a suitable photodetector 10 can be used. The photodetector can be associated with processing equipment to distinguish between the types of signals shown in Figures 3 to 6 inclusive.

The present invention has been described above purely by way of example, and modifications can be made within the invention.

Claims (17)

Claims

1. A method of examining a gemstone, comprising: irradiating the gemstone; observing the gemstone through a narrow band pass filte that passes radiation of a first wavelength corresponding substantially to a Raman emission of diamond; altering the wavelength passed by the filter; observing the object at. at least one reference wavelength; and comparing the observations at the first and reference wavelengths and thereby classifying the object as diamond or not.

2. A method of examining a gemstone, comprising: irradiating a gemstone; viewing the light emitted by the gemstone through two band pass filters in series, one of which passes a substantially narrower band than the other, in order to observe the gemstone at s Raman emission wavelength of diamond; altering the band passed by the narrower band filter by tilting it about an axis so that at. first setting radiation of the Raman wavelength passes through the narrower band filter and the broader band filter, and at a second setting, the radiation of the Raman wavelength cannot pass through the narrower band filter; and detecting the change in radiation passing through both filters as the narrower band filter is altered, and thereby classifying the gemstone as r'iamond or not.

3. The method of Claim 1, wherein the gems tone is viewed through a broad band pass filter which passes radiation of the first wavelength and the reference wavelength.

4. A. The method of Claim 2 or 3, wherein the broad band pass filter is a laser blocking filter, the gemstone being irradiated using laser radiation.

5. The method of Claim 2, wherein the gemstone is irradiated with stimulating radiation and the luminescence of the gemstone is examined.

6. The method of any of the preceding claims, wherein the narrow band filter is tilted about an axis normal to its optical axis to alter the band of radiation passed by the filter.

7. The method of any of the preceding Claims, wherein the narrow^band filter is tilted to and fro between a first position and a second position.

8. The method of Claim 7, wherein the first position corresponds to the axis of the narrower band filter being normal to the optical axis.

9. The method of any of the preceding claims, wherein the Raman emission is visible light.

10. The method of Claim 9, wherein the gemstone is observed by eye..

11. 1.1. Apparatus for examining a gemstone, comprising: means for irradiating the gemstone; a narrow band pass filter which passes radiation at a first wavelength corresponding substantially to a Raman emission of diamond; means for altering the band passed by the filter to a reference wavelength different from the first wavelength; means for observing the gemstone through the narrow band pass filter, whereby the gemstone can be .classified as diamond or not.

12. Apparatus for examining a gemstone, comprising: means for irradiating the gemstone; two band pass filters, one of which passes a substantially narrower band than the other, through both of which in series the gemstone may be observed; means for altering the band passed by the filter which passes the narrower band, so that at a first setting the narrower band filter passes radiation of a Raman emission wavelength of diamond and at a second setting the narrower band filter passes radiation of a second wavelength different to the Raman emission wavelength, the Raman wavelength and second wavelength passing through the broader band filter, whereby the gemstone may be classified as diamond, or not.

13. The apparatus of Claim 11 or 12, wherein the narrow band filter is tilted about an axis normal to its optical axis to alter the band passed.

14. The apparatus of Claim 11, 12 or 13 wherein the narrow band filter passes radiation of the Raman emission wavelength when it is normal to the optical axis.

15. The apparatus of any of Claims 11 to 14, configured to carry out the method of any of Claims 3 to 10.

16. method of examining a gemstone as defined one of the Claims 1 through 10 and substantially as herein described with reference to the accompanying drawings.

17. Apparatus for examining a gemstone, as defined in one of Claims 11 through 15 and substantially as herein described with reference to the accompanying drawings. For the Applicants, Patent Attoreny