Optics: measuring and testing – Crystal or gem examination
Patent
1988-11-15
1990-02-13
Evans, F. L.
Optics: measuring and testing
Crystal or gem examination
356301, G01J 344, G01N 2165, G01N 2187
Patent
active
049001477
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to diamond mapping and in particular to a method for mapping the crystal structure of a diamond.
Diamonds have long been recognised as being of great value not only for decorative and industrial purposes but also as an investment. Their increasing value has presented problems of security for owners, insurance companies and police authorities. Identification of a recovered diamond which has been lost or stolen is made difficult because the superficial appearance of the diamond may have been changed by cutting, repolishing and the like. Conventionally, the recognition of diamond has been achieved on the basis of a record of characteristics comprising carat weight, cut, colour type, clarity etc.
More recently, X-ray topography has been used for identifying diamonds, as is disclosed in UK Patent No. 1,547,371. In this technique a set of records is produced by X-ray topography to provide an overall point-by-point three-dimensional representation of the diamond. The set of records of a recovered diamond that has been lost or stolen may be compared with the sets of records of known diamonds as a means of identifying the recovered diamond. However, this technique may be slow and requires interpretation of the diffraction patterns to produce the sets of records of defects.
The Raman signal of diamond is much stronger than that of other materials because diamond only contains carbon to carbon bonding and its Raman signal occurs at a position well separated from those of other minerals. Also, as diamond only contains one type of carbon to carbon bond, there is only a single Raman signal which can be readily distinguished from associated broad band fluorescence. Thus the Raman signal is highly specific for diamond. The intensity of the Raman signal is affected by the crystal structure of the diamond and hence by the presence or otherwise of imperfections or inclusions in the diamond. For example, it has been found that imperfections cause a broadening of the diamond Raman signal and inclusions do not give a diamond Raman signal.
Thus according to the present invention there is provided a method for mapping the crystal structure of a diamond, the method comprising the steps of (a) placing the diamond in a beam of monochromatic laser radiation capable of causing Raman radiation to be scattered from the diamond, (b) passing the scattered Raman radiation from the diamond through a filter adapted to pass only scattered Raman radiation characteristic of diamond, and (c) measuring the intensity of the filtered Raman radiation at one or more different orientations of the diamond.
Also, according to the present invention there is provided a method for producing a record of a diamond, the method comprising the steps of (a) placing the diamond in a beam of monochromatic laser radiation capable of causing Raman radiation to be scattered from the diamond, (b) passing the scattered Raman radiation from the diamond through a filter adapted to pass only scattered Raman radiation characteristic of diamond, (c) measuring the intensity of the filtered Raman radiation, and (d) recording the intensity of the filtered Raman radiation at one or more different orientations of the diamond.
The invention also includes records whenever produced by the method as hereinbefore described.
According to the present invention there is also provided an apparatus for producing a record of a diamond, the apparatus comprising in combination (a) means for holding the diamond in a beam of monochromatic laser radiation capable of causing Raman radiation to be scattered from the diamond, (b) means for filtering the resultant scattered Raman radiation, said filter being adapted to pass only scattered Raman radiation characteristic of diamond, (c) means for measuring the intensity of the filtered Raman radiation, and (d) means for recording the intensity of the filtered Raman radiation at one or more different orientations of the diamond to produce a record of the diamond.
Preferably, the Raman intensities are recorded at three mut
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patent: 4397556 (1983-08-01), Muller
Field, J. E., The Properties of Diamond, "Colour Centres and Optical Properties" (Academic Press, 1979) pp. 23-77.
"Image Visualization", Physical Encyclopaedical Dictionary, Moscow Soviet Encyclopaedia (1983), p. 76. (with translation).
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Long, D. A., "The Renaissance of Raman Spectroscopy", Chemistry in Britian (Jun. 1989), pp. 589-590.
Long, D. A., "Linear and Nonlinear Raman Effects: The Principles", Chemistry in Britain (Jun. 1989), pp. 592-596.
Corset, J. et al., "Raman Microscopy", Chemistry in Britain, (Jun. 1989), pp. 612-616.
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Bowley Heather J.
Gerrard Donald L.
Curatolo Joseph G.
Evans F. L.
Evans Larry W.
Moxon II George W.
The British Petroleum Company p.l.c.
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