Optics: measuring and testing – Crystal or gem examination
Patent
1988-09-12
1990-03-13
Evans, F. L.
Optics: measuring and testing
Crystal or gem examination
209581, 356301, G01J 344, G01N 2165, G01N 2187
Patent
active
049078755
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the assessment of diamond colour and more particularly relates to the assessment of diamond colour by means of laser Raman spectroscopic techniques.
The colour of valuable gems such as diamonds is generally determined in a subjective manner. Thus a person allegedly expert in the art will examine the diamond by eye and then express an opinion as to its colour. However, the procedure is slow, requires considerable skill and in view of its subjectivity could be open to error.
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. Thus the Raman signal is highly specific for diamond. Also, because 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.
Laser Raman spectroscopy may be used for the separation of diamonds from a diamondiferous material and a method using this technique is disclosed in our UK patent no. GB 2140555B. Laser Raman spectroscopy may also be used for assessing the quality of a diamond in an objective manner and a method using this technique is disclosed in our PCT patent application no. WO 87/03963. The method comprises the steps of calibrating a laser Raman spectrometer with diamonds of known quality characteristics, placing a diamond of unknown quality characteristic in a fixed orientation, passing incident laser radiation of known frequency and intensity onto the diamond, and monitoring the intensity of the scattered Raman signal for one or more orientations of the diamond of unknown quality.
It is an object of the present invention to provide means for assessing the colour type of a diamond in a less subjective manner than the known method of judgement by a human observer and it has been found that laser Raman spectroscopy may be used as a basis for an improved method of assessing the colour type of diamonds.
Thus according to the present invention there is provided a method of assessing the colour type of diamonds comprising the steps of (a) passing incident laser radiation of pre-determined intensity at two or more different, pre-determined wavelengths onto a diamond, the laser radiation being capable of causing Raman radiation to be scattered from the diamond, (b) measuring the intensity of the scattered Raman radiation from the diamond for each of the incident laser radiation wavelengths, (c) generating a numerical value characteristic of the colour type of the diamond based on the measured intensities of scattered Raman radiation for the two or more different incident laser radiation wavelengths.
According to a further aspect of the invention there is provided an apparatus for assessing the colour type of diamonds comprising, (a) means for irradiating a diamond with laser radiation of pre-determined intensity at two or more different, pre-determined wavelengths, the laser radiation being capable of causing Raman radiation to be scattered from the diamond, (b) means for measuring the intensity of the scattered Raman radiation from the diamond for each of the incident laser radiation wavelengths, (c) means for generating a numerical value characteristic of the colour type of the diamond based on the intensities of scattered Raman radiation for the two or more different incident laser radiation wavelengths.
It is preferred that the diamonds are of similar size range and thus it may be necessary to sort diamonds into size ranges prior to using the method and apparatus according to the present invention. The present invention may be used for the assessment of both cut and uncut diamonds.
The diamond may be irradiated with laser radiation of two or more wavelengths, either simultaneously or sequentially.
The source of laser radiation may be a single laser adapted to operate at two or more discrete, pre-determined wavelengths, simultaneously or sequentially. Alternatively two or more
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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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