Optics: measuring and testing – Crystal or gem examination
Patent
1987-08-06
1989-10-24
Evans, F. L.
Optics: measuring and testing
Crystal or gem examination
356301, G01J 344, G01N 2165, G01N 2187
Patent
active
048757713
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the assessment of diamond quality and more particularly relates to the assessment of diamond quality by means of Raman spectroscopic techniques.
The quality of valuable gems such as diamonds is generally determined in a subjective manner. Thus, a person allegedly expert in the art will examine the gem by eye and then express an opinion on the quality and value of the gem. However, it frequently occurs that the quality of the gem is differently judged by different persons. Also the procedure is slow, requires considerable skill and in view of its subjectively is not always consistent.
Laser Raman spectroscopy may be used for the separation of diamonds from a diamondiferous material and is disclosed in our co-pending GB patent application No. 2140555A. The method comprises the steps of passing discrete units of gangue through a beam of laser radiation capable of causing Raman spectral activation, detecting the scattered Raman radiation by means of a detector, the detector being adapted to actuate means for separating discrete units of diamond containing material from the discrete units of non-diamond containing material and collecting the separated discrete units.
The Raman signal of diamond is much stronger than that of other materials because diamond only contains C--C 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 C--C bond, there is only a single Raman signal which can be readily distinguished from associated broad band fluorescence.
It is an object of the present invention to provide means for effectively assessing the quality of a diamond in an objective manner so as to avoid resorting to subjective judgement by a human observer and it has now been found that laser Raman spectroscopy may be used as a basis for assessment or classifying diamonds of differing quality.
Thus according to the present invention there is provided a method for assessing diamonds of differing quality comprising the steps of (a) calibrating a laser Raman spectrometer with diamonds of known quality characteristics, (b) placing a diamond of unknown quality characteristic in a fixed orientation, (c) passing incident laser radiation of known frequency and intensity onto the diamond, and (d) monitoring the intensity of the scattered Raman signal for one or more orientations of the diamond of unknown quality characteristic.
It is preferred that the diamonds of differing quality are of a similar size range and thus it may be necessary to separate the diamonds into size ranges prior to using the method as hereinbefore described.
A laser Raman spectrometer adapted to emit a single frequency of radiation is calibrated by monitoring the intensity of the scattered Raman signal for a series of diamonds of known but differing quality. Preferably the diamonds of known but differing quality are of a single colour type e.g. white, yellow, green. The diamonds of known but differing quality may be assessed by conventional subjective means e.g. by use of an expert observer.
The diamonds used for calibration and those the quality of which are to be determined are mounted in a suitable holder and are assessed individually. Preferably the orientation of the diamonds with respect to the incident laser radiation is similar in each case but the method is not as sensitive to this requirement as certain other techniques.
The source of laser radiation is adapted to operate in a single frequency mode. The frequency of the radiation chosen is dependent on the colour type of the diamond being assessed.
The scattered Raman radiation from the diamond being assessed is filtered from other types of radiation by a suitable optical arrangement such as a collection optic and monochromator. A detector such as a photomultiplier or multichannel detector (e.g. diode array detector) may be used to detect the scattered Raman radiation, the intensity of the scattered radiation being capable of
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S. A. Solin and K. A. Ramdas, Raman Spectrum of Diamond, Physical Review vol. 1, #4, pp. 1687-1698.
Bowley Heather J.
Gerrard Donald L.
Evans F. L.
The British Petroleum Company p.l.c.
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