Patent GB-1416568-A (GB)
Title: Method of and apparatus for evaluating registering and  identifying gemstones
IPC: G01N21/87
Filed: 1973-10-19
Abstract: 1416568 Gemstone parameter measuring system S S WILSON 19 Oct 1973 [20 Oct 1972] 48876/73 Heading G1A Gemstones are measured, registered and identified by means of a central data processor connected to a plurality of terminals each of which can develop signals representing given parameters of the gemstone. The terminals are connected to the central processor over telephone lines and each contains a weighing station and a dimension and optical parameter measuring device connected to a terminal computer which is also provided with an output printer a CRT display and a typewriter. The dimension and optical parameter measuring device comprises a platform on which the gemstone (exemplified as a brilliant cut diamond) is placed on its table facet. The device is provided with a sequential timer which controls the performance of the following operations. An iris diaphragm (104) is closed round the gemstone to centre it on the platform and to measure its girdle diameter by means of a sensor (109). A probe (114) is lowered into contact with the cullet facet to provide a height measurement by means of a second sensor (117) and the iris is then retracted followed by the probe (114). The platform is then set into rotation. Index facet identification. An infrared laser source 83 is energized to illuminate the gemstone through its table facet the light being internally reflected and refracted in such a way that it emerges from the bezel facets. The platform 39 comprises two sheets of glass sandwiching a reflective layer (128) which reflects the emerging light upwards to a photo-detector array 91 formed as a part conical surface. The array 91 comprises a masking plate having a plurality of slits in ten columns (sectors) of twelve slits arranged so that each slit corresponds to a different position of the reflected beam. Behind each slit is mounted a photo-detector and as the gemstone rotates the highest photo-detector energized in any sector is noted and the facet producing this signal is used as the index facet to which the subsequent measurements are related. The signals from the photodetectors are sequentially scanned to identify which sector and which cells within the sector are illuminated at any instant the signals being passed through a discriminator to eliminate the effects of weak images produced by other facets. Bezel facet angle. The same optical system is then used to determine the angle of each of the bezel facets in turn. As the image of each facet traverses the detector array the position within the sectors of the highest and lowest photo-detectors illuminated are stored in two registers (176, 177) and the position of the sectors containing these detectors are stored in two further registers (182, 183). A coarse value of bezel angle is obtained by subtracting the detector position co-ordinates from one another, dividing by two and adding the position co-ordinate of the lower detector. This value is then corrected for the size of the gemstone by adding a signal which is a function of the previously measured girdle diameter and the resulting value may be offset by the value which would be obtained from a perfect gemstone. A fine connection value is then obtained from the stored sector co-ordinates and is added to produce a measure of the bezel facet angle. Pavilion facet angle. A second layer (84) is then energized to illuminate the top portion of the gemstone from above at an angle close to the vertical and light reflected from the pavilion facets is received by a second photo-detector array 92 constructed in a similar manner to and used in the same way as the first array 91 to determine the pavilion facet angles. Colour. A source 86 is energized to emit IR radiation at a wavelength of 7.8Á which is absorbed to a greater extent by the nitrogen content of the diamond than is the radiation at 0.9Á emitted by source 83 which is also utilized. A chopper 204 periodically interrupts the beam from source 86 and at these times source 83 pulsed at  high frequency. The radiation from sources 86 and 83 is therefore directed alternately on to the table facet of the gemstone, emerges from the cullet facet 72 and is received by a detector assembly 93 comprising two photo-detectors in tandem the first detector 208 being more sensitive to 7.8Á radiation and transparent to 0.9Á radiation and the second detector 209 being more sensitive to 0.9Á radiation. The output of the detectors are compared to provide a signal proportional to the nitrogen content and this is normalized by dividing by the height of the gemstone as previously determined to produce a signal representing the colour. The system is calibrated in the absence of a gemstone to produce a zero colour signal. Clarity. A further source (87) illuminates the gemstone from above with a divergent beam centred approximately normal to the pavilion facets so that any flaws scatter light down through the table facet and via a half silvered mirror 248 to a photo-detector 243. The gemstone is rotated and at the same time a radial scan is produced by lateral movement of the assembly 253 carrying mirror 248 and detector 243. The plane within the diamond which is being scanned is selected by an adjustable focus system 251 and by this means the whole volume of the gemstone is scanned and the co-ordinates of any flaws stored.  Analogue signals produced in the various measurements are converted to digital form and stored for display and transmission to the central data processor which can be used to register the parameters and record the history of individual stones and identify stolen gemstones.