Main Angles の組み合わせと Brilliancy
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概要
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In GIA's diamond grading system, Tolkowsky cut is taken as standard, and the deviations from this are used to evaluate the proportions of a test stone. It is a general practice to measure the proportions by visual observation. For instance, a pavilion proportions is evaluated by visually measuring the ratio of the radius of circumcircles of reflection of table facet on the pavilion surface and that of the table area (Fig.1, P'G:P' C), when viewed face-up^<(1)>.Effect of the crown main facet angle and the table percentage are not taken into consideration in this treatment, in spite of that these factors give significant effects upon the ratio. Therefore the method is not only too crude to properly estimate the proportion but also principally incorrect. This is explained in Fig.2 and 3. The author previously^<(2)> proposed the use of F-values to evaluate brilliancy. Other factors such as dispersion, scintillation, were also analysed on this basis. From the analyses, he proposed F-cut(40.75°) as an ideal cut, and compared it with Tolkowsky cut. In this paper, it is demonstrated how true brilliancy can be evaluated by using the F-values. The F-values are calculated and diagramatically presented in relation to various combinations of main facet angles of pavilion (α) and crown (β) (Fig.4, which was drawn using the values given in Table 1) . It is shown that the F-cut gives the best brilliancy (Table 2) . The dispersion,dispersion area and volume are also analysed so that the grading of brilliancy can be improved (Fig.5,6,7). An Appendix is given to demonstrate how the practice in GIA method by visually measuring the table reflection in the area of pavilion may lead to an erroneous result in evaluating the proportions. The table reflection corresponds to the area of S_1(P'G, Fig.1), and only loosely related to real proportion. Similarly, the dark center seen in a lumpy stone can be a mere coloured incident ray from the dark surface of a magnifying lens (Fig.8), and not due to the so-called massive unplanned light leakage. These can be shown by a simple experiment.
- 宝石学会(日本)の論文
- 1991-07-30
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