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Figure 7 shows, in sequence, the appearance of a diamond as the low-, medium- and high-illumination ray angles are removed. The first row of images (a, b, and c) are actual
photographs; the second row (d, e and f) are ray-tracing simulations. There is not complete agreement between the images because of a small error in acquiring the gem’s geometry as represented by the modeling abilities of the software
package. Nevertheless, the images show how powerful ray tracing can be as a tool for the study of light behavior in gems. The colorful image in Fig. 7(g) is an actual photograph that for purposes of
evaluation combines the low (blue), medium (red) and high (black) angles. Color hues are caused by the multiple ray refractions and by Fresnel ray splitting. This image shows a balanced distribution of each angular range in the gem crown so that light return and contrast are maximized to produce
brilliance — the signature of a superior stone.
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Gemstone fabrication and characterization technology have made important
advances. Today, some cutters can produce stones in which each facet can be
located to less than 0.01 mm and to 0.2°, a remarkable achievement. And
variations on this order can indeed change a gemstone’s appearance. Sarin
Technologies Ltd.16 produces a machine that is capable of acquiring gem
proportions, facet angles and locations, and generating CAD files that can be
imported into optical design and analysis programs.
An elegant way to test a gem is the Firescope, invented in Japan in 1984.17
The Firescope is a simple device that uses a small cap, painted red on the inside, a
diffused white-light source and a 10X eyepiece [see Fig. 8(a)]. The Firescope
does not measure fire but instead shows the gem’s ability, per crown zone, to
bring light to an observer’s eye [see Fig. 8(b)]. Under a Firescope, light leakage
through the pavilion appears white and high angles appear black. Red or red
hues indicate that the stone can bring different amounts of light from a given
angular range. Both the spatial view of a gem and the angular spectrum are
complementary representations. There is not a one-to-one mapping between
these representations because, given Fresnel reflections, more than one ray
direction can light a facet.
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Progress in gemstone CAD modeling, precision facet cutting and optical
characterization are bringing new excitement to the gemstone industry and to the
optical design community. Thanks to these advances, jewelers are designing,
enhancing and creating more beautiful gems. There are still many challenges to
be met in areas including the automatic optical design of gems, grading, cutting
and characterization, and understanding subtle details of the light- handling
abilities of gems. From an optical designer’s point of view, the optical design of
gem-stones is a refreshing and challenging endeavor.
José M. Sasián (jose.sasian@optics.arizona.edu)
is a
professor in the Optical Sciences Center of the
University of Arizona.
Peter Yantzer (pyantzer@agslab.com)
is the director of the
American Gem Society Laboratories, a subsidiary of
the American Gem Society (www.ags.org).
Tom Tivol is the president of Tivol Jewels, Kansas City,
Missouri, and a member of the American Gem
Society (www.ags.org),
where he serves on the
board of directors. |
References
1. M. Tolkowsky, Diamond Design: A Study of the Reflection and Refraction of Light in a
Diamond, Spon & Chamberlain, New York, 1919. http://www.folds.net/diamond_design/index.html
2. M. Cowing, “Diamond Brilliance: Theories, Measurements and Judgment,”
Gemmological Association of Great Britain, J. Gemology, October 2000.
http://www.acagemlab.com
3. T. S. Hemphill, I. M. Reinitz, M. L. Johnson and J. E. Shigley, Gems & Gemology,
34(3), 158-83, Fall 1998.
4. I. M. Reinitz, M. L. Johnson, T. S. Hemphill, A. M. Gilbertson, R. H. Geurts, B. D.
Green and J. E. Shigley, Gems & Gemology, 37(3) 174-97, Fall 2001.
5. S. Sivovolenko, Y. Shelementiev and A. Vasiliev, “4. The Results of Diamond Cut
Studies Carried Out in Moscow State University”, 1999.
http://www.cutstudy.com/cut/english/document4.htm
6. S. Sivovolenko, Y. Shelementiev, V. Onischuk G. Holloway, “On the Problem of Grading the Quality of
Diamond Cut: An Analysis of Some Aspects of the Problem,” 2002.
http://www.cutstudy.com/cut/english/grading1/index.htm
7. A. B. Meinel and E. S. Meinel, “Investigation of the Optical Behavior of Brilliant Cut Diamonds,” Report to
Helio Associates, 1974.
8. GemCad Software by Gemsoft Enterprises, http://www.gemcad.com/.
9. DiamCalc software by Octonus, http://www.gemology.ru/octonus/english/diamcalc/.
10. ASAP by Breault Research Organization, http://www.breault.com/.
11. FRED by Photon Engineering, http://www.photonengr.com/.
12. LightTools by Optical Research Associates, http://www.opticalres.com/.
13. TracePro by Lambda Research, http://www.lambdares.com/.
14. ZEMAX by FocusSoftware, http://www.zemax.com/.
15. Sarin Technologies Ltd., http://www.sarin.com/viewer/.
16. History of the Firescope, http://www.christian-bernard.com/diamonds/histbril.htm. (Firescope is a
registered trademark of Eightstar, Inc.)
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©
2003 Copyright Optical Society of America
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