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Magnification Lens Shrinks Diamond Spectrophotometer

Photonics Spectra
Jul 2000
Daniel C. McCarthy

Diamonds are valued by their carat weight, clarity, color and cut. Weight and clarity are integral to the quality of the stone and can be measured by empirical standards. Valuation by color also has become less interpretive, thanks in part to photonics technology.

Cut, however, has resisted systematic methods of valuation. The closest the diamond market has come to a standard is the Tolkowsky Ideal, a mathematical model developed in 1919 that postulates the ideal cut. In theory, the more closely a diamond's cut adheres to the Tolkowsky Ideal, the more it will sparkle and scintillate.


GemEx's diamond spectrophotometer penetrates conventional valuation based on a diamond's exterior cut to establish its value by how much white light it returns. Courtesy of GemEx Systems Inc.

"But in everyday life, it doesn't happen that way," explained Kurt Schoeckert, vice president of GemEx Systems Inc. "The facets may be misaligned, or the graining of the diamond could counteract the cut, or the location of the inclusions could affect appearance." These factors can make a diamond that has an excellent cut appear less than optimal or, conversely, a yellow diamond will scintillate beautifully if it is cut well.

Last month, GemEx introduced the Brilliancescope, an imaging spectrophotometer for valuing a diamond by its cut. The principle behind the instrument penetrates the exterior-based criteria of the Tolkowsky Ideal to value a diamond's beauty by the bottom line: how much white light it returns.

"Up until today, it's been about angles," said Schoeckert. "We're moving a market based on color and clarity back to a market based on beauty, where it used to be."

GemEx's spectrophotometer measures light returning from a diamond at several angles of incidence. The diamond's image passes through an enlarging lens from Rodenstock Precision Optics Inc. before reaching a red, green and blue filter and a black-and-white CCD detector. A computer separates the colors and reports how much fire, brilliance and scintillation the diamond has.

To market its spectrophotometer to diamond merchants, GemEx had to ensure that it was small enough to fit on most countertops. More than any other attribute, focusing length determines the dimensions of the instrument. GemEx had sampled other lenses but found the focusing distance too long. Rodenstock's 8× magnification fixed stop lens provided the shortest offset between the CCD and the diamond, enabling GemEx to keep its box height to 18 in.

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