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Microspectrometer Is a Diamond's Best Friend

Photonics Spectra
Mar 2000
Daniel C. McCarthy

Along with weight, clarity and cut, a diamond's value is linked to its color. A D-grade stone -- the most valuable -- is almost perfectly white. Add a faint tinge of yellow, and a diamond's grade drops a notch to E, and its value plunges by 40 percent.

The value-curve is less precipitous as diamond grades continue alphabetically to Z-class stones. But whatever the grade, fast, accurate and repeatable appraisal is important. One solution, a diamond colorimeter from Gran Computer Industries Ltd., relies on a microspectrometer from Steag microParts GmbH.

The low cost of Steag microParts' device spurred its selection, according to Paul Gran, director of Gran Computer Industries. "It cost less than half of the closest competitor's," he said of the instrument. "In a commercial market like diamond grading, where your product cost gets multiplied by a factor of four -- from the manufacturing cost to the distributor to the end user -- everyone has a markup, so that a difference of $500 in the parts means $2000 to the end user. No jeweler is willing to pay for that."

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Gran Computer Industries' diamond colorimeter uses Steag microParts GmbH's microspectrometer to quickly grade diamonds based on color and tint. Courtesy of Gran Computer Industries Ltd.

The actual cost of Steag microParts' spectrometer varies depending on the quantity ordered. The price begins at about $250 for small orders and drops for volume purchases, according to Sven Schönfelder, head of micro-optics at Steag microParts.

Gran's earlier diamond colorimeters required filters that limited measurement to the apexes of the red, green and blue triangle in the CIE 1931 color chart. The blue filter, in particular, presented problems. "We could not cut off the ultraviolet without sacrificing the color gain close to blue -- 400 to 450 nm -- where everything exciting happens in diamonds. So there's a sharp area where we needed a good response, but we didn't want any UV there because it would cause fluorescence."

Steag microParts' low-cost spectrometer has some trade-offs in resolution. Rather than the standard slit, the optical fiber separates wavelengths, providing a 3-nm step in the array and 8 nm in the bandwidth.

But Gran pointed out that a loss in resolution is not an issue for color measurement applications, where the priorities are repeatability, stability and color resolution. The microspectrometer provides a larger spectrum of tints, covering the CIE chart.

Also, Gran observed that advances in software put less of a demand on a system's hardware. It can, for instance, compensate for dark current and even for stray light.


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