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Accuracy of Proportion Measurements
The accuracy of digital proportion measurements is far superior to what was achieved with optical devices a decade or two before.
Sarin Technologies and Ogi Corporation are two of the main companies who manufacture proportion-measuring devices. The base models from both companies have limited accuracy but more advanced models incorporate multi-lens systems that use a larger portion of the field of view to measure different size stones. Measuring a 3mm stone with a lens designed to measure 20mm diamonds reduces the accuracy by a factor of three compared to using a lens designed for a 7mm maximum stone diameter.
Sarin claim accuracy of 20 microns or ±0.2 degree. The proprietary system they designed measures facet inclination by plotting interceptions. Shorter facet edges have a greater margin for error. Because the crown angle is shorter it has twice the potential error when compared to pavilion facets. The pavilion angle is four or five times more critical than crown angle (or any other proportion) in its effect on HCA scores.
A ±0.2 degree variation in angle data can give rise to as much as 0.6 variations in HCA score in some critical cases, or as little as 0.1 in less critical zones.
Observers often note Sarin proportions depth percentages do not add up to the same total depth as calculated by dividing depth by diameter. This is more to do with the various positions that various labs choose to measure girdle thickness. Some labs measure the thinnest part, and others measure the thicker part. Sarin is less accurate at measuring depth and diameter, because this data is actually a ‘construction’ of ‘join the dots’.
The Ogi system measures dimensions more accurately but then extrapolates crown and pavilion angles. In the same way that Sarin have limitations measuring dimensions, Ogi are less reliable at measuring angles, especially pavilion angles. Any very small culet tends to lead to an understatement of pavilion angle. At this time we believe all major laboratories choose to use Sarin devices.
We would prefer the source of Sarin information for HCA came from a laboratory that accurately calibrates and maintains its Sarin devices and employs well trained personal. We also recognise this would reduce the potential for deliberate interference of data. We encourage the GIA-GTL to list crown and pavilion information on certificates – something we believe will happen early in 2002. We also wish to advise that the firm or individual who originally submitted a diamond to GIA for certification can request crown and pavilion angle data.
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