Optics: measuring and testing – Crystal or gem examination
Reexamination Certificate
2000-02-16
2002-10-29
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Crystal or gem examination
C356S445000, C356S446000
Reexamination Certificate
active
06473164
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to systems, apparatuses and methods for gem color measurement and analysis, and more particularly systems, apparatuses and methods for measuring and analyzing the color of a diamond in a manner that approximates visual measurement and analysis methodology.
BACKGROUND OF THE INVENTION
Diamonds and other gemstones are often analyzed based upon their visual appearance to the human eye. Indeed, a diamond's visual appearance to the human eye under natural or daylight-approximating light is a primary indicator of the quality of the diamond. Accordingly, because diamond quality is substantially based on human visual perception, diamond analysis requires the exercise of judgment, the formation of opinions and the ability to draw fine distinctions based on visual comparisons.
In practice, diamond quality analysis is optimally performed by a team of trained individuals who visually inspect a diamond for features such as inclusions and structural flaws. This time-intensive process involves numerous inspections, measurements and checks by each individual. The process also involves quality control and may include a variety of non-destructive tests to identify treatments, fillings or other defects that may affect the quality of a specimen. Finally, the process includes intensive visual comparison of the diamond with a reference set of diamond master stones that serve as a historical standard with respect to diamond color and clarity.
The foundation of diamond analysis comprises analysis of the Four C's (color, clarity, cut and carat weight), a method of analysis invented by the Gemological Institute of America (GIA). Two of the Four C's, color and clarity, are evaluated along a scale or continuum. In the case of colorless to light-yellow colored diamonds, an analysis is made along what is commonly referred to as the GIA D to Z scale. The GIA D to Z color scale, ranging from colorless to yellow, is an international standard which has been calibrated to GIA's master diamonds since its development.
As described above, the visual inspection process of diamond analysis is subtle, time consuming and requires trained and experienced individuals. As a result, many members of the jewelry and gem field have stated a need for an instrument that can approximately analyze a diamond's color according to the D to Z standard. Over the years, numerous mechanical instruments have been proposed to “measure” the color of a polished diamond. Yet, apart from the problems of calibration errors and electronic drift, these instruments have not reached the desired level of accuracy and repeatability due to various inadequacies. Moreover, these instruments have not approximated visual color analysis methodology in a manner that makes their results meaningful within the context of the historical analysis standards.
The history of mechanical gemstone color grading instruments dates at least back to the 1940's when Dr. Robert M. Shipley, founder of GIA, developed a simple calorimeter comprising a light source and an incrementally moveable, colored plastic wedge. The Shipley device positioned the colored wedge behind a static diamond mount, allowing the user to compare the color of the diamond against the colored wedge backdrop. The Shipley device thus acted as an aid to visual inspection, relying on the human eye instead of a mechanical light detector and the human brain instead of an optical measurement device and processor.
In the 1950's, Dr. Shipley invented the first non-visual gem color analysis instrument, a modified color comparator, comprising a tungsten filament lamp for a light source, a photo cell for a light detector, blue and yellow light filters, a static stone holder and an iris diaphragm that passes light through to the photo cell. According to the method of the Shipley Colorimeter, the instrument user placed the diamond table down over a diffuser plate so that the tungsten light was transmitted first through the diamond and then through the iris diaphragm to the photocell. The instrument user then made sequential measurements of transmitted light, first deploying the blue filter and then deploying the yellow filter. According to the method of the invention, the instrument user subsequently compared the two transmission magnitudes detected by the photocell and looked up the results in a table organized along the D to Z scale to determine the index of the diamond's color.
Although the Shipley Colorimeter provided a type of color standard for many years, that standard did not precisely correlate with historical visual analysis standards for several reasons. First, the geometric relationship between the diamond, the light source and the detector did not approximate that of visual diamond analysis. Second, the tungsten filament lamp, though of fair output stability, did not provide the type of daylight conditions which have been the standard for visual analysis of diamonds and other gemstones. Third, the photocell detector did not register each individual frequency in the spectrum of visible light, like the human eye, but rather tracked the change in an overall spectrum magnitude resulting from the change in light filters. Thus, although the Shipley Colorimeter provided a highly useful and innovative instrument for non-visual diamond color analysis, it did not precisely approximate visual analysis methodology. Moreover, during analysis, the diamond was static, not rotated, and the device did not average color over a 360° rotation.
In the 1970's, the Eickhorst Colorimeter and the Okuda Colorimeter introduced two new varieties of color measurement instruments. Although still based on the color comparator method of the Shipley Colorimeter, Eickhorst disclosed the concept of using a fiber optic couple to direct light to the light detector. Okuda disclosed the concept of a voltage-stabilized tungsten light source and an integrating sphere to direct light on the diamond. However, like the Shipley Colorimeter, these instruments relied on the tungsten filament lamps for their light source. Moreover, like the Shipley Colorimeter, the devices compared the overall magnitude of light transmitted by the diamond in response to the use of two different frequency filters. The Eickhorst and Okuda instruments furthermore directed the tungsten light into the crown facets of the diamond, rather than illuminating the diamond into its pavilion side, and did not subsequently measure the light coming out of the pavilion side, as is the case with visual diamond analysis.
In the 1980's, U.S. Pat. No. 4,508,449, to Okazaki, disclosed an apparatus for measuring the color of a brilliant cut diamond by using a spectrophotometer to measure a limited spectrum of light coming from a diamond. The instrument included an arithmetic unit for deriving tristimulus values X, Y and Z from the measured spectrum. Okazaki further disclosed the use of a xenon or halogen white light source and a type of filter (monochromater) to provide a beam of monochromatic light that is sequentially varied in frequency over a spectral band of interest. Okazaki further disclosed a method of recording the magnitude of light emanating from the diamond in response to sequentially changing frequencies within the spectral band of interest. Okazaki taught away from directing light into the pavilion side of the diamond in the manner of visual analysis (Col. 1, 11.38-39) and did not detect, either directly or indirectly, a specific angle of light coming from the diamond. Additionally, Okazaki's use of a photomultiplier tube and his sequential measurement of frequency response within the spectral band of interest creates an undesirable time delay in recording the transmission spectrum.
The 1990's have seen several variations in diamond color analysis instruments. For example, the Austron Colorimeter and the Gran Colorimeter disclosed the use of a photodiode as a light detector. Like their predecessors, the Austron and Gran Colorimeters used, respectively, halogen
De Jong Peter
Geurts Ronald
Font Frank G.
Gemological Institute of America, Inc.
Gray Cary Ware & Freidenrich
Punnoose Roy M.
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