In attempting to replace grading with the naked eye by the employment measuring instruments, in order perhaps to achieve a more objective judgment of the color, measuring techniques have been developed to determine precisely the color of diamonds. For this purpose a number of photometric instruments of various types of construction are available today, of which the most important should be briefly named.
The basis of these instruments is the measurement of light absorption of certain wave-lengths in diamond. The behavior of light absorption In the blue and yellow wave-length regions has proved particularly Important. If a diamond transmits more yellow wave lengths and absorbs a large proportion of the wave-lengths out of white light, then the whole stone appears yellowish. This yellow saturation is so much stronger, the greater the absorption in the blue region.
If on the other hand yellow is more strongly absorbed, the stone looks colorless. The degree of color saturation is therefore dependent on the relationship between the yellow- and blue-absorption in a diamond.
The first instrument used for such measurements was the Electronic Colorimeter constructed by R. SHIPLEY in the USA in 1955. This instrument measured the light transmitted by the brilliant after traversing a blue and a yellow color filter. These two color components, blue and yellow, when mixed, produce the actual body color of the diamond. The values of the blue and yellow quantities of light are measured by a photo-electric cell. This transmits them onwards to a calculator, built into Colorimeter, which relates the values to one another. The numerical result is converted into the color-grade.
As a consequence of several inaccuracies and technical as well as constructional deficiencies the instrument was withdrawn in the middle sixties from the trade and has so far not been replaced by a better one in the USA. Its use was confined to brilliant-cut and non-fluorescent diamonds.
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