The color matching method is very common in the process of gemstone color evaluation, using master stone or standard color card. When the luster of gemstone is obviously different from that of reference material, how to exclude the influence of surface luster and evaluate the color accurately ?
Dear Guo Ying,
Many standardized modes of illumination and viewing have been published for visual assessment, see CIE publication 15:2004. The simplest is known as 0/45 geometry where the numbers are degrees of angle departure from the perpendicular relevant to an essentially flat surface. The first number is the angle of illumination and the second is the angle of viewing. The numeric indication may also be ‘d’ indicating diffuse mode illumination or viewing. A light-integrating sphere up to a meter in diameter with well positioned sample and viewing ports is needed for diffuse light management and it must have a white powder coating to enable light-ray integration by multiple reflection.
For lustre free colour comparison the gems should be carefully oriented to a common surface angle at the sample port, viewing should be diffuse mode, and the illumination should be 0/5 to allow the lustrous spectral reflectance to exit the sphere without affecting the assessment.
David Oulton thanks for your valuable contribution. I know that different modes and specific angles can be chosen for color testing. Gemstones usually have a strong luster and the human eye's perception of the object's color combines surface effects. I would like to ask if there are any skills that can be used in visual evaluation to subjectively peel off the perception caused by luster?
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