Application On Colorimetry Of Rubies' Color Assessment

Being as one of five previous gemstones, ruby quality grading system of rubies still doesn't reached a consensus, which plays a negative effect on promotion of rubies and healthy development of industry at home and abroad.Chromatic quantitative analysis is the premise of quality grading and evaluation for all different kinds of gemstones. Since 1950s, ruby's color quantitative determination and contrastive analysis had been become one of important research subjects in gemology. Many research institutions, such as GIA, GIT, have quantitatively analyzed chroma and saturation of rubies. It doesn't apply uniform color space so that it cannot reflect the real difference between the similar colors. Recent research has presented color quantitative grading of rubies, but it doesn't put the ruby specimen on the direct measurement, whose data is not worthy of reliability.In this case, this paper selects CIE1976 uniform color space recommended by CIE, and CIELCH as expression system. The external reflected signal of specimen by X-rite I5 spectrophotometer is firstly adopted, and then converted into the parameter indices in CIELCH which will realize the quantitative grading of rubies' color. Common ruby specimens are divided into red, pink and amaranth series by hue in D65-10 illuminant, into grey, shallow, moderate, deep, vivid by saturation, and into darker, dark, middle, bright and brighter by brightness. Record the color of ruby in the objective way of "brightness /saturation /hue". Consider comprehensively three color elements, and construct ruby color grading system in uniform color space. There are eight levels fromⅠtoⅧ, which the colorimetric parameter of the superlative level is ho∈( 0°,14°), C*>29 , L∈(48-53). The notion " chromatism" is introduced to represent the accurate difference between two similar colors, and CIE DE2000 chromatism formulae is selected to construct ruby color quality system.In this paper, The quantitative effect of different illuminants on rubies' color difference is also investigated, which will provide theoretical basis for the display and sales of rubies. It is revealed that when the color temperature of the illuminants deceases, brightness value L will increase slowly; red/green value a* and saturation C* are independent of color temperature difference;yellow / blue value b* and hue angle ho increases largely with the decreasing temperature difference. It can confirm that U30 illuminant is the best illuminant for all varieties of rubies' display, comparing to other common illuminants. Furthermore, it is obtained that amaranth ruby looks more beautiful at U30 illuminant rather than D65 illuminant;red ruby looks more beautiful at D65 illuminant rather than U30 illuminant;parts of pink ruby looks better at U30 illuminant ,but the rest are not decided.