| The theoretical research of the relationship between the optical effects and cut of diamonds has been carried out for several hundred years.Until now,the dominant method of assessing the optical effects of diamonds is to use cut levels for indirect evaluation.However,there are obvious differences in the grading system of diamond cutting among the various evaluation institutions.This means that the relationship between the optical effects of diamonds and the diamond cut is very complex,and whether the current grading system of diamond cutting can accurately reflect the level of optical effect of diamond remains to be verified.Currently,Sarine(Israeli company),De Beers,and Gemex(American company)have developed instruments to test the optical effects of diamonds directly.In China,Dr.Liu Pingping from the Jewelry Institution of China University of Geosciences(Wuhan)studied the method of quantitative analysis of diamond optical effect under six ring light sources in 2014.In order to study the optical effect of diamond,a high-precision narrow ring light source is used as the test light source in this thesis.The light source,the diamond and the central axis of the camera are on a straight line.The light source penetrates into the diamond sample by moving vertically up and down to form continuously varying light from different incident Angles.The incident Angle of the narrow ring light source gradually increases from 9° to 34°,and 137 optical images of the diamond are collected directly and continuously by using a digital camera to simulate the human eye.The color information of the image is obtained by digital image processing technology and the optical effect of diamond is quantitatively analyzed based on the principle of machine vision.In order to ensure the stability of the test results,the diamond sample was tested once every 90° for a total of four times and the quantitative results were averaged.A total of 548 digital images were collected for each diamond.Step-scan mode narrow-ring light source instrument can achieve highly stable and repeatable values: the same diamond is tested multiple times on two instruments,and the maximum relative error of the measured values(brightness,fire,scintillation)is within1%.Accurate test provides accurate and reliable data for this study.The brightness of diamond is caused by the reflection(including internal and external reflection)and refraction of light,and the intensity of the light from the crown of diamond determines the visual brightness of diamond.Therefore,this thesis converts the RGB values of all the pixels in the optical image into YUV color space and adds up the Y brightness values of all the bright points.The brightness value of the diamond was characterized by the average value of all the tested images with diameter normalization.The cause of fire is that when white light propagates inside diamonds with high dispersion rate,monochromatic light of different wavelengths is transmitted out from different positions on diamond facet or from different directions.This gives the diamond a colorful appearance.Therefore,the RGB value of the image pixel is converted into HSV(hue,saturation,brightness value)color space.Based on this,the color brightness index Z=S×V is established,and the point with Z>13 is regarded as the fire color point.For the angular range of hue,different coefficients W(h)are used to weight the color vividness Z.The fire value of a unit pixel follows the formula f=Z×W(h).In order to make the calculation results in consistent with the naked eye observation,this thesis uses the Laplace edge operator of machine vision to eliminate the unclear color edges around the white areas of the image,and adds up the fire values of all the remaining fire points in the image.The fire value of the diamond was characterized by the average value of all the tested images with diameter normalization.Scintillation refers to the flashing or alternating between light and dark of the reflected light when the diamond,light source or observer moves.In order to analyze the brightness change of the images,the simple color difference formula is used to calculate the color difference value of the same pixel in the two adjacent images.The color difference values of all points in the image were accumulated.Color difference values of overall 137 photos were formed into a curve,and the average of the data larger than the mean value of the curve was used as the scintillation value.This article compares the test scintillation level of 48 diamonds with the naked-eye observation level of two observers.The complete agreement reached 30% and the approximate agreement reached 80%.This indicates that the naked eye observation has errors due to subjective factors and is not accurate and objective.In this thesis,1098 diamond samples with different GIA 4C(Color,Clarity,Cut and Carat weight)grade were tested using the method based on the direct measurement of optical effects(brightness,fire and scintillation).The conclusions drawn are as follows:1.The values of brightness,fire and scintillation of diamonds are basically close to the normal distribution,with the characteristics of random variables.The number of diamond samples with medium brightness was the most,while the number of diamond samples with small brightness or large brightness was less.The brightness,fire and scintillation values of diamonds are complex and varied.But only 14 diamonds,or 1.4percent of all 1,000 diamonds,were EX in brightness,fire and scintillation.2.The brightness of each 4C grade diamond is close to the normal distribution,and the value range of the brightness of different kinds of diamonds overlaps.Of the 3EX diamonds,only about 3% are capable of reaching top 20% brightness.3.This thesis uses the statistical method of multiple linear regression to confirm that there is a statistically significant negative correlation between brightness and table size,crown angle,pavilion angle,and lower girdle facet.There is a significant positive correlation between brightness and star length and symmetry.There is no significant correlation between brightness and polish grade.There is a significant negative correlation between fire and table size,crown angle,pavilion angle and lower girdle facet.There is no significant correlation between fire and star length,symmetry and polish grade.There is no significant correlation between scintillation and the cut parameters.4.In this thesis,the linear scanning narrow ring light source is used to move vertically up and down,and several images are collected continuously.By comparing the correlation coefficient of brightness,fire and scintillation arrays of different diamonds,the optical fingerprint identification of one thousand diamonds can be realized.The existing 4C grading standards for evaluating diamonds from the aspects of color,clarity,cut and carat weight cannot fully reflect the optical information of diamonds.There is indeed a certain internal connection between the optical brightness of diamonds and the 4C parameters.However,due to the insufficient comprehensiveness and inaccuracy of the 4C parameters,the optical brightness of each diamond cannot be accurately predicted by 4C.Direct measurement of the optical effect of a diamond can be independent of the existing 4C grading system and become another5 C that reflects the quality of diamonds from the optical perspective. |
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