CN115128078B - Gem identification method - Google Patents

Abstract

The invention discloses an emerald green identification method which is characterized by comprising the following steps of: 1) Collecting a transmittance spectrum and a reflection spectrum of emerald in a range from 380nm to 780 nm; 2) Carrying out spectrum modulation on the spectrum data acquired in the step 1) through an algorithm; 3) Converting the spectrum data obtained by spectrum modulation in the step 2) into chromaticity coordinates through chromaticity calculation; 4) And identifying the emerald according to the distribution position of the chromaticity coordinates. The method for identifying the emerald green combines the means of colorimetry and spectroscopy, improves the accuracy of identification, is used for identifying the natural emerald green and the synthetic emerald green, and makes up for the defects of the conventional identification means such as visual observation and the like.

Description

A method for identifying gemstones

Technical Field

The present invention relates to the technical field of jewellery identification, and in particular to a method for gemstone identification.

Background Art

The Charles filter is the most commonly used filter in gem identification, also known as the "emerald filter". This filter was developed by Anderson and Payne of the British Gem Testing Laboratory and was first used in the Charles Industrial School, hence the name "Charles filter". The original design was to quickly distinguish natural emeralds from their imitations. Although emeralds are green, they allow some red light to pass through because they contain the coloring element Cr, so under the Charles filter, these emeralds appear red or pink. But later it was discovered that many emeralds from new origins, especially emeralds from South Africa, did not turn red under the Charles filter. In addition, with the large-scale listing of synthetic emeralds (synthetic emeralds also appear red under the Charles filter), the role of the Charles filter in identifying emeralds has become increasingly limited. It can be seen that the Charles filter is no longer sufficient to distinguish natural emeralds from artificial synthetic emeralds under simple naked eye observation (whether it turns red or not), and it is very important to find a more accurate identification method.

Summary of the invention

The present invention aims to solve at least one of the technical problems existing in the prior art, and proposes a method for gem identification, which is mainly used for distinguishing natural emeralds from synthetic emeralds. The method comprises the following steps:

1) Collect the transmittance spectrum and reflectance spectrum of emerald in the range of 380nm to 780nm;

2) spectrally modulating the spectral data collected in step 1) through an algorithm;

3) converting the spectral data obtained by spectral modulation in step 2) into chromaticity coordinates through colorimetric operation;

4) Identify emeralds based on differences in the distribution of chromaticity coordinates.

Optionally, the reflectance spectrum of the emerald is collected when the minimum transmittance of the emerald in the wavelength range of 380-780nm is less than 5%, and the transmittance spectrum of the emerald is collected when the minimum transmittance of the emerald in the wavelength range of 380-780nm is not less than 5%.

Optionally, the algorithm in step 2) is to multiply the collected emerald spectrum data by the difference between the filter transmittance at the corresponding wavelength and the gem cut correction factor.

Optionally, the color filter only allows light in two wavelength ranges between 380-780nm to pass through, the first wavelength range is 650-780nm, the maximum transmittance is 70%-95%, and the second wavelength range is 540-580nm, the maximum transmittance is 0.5%-5%.

Optionally, the spectral data modulated in step 3) is converted into CIEx and CIEy values in the CIE 1931 chromaticity coordinate system.

Optionally, the chromaticity coordinate distribution position based on which the emerald identification in step 4) is based on a data standard established based on 1000 color coordinates obtained by performing steps 1) to 3) on 500 known natural emerald samples and 500 synthetic emerald samples.

Optionally, the cut shapes of the gemstone include the commonly seen round multi-faceted shapes, elliptical faceted shapes, emerald shapes, heart-shaped faceted shapes, pear-shaped faceted shapes, and cabochon shapes on the market; the cut correction factor is equal to the difference between the theoretical calculated value of the spectrum and the actual measured value of the spectrum divided by the emerald spectrum value measured in step 1); the theoretical calculated value of the spectrum is equal to the emerald spectrum value measured in step 1) multiplied by the transmittance spectrum value of the color filter; the actual measured value of the spectrum is the spectrum value measured by the instrument when the gemstone table is in direct contact with the color filter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a flow chart of the emerald identification method.

Figure 2 shows the transmittance spectrum of a round, multi-faceted natural emerald.

Figure 3 shows the transmittance spectrum of a round multi-faceted synthetic emerald.

Figure 4 shows the transmittance spectra of commonly used color filters.

FIG5 is a spectrum obtained by algorithm modulation of the transmittance spectrum of a round multi-faceted natural emerald.

FIG6 is a spectrum obtained by algorithm modulation of the transmittance spectrum of a round multi-faceted synthetic emerald.

Figure 7 shows the collected spectrum data of a round polyhedral synthetic emerald, the color filter spectrum data, the round polyhedral correction factor, and the spectrum data obtained by modulation.

Figure 8 shows the judgment criteria for natural and synthetic gemstones based on the calculated color coordinates of 500 natural emeralds and 500 synthetic emeralds, as well as the classification results of 10 known synthetic emeralds and 20 known natural emeralds.

DETAILED DESCRIPTION

In order to enable those skilled in the art to better understand the technical solution of the present invention, a method for gem identification disclosed in the present invention is described in detail below with reference to the accompanying drawings.

FIG1 is an identification flow chart of an emerald identification method. According to the flow chart, the first step is to collect the transmittance spectrum and reflection spectrum of the emerald in the range of 380nm to 780nm. For transparent emeralds, that is, when the minimum transmittance of the emerald in the wavelength range of 380-780nm is not less than 5%, the transmittance spectrum of the emerald is collected, and the absorption spectrum can also be collected. The transmittance spectrum and the absorption spectrum can be converted accordingly. For emeralds with low transparency, that is, when the minimum transmittance of the emerald in the wavelength range of 380-780nm is less than 5%, the reflection spectrum of the emerald is collected. The spectrum can be collected with a data point every 1nm, 2nm or 5nm. The denser the data points are collected, the higher the accuracy of the results obtained by subsequent calculations. FIG2 is a transmittance spectrum of a round multi-faceted natural emerald collected, and FIG3 is a transmittance spectrum of a round multi-faceted synthetic emerald collected. In this step, the transmittance spectrum value of the selected color filter is collected corresponding to the collection accuracy of the emerald spectrum (interval 1nm, 2nm or 5nm). The transmittance spectrum of the color filter can also be obtained directly from the color filter manufacturer. The color filter only allows light in two wavelength ranges between 380-780nm to pass through. The first wavelength range is 650-780nm, with a maximum transmittance of 70%-95%, and the second wavelength range is 540-580nm, with a maximum transmittance of 0.5%-5%. Figure 4 shows the transmittance spectrum of the Charles filter (a color filter commonly used in gem identification) on the market. The first wavelength range that the Charles filter allows light to pass through is 650-780nm, with a maximum transmittance of 85%-90%, and the second wavelength range that allows light to pass through is 540-580nm, with a maximum transmittance of 1%-2%.

The second step is to modulate the spectrum of the collected emerald spectrum data by calculation. The specific algorithm is: multiply the collected emerald spectrum data by the difference between the filter transmittance at the corresponding wavelength and the gem cut correction factor. The purpose of this step is to eliminate the need to accurately measure the spectrum of light after it passes through the emerald and then through the filter through the establishment of an algorithm. Because it is impossible to achieve ideal contact between the emerald and the filter, the light needs to go through the propagation path of air → emerald → filter → air. It is necessary to ensure that the emerald and the filter are in good contact to minimize the loss of light propagating between different media. By pre-measuring and establishing the correction factor for optical loss, only the spectrum of the emerald can be measured, avoiding experimental errors caused by changes in test conditions, which affect the accuracy of the final emerald identification.

In addition, by modulating the emerald spectrum in two bands of the color filter, the common part of the emerald spectrum can be ignored and the different part of the emerald spectrum can be amplified. Figure 5 shows the spectrum of the natural emerald transmittance spectrum obtained in the first step tested by algorithm modulation, and Figure 6 shows the spectrum of the synthetic emerald transmittance spectrum obtained in the first step tested by algorithm modulation. After modulation, the transmittance spectrum of the emerald only retains the light transmission around 650-780nm and 560nm. Figure 7 shows the spectrum data from the collected emerald transmittance spectrum data to the final modulation in the range of 380-780nm at intervals of 10nm.

Emeralds of different cuts have different light losses during propagation. Therefore, corresponding correction factors are established for the cuts of common emeralds on the market, such as round faceted, elliptical faceted, emerald, heart-shaped faceted, pear-shaped faceted, and cabochon. In actual calculations, the corresponding correction factor is selected according to the cut of the measured emerald. The cut correction factor is equal to the difference between the theoretical calculated value of the spectrum and the measured value of the spectrum divided by the emerald spectrum value measured in step 1). The theoretical calculated value of the spectrum is equal to the emerald spectrum value measured in step 1) multiplied by the transmittance spectrum value of the color filter. The measured value of the spectrum is the spectrum value measured by the instrument when the gem table is in direct contact with the color filter. In order to ensure good contact between the gem table and the color filter, a small amount of refractive oil (diiodomethane plus sulfur) is added between the gem table and the color filter.

The third step is to convert the modulated spectral data into chromaticity coordinates through colorimetric calculation. Because different emeralds have different thicknesses, the reflection on the spectrum of the emerald is the overall movement of the transmittance in the visible light range, that is, uniform increase or uniform decrease. In order to avoid the influence of parameters such as thickness that affect the transmittance of emeralds on the identification of emeralds, the present invention uniformly converts the modulated spectral data into CIEx and CIEy values in the CIE 1931 chromaticity coordinate system. Each spectrum corresponds to a unique CIEx and CIEy. Only the hue and saturation of the converted color are considered and displayed on the CIE1931 "horseshoe" chromaticity diagram. The method of converting spectral data into chromaticity data refers to calculation formulas (1), (2), (3), and (4):

(1)

(2)

(3)

(4)

In the above calculation formula, The three values represent the stimulation values of the three primary colors at the corresponding wavelengths. The corresponding values can be found on the official website of the CIE International Commission on Illumination. By integrating the wavelengths, you can get X (red primary color stimulation amount), Y (green primary color stimulation amount) and Z (blue primary color stimulation amount). is the intensity distribution of the transmittance spectrum, The intensity distribution of the standard lighting source is D65. The intensity distribution of the standard D65 light source used in the present invention can also be obtained from the official website of the CIE International Commission on Illumination.

The corresponding CIE XYZ value can be calculated based on the spectrum data modulated in the third step, and then converted into CIEx and CIEy coordinates in the CIE 1931 chromaticity coordinates according to the color coordinate conversion formula x=X/(X+Y+Z), y=Y/(X+Y+Z). According to the above algorithm, the color coordinates calculated for the modulated spectrum of the natural emerald tested in the first step are (0.3786, 0.4746), and the color coordinates calculated for the modulated spectrum of the synthetic emerald tested are (0.4393, 0.4424).

The fourth step is to identify the emerald according to the position of the chromaticity coordinates. This step is based on the pre-established judgment standard for natural emeralds and artificial synthetic emeralds, which is established based on 500 known natural emerald samples and 500 artificial synthetic emerald samples. Through the methods of the first to third steps mentioned above, the spectra of 500 natural emerald samples and 500 artificial synthetic emerald samples were converted into corresponding 1000 sets of CIEx values and CIEy values, and recorded on the CIE 1931 chromaticity diagram. Then, according to the statistical law, 15 natural emerald color coordinates (distributed in the synthetic emerald area) and 22 synthetic emerald color coordinates (distributed in the natural emerald area) that do not conform to the distribution law are eliminated. As shown in Figure 8, the remaining 963 color coordinates are divided into three areas on the CIE 1931 chromaticity diagram. The color coordinate distribution area of synthetic emerald is located on both sides of the natural emerald color coordinate distribution area. In order to ensure the randomness of the samples, the 1,000 samples include round multi-faceted, oval faceted, emerald, heart-shaped faceted, pear-shaped faceted, and cabochon types. Among them, the origins of 500 natural samples include Colombia, Russia, China, Pakistan, Afghanistan, Brazil, and South Africa, and 500 synthetic emerald samples include hydrothermal synthetic emeralds and flux synthetic emeralds. In order to verify the accuracy of the judgment standard again, 10 synthetic emerald samples and 20 natural emerald samples were additionally selected for identification by the identification method of the present invention. As can be seen from Figure 8, the color coordinates of the 10 synthetic emeralds are distributed in the synthetic emerald area, and the 20 natural emeralds are also distributed in the natural emerald area, which shows that the method provided by the present invention can effectively distinguish natural emeralds from synthetic emeralds. The accuracy of the gem identification method provided by the present invention can also be improved as the number of samples increases.

It is to be understood that the above embodiments are merely exemplary embodiments used to illustrate the principles of the present invention, but the present invention is not limited thereto. For those of ordinary skill in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims (5)

1. A method for identifying emerald, comprising the following steps: 1) Collect the transmittance spectrum and reflectance spectrum of emerald in the range of 380nm to 780nm; 2) The spectrum data collected in step 1) is spectrally modulated by an algorithm, wherein the algorithm is to multiply the emerald spectrum data collected in step 1) by the difference between the color filter transmittance at the corresponding wavelength and the gem cut correction factor, the gem cuts include the common round multi-faceted type, elliptical faceted type, emerald type, heart-shaped faceted type, pear-shaped faceted type, and cabochon type on the market, the cut correction factor is equal to the difference between the spectrum theoretical calculation value and the spectrum measured value divided by the measured emerald spectrum value, the spectrum theoretical calculation value is equal to the measured emerald spectrum value multiplied by the filter transmittance spectrum value, and the spectrum measured value is the spectrum value directly measured by the instrument when the gem table is in contact with the filter; 3) converting the spectral data obtained by spectral modulation in step 2) into chromaticity coordinates through colorimetric operation; 4) Identify emeralds based on differences in the distribution of chromaticity coordinates. 2. The emerald identification method according to claim 1 is characterized in that, in step 1), the reflectance spectrum of the emerald is collected when the minimum transmittance of the emerald in the wavelength range of 380-780nm is less than 5%, and the transmittance spectrum of the emerald is collected when the minimum transmittance of the emerald in the wavelength range of 380-780nm is not less than 5%. 3. The emerald identification method according to claim 1 is characterized in that the color filter only allows light in two wavelength ranges between 380-780nm to pass through, the first wavelength range is 650-780nm, the maximum transmittance is 70%-95%, and the second wavelength range is 540-580nm, the maximum transmittance is 0.5%-5%. 4. The emerald identification method according to claim 1, characterized in that the spectral data modulated in step 3) is converted into CIEx and CIEy values in the CIE 1931 chromaticity coordinate system. 5. The emerald identification method according to claim 1 is characterized in that the chromaticity coordinate distribution position based on which the emerald identification in step 4) is based on 1000 color coordinates obtained by performing steps 1) to 3) on 500 known natural emerald samples and 500 synthetic emerald samples.