RU2765213C1 - Device for diagnostic stones in jewelry - Google Patents

Abstract

FIELD: measuring technology.

SUBSTANCE: invention relates to measuring technology, in particular to optical means for diagnosing the quality of precious stones for jewelry. The device for diagnostics by laser spectroscopy includes a laser radiation source for illuminating a stone, a spectrometer for recording radiation scattered by a stone, containing a monochromator, a photomultiplier tube installed at the output of the monochromator operating in the photon counting mode, a spectrometer control and signal processing unit, a computer and a display. The monochromator includes two sequentially installed acousto-optic cells connected to a frequency synthesizer, the control input of which is connected to the control output of the control and signal processing unit. The monochromator in the optical path includes three polarizers, the first of which is installed at the input of the first acousto-optic cell, the second at the output of the second acousto-optic cell, and the third central polarizer, made from the condition of ensuring the incidence of the radiation beam close to the Brewster angles, is located between the output of the first and the input of the second acousto-optic cells.

EFFECT: increasing the resolution while reducing the requirements for tuning and aligning the optical path of the device.

6 cl, 10 dwg

Description

The invention relates to measuring technology, in particular to optical means for diagnosing the quality of precious stones for jewelry.

The art of growing crystals has reached heights that make it possible to produce most of the gemstones in laboratories. Accordingly, there is a need for equipment for diagnosing the authenticity and quality of natural gems, which includes optical spectroscopy methods, including those based on the use of Raman scattering [K. Nassau. Raman spectroscopy as gemstone test. - J. Gemmol. 1982.vol. XYII, N5, p. 306-320].

A known method for determining the genetic type of diamond grains (SU 1762628 (A1), VNIGI named after A.P. Karpinsky, 05/20/1999), including illumination of each grain with a beam of optical monochromatic radiation, in which the Raman scattering line of the diamond grain is recorded in the region of 1332 ± 50 cm ^-1 , measure the width of the contour of the indicated line at half the height of its maximum, determine the spectral position of the maximum of the Raman scattering line, and according to the data obtained, using a pre-built diagram, the genetic type of the diamond grain is established. The disadvantage of this method is that it can only be used to diagnose diamonds.

The currently known setups for studying Raman scattering of light, as a rule, are aimed at solving unique scientific problems; therefore, they are rather cumbersome, require adjustment, testing, and adjustment before starting measurements, which significantly limits the possibility of their mass use for diagnostic purposes.

Thus, a device for determining diamonds by the method of coherent laser spectroscopy (RU 2180108 (C2), BIRZ KONSOLIDEJTED MAJNS LTD D, February 27, 2002) is described, in which the analyzed diamonds are irradiated with radiation formed by focusing a plurality of laser beams. At least two beams have frequencies that differ from each other in accordance with the characteristics of the diamond, and must be coherently phase-matched. The device for implementing the process includes an auxiliary means for determining whether the scattering signal emitted by each particle is a characteristic coherent Raman signal for diamond. The disadvantage of the device is the need to use two lasers, the frequencies of which differ by a value characteristic of diamond (1332 1/cm), and it is necessary that they be coherently matched in phase. On the one hand, this determines the complexity of the optical scheme, and on the other hand, it makes it possible to detect only diamonds with a strictly fixed isotopic composition. The latter circumstance is related to the fact that the Raman frequency shift characteristic of diamond can vary by 50 1/cm depending on the isotopic composition of diamond, i.e. from the era and place of formation.

A device for the identification of diamonds is described (EA 035897 V1, JSC ALROSA, FSBI TISNUM, 08/28/2020). A device for identifying a cut diamond contains a measuring point with a measuring aperture in which a cut diamond to be examined is fixed; mobile optical system, including a spectrometer, two radiation sources at wavelengths of 250-280 nm and 350-380 nm, respectively, and these two radiation sources and the spectrometer are connected to the measurement site by optical fibers to input radiation into a diamond and an optical fiber to output radiation from faceted diamond; as well as a source of laser radiation at a wavelength of 532 nm and a microcontroller that is configured to control the alternating operation of radiation sources in a given time sequence, move the optical system to inject radiation into a cut diamond, and process spectrometer data. The disadvantage is the need to use a fixed place with a measuring hole on which the test diamond is placed, as well as a moving optical system containing collimating elements and two pieces of optical fiber that provide input and output of radiation, which leads to the complexity and multi-element design of the device. In addition, radiation sources in the UV region of the spectrum (250-280 and 350-380 nm) are used, which implies the use of a closed chamber to locate the diamond under study, and the research procedure itself includes three successive stages and requires adjustment to different radiation sources.

A device is known for identifying a diamond (RU 2679928 (C1), FGBNU TISNUM, February 14, 2019), for separating natural diamonds from synthetic ones and for identifying disputed type Na diamonds, which may have been subjected to thermobaric treatment in order to improve color. The claimed device for diamond identification contains two sources of radiation: a deuterium lamp and a laser with a wavelength of 405 nm. The radiation from these sources, which has passed through the diamond, is recorded by a spectrometer. The device is also equipped with a notch filter input-output mechanism between the collimating lenses of the receiving fiber, which guides the radiation that has passed through the diamond sample and exited it into the spectrometer, which is connected to a microprocessor controller that analyzes and interprets the obtained data. The disadvantage of the device is that it does not involve the diagnosis of other types of gems, requires a closed chamber for the location of the samples under study and complicates the orientation of the laser radiation of the second source to a specific sample, and the device is supposed to work with single gems not in a jewelry item, since it is used a limited isolated volume for the location of the test sample.

A device for measuring the parameters of a faceted gemstone is described (RU 2664910 (C2), DE BEERS UK LTD, 23.08.2018). The device consists of a set of radiation sources, a receiver of two spectrometers, the first of which is designed to measure absorption, and the second to measure the Raman spectrum, while the cut gemstone is located in the same measuring position. The disadvantage of the device is that it is aimed at "increasing the reliability and speed of sorting cut gems" and is not intended for diagnosing them in complex jewelry. In addition, the device contains either a broadband ultraviolet radiation source and a second narrow-band visible frequency source (laser) and two corresponding spectrometers, or several laser sources and a corresponding number of receiving spectrometers, which significantly complicates the device. In particular, the unit for supplying and receiving radiation to a sample via a stranded fiber requires precise positioning of a particular emitter-receiver pair. The use of an ultraviolet source implies the presence of a closed space for the location of the test sample and requires its "hard" fixation so as not to disturb the alignment when the working space is closed.

An integrated optical tester for diamond identification is described (CN 110455804 (A), NANJING JIANZHI INSTR AND EQUIPMENT CO LTD, 11/15/2019). Consists of Raman excitation light source, UV light source, broadband light source, light guide, CCD camera, UV filter plate, spectrometer and built-in optical probe. However, the device cannot effectively exclude the influence of background fluorescence and phosphorescence, moreover, the integrated optical tester, according to the principles laid down in its operation, can only identify diamonds, and in the "tester" mode, i.e. on a "yes" or "no" basis. The use of a CCD camera for recording a signal involves fixing not the full spectrum of reflected (passed) radiation, but individual narrow sections. This simplifies and reduces the cost of the implementation of the device, but sharply reduces its versatility.

Closest to the patented device is a tool for identifying gems based on Raman spectroscopy (CN 106053425 (A) - Raman spectrum gem and jade appraising device and method, UNIV NANJING SCIENCE & TECH, 10/26/2016 - prototype). It includes a power supply, an optical part, an analysis and data management module. The optical part includes a power supply for excitation of the laser, an external optical path, a high voltage power supply, a photomultiplier, a monochromator, a photon counter, and a data analysis and control module that includes a main control driver, a computer, and a display. The control unit includes: Raman spectra database module for precious stones; a spectrum analysis module for comparing with a database spectrum to determine the authenticity classification; a module for calculating the composition of artificial gems and jade, and also helps in assessing their quality.

The disadvantage of the prototype device is the need to use a camera with a holder to accommodate the stone under study, rigidly connected to the monochromator of the external optical path, as well as the complexity and multi-element of the monochromator itself, which necessitates the alignment of the optical path.

The present invention is aimed at simplifying the design of the device while increasing the spectral resolution, which expands the functionality of diagnosing jewelry stones.

The patented device for diagnosing gemstones by laser spectroscopy includes a laser source of radiation for illuminating the stone, a spectrometer for detecting radiation scattered by the stone, containing a monochromator, a photomultiplier installed at the output of the monochromator operating in the photon counting mode, a control unit and signal processing of the spectrometer, a computer and a display .

The difference is as follows. The monochromator includes two acousto-optic cells installed in series connected to a frequency synthesizer, the control input of which is connected to the control output of the control and signal processing unit.

The monochromator includes three polarizers, the first of which is installed at the input of the first acousto-optic cell, the second - at the output of the second acousto-optic cell, and the third central polarizer, made to ensure the incidence of the radiation beam close to the Brewster angles, is placed between the output of the first and the input of the second acousto-optic cells.

The monochromator also contains a temperature sensor connected to the first information input of the control and signal processing unit, the second input of which is connected to the output of the photomultiplier, and the input-output of the control and signal processing unit is connected to a computer connected to the database of Raman spectra of standards.

The device may be characterized in that the optical output of the laser radiation source and the optical input of the spectrometer for recording scattered radiation are connected to separate optical fibers combined by a flexible shell, the ends of which form the contact zone of the measuring probe, and the lighting fibers are placed in the center of the specified contact zone, and the optical fibers for registration scattered radiation - along the periphery of the specified contact zone.

The device can also be characterized by the fact that the acousto-optic cell contains a calcium molybdate monocrystal sound guide and a lithium niobate piezoelectric transducer connected to the sound guide with the possibility of excitation of ultrasonic vibrations in it in the frequency range of 40-120 MHz.

The device can also be characterized by the fact that the third central polarizer is made of two isosceles calcite prisms with an apex angle of 71°, and also by the fact that the laser source of radiation for illuminating the stone has a wavelength of 532 nm, in addition, by the fact that at the input of the spectrometer notch filter installed.

The technical result of the invention is to increase the spectral resolution while reducing the number of optical elements both in the monochromator and in the external optical path, which greatly simplifies the path;

- the used optical elements are monolithic crystalline acousto-optic cells, so the device is resistant to vibrations, is portable and does not require adjustment before measurements;

- the use of a flexible fiber probe eliminates the need for a special chamber with a sample holder, which limits the size of the jewelry (necklace, bracelet ...) and allows you to quickly rebuild (due to "manual" contact) from one diagnosed stone to another;

- the availability of the possibility of replenishing the database in the control and signal processing unit allows you to directly enter spectral information about new reference samples.

The device is made in a single-block implementation and includes a single-frequency narrow-band laser radiation source in the visible range and an acousto-optic spectrometer, as well as a flexible fiber probe that allows you to examine jewelry of any size. Low intensity radiation is used, which is safe for human vision. To study a specific gem in a complex piece of jewelry, the measuring end of a flexible fiber probe is directly contacted, which makes it possible to quickly change from one stone to another. The measurement result is the Raman spectrum of the sample, which allows, in the presence of a reference database on the spectra of various gems, to determine whether this stone is precious or if it is an imitator, for example, colored glass, plastic, etc. Under the term "complex jewelry" the present invention refers to an article containing a plurality, possibly different, of precious stones distributed over a large area (bracelets, necklaces, brooches, etc.).

The essence of the invention is illustrated in the drawings, where:

fig. 1 - block diagram of the device;

fig. 2 - optical scheme of the monochromator;

fig. 3 - design of the fiber probe;

fig. 4 - layout of fibers in the probe;

fig. 5 - device assembled;

fig. 6 - Raman scattering spectrum of natural gem-quality diamond (a characteristic peak is visible at a frequency of 1332 cm ^-1 );

fig. 7. - normalized spectrum of Raman scattering of fianite crystal;

fig. 8 - spectrum of Raman scattering of a rock crystal;

fig. 9 - Raman spectrum of two different scheelite crystals;

fig. 10 - Raman spectra for a diamond sample, which is a twin.

The block diagram of the device is shown in Fig. 1. The device contains a laser 1, a fiber optic probe 2, a spectrometer 3, a personal computer 4. The spectrometer 3 contains a monochromator 30 containing two acousto-optic (AO) cells 31 and 32 installed in series, a temperature sensor 33 of the monochromator 30, a PMT photomultiplier 34, installed at the output of the monochromator 30. The spectrometer 3 includes a frequency synthesizer 35 to ensure the operation of the AO cells 31 and 32, as well as a control and signal processing unit 36. The inputs of the piezoelectric transducers 311, 321 of the AO cells 311 and 312 are connected to the outputs of the synthesizer 33, which generates electrical oscillations in the frequency range of 40-120 MHz, necessary to excite ultrasonic oscillations in the AO cells.

The operation of the spectrometer 3 is provided by the control and signal processing unit 36, the inputs of which are connected to the temperature sensor 33 and the output of the PMT multiplier 34.

The control and signal processing unit 36 is connected by a bus 341 for information exchange and control with a personal computer 4 connected to the database 41 of the standards database via bus 411. A notch filter 37 is installed at the input of the spectrometer 3, which prevents direct radiation from the laser 1.

In FIG. 2 shows the optical layout of the monochromator. The monochromator 30 along the direction of propagation of the analyzed scattered radiation contains an input polarizer 301, a corrective prism 302, installed at the input of the AO cell 31. At the output of the first AO cell 31, there is a central polarizer 303, connected to the second AO cell 32, at the output of which a corrective prism 304 is placed Next, there is an output polarizer 305, a light filter 306, a focusing lens 307 and a diaphragm 308, through which the radiation enters the photomultiplier 32. The direction of propagation of the analyzed scattered radiation is indicated by pos. 309.

Sound guides 3111, 3211 of cells 31, 32 are made of optically transparent single crystals of calcium molybdate CaMoO ₄ , have a cross section of 14×12 mm and a length of about 7 mm. Piezoelectric transducers 311 and 321 are made of lithium niobate LiNbO ₃ and provide excitation of ultrasonic vibrations in the sound ducts 3111, 3121 in the frequency range of 40-120 MHz.

To reduce the dimensions of the monochromator 30 at the input of the first 31 and the output of the second 32 AO cells, corrective prisms 302 and 304 are installed, made of glass and installed in such a way that the light beam in the monochromator 30 does not change its direction. One of the faces of these prisms 302 and 304 is coated, the light loss on the second face of the prisms is negligible, because the angle of incidence of light on this face is close to the Brewster angle. Input 301 and output 304 polarizers are made according to the Glan scheme, and the input prism is made of CaCO ₃ calcite, and the output prism is made of glass. The vertex angle for both types of prisms is 40°. The input and output faces of the polarizers 301, 305 are coated.

The central polarizer 303 consists of two isosceles calcite prisms 3031, 3032 with an apex angle of 71°. Such a polarizer has a large angular aperture, with the angles of incidence of the "axial" beam close to the Brewster angles, so that these beams pass through the polarizer 303 without loss. A light filter 306 is installed at the output of the monochromator 30, designed to additionally suppress the radiation of laser 1. Behind the light filter 306 there is a separator 307 of the transmitted radiation, which, using a diaphragm 308 located in the focal plane of the lens 307, cuts off the radiation emerging from the monochromator at an angle of more than 1.5 ° to the propagation axis 309.

Measurements have shown that the instrumental function of the monochromator 30 in the device at the wavelength of the 532 nm laser used has a spectral width of 2 angstroms with an out-of-band rejection of about 50 dB. Such results make it possible to record features in the spectra of the studied gemstones with a high spectral resolution and, thereby, provide a high reliability of diagnostics.

Fiber probe 2 (Fig. 3) is designed to transmit radiation in the visible range from laser 1 to the studied stone K and radiation scattered from it to spectrometer 3. The design of such flexible probes, which are used to illuminate an object and then analyze the received radiation for various applications, known from the prior art (see, for example, US 5202558 (A), BARKER LYNN M., 04/13/1993; RU 138056 U1, INLIFE LLC, 02/27/2014), and is used for a known purpose.

In a specific implementation, the probe 2 is a densely packed bundle 21 of optical fibers with an ordered stacking, including lighting 22, receiving 23 and non-working bundles 24, forming a flexible Y-shaped fiber probe. The diameter of the fibers is about 200 microns, their total number is 37 pieces, of which 7 are lighting pieces, and 30 are receiving ones. Each fiber is covered with a polyamide protective sheath. The bundles of 21 fibers are protected from the outside by a metal hose. The arrangement of fibers in tows is shown in Fig. 4. Illumination fibers are located in the center of the probe, receiving fibers are located along the periphery. The harness 21 is enclosed in a cylindrical needle-shaped holder 26 with a plastic handle. The total length of the probe 2 is 1.55 m, including the lighting end - 0.5 m, the receiving end - 0.5 m, with the length of the working end - 1 m. The lighting and receiving ends of the probe end with standard optical connectors 27, 28 .Working temperature range from -150°С to +60°С, spectral range - 0.5-0.9 microns.

The power supply for the functioning of the device is carried out using known technical means and is not given in this description.

In FIG. 5 shows the implementation of the device in a single-block design. In the case with a total size of 35 × 25 × 17 cm ^{3, a box 50 with power supplies and a laser is placed on the} bottom, a monochromator 30 with a photomultiplier 34 attached to it is mounted on the box. Output (to 27) and input (to 28) sockets are located on the box and monochromator for connecting a flexible Y-shaped fiber probe.

The device for diagnosing gemstones works as follows. The piece of jewelry is placed on any surface so that the facets of the gemstones are on top. The power supplies of the laser 1, the PMT 34, the control and signal processing unit 36, the frequency synthesizer 35 and the PC4 personal computer are switched on, thereby bringing the laser, spectrometer and PC into working condition. The measuring end 25 of the Y-shaped fiber probe 1 is “manually” brought into direct contact with the gemstone K. In this case, the radiation from the solid-state single-frequency narrow-band laser 1 is fed through the illumination bundle 22 of the fiber probe 2 to the analyzed gemstone K, and the scattered radiation is fed through the receiving bundle 23 of the same probe enters the input of spectrometer 3.

Passing through the notch filter 301, which suppresses the spectral component corresponding to the frequency of the laser 1, the scattered radiation enters the input of the monochromator 30, sequentially passes through the acousto-optic cells 31 and 32, and enters the photomultiplier 34 operating in the photon counting mode.

The electrical signal from the PMT 34 output is fed to the control and signal processing unit 36, where a standard ADC (not shown in the diagram) is converted to digital form and transmitted to PC4, where the current data file is automatically created for further presentation in graphical form on the PC4 monitor ..

Scanning over the spectrum of the scattered radiation supplied to the monochromator 30 is carried out by the commands of the program recorded in the memory of the control and signal processing unit 36 in the form of appropriate codes that allow you to set the frequency of the variable control signal supplied by the frequency synthesizer 35 to the ultrasonic transducers 311, 321 of the acousto-optic cells 31 and 32 , as well as the magnitude and time moments of switching on/off this control signal. Thus, with a point-by-point change in the frequency of the control signal, due to the successive diffraction of the transmitted radiation in acousto-optic cells, its spectral components are recorded.

The presence of a temperature sensor 33 in the monochromator circuit makes it possible during measurements to carry out self-calibration and introduce the necessary frequency correction of the synthesizer 35 to take into account the temperature change in the parameters of the cells.

As a result of the described procedure, a file is created in a PC that allows you to display the spectrum of radiation scattered from a precious stone on the monitor and, using the available database of 41 standards, either visually or programmatically determine whether this stone is precious or an imitator.

The device makes it possible to determine the quality of fairly small diamonds, since due to the design of the fiber probe 2, the scattered radiation leaves the stone in a wide aperture of the corners and, therefore, always enters several fibers of the receiving bundle 23.

To confirm the achievement of the technical result, the identification of cut diamonds (diamonds) and basic imitators in jewelry, as well as natural uncut diamonds and scheelites, was carried out. The device made it possible to distinguish between diamond samples and imitators, and confirmed the ability to differentiate natural diamonds from synthetic stones.

It is known that diamond crystals have a cubic lattice in which carbon atoms are linked by strong covalent bonds. Such crystals are characterized by one group vibration. Consequently, the Raman spectrum should contain only one pronounced peak, broadened to the extent of the thermal vibration of the lattice, which is demonstrated in Fig. 6.

The closest imitator of diamond is cubic zirconia. According to the chemical composition, cubic zirconia is an oxide of zirconium and hafnium in combination with rare earth elements. The crystal lattice of fianite contains ordered atoms of different elements, which leads to the formation of several frequencies of group vibrations and the Raman scattering spectrum is much richer than for diamond (see Fig. 7).

Another long-used imitation diamond is rock crystal. It is silicon dioxide (SiO ₂ ) crystals. In FIG. 8 shows the Raman spectrum of a quartz crystal. A clearly defined single characteristic peak is visible (which is also typical for diamond), but its frequency is 466 cm ^-1 and, therefore, the diagnosis of a stone in a piece of jewelry is unambiguous in this case - it is an imitator.

Shown in FIG. 9 Raman spectra for two different scheelite crystals, mined at different times and in different places, demonstrate another possibility. It can be seen from the graph that regardless of the stone (whether it is processed or not, mined from different deposits or from the same one, but at different times), the main characteristic peaks in the spectra coincide. This makes it possible to unambiguously determine the interspecies type of the studied mineral.

In FIG. 10 shows the Raman spectra in three different regions of diamond, which is a twin. The shift of the Raman peak is due to the different ratios in the isotopic composition of different parts of a given crystal that grew in different geological epochs. This circumstance makes it possible to distinguish natural diamonds formed in early geological epochs from those artificially grown.

Thus, the presented data indicate the achievement of a technical result - an increase in resolution while reducing the requirements for tuning and aligning the optical path of the device.

Claims (9)

1. A device for diagnosing precious stones by laser spectroscopy, including a laser source of radiation for illuminating a stone, a spectrometer for recording radiation scattered by a stone, containing a monochromator, a photomultiplier tube installed at the output of the monochromator operating in the photon counting mode, a control unit and signal processing of the spectrometer, a computer and display characterized in that the monochromator includes two acousto-optic cells installed in series connected to a frequency synthesizer, the control input of which is connected to the control output of the control and signal processing unit, while the monochromator includes three polarizers, the first of which is installed at the input of the first acousto-optic cell, the second - at the output of the second acousto-optic cell, and the third central polarizer, made to ensure the incidence of the radiation beam close to the Brewster angles, is placed between the output of the first and the input of the second acousto-optic cells; the monochromator contains a temperature sensor connected to the first information input of the control and signal processing unit, the second input of which is connected to the output of the photomultiplier, and the input-output of the control and signal processing unit is connected to a computer connected to the database of Raman spectra of standards. 2. The device according to claim 1, characterized in that the optical output of the laser radiation source and the optical input of the spectrometer for recording scattered radiation are connected to separate optical fibers combined by a flexible shell, the ends of which form the contact zone of the measuring probe, and the lighting fibers are placed in the center of the specified contact zones, and light guides for registration of scattered radiation - along the periphery of the specified contact zone. 3. The device according to claim 1, characterized in that the acousto-optic cell contains a sound guide made of a single crystal of calcium molybdate and a piezoelectric transducer made of lithium niobate, connected to the sound guide with the possibility of excitation of ultrasonic vibrations in it in the frequency range of 40-120 MHz. 4. The device according to claim 1, characterized in that the third central polarizer is made of two isosceles calcite prisms with an apex angle of 71°. 5. The device according to claim 1, characterized in that the laser source of radiation for illuminating the stone has a wavelength of 532 nm. 6. Device according to claim 1, characterized in that a notch filter is installed at the spectrometer input.

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