IL108513A - Meth d and appar tus f r exa ining an object - Google Patents

Description

METHOD AND APPARATUS FOR EXAMINING Al OBJECT THE APPLICANT:-GERSAN ESTABLISHMENT, A LIECHTENSTEIN COMPANY, STAEDTLE 36, 9490 VADUZ, LIECHTENSTEIN.

THE INVENTORS: 1. SMITH, MARTIN PHILLIP 18, HAMILTON ROAD, WARGRAVE, BERKSHIRE RGIO 8EE, ENGLAND 2. SMITH, ROBIN WYNCLIFFE 79, OUTRAM ROAD, CROYDON, SURREY CRO 6XJ, ENGLAND 3. WELBOURN, CHRISTOPHER MARK TYTHEBARN COTTAGE, BREADCROFT LANE, LITTLEWICK GREEN, MAIDENBEAD, BERKS, SL6 30F ENGLAND METHOD AND APPARATUS FOR EXAMINING AN OBJECT Background of the Invention This is a third division from Patent Application No. 097947.

In general terms, the invention relates to examining or classifying an object by detecting the spectral properties of the object. The invention is particularly, but not exclusively, concerned with identifying gemstones such as diamonds, e. g. distinguishing diamonds from diamond-like simulants and disti guishing natural diamonds from synthetic diamonds.

WO 86/07457 discloses a method for distinguishing diamond from diamond like simulant, by visually detecting the Raman signal emitted from a specimen which is irradiated with suitable exciting radiation. The Raman emission has two peaks, one on either side of the wavelength of the exciting radiation, termed the Stokes signal and the anti -Stokes signal. The Stokes signal is much stronger than the anti-Stokes signal, but it is still very .weak. One of the problems is that if a diamond-like simulant luminesces, it is very hard to discern the appropriate Raman peak against the luminescent background.

Diamond simulant comprises dense non-diamond material (eg. metal oxides, particularly zirconium dioxide) which has similar refractive properties to diamond. Synthetic diamond comprises diamond material (ie. crystalline carbon) produced by an industrial process.

The technique, disclosed by WO 86/07457 is only suitable for distinguishing diamond from diamond-like simulant. All diamonds, · natural or synthetic, show the Raman emission when irradiated with suitable exciting irradiation, and cannot be distinguished by this technique.

Independent inventive features f the apparatus described herein are described in the following co-pending British patent applications. British patent application number 9325532. 1 describes a method and apparatus for examining a diamond in which the intensity of radiation absorbed by the diamond at a wavelength substantially equal to 415.5 nm is observed, normalised to allow for the size of the diamond and used to classify the diamond as natural or synthetic.

British patent application number 2244329 describes a method and apparatus for examining an object where the object is irradiated with radiation of a plurality of wavelengths so that its transmission spectrum can be observed at a characteristic wavelength and at at least two reference wavelengths which are close to the characteristic wavelength, the observations being combined to produce a signal substantially independent of the size of the object which can be used to classify the object.

The Invention.

The invention provides a method of and apparatus for classifying an object' as set out in Claim 1, 16, 21, or 22 Preferred and/or optional features are set out in Claims 2 to 15 and 17 to 20.

At least in its preferred forms, the invention enables diamonds and other suitable gemstones to be examined and classified by operators with little scientific or technical training.

The object rr'=iy. be irradiated with stimulating radiation and the emission/luminescence of the object is examined. If it is desired to distinguish diamond from diamond simulant, it is preferable to use a laser that will cause Raman activation in the visible spectrum; a suitable Raman wavelength is about 552.4 nm which can be produced by an argon ion laser operating at 514.5nm, and in general terms the laser wavelength may lie between 450 nm and 1064 nm, but may be outside this range.

As the band passed by the alterable filter is altered, icy n ¾ jz. 4 the signal detected will be markedly different, depending on whether the object or gemstone is say a diamond, or a diamond simulant which does not exhibit Raman emission at the correct wavelength, and does not luminesce, or a diamond simulawt which luminesces. This is explained later with reference to the drawings.

Though it may be possible to use other methods, a simple method of altering the band passed is by tilting the filter about an axis normal to its optical axis. With narrow band pass filters, the cut-offs are most clearly defined when the filter is correctly orientated with its optical axis; as the filter is tilted, the centre of the band passed changes, and the band widens -.this widening is not essential to the preferred embodiment of the invention, but is an incidental effect.

The obje may alternatively be irradiated . ith light in the longwave ultraviolet/visible part of the spectrum and the absorption spectrum of the object may be studied by measuring the intensity of light absorbed by the obj ect.

The object may be illuminated with a lamp running oif a mains electricity supply. A change in the lamp supply voltage can alter the temperature of the emission source of the lamp and thus the spectral distribution of its output energy may vary. Provision should be made to detect this variation so that parameters 'such as ratios between transmitted wavelengths can be corrected for errors introduced, by the spectral variation. By making more than two observations of the absorption spectrum of the object, any spectral shift d--e to lamp variation- can be detected and compensated for.

If natural diamond is to be distinguished from synthetic diamond, the' absorption (or, equivalently, transmission) spectrum may be observed by measuring the absorption at 415. 5nm and at least two slightly different reference wavelengths, say 4l0nm and 418.5nm. The absorption should be measured at three wavelengths very clc.?e together, as this will help to clearly identify a characteristic absorption. 415.5nm is a very strong absorption, characteristic of diamonds of type IaAB. 418. 5nm and 4l0nm fall outside the absorption peak for this characteristic absorption and so the absorption is relatively low. In general, diamonds of different types to IaAB do not show a very strong absorption at 415. 5nm, and even if there is some absorption, it vill not be very ...uch different from the absorption at 10r.m and 418. 5nm.

Accordingly, diamonds of type IaAB can be positively distinguished, and as diamonds of this class of diamonds are effectively always natural in origin, the second embodiment of the first aspect of the invention allows all diamonds encountered to be classified as belonging to a class comprising definitely natural diamonds or a class containing diamonds which may or may not be natural. This will be explained further below.

The apparatus may be. very simple to use and construct, as it only has a small number of components. The whole apparatus may only occupy a space of about 25 x 10 x 15 cm, being suitable for use on a bench top. The method does not require any great skill on the part of the operator and is suitable for producing an answer very quickly.

The Drawings.

The invention will be further described by way of example, with reference to the accompanying drawings, in which; - Figure 1 shows an example of a portion of the absorption spectrum of type lb diamond; Figure 2 shows an example of a portion of the absorption spectrum of type IaAB diamonds; Figure 3 shows a high resolution transmission spectrum for a type IaAB diamond, between 4i0nm and 420nm; Figure 4 shows an apparatus for observing a gemstone according to a first embodiment of the invention; Figure 5 shows the filter of the apparatus of Figure 12 in first and second positions; Figure 6 shows the variation with angle of incidence of band' ass characteristics of the filter of Figures 4 and 5; Figure 7 shows the use of three observations to fit a curve; Figure 8 is a schematic illustration of apparatus according to a second embodiment of the invention; Figure 9 shows a flow chart for use with the i vention.

Figures 1 - to 3 One way of classifying diamonds is according to their spectros opic properties. The absorption spectrum of a diamond in the visible region will determine its colour. To a certain extent, it is possible to associate each type of diamond with a range of structure, concentration and composition of Impurity defects. An analysis of diamonds in this manner gives the following classi ication.

Type I This general type class is defined as the class of ' diamonds which have a measurable defect induced infra-red absorption in the 1-phonon region (below 1332 cni The defects result from the incorporation of nitrogen atoms into the crystal lattice substituting for carbon- atoms during growth of the diamond. Natural type I diamonds will typically contain several hundred to a few thousand ppm of nitrogen. The content of nitrogen in synthetic diamonds can be controlled during the process of synthesis ing the diamonds. This gives a range of nitrogen atom content of a few hundred ppm to practically zero in synthetic diamonds.

The general class type I is divided into the following subtypes: Type lb In this type of diamond single nitrogen atoms are substituted, for single carbon atoms at random throughout the lattice. This gives rise to an optical absorption starting at about 600nm which continues with increasing strength into the longwave ultra-violet region (Figure 1). This gives rise to the so-called canary yellow colour shown by some diamonds. Type lb diamonds represent a ncn-equilibrated form of diamond. Diamonds are formed at conditions of very high temperature and, pressure, arid if the diamond is maintained at these conditions impurity nitrogen atoms will tend to aggregate. Natural diamonds were usually maintained at these equilibrating conditions for geologically significant periods of time and accordingly type lb diamonds are rare in nature (much less than 1% of all natural diamonds). On the other hand, synthetic diamonds are not maintained at equilibrating conditions and accordingly most synthetic diamonds are type lb.

Type la This class comprises diamonds in which the nitrogen has migrated to form -more complex defects. There are two principal forms of nitrogen defect which are found in type la diamonds, the A form and the B form. The A form comprises 'irs of nitrogen atoms on nearest-neighbour substitutional sites. The B form of nitrogen is believed to comprise a complex of four substitutional nitrogen atoms surrounding a vacancy. The ratio of the concentration of A type defects to B type defects varies continuously, the extreme ends of the sequence being labelled type IaA and type IaB. Pure type laB diamonds are very rare. Synthetic type lb diamonds can be converted to type IaA by a high-temperature and high-pressure treatment. lo Type IaA diamonds have no absorption in the visible region of the spectrum so they are colourless. There is very little visible absorption associated with B centres, and as a result IaB diamonds are colourless. Most natural diamonds contain both A and B centres and are known as type IaAB. In addition to the two principle forms of nitrogen defect, the contain two "by-products" of the nitrogen aggregation process: platelets and N3 centres. Platelets are interstitial planar defects, a few tens of nanometres in- diameter lying on cube planes. These give rise to a peak in the infra-red spectrum. N3 centres comprise three co-planar nitrogen atoms probably surrounding a vacancy. N3 centres give rise to absorption between 490nm and 350nm with a sharp sero-phonon line at 415.5nm. This absorption in the blue/violet region causes the so-called cape yellow colour exhibited to a greater or lesser extent by the vast majority of natural diamonds (Figure 2). Figure 3 is a high resolution transmission spectrum showing the 415.5nm absorption of a type IaAB diamond in more detail. It can be seen that there is a strong decrease in transmission, at . about 415.5nm, transmission being much higher at other wavelengths, for example 410nm.

Type II a This class comprises diamond in which nitrogen is only present in trace amounts, of the order of 1 ppm. There is often a form of background absorption at the shorter wavelength end of the visible spectrum, giving some of these diamonds a generally brown colour. This neat-absence of nitrogen in diamonds rarely occurs in nature (less than 2% of natural diamonds are type Ila) but can be assurred in .the production of synthetic diamonds.

Type lib This, is a very rare class of semiconducting diamonds in nature. The dia.monds contain trace amounts of substitutional boron as semiconductor acceptor centres which give the diamonds a bluish colour due to the tail of the photoionization spectrum at the acceptor centre. Type lib diamonds are generally natural, but synthetic diamonds containing added boron can be produced.

In all, most natural diamonds are type IaAB and iaA, only about 2% being II, lb or laB.

Figure 4 Figure 4 is a schematic drawing of a first embodiment of apparatus according to the invention, which is set up to classify a finished diamond as definitely natural or not definitely natural. Α diamond 1 is illuminated with radiation generated by a halogen lamp 2. of a suitable wavelength. The illuminating radiation is fed to the diamond via a fibre optic 3 and, in the case of a brilliant cut diamond, the light is fed in through the table 4 of the diamond. A brilliant cut diamond is intended to be viewed through the table 4 and is so shaped that the maximum amount of light is reflected by the lower faces ' of the diamond back out of the table 4.

In order to study the absorption spectrum of zhe diamond, a second fibre optic 5 is provided to collect light leaving the diamond via the table 4. Transmitted light is fed via the fibre optic 5 into detector apparatus 6 which includes a filter 7. A photomultiplier tube or other photodetector 8 is provided to give a signal dependent upon the intensity of light passed by the filter 7, which signal is fed to an amplifier 9 ' and then to a microprocessor 10.

The filter 7 is rotatable about an axis 11 to transmit light at different wavelengths, being driven by a motor 12. The motor 12 can be controlled by the. microprocessor 10, a transducer 13 comprising a shaft encoder or the like being provided to give a- signal indicating the position of the filter 7. In order for' the readings taken by the apparatus to be simply presented and easily understood, a visual displo/ unit tor π Α 13 (VDU) 14 may be provided receiving signals from the microprocesso 10.

As shown in Figure 5, the filter 7 can' be rotated about an axis 11 normal to its optical axis 15 into a tilted position (as shown at T ). The band pass characteristics of the filter 7 vary with the angle 0 between- the optical axis 15 of the filter and the direction of incident light 16. Figure 6 shows the band pass characteristics of a CWL = 418.5nm filter at various values of Θ. I . may be seen that as 0 increases, the maximum of the transmission moves to lower wavelength, the transmission maximum decreases in intensity and the width of the band passed increases. The full width at half maximum for the filter where Θ = 0 is Inm. Such a filter is manufactured by Omega Optical Company in the USA.

Thus, if the filter 7 is tilted through a variety of angles Θ by the motor 12, a region of the absorption spectrum of the diamond 1 may be scanned and sampled.

The apparatus shown in Figure 4 can be used to classify a diamond as belonging to type IaAB or not. A filter 7 having the band pass characteristic shown in Figure 6 is used, so that a signal can be derived representive of the absorption of light at 415.5nm. On its own, this signal does not give much useful information unless it is normalized, because' the signal will vary with the size of the diamond. Furthermore, diamonds of type laAB will vary greatly in the absorption co-efficient at 415.5nm between themselves, and no positive range can be assigned to clearly identify a diamond of type laAB on, the basis of this one uncorrected absorption signal alone. Accordingly, a second measurement is taken at a reference wavelength of 410nm for example. This lies completely outside the absorption peak at 415.5nm and. is of higher energy..

The lamp 2 used to illuminate the diamond may be an halogen lamp, for example a 12 volt, 12 watt Thorn type M64 with a Spindler and Hoyer lens 063097. This form of lamp operates at about 3,000K with a peak towards the red end of the visible spectrum. The wavelengths to be observed lie on a steep part of the thermal radiation curve. Accordingly,, if the temperature of the lamp shifts to, say, 3, 200K due to a perturbation in the power supply, the shape of the curve will vary and the intensity of light at the wavelengths to be observed will vary quite markedly, the ratio of the intensities between the two irradiating wavelengths will vary, and so the reading based upon the intensity of radiation absorbed at these wavelengths by the diamond can be in error. In order to detect this error, a third measurement is made, for example at a wavelength- of 418.5nm.

Preferably, a series of measurements are made in the region 418.5 to 410nm, and the absorption results interpreted by a curve fitting technique, operated by the microprocessor 10 to detect if a 415.5nm absorption is in fact present.

The filter 7 is rotated at high speed (3,000 rpm) about its axis 11 and the absorption of light at various wavelengths (deducible from the angle Θ of the filter, measured by transducer 1 ,:, ) measured many times over and stored by the microprocessor. Thus instead of just three readings, a mass of data is obtained quickly and simply which can be analysed by a. statistical technique to provide more accurate information on the absorption characteristics of the diamond. This improves the repeatability of the test and reduces the error.

The microprocessor 10 can be programmed to compare the readings directly and to produce a signal representative of whether the diamond is natural or should be tested further, or all the readings may be shown numerically, or graphically on the VDU 14.

Figure 7 shows how the three measurements at 410, 415.5 and 418.5 m are used in the microprocessor to plot a curve showing the absorption characteristics of the diamond in this region of the spectrum, so that an absorption at 415..5nrrr can be clearly identified.

Three absorption curves are shown, for diamonds of type IaAB showing the cape yellow colour with varying degrees of intensity.

Being able to classify a diamond as type IaAB or not will be useful to the jeweller or other craftsman in identifying natural diamonds, as the vast .maj ority of natural diamonds belong to class IaAB and (because of. the complexity of the defects and the fact that they take a long time to develop) synthetic diamonds are effectively never type IaAB. Thus the apparatus of the invention can be set up as above, to divide all diamonds into one of two classes: definitely natural; possibly natural, possibly synthetic; The number of natural diamonds classified in the second class by the apparatus of Figure 4 will be very small (about 2%), comprising type la, lb, Ila, lib and laA or IaB diamonds, which are all very rare.

The apparatus of Figure 4 can also be used to measure the colour of a cape diamond, by measuring the strength of' the 415.5 m absorption.

In the apparatus shown- in Figure 4, the lamp 2 may be a 12 volt 12 watt halogen lamp manufactured by Thorn, type 64, using a lens 063097 manufactured by Spindler and Hoyer Limited. Suitable fibre optic cable is manufactured by Schott. The lenses shown in the detector 6 are, from left to right, a lens 063097 and lens 063045 respecti ely, both manufactured by Spindler and Hoyer. The photomultiplier tube 8 can be of the type manufactured by Hamamatsu KK.

Figure 8 Figure 8 shows a second embodiment of apparatus for classifying a diamond according to the invention. In this apparatus, radiation is produced by a source 17 and fed into the system via. mirror 18 through a wheel 19 having a broad band pass filter 20 for excluding infra red radiation or aperture 21 with infra red filter for controlling dynamic range. The wheel 19 can be rotated by motor 22 to present a different aperture size 23, also having an infra red filter. The radiation passes along a fibre optic system which has an input arm 24 and an output arm 25 in a similar manner to the arrangement of Figure 4. The diamond 1 is irradiated with radiation and the transmitted radiation is collected by the fibre optic system and. fed to a photomultiplier tube 26 through a narrow band pass filter 27 that passes radiation of wavelength 415.-5nm. The signal from the photomultiplier tube 26 is amplified at 2.8 and fed to a microprocessor 29 which can operate with a visual display unit 30. Using the apparatus of Figure 8, instead of tilting the filter, second and third narrow band pass .filters 31 and 32 of slightly different bandpass characteristics, say 410nm and 413.5nm, may be interposed between the diamond 1 and the photomultiplier tube 26 by rotating the filter wheel 33 using motor 34. The filter wheel 33 may be rotated at high speed (for example 3, OOOrpm) to obtain a large number of measurements as in the apparatus of Figure 4. Thus in a similar manner to the apparatus of Figure 4, a signal representative of the absorption at the characteristic wavelength 415.5nm and at a reference wavelengths 410nm and 418.5nm can be obtained and compared. If the first signal is higher than the second and third signals, a strong absorption at 415.5nm. has been identified and the diamond is accordingly a type laAB diamond and is classified as natural. Diamonds of other types do not give very different 415.5nm, 418.5nm and 410nm signals and are classified as possibly natural or possibly synthetic.

The lamp 17, the fibre optic cable 24, 25 and the photomultiplier 26 used in the apparatus Figure 8 may be the same as those used in the specific embodiment shown in Figure 4. A Spindler and Hoyer mirror 18 may be used.

Figure 9 ■ Figure 9 shows a flow chart for classifying a finished diamond using apparatus according to the invention. The diamond is first analysed in terms of its colour at 36. Two classes 37, 38 are produced, consisting of the following colour types (with their estimated occurence, as a percentage, derived from intake figures for +0.5ct rough diamonds):' Class 1 Tinted white to yellow (72%) Fancy yellow (less than 0.1%) Brown (approximately 1%) Green and yellow green (less than 0.1%) Pink (less than 0.1%) Class 2 Colourless (27%) Blue (less than 0.1%) Diamonds of Class 1 are subjected to the 415.5nm test a*" 39 to produce a class of diamonds which are definitely natural (type IaAB) and a class of diamonds which are not definitely natural or 40 and 41. Class 2. diamonds are rejected as not definitely natural.

The present invention has been described above purely by way of example, and modifications can be made within the invention.

Claims (18)

1. Clai ms 1. A method of classifying an object, comprising irradiating the object; . . observing the object through a narrow band pass filter that passes radiation of a first wavelength corresponding substantially to a characteristic wavelength of a particular class of objects;, cycling the wavelength passed by the filter a number of times through each wavelength of a set of wavelengths including the characteristic wavelength and at least two reference wavelengths; making a plurality of observations of the object at each of the wavelengths of said set; and using the observations at the wavelengths of said set to classify the object as belonging o the particular class of objects or not.

2. The method of Claim 1, wherein the filter is tilted about an axis normal to its optical axis to alter the band of radiation passed by the filter.

3. The method of Claim 2, wherein the filter is tilted to and fro between a first position and; a second position.

4. The method of Claim 3, wherein the first position corresponds to the filter being normal to the optical axis.

5. The method of Claim 1 to 4, wherein the intensity of radiation absorbed by the object at the first wavelength and at the reference wavelengths is observed, and wherein the reference wavelengths lie substantially outside the absorption peak corresponding to the characteristic wavelength in the absorption spectrum of an object of the particular class.

6. The method of Claim 5, wherein the object is classified belonging to the particular class of objects if the' intensity of radiation absorbed at the first wavelength is greater than at the reference wavelengths.

7. The method of any of the Claims 1 to 6, wherein the method is for exazr.ining diamonds.

8. The method of Claim 7, wherein the first wavelength is substantially equal to 415 nm, and the particular class of object comprises type laAB diamonds.

9. The method of Claim 8, wherein one of. the reference wavelengths is about 410nm. -

10. The method of any of the preceding claims, further comprising correcting the observations at the first wavelength and at one of the reference wavelengths to allow for spectral variations in the radiation irradiating the object.

11. The method of any of claims 1 to 4, wherein the object is viewed through a broad band pass filter which passes radiation of the first wavelength and the reference wavelengths.

12. The method of Claim 11, wherein the broad band pass filter is a laser blocking filter, the object being irradiated using laser radiation.

13. The metaod of Claim 11 or 12, wherein the characteristic radiation is visible light.

14. The method of Claim 13, wherein the object is observed by eye.

15. The method of any of Claims 11 to 14, wherein the first wavelength corresponds to the Raman emission of a di amond.

16. Apparatus for classifying an object, comprising: means for irradiating the object; a filter which passes radiation at a first wavelength substantially corresponding to a characteristic wavelength of a particular class of objects; means for cyclically altering the band of radiation passed by the filter through each of a set of wavelengths including the first wavelength and at least two reference wavelengths different from the first wavelength; means for giving signals dependent upon the intensity of radiation passing through the filter, and means for classifying the object as belonging to the first class of objects or not on the basis" of the signals.

17. The apparatus of Claim 16, wherein the filter is tilted about, an axis normal to its optical axis to alter the band of radiation passed by the filter.

18. The apparatus of Claim 17, wherein the the filter-passes radiation of the first wavelength when it is normal to the optical axis. ?,9. The. apparatus of any of Claims 15 to 18, configured to carry out the method of any of Claims 2 to 15. 20. A method of examining an object, substantially as herein described with reference to Figures 1 to 3, 4 to 7, 8 and 9 of the accompanying drawings. 21. Apparatus for examining an object, substantially as herein described with reference to Figures 1 to 3, 4 to 7, 8 and 9 of the accompanying drawings.. F R THE APPLICANTS, PATENT ATTORNEY .