GB2274165A

GB2274165A - Method and apparatus for examining an object

Info

Publication number: GB2274165A
Application number: GB9325532A
Authority: GB
Inventors: Martin Phillip Smith; Robin Wyncliffe Smith; Christopher Mark Welbourn
Original assignee: Gersan Ets
Current assignee: Gersan Ets
Priority date: 1990-04-24
Filing date: 1993-12-14
Publication date: 1994-07-13
Anticipated expiration: 2013-12-14
Also published as: GB2274165B; GB9325532D0

Description

METHOD AND APPARATUS FOR EXAMINING AN OBJECT.

Background of the Invention In general terms, the invention relates to examining cr 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 distinguishing 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 actually 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.

The Invention.

The first aspect of the invention provides a method of and apparatus for classifying an object as set out in Claims 1, 13, 46 or 47 and a method of and apparatus for examining an object as set out in Claims 22,33, 42 or 43. Preferred and/or optional features are set out in Claims 2 to 17,19 to 21,23 to 32 and 34 and 35.

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.

According to a first embodiment of the first aspect of the invention, the object is irradiated with stimulating radiation and the emission/luminescence of the object is examined. If it is desired to distinguish diamond fror. 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, 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 simulant which luminesces-This is explained later witn 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.

According to a second embodiment of the first aspect of the invention, the object is irradiated with 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 object.

The object may be illuminated with a lamp running off 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 due 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 410nm and 418.5nm. The absorption

should be measured at three wavelengths very close together, as this will help to clearly identify a characteristic absorption. 415.5nm is a very strong absorption, characteristic of diamonds of type laAB.

418. 5nm and 410nm fall outside the absorption peak for this characteristic absorption and so the absorption is relatively low. In general, diamonds of different types to laAB do not show a very strong absorption at 415. 5r. : n, and even if there is some absorption, it will not be very much different from the absorption at 410nm 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 second aspect of the present invention provides a method of and apparatus for classifying a diamond as definitely natural or not definitely natural as set out

n Claims 36, 38, 44 or 45. Preferred and/or optional features of the invention are set forth in Claims 37 and 39 to 41.

: n a similar manner to the second embodiment of the first aspect of the invention, a measurement of the absorption at 415.5nm is taken for the diamond and normalized on the basis of some parameter representative of the size of the diamond, for example the weight, or the absorption at at least one reference wavelength. As above, two or more reference wavelengths should be studied for the best results. For example, the absorption at 410nm and 418. 5nm can be measured.

Thus any spectral shift, due to illuminator variation, can be detected and compensated for. Alternatively, a single reference wavelength (for example 410nm) may be measured, and spectral variations due to temperature variations of the lamp can be detected by sampling radiation directly from the lamp.

The invention also provides combined apparatus for performing any or all of the operations of the methods of the first and second aspects of the invention.

The Drawings.

The invention will be further described by way o, ; : example, with reference to the accompanying drawings, in which :- Figure 1 is a schematic side view of apparatus in accordance with the first embodiment of the first aspect of the invention ; Figure 2 illustrates the bands passed by the two filters in Figure 1; Figures 3a to 3c illustrate what occurs in figure 1 when a diamond with no luminescence is examined, showing the signal position, the no signal position., and the result of rocking ; Figures 4a to 4c correspond, but show the situation for a diamond with luminescence; Figures 5a to 5c correspond, but show the situation with a diamond simulant without luminescence; and Figures 6a and 6c correspond, but show the situation with a diamond simulant with luminescence ;

Figure 7 shows an example of a portion of the absorption spectrum of type Ib diamond ; Figure 8 shows an example of a portion of the absorption spectrum of type IaAB diamonds; Figure 9 shows a high resolution transmission spectrum for a type IaAB diamond, between 410nm and 420nm ; Figure 10 shows an apparatus for observing a gemstone according to the second embodiment of the first aspect of the invention; Figure 11 shows the filter of the apparatus of Figure 12 in first and second positions ; Figure 12 shows the variation with angle of incidence of band pass characteristics of the filter of Figures 10 and 11 ; Figure 13 shows the use of three observations to fit a curve; Figure 14 is a schematic illustration of apparatus according to the second aspect of the invention ; Figure 15 shows an alternative embodiment of the second aspect of the invention;

Figure 16 shows a flow chart for use with-he first or second and third aspects of the invention.

Figures 1 to 6 Figure 1 shows a stone 1 on a dop 2, the stone 1 being illuminated with a laser 3 which may be an argon ion

laser operating a514. Snm (shown as 9 in Figure 2), t thereby stimulating luminescence of the stone 1 if the stone 1 is capable of it at the stimulating wavelength.

The stone 1 is viewed by eye through a laser blocking filter 4 which rejects light at laser wavelength (for example a Schott OG 530 rejects the laser light at 514.5nm) and a narrow band pass filter 5, for example a inm FWHM 4 cavity design centred at 552. 4 nm. manufactured by the Andover Corporation in the USA. The narrow band pass filter 5 can be tilted about an axis 6 normal to its optical axis, and a drive 7 is shown for rocking or oscillating the filter 5.

The band passes of the narrow band pass filter 5 are shown in Figure 2, which shows graphs of transmissivity T against wavelength X for incident light parallel to and for incident light inclined to the optical axis of the filter 5. The continuous lines indicate the bands passed when the filter is set up properly, nornal

to the optical or viewing axis. The filter 5 passes a

% N-lalb narrow band of about 1 nm (measured at the height of

half maximum transmissivity), centered on the Raman emission (Stokes) at 552.4 nm. It can be seen that with this set-up a narrow band of detectable radiation wavelengths can pass-in this case, detectable radiation is radiation that can be detected by the eye.

As the filter 5 is tilted about the axis 6, its optical characteristics alter in that its cut-on moves to lower wavelengths and the band passed widens-this is illustrated in the dashed lines in Figure 2.

The filter 5 can be set-up so that it is rocked from a first setting represented by the continuous line in Figure 2 to a second setting represented by the dashed line in Figure 2. This rocking can be done by hand, or, as indicated in Figure 1, by a motor drive 7.

Figures 3a-3c illustrate what occurs when the stone 1 is a non-luminescing diamond. Figures 3a and 3b are graphs of. against X, while Figure 3 is a graph of intensity i against time t. The Stokes emission (illustrated as the peak in the emission 8) is passed when the filter 5 is in its first setting, and thus there is a signal (Figure 3a). When the filter 5 is in its second setting, the Stokes emission 8 is blocked and

there is no signal (Figure 3b). As the filter 5 is rocked between the first and second settings, there is a an approximately square-wave signal with no dc shift, i. e. the troughs are a--zero intensity (Figure 3c).

If the stone 1 is a diamond with luminescence, there will be a signal when the filter 5 is at normal incidence (Figure 4a) comprising the Raman and luminescence background. When the filter 5 is tilted (Figure 4b) a signal corresponding to luminescence background is produced. As the filter 5 is rocked between the first and second settings, there is an approximately square-wave signal with the troughs at non-zero intensity (Figure 4c).

If the stone 1 is a diamond simulant with no

luminescence, there is no signal at all (Figures 5a to 5).

If the stone 1 is a diamond simulant with luminescence, in the first setting of the filter 5 there will be a signal. However, as the filter 5 is gradually moved from the first setting to the second setting, its band pass width slightly increases whilst its peak transmission drops slightly; in many cases, to a good first approximation the total integrated light transmission is

unchanged and the total signal from the background luminescence spectrum is approximately constant. Thus, the signal is as in Figure 6c.

Instead of the human eye, a suitable photodetector 10 can be used. The photodetector can be associated with processing equipment to distinguish between the types of signals shown in Figures 3 to 6 inclusive.

Figures 7 to 9 One way of classifying diamonds is according to their spectroscopic 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 classification.

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 1332cm). 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 synthesising 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 Ib 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 7). This gives rise to the so-called canary yellow colour shown by some diamonds. Type Ib diamonds represent a non-equilibrated form of diamond. Diamonds are formed at conditions of very high temperature and pressure, and 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 Ib 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 Ib.

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 Ia diamonds, the A form and the B form. The A form comprises pairs 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 IaB diamonds are very rare. Synthetic type Ib diamonds can be converted to type IaA by a high-temperature and high-pressure treatment.

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, they 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 zero-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 8). Figure 9 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 IIa 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 near absence of nitrogen in diamonds rarely occurs in nature (less than 2% of natural diamonds are type IIa) but can be assurred in the production of synthetic diamonds.

Type IIb This is a very rare class of semiconducting diamonds in nature. The diamonds 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 IIb diamonds are generally natural, but synthetic diamonds containing added boron can be produced.

In all, most natural diamonds are type IaAP and IaA, only about 2% being II, Ib or IaB.

Figure 10 Figure 10 is a schematic drawing of apparatus according to the second embodiment of the first aspect of the invention, which is set up to classify a finished diamond as definitely natural or not definitely natural.

A diamond 12 is illuminated with radiation generated by a halogen lamp 13 of a suitable wavelength. The illuminating radiation is fed to the diamond via a fibre optic 14 and, in the case of a brilliant cut diamond, the light is fed in through the table of the diamond. A brilliant cut diamond is intended to be viewed through the table and is so shaped that the maximum amount of light is reflected by the lower faces of the diamond back out of the table 15. In order to study the absorption spectrum of the diamond, a second fibre optic 16 is provided to collect light leaving the diamond via the table 15. Transmitted light is fed via the fibre optic 16 into detector apparatus 17 which includes a filter 18. A photomultiplier tube or other photodetector 19 is provided to give a signal dependent upon the intensity of light passed by the filter 18, which signal is fed to an amplifier 20 and then to a microprocessor 21. The filter 18 is rotatable about an axis 25 to transmit light at different wavelengths.

Rotation can be driven by a motor 22 or by hand. The motor 22 can be controlled by the microprocessor 21, a transducer 23 comprising a shaft encoder or the like being provided to give a signal indicating the position of the filter 18. In order for the readings taken by the apparatus to be simply presented and easily understood, a visual display unit (VDU) 24 may be provided receiving signals from the microprocessor 21.

As shown in Figure 11, the filter 18 can be rotated about an axis 25 normal to its optical axis 26 into a tilted position (as shown at 18'). The band pass characteristics of the filter 18 vary with the angle 9 between the optical axis 26 of the filter and the direction of incident light 27. Figure 14 shows the band pass characteristics of a CWL = 418.5nm filter at various values of e. It may be seen that as 9 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 8 = 0 is lnm. Such a filter is manufactured by Omega Optical Company in the USA.

Thus, if the filter 18 is tilted through a variety of angles 8 by the motor 22, a region of the absorption spectrum of the diamond 12 may be scanned and sampled.

The apparatus shown in Figure 10 can be used to classify a diamond as belonging to type IaAB or not. A filter 18 having the band pass characteristic shown in Figure 12 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 IaAB 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/41s. snm and is of

higher energy. In general, for non type laAB diamonds measurements at 415.5nm and 410nm would be expected to be very similar in size, the absorption at higher wavelength being lower if there were any difference. In the case of diamonds of type IaAB, the absorption at longer wavelength is expected to be much stronger.

Thus, a diamond which gives a much stronger absorption at 415.5nm than 410nm may be classified as type IaAB.

The lamp 13 used to illuminate the diamond may be an halogen lamp, for example a 12 volt, 12 watt Thorn type M54 with a Spindler and Hoyer lens 063097. This form of lamp operates at about 3, 000K with a peak in the middle part 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 may be made at a wavelength of, for example 418. 5nm. Preferably, a whole 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 21 to detect if a 415.5nm absorption is in fact present.

In a more advanced application of this technique, the filter 18 is rotated at high speed (3, 000 rpm) about its axis 25 and the absorption of light at various wavelengths (deducible from the angle 6 of the filter, measured by transducer 23) 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 21 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 24.

Figure 13 shows how the three measurements at 410, 415. 5 and 418.5nm can be 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. 5am can be clearly identified.

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

Instead of correcting the first and second wavelength measurements using a third wavelength and a curve fitting technique, the spectrum of the lamp''ray be sampled directly, using a reference channel. A third fibre optic may be used, leading directly from the lamp 13 to a detector, which passes data to the microprocessor 21.

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 majority 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 laAB. 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 10 will be very small (about 2%), comprising type la, Ib, IIa, IIb and IaA or IaB diamonds, which are all very rare.

The apparatus of Figure 10 can also be used to measure the colour of a cape diamond, by measuring the strength of the 415.5nm absorption.

In the apparatus shown in Figure 10, the lamp 13 may be a 12 volt 12 watt halogen lamp manufactured by Thorn, type M64, using a lens 063097 manufactured by Spindler and Hoyer Limited. Suitable fibre optic cable is manufactured by Schott. The lenses shown in the detector 17 are, from left to right, a lens 063097 and lens 063045 respectively, both manufactured by Spindler and Hoyer. The photomultiplier tube 19 can be of the type manufactured by Hamamatsu KK.

Figure 14 Figure 14 shows apparatus for classifying a diamond according to the second aspect of the invention. In this apparatus, radiation is produced by a source 23 and fed into the system via mirror 29 through a wheels 30 having a broad band pass filter 30a for excluding infra red radiation or aperture 30b for controlling dynamic range. The wheel 30 can be rotated by motor 31 to present a different aperture size 30c. The radiation passes along a fibre optic system which has an lnput ar 32 and an output arm 33 in a similar manner to the arrangement of Figure 10. The diamond 34 is irradiated with radiation and the transmitted radiation is collected by the fibre optic system and fed to a photomultiplier tube 36 through a narrow band pass filter 35a that passes radiation of wavelength 415.5nm.

The signal from the photomultiplier tube 35 is amplified at 37 and fed to a microprocessor 38 which can operate with a visual display unit 39. Using the apparatus of Figure 14, instead of tilting the filter, second and third narrow band pass filters 35b and 35c of slightly different band pass characteristics, say 410nm and 418.5nm, may be interposed between the diamond 34 and the photomultiplier tube 36 by rotating the filter wheel 35 using motor 31. The filter wheel 35 may be rotated at high speed (for example 3, OOOrpm) to obtain a large

number of measurements as in the apparatus of figure 10. Thus in a similar manner to the apparatus of Figure 10, 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 28, the fibre optic cable 32,33 and the photomultiplier 36 used in the apparatus Figure 14 may be the same as those used in the specific embodiment shown in Figure 10. A Spindler and Hoyer mirror 29 may be used.

Figure 15 shows an alternative embodiment of apparatus for use with the second aspect of the invention, in which the reference numerals have the same meaning as in Figure 14 but only two narrow band pass filters are included in wheel 35, and only two absorption measurements are made. An additional reference channel 40 is provided which directs light from the source 28

through the filter wheel 35 to the photomultiplier tube 36. Information can then be given directly to the microprocessor 38 on the spectral characteristics of the lamp 28 when a measurement is taken.

Figure 16 Figure 16 shows a flow chart for classifying a finished diamond using apparatus according to the first and second aspects of the invention. The diamond is first analysed in terms of its colour at 41. Two classes 42, 43 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 at 44 to produce a class of diamonds which are definitely natural (type lab) and a class of diamonds which are not definitely natural or 45 and 46. 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 spirit of the invention. The invention also consists in any individual features described or implicit herein or shown or implicit in the drawings or any combination of such features or any generalisation of any such features or combination.

Claims

Claims 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 ; altering the wavelength passed by the filter ; observing the object at at least one reference wavelength ; and comparing the observations at the first and reference wavelengths and thereby classifying the object as belonging to 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.

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4. The method of any of Claims 1 to 3, wherein the intensity of radiation absorbed by the object at the first wavelength and at the reference wavelength is observed, and wherein the reference wavelength lies substantially outside the absorption peak corresponding to the characteristic wavelength in the absorption spectrum of an object of the particular class.

5. The method of Claim 4, wherein the object is classified as belonging to the particular class of objects if the intensity of radiation absorbed at the first wavelength is greater than at the reference wavelength.

6. The method of any of the Claims 1 to 5, wherein the method is for examining diamonds.

7. The method of Claim 6, wherein the first wavelength is substantially equal to 415. 5nm, and the particular class of object comprises type IaAB diamonds.

8. The method of Claim 7, wherein the reference wavelength is about 410nm.

9. The method of any of the preceding claims, further comprising correcting the observation at the first wavelength and the reference wavelength to allow for

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spectral variations in the radiation irradiating the object.

10. The method of any of the preceding claims, wherein the object is observed at at least two reference wavelengths.

11. The method of Claim 10, wherein :

c the wavelength passed by the filter is cycled a number of fT times through each wavelength of a set of wavelengths including the first and second reference wavelengths; a plurality of observations of the object are made at each of the wavelengths of said set; and the observations at the wavelengths of said set are used to classify the object as belonging to the particular class of objects or not. l The method of Claim 3, wherein the first position corresponds to the filter being normal to the optical axis.

13. The method of any of Claims 1 to 3, wherein the object is viewed through a broad band pass filter which

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passes radiation of the first wavelength and the reference wavelength.

14. The method of Claim 13, wherein the broad band pass filter is a laser blocking filter, the object being irradiated using laser radiation.

15. The method of Claim 13 or 14, wherein the characteristic radiation is visible light.

16. The method of Claim 15, wherein the object is observed by eye.

17. The method of any of Claims 13 to 16, wherein the first wavelength corresponds to the Raman emission of a diamond.

18. 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 altering the band passed by the filter to a reference wavelength different from the first wavelength;

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means for observing the object through the narrow band pass filter, whereby the object can be classified as belonging to the first class of objects or not.

19. The apparatus of Claim 18, wherein the filter is tilted about an axis normal to its optical axis to alter the band of radiation passed by the filter.

20. The apparatus of Claim 19, wherein the filter passes radiation of the first wavelength when it is normal to the optical axis.

21. The apparatus of any of Claims 18 to 20, configured to carry out the method of any of Claims 2 to 17.

22. A method of examining an object, comprising: viewing the object through two band pass filters, one of which passes a substantially narrower band than the other, in order to detect a characteristic radiation wavelength which passes through both filters; altering the band passed by one of the filters so that at a first setting the characteristic radiation passes through both filters, and at a second setting, the characteristic radiation cannot pass through both filters ; and

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detecting the change in radiation passing through both filters as the respective filter is altered.

23. The method of Claim 22, wherein the filter passing the wider band is a laser radiation blocking filter.

24. The method of Claim 22 or 23, wherein the filter passing the narrower band is a narrow band pass filter, and the band of the narrow band pass filter is altered.

25. The method of any of Claims 20 to 22, wherein the band passed is altered by tilting the filter about an axis normal to its optical axis.

26. The method of Claim 25, and arranged such that when the alterable filter is normal to its optical axis, the characteristic radiation is passed through both filters.

27. The method of any of Claims 22 to 26, wherein the band passed by the alterable filter is altered to and fret between two settings.

28. The method of any of Claims 22 to 27, wherein the characteristic radiation is visible light.

29. The method of Claim 28, wherein the characteristic radiation is directly detected by eye.

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30. The method of any of Claims 22 to 29, wherein the object is irradiated with stimulating radiation and the luminescence of the object is examined.

31. The method of Claim 30, wherein the Raman emission of the object is examined.

32. The method of any of Claims 22 to 31, and used for ascertaining whether the object is diamond or not.

33. Apparatus for examining an object, comprising : two band pass filters, one of which passes a substantially narrower band than the other, in order to detect a characteristic radiation wavelength which passes through both filters ; and means for altering the band passed by one of the filters so that at a first setting, the characteristic radiation passes through both filters, and at a second setting, the characteristic radiation cannot pass through both filters; whereby the change in radiation passing through both filters as the respective filter is altered, can be detected.

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34. The apparatus of Claim 33, wherein the filter passing the broader band is a laser radiation blocking filter.

35. The apparatus of Claim 33 or 34, and arranged to carry out the method of any of Claims 23 to 32.

36. A method of classifying a diamond, comprising: irradiating the diamond; deriving a signal dependent upon the intensity of radiation absorbed by the diamond at a wavelength substantially equal to 415.5nm ; normalizing said signal on the basis of a parameter representative of the'size of the diamond ; and classifying the diamond as belonging to a class of diamonds which may be natural or synthetic or as belonging to a class of diamonds which are definitely natural, on the basis of the normalized signal.

37. The method of Claim 36, wherein said signal is normalized by comparing said signal with at least one reference signal, wherein said reference signal depends upon the intensity of radiation absorbed by the diamond

I

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at a wavelength substantially outside the absorption peak corresponding to 415.5nm in the absorption spectrum of type IaAB diamonds.

38. Apparatus for classifying a diamond, comprising: means for irradiating the diamond; means for giving a signal dependent upon the intensity of radiation absorbed by the diamond at a wavelength substantially equal to 415.5nm ; means for normalizing said signal on the basis of a parameter representative of the size of the diamond, so that the diamond can be classified as belonging to a first class comprising natural or synthetic diamonds or as belonging to a second class comprising natural diamonds only, on the basis of the normalized signal.

39. The apparatus of Claim 38, wherein tne means for giving a signal dependent upon the intensity of radiation absorbed at 415.5nm comprises a filter passing radiation at a wavelength substantially equal to 415. 5nm.

40. The apparatus of Claim 39, comprising means for giving a reference signal dependent upon the intensity of radiation absorbed by the diamond at a wavelength

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substantially outside the absorption peak at 415.5nm in the absorption spectrum of type IaAB diamonds; and means for comparing said signal and reference signal, the gemstone being classified as belonging to the second class if said signal is greater than said reference signal.

41. The apparatus of Claim 40, comprising means for tilting the filter in order to pass light of a wavelength substantially different to 415.5nm.

42. A method of examining an object, substantially as herein described with reference to Figures 1 to 6 of the accompanying drawings.

43. Apparatus for examining an object, substantially as herein described with reference to, and as shown in Figures 1 to 6 of the accompanying drawings.

44. A method of classifying a diamond as definitely natural or not definitely natural, substantially as herein described with reference to, and as shown in Figures 10 to 14 and 16 of the accompanying drawings.

45. Apparatus for classifying a diamond as definitely natural or not definitely natural, substantially as herein described with reference to, and as shown in Figures 10 to 14 of the accompanying drawings.

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46. Apparatus for classifying an object, substantially as herein described with reference to, and as shown in any of Figures 1 to 6, or 10 to 14 of the accompanying drawings.

47. A method of classifying an object, substantially as herein described with reference to, and as shown in

or 10 to IA Figures 1 to 6, or 10 to 14 of the accompanying drawings. 48. Apparatus for classifying an object, substantially as herein described with reference to, and as shown in Figures 15 of the accompanying drawings.

49. Method of classifying an object, substantially as herein described with reference to, and as shown in Figure 15 and 16 of the accompanying drawings,