WO2012031625A2

WO2012031625A2 - Optical probe

Info

Publication number: WO2012031625A2
Application number: PCT/EP2010/063148
Authority: WO
Inventors: Tony Maddison
Original assignee: Foss Analytical Ab
Priority date: 2010-09-08
Filing date: 2010-09-08
Publication date: 2012-03-15
Also published as: WO2012031625A3

Abstract

An optical probe comprises a probe head (32) housing a transmission element(4), preferably comprising a fibre-optic (46) and a collection element (10), preferably comprising a fibre optic (48) each having a distal end (6,12) at which optical radiation is respectively transmitted into and collected from a sample receiving volume 8 defined, at least in part, by non parallel opposing side-walls (34, 36) of the probe head (32) in which the respective distal ends (6, 12) terminate. The respective distal ends (6,12) are disposed in an off-axis configuration in which the distal end (12) of the collection element (10) is located outside the acceptance cone (14) of the transmission element (4) to avoid collection at the distal end (12) of the collection element (10) of uninterrupted radiation emitted from the distal end (6) of the transmission element (4).

Description

Optical Probe

[0001] The present invention relates to an optical probe and in particular to a

fiber-optic probe for use in the spectrometric analysis of material.

[0002] Spectrometric analysis, particularly in the infrared wavelength region, is a known technique which is used to examine stationary or moving samples and for in-line process control and monitoring. Typically, optical radiation of a known spectral composition is directed at the sample which absorbs and scatters some of the radiation at various wavelengths. The diffusely scattered portion of the radiation is collected and analyzed, for example using a monochromator, an interferometer or other spectrometric instrument capable of resolving wavelength dependent intensity variations, in order to characterise the sample.

[0003] As used herein optical radiation refers to radiation within some or all of the wavelength region between ultraviolet and infra-red. Optics, optical probes and similar words and phrases are to be interpreted accordingly.

[0004] Optical probes, particularly fiber-optic probes, are frequently used in

combination with such instruments in order to direct light towards the sample and to collect the scattered light after passing through the sample. When utilizing the shorter wavelength infrared region, typically the near infrared (NIR) region at wavelengths below 1 100 nanometers (nm), the optical probe is traditionally disposed in a transmission configuration in which optical radiation is emitted from a distal end of a transmission element, through a sample to be measured which is located in a

measurement region and is collected at a distal end of a collection element. The amount of sample through which the radiation is transmitted depends on the absorption properties of the sample and in the NIR region this may be say around 10mm for cheese and around 20mm for cereal grain. The spectrometric instrument is optically coupled to a proximal end of the collection element to receive the collected light for analysis.

Typically, one or both of the transmission element and the collection element may comprise a fibre-optic element. [0005] One such probe is disclosed in US 6,137,108 of DeThomas et al. Here the probe comprises a slot defining a measurement region into which a sample is received. In one embodiment a transmission fibre-optic bundle and two collection fibre optic bundles are all disposed at a same side of the slot. A reflection element is located at the opposing side of the slot to these bundles and is disposed to reflect light from the transmission fibre- optic bundle towards the collecting fibre-optic bundles after it has traversed the slot. In an alternative embodiment, the transmission fibre- optic bundle and the collection fibre-optic bundle are disposed facing one another at opposite sides of the slot. A common feature of these

embodiments is that the transmission fibre-optic bundle and the collection fibre-optic bundle are located Όη-axis' relative to one another, that is, the collection element lies on an axis parallel to the direction of uninterrupted transmission of light between distal ends of the transmission and the collection elements.

[0006] Another such probe is disclosed in US 6,737,649 of Webster. Here the distal ends of the transmission and collection elements, exemplified as fibre-optic bundles, are located in an on-axis configuration in which the distal ends of the collection fibres are disposed in a ring around and set back from the distal ends of the transmission fibres. By this configuration collection of directly reflected light from the sample is significantly reduced.

[0007] It is well known that the accuracy and sensitivity of the analysis is

dependent upon the quality and quantity of the diffusely scattered light collected from the sample by the probe. It is generally accepted that good collection efficiency results in higher sensitivity. Moreover, the more the light has interacted with a sample the more representative of the bulk sample the information obtainable form the collected light will be.

[0008] Generally, inconsistent packing of the sample into the measurement

region between the probes creates many disturbances or bad spectral scans that corrupt the data and which have to be filter out and discarded. For example, when performing analysis on cereal grains, milk powder, pharmaceutical products and the like, the relatively large particle size may result in a 'pin hole' effect where the particles do not completely fill the measurement region and voids are formed in the sample to be measured which allows the light to directly pass through the spaces to the distal end of the collection element and causes blinding (or saturation) of the detector.

[0009] In process applications an opposing transmission/collection pair of fiberoptic probes which protrude into a process conduit through which a material, possibly a relatively viscous material such as butter, yoghurt or some oils, flows has shown to create problems such as physical integration into the conduit; obstruction of the flow; and alteration of the flow characteristics that may affect the measurements, such as turbulence creating air bubbles. Moreover, particularly in transmission configurations where the distance between distal ends of the of the optical probe is limited by the absorption properties of the material in the process line, trapping of the material between the distal ends of the transmission and collection elements of the probe may occur. This makes it uncertain that the sample is being exchanged with new material as it flows through the process conduit.

[0010] It is an aim of the present invention to at least alleviate one or more of the above mentioned problems associated with known optical probes.

[001 1] According to a first aspect of the present invention there is provided an optical probe comprising a transmission element and a collecting element each having a distal end at which optical radiation is respectively transmitted into and collected from a sample region of the probe characterised in that the respective distal ends are disposed in an off-axis configuration in which collection at the distal end of the collection element of uninterrupted radiation emitted from the distal end of the transmission element is avoided and in which the distal end of the collection element is located outside an acceptance cone of the transmission element (being generally understood by those skilled in the art to be delimited by the maximum angle of emission, i.e. acceptance angle, from the transmission element). In this manner light from the transmission element which does not interact with material in the sample region is not collected by the collection element and a gap between the distal ends of the transmission and collection elements may be provided which is not delimited by absorption properties of the sample.

[0012] Usefully, the optical probe may further comprise a probe head having

opposing non-parallel surfaces configured as wall portions of a slot for receiving the sample, which slot may be produced either as an open ended or as a closed ended slot; a first optically transparent window disposed in a one of the non parallel surfaces to form an optical interface between received sample and a first channel disposed internal the probe head; and a second optically transparent window disposed in the other of the non parallel surfaces to form an optical interface between received sample and a second channel disposed internal the probe head. The probe head may provide a degree of mechanical protection for the transmission and collection elements, is able to define the relative angle between these elements in a convenient and reproducible manner to facilitate production of the probe and may also be provided with at least a portion of its external surface which is shaped to enhance the flow of material past these elements and/or reduce the influence of the probe on the flow of bulk material in a process line.

[0013] According to a second aspect of the present invention there is provided a method of performing optical analysis of a sample comprising the steps of introducing into a sample distal ends of a transmission element and a collection element of an optical probe; by means of the optical probe, illuminating the sample with optical radiation emitted from the distal end of the transmission element; collecting through the distal end of the collection element the optical radiation after its interaction with the sample; and optically coupling a spectrometric instrument to a proximal end of the collection element of the optical probe to receive collected light for spectrometric analysis wherein the optical probe consist of an optical probe according to the first aspect. Thus the method has associated with it those advantages provided by the probe.

[0014] According to a third aspect of the present invention there is provided an optical analyser comprising a spectrometric instrument; a source of optical radiation and an optical probe having a transmission element and a collection element, the transmission element having a proximal end optically coupled to the source and a distal end for emitting radiation from the source into a sample, the collection element having a proximal end optically coupled to the spectrometric instrument and a distal end for collecting emitted radiation after its interaction with the sample; wherein the optical probe consist of an optical probe according to the first aspect of the present invention and wherein the analyser may be operated to perform the method according to the second aspect of the present invention.

[0015] Exemplary embodiments of the present invention will now be described with reference to the drawings of the accompanying figures of which:

[0016] Fig. 1 illustrates possible configurations of the transmission and the

collection elements of an optical probe according to the present invention;

[0017] Fig. 2 illustrates an optical analyser according to the present invention comprising an optical probe according to Fig.1 ;

[0018] Fig. 3 illustrates an embodiment of an optical probe usable in the optical analyser of Fig. 2;

[0019] Fig. 4 illustrates a cross-section of the probe head of Fig. 3; and

[0020] Fig. 5 illustrates a further embodiment of an optical probe according to the present invention.

[0021] Considering now the optical probe 2 illustrated in Fig.1 , a transmission element 4 is provided with a distal end 6 from which optical radiation is intended to be emitted to pass through a sample region 8 in an

uninterrupted direction parallel to an axis TV, which for a fibre optic is the fibre core axis (an axis passing through the centre of the optical fibre or other optical guiding element). The probe 2 also comprises a collection element 10 which has a distal end 12 for receiving optical radiation emitted by the transmission element 4 after its interaction with a sample in the sample region 8. Essential to the invention is that the distal end 12 of the collection element is located relative to the distal end 6 of the transmission element 4 so that light passing from the transmission element 4 cannot reach the collection element 10 without first having interacted with a sample in the sample region 8. [0022] In the present embodiment both the transmission element 4 and the collection element 10 are conveniently realised as fibre-optic elements, each one of which may comprise a cooperating bundle of optical fibres. It will be appreciated that other elements that can guide optical radiation to and from the sample region 8 may be substituted for the whole or a part of the fibre-optic elements without departing from the invention as claimed.

[0023] Where such fibre optic elements are employed then the distal end 12 of the collection element 10 is disposed in an off-axis arrangement relative to the axis A at a location outside a so-called acceptance cone 14 of the transmission element 4 such that collection of emitted radiation which passes through the sample region 8 uninterrupted by any sample is avoided. It is well known that light exits a fibre optic (here transmission element 4) within a certain cone known as the acceptance cone 14 of the fibre 4. The half angle Θ of this cone is called the acceptance angle. The acceptance angle Θ may be calculated from a knowledge of the indices of refraction of the core (n-i) and the cladding (n2) of the fibre optic element 4 and of the material into which the light will pass (n). This may be expressed as:

[0024] n sin© = (ni2 - n₂ ²)^½ (1 )

[0025] For other optical arrangements the maximum angle over which the optical element can emit or accept light is given by the equivalent, so-called, numerical aperture. Whilst in the majority of embodiments the relative orientation of the transmission element 4 and the collection element 10 which fulfils the criterion that light passing from the transmission element 4 cannot reach the collection element 10 without first having interacted with a sample in the sample region 8 may be calculated such relative orientations may also be simply derived experimentally dependent on the material being measured and the application in order to maximise rejection of light that has not interacted with a sample whilst defining a sampling region 8 through which a sample may easily pass.

[0026] In the present exemplary embodiment the collection element 10 is

illustrated as having a fibre core axis, B, located at 90° to the axis A but other possible relative dispositions of the collection element 10 according to the present invention with respect to the axis A are illustrated by broken line constructions of the collection element 10(i) and 10(ii). It will be appreciated that these alternative dispositions are provided by way of example only and are not intended to limit the optical probe 2 according to the present invention.

[0027] Considering now an embodiment of an optical analyser 16 according to the present invention which is illustrated in Fig. 2. The analyser 16 comprises an optical probe, in this exemplary embodiment the optical probe 2 of Fig. 1 , and a spectrometric instrument 18. The instrument 18 is operably connected to a proximal end 20 of the collection element 10, here a fibre-optic element, to receive light there from and to resolve wavelength dependent intensity variations in the received light. In use, a proximal end 22 of the transmission element 4, here also a fibre-optic, optically couples to a source of optical radiation, typically a source of NIR optical radiation 24, and functions to guide optical radiation emitted by the source 24 into a sample located in the sample region 8 between the distal ends 6,12 of the transmission fibre-optic 4 and the collection fibre-optic 10 respectively which, by way of example only, are located internal a process line 26 through which a material to be analysed is intended to flow in direction illustrated by the arrow of Fig. 2.

[0028] Such material may be, for example:

- Agriculture products such as grain for both unprocessed such as whole grain analysis of wheat, barley, soy beans and others and further processed product such as ground grain in animal feed production;

- Dairy products such as milk, cheese, processed cheese, yoghurt, quark, whey, and others;

- Meat and meat emulsions;

- Pharmaceutical products and intermediaries;

- Bio-fuel: raw, intermediate and/or final product.

[0029] Installations in process lines will typically require installation in pipes of different diameters, or in a vessel, tank or storage container with a flat or curved surface. The optical probe of the present invention allows for product to easily flow between the selected angle, such as a 'V shape between a 90° relative disposition to allow grain to easily flow in the sample region 8.

[0030] Further, it will be appreciated by those skilled in the art that other products which are today measured by NIR transmission are potential candidates for measurement with an optical probe and analyser according to the present invention. Moreover, the present invention is not limited to applications for in-line analysis in process lines but can also be used to monitor flowing material outside a process line, such as bulk grain on a conveyor moving to or from a storage silo or as it is unloaded from a bulk transporter; in the design of bench analysers of stationary or flowing material or in the design of hand-held analysers of stationary bulk materials such as say grain at grain receiving stations or in bulk transport or storage containers.

[0031] Selecting the optimal angle for a particular process, product and

application allows optimization of product flow and a lowering of the chance of blockage of product flow between the distal ends 6,12 of the probes 4,10.

[0032] Optically selecting the optimal angle for a particular process and product allows optimization to eliminate disturbances that can occur in the spectra such as turbulence causing air bubbles. With samples with a large particle size such as grain, for example wheat, the optical probe and analyser according to the present invention is immune to the 'pin hole' effect as only light that is scattered through the sample in the sample region 8 will reach the collection fiber 10 (hence reach the detector of the spectrometric instrument).

[0033] Preferably the optical probe according to the present invention may also include a probe head which advantageously provides a degree of mechanical protection for the transmission and collection elements, defines the relative angle between these elements in a convenient manner and also defines a sample region which can be shaped to enhance the flow of material past these elements whilst reducing the influence on the flow of bulk material in a process line. An exemplary embodiment of such an optical probe is illustrated in part in Fig. 3 and Fig. 4 with reference to the optical analyser of Fig. 2 and wherein the corresponding elements are provided with the same reference numerals. Considering now the optical probe 28 of Fig. 3 located, in use and by way of example, in a process pipe-line 26 through which bulk material to be analysed will flow in a direction generally indicated by the arrow. The optical probe 28 comprises a transmission element 4 and a collection element 10 which are both optically coupled at their distal ends to a probe head 32 that projects into the pipe-line 26 in either permanent or temporary sealing engagement so as to prevent egress of material from the pipe-line 26. The probe head 32 comprises opposing non-parallel surfaces 34, 36 in which surfaces 34,36 are provided windows 38,40 for providing an optical coupling between internal and external the probe head 32.

[0034] The surfaces 34,36 are inclined relative to one another to form an open- ended slot or groove 49 which delimits the sample region 8 between the windows and through which sample material may readily flow. Most usefully, and as illustrated in the present exemplary embodiment, the probe head 32 may be provided with a surface 39 ("leading edge") generally perpendicular to and facing the direction of flow shaped to reduce any adverse influence that the probe head 32 may have on the flow of material through the pipe-line 26 and/or aid flow of material through the sample region 8 by acting to preferentially direct material to that region 8.

[0035] The probe head 32 is illustrated in partial cross-section in Fig. 4. As can be seen, an optical window 38 and 40 is located in and parallel to a

respective surface 34 and 36 of the non-parallel opposing surfaces

34,36of the probe head 32. Each window 38, 40 is located in a sealing engagement with an associated channel 42,44 which passes through the probe head 32. In the present embodiment optical fibres 46,48 of the transmission element 4 and the collection element 10 respectively pass along an associated channel 42, 44 and terminate with their distal ends 6,12 at or close to the associated window 38,40. In other embodiments each channel 42, 44 may itself act as an optical waveguide to guide optical energy into and out of the sample region 8. [0036] The non-parallel opposing surfaces 34,36 are configured to form a groove 49 and are disposed relative to one another such that the fibre core axes A,B of the transmission fibre-optic 46 and the collection fibre-optic 48 respectively are substantially perpendicular to one another so that collection is minimised of optical irradiation by fibre-optic 48 of the collection element 10 which passes from the fibre-optic 46 of the transmission element 4 uninterrupted by any sample in the sample region 8. As may be seen from Fig. 4, in the present embodiment this is realised by locating the window 40 associated with the collection element 10 outside of the acceptance cone 14 of the transmission element 4, as previously described with respect to the embodiment of Fig. 1.

[0037] In the present embodiment the probe head 32 is also provided with a

peripheral flange 51 and compressible Ό' ring 53 by which it is intended to releasably seal the probe head 32 in connection with the pipe-line 26 (or similar wall portions of other material containers such as reaction vessels, ingredient, intermediate final or waste product storage tanks). The flange 51 is here provided with through holes 55 for receiving bolts to engage with threaded portions of the pipe-line 26. Such a releasable connection facilitates the removal of the probe head 32 for cleaning, repair or replacement. In other embodiments the probe-head 32 may be sealed in a more permanent manner to the appropriate wall section by means of welds or the like.

[0038] A further embodiment of an optical probe 50 according to the present

invention is illustrated in Fig. 5 (not to scale). The optical probe 50 comprises an elongate, here generally cylindrical, rigid body portion 52 having, at its distal end a generally conical probe head 54 forming a 'tip' that, due to its shape, can provide a reduced resistance to insertion into a sample material. Usefully in some embodiments the probe head 54 may be formed from or provided with a coating of low friction material (PTFE for example).

[0039] The probe head 54 has, in the present embodiment, formed in it a closed slot 56 which is delimited by opposing, non-parallel surfaces 58,60. An optically transparent window 62 is located in sealing connection with a one of the non-parallel surfaces 58 to form a protective optical interface between a distal end of an internal transmission fibre-optic 66 and a sample region 68 within the slot 56. The transmission fibre-optic 66 is provided to transmit optical radiation from external the probe 50, along internal the body portion 52 to exit the optical window 62 and illuminate sample within the sample region 68.

[0040] Similarly, an optically transparent window 64 is located in a sealing

connection with the other non-parallel surface 60 to form a protective optical interface between a distal end of an internal collection fibre-optic 70 and the sample region 68. The collection fibre-optic 70 is provided to transmit optical radiation emitted from the transmission fibre-optic 66 after its interaction with sample material in the sample region 68 along internal the body portion 52 to exit the optical probe 50 at a proximal end 72.

[0041] The relative inclination of the two non-parallel surfaces 58,60 and the

locations of their associated windows 62,64 are selected such that the associated fibre core axes are not parallel with one another and such that window 64 associated with the collection fibre-optic 70 lies outside the acceptance cone of the transmission fibre-optic 66. In this configuration the collection by the collection fibre-optic 70 of radiation which passes uninterrupted through the sample region 68 from the transmission fibre- optic 66 is at least reduced and preferably avoided. Moreover, and in common with other embodiments of the present invention, the size of the slot is not limited by the absorption properties of the material being sampled and so the risk of sample failing to be exchanged as the material under analysis and the optical probe are moved relative to one another is reduced.

[0042] It will be appreciated that some or all of the transmission fibre-optic 66 and the collection fibre-optic 70 may be replaced with any suitable optical waveguide without departing from the invention as claimed. It will be further appreciated that the probe head 54 may be formed with an open- ended slot 49 similar to that of the head 32 described in relation to Fig. 3 and Fig.4 and that the probe head 32 of Figs. 3 and 4 may be formed with a closed slot similar to that slot 56 described in respect of the head 50 of Fig. 5.

[0043] In the present embodiment a flexible optical coupling 74 is optically

coupled to the proximal end 74 of the optical probe 50 and is provided with optical waveguides, most suitably fibre-optic guides 76,78, which provide an optical connection between the transmission fibre-optic 66 and a source of optical radiation (not shown) and between the collection fibre- optic 70 and a spectrometric instrument or other optical analyser (not shown).

[0044] In one embodiment this probe 50 may be formed as a 'lance' in which the rigid body portion 52 may be of the order of 1 meter of more for insertion, for example into stationary bulk granular material in a silo or container. In another embodiment this probe 50 may be formed as a 'pen' in which the rigid body portion 52 may be of the order of a few tens of centimetres for insertion into small, bench-top, sample volumes of granular or liquid material.

[0045] It is intended that in use the optical probe according to the present

embodiment is employed in the optical analysis of a sample the probe head 54 is introduced into a sample causing sample to flow into the sample region 68 formed by slot 56 whereby distal ends of the

transmission element 66 and the collection element 70 of the optical probe are optically coupled to the sample via associated windows 62,64. The sample is then illuminated with optical radiation emitted from the distal end of the transmission element 66 which is collected through the distal end of the collection element 70 after its interaction with the sample in the sample region 68. Relative movement of the probe and the sample will cause new sample material to be exchanged with the old sample material that was in the sample region 68. A spectrometric instrument is provided optically coupled to a proximal end of the collection element of the optical probe to receive collected light for spectrometric analysis.

Claims

1. An optical probe (2,28,50) comprising a transmission element (4,46,66) and a collection element (10,48,70) each having a distal end (6,12) at which optical radiation is respectively transmitted into and collected from a sample

characterised in that the respective distal ends (6,12) are disposed in an off- axis configuration in which the distal end (12) of the collection element is located outside an acceptance cone (14) of the transmission element (4,46,66) to avoid collection at the distal end (12) of the collection element (10,48,70) of uninterrupted radiation emitted from the distal end (6) of the transmission element (4,46,66) .

2. An optical probe (2,28,50) as claimed in claim 1 characterised in that one or both the light transmitting element (4,46,66) and the light collecting element (10,48,70) comprises a fibre-optic.

3. An optical probe (2,28,50) as claimed in claim 1 or claim 2 characterised in that the distal ends (6,12) are arranged at an angle of 90 degrees to one another.

4. An optical probe (28,50) as claimed in any preceding claim characterised in that the probe further comprises a probe head (32,54) having opposing non- parallel surfaces (34,36,58,60) configured as wall portions of a slot (49,56) and defining, at least in part, a sample receiving volume (8,68); a first optically transparent window (38,62) disposed in a one of the non parallel surfaces (34,58) to form an optical interface between the sample receiving volume (8,68) and the transmission element (4,66) disposed internal the probe head (32,54); and a second optically transparent window (40,64) disposed in the other of the non parallel surfaces (36,60) to form an optical interface between the sample receiving volume (8,68) and the collection element (10,70) disposed internal the probe head (32,54).

5. An optical probe as claimed in claim 4 characterised in that at least a portion of the transmission element (4) comprises a fibre-optic (46,66) located within the probe head (32,54) with its distal end (6) terminating proximate the first window (38,62) and in that at least a portion of the collection element (10) comprises a fibre-optic (48,70) located within the probe head (32,54) with its distal end (12) terminating proximate the second window (40,64).

6. An optical probe (28) as claimed in claim 4 or claim 5 characterised in that the probe head (32) is configured with the slot as an open-ended slot (49).

7. An optical probe (50) as claimed in claim 4 or claim 5 characterised in that the probe head (54) is configured with the slot as a closed-ended slot (56).

8. An optical probe (50) as claimed in any of the claims 4 to 7 characterised in that the optical probe further comprises an elongate rigid body (52) for housing the transmission (70) and the collection element (66) and having a distal end formed as the probe head (54).

9. An optical probe (50) as claimed in claim 8 characterised in that the probe

head (54) is externally shaped to reduce resistance to its insertion into a sample material

10. An optical probe (50) as claimed in claim 9 characterised in that the probe head (54), at least at its external surface, is formed of a low friction material.

1 1. An optical probe (28) as claimed in any of the claims 4 to 7 characterised in that the probe head (32) is formed with at least a portion of an external body section (39) shaped to aid the flow of a sample towards the sample receiving volume 8.

12. A method of performing optical analysis of a sample comprising the steps of introducing into a sample distal ends of a transmission element and a collection element of an optical probe; by means of the optical probe, illuminating the sample with optical radiation emitted from the distal end of the transmission element; collecting through the distal end of the collection element the optical radiation after its interaction with the sample; and optically coupling a spectrometric instrument to a proximal end of the collection element of the optical probe to receive collected light for spectrometric analysis characterised in that the optical probe consist of an optical probe (2,28,50) as claimed in any of the preceding claims.

13. A method as claimed in claim 12 characterised in that the step of introducing the optical probe comprises introducing the distal end of the optical probe into a flow conduit in which the sample is intended to flow.

14. An optical analyser (16) comprising a spectrometric instrument (18) and an optical probe (2) having a transmission element (4) and a collection element (10), the transmission element (4) having a proximal end (22) for optically coupling to a source of optical radiation (24) and a distal end (6) for emitting radiation from the optically coupled source (24) into a sample receiving volume (8), the collection element (10) having a proximal end (20) optically coupled to the spectrometric instrument (18) and a distal end (12) optically coupled to the sample receiving volume (8) for collecting emitted radiation after its interaction with the sample; characterised in that the optical probe consist of an optical probe (2,28,50) as claimed in any of the claims 1 to 1 1 .