WO2006032910A1

WO2006032910A1 - Measuring device and system for measuring spectral reflectance characteristics

Info

Publication number: WO2006032910A1
Application number: PCT/GB2005/003678
Authority: WO
Inventors: Gary C. Stevens; Henryk Herman; Patrick Baird
Original assignee: The University Of Surrey
Priority date: 2004-09-25
Filing date: 2005-09-23
Publication date: 2006-03-30
Also published as: EP1800111A1; GB0421374D0

Abstract

A measuring device for measuring spectral reflectance characteristics of an area of material comprises illumination means (44, 46) arranged to illuminate an area of material. The measuring device also comprises a hollow integrating sphere (24) which has a sample port (26) arranged to allow light reflected from the area of material to enter the sphere (24). A light collection means (32, 34) is arranged to collect reflected light from the integrating sphere (24). A housing (12) is arranged to house both the illumination means (44, 46) and the integrating sphere (24).

Description

MEASURING DEVICE AND SYSTEM FOR MEASURING SPECTRAL REFLECTANCE CHARACTERISTICS

The present invention relates to a measuring device and system for measuring spectral reflectance characteristics of an area of material. It is suitable for a wide range of applications, in particular to measure degradation of certain materials such as transformer coil electrical insulation material.

It is possible to analyse reflectance characteristics of an area of material to determine the condition of that material. It is important in applications such as transformer coil electrical insulation materials in order to predict material failure. In order to determine the condition of such a material via its reflectance characteristics, it is necessary to analyse reflected light from the material over a range of wavelengths for example 350nm to 2500nm.

Conventional optical probes which are used are based upon specular reflectance or upon collecting diffuse reflectance over a limited solid angle. These suffer from coherent effects such as speckle or angular effects due to finite sampling of the material. The consequence of this is that large intensity variations in spectral signal can be found for small changes in position across the sample and this leads to problems in comparing and analysing spectral data from such samples. ^■

According to a first aspect of the invention, there is provided a measuring device for measuring spectral reflectance characteristics of an area of material comprising: illumination means arranged to illuminate an area of material; a hollow integrating sphere having a sample port arranged to allow light reflected from the area of material to enter the sphere; light collection means arranged to collect reflected light from the integrating sphere; and a housing arranged to house both the illumination means and the integrating sphere.

According to a second aspect of the invention, there is provided a measurement system for measuring spectral reflectance characteristics of an area of material comprising: the measuring device of the first aspect of the invention; a spectrometer arrangement arranged to analyse the light from the integrating sphere and thereby provide data relating to the area of material; and processing means arranged to process the data.

The present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a schematic side section through a measuring device according to a first embodiment of the invention;

Figure 2 is a schematic diagram of a measuring system including the measuring device of Figure 1 according to the invention;

Figure 3 shows a schematic side section through a measuring device according to a second embodiment of the invention; and

Figure 4 is a schematic diagram of a measuring system including the measuring device of Figure 3 according to the invention.

Referring to Figure 1, according to an embodiment of the present invention a measuring system for measuring spectral reflectance characteristics of an area of material includes a measuring device in the form of a hand-held portable probe 10, which comprises a housing 12 and a handle 14 attached to the housing. The housing 12 is generally cylindrical, having a longitudinal axis and comprises a sphere housing 16 and a lamp housing 18. The sphere housing 16 and lamp housing 18 are fixed relative to each other. The sphere housing 16 is located at a front end 20 of the probe 10 and the lamp housing 18 is of a larger diameter and located at the rear end 22 of the probe 10. The length of the sphere housing 16 is approximately 60mm and the length of the lamp housing 18 is approximately 200mm. The length of the handle 14 is approximately 150mm.

An integrating sphere 24 is securely mounted within the sphere housing 16. The internal surface of the sphere 24 is coated with a white Teflon based diffuse material. The integrating sphere 24 has an aperture forming a sample port 26 at its front end and a further aperture forming an illumination port 28 near its rear end. The integrating sphere 24 has an internal diameter of 50mm and its sample port 26 is circular and has a diameter of 10mm. The sample port 26 is covered by a flat calcium fluoride window 30 mounted against the outside of the sphere 24.

Targeting means in the form of cross hairs 100 are located at the front end of the probe 10. These are attached to the front of the sphere housing 16 and centred at the centre of the sample port 26.

The illumination port 28 is offset from the longitudinal diameter of the sphere 24, which is an imaginary line running through the centre of the sphere and parallel to the longitudinal axis of the probe 10. The illumination port 28 is offset from this imaginary line by about 8° in a generally vertical direction as shown in Figure 1.

The integrating sphere 24 further comprises an annular light collection channel 32, which extends around the sphere symmetrically about the longitudinal axis of the probe. The light collection channel is formed from a longitudinal section 32a the rear end of which opens into the interior of the sphere 24 and a radial section 32b extending outwards from the front end of the longitudinal section 32a. The light collection channel 32 opens into the sphere 24 at a position spaced from the sample port 26, being about half way between the sample port 26 and the centre of the sphere 24.

Light conveying means in the form of a first 600 μm NIR optical fibre 34 and a second 400 μm visible range optical fibre (not shown separately) extend rearwards from the bottom of the light collection channel 32. The first fibre 34 and the second fibre are rigid and completely enclosed within the housing 12 and extend beneath the sphere 24 to a connector 37 near the front end of the lamp housing 18. A mirror 38 is arranged in the bottom of the light collection channel 32 to direct light from the light collection channel 32 into the first and second optical fibres 34.

The sphere 24 further comprises a specular light trap 40 in the form of a small cavity, the inner surface of which is blackened with light-absorbing material which effectively "traps" the specular component of the light reflected from the sample so that it is not present in the signal collected. Since the specular component is reflected from the sample at an angle equal to the angle of incidence, it must make the same angle 8° with the normal to the sample on the other side (relative to the source position) . The specular light trap 40 is therefore located on the inner surface of the sphere 24 offset from the longitudinal diameter of the sphere 24 by about 8° (i.e. the same amount as the illumination port 28) and is positioned on an opposite side of that diameter to the illumination port 28.

A reference block housing 41 is arranged to be removably attached to the front end 20 of the probe so that it extends over the sample port 26. The reference block housing 41 supports a reference block 42 which is held in front of the sample port 26 against the outside of the window 30, when the reference block housing is in place.

A light source 44 and light directing system 46 are securely mounted within the lamp housing 18. The light source comprises a ceramic type tungsten filament source. The light directing system 46 comprises a parabolic reflector 48 and a filament protective NIR window 50 which reduces the amount of dust reaching the parabolic reflector 48.

The light directing system 46 further comprises a main lens 46a, an aperture stop 46b, and a light beam trim lens 46c arranged to allow under-filling of the sample port. These are arranged to focus light emitted from the light source 44 such that it passes through the illumination port 28 before reaching the sample port 26. The components of the light directing system 46 are aligned along an optical axis that is at an angle of about 8° to the horizontal and passes through the illumination port and the sample port. The illumination system further comprises a shutter 52 located between the trim lens 46c and the illumination port 28. The shutter 52 is selectively operable to allow light to pass from the light source 44 through the illumination port 28 or to block this light.

The light source 44 and components of the light directing system are securely mounted to a wall (not shown) which extends along a central axis of the housing 12. The housing 12 further comprises a removable side panel (not shown) to facilitate access to these components, which are adjustably mounted such that their positions can be manually adjusted, for example to adjust focusing of the light beam.

The handle 14 extends perpendicularly to the longitudinal probe axis and facilitates handling and operation of the probe 10. The handle 14 extends downwardly from the rear end of the sphere housing 16. Three trigger buttons comprising a scan button 54, a background button 56 and a dark button 58 are located on the handle in vertical alignment. Associated with each trigger button 54, 56, 58 is a first, second, third switch 60, 62, 64 respectively so that when a button 54, 56, 58 is pressed, its associated switch 60, 62, 64 is operated. The function of these switches will be described below.

At the rear end 22 of the probe 10 inside the lamp housing 18, there is located a cable connector block 66. Electrical connections in the form of wires extend from the first switch 60, the second switch 62 and the third switch 64 to the cable connector block 66. A flexible 600μm optical fibre 68 extends from the first rigid 600μm fibre 34 and a flexible 400μm optical fibre (not shown separately) extends from the rigid second 400μm optical fibre. Both the 600μm fibre 68 and the flexible 400μm fibre extend to the cable connector block 66. Extending from the cable connector block 66 within the lamp housing 18 are connections to first, second and third LEDs 72, 74, 76. These LEDs indicate the status of the system as described below and act independently of the state of the switches. The LEDs 72, 74, 76 are located on the outer surface of a removable rear panel at the rear end of the lamp housing 18. Also on this outer surface of the lamp housing there is located a light source and ventilation power switch 78 and a shutter switch 80. These are electrically connected to the cable connector block 66.

An umbilical cable 82 extends from the cable connector block 66 externally from the lamp housing 18. The umbilical cable 82 contains optical connections to the flexible optical fibres 68, electrical power connections for the light source, the shutter and the LEDs 72, 74, 76, and signal lines arranged to carry signals from the probe indicating whether the trigger buttons 54, 56, 58 have been depressed and signals to the probe to control the LEDs 72, 74, 76.

Referring to Figure 2, the measuring system also comprises a portable rack 84. A first ultraviolet (UV) /visual range spectrometer 86 and a second near infrared (NIR) range spectrometer 88 are mounted on the portable rack 84. Also mounted on the portable rack are a 24V light source power module 90 and a 5 V shutter, LED and switch power module 92. The power modules 90, 92 are in the form of batteries. The 24V power module 90 is arranged to power the light source 44 and the 5V power module 92 is arranged to power the shutter 52, the LEDs 72, 74, 76 and the switches 60, 62, 64. The spectrometers 86, 88 and power modules 90, 92 are removably mounted on the rack 84.

The portable rack 84 has an umbilical connection port 94 which is arranged to receive the umbilical cable 82. From the umbilical connection port 94 suitable connections provide a power connection between the light source power switch 78 and the 24V power module 90 such that operation of the light source switch 78 controls the supply of power to the light source 44. Similarly, a further power connection is provided between the shutter switch 80 and the 5V power module 92 such that the shutter 52 can be selectively operated using the shutter switch 80. Similarly, power connections for the LEDs 72, 74, 76 are provided.

Optical connections from the umbilical connection port 94 also extend to the first 86 and the second 88 spectrometers such that the 600μm optical fibre 68 is connected to provide a light input to the NIR range spectrometer 88 and the 400μm optical fibre is connected to provide a light input to the UV/visual range spectrometer 86. The portable rack 84 may comprise a handle, wheels or any other similar mechanism to make it easily portable.

The measuring system also comprises processing means in the form of a computer 96. The computer is connected to the signal lines in the umbilical cable in through the umbilical connection port 94. In addition, the computer 96 is directly connected to each of the first 86 and second

88 spectrometers so that it can receive spectral data from them and send commands to them to control their operation. The computer 96 is a portable computer, such as a laptop. The computer 96 has a software application running on it which is able to analyse the data received from the spectrometers 86, 88 as described below.

In use, a user wishing to measure spectral reflectance characteristics of an area of material needs to take a background measurement and a dark measurement to calibrate the system. The user therefore first attaches a white reference block 42 by clip fitting the reference block housing to the front end 20 of the probe 10. The user then operates the light source power switch 78 which causes the 24V power module 90 to provide power to the light source 44. Once the light source has warmed up and stabilized, it is left on and the shutter is opened and closed using the shutter switch 80 to control whether or not light from the source 44 reaches the sample. The light source 44 emits light which is focused by the light directing system 46. The shutter switch 80 is operated causing the shutter 52 to open so that the focussed light passes through the illumination port 28 of the integrating sphere 24 and onto the sample port 26. The light directing system 46 is arranged such that all of the incident light hits the sample port 26 and none of it is incident upon the internal surface of the integrating sphere 24 surrounding the sample port 26. This is so that the maximum amount of incident light reaches the sample port 26 which optimizes the signal to noise ratio of the measurement. The white reference block 42 is located adjacent the sample port 26 and incident light is reflected from the reference block 42. The white reference block 42 is chosen since it has a known reflectance close to 100% over the range of wavelengths analysed by the two spectrometers. The background button 62 is pressed by the user to indicate that a measurement can be taken and that the white block 42 is in place. The signal line through the umbilical 82 provides an input to the computer 96. The software application running on the computer 96 recognises from this input that the second switch 62 has been activated and records background reference spectral data from the two spectrometers. While the scanning is taking place the computer sends signals to the LEDs to indicate whether or not the system is scanning, or has finished a scan, or if the scan is to be rejected as bad data.

As indicated previously, specular reflection is undesirable for measurement purposes and reflected specular light enters the specular light trap 40 and does not re-emerge from the light trap 40. Diffuse reflected light is reflected around the internal surface of the integrating sphere and is homogenised. The homogenised reflected light is not prone to coherent optical effects such as speckle and partial polarisation - these would lead to spectral and intensity errors.

Some of the diffuse reflected light enters the annular light collection channel 32. Some of this light is reflected by the mirror 38 and enters the NIR optical fibre 34 and some enters the visible range optical fibre. The light then passes via the flexible fibres 68 and the umbilical cable 82 to the relevant spectrometer 86 or 88. Each spectrometer 86, 88 analyses the light in a standard manner to provide information on the intensity of light at any particular wavelength within the range across which the spectrometer operates. The computer 96 analyses this information as indicated above, the software application has recognised that this information is for reflectance from the white block 42 of known reflectance close to 100%.

To obtain a dark reference, the illumination is switched off using the shutter, and the dark button 58, is depressed. The resulting dark reference spectral data from the spectrometers 86, 88 are recorded under control of the computer. During the collection of the dark data, the shutter is used to block the illumination and the white reference block is kept in place so that no light can enter from outside the measurement port. This prevents any light entering the sphere, which is sufficient for a dark signal. The frequency at which dark and backgrounds need to be taken depends on the level of electronic noise and drift in the system.

In this way, the measuring system can be calibrated using the reference data before taking a measurement of the spectral reflectance characteristics of the area of material.

Once the system has been calibrated, the shutter is opened and the user then manoeuvres the probe 10 so that the area of material to be analysed is adjacent to the sample port 26. The scan button 54 is then depressed by the user. This is detected by the computer, which records sample spectral data from the spectrometers 86, 88. The pushbutton switch only needs to be pressed once to initiate each scan and can then immediately be released, the signal to the computer being activated on a rising (or possibly trailing) edge of the switch signal.

The software application running on the computer 96 is able to compare the calibrated spectral reflectance characteristics obtained from the area of material to standard reflectance characteristics for such a material according to certain parameters (e.g. age, environment etc.) - this information is stored as a spectrum (ASCII file) on the computer 96 which the software application is able to interrogate.

Referring to Figure 3, according to a further embodiment of the present invention a measuring system for measuring spectral reflectance characteristics of an area of material includes a measuring device in the form of a handheld probe 210. The probe 210 includes many similar features to the handheld portable probe 10 of the embodiment shown in

Figures 1 and 2. Features of the probe 210 similar to features of the probe 10 have corresponding reference numerals increased by two hundred. Some of the differences between the probe 210 and the probe

10 are described below.

The probe 210 comprises a housing 212 and a handle 214. The housing 212 is not cylindrical in this embodiment and is smaller in size than the housing 12 of the probe 10. The length of the lamp housing 218 is approximately 150mm and the length of the handle is approximately 100mm. This provides an even more compact handheld portable probe 210 than the probe 10 of the previously described embodiment.

The sample port 226 is covered by a flat calcium fluoride window 230 which has a thickness of lmm and is mounted against the outside of the sphere 224. Targeting means in the form of cross hairs 300 are located at the front end of the probe 210 and these are attached near to the front end of the sphere housing 216 and centred at the centre of the sample port 226.

The integrating sphere 224 does not comprise an annular light collection channel to collect light as in the integrating sphere 24. Instead the integrating sphere 224 has a collection port 232 which is situated at the bottom of the integrating sphere 224 at its centre. Light conveying means in the form of a bundle 234 of NIR and visible range optical fibres extend rearwardly from the light collection port 232. The bundle 234 of fibres is enclosed in the housing 212 and extends from the bottom of the sphere 224 to a fixture 237 near the front end of the lamp housing 280.

The reference block housing 241 is removably attached to the front end 220 of the probe 210 by a reference block fitting 243 and retaining clip 245 arrangement. The clip 245 attaches the fitting 243 to the probe 210 in the region of the front end of the lamp housing 218 so that the reference block housing 241 extends over the sample port 226.

The light source 244 and light directing system 246 are securely mounted within the lamp housing 218. The light directing system 246 of the probe 210 is slightly different from the light directing system 46 of the probe 10. The light directing system 246 comprises a lens (in other embodiments more than one lens may be used) and an aperture. A shutter 252 is located between the light directing system 246 and the illumination port 228. In this embodiment the light source 244 and the components of the light directing system 246 are set back from the central axis of the housing 212 - they do not extend along the central axis as in the probe of the previously described embodiment. In this embodiment a ventilator 251 is located near the light source 244 and arranged to cool the light source 244 and surrounding area/components. The ventilator 251 is located in a top panel of the lamp housing 218. A grille 253 is located in panels near a bottom end of the lamp housing 218 and the ventilator 251 and grille 253 are arranged to produce a flow of air around the lamp housing 218 to extract heat away from the lamp housing 218 to the environment e.g. surrounding air. The grille 253 works in conjunction with the ventilator 251 to increase the efficiency of the probe and amount of time for which the portable probe 210 can be used. This cooling system is particularly advantageous as the probe 210 may be in use for long periods of time and overheating needs to be prevented. This is particularly important for a portable probe since there will often be no immediately available replacement probe if a portable probe breaks down in use. The power switch 278 on the outer surface of the lamp housing is a ventilation power switch as well as a light source power switch - when power is provided to the light source power is also provided to the ventilation system automatically. Advantageously a user is not required to separately activate the ventilation system.

Umbilical cables 282 extend from the cable connector block 266 and contain the flexible optical fibres 268 along with electrical power connections for the light source 244 and ventilator 251, the shutter and the LEDs 272, 274, 276 and signal lines arranged to carry signals from the probe 210 indicating whether the trigger buttons 254, 256, 258 have been depressed and signals to the probe to control the LEDs 272, 274, 276.

Referring to Figure 4, the measuring system . includes the portable rack 284. On the portable rack 284 the power module 292 which provides power to the shutter 252 also provides power to the ventilator 251 and is a 12V power source. An electronic interface 293 is arranged to control and process signals associated with the probe received from the switches and sent to the LEDs. The LEDs 272, 274, 276 and the switches 260, 262, 264 are powered using the 5 V power line from the computer 296. The umbilical connection port 294 which is arranged to receive the umbilical cable 282 has suitable connections providing power from the 24V power module 290 to the light source 44. A similar connection is provided so that activation of the same switch 278 arranges for the 12V power module 292 to provide power to the ventilator 251 simultaneously with control of the light source power as previously described. A further power connection is provided between the shutter switch 280 and the 12V power module so that the shutter 252 can be selectively operated using the shutter switch 180. Optical connections from the umbilical connection port 294 extend to the first 286 and the second 288 spectrometers such that a bifurcated part of the fibre bundle (600μm optical fibres) is connected to provide a light input to the NIR range spectrometer 288 and a second bifurcated part of the fibre bundle 234 (400μm optical fibres) is connected to provide a light input into the UV/visual range spectrometer 286. The computer 296 is connected to signal lines in the umbilical cable via the electronic interface 293. In use the diffuse reflected light arrives at the collection port 232 and enters the fibre bundle 234. The light then passes through the fibre bundle 234 and the umbilical cable 282 to the relevant spectrometer 286 or 288. Each spectrometer 286, 288 analyses the light in a standard manner to provide information on the intensity of light at all wavelengths within the range across which the spectrometer operates.

The software application running on the computer 296 is able to compare the calibrated spectral reflectance characteristics obtained from the area of material to standard reflectance characteristics available for such a material according to certain parameters. The spectrum is stored on the computer 96 which the software application is able to interrogate. Various properties of the material being measured can be estimated from its spectrum using calibration models relating to similar material that has been obtained using analysis methods applied to a database consisting of calibration spectra and material property data.

Various modifications may be made to the present invention without departing from its scope. For example, a different size of integrating sphere may be used. Different types of optical fibres and spectrometers may be used in order to obtain information on different ranges of wavelengths. A single spectrometer may be used having more than one channel to analyse input from two or more different fibres. The computer may be mountable on the portable rack. The window covering the sample port may be made from any material which allows light of the required wavelengths to pass through. Any combination, or all, of the spectrometer (s) or power modules or computer may be fixedly mounted on the portable rack.

Claims

1. A portable measuring device for measuring spectral reflectance characteristics of an area of material comprising: illumination means arranged to illuminate an area of material; a hollow integrating sphere having a sample port arranged to allow light reflected from the area of material to enter the sphere; light collection means arranged to collect reflected light from the integrating sphere; and a housing arranged to house both the illumination means and the integrating sphere.

2. A measuring device according to Claim 1 further comprising a handle.

3. A device according to Claim 2 wherein the handle extends downwards from the housing.

4. A measuring device according to any preceding claim, in which the housing comprises a first portion housing the sphere fixed to a second portion housing the illumination means.

5. A measuring device according to Claim 4, in which the first and second housing portions are separately formed.

6. A measuring device according to any preceding claim, in which the housing comprises sphere mounting means arranged to securely mount the sphere to the housing.

7. A measuring device according to any preceding claim, in which the housing comprises illumination mounting means arranged to securely mount the illumination means to the housing.

8. A measuring device according to Claim 6 or Claim 7, in which the illumination mounting means is adjustable.

9. A measuring device according to any preceding claim, in which the illumination means comprises a light source and light directing means arranged to focus light from the light source onto the area of material.

10. A measuring device according to Claim 9, in which the sphere is located towards a front end of the housing, the illumination means is located rearwardly of the sphere, the sample port is located at the front end of the sphere and the sphere further comprises an illumination port towards its rear end wherein the light directing means is arranged to direct light through the illumination port and the sample port.

11. A measuring device according to Claim 9 or Claim 10, in which the light directing means is arranged to direct light from the light source through the sample port such that none of said light is incident upon an inner surface of the hollow sphere directly from the light source.

12. A measuring device according to any of Claims 9 to 11, in which the light directing means comprises one or more lenses and/or apertures.

13. A measuring device according to any preceding claim, in which the light collection means comprises a light collection channel.

14. A measuring device according to Claim 13, in which the light collection channel is an annular channel.

15. A measuring device according to any of Claims 1 to 12, in which the light collection means comprises a light collection port.

16. A measuring device according to any preceding claim, in which the light collection means comprises an optical fibre or fibres.

17. A measuring device according to Claim 16 when dependent on any of Claims 13 to 15, in which the optical fibre (s) extends from the light collection means.

18. A measuring device according to any preceding claim, in which the light collection means is arranged to collect light from a position within the sphere spaced from the sample port.

19. A measuring device according to any preceding claim, further comprising a window arranged to cover the sample port.

20. A measuring device according to Claim 19, in which the window is formed of calcium fluoride.

21. A measuring device according to Claim 19 or Claim 20, in which the window is flat.

22. A measuring device according to any preceding claim, further comprising fixing means arranged to releasably fix a reference block at or adjacent the sample port such that light reflected from the reference block can enter the sphere through the sample port.

23. A measuring device according to any preceding claim, further comprising user input means arranged to enable a user to control operation of the measuring device.

24. A measuring device according to Claim 23, in which the user input means is arranged to enable the user to control the collection of reference data and sample data.

25. A measuring device according to Claim 23 or Claim 24 when dependent on Claim 2 wherein the user input means is mounted on the handle.

26. A measuring device according to any preceding claim, in which the direction in which the light source directs light towards the sample is inclined to the longitudinal axis of the probe.

27. A measuring device according to any preceding claim, in which the sphere further comprises a trap for specular reflected light.

28. A measuring device according to any preceding claim, further comprising targeting means arranged to facilitate alignment of the sample port with the area of material.

29. A measuring device according to Claim 28, in which the targeting means comprises cross-members aligned with the sample port.

30. A measuring device according to any preceding claim, in which the illumination means comprises a shutter mechanism operable to selectively illuminate the area of material.

31. A measuring device according to Claim 30, in which the shutter mechanism is controlled by a manually operable switch located on the housing.

32. A measurement system for measuring spectral reflectance characteristics of an area of material comprising: the measuring device of any of Claims 1 to 31; a spectrometer arrangement arranged to analyse the light from the integrating sphere and thereby provide data relating to the area of material; and processing means arranged to process the data.

33. A measurement system according to Claim 32, in which the spectrometer arrangement is arranged to analyse light samples varying in wavelength from UV to near infrared.

34. A measurement system according to Claim 32 or Claim 33, in which the spectrometer arrangement comprises two spectrometers.

35. A measurement system according to any of Claims 32 to 34, in which the spectrometer arrangement is mounted on a portable rack.

36. A measurement system according to Claim 35, in which the processing means is mounted on the rack.

37. A device or system substantially as hereinbefore described with reference to Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.