WO2003078945A1

WO2003078945A1 - Calibration of a spectrometer

Info

Publication number: WO2003078945A1
Application number: PCT/GB2003/001114
Authority: WO
Inventors: Andrew Mark Woolfrey
Original assignee: Renishaw Plc
Priority date: 2002-03-18
Filing date: 2003-03-18
Publication date: 2003-09-25
Also published as: AU2003216823A1; GB0206341D0; WO2003078945A8

Abstract

An optical system for use in an apparatus capable of spectral analysis comprises a housing for housing a reference sample, and a reference sample collector within the housing wherein the reference sample collector collects light scattered from the reference sample. The housing may include a director for directing light from a source towards the reference sample. The reference sample may be mounted for movement to provide different distances between the reference sample collector, perhaps a focusing optic, and the reference sample. A further self illuminating reference source may be provided. The reference sample may comprise a photodiode and means to sense electrical current at the p-n junction of the photodiode. Also disclosed is a spectrometer comprising a reference sample wherein light from a source within the spectrometer may be optionally directed by a director towards the reference sample and light scattered from the reference sample collected by a reference sample collector.

Description

CALIBRATION OF A SPECTROMETER

This invention relates to the calibration of a spectrometer and, in particular the use of internal calibration sources.

Spectroscopic systems that rely on analysis of a spectrum of radiation received from a sample require calibration in order that different results can be compared or, that materials can be correctly identified from characteristic features.

Raman spectroscopy is an example of such a spectroscopic system. Monochromatic light is focused on a sample, the majority of recovered light is of the same wavelength as the incident light (Rayleigh scattered) however, a small percentage is at different wavelengths (Raman scattered) . These different wavelengths are characteristic of the composition of the material of the sample and are due to changing molecular motions within the sample. The reflected light is filtered to remove the Rayleigh scattered wavelength and the resultant light is manipulated by, for example, a dispersive device, across a multi- channel detector such as a charge coupled device (CCD) .

In the Raman system, there are various parameters that can be measured and used to calibrate the spectroscopic system. Firstly, wavelength or wavenumber calibration is important when attempting to identify the composition of a sample. This parameter is less critical when comparing different samples (on the same machine) as the shape of the spectrum is not affected by this parameter merely its position along the x-axis. The apparatus is calibrated using a sample with specific characteristic emission lines, such as neon or silicon.

Secondly, the intensity of the resultant spectrum needs calibrating to minimise or eliminate the spectral effects of the light path of the spectroscopic system. Each optical element in the light path has transmission and reflection characteristics which would, without calibration, influence the shape of the resultant sample spectrum. In order to characterise a material, ratios of the different peaks within the sample spectrum may be compared to a library or database. A system influenced spectrum could affect these ratios and cause misinterpretation of results. White light of either known or uniform spectrum or a sample such as silicon, which has characteristic peaks of different intensities, is used to create a function which is applied to a spectrum to alleviate or remove the characteristics of the system.

Traditionally, when calibrating spectroscopic systems, the calibrating or reference samples have to be manually positioned replacing a real' sample. This is not a particular problem when using a microscopic system, but when a probe is used, for example within a process pipeline, it would mean either the removal of the probe or the insertion of the reference sample into that environment neither of which may be practical. One other problem when using external calibration sources is that they could get damaged or contaminated which could affect the reliability and reproducibility of the calibration process. When calibration is carried out using a self radiating reference, for example neon or white light, a monochromatic light source is not required to interact with the sample to produce spectral emission. When a reference which needs to be irradiated is used, for example silicon, then a monochromatic light source is required. One problem with this is that the spectroscopic system will often include a number of sources of radiation of different wavelengths as this increases the flexibility of the system. This means that the optical elements encountered within the system will either have to be changed or, a sample position altered, in order to correctly focus radiation of different wavelengths (in order to calibrate the system, each source of radiation will need to be calibrated individually) . Having the radiation source correctly focused on a sample or reference will result in better light collection from the sample or reference which is important for the collection of Raman scattering and when calibrating intensity. This use of different wavelengths makes it difficult to use internal references that need to be irradiated by a radiation source.

Recent developments have centred on the use of internal self radiating calibration sources. US Patent No. 6,067,156 discloses the introduction of calibration wavelength (s) which are near to the laser wavelength from, for example a neon lamp, into the optical path between the filtering step which removes Rayleigh scattering and the detector. This allows for calibration in respect of wavelengths that are close to the incident light wavelength. However, the calibration wavelengths do not experience the same optical path as light reflected from a sample which can introduce errors which may cause difficulty when comparing results from different systems .

According to a first aspect of the present invention there is provided an optical system for use in an apparatus capable of spectral analysis comprising a housing for housing a reference sample and a reference sample collector wherein the reference sample collector collects light scattered from the reference sample.

Preferably, the housing includes a director capable of directing light from a source towards the reference sample .

According to a second aspect of the invention there is provided a spectrometer comprising a source of substantially monochromatic light; means to direct the substantially monochromatic light towards a sample holder; a collector which collects light scattered from the sample holder; an ancillary coupling optic; a selector which selects light from the collected light after the collected light has passed through the ancillary coupling optic; and a detector which detects light from the selected light wherein a reference sample is provided within the spectrometer and light from the source may be optionally directed by a director towards the reference sample and light scattered from the reference sample collected by a reference sample collector.

Preferably, the director is provided between the source of substantially monochromatic light and the selector. By having an internal or integral reference sample, the chance of damage or contamination of the reference sample is reduced so there can be more confidence in the calibration process.

In a preferred embodiment, the reference sample is mounted for movement to provide different distances between the reference sample collector and the reference sample.

By enabling the distance between the collection means and the reference sample to be altered, the collection of radiation from a number of radiation sources (having different spectral characteristics, for example wavelength) may be optimised without changing the optic path or introducing additional optical elements which would affect the spectrum obtained.

According to a third aspect of the present invention, there is provided an optical system for use in an apparatus capable of spectral analysis comprising a housing for housing a reference sample and a collector wherein the collector collects light scattered from the reference sample and wherein the reference sample is a photodetector which indicates the intensity of the incident light.

The photodetector may comprise a phototransistor a photovaristor or, in a preferred embodiment, a photodiode.

A photodiode is a semiconductor diode which may have a lens which focuses incident radiation on the p-n junction of the semiconductor diode producing a current. This reference sample indicates the intensity and so power of the radiation source. Thus it provides information regarding the working life of the radiation source .

Conveniently, the reference sample of the first or second aspects incorporates a photodiode of the third aspect. This requires that the reference sample is a semiconductor, for example silicon. As the p-n junction of the photodiode is not at the surface of the reference sample on which the substantially monochromatic light is incident, the doping agents do not affect the characteristic spectrum from the reference sample.

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

Figure 1 shows a schematic of a Raman spectroscopy instrument;

Figure 2 shows an internal reference source according to the present invention;

Figure 3 shows schematically, a device for changing optical elements in the laser beam path; and Figure 4 shows an alternate reference source according to the invention.

Figure 1 shows a schematic of a Raman spectroscopy instrument. A laser beam 10a from a source (not shown) is incident on a mirror 14 and directed to a dichroic filter arrangement 16 which reflects light of the wavelength of the laser beam and transmits all other wavelengths. A further mirror 18 directs the laser beam 10b through sample focusing optics 20 and onto a sample 22. Between the filter 16 and the further mirror 18 are one or more ancillary coupling optics (not shown) . A microscope 8 is positioned in line with the further mirror 18 and the sample 22 enabling visual inspection and manipulation of the sample. A beam splitter 6 lies in the path of the laser beam 10b and directs a portion of the radiation collected from the sample to a video camera 7. Light reflected from the sample 22 is guided along the same optical path as the incident laser beam 10b, 10a to the dichroic filter 16 where light of the same wavelength as the laser beam is reflected and all other wavelengths, the Raman scattered light, is transmitted to an analyser 24 which manipulates the different wavelengths of Raman scattered light prior to the light being focused 26 onto a CCD camera unit 28.

The CCD camera unit 28 is a two-dimension array of photodetectors attached to a suitable imaging device, in this case a camera. The person skilled in the art will appreciate that there are alternative devices that could be used in place of a CCD camera unit.

Figure 2 shows a first internal reference source 30 which is positioned between the dichroic filter, indicated by arrow 16 and the sample focusing optics indicated by arrow 20. A second internal reference source 36 which includes a neon lamp and a white light source of known spectrum is positioned in line with the laser beam when the beam is between the dichroic filter, indicated by arrow 16, and the further mirror 18 i.e. the second reference source is behind the reflecting surface of the mirror. In order to divert the laser beam 10b from its normal path indicated by arrow 20 and onto the first reference source 30, a change in optical elements within the normal beam path is required.

The first reference source 30, is a flat piece of silicon 31 mounted at an angle on a rotatable holder 32 having a rotatable axis 33, a 45° diverting mirror 34 is inserted into the beam path after the beam has been reflected by further mirror 18 diverting the beam at a right-angle 35 to its normal path 10b. A static focusing objective 38 is placed on the diverted beam path in-between the 45° mirror 34 and the silicon 31. This optic focuses the incident radiation at about the surface of the reference sample and collects a proportion of the scattered radiation for return along the optic path for analysis. This diverted beam 35 travels along a path parallel to the rotational axis 33 of the holder 32 however, the rotational axis 33 is not in line with the diverted beam path 35, it is offset. Thus, when the rotatable holder 32 is rotated, the position of the laser beam spot on the inclined plane of the silicon 31 changes i.e. the distance between the focusing objective 38 and the surface of the silicon 31 changes. This allows laser beams of different wavelength to be correctly focused on the silicon sample 31 without changing or adding optical elements to the diverted beam path 35.

This is important for two reasons, firstly, in order to calibrate the intensity of a laser beam, it is desirable to have the beam correctly focused on a reference surface as this results in better light collection from the reference (or indeed a sample) giving higher intensity so more photons. Secondly, in order that the calibration has some meaning when related to sample results, it is important that the optical beam path of the reference is as identical as possible to that experienced by light reflected from a sample. By enabling calibration of various wavelengths using just two optical elements the present invention minimises the effect of extra optical elements.

The first reference source may be positioned anywhere between the dichroic filter and the sample focusing optics however, for reasons described above, it is preferred that the first reference source 30 is placed after any optical elements (such as further mirror 18) that would direct the laser beam onto its normal beam path 10 towards a sample, indicated by arrow 20.

It is convenient that the first reference source is located in line with the video camera 7 as this enables the video camera 7 not only to View' actual samples but, also the first reference source. For this set-up, the diverting mirror 34 replaces beam splitter 6 (see Fig 1) when the reference sample is being used and is required to be partially transmissive .

Ideally, the static focusing objective 38 is set up such that the median wavelength of the optical system is focused at the midpoint of the travel between the focusing optic 38 and the surface of the silicon 31. An example of a focusing optic is a lens.

As an alternative to the silicon 31 being flat and mounted on the rotatable holder at an angle, the person skilled in the art will appreciate that any configuration that changes the distance between the surface of the silicon and the collecting optic 38 on rotation of the holder 32 is acceptable. For example, the silicon could be wedge shaped or the holder surface could undulate or be inclined with respect to its rotation axis 33 which may be achieved during manufacturing of the holder or installation. At short wavelengths, a sample of silicon must be coated with a fluorescent material to enable measurement. In order to enable calibration of 244-830nm and to maintain focus on said internal reference, the distance between the surface of the silicon and the first diverting mirror must change in the order of hundreds of micrometres. It should be noted that this change in the distance is not a linear function with respect to wavelength.

The first reference source does not have to be silicon it has to meet the requirements that it has characteristic peaks of differing intensities. In fact, the first reference source could be a fluid encapsulated in a suitable container. A transparent fluid would have the added benefit that, if there were a sufficient amount (or depth) of fluid within the path of the first diverted beam i.e. a few hundred or a thousand micrometres, then the need to refocus for the calibration of different wavelengths is obviated. If however, this amount of fluid were not a practical option, then the sample could be mounted at an angle to the rotational axis of the holder as before.

For the second reference source 36, which, in this case includes a neon lamp and a white light source of known spectral output, the laser beam is not required as the references are self illuminating however, the light from these sources requires a path through the dichroic filter to the analyser and CCD camera unit, shown at arrow 16. In order to provide the path 37 from the second reference source 36, the further mirror 18 is removed from its normal position and thus out of the laser beam path. The normal position of further mirror 18 being defined as the position of that mirror during sample examination. The light 37 from the second reference source 36 continues past the position where the mirror 18 normally occupies and on to dichroic filter, shown by arrow 16.

Although the second reference source 36 in this case contains both neon and white light sources, it could contain either one of them and optionally any other suitable self illuminating reference source.

It will be apparent to the person skilled in the art that the positions of the first 30 and second 36 reference sources could swapped or indeed, both sources could be positioned within the laser beam path either before or after mirror 18.

Figure 3 shows schematically, a device 40 for changing optical elements in the laser beam path. The device 40 comprises a rotary mounted wheel 41 having a number of defined positions 42, 43, 44, 45 in spaced relationship around the wheel, each position having its centre 46 at substantially the same radius from the centre of the wheel 47. The wheel is required to have two positions, a first position wherein most of the substantially monochromatic light passes through undirected towards the sample holder and a second position wherein most of the substantially monochromatic light is directed towards the reference sample by, for example, a mirror.

The wheel is rotated by a motor (not shown) although it could be rotated manually, between each of the defined positions 42, 43, 44, 45. The centre 46 of each of the defined positions 42, 43, 44, 45 corresponds to the centre of the laser beam path 10.

As an alternative to a wheel, a linear slide may be used to change the optical element which lies within the beam path.

Figure 4 shows an alternate reference source which is a silicon photodiode 50. The photodiode 50 includes a doped silicon semi-conductor with a p-n junction (not shown) . Electrical connections 52 on the p-n junction are connected to an ammeter (not shown) . The intensity of the beam and thus the power of the laser source is indicated by the current flowing through the p-n junction. The photodiode 50 may include a lens to accurately focus the incident laser beam on the p-n junction.

As the photodiode 50 is made from silicon, advantageously, this further reference source could be incorporated into the first silicon reference source. In this case, the holder 56 on which the photodiode is placed is inclined with respect to its rotational axis 57.

A further advantage of using a reference source with the surface on which light is incident being inclined with respect to the axis of that light thus enabling accurate focusing of incident light of different wavelengths is that for the different devices (microscope, video camera, CCD camera unit) that need to be in the line of sight of a sample 20, their alignment with each other can be checked.

Referring now to figure 1, the microscope 8, video camera 7 and CCD camera unit 28 all lie on different optical paths. In order to check the registration of the microscope eyepiece 8 crosshairs with the video camera 7 crosshairs and the laser spot, a sample 22 is first positioned under the microscope eyepiece 8 crosshairs. The CCD camera unit 28 is then aligned with the sample by focusing the CCD camera unit 28 onto a feature on the sample and moving the line of sight of the CCD camera unit 28. This ensures that the feature on a sample that is being viewed is also the feature for which Raman scattering is being recorded on the CCD camera unit 28. The video camera 7 is aligned by moving the optical elements 6 associated with the video camera 7 optical path until the feature on the sample is approximately in the centre of the field of view of the video camera 7. The crosshairs (not shown) on the video camera 7 are then set on this feature at the approximate centre of the video camera 7 field of view. The video camera connects to a PC for data manipulation, automatic focusing on objects etc. (not shown) and collects images when either the sample 20 or first reference beam path is used. The laser spot is viewed on the reference source (not shown) having the surface on which light is incident being inclined with respect to the axis of that light through the video camera 7. A further crosshair (not shown) of the video camera 7 is then set on the laser spot. When the wavelength of the laser light is changed, if the laser spot is still aligned to the further crosshair, then this indicates that the various devices are in approximate alignment.

If the further crosshairs are not in alignment, then this indicates that, in order to be confident of the feature that is being measured by the system, the devices should be realigned. One use for having a video camera associated with the first reference source is that complicated beam steering manipulation when this occurs can be automated via the PC using motorised mirrors in the beam path. The confirmation of crosshair alignment can be carried out as a part of a calibration process of the system.

The person skilled in the art will appreciate that there are a number of alternatives to the optical elements that have been described above for example, a mirror may be replaced by an appropriate beam splitter and vice versa, and that the use of such alternatives can be effected within the spirit and scope of the invention.

Claims

1. An optical system for use in an apparatus capable of spectral analysis comprising a housing for housing a reference sample, and a reference sample collector within the housing wherein the reference sample collector collects light scattered from the reference sample.

2. An optical system according to claim 1 wherein the housing includes a director capable of directing light from a source towards the reference sample.

3. An optical system according to claim 2 wherein the director comprises a wheel having a plurality of positions wherein each of the positions may be aligned with the light.

4. An optical system according to any preceding claim wherein the reference sample is mounted for movement to provide different distances between the reference sample collector and the reference sample.

5. An optical system according to any preceding claim wherein the collector is a focusing optic.

6. An optical system according to claim 5 wherein the focusing optic is a lens.

7. An optical system according to any preceding claim wherein a second self illuminating reference source is provided within the housing.

8. An optical system according to any preceding claim wherein the reference sample comprises a photodiode and means to sense electrical current at the p-n junction of the photodiode.

9. A spectrometer comprising a source of substantially monochromatic light; means to direct the substantially monochromatic light towards a sample holder; a collector which collects light scattered from the sample holder; a selector which selects light from the collected light; and a detector which detects light from the selected light wherein a reference sample is provided within the spectrometer and light from the source may be optionally directed by a director towards the reference sample and light scattered from the reference sample collected by a reference sample collector.

10. A spectrometer according to claim 9 wherein the director is provided between the source of substantially monochromatic light and the selector.

11. A spectrometer according to claim 9 or claim 10 wherein the reference sample is mounted for movement to provide different distances between the reference sample collector and the reference sample.

12. A spectrometer according to any of claims 9 to 11 wherein the director comprises a wheel having a plurality of positions wherein each of the positions may be aligned with the light.

13. A spectrometer according to claim 12 wherein the wheel has a first position wherein most of the substantially monochromatic light passes through undirected towards the sample holder and a second position wherein most of the substantially monochromatic light is directed towards the reference sample.

14. A spectrometer according to any of claims 9 to 13 wherein the collector is a focusing optic.

15. A spectrometer according to claim 14 wherein the focusing optic is a lens.

16. A spectrometer according to any of claims 9 to 15 wherein a second self illuminating reference source is provided.

17. A spectrometer according to any of claims 9 to 16 wherein the reference sample comprises a photodiode and means to sense electrical current at the p-n junction of the photodiode.

18. A spectrometer according to any of claims 9 to 17 wherein a plurality of light sources are provided each having different spectral characteristics.

19. An optical system for use in an apparatus capable of spectral analysis comprising a housing for housing a reference sample and a collector, wherein the collector collects light scattered from the reference sample and wherein the reference sample is a photodetector which indicates the intensity of the incident light.