GB2424696A

GB2424696A - Spectrometer fault detection using optical reference source

Info

Publication number: GB2424696A
Application number: GB0506240A
Authority: GB
Inventors: Domenico Michele Dav Marseglia
Original assignee: SPECTRUM MEDICAL LLP
Current assignee: SPECTRUM MEDICAL LLP
Priority date: 2005-03-29
Filing date: 2005-03-29
Publication date: 2006-10-04
Also published as: GB0506240D0

Abstract

A spectrometer for receiving a multiwavelength beam of radiation to be analysed has a photodetector 6 comprising an array of detection elements producing electrical output signals representative of radiation detected by detection elements, and a splitter 4 splitting the beam into components of different wavelengths and directing the components onto the detection elements. To increase the reliability of measurement readings, the spectrometer additionally includes an optical reference source A, B or C for illuminating the detection elements with reference radiation, and a monitoring arrangement indicating an error condition in dependence on (i) first electrical output signals from the array in response to detection of the components of different wavelengths in the absence of illuminating reference radiation, and (ii) second electrical output signals from the array in response to detection of the components of different wavelengths together with illuminating reference radiation. The inclusion of such a reference source enables the diagnosis of faulty detection elements of the array as well as enabling more reliable analysis of the incoming light spectrum. Other in-field and production failures may also be diagnosable more easily by the inclusion of such a reference source. The system may be used to monitor oxygen saturation in blood.

Description

M&C Folio No P53407GB 1 Spectrometers This invention relates to a

spectrometer for analysing a multiwavelength beam of radiation, for example to monitor the level of a predetermined substance, such as oxygen, in blood.

The standard way to measure blood oxygen saturation in a patient is to direct light onto a target area of the patient's body containing the blood, and to measure the intensity of light reflected by or transmitted through the blood in the target area, either at discreet wavelengths or over a substantially continuous spectral range, and then to calculate the SO2 as a function of the measured light intensity values. Such measurement apparatus is known from WO 94/03102 for example. However many factors reduce the accuracy of such known SO2 measurement apparatus. One significant factor is the degradation of the accuracy of the apparatus due to patient movement, that is due to motion that leads to a change in the path length of the light through the biological tissue and hence to variation in the intensity of the detected light. This problem is particularly severe in critical health care applications where continuous monitoring is essential. Another factor that reduces the accuracy of such measurement apparatus is skin pigmentation in that, for measurements made at the patient's skin, the quantity of detected light will vary depending on skin colour which ranges from fair to dark as the concentration of melanin increases.

Furthermore WO 00/09004 describes SO2 measurement apparatus that operates by passing light through biological tissue and monitoring the transmitted or reflected light continuously by means of a photodetector. However, the subsequent analysis of the detected light is based on the use of only a limited number of wavelengths, including some isosbestic wavelengths at which oxygenated blood and deoxygenated blood absorb the same amount of light. The fact that only a limited number of wavelengths is used in such analysis limits the accuracy of the SO2 determination. Furthermore the apparatus is sensitive to ambient light interference, for example due to the use of fluorescent lighting.

M&C Folio No P53407GB 2 Furthermore WO 00/01294 describes apparatus for measuring one or more analytes in blood in a patient's body which comprises a light transmitter comprising a plurality of transmitting fibres positioned to transmit light to the body, and a light detector comprising a plurality of light detector fibres positioned to detect light transmitted through or reflected from the body. Light of only certain discrete wavelengths, including some of the isosbestic wavelengths, is transmitted to and reflected from the blood, and accordingly such apparatus also suffers from limited accuracy and is sensitive to ambient interference.

Such apparatus is also sensitive to the failure of individual detecting elements of the array in that, should an individual element or group of elements of the array fail in such a manner that the output signal from the or each such element is severely attenuated, a false indication will be given that certain wavelengths have been absorbed at the target area, and as a result an erroneous reading will be given.

It is an object of the invention to provide a spectrometer that is suitable inter alia for monitoring the level of a predetermined substance in blood and that provides an output of inherently greater reliability in use.

According to one aspect of the present invention there is provided a spectrometer comprising: an input for receiving a multiwavelength beam of radiation to be analysed; a photodetector comprising an array of detection elements for producing electrical output signals representative of radiation detected by detection elements; a splitter for splitting the beam into components of different wavelengths and for directing the components onto the detection elements; an optical reference source for illuminating the detection elements with reference radiation; and monitoring means for indicating an error condition in dependence on (i) first electrical output signals from the array in response to detection of the components of different wavelengths in the absence of illuminating reference radiation, and (ii) second M&C Folio No P53407GB 3 electrical output signals from the array in response to detection of the components of different wavelengths together with illuminating reference radiation.

According to another aspect of the present invention there is provided a method of analysing a multiwavelength beam of radiation comprising: splitting the beam into components of different wavelengths; detecting the components by means of an array of detection elements producing electrical output signals representative of the detected components; illuminating the detection elements with reference radiation from an optical reference source; and indicating an error condition in dependence on (i) first electrical output signals from the array in response to detection of the components in the absence of illuminating reference radiation, and (ii) second electrical output signals from the array in response to detection of the components together with illuminating reference radiation.

The invention provides significant advantages in a wide range of applications as compared with known techniques, such as when monitoring the level of a predetermined substance, such as oxygen, in blood. For example the inclusion of the optical reference source may enable the diagnosis of faulty detection elements of the array. En addition the inclusion of the optical reference source may enable more reliable analysis of the incoming light spectrum. Other in-field and production failures may also be diagnosable more easily by the inclusion of the optical reference source. For example, if the photodetector responds to illumination from the optical reference source but not to external illumination, this may indicate that the external light source or fibre optic delivery system is suspect. If the photodetector does not respond to illumination from the optical reference source, the associated electronics may be suspect.

In one embodiment of the invention compensating means are provided for compensating an electrical output indicative of the relative intensities of the components of different wavelengths in dependence on the error condition indication from the monitoring means.

M&C Folio No P53407GB 4 Furthermore the compensating means may serve to exclude from said electrical output electrical output signals from selected detection elements of the array when an error condition indication is received from the monitoring means that indicates that the selected detection elements are faulty.

The monitoring means may provide various indications of fault conditions. For example the monitoring means may provide an indication that the photodetector is faulty. Alternatively or additionally the monitoring means may provide an indication that a transmission path for transmitting the multiwavelength beam to be analysed to the input is faulty. As a further alternative the monitoring means may provide an indication that the optical reference source is faulty.

In a preferred embodiment the optical reference source comprises a light pipe extending through a wall for directing a beam of reference radiation onto the array.

Additionally control means may be provided for switching the optical reference source on and off during detection of the components of different wavelengths so as to produce said first and second electrical output signals.

In order that the invention may be more fully understood, several embodiments of spectrometer in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a known Czerny Turner type spectrometer; Figure 2 is a graph of the extinction coefficients of Hb and Hb02 against wavelength; Figure 3 is a schematic representation from above of a first embodiment of spectrometer in accordance with the invention showing three alternative positions for the reference light source; M&C Folio No P53407GB 5 Figure 4 is a schematic representation from above of a second embodiment of spectrometer in accordance with the invention showing a further possible position for the reference light source; Figure 5 is a schematic representation from one side of a third embodiment of spectrometer in accordance with the invention showing a further possible position for the reference light source; and Figure 6 is a schematic representation of the overall measurement apparatus including the reference light source.

The following description is given with reference to the measurement of blood oxygen saturation in a patient. However it should be understood that the described spectrometer arrangements can also be used with advantage in a wide range of the other applications, such as in colour measurement, colour mixing and matching, gemmology, thin film measurement, monitoring of coating processes, irradiance and fluorescence measurements, and various biomedical applications.

Figure 1 diagrammatically shows a Czerny Turner type spectrometer for monitoring the oxygenation of blood, as used in SO2 measurement apparatus. In such apparatus a sensor has a LED at one end for directing white light onto a target area of a patient's body or a tube carrying the patient's blood, and an optical fibre for receiving the light reflected by or transmitted through the blood in the target area and for transmitting this to a monitoring unit incorporating the spectrometer. The sensor is coupled to the monitoring unit by a lead incorporating the optical fibre and the necessary electrical wiring for supplying power to the LED. In practice two or more sensors will be coupled to the monitoring unit, including, in one implementation, a sensor for monitoring the venous absorption spectrum and a sensor for monitoring the arterial absorption spectrum. In alternative implementations of the invention a substantially greater number of sensors may be provided for measuring the absorption at a number of points on the patient's skin.

M&C Folio No P53407GB 6 The transmitted or reflected light from the target area is transmitted by the optical fibre and is inputted to the spectrometer I by way of a slit 2. The light is then focused by a first curved mirror 3 onto a splitter in the form of a diffraction grating 4 that serves to split the light into its constituent spectral components by reflecting each wavelength of light by a corresponding angle. The spectral components of the light are then focused by a second curved mirror 5 onto a linear CCD photodetector array 6 comprising a series of pixels spaced apart along a detection axis so that spectral components of different wavelengths different wavelengths are incident on physically different pixels, and allow the spectrum of the incoming light to be measured. As a result electrical output signals are obtained from the different pixels of the photodetector array 6 that are indicative of the relative intensities of the detected spectral components. The light transmitted or reflected by the venous blood and the arterial blood may be detected separately by a common detector assembly or by means of two separate detector assemblies. Other types of spectrometer can be used in alternative implementations of the invention.

In a typical measurement cycle, a "light sample" measurement is first obtained utilising the transmitted or reflected light from the target area as described above, followed by a "dark sample" measurement obtained under the same conditions as the "light sample" but with the light from the LED turned off. The output signals from the array for the dark sample are then subtracted, pixel by pixel, from the output signals from the array for the light sample. This compensates for first and second aberrations in the detected data. The first aberration is a feature of CCD electronic detectors in that they give an output even when no light is present, and this output is a function of integration time, temperature and physical properties of the individual CCD detector. The second aberration is ambient light entering the system. The light-dark sample obtained in this manner is known as a remittance spectrum, and an absorbance spectrum is obtained from the remittance spectrum by applying the following formula A = -log (Rb / Rw) on a pixel by pixel basis, where Rb is the remitted blood sample as described above, and Rw is a remitted white reference sample.

M&C Folio No P53407GB 7 Conceptually Rw is the sample that would have been obtained by measurement of the remitted spectrum of a pure white target, although in practice such a sample is never actually measured directly, due to the complex calibration procedure used in this technique. The inclusion of the white reference sample here compensates for the nonuniform nature of various parts of the apparatus, namely the detector, the light source etc. The log transform, using the mathematical identity log (AX) = log (A) + log (X) or log (AX) = k + log(X) where k = log (A), turns scalar changes in Rw and Rb into linear shifts in the absorbance spectrum so that, if, instead true remittance being measured, a constant fraction of remittance is measured, the log curve corresponds to the true absorbance plus a (different) constant. Thus, provided that the SO2 calculation can compensate for linear shifts in the absorbance curve, it is never necessary to know the illumination levels in absolute terms, and the same result is obtained regardless of how much of the remitted light is actually collected.

The generation of reference spectra corresponding to high and low levels of oxygenation of the blood in this method is substantially the same as in the method described in WO 00/09004 in that high oxygen saturation of a sample of blood is artificially induced, after which, when stability has been reached, the current absorbance curve is taken as the high level reference spectrum, along with a corresponding SO2 value obtained from a blood gas analyser. Blood gas analysers are both invasive and destructive, but give much higher precision. The measurement step is then repeated with a sample of blood of low oxygen saturation to obtain the low level reference spectrum.

Figure 2 is a graph of the light absorption by blood over a range of wavelengths expressed in terms of the extinction coefficient at each wavelength for fully deoxygenated blood Hb and fully oxygenated blood Hb02. In practice the absorption spectra, corresponding to the low level reference spectrum and the high level reference spectrum referred to above, measured for an actual patient will always be for blood that has some intermediate level of oxygenation between 0% and 100%, and the detected values obtained for venous and arterial blood will correspond to two different levels of oxygenation between 0% and 100%. The function of the apparatus is to determine what levels of oxygenation correspond to the detected spectral values.

M&C Folio No P53407GB 8 Referring to the two absorption spectra 10 and 11 of Figure 2 corresponding to the extinction coefficients of Hb and Hb02, it will be appreciated that these exhibit various peak values and additionally a number of wavelengths, that is the so-called isobestic wavelengths, at which the light absorption of the blood is independent of the degree of oxygenation. In analysing the detected spectral values obtained with a particular sample of blood, it is necessary to determine the relative contributions to the absorption spectrum of deoxygenated haemoglobin Hb and oxygenated haemoglobin Hb02. It is known to match the measured absorption, reflectance or transmission spectrum against two or more reference spectra representing different predetermined levels of blood oxygenation such that the match yields a measure of actual blood oxygen saturation.

However the photodetector array 6 is a sensitive electronic device, and has a number of undesirable properties and failure modes that may prejudice the reliability of the measurement readings. Accordingly a technique will now be described with reference to Figures 3, 4, 5 and 6 by which a number of these undesirable properties can be corrected and by which some of the failure modes can be detected and compensated for.

The same reference numerals are used as in Figure 1 to denote like parts in these figures.

Figure 3 is a schematic representation a first embodiment of spectrometer in accordance with the invention showing three alternative positions A, B and C for a light reference source. The light reference source is introduced into the system at a point after the diffraction grating 4 but before the photodetector array 6. The light from the light reference source therefore illuminates the whole of the photodetector array 6, and not just the pixels of the array that would be illuminated by the appropriate colour of light had the light entered by way of the diffraction grating 4.

The light reference source can be placed in a number of different positions, depending on the optical geometry of the spectrometer. The positions A, B and C shown in Figure 3 are exemplary positions for direct illumination of the photodetector array 6 when a large LED mounted within the spectrometer is used as the light reference source. Any M&C Folio No P53407GB 9 position within the spectrometer from which the whole array can be illuminated would

however be suitable.

Figure 6 is a schematic representation of the measurement apparatus incorporating two sensor LED's 10, 11 for directing white light onto two target areas 12 and 13 of the patient's body for measurement of the venous and arterial SO2 under control of an illumination control unit 14, and two optical fibres 15, 16 for conducting the reflected or transmitted light to the spectrometer 17. The light reference source 18 is controllable by the illumination control unit 14 in sequence with reading of measurement data by the spectrometer 17, and should be of a colour that the photodetector array of the spectrometer 17 is sensitive to. There is no need for the light reference source 18 to have any particular colour profile or even to emit light of a colour range normally detected by the spectrometer 17 since the colour splitting parts of the spectrometer are bypassed by the light from the light reference source 18. There is no need for the light reference source 18 to be perfectly uniform as comparisons between detected light intensity levels are only made from one scan to the next, so it is the differences between detected light intensity levels that are of significance, not the absolute light intensity levels. The intensity of the light reference source 18 would typically be much greater than the light levels otherwise detected by the photodetector array, so that light-on measurements (that is measurements taken when the light reference source 18 is on) can be taken without the need to exclude light entering by way of the diffraction grating. A low cost light emitting diode is an ideal light reference source for this purpose.

In use of the spectrometer 17 two measurement readings are taken for each pixel of the array as discussed above, that is a "light sample" measurement obtained utilising the transmitted or reflected light from the target area, and a "dark sample" measurement obtained under the same conditions as the "light sample" but with the light from the sensor LED 10 or 11 turned off, in order to obtain the remittance spectrum. The "dark sample" is largely a function of thermal drift within the photodetector array and associated electronics, and must be subtracted from the light sample in the processing electronics 19 to eliminate thermal offset drift and give a true remitted signal. During a calibration step, a pair of samples is also taken where a known illumination is used, and M&C Folio No P53407GB 10 this gives a reference white level which maps the spectral response of the spectrometer to a controlled light source.

When the actual measurement readings are to be taken, the reference white level is used by the processing electronics 19 to compensate the readings for the inherent non-linear response profile of the spectrometer optics and electronics.

The addition of the internal light reference source 18 enables a sample of a consistent light level to be applied to the photodetector array at any time, that is both during calibration and when the actual measurement readings are being taken. The readings taken during illumination by the internal light reference source 18 can be used by the processing electronics 19 to compensate for thermal drift in the amplitude of the output signal on a pixel by pixel basis during such measurements.

If a number of readings are taken whilst the array is illuminated by the light reference source in the measurement phase, usually interspersed with the normal measurement readings, the measurement readings can be compared against each other by the processing electronics 19. Most of these measurement readings will be the same for each pixel, as they are taken under the same optical conditions. However some pixels will be statistically much noisier than others, and these pixels can be therefore be identified as faulty by the processing electronics 19. If there are only a few such faulty pixels, the instrument can continue to be used with known degraded performance.

However, if a large number of faulty pixels are found, the instrument will be diagnosed as faulty and in need of replacement. It should be noted that this diagnostic capability continues after normal calibration of the instrument and on into the field where many individual pixel failures occur.

The light reference source greatly enhances the diagnosis of faulty detector pixels. It also enables more reliable analysis of the incoming light spectrum in that the normal incoming light supplied to the instrument may itself be variable. Also, depending on the application, the incoming light source may not form a continuum. The use of the M&C Folio No P53407GB ii internal light reference source prevents gaps in the response of the photodetector from being confused with gaps in the incoming light spectrum.

Other in-field and production failures are also diagnosable more easily by the inclusion of the light reference source. For example, if the photodetector responds to internal illumination but not to external illumination, this indicates that the external light source or fibre optic delivery system is suspect. if the photodetector does not respond to internal illumination, the electronics are suspect.

Figure 4 is a schematic representation of a second embodiment of spectrometer in accordance with the invention showing an alternative position D for the light reference source. In position D the light reference source is positioned so as to direct its light beam onto the second curved mirror 5 that reflects the light and produces a light beam that illuminates the whole of the photodetector array 6.

Figure 5 is a schematic representation from one side of a third embodiment of spectrometer in accordance with the invention incorporating an external light reference source in the form of an LED 7 and a light pipe 8 extending through a hole in a base plate 9 of the instrument and having an end facet that directs the light onto the photodetector array 6. in this case the reference light enters the spectrometer by way of the light pipe 8 from underneath the base plate 9. This arrangement has a number of benefits in that the light pipe 8 can be shaped to form a lens to direct the light more accurately either at the photodetector array 6 or at the second curved mirror 5. The finish of the end facet of the light pipe 8 can be such that it doubles up as a form of focussing and diffusing lens.. Also, since the light reference source 7 is physically mounted outside the spectrometer casing, the electrical connections to the light reference source 7 can be made in a straightforward manner from outside the spectrometer, and, if the light pipe 8 has a low enough profile, the light reference source 7 can be placed in line with the normal light beam without obscuring the normal light path from the external source. The light pipe could alternatively extend from above through an upper cover of the spectrometer.

Claims

M&C Folio No P53407GB 12 CLAIMS: 1. A spectrometer comprising: an input

for receiving a multiwavelength beam of radiation to be analysed; a photodetector comprising an array of detection elements for producing electrical output signals representative of radiation detected by detection elements; a splitter for splitting the beam into components of different wavelengths and for directing the components onto the detection elements; an optical reference source for illuminating the detection elements with reference radiation; and monitoring means for indicating an error condition in dependence on (i) first electrical output signals from the array in response to detection of the components of different wavelengths in the absence of illuminating reference radiation, and (ii) second electrical output signals from the array in response to detection of the components of different wavelengths together with illuminating reference radiation.
2. A spectrometer according to claim 1, wherein compensating means are provided for compensating an electrical output indicative of the relative intensities of the components of different wavelengths in dependence on the error condition indication from the monitoring means.
3. A spectrometer according to claim 2, wherein the compensating means serves to exclude from said electrical output electrical output signals from selected detection elements of the array when an error condition indication is received from the monitoring means that indicates that the selected detection elements are faulty.
4. A spectrometer according to claim 1, 2 or 3, wherein the monitoring means provides an indication that the photodetector is faulty.
5. A spectrometer according to any preceding claim, wherein the monitoring means provides an indication that a transmission path for transmitting the multiwavelength beam to be analysed to the input is faulty.

M&C Folio No P53407GB 13
6. A spectrometer according to any preceding claim, wherein the monitoring means provides an indication that the optical reference source is faulty.
7. A spectrometer according to any preceding claim, wherein the optical reference source is a light-emitting diode (LED).
8. A spectrometer according to any preceding claim, wherein the reference radiation from the optical reference source detected by the array is of much greater intensity than the radiation from the input detected by the array.
9. A spectrometer according to any preceding claim, wherein the optical reference source is positioned so as to direct a beam of reference radiation directly onto a detection surface of the array.
10. A spectrometer according to any preceding claim, wherein the optical reference source comprises a light pipe extending through a wall for directing a beam of reference radiation onto the array.
11. A spectrometer according to any preceding claim, wherein control means is provided for switching the optical reference source on and off during detection of the components of different wavelengths so as to produce said first and second electrical output signals.
12. A spectrometer according to any preceding claim, wherein the monitoring means is arranged to subtract the first electrical output signal from the second electrical output signal in order to obtain a resultant signal.
13. A spectrometer according to any preceding claim, wherein a focussing unit is provided for focussing the components of different wavelengths onto respective detection elements of the array.

M&C Folio No P53407GB
14 14. A spectrometer according to claim 13, wherein the optical reference source is positioned in a plane intermediate the focussing unit and the array.
15. A spectrometer according to claim 13 or 14, wherein the focussing unit comprises a curved mirror.
16. A spectrometer according to any preceding claim, wherein a further focussing unit is provided for focussing the beam from the input onto the splitter.
17. A spectrometer according to any preceding claim, which is a Czerny Turner type spectrometer.
18. A method of analysing a multiwavelength beam of radiation comprising: splitting the beam into components of different wavelengths; detecting the components by means of an array of detection elements producing electrical output signals representative of the detected components; illuminating the detection elements with reference radiation from an optical reference source; and indicating an error condition in dependence on (i) first electrical output signals from the array in response to detection of the components in the absence of illuminating reference radiation, and (ii) second electrical output signals from the array in response to detection of the components together with illuminating reference radiation.
19. A spectrometer substantially as hereinbefore described with reference to the accompanying drawings.
20. A method of analysing a multiwavelength beam of radiation substantially as hereinbefore described with reference to the accompanying drawings.