Spectroscopy

Abstract

A spectrometer comprises an optical input 137 for receiving spectral light and a spectral dispersive optic 143 to disperse the spectral light into a spectrum. A device 145 is arranged to direct the sp

Description

Field of Invention This invention concerns spectroscopy and, in particular, spectroscopes, spectroscopy apparatus and methods for generating spectral data representative of an accumulation of multiple photon arrivals at a photodetector and for carrying out time separation of single-photon arrivals at a photodetector. Background The Raman Effect is a phenomenon in which a sample scatters excitation light of a given frequency into a frequency spectrum, which has characteristic peaks caused by interaction of the excitation light with the molecules making up the sample. Different molecular species have different characteristic Raman peaks, and so the effect can be used to analyse the molecular species present. For some samples, the excitation radiation gives rise to fluorescent radiation together with the Raman radiation. Since the Raman signals are emitted from the sample in a shorter time frame (less than a picosecond) compared to the fluorescence signals (a few thousand picoseconds or even tens of milliseconds), a fast-time resolved detector can be used to distinguish spectra from the two processes. It is known to use time correlated single photon counting (TCSPC) to separate out the Raman and fluorescence signals. The Raman and fluorescent signals will typically overlap because of the statistical nature of the processes and the finite width of the excitation pulse. US2012 / 0194815 Al discloses apparatus comprising a single-photon detector, which may be a single photon avalanche diode (SPAD) array, that functions in a “Geiger-mode”, wherein a single photon impacting on the photodetector causes an output pulse to be generated (likened to a “click” of a Geiger counter). Detecting Raman radiation in a pulsed mode effectively filters the background noise and fluorescence out, since outside the detecting or registration period no optical nor electric pulses are taken into account. US2013 / 0342835 Al discloses a time-resolved Raman spectroscopy apparatus that uses a SPAD with optical or electrical triggering. EP2956748 Al discloses apparatus for measuring Raman radiation from an object comprising a detector having a plurality of single-photon avalanche diode (SPAD) elements. A counter measures the individual timing of each detection made in the detector array with respect to an excitation pulse. The counter counts a number of detections in a Raman time window and one or more fluorescence time windows. The counter forms a number of Raman detections by eliminating an estimate of a number of detections of fluorescence photons in the Raman time window on the basis of a number of detections in the at least one fluorescence window. Summary of Invention According to a first aspect there is provided a spectrometer comprising an optical input for receiving spectral light and a single photon detector element. The spectrometer may further comprise an array, such as a two-dimensional array, of multi-photon detector elements. The spectrometer may further comprise a spectral dispersive optic arranged to disperse the spectral light into a spectrum. The spectrometer may further comprise a device arranged to: i) direct the spectrum to both the single photon detector element and the array of multi-photon detector elements; and / or ii) selectively direct the spectrum from the optical input to either one of the single photon detector element and the array of multi-photon detector elements. In this way, the array of multi-photon detector elements can be used for detection of the spectrum in circumstances where the capabilities of the array of multi-photon detector elements is required instead of or in addition to the capabilities of the single photon detector element. In particular, a measurement derived from the single photon detector element may have a lower signal-to-noise ratio than the multiphoton detector elements because fewer photons of the spectral light that impact the single photon detector element may be detected. In particular, single photon detector elements, such as SPADs, may not be able to detect photons during an “off’ period after detection of an initial single photon, such that the single photon detector element may detect a smaller proportion of the spectral light compared to multi-photon detector elements. Furthermore, multi-photon detector elements may allow collection methods to be carried out that are not achievable with the single photon detector elements, such as SPADs. For example, techniques that require use of a two dimensionality of the array, the higher fill factor provided by typical arrays of multi-photon detector elements, such as CCDs, and the accumulation of electric charge on and shifting of electric charge between the multi-photon detector elements of the array, such as disclosed in WO2008 / 090350, WO2009 / 093050, WO2015 / 049494, all of which are incorporated herein in their entirety by reference. However, the single photon detector element allows the apparatus to be used for time correlated single photon counting for a particular region of the spectrum. In this way, the spectrometer can provide time correlated single photon counting whilst also having the capabilities of the array of multi-photon detector elements for spectral light arriving at the optical input from the same source / spectrometer setup (e.g. spectral light generated with the same source of excitation light, same microscope, same sample, etc). This may allow cross-correlation between spectral measurements made using the single photon detector element and measurements made using the array of multi-photon detector elements. The spectrum is light spatially separated by wavelength. The spectrometer may comprise a processor. The processor may be configured to correlate the output from the single photon detector element with the output from the two-dimensional array of multi-photon detector elements. In one cross-correlation technique, an estimate of a fluorescence signal from the sample may be determined from detections by the array of multi-photon detector elements and a Raman signal from the sample may be determined by eliminating the estimate of the fluorescence signal from detections by the single photon detector. The elimination may comprise subtracting from the detections by the single photon detector a number of detections based on the estimate of the fluorescence signal or attributing a portion of the detections by the single photon detector to fluorescence based on the estimate the fluorescence signal, such as in a fitting process. The array of multi-photon detector elements may generate signals that are a result of accumulation of photon detections over a period that includes the time at which a sample will fluoresce. Hence, a spectrum constructed from the detections by the array of multi-photon detector elements will be dominated by fluorescence from the sample (because the Raman effect is a weak effect compared to fluorescence). Therefore, an estimate of the fluorescence signal may be made from the detections made by the multi-photon detector elements. The single photon detector generates timed single photon detections, some of which are photons generated by the Raman effect and some of which are generated as a result of fluorescence. There is an overlap in the time periods over which the Raman and fluorescence photons are generated. Accordingly, it may be desirable to eliminate detections due to fluorescence from other detections generated by the single photon detector in order to determine the Raman signal. In an embodiment of the invention, detections by the array of multi-photon detector elements are used to estimate a spectral form of the fluorescence from the sample, and the estimate of the spectral form of fluorescence is used to determine a number of counts to eliminate from detections made by the single photon detector for a particular time (e.g. a temporal bin of a histogram). The form of the fluorescence signal determined from the array of multi-photon detector elements may be used in a fitting process, wherein reference spectra of a known form, including a fluorescence spectrum determined from the array of multi-photon detector elements, are fitted to detections generated by the single photon detector for a particular time to determine components and / or an amount of each component in the sample. For example, the fitting process may be a direct classical least squares fitting process, such as disclosed in WO 2012 / 156667. This makes an assumption that the spectral form of the fluorescence spectrum from the sample remains the same even as an intensity of the fluorescence spectrum changes with time. In an alternative embodiment, such a correlation of the output from the single photon detector element with the output from the two-dimensional array of multiphoton detector elements may be carried out on a processor separate and / or remote from the spectrometer. The term “single photon detector element” is used herein to mean a photodetector element arranged to generate an output signal when a single photon is detected by the photodetector such that detection of each photon can be distinguished. For example, the single photon detector element may generate a signal pulse for each photon that is detected by the photodetector. The single photon detector element may be arranged to generate an output signal when a single photon is detected by the photodetector and, for a “off’ period after detection of the single photon, the single-photon detector is unable to detect further photons that impact the photodetector. The “off’ period may be at least ten nanoseconds. The single photon detector may comprise a single photon avalanche diode (SPAD). The term “multi-photon detector element” is used herein to mean a photodetector element arranged to generate an output signal that varies (e.g. accumulates) with the number of photons that impact on the photodetector element. Each multi-photon detector element may generate a signal that is proportional to a number of photons absorbed by the detector element over a period of time. For example, the array of multi-photon detector elements may be a charge coupled device (CCD), wherein electron charge accumulates at each detector element as photons are absorbed until the charge is transferred to another of the elements and / or read out. In an alternative embodiment, the array of multi-photon detector elements is a CMOS sensor. The device may comprise an optical element, such as a spectral light diversion mirror, that is operable to selectively direct the spectral light from the optical input to either one of the single photon detector element and the array of multi-photon detector elements. For example, the optical element may be movable to selectively direct the spectral light from the optical input to either one of the single photon detector element and the array of multi-photon detector elements, such as movable from a position inserted into an optical path of the spectral light to a position out the optical path. The device may be a mechanism configured to move the single photon detector element and the array of multi-photon detector elements in and out of an optical path of the spectral light to selectively direct the spectral light from the optical input to either one of the single photon detector element and the array of multi-photon detector elements. The device may be a beam splitter arranged to split the spectral light received by the optical input between the single photon detector element and the array of multiphoton detector elements. In this way, both the single photon detector element and the array of multi-photon detector elements may generate a signal arising from a common excitation of the sample. In one embodiment, the spectrometer comprises an array of single photon detector elements. For example, the array of single photon detector elements may comprise a one- or two-dimensional array of single photon detector elements. The spectral dispersive optic may be arranged to disperse the spectral light into a spectrum directed across the array of single photon detector elements when the device directs the spectrum to the array of single photon detector elements. The spectrometer may comprise a two-dimensional array of single photon detector elements; the spectral dispersive optic arranged to disperse the spectral light into a spectrum in a spectral plane such that the spectrum is spread across the two-dimensional array of single photon detector elements; and an optical arrangement in the optical path for spectral light between the spectral dispersive optic and the two-dimensional array of single photon detector elements. The optical arrangement may be configured to converge and / or diverge the spectral light in a spatial plane, perpendicular to the spectral plane, to and / or from a spatial focal point, wherein, in the spectral plane, the spectral light converges to a spectral focal point that is at a different location to the spatial focal point. The spatial focal point may be out of a detector plane of the two-dimensional array of single photon detector elements. The spectral focal point may be in or closer to the detector plane than the spatial focal point. In an alternative arrangement, the optical arrangement includes a structured diffuser configured to diffuse the spectral light in a spatial plane, perpendicular to the spectral plane. The structured diffuser may be arranged to diffuse light in one dimension only. In this way, a high spectral resolution can be achieved by the spectrometer whilst spreading the spectrum across multiple single photon detector elements in a spatial direction / the spatial plane. Therefore, if a single photon detector element is not useable, another single photon detector element for that spectral location can be used to detect the spectral light. The optical arrangement may be configured to focus the spectral light in the spectral plane. The optical arrangement may be arranged to locate a focal point of the spectral light such that the spectral light is incident on two or more single photon detector elements of the array in the spatial direction. The optical arrangement may be arranged to spread out spectral light received from the spectral dispersive optic in a spatial direction perpendicular to a spectral plane in which the spectral light is dispersed by the spectral dispersive optic across the two-dimensional array of single photon detector elements. The optical arrangement may comprise a diverging optic. The term “diverging optic” as used herein means an optic that refracts and / or reflects received light to spread the received light out perpendicularly to an optical path to a greater extent than would be the case if the optic was not in the optical path of the light. The optical arrangement may be a converging optic arranged to locate the focal point of the spectral light out of a detector plane of the two-dimensional array of single photon detector elements. The converging optic may locate the focal point in front of the detector plane of the two-dimensional array of single photon detector elements. The term “converging optic” as used herein means an optic that refracts and / or reflects the light to focus the light to a greater extent than would be the case if the optic was not in the optical path of the light. The defocussing optic or converging optic may be arranged not to spread light in the spectral plane. In this way, spectral light is out of focus in the spatial plane / direction, on the two-dimensional array of single photon detector elements. The optical arrangement may comprise a non-circular optic having an optical surface that is non-circularly symmetric about an optical axis of the spectral light. The optical arrangement may comprise a lens or mirror, such as a cylindrical lens or mirror, an axicon, or lenticular array. The non-circular optic may be oriented with a longitudinal axis of the non-circular optic aligned (i.e. parallel ) with the spectral direction / plane of the spectral light incident on the non-circular optic. The term “spectral plane and “spectral direction” are used herein to refer to a plane or direction in which the spectrum is dispersed at the specified point in the optical path of the spectral light, e.g. at the detector or at the non-circular optic. The non-circular optic may have a plane of symmetry having a normal orthogonal to an optical axis and the longitudinal axis of the non-circular optic. It will be understood that between points along the optical path, an orientation of the spectral plane or spectral direction may be transformed by optics present in the path. Accordingly, the spectral plane or spectral direction at the detector may not have the same orientation as the spectral plane or spectral direction at the non-circular optic. Definitions of other planes / directions, such as the spatial direction / plane, with reference to the spectral plane should be taken to mean relative to the orientation of the spectral plane at the same point along the optical path, e.g. an orientation of the spatial plane at the detector relative to the spectral plane also at the detector or an orientation of the spatial plane at the non-circular optic relative to the spectral plane also at the non-circular optic. The optical arrangement may be arranged not to cause spectral dispersion of the light. The spectral dispersive optic and the optical arrangement may be configured to direct the spectral light onto the array of single photon detector elements such that a width of the spectral light (at the detector) in a spatial direction orthogonal to the spectral plane is equal to or greater than a width of two or more single photon detector elements in the spatial direction. The two-dimensional array of single photon detector elements may be divided into pixels, each pixel comprising a plurality of single photon detector elements in a spatial direction orthogonal to the spectral plane, the dispersive device and the optical arrangement may be configured such that the spectral light is spread in the spatial direction across two or more of the single photon detector elements of each pixel. The optical arrangement may be configured to direct the spectral light onto the array of single photon detector elements such that a width of the spectral light (at the detector) in a spatial direction orthogonal to the spectral plane (in which the spectrum is spread by the spectral dispersive optic) is equal to or greater than a pixel width in the spatial direction. A pixel width of each pixel of the two-dimensional array of single photon detector elements may be larger in the spatial direction than a width of a pixel of the multiphoton detector elements in the spectral plane. Each pixel may comprise a set of single photon detector elements, each single photon detector element of the set for detecting light in the same wavelength range. For the array, detection of photons by the single photon detector elements are recorded on a per-pixel basis, i.e. there is no separate counting of photons detected by different ones of the single photon detector elements that make up the pixel. The array of single photon detector elements may be a line sensor (a onedimensional array of pixels). The line sensor may be oriented to only have a single pixel width in the spatial direction and a plurality of pixels in the spectral direction. For example, the line sensor may be as described in “A CMOS SPAD Line Sensor With Per-Pixel Histogramming TDC for Time-Resolved Multispectral Imaging”, IEEE Journal of Solid State Circuits, Volume: 54, Issue: 6, June 2019, incorporated herein in its entirety by reference. In another embodiment, the array of single photon detector elements comprises a two-dimensional array of pixels. In a typical array of single photon detector elements, different ones of the single photon detector elements can have different responses. In particular, it has been found that, in a typical array of single photon detector elements, the single photon detector elements have a varying dark count with some producing so many dark counts that the spectral, such as Raman, counts are swamped out. As a result, typically not all single photon detector elements of a pixel can be used for spectrometry. This means that the single photon detector elements useable for spectroscopy are not in the same position in each pixel. As a result, there can be significant variation in sensitivity of each pixel in the spatial direction. This can result in measurement inaccuracies if an extent of the spectral light at the detector (in the detector plane) in the spatial direction is of the order of a width of a single photon detector element as, for some pixels, the spectral light may fall on a less sensitive single photon detector element. By configuring the optics of the spectrometer to direct the spectral light onto the array of single photon detector elements such that a width of the spectral light in a spatial direction orthogonal to the spectral plane (in which the spectrum is spread by the spectral dispersive optic) is equal to or greater than a pixel width or equal to or greater than two or more single photon detector elements, such measurement inaccuracies resulting from variations in sensitivity of single photon detector in each pixel can be mitigated. The optical arrangement may be provided between the optical element and / or the beam splitter and the array of single photon detector elements. The spectral dispersive optic may be located between the optical input and the optical element and / or the beam splitter. In this way, the spectrometer may direct the spectrum on to the two-dimensional array of multi-photon detector elements with a different spreading in the spatial direction than the spectrum directed on to the array of single photon detector elements. The spectral dispersive optic may be a diffraction grating or a prism. The spectral dispersive optic may comprise a first diffraction grating or prism for dispersing the spectral light into a spectrum across the array of multi-photon detector elements and a second diffraction grating or prism for dispersing the spectral light into a spectrum across the array of single-photon detector elements. The diffraction grating disperses light into a spectrum at orders other than the zeroth order, where light passes through or is reflected by the diffraction grating with no wavelength separation. The processor may be configured to record a spectrum from a subset of the total number of single photon detector elements in each pixel. The subset may comprise the same number of single photon detector elements for each pixel. The subset of single photon detector elements in each pixel may be selected based on a figure of merit that measures the size of dark counts generated by each single photon detector element. Accordingly, the single photon detector elements used for each pixel may be located at different locations in the pixel. Hence, directing the spectrum of the spectral light onto the array of single photon detector elements such that a width of the spectral light in a spatial direction orthogonal to the spectral plane is equal to or greater than a pixel width in the spatial direction, ensures that the spectrum falls on the selected subset of single photon detector elements. According to a second aspect of the invention there is provided a spectrometer comprising: a spectral analyser; a detector; and a non-circular optic having an optical surface that is non-circularly symmetric about an optical axis, such as a cylindrical lens or mirror, the spectral analyser comprising an optical input port for receiving spectral light, a spectral dispersive optic arranged to disperse the spectral light into a spectrum and an optical output port to which the spectrum is delivered; the detector comprising an array of pixels, the detector mounted to the optical output port such that the spectrum is dispersed in a spectral plane across the array of pixels, wherein the non-circular optic is located on the optical path between the spectral dispersive optic and the detector. Each pixel may comprise a plurality of single photon detector elements in a spatial direction orthogonal to the spectral plane, and the non-circular optic may be configured to spread the spectral light in the spatial direction across two or more of the single photon detector elements of each pixel. The array of pixels may comprise a one-dimensional array of pixels (a line array). The spectral analyser may comprise a further optical output port. The spectrometer may comprise a further detector. The further detector may be a one- or two-dimensional array of multi-photon detector elements. The further detector may comprise a charge-coupled device (CCD). The spectral analyser may comprise a device arranged to: i) direct the spectral light from the optical input to both the optical output port and the further optical output port; and / or ii) selectively direct the spectral light from the optical input to either one of the optical output port and the further optical output port. The non-circular optic may be part of the spectral analyser. For example, the noncircular optic may be located between the spectral dispersive optic and the optical output port. According to a third aspect of the invention there is provided a spectroscopy apparatus comprising a spectrometer according to the first aspect or second aspect of the invention and a spectral path from a sample to the spectrometer for detecting spectral light emitted from the sample when irradiated by excitation light. The spectroscopy apparatus may comprise a source of excitation light. The source of excitation light may be configured to generate pulses of excitation light, for example a pulsed laser such as a pulsed laser diode. The source may emit pulses at a frequency of between IKHz and 100MHz. A pulse length may be of the order of tens or hundreds of picoseconds. According to a fourth aspect of the invention there is provided a spectroscopy method comprising detecting spectral light arising from excitation of a sample with excitation light with a single photon detector element and detecting spectral light arising from excitation of the sample with the excitation light with an array, such as a two-dimensional array, of multi-photon detector elements. The spectral light may be detected sequentially or simultaneously by each detector. An output of the single photon detector element may be used for time correlated single photon counting (TCSPC), for example to separate out Raman and fluorescence signals. The method may comprise correlating the output from the single photon detector element with the output from the two-dimensional array of multi-photon detector elements. The spectroscopy method may use a spectrometer according to the first aspect or second aspect of the invention. The method may comprise determining an estimate of a fluorescence signal from the sample from detections by the array of multi-photon detector elements and determining a Raman signal from the sample by eliminating the estimate of the fluorescence signal from detections made by the single photon detector. The elimination may comprise subtracting from the detections by the single photon detector a number of detections based on the estimate of the fluorescence signal or attributing a portion of the detections by the single photon detector to fluorescence based on the estimate the fluorescence signal, such as in a fitting process. The method may comprise estimating a spectral form of the fluorescence from the detections by the array of multi-photon detector elements, and estimating a number of Raman detections for a particular time or time period from an exposure of the sample to excitation radiation by eliminating detections made by the single photon detector allocated to the particular time or time period (e.g. a time bin of a histogram or Raman window) based on the estimate of the spectral form of the fluorescence. The form of the fluorescence may be used in a fitting process, wherein spectra of a known form, including a fluorescence spectrum determined from the array of multiphoton detector elements, are fitted to each spectrum generated by the single photon detector for the particular time or time period to determine components and / or an amount of each component in the sample. For example, the fitting process may be a direct classical least squares fitting process, such as disclosed in WO 2012 / 156667. This makes an assumption that the spectral form of the fluorescence spectrum from the sample remains the same even as the intensity of the fluorescence spectrum changes with time. According to a fifth aspect of the invention there is provided a method of analysing spectral data collected by a spectrometer according to the first aspect of the invention, the method comprising receiving first spectral data for a sample, the first spectral data detected by the single photon detector element; receiving second spectral data for the sample, the second spectral data detected by the array of multiphoton detector elements and correlating the first spectral data with the second spectral data. The step of correlating the first spectral data with the second spectral data may be in accordance with the method described with respect to the fourth aspect of the invention. The method of the fourth and fifth aspects of the invention may comprise analysing the Raman signal to identify a property of the sample and controlling a process and / or carrying out further processing on the sample based upon the identified property. For example, the process may be a manufacturing process. The sample may be a sample of one or more manufactured products and the identified property may be used to determine if the manufactured products meet a required specification. Failure to meet the required specification may require an adjustment of the process such that products are manufactured to the required specification. The sample may be a tissue sample and the process may be treatment of a patient providing the tissue sample. The method may be used as part of a checking procedure, for example a security procedure or a quality control procedure), comprising generating an alarm based upon the analysis of the spectral data. According to a sixth aspect of the invention there is provided a data carrier having instructions thereon, which, when executed by a processor, causes the processor to carry out the method of the fifth aspect of the invention. The data carrier may be a non-transient data carrier, such as volatile memory, e.g. RAM, non-volatile memory, e.g. ROM, flash memory and data storage devices, such as hard discs, optical discs, or a transient data carrier, such as an electronic or optical signal. According to a seventh aspect of the invention there is provided apparatus comprising receiving means configured to receive first spectral data for a sample, the first spectral data comprising first detections made by the single photon detector element and second spectral data for the sample, the second spectral data comprising second detections made by the array of multi-photon detector elements, and correlating means configured to correlate the first spectral data with the second spectral data. The correlating means may be configured to determine an estimate of a fluorescence signal from the sample from the second detections and determine a Raman signal from the sample by eliminating the estimate of the fluorescence signal from the first detections. The elimination may comprise subtracting from the first detections a number of detections based on the estimate of the fluorescence signal or attributing a portion of the first detections to fluorescence based on the estimate the fluorescence signal, such as in a fitting process. The correlating means may be configured to estimate a spectral form of the fluorescence from the second detections, and estimate a number of Raman detections for a particular time or time period from an exposure of the sample to excitation radiation by eliminating first detections allocated to the particular time or time period based on the estimate of the spectral form of the fluorescence. The correlator means may be configured to use a fitting process, wherein spectra of a known form, including a fluorescence spectrum, are fitted to each spectrum generated by the single photon detector for the particular time or time period to determine components and / or an amount of each component in the sample. For example, the fitting process may be a direct classical least squares fitting process, such as disclosed in WO 2012 / 156667. According to an eighth aspect there is provided a spectral analyser comprising means for receiving spectral light; means for detecting photons and generating a signal on a per-photon basis; means for detecting photon across an array of photodetector elements, each photodetector element generating a signal that varies (e.g. accumulates) with the number of photons that impact on the photodetector element; means for dispersing spectral light into a spectrum across the array of photodetector elements; and means for: i) directing the spectrum to both the means for detecting individual photons and the array of photodetector elements; and / or ii) selectively directing the spectrum to either one of the means for detecting individual photons and the array of photodetector elements. According to a ninth aspect of the invention there is provided a spectroscopy method to be carried out with a detector comprising a row of pixels, each pixel comprising a column of at least two single photon detector elements perpendicular to the row of pixels, the method compri sing:-receiving spectral light; dispersing the spectral light onto a spectrum across the row of pixels; moving the spectrum along the columns to direct the spectrum to different single photon detector elements of each pixel; and recording detections made by the pixels for different positions of the spectrum on each pixel. The method may comprise generating spectral data by using a subset of the recorded detections by the different single photon detector elements of each pixel. The subset of the recorded detections may be made by the single photon detector elements of each pixel determined to have a required level of performance. The required level of performance may be a number of dark counts below a threshold. The required level of performance may be that the single photon detector element is one or a preset number of single photon detector elements of the pixel having the lowest number of dark counts. For example, the subset of the recorded detections may be made by four single photon detector elements of the pixel having the lowest number of dark counts. According to a tenth aspect of the invention there is provided a data carrier having instructions thereon, which, when executed by a processor, causes the processor to carry out the method of the ninth aspect of the invention. According to an eleventh aspect of the invention there is provided a spectrometer compri sing:- an optical input for receiving spectral light; a detector comprising a row of pixels, each pixel comprising a column of at least two single photon detector elements perpendicular to the row of pixels; a dispersive optic for dispersing the spectral light onto a spectrum across the row of pixels; and a movable optic for moving the spectrum along the columns to direct the spectrum to different single photon detector elements of each pixel. The spectrometer may comprise a confocal microscope comprising a confocal slit or pinhole along the optical path for the spectral light, wherein the movable optic is provided after the confocal slit or pinhole. The movable optic may comprise a movable mirror. The movable optic may be rotatable to change an angle of incidence of the spectral light on the movable optic. The spectrometer may comprise a controller arranged to control the spectrometer to carry out the method of the ninth aspect of the invention. Description of Drawings FIGURE 1 is a schematic representation of spectroscopy apparatus according to an embodiment of the invention; FIGURES 2a and 2b are schematic representations of a spectral analyser and detectors of the spectroscopy apparatus; FIGURE 3 is a schematic representation of a detector comprising an array of single photon detector element; FIGURE 4 is a schematic representation of the detector shown in Figure 3 showing how the spectrum is spread in a spatial plane by the diverging cylindrical lens; FIGURE 5 is a schematic representation of an optical arrangement according to another embodiment of the invention; FIGURE 6 is a schematic representation of a spectral analyser and detectors of spectroscopy apparatus according to another embodiment of the invention; FIGURE 7 is a histogram exemplifying detections made by the singlephoton detector; and FIGURE 8 is a plot exemplifying the form of detections made by the multi-photon detector. Description of Embodiments Referring to Figure 1, the spectroscopy apparatus comprises a source 110 of excitation light, a confocal microscope 118 for delivering the laser beam to a sample 124 and spectral light, such as Raman-shifted light, to a spectral analyser 128 of a spectrometer 117. The spectral analyser spatially disperses the received spectral light by wavelength into a spectrum and delivers the spectrum to a selected one of two detectors 130, 131. Detector 130 comprises an array of single-photon detector elements, for example a two-dimensional array of single photon avalanche diodes (SPADs). Detector 131 comprises a two-dimensional array of multi-photon detector elements 134, such as a charge coupled device (CCD). In this embodiment the light source 110 is a pulsed laser configured to generate laser pulses along laser optical input path 113 to an optical device. The optical device comprises a mirror 114 and a Rayleigh filter 116. In this embodiment, the Rayleigh filter 116 is a notch or edge filter, which acts as a dichroic beam splitter. The mirror 114 and Rayleigh filter 116 reflect the laser beam directed along the optical input path 113 to a sample 124. The laser beam directed to the sample 124 enters into the microscope 118, wherein the laser beam is deflected by an optic 120 through an objective lens 122 and focused on to a sample 124. Raman scattering takes place at the sample, producing Raman-shifted light at different wavenumbers from the incident laser line. The sample is mounted on a stage 123 that can be moved in two dimensions (x,y) perpendicular to the laser beam. Movement of the stage 123 allows the laser beam to be positioned at different locations on the sample 124 enabling spectroscopy data to be gathered at each of these locations. The Raman-shifted light is collected by the objective lens 122 and passed back along a spectroscopy optical path 115 via the optic 120 to the Rayleigh filter 116. Whereas the Rayleigh filter 116 reflects light of the laser wavelength, it transmits the Raman-shifted wavenumbers. While doing so, it rejects the much more intense laser line. The Raman-shifted light then passes through a Raman analyser 128 and to the selected detector 130, 131. Referring to Figures 2a and 2b, the spectral analyser 128 comprises a housing 135 having an optical input port 137 and two optical output ports 139 and 141. In this embodiment, the optical input port 137 is an entrance slit, which functions as a confocal slit 129. However, in other embodiments, a confocal slit or pinhole 129 could be located elsewhere in the system and separate from the optical input port 137. The housing 135 contains a collimating lens 142, a spectral dispersive optic, in this embodiment a diffraction grating 143, a focussing optic 144, a spectral light diversion mirror 145 and an optical arrangement configured to diverge the spectral light in a spatial plane, perpendicular to the spectral plane, from a virtual spatial focal point, 147. In this embodiment, the optical arrangement is a noncircular, diverging optic having a plane of symmetry 155 in the form of a cylindrical lens 146. The diffraction grating 143 disperses the spectrum in a spectral direction / plane, X. A spatial plane, such as shown in Figure 4, is perpendicular to the spectral direction / plane, X. The CCD 131 is mounted to the spectral analyser 128 to receive spectral light from optical output port 141 and the array 130 of single photon avalanche diodes 133 is mounted to the spectral analyser 128 to receive spectral light from optical output port 139 when the mirror 145 is moved into the path of the light dispersed by the diffraction grating 143 (as shown in Figure 2b). As can be appreciated, the spectral direction in which the spectrum is spread is rotated by mirror 145 (although the spectral plane remains the same). However, mirror 145 could be angled to direct the spectral plane out the plane shown in Figure 2b (i.e. out of the plane of the paper). When the mirror 145 is out of the optical path, light dispersed by the diffraction grating 143 is directed to the multi-photon detector 131 (as shown in Figure 2a). The cylindrical lens 146 diverges the spectral light in the spatial plane from the virtual spatial focal point 147. This virtual focal point 147 is at a different location to a spatial focal point 149 to which spectral light in the spectral plane is converged to by focussing optic 144. The focussing optic, in this embodiment a focussing lens 144, is arranged to focus the spectrum onto the CCD with a required resolution. In this embodiment, the focussing lens 144 is arranged to produce a spectral line on the CCD 131 that is about one pixel (of the CCD) wide in the spatial direction (for example, because it focusses an image of a confocal slit, through which the Raman light passes, to a width of about one pixel). The spectral light diversion mirror 145 is movable into the optical path of the spectral light dispersed by the dispersive optic 143 to redirect the dispersed spectrum of light to the detector 130. The diverging cylindrical lens 146 is located on the optical path between the dispersive optic 143 (and, in this embodiment, also the spectral light diversion mirror 145) and the detector 130 and is oriented to spread the spectrum out across the detector 130 in a spatial direction, S. The solid lines 152 in Figure 4 illustrate the light diverged by the cylindrical lens and the dotted lines 154 illustrate the converging path the light would take in the absence of the cylindrical diverging lens 146. The cylindrical lens 146 diverges the light in a spatial plane perpendicular to a longitudinal axis B-B of the cylindrical lens 146 and leaves it unaltered in the direction parallel to the longitudinal axis B-B. Accordingly, cylindrical lens 146 is oriented such that its longitudinal axis is parallel with the spectral direction, X. As the cylindrical lens diverges the spectral light, a (virtual) focal point 147 of the diverging light is out of a detector plane 148 of the detector 130. Referring to Figure 3, the detector 130 comprises single photon detector elements 133 in a two-dimensional array. The single photon detector elements 133 are grouped into pixels Pi to Pn. Each pixel comprises a plurality of single photon detector elements 133, including a plurality of single photon detector elements 133 in a spatial direction, S orthogonal to the spectral direction, X. (The dotted box encloses the single photon detector elements 133 that make up pixel Pi. Each other pixel Pi to Pn comprises a corresponding set of single photon detector elements 133.) In this embodiment, each pixel comprises single photon detector elements 133 in the spatial direction and two single photon detector elements 133 in the spectral direction. Each single photon detector element 133 is a similar in size to a detector element 134 of the CDD. Accordingly, without the cylindrical diverging lens 146, the spectrum would be focussed onto fewer than all of the single photon detector elements 133 of each pixel Pi to Pn. However, as shown in Figure 4, with the cylindrical diverging lens 146, the spectral light is spread over all single photon detector elements 133 of each pixel Pi to Pn. A cylindrical lens 146 is selected having a focal length that achieves this purpose. For a typical array of single photon detectors 133, a number of the single photon detectors 133’ may operate in a manner that is not suitable for detecting Raman signals. For example, some of the single photon detectors 133’ may produce a dark signal that swamps any Raman signal that may be detected. However, as the spectral light is spread out across all single photon detectors 133 of each pixel Pi to Pn, the spectrum is more likely to be incident on at least one single photon detector 133 of each pixel Pi to Pn having the required sensitivity to Raman light / a sufficiently low dark current. The apparatus may be operated to only use the single photon detectors 133 of each pixel Pi to Pn deemed to have a satisfactory operation, e.g. a small enough dark current. The apparatus may be operated to only use a set number, such as four, of the single photon detectors 133 of each pixel Pi to Pn deemed to have the smallest dark current. As the spectral light is spread out across all of the single photon detectors 133 of each pixel Pi to Pn, one can select the set number of single photon detectors 133 from any of the single photon detectors 133 of the pixel Pi to Pn. In an alternative optical arrangement shown in Figure 5, a converging cylindrical lens 246 is used instead of the diverging cylindrical lens 146. The converging cylindrical lens 246 converges spectral light in the spatial plane to a spatial focal point 247 that is out of the detector plane 148. Accordingly, in the spatial plane, the spectral light is spread out across the array of single photon detector elements 133 of each pixel Pi to Pn. In the spectral plane, the spectral focal point, typically on the detector plane 148, remains substantially unchanged. Detector 130 is linked to the laser 110 via a trigger line 132 used to synchronise operation of the detector 130 with the pulses of the laser 110. Further details of the operation and timing of the detector 130 can be found in “A CMOS SPAD Line Sensor With Per-Pixel Histogramming TDC for Time-Resolved Multispectral Imaging”, IEEE Journal of Solid State Circuits, Volume: 54, Issue: 6, June 2019. In particular, after detecting a photon, each single photon detector element is unable to detect a further photon at least until the next laser pulse. The detector 130 stores a histogram of the photon detections on a per pixel basis detected over a number of pulses (exposures) of the sample to the laser beam. The detector 130 allocates detections by each pixel to appropriate bins of a corresponding histogram based on the measured time of arrival of the photon. Hence, the resultant histogram for each pixel plots time of arrival versus number of detections (counts). Each pixel corresponds to a wavenumber based on the spectral dispersion at the detector 130. An example of a histogram produced is shown in Figure 7, (only one histogram is shown but it will be understood that a histogram per pixel is produced as illustrated by the wavenumber axis). A Raman spectrum and / or fluorescence spectrum can then be determined, for example by controller 150, from the histograms based on time of arrival of the photons after an excitation pulse. In one embodiment, the controller 150 sends the spectral data to another computer, separate from the spectrometer 117, and the Raman spectrum and / or fluorescence spectrum is determined by this other computer. In use, a user can select to carry out detection using detector 130 or detector 131. In particular, the user may use detector 130 to carry out time resolved spectral imaging of a sample, whereas detector 131 may be used to carry out spectral imaging of the sample that is not time resolved. The user may select to use detector 131 as the increased signal to noise ratio achievable with the charge coupled device CCD may be desirable for a particular application, or the ability to shift the charge between pixels of the CCD synchronously with movement of the laser spot on the sample may be desirable in certain spectral collection techniques. However, time resolved spectral imaging may be desirable for other applications or in addition to using detector 131. The user may input into a controller 150 an indication of which detector 130,131 the user wants to use for detection and the controller 150 generates a command that causes the spectral analyser 128 to move the spectral light diversion mirror 145 into or out of the optical path of the dispersed spectrum. The user may use both detectors 130 and 131 to detect Raman and / or fluorescence spectra from the same sample and analyse the spectra from both detectors 130 and 131 to determine a characteristic of the sample. For example, the spectra gathered by each detector 130 and 131 may be analysed together to find correlations that relate to characteristics of the sample. In one embodiment, the results from one detector 130, 131 may be used to modify and / or in a spectral model of the results from the other detector 130, 131. For a sample that produces Raman light and fluorescence, detections by each detector 130, 131 will be a combination of the photons from both these effects. The photons produced by the Raman effect will tend to arrive earlier than the photons from fluorescence of the sample. Accordingly, for the single photon detector 130, this may result in a distribution of detections for a pixel as shown in Figure 7 where a first peak of counts corresponding to the Raman light is followed by a second peak of counts corresponding to the fluorescence. The two peaks may overlap as illustrated by the dashed line 160 illustrating the Raman peak, which typically has a temporal form similar to that of the excitation light, and the dashed and double dotted line 161 illustrating the fluorescence peak. The histogram shown in Figure 7 is shown without noise for clarity but, in a practical application, significant noise is present in the data, for example from dark counts, obscuring the peaks shown. To analyse the sample based on the Raman or fluorescence peak, the data is post-processed to remove or otherwise eliminate from the analysis the other of the fluorescence or Raman peak. For the multi-photon detector 131, there is no time division of the counts. Counts are accumulated by the multi-photon detector over a time period that includes both the Raman peak and the fluorescence peak. Accordingly, the counts for each wavenumber recorded by the multi-photon detector will be a sum of detected photons produced by the Raman effect and detected photons produced by fluorescence. The Raman effect is typically a weak effect relative to fluorescence and therefore, a number of counts due to fluorescence of the sample will be much greater than a number of counts due to the Raman effect. Accordingly, the form of spectrum recorded by detector 131, an example of which is shown in Figure 8, can be attributed to fluorescence as the impact of Raman photons on the spectrum is negligible. Furthermore, the multi-photon detector 131 will likely record a greater number of total counts over the collection period and suffer from less noise, such as dark counts, than the single-photon detector. Accordingly, errors in the form of fluorescence spectrum determined from the detections made by detector 131 are likely to be less than if the form of the fluorescence spectrum is determined from detections made by the single-photon detector 130. However, to determine a Raman spectrum of the sample, the output from the single-photon detector 130 is analysed. In one embodiment, the fluorescence spectrum determined from the multi-photon detector is used to determine a number of counts at each wavenumber to remove from detections made by the single photon detector 130. In particular, the form of the fluorescence spectrum as determined from the multi-photon detector 131 can be used as a component (a reference spectrum) in a fitting process, such as a direct classical least squares fitting process, wherein a number of reference spectra of known form are fitted to the measured spectral data to determine the components and their magnitude that replicate the measured spectral data. The fitting process may scale the selected reference spectra, including the fluorescence spectrum, to produce a best fit to the spectral data in accordance with a figure of merit (e.g. least squares, Bayesian information criteria (BIC), or the like). Such a process eliminates, at least partially, a contribution to the number of counts recorded by the single-photon detector as a result of fluorescence, providing for a more accurate determination of components and / or amounts of components present in the sample from the Raman effect. The fitting process could be applied to each temporal bin or to a block of counts across multiple temporal bins. For example, counts within a Raman time window 162 may be summed together and the fitting process carried out on the resulting summed counts from the Raman time window. It will be understood that the analysis for the detections made by the detectors 130, 131 is not limited to the above-described techniques but other analysis techniques may be used, such as the analysis technique described in EP2956748 Al. In a further embodiment shown in Figure 6, rather than a cylindrical lens, means is provided for moving the spectrum across the detector 130 in the spatial direction, S. For example, an optic, such as a mirror, for example mirror 145 may be mounted for rotation about an axis A-A aligned with the spectral direction, X. A motor 151 is provided to rotate the mirror 145 about the axis A-A such that the spectrum (for example, having a spatial focal point at the detector plane) is moved across multiple single photon detector elements 13 of each pixel Pi to Pn. The motor 151 may rotate the mirror 145 at a speed such that, for a given point of the laser beam spot on the sample, multiple single photon detector elements 13 of each pixel Pi to Pn receive photons resulting from the excitation of that given point of the sample. Pulses of the laser may excite a given region for a time that is of the order of milliseconds before the laser is directed to another given region of the sample. Accordingly, the motor 151 may rotate the mirror 145 at a speed such that the spectrum is directed to all single photon detector elements 13 of each pixel Pi to Pn within a millisecond, tens of milliseconds or hundreds of milliseconds. A spectrum can then be formed from the selected single photon detector elements 13 of each pixel Pi to Pn deemed to have a sufficiently low dark count. It will be understood that other optical components may be motorised to move the spectrum over the detector 130. In a confocal microscope, the motorised optic is located after the slit. Modifications and alterations may be provided to the above-described embodiments without departing from the invention as defined herein. In a further embodiment, the locations of the detectors 130 and 131 are reversed such that insertion of the spectral light diversion mirror 145 into the optical path of the light dispersed by the dispersive optic 143 directs the dispersed spectrum of light to the detector 130. The movable spectral light diversion mirror 145 may be moved in and out of the optical path of the spectral light through linear or rotary motion. Rather than moving the spectral light diversion mirror 145 in and out of the optical path of the spectral light, the spectral light diversion mirror 145 may move in the optical path to change the angle of deflection to selectively direct the light to the different ports 139, 141. In an alternative, rather than moving a movable spectral light diversion mirror 145, the detectors 130 and 131 may be moved to selectively locate a one of the detectors 130 and 131 at a port, for example port 141. In such an embodiment, the detector 130 and cylindrical lens 145 may be configured to move together as a single unit and the movable spectral light diversion mirror 145 may be omitted (as can the second port). In another embodiment, the movable spectral light diversion mirror 145 may be replaced by a beam splitter such that both detectors 130 and 131 can be used simultaneously to detect spectral light during exposure of a sample to a train of pulses. In a further embodiment, another non-circular lens or mirror other than a cylindrical lens or mirror may be used. For example, the non-circular lens or mirror may converge or diverge the spectral light in the spectral plane but to a different extent to the convergence or divergence of the spectral light in the spatial plane to locate the spatial and spectral focal points at different locations (e.g. the spatial focal point located out of the detector plane and the spectral focal point located closer to the detector plane).

Claims

1. A spectrometer comprising an optical input for receiving spectral light, a single photon avalanche diode, an array of multi-photon detector elements, a spectral dispersive optic arranged to disperse the spectral light into a spectrum and a device arranged to:i) direct the spectrum to both the single photon avalanche diode and the array of multi-photon detector elements; and / orii) selectively direct the spectrum to either one of the single photon avalanche diode and the array of multi-photon detector elements.

2. A spectrometer according to claim 1, wherein the array of multi-photon detector elements is a charge coupled device.

3. A spectrometer according to any one of the preceding claims, wherein the device comprises an optical element that is operable to selectively direct the spectral light from the optical input to either one of the single photon avalanche diode and the array of multi-photon detector elements.

4. A spectrometer according to claim 3, wherein the optical element is a spectral light diversion mirror.

5. A spectrometer according to claim 3 or claim 4, wherein the optical element is movable to selectively direct the spectral light from the optical input to either one of the single photon avalanche diode and the array of multi-photon detector elements.

6. A spectrometer according to claim 5, wherein the optical element is movable from a position inserted into an optical path of the spectral light to a position out the optical path.

7. A spectrometer according to any to claim 1 or claim 2, wherein the device comprises a beam splitter arranged to split the spectral light received by the opticalinput between the single photon avalanche diode and the array of multi-photon detector elements.

8. A spectrometer according to any one of the preceding claims, comprising an array of single photon avalanche diodes.

9. A spectrometer according to claim 8, wherein the spectral dispersive optic is arranged to disperse the spectral light into a spectrum directed across the array of single photon avalanche diodes when the device directs the spectrum to the array of single photon avalanche diodes.

10. A spectrometer according to claim 9, comprising a two-dimensional array of single photon avalanche diodes; the dispersive optic arranged to disperse the spectral light into a spectrum in a spectral direction across the two-dimensional array of single photon avalanche diodes; and a diverging optic between the dispersive optic and the two-dimensional array of single photon avalanche diodes.

11. A spectrometer according to claim 10, wherein the two-dimensional array of single photon avalanche diodes is divided into pixels, each pixel comprising a plurality of single photon avalanche diodes in a spatial direction orthogonal to the spectral direction, the dispersive device and diverging optic configured such that the spectrum is spread in the spatial direction across two or more of the single photon avalanche diodes of each pixel.

12. A spectrometer according to claim 11, wherein a pixel width of each pixel of the array of two-dimensional array of single photon avalanche diodes is larger in the spatial direction than a width of a pixel of the multi-photon detector elements.

13. A spectrometer according to claim 11 or claim 12, comprising a processor configured to record a spectrum from a subset of the total number of single photon avalanche diodes in each pixel.

14. A spectrometer according to claim 13, wherein the subset for each pixelcomprises the same number of single photon avalanche diodes.

15. A spectroscopy apparatus comprising a spectrometer according to any one of the preceding claims and a spectral path from a sample to the spectrometer for detecting spectral light emitted from the sample when irradiated by excitation light.

16. A spectroscopy method comprising detecting spectral light arising from excitation of a sample with excitation light with a single photon avalanche diode and detecting spectral light arising from excitation of the sample with the excitation light with an array of multi-photon detector elements.

17. A spectroscopy method according to claim 16, wherein the spectral light is detected sequentially or simultaneously by each detector.

18. A spectroscopy method according to claim 16 or claim 17, comprising correlating the output from the single photon avalanche diode with the output from the two-dimensional array of multi-photon detector elements.

19. A spectroscopy method according to any one of claims 16 to 18, wherein a spectrometer according to any one of claims 1 to 14 is used to detect the spectral light.

20. A method of analysing spectral data collected by a spectrometer according to any one of claims 1 to 14, the method comprising receiving first spectral data for a sample, the first spectral data detected by the single photon avalanche diode; receiving second spectral data for the sample, the second spectral data detected by the array of multi-photon detector elements and correlating the first spectral data with the second spectral data.IntellectualPropertyOfficeApplication GB2503128.7Search report under Section 17 of the Patents Act 1977Date search completed: 14 August 2025Claims searched: 1-20International classificationSubclass and subgroup Valid from G01J3 / 02 01 / 01 / 2006 G01J3 / 18 01 / 01 / 2006 G01J3 / 28 01 / 01 / 2006 G01J3 / 44 01 / 01 / 2006Field of searchWorldwide search of patent documents classified in the following areas of the IPC:G01J, G01NDatabases used in the preparation of this search report:SEARCH-PATENTDocuments considered to be relevantPatent literatureCategory Relevant Document of relevanceclaimsX 1-8 &15 20 US 2022 / 0042916 A1 (NOTINGHER), See Fig. 1 and para. 0054-0059 X 1-3, 7-9 &15-20 WO 2023 / 033744 A2 (NAT UNIV SINGAPORE), See Figs 11, 14 and related text Non-patent literature Category Relevant claims Document of relevanceCategoriesLetter or symbol Description X Document indicating lack of novelty or inventive step.Y Document indicating lack of inventive step, if combined with anotherdocument of the same category.& Member of the same patent family. A Document indicating technological background. P Document published on or after the priority date but before the fling date of the present application. E Earlier application published on or after the filing date of the present application.

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