Abstract:
Spectroscopy including a source generating light profile on the sample. The light profile is movable relative to the sample. An optical input for receiving light from sample interaction with light from the source, a detector including two-dimensional array of photodetector elements, a dispersive device between the optical input and detector to spectrally disperse light from the optical input in spectral direction across the detector, an optical splitter in optical path between the optical input and detector to split light based upon wavenumber so, for a spectrum generated by a point on the sample, the spectrum first and second portions are dispersed across photodetector elements of different array row/column. Controller shifting data between photodetector elements in spatial direction, perpendicular to spectral direction, synchronously with relative movement between light profile and sample so data is accumulated on the spectrum first and second portions across different sets of photodetector elements during relative movement.

Description:
FIELD OF INVENTION 
       [0001]    This invention relates to spectroscopy apparatus and methods. It is particularly useful in Raman spectroscopy, though it can be used in other forms of spectroscopy. 
       BACKGROUND 
       [0002]    The Raman Effect is a phenomenon in which a sample scatters incident light of a given frequency into a frequency spectrum, which has characteristic peaks caused by interaction of the incident 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. 
         [0003]    A prior Raman analysis apparatus is described in European Patent Application No. EP 0543578. A sample is illuminated by a laser beam, and the resulting Raman scattered light is analysed, and then detected. The detector may be a charge-coupled device (CCD) comprising a two-dimensional array of pixels. The analysis of the Raman spectrum may be carried out by a dispersive device such as a diffraction grating, which disperses the spectrum produced from a point or line on the sample across the width of the CCD. The apparatus may be arranged to disperse the spectrum widely across the CCD, to provide high spectral resolution. 
         [0004]    For a CCD of a given width, however, only a part of the spectrum can then be detected at any one time. To acquire data from a wider spectrum, one possible method is to expose one part of the spectrum onto the CCD for a sufficient time, and then to read all of the data relating to that part of the spectrum from the CCD into a computer. Next, the diffraction grating is indexed to a new position, so that a second part of the spectrum is received by the CCD. Again, sufficient exposure time is allowed, and all the data from the second part of the spectrum is read into the computer. This process is repeated as often as necessary. Exposing the separate parts of the spectrum sequentially increases the time required to analyse the complete spectrum compared with a lower resolution system in which the whole spectrum of interest is dispersed more narrowly across the width of the CCD. 
         [0005]    A further method, as disclosed in U.S. Pat. No. 5,638,173, is to split the spectrum into separate optical paths using edge filters and a mirror. These components are tilted at different angles so that, after the beams have been dispersed by a diffraction grating, partial spectra are formed on the detector, one above the other. This allows several consecutive parts of the spectrum to be widely dispersed and viewed simultaneously. 
         [0006]    If it is desired to map an area of the sample, rather than just a single point or line, then it is known to mount the sample on a stage which can be moved in orthogonal directions X, Y. Alternatively, movable mirrors may deflect the light beam across the surface of the sample in X and Y directions. Thus, a raster scan of the sample can take place, giving Raman spectra at each point in the scan 
         [0007]    To reduce the time taken for a scan, it is known to illuminate the sample not with a point focus, but with a line focus. This enables the acquisition of spectra from multiple points within the line simultaneously. On the CCD detector, it is arranged that an image of the line extends orthogonally to the direction of spectral dispersion. This enables efficient use of the two-dimensional nature of the detector to acquire multiple spectra simultaneously. . The spectral data obtained for each location of the point or line focus can be combined in a “step and stitch” process to generate a spectral map of the area. 
         [0008]    International Patent Application No. WO 2008/090350 describes a continuous collection process, in which the sample is scanned in the longitudinal direction of the line focus. Synchronously with this, charge in the CCD is shifted from pixel to pixel in the same direction, towards an output register of the CCD, and continues to accumulate as the scan proceeds. This longitudinal line scan is repeated at laterally spaced locations. This enables a spectral map of an area of a sample to be obtained without performing the “step and stitch” process (in one axis), hence helping to avoid discontinuities. 
       SUMMARY OF INVENTION 
       [0009]    According to a first aspect of the invention there is provided spectroscopy apparatus comprising:
       a support for a sample;   a light source arranged to generate a light profile on the sample;   the support and light source arranged such that the light profile is movable relative to the sample;   an optical input for receiving light generated by interaction of the sample with light from the light source;   a detector comprising a two-dimensional array of photodetector elements;   a dispersive device arranged between the optical input and the detector to spectrally disperse light received by the optical input in a spectral direction across the detector;   an optical splitter located in an optical path between the optical input and the detector to split light based upon wavenumber such that, for a spectrum generated by a given point on the sample, each of a first portion and a second portion of the spectrum is dispersed across photodetector elements of a different row or column of the array; and   a controller arranged to control shifting of data between the photodetector elements in a spatial direction, perpendicular to the spectral direction, synchronously with relative movement between the light profile and the sample so that data is accumulated on each of the first and second portions of the spectrum across different sets of photodetector elements of the detector during the relative movement.       
 
         [0018]    According to a second aspect of the invention there is provided spectroscopy apparatus comprising:
       a support for a sample;   a light source arranged to generate a light profile on the sample;   the support and light source arranged such that the light profile is movable relative to the sample;   an optical input for receiving light generated by interaction of the sample with light from the light source;   a detector comprising a two-dimensional array of photodetector elements;   a dispersive device arranged between the optical input and the detector to spectrally disperse the light received by the optical input in a spectral direction across the detector;   an optical splitter arranged in an optical path between the optical input and the detector to split light based upon wavenumber such that first and second portions of a spectrum generated by a given point on the sample, which occur across a spectral range that is greater than the spectral range provided by a width of the detector in the spectral direction, are directed to photodetector elements of the detector; and   a controller arranged to control shifting of data between the photodetector elements in a spatial direction, perpendicular to the spectral direction, synchronously with relative movement between the light profile and the sample so that data is accumulated on each of the first and second portions of the spectrum across different sets of photodetector elements of the detector during the relative movement.       
 
         [0027]    In the second aspect of the invention, each of the first and second portions may be dispersed across photodetector elements of a different row or column of the array or may be dispersed across different photodetector elements of the same row or column of the array. 
         [0028]    The invention may allow a spectral map to be obtained for selected portions of the spectrum that are of interest. In particular, the two portions of the spectrum that are of interest can be collected using a “Continuous Collection” process, in which data is accumulated on each of the first and second portions as data is shifted between the photodetector elements in the spatial direction synchronously with relative movement between the light profile and the sample, even though these two portions may be divided across the detector in the spatial direction and/or occur across a spectral range that is greater than the spectral range provided by a width of the detector in the spectral direction (e.g. there is a spectral gap between the first and second portions). 
         [0029]    The optical splitter may comprise at least one splitting filter, such as an edge filter, short pass filter or the like, arranged between the optical input and the detector, the filter transmitting the first portion of the spectrum along a first optical path to the detector and reflecting the second portion of the spectrum along a second, different optical path to the detector. 
         [0030]    The apparatus may further comprise a blocking filter arrangement for filtering light received by the optical input, the blocking filter arrangement filtering light wavenumbers directed by the optical splitter along the first optical path and wavenumbers directed by the optical splitter along the second optical path. The blocking filter arrangement may comprise at least one blocking filter, such as a notch filter, before the optical splitter, the at least one blocking filter arranged to filter wavenumbers that would otherwise be directed along the first optical path and wavenumbers that would otherwise be directed along the second optical. The blocking filter arrangement may comprise a first blocking filter, such as an edge or notch filter, in the first optical path for blocking light, based upon wavenumber, directed by the optical splitter along the first optical path and a second, different blocking filter, such as an edge or notch filter, in the second optical path for blocking light, based upon wavenumber, directed along the second optical path by the optical splitter. The first and second blocking filters may be complimentary such that each of the first and second blocking filters blocks light that would otherwise be dispersed across photodetector elements of the detector in which data on the second and first portions, respectively, of the spectrum are accumulated. The apparatus may comprise a plurality of complimentary pairs of first and second blocking filters arranged to be moved in and out of the first and second optical paths such that different portions of the spectrum can be selected (for collection in the Continuous Collection process). Alternatively, each of the first and second blocking filter may be tuneable to block a desired range of wavenumbers. Accordingly, the user can selective partial data on each of the first and second optical paths (arms) for collection in the Continuous Collection process. 
         [0031]    The one or more blocking filters may be arranged to be moved in and out of the optical path(s). The apparatus may further comprise a first shutter movable in and out of the first optical path for completely blocking light from travelling along the first optical path to the detector and a second shutter movable in and out of the second optical path for completely blocking light from travelling along the second optical path to the detector. In this way, the user can select to collect full spectra for one of or sequentially for each of the first and second optical paths or simultaneously collect spectra for selected wavenumber regions on each of the optical paths. 
         [0032]    According to a third aspect of the invention, there is provided a spectroscopy method comprising:
       illuminating a sample to generate a light profile on the sample;   moving the light profile relative to the sample;   spectrally dispersing received light generated by interaction of the sample with light of the light profile in a spectral direction across a detector, the detector comprising a two-dimensional array of photodetector elements,   splitting the received light based upon wavenumber such that, for a spectrum generated by a given point on the sample, each of a first portion and a second portion of the spectrum is dispersed across photodetector elements of a different row or column of the array; and   shifting data between the photodetector elements in a spatial direction, perpendicular to the spectral direction, synchronously with relative movement between the light profile and the sample so that data on each of the first and second portions of the spectrum accumulates across different sets of photodetector elements of the detector during the relative movement.       
 
         [0038]    According to a fourth aspect of the invention, there is provided a spectroscopy method comprising:
       illuminating a sample to generate a light profile on the sample;   moving the light profile relative to the sample;   spectrally dispersing received light generated by interaction of the sample with light of the light profile in a spectral direction across a detector, the detector comprising a two-dimensional array of photodetector elements,   splitting light based upon wavenumber such that first and second portions of a spectrum generated by a given point on the sample, which occur across a spectral range that is greater than the spectral range provided by a width of the detector in the spectral direction, are directed photodetector elements of the detector; and   shifting of data between the photodetector elements in a spatial direction, perpendicular to the spectral direction, synchronously with relative movement between the light profile and the sample so that data on each of the first and second portions of the spectrum accumulates across different sets of photodetector elements of the detector during the relative movement.       
 
         [0044]    According to a fifth aspect of the invention there is provided spectroscopy apparatus comprising:
       a support for a sample;   a light source arranged to generate a light profile on the sample;   an optical input for receiving light generated by interaction of the sample with light from the light source;   a detector comprising a row of photodetector elements;   a dispersive device arranged between the optical input and the detector to spectrally disperse the light received by the optical input across the row of the detector;   an optical splitter arranged in an optical path between the optical input and the detector to split light based upon wavenumber such that spectrally separated first and second portions of a spectrum generated by a given point on the sample, which occur across a spectral range that is greater than a spectral range provided by the row of the detector, are simultaneously directed to photodetector elements of the row.       
 
         [0051]    In this way, two portions of the spectrum of interest that are spectrally separated can be simultaneously recorded without reducing the spectral resolution. 
         [0052]    The light profile may be a line focus. The detector may comprise a two-dimensional array of photodetector elements. The dispersive device may be arranged to spectrally disperse the light generated by interaction of the sample with the line focus across multiple rows of the detector such that, for each given point on the sample illuminated by the line focus, spectrally separated first and second portions of the spectrum generated by the given point, which occur across a spectral range that is greater than a spectral range provided by the row of the detector, are simultaneously directed to photodetector elements of the same row. 
         [0053]    According to a sixth aspect of the invention, there is provided a spectroscopy method comprising:
       illuminating a sample to generate a light profile on the sample;   spectrally dispersing received light generated by interaction of the sample with light of the light profile in a spectral direction across a detector, the detector comprising a row of photodetector elements,   splitting the received light based upon wavenumber such that, for a spectrum generated by a given point on the sample, spectrally separated first and second portions of the spectrum, which occur across a spectral range that is greater than a spectral range provided by the row of the detector, are simultaneously directed across photodetector elements of the row of the detector.       
 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0057]      FIG. 1  is a schematic representation of a Raman spectroscopy apparatus according to an embodiment of the invention; 
           [0058]      FIG. 2  is a perspective view of an analyser, lens and detector of the Raman spectroscopy apparatus shown in  FIG. 1 ; 
           [0059]      FIG. 3  is a schematic of shifting of charge across the CCD of the Raman spectroscopy apparatus synchronously with movement of a line focus across a sample; 
           [0060]      FIG. 4 a    is a schematic representation of a Raman spectrum and  FIG. 4B  is a schematic representation of the Raman spectrum dispersed across the CCD without filtering of the light along either optical path; 
           [0061]      FIG. 5 a    is a schematic representation of a wavenumber profile filtered by filters in first and second optical paths of the apparatus and  FIG. 5 b    is a schematic representation of first and second portions of the spectrum, transmitted by these filters, dispersed across the CCD; 
           [0062]      FIGS. 6 a    is a schematic representation of an alternative wavenumber profile filtered by filters in the first and second optical paths and  FIG. 6 b    is a schematic representation of first and second portions of the spectrum, transmitted by these filters, dispersed across the CCD; and 
           [0063]      FIG. 7 a    is a schematic representation of a further wavenumber profile filtered by filters in the first and second optical paths and  FIG. 7 b    is a schematic representation of an alternative embodiment wherein first and second portions of the spectrum, transmitted by filters in the first and second optical paths, are dispersed across different photodetector elements the same row of the array of the photodetector elements. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0064]    Referring to  FIG. 1 , Raman spectroscopy apparatus according to an embodiment of the invention comprises a dichroic filter  12  arranged at 45 DEG to an incoming laser beam  10 . The filter  12  reflects light of the wavelength of the laser beam  10  but transmits all other wavelengths. The laser beam  10  is thus reflected through 90 DEG into an optical path  13 , and is focused by a microscope objective lens  16  onto a sample supported on stage  18 . Typically, the laser beam  10  will be focused onto the sample to form a specific light profile  19 , such as a spot or line, on the sample. Light of various wavelengths scattered from the sample (e.g. as a result of Raman scattering) is collected by an optical input, in this embodiment comprising the lens  16 , and passes back through the system. The dichroic filter  12  rejects reflected and Rayleigh scattered light at the same wavelength as the incoming laser beam, and transmits light which has been shifted to other wavelengths. The received Raman light transmitted by the filter  12  is optically processed by an analyser  20  to disperse a spectrum of the Raman light in a spectral direction, the spectrum focused by a lens  22  onto a detector, such as a charge-coupled device (CCD)  24 . The signals detected by the CCD  24  may then be acquired by a computer  25  for further processing. 
         [0065]      FIG. 2  shows the analyser  20  in more detail. The analyser  20  comprises an optical splitter  38  that divides the incoming beam  36  into two beams  48 A and  48 B based upon wavenumber and a diffraction grating  44 , which disperses the beams into a spectrum in the spectral direction, S. 
         [0066]    In this embodiment, the optical splitter  38  comprises an edge filter  38 A and a mirror  38 B arranged in the path of the incoming light  36  before the diffraction grating  44 . The edge filter  38 A is a short pass filter tilted with respect to the incoming beam  36  to split the incoming beam  36  into two beams having different wavenumber ranges that travel to the detector  24  along different optical paths  48 A and  48 B. The edge filter  38 A passes the low Raman shift wavenumbers, but reflects higher Raman shift wavenumbers as beam  48 A. The mirror  38 B reflects the higher Raman shift wavenumbers as beam  48 B. The optical paths  48 A,  48 B are arranged, for example, through appropriate positioning of the filter  38 A and mirror  38 B, such that a spectrum generated by a given point on the sample is dispersed across photodetector elements  104  of two rows of the detector  24 . 
         [0067]    As the spectrum is split across two rows of the detector  24 , the grating  44  is arranged to disperse the spectrum of interest more widely than can be accommodated within a single width of the CCD  24 . The diffraction grating  44  is arranged to disperse beams  48 A and  48 B in the spectral direction, S, such that the spectrum of each beam  48 A,  48 B occupies the full width of the detector  24  in the spectral direction, but with the spectra spaced in the spatial direction, D. As shown in  FIG. 4B , first beam  48 A comprises a first set of wavenumbers A to C and the second beam  48 B comprises a second set of wavenumbers D to E, the diffraction grating arranged to disperse the beams  48 A,  48 B such that the full spectral ranges, A to C and D to E, are captured by the detector  44 . 
         [0068]    To collect the spectrum of either beam  48 A,  48 B over the full spectral range A to C or D to E, using the method as described in WO 2008/090350, the other beam  48 B,  48 A has to be blocked. To this end, the analyser  20  comprises blocking shutters  51 A,  51 B movable in and out of the beam  48 A,  48 B under the control of motors (not shown) to block one of the beams  48 A,  48 B whilst spectra for the other beam  48 A,  48 B is collected. Collection of spectra across the entire spectral range A to E can be achieved by sequentially collecting each part A to C and D to E of the spectrum of incoming beam  36 . 
         [0069]    The analyser  20  further comprises blocking filters  49 A,  49 B movable in and out of beams  48 A,  48 B under the control of motors (not shown). The blocking filters  49 A,  49 B are, for example edge or notch filters, for blocking the passage of selected wavenumbers of beams  48 A and  48 B, respectively. More specifically, the blocking filters  49 A and  49 B are arranged to block wavenumbers of the beams  49 A and  49 B such that, first and second portions  50 A,  50 B of a spectrum generated by a given point on the sample that are transmitted through the blocking filters  49 A,  49 B, are dispersed across different sections of the detector  24  divided along a line in the spatial direction, D. In this way, each portion  50 A,  50 B can be simultaneously collected, wherein data for each portion  50 A,  50 B of the spectrum is accumulated in the detector  24  as charge is shifted between the photodetector elements in the spatial direction synchronously with relative movement between the light profile  19  and the sample  18 . Data is accumulated on each of the first and second portions  50 A,  50 B of the spectrum across different (mutually exclusive) sets of photodetector elements  104  of the detector  24  during the relative movement. 
         [0070]      FIG. 5B  illustrates how the beams  48 A,  48 B may be filtered by filters  49 A,  49 B having a combined filtering profile as shown in  FIG. 5A , with data on a first portion  50 A of the spectrum, extending across the spectral range D to E, being accumulated in a first half of the detector  24  to the left of line N-N and a data on a second portion  50 B of the spectrum, extending across the spectral range B to C, being accumulated in a second half of the detector  24  to the right of line N-N. 
         [0071]    Collection of data on the portions  50 A and  50 B will now be described in more detail with reference to  FIG. 3 .  FIG. 3  shows part of area  124  of sample  102  illuminated by the line focus  110 . Y shows the direction of movement of the sample  102  and arrow  127  the direction that charge is shifted on the CCD array  24 . For each region  132  (hereinafter referred to as a point) on the line focus  110 , a Raman spectrum (indicated by the shaded area) is dispersed across the CCD array  24  in a spectral direction S, perpendicular to spatial direction D. The spectrum generated by an illuminated point  132  along a line focus  110  is dispersed across the detector  24  with a first portion  50 A of the spectrum dispersed across photodetector elements  104  of a different row  118 A of array  24  to that  118 B across which the second portion  50 B is dispersed. 
         [0072]    It should be understood that the size of the points  132  and photodetector elements  104  have been exaggerated in  FIG. 3 . In reality there are many more times this number of points and many more times this number of rows  118  of photodetector elements on the CCD  24  for such a size of CCD  24 . 
         [0073]    The exposure of the CCD  24  to light results in the accumulation of charge in each photodetector element  104 . This charge represents a spectral value (or bin) for the Raman spectrum and is in proportion to the amount of light it has received during the exposure. 
         [0074]    The sample  102  moves continuously relative to the line focus  110  simultaneously with shifting of the charge between the rows  118  of the CCD  24  in direction  127 . Charge steadily accumulates for scattered light originating from a given region on the sample  102  in successive photodetector elements  104  of the array  24 . For portion  50 A, charge accumulates in the photodetector elements to the right of line L-L and, for portion  50 B, charge accumulates in the photodetector elements to the left of line L-L. The shifting of charge continues until the charge is shifted into readout register  134 . The charge in readout register  134  is read out to computer  25 . Thus, between shifts in the charge on the CCD  24 , the shift register  134  holds data for portion  50 A of a first spectrum for light scattered from a first point  132  and a portion  50 B of a second spectrum for light scattered from a second, different point  132 . 
         [0075]    In another embodiment, blocking filters  49 A,  49 B may be tuneable to allow alterations in the filtering profile. For example, as shown in  FIGS. 6A and 6B  the first portion and second portion are accumulated across different sets of photodetector elements of the detector  24  sectioned by a line M-M. 
         [0076]      FIGS. 7 a  and 7 b    shows a further embodiment, wherein the first and second portions of a spectrum are directed towards different photodetector elements  104  of the same row of the detector array  24 . In  FIG. 7 b   , the first and second portions occur across a spectral range A-F that is greater than the spectral range provided by a width of the detector  24  in the spectral direction, S. 
         [0077]      FIG. 7 b    includes arrows to show that the data on the first and second portions is accumulated across the detector synchronously with relative movement of a line focus across the sample. 
         [0078]    In an alternative embodiment, not shown, data may be gathered on first and second portions of a spectrum, dispersed across the detector as shown in  FIG. 7 b   , generated from a focal point or line focus that does not move relative to the sample. 
         [0079]    For example, this may be carried in a step and stitch method or when a mapping of a sample is not required. In such an embodiment, data on the spectrum generated from a given point would not be accumulated as charge is shifted across the detector. Data on first and second portions of a spectrum generated by a given point may be accumulated in a single row  118  of the detector  24 , wherein at the end of sampling the charge is shifted to the read-out register in order for the data to be read-out to the computer. Such an embodiment may allow high resolution partial spectra to be collected over the full height of the detector in the spatial direction. 
         [0080]    The apparatus may allow the user to select portions of the whole spectrum that are of interest and collect data on these portions using a Continuous Collection technique without reducing resolution. 
         [0081]    Alternations and modifications may be made to the described embodiment without departing from the invention as defined herein. For example, the incoming beam may be split into three or more beams based upon wavenumber and, each one of the three or more beams may be dispersed across photodetector elements in different rows of the detector. Filters may be provided to allow the user to select a portion of each of the three or more beams that is of interest for collection by accumulation of data across the detector synchronously with portions in the other beams. 
         [0082]    The filter(s) for isolating the portions of the spectrum of the incoming beam that are of interest may be provided before the optical splitter  38  or after the diffraction grating  44 . The optical splitter may also be provided after the diffraction grating  44 .