Spectrometer device with background compensation

Abstract

A spectrometer device comprises a light source (10), a spectrometer section (4), and a control unit (16, 38, 72). The spectrometer section (4) has a dispersive element (22a, 22b, 30, 32), such as a grating or prism, spatially redistributing incoming light as a function of its wavelength. It further has a camera (20) receiving the spatially redistributed light from the dispersive element (22a, 22b, 30, 32). The pixels (36) of the camera comprise a photosensitive element (42) and integration circuitry (44, 44-1, 44-2). The integration circuitry (44, 44-1, 44-2) can be controlled with an integration control input that identifies integration periods with first and second integration modes. The control input further identifies a first and a second integration mode. The integration circuitry (44, 44-1, 44-2) integrates, over the integration periods, the signal from the photosensitive element (42). In the first integration mode, it either integrates the signal in a first integrator (44-1), or it integrates the signal positively in a differential integrator (44). In the second integration mode, it either integrates the signal in a second integrator (44-2), or it integrates the signal negatively in the differential integrator (44).

Description

[0001] Spectrometer Device with Background Compensation

[0002] Technical Field

[0003] The invention relates to a spectrometer device and to a method for operating a spectrometer device.

[0004] Background Art

[0005] US8022349B2 describes a spectrometer device having a dispersive element, such as a grating, to spatially redistribute incoming light as a function of its wavelength and a CCD camera positioned to receive spatially redistributed light from the dispersive element. The camera has an array of pixels, with the pixels comprising a sensing element and an integrator, with the latter integrating the signal, namely the charges, from the sensing element.

[0006] R. Smith et al., "Raman spectroscopy: an evolving technique for live cell studies", Analyst, 2016, 141, 3590-3600 (DOI: 10.1039 / c6an00152a) describe a spectrometer device with a spectrometer section including a dispersive element and a CCD camera. The device further includes a light source used to illuminate a target. The spectrum of Raman-scattered light from the target is analyzed in the spectrometer section.

[0007] Disclosure of the Invention

[0008] The problem to be solved by the present invention is to provide a spectrometer device and method for operating such a devices that reduce adverse effects due to background illumination (e.g., ambient light and other light sources).

[0009] This problem is solved by the device and method of the independent claims.

[0010] Accordingly, the invention relates to a spectrometer device comprising at least the following elements:

[0011] - A light source: The light source may be adapted to send excitation light into a target in order to evoke a response. - A spectrometer section: The spectrometer section may be adapted to analyze the response from the target.

[0012] - A control unit.

[0013] The spectrometer section comprises at least the following elements:

[0014] - A dispersive element: This element may be adapted dispersively process the incoming light received by the spectrometer section in order to spatially redistribute it as a function of its wavelength.

[0015] - A camera: The camera is positioned to receive the spatially redistributed light from the dispersive element. It comprises several pixels. At least some of the pixels (in particular a plurality of the pixels) comprise at least the following elements: a) A photosensitive element: This may be any suitable photosensor. Various examples are provided below. b) Integration circuitry: The integration circuitry has an integration control input that is adapted to identify integration periods with first and second integration modes. The integration circuitry can be implemented in two variants i) or ii). Namely, it is adapted to integrate, over the integration periods, the signal from the photosensitive element: i) in a first integrator in the first integration mode and in a second integrator in the second integration mode or ii) in a differential integrator adapted to integrate a weighted difference between integrals of the signal from the photosensitive element in the first and second integration modes. As described below, the weighted difference may be an equally weighted or a non-equally weighed difference.

[0016] The control unit of the device is adapted to perform at least the following:

[0017] - Operate the light source to generate light pulses: The light pulses define "illumination periods".

[0018] - During measurement periods, with each measurement period at least overlapping with an illumination period, the control unit applies an integration control signal to the integration control input of the pixels that indicates an integration period in the first integration mode.

[0019] - During reference periods, with each reference period being between the illumination periods, the control unit applies an integration control signal to the integration control input of the pixels that indicates an integration period in the second integration mode. In other words, the integration circuitry is operated in its first integration mode during the measurement periods that overlap at least partially with the illumination periods. Hence, in the first integration mode, the integrated signal in the first integration mode is a signal in the presence of illumination from the light source. During the reference periods, which are timed between the illumination periods, the integration circuitry is operated to integrate in its second integration mode. Hence, the integrated signal in the second integration mode is a signal without illumination from the light source.

[0020] This design is based on the concept of operating this kind of spectrometer in pulsed mode and to record light during the illuminated and non -illuminated phases at a pixel level, which allows to compensate for background illumination.

[0021] In some embodiments, the control unit may be adapted to pulse the light source with a duty cycle of less than 10% or even less than 1%. In other words, the light source is in its on-state during less than 10% or even less than 1% of each measurement cycle. This is based on the understanding that, in this case, much higher light intensities may be used during the illumination periods without exceeding a given average light power level, i.e., without risk of damaging the target. This allows to reduce the signal-to-noise ratio (SNR), in particular due to shot noise. Also, it further reduces the influence of background illumination.

[0022] In some embodiments, in a simple design, the integration circuitry may comprise a first integrator adapted to integrate the signal from the photosensitive element in the first integration mode and a second integrator adapted to integrate the signal from the photosensitive element in the second integration mode. In other words, there are separate integrators for the two integration modes.

[0023] In this case, the spectrometer device further comprises at least one subtractor adapted to calculate a difference parameter between values of the first and the second integrator. This difference parameter may, e.g., be an equally -weighted difference, a weighted difference, or a parameter depending on such a difference between the values of the first and second integrator.

[0024] In a second aspect, a method for operating a spectrometer device is provided, such as for operating the spectrometer device outlined herein. This spectrometer device comprises at least the following elements:

[0025] - A light source: The light source may be adapted to send excitation light into a target in order to evoke a response.

[0026] The spectrometer section comprises at least the following elements: - A dispersive element: This element may be adapted dispersively process the incoming light received by the spectrometer section in order to spatially redistribute it as a function of its wavelength.

[0027] - A camera: The camera is positioned to receive the spatially redistributed light from the dispersive element. It comprises several pixels. At least some of the pixels (in particular a plurality of the pixels) comprise at least the following elements: a) A photosensitive element: This may be any suitable photosensor. Various examples are provided below. b) Integration circuitry: The integration circuitry has an integration control input that is adapted to identify integration periods with first and second integration modes. The integration circuitry can be implemented in two variants i) or ii). Namely, it is adapted to integrate, over the integration periods, the signal from the photosensitive element: i) in a first integrator in the first integration mode and in a second integrator in the second integration mode or ii) in a differential integrator adapted to integrate a weighted difference between integrals of the signal from the photosensitive element in the first and second integration modes. As described below, the weighted difference may be an equally weighted or a non-equally weighed difference.

[0028] The method comprises at least the following:

[0029] - operating the light source to generate light pulses defining illumination periods,

[0030] - during measurement periods, with each measurement period at least overlapping with an illumination period, applying an integration control signal to the integration control inputs indicating an integration period in the first integration mode,

[0031] - during reference periods, with each reference period being between the illumination periods, applying an integration control signal to the integration control inputs indicating an integration period in the second integration mode.

[0032] In variant i) of the integration circuitry, the method may further comprise

[0033] - calculating, by means of a subtractor of the spectrometer device, a difference parameter between values of the first and the second integrator.

[0034] The spectrometry method and device may, e.g., be used for measuring a Raman spectrum or a fluorescence spectrum of a target by sending the light pulses into a target and spectrally analyzing, by means of the spectrometer section, the spectrum of light returning from the target.

[0035] Brief Description of the Drawings

[0036] The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein:

[0037] Fig. 1 illustrates a possible design of a spectrometer device,

[0038] Fig. 2 illustrates another implementation of the spectrometer section,

[0039] Fig. 3 illustrates yet another implementation of the spectrometer section,

[0040] Fig. 4 shows a possible architecture of a camera,

[0041] Fig. 5 shows a block diagram of some embodiments of a pixel,

[0042] Fig. 6 shows an analog implementation of a pixel of Fig. 5,

[0043] Fig. 7 shows a digital implementation of a pixel of Fig. 5,

[0044] Fig. 8 shows a possible timing and the corresponding signals of consecutive measurement cycles,

[0045] Fig. 9 shows the measurement, readout, and idle cycles in embodiments using burst mode,

[0046] Fig. 10 compares different integration techniques,

[0047] Fig. 11 shows a block diagram of pixel embodiments based on on- pixel subtractors,

[0048] Fig. 12 shows an analog implementation of a pixel,

[0049] Fig. 13 shows a digital implementation of a pixel,

[0050] Fig. 14 shows another digital implementation of a pixel,

[0051] Fig. 15 shows a possible timing and corresponding signals of consecutive measurement cycles and the implementation of integrator swapping,

[0052] Fig. 16 compares measured data without and with integrator swapping,

[0053] Fig. 17 shows an implementation of a pixel with a differential integrator, and

[0054] Fig. 18 shows a possible timing and the corresponding signals of consecutive measurement cycles in a pixel with a differential integrator, Fig. 19 shows a type of implementation of a pixel with an analog differential integrator, and

[0055] Fig. 20 shows another type of implementation of a pixel with an analog differential integrator.

[0056] Modes for Carrying Out the Invention

[0057] Definitions

[0058] The term "measurement cycle" spans one measurement period (where the integration circuitry integrates a signal under illumination) and one reference period (where the integration circuitry integrates a signal without illumination).

[0059] The term "integration cycle" designates a period spanning several measurement cycles and the subsequent readout of the pixels. If the device is operated in burst mode, the integration cycle may comprise one or more burst phases, but in many cases it contains one bust phase only.

[0060] Example Design

[0061] Fig. 1 illustrates the design of some embodiments of a spectrometer device, such as it can, e.g., be used for Raman spectroscopy, fluorescence spectroscopy, or other types of spectroscopy.

[0062] The shown device may comprise functional sections including an illumination section 2 and a spectrometer section 4.

[0063] Illumination section 2 comprises a light source 10 generating light in an illumination wavelength range.

[0064] The light source 10 may be a semiconductor light source for ease of integration with modern electronics. In particular, it may be a semiconductor laser for generating high-intensity narrow-bandwidth light suitable for many spectrometrytype applications. For example, it may be a vertical -cavity surface-emitting laser (VCSEL) array, either a single-mode VCSEL or a multi-mode VCSEL.

[0065] In the shown type of embodiment, the light from light source 10 is collimated by one or two lenses 12a, 12b and fed to a dichroic mirror 14. Dichroic mirror 14 is transparent for light in the illumination wavelength range, so the light is transmitted. It passes objective optics of one or two lenses 12c, 12d and is focused towards a target 15. Light returning from target 15, such as elastically scattered light, Raman-scattered light, and / or fluorescent light, passes the objective optics 12d, 12c and arrives at dichroic mirror 14.

[0066] Dichroic mirror 14 is reflective for light in a spectrometer wavelength range, such as for at least some of the Raman-scattered and / or fluorescent light. Hence, that light is reflected and fed to spectrometer section 4. Typically, the spectrometer wavelength range does not overlap the illumination wavelength range.

[0067] Spectrometer section 4 is described in more detail in the next section.

[0068] The spectrometer device also comprises a control unit 16, which controls the operation of the device, such as the timing of the various operations therein. Control unit 16 may be implemented in hardware and / or software. Some of its aspects are described in more detail below.

[0069] It must be noted that both the illumination section 2 and the spectrometer section 4 of Fig. 1 are mere examples of possible embodiments.

[0070] In other embodiments, for example, dichroic mirror 14 may be reflective at the illumination wavelength range and non-reflective at the spectrometer wavelength range, and the positions of light source 10 and spectrometer section 4 may be swapped. Yet other types of beam splitter may be used. In yet other examples, target 15 may be illuminated from one direction, and the returning light may be recorded from a second, different direction, in which case no beam splitter is required.

[0071] Spectrometer Section

[0072] Spectrometer section 4 comprises a dispersive element adapted to spatially redistribute the incoming light from illumination section 2 as a function of its wavelength, i.e., the spectral components of the light from illumination section 2 are spatially distributed in different ways.

[0073] Further, spectrometer section 4 comprises a camera 20 positioned to receive the spatially redistributed light from the dispersive element, such that the light distribution on camera 20 is a function of the spectrum of the incoming light.

[0074] In the following, some examples for a dispersive element are described. The design of camera 20 is described in the further sections below.

[0075] In the type of embodiment of Fig. 1, the dispersive element is formed by diffractive gratings 22a, 22b forming the end-of-arms mirrors of a Michelson interferometer. The Michelson interferometer further comprises a beam splitter 24 splitting the incoming light 18 into the two arms and recombining the returning, diffracted light to send it to camera 20. Prisms 26a, 26b in the arms of the interferometer may improve the overlap of the spectral components on camera 20. The interferometer and the camera 20 together form a Spatial Heterodyne Spectrometry (SHS) device, as, e.g., described in the article of R. Smith mentioned in the Background Art section above. As shown there (but not in Fig. 1), camera optics may be arranged between beam splitter 24 and camera 20, again for generating a better overlap of the spectral components on camera 20.

[0076] In SHS spectrometers, the different spectral components of the incoming light form stripe interference patterns on camera 20, with the spatial frequency of the interference patterns being a function of the wavelength of the spectral components.

[0077] Spectrometer section further comprises a collimator 28 for improving the collimation of the incoming light and generating a light field with a defined aperture.

[0078] Fig. 2 illustrates an alternative design of a spectrometer embodied by a standard prism spectrometer. Here, the dispersive element is formed by a prism 30 having an index of refraction that varies strongly over the spectrometer wavelength range. The incoming, collimated light 18 is refracted at prism 30. Thereby, the incoming light is redistributed as a function of its wavelength. The refracted light is sent to camera 20.

[0079] Fig. 3 illustrates yet another alternative design of a spectrometer based on a diffractive grating 32, e.g., operating in reflection, as its dispersive element. Again, the incoming light 18 is redistributed as a function of its wavelength. The refracted light is sent to camera 20.

[0080] Hence, in more general terms, in a simple embodiment, the dispersive element may comprise a refractive prism 30 or a diffractive grating 22a, 22b or 32

[0081] The resolution of camera 20 of spectrometer section 4 and the dispersion of dispersive element 22a, 22b, 30, 32 may be selected such that the camera can distinguish between a large number of different spectral components (spectral strength at different wavelengths) of the incoming light 18 over the spectrometer wavelength range, e.g., between at least 100, in particular at least 1000 different spectral components, in order to obtain detailed spectrometric information about the light to be analyzed.

[0082] The present method and use of the device may comprise deriving the spectral composition of the incoming light from the spatial distribution of the signals received by the pixels of camera 20. Said derived spectral composition may include at least 100, in particular at least 1000, different spectral components (i.e., the spectral composition indicates the spectral strength of the oncoming light for at least 100, in particular at least 1000, wavelength components).

[0083] In the embodiments of Figs. 2 and 3, the row numbers of the pixels of camera 20 may, e.g., be a function of the wavelength over the spectrometer wavelength range. In the embodiment of Fig. 1, as mentioned, the amplitudes of the spatial frequencies recorded by the array of pixels of camera 20 are a function of the light spectrum over the spectrometer wavelength range.

[0084] In the spectrometer device, the spatial distribution of the light on camera 20 depends, as mentioned, on the spectral composition of the incoming light. To ease analysis, however, the spatial distribution of the light on camera 20 may be independent of the location where it was scattered by target 15 in the sense that target 15 and camera 20 are not in conjugate optical planes, i.e., there is no optical projection between target 15 and camera 20.

[0085] Camera

[0086] Fig. 4 illustrates some embodiments of camera 20.

[0087] Camera 20 comprises an array 34 of pixels 36. Array 34 may be a two-dimensional array as shown, or it may be a one-dimensional array. For ease of manufacture, the array 34 may be integrated on a semiconductor chip.

[0088] Camera 20 further comprises a control circuitry 38, which may be co-integrated on the same semiconductor chip as array 34 or located, at least in part, outside such a chip.

[0089] In Fig. 4, control unit 38 is shown to be located outside array 34. However, at lest part of control circuitry 38 may also comprise components at pixellevel.

[0090] Control circuitry 38 forms a functional part of control unit 16 of the spectrometry device.

[0091] Camera 20 may further comprise interface circuitry 40a, 40b for controlling the pixels in array 34. For example, such interface circuitry may include row and column interface circuitry 40a and 40b, respectively, as shown.

[0092] Pixel Architecture

[0093] Fig. 5 illustrates the architecture of possible embodiments of a pixel 36.

[0094] The pixel 36 as shown comprises a photosensitive element 42, such as a pinned photodiode (PPD) or a single-photon avalanche diode (SPAD) as described in more detail below. However, other types of photosensitive elements 42 may be used as well, such as regular photodiodes operated in photovoltaic or photo- conductive mode.

[0095] Pixel 36 further comprises integration circuitry 44 having, in the embodiments of the type of Fig. 5, N > 1 integrators 44-1, 44-2..., including at least a first integrator 44-1 and a second integrator 44-2. Each integrator 44-i is adapted to integrate the signal from photosensitive element 42 over integration periods, with the integration periods being defined by a gate input 46-1, 46-2... as described in the following.

[0096] The integration circuitry 44 has an integration control input. In Fig. 5, the integration control input comprises first and second gate inputs 46-1, 46-2 to control gate circuitry 48-1, 48-2 (such as an analog or digital gate, see below).

[0097] Further, the integration circuitry 44 has a reset input adapted to reset the integration circuitry. In Fig. 1, each integrator 44-i comprises a reset input 50-1, 50-2... controlling reset circuitry 52-1, 52-2... that is adapted to reset the integrator.

[0098] Finally, the integration circuitry 44 comprises integration hardware 54-1, 54-2, such as a charge storage or a digital counter as described below, where integration takes place.

[0099] Pixel 36 also comprises read-out circuitry 56 for reading out the results of the integration circuitry 44, either directly or in pre-processed form. Read-out circuitry 56 is controlled by a read-out control input 58 that defines when and what data is to be read out.

[0100] In a two-dimensional array of pixels, read-out circuitry 56 is, e.g., sending its signal to a read-out bus line 60 extending along each column of the array. The gate inputs 46-1, 46-2, the reset inputs 50-1, 50-2, and the read-out control inputs 58 may, e.g., be interconnected between the pixels 36 along a row of the array.

[0101] The following sections provide more detailed examples of some possible implementations of pixel 36.

[0102] Analog Implementation

[0103] Fig. 6 illustrates first type of implementations of pixel 36.

[0104] Here, photosensitive element 42 may be a pinned photodiode operating in photovoltaic mode.

[0105] Light impinging on PPD 42 generates charges, such as electrons, that accumulate at the cathode. These charges can be selectively transferred to one of the integrators 44-1, 44-2 by switching on one of the gate circuits 48-1, 48-2 by means of respective gate-inputs 46-1, 46-2.

[0106] When a gate circuit 48-1 or 48-2 is enabled, the charges from PPD 42 are transferred to the integration circuitry 54-1 or 54-2, which may be implemented as, e.g., a pinned diode or a capacitor.

[0107] The read-out circuitry 56 is, in this embodiment, formed by a first read-out section 56-1 and a second read-out-section 56-2, and the read-out control signals are fed read-out control inputs 58-1, 58-2, which can be used to selectively connect one of the integrators 44-1, 44-2 (e.g., one of the capacitors 51-1, 54-2) to the bus line 60.

[0108] The reset circuitry is formed by a transistors 52-1, 52-2. When switched on by the reset signal applied reset input 50, i.e., the gates of the transistors, it can be used to remove the charges from the integration circuitry 54-1, 54-2.

[0109] Pixel 36 further comprises a detector restart circuit 64 implemented as a transistor and selectively enabled by a restart signal 65, which can be used to discharge the PPD, e.g., prior to a measurement period or reference period as described below.

[0110] Instead of using a pinned photodiode, any other photodiode type adapted to generate charges proportional to the received light may be used.

[0111] Another design for an analog implementation with analog in-pixel integration is, e.g., described by L. Stark et al., "Back-illuminated voltage-domain global shutter CMOS image sensor with 3.75pm pixels and dual in-pixel storage nodes," 2016 IEEE Symposium on VLSI Technology, Honolulu, HI, USA, 2016, pp. 1-2, doi: 10.1109 / VLSIT.2016.7573451, see Fig. 1 therein. That one is based on using capacitors for accumulating charges from the photosensitive element.

[0112] Another embodiment using capacitors for accumulating charges from the photosensitive element is described below in reference to Fig. 12.

[0113] Hence, in an analog pixel design, the integrators 44-1, 44-2 may be charge integrators, such as the pinned diodes or capacitors mentioned above adapted to accumulate charges from the photosensitive element 42.

[0114] Hence, in general terms, the integrator(s) 44-1, 44-2 may be analog charge integrator(s).

[0115] Digital Implementation

[0116] Fig. 7 illustrates another class of embodiments of pixel 36. Here, photosensitive element 42 may be a single-photon avalanche photodiode SPAD or another element suited to generate pulses with a number proportional to the amount of detected light.

[0117] The circuit of Fig. 7 is, e.g., as described, for a different application, by C. Niclass et al., "Single-Photon Synchronous Detection ", IEEE Journal of Solid- State Circuits • August 2009 (DOI: 10.1109 / JSSC.2009.2021920).

[0118] It comprises digitization circuitry 66 generating a digital pulse, e.g., for each detected photon.

[0119] These pulses can be selectively transferred to one of the integrators 44-1, 44-2, by switching on one of the gate circuits 48-1, 48-2, which are, in this embodiment, AND-gates switched-on by the respective gate-inputs 46-1, 46-2.

[0120] When a gate circuit 48-1, 48-2 is enabled, the pulses are transferred to the respective integrator 54-1, 54-2, which may be implemented, e.g., as digital counters.

[0121] The read-out circuitry 56 is, in this embodiment, formed by a demultiplexer and bus driver circuit, with a read-out control input 58 that selects one of the integrators 44-1, 44-2 and indicates that its value is to be fed to bus line 60, e.g., in serialized form.

[0122] The reset circuitry is, in these embodiments, part of the digital counters 54-1, 54-2, and the reset signals 50-1, 50-2 may be used to reset the counters.

[0123] Pixel 36 further comprises a detector restart circuit 64 implemented as a transistor and selectively enabled by a restart signal 65, which can be used to quench the SPAD, e.g., prior to or after a measurement period or reference period as described below.

[0124] Hence, in more general terms, the photosensitive element 42 may be SPAD photodiode and the integrators 44-1, 44-2 may be digital counters.

[0125] Operation

[0126] In the following, some embodiments of the operation of the spectrometry device are described. The operation is controlled by control unit 16, which generates signals for driving light source 10 as well as the various control signals for the pixels 36 and read-out circuitry of camera 20.

[0127] As shown in Fig. 8, control unit 38 operates the spectrometry device in repetitive measurement cycles C.

[0128] Graph (8.1) shows a light control signal L generated by control unit 38 that controls light source 10. In each measurement cycle C, one light pulse may be generated, e.g., between times tO and tl. Each light pulse defines an illumination period Pi.

[0129] Graphs (8.2) and (8.3) show first and second gate signals Gl, G2 applied to the gate inputs 46-1, 46-2, which control the integration performed by the first and second integrators 44-1, 44-2.

[0130] In general terms, the first and second gate signals define integration periods of the integration circuitry 44 in a first and second integration mode. In the embodiments shown so far, in the first integration mode, the integration circuitry 44 integrates the signal from the photosensitive element 42 in the first integrator 44-1 and, in the second integration mode, the integration circuitry 44 integrates the signal from the photosensitive element 42 in the second integrator 44-2.

[0131] As can be seen, the first gate signal Gl operates first integrator 44-1 to integrate the signal from photosensitive element 42 over measurement periods Pm.

[0132] The measurement periods Pm are timed to at least overlap with the illumination periods Pi. To have a strong signal -to-background ratio in response to the illumination, this overlap may be large in the sense that at least 80%, in particular 100%, of each measurement period Pm overlaps with an illumination period Pi.

[0133] For example:

[0134] - The measurement periods Pm may be timed to extend between times tO and tl, i.e., they may coincide with the illumination periods Pi.

[0135] - The measurement periods Pm may be timed to be shorter than the illumination periods Pi and lie fully within the times tO and tl. In this case, again, each measurement period Pm fully overlaps with an illumination period.

[0136] - The measurement periods Pm may be timed to lie partially outside an illumination period Pi, e.g., to start before tO and / or to extend after tl. However, also in this case, the overlap may be chosen to be large as defined above.

[0137] As can further be seen, the second gate signal G2 operates second integrator 44-2 to integrate the signal from photosensitive element 42 over reference periods Pr. The reference periods Pr lie between the illumination periods Pm, i.e., the reference periods and the illumination periods do not overlap, and second integrator 44-2 integrates during times where light source 10 is off.

[0138] In the shown embodiment, the reference periods Pr are between times t2 and t3, with t2 > tl .

[0139] The duration t3 - 12 of the reference periods Pr may be equal to the duration tl - tO of the measurement periods Pm to simplify the background correction as described below. Graphs (8.4) and (8.5) show the values II, 12 as integrated by the first and second integrators 44-1, 44-2, respectively. As can be seen, signal II increases during the measurement periods Pm and signal 12 increases during the reference periods Pr.

[0140] After a fixed or variable number N > 0 of measurement cycles, a difference parameter D between the values II, 12 is calculated by means of at least one subtractor. This subtractor may, e.g., be part of control circuitry 38, as shown with reference number 68 in Fig. 4, or it may, e.g., be arranged in another part of control unit 16. There may, e.g., also be a plurality of subtractors, such as one subtractor for each pixel 36 as, e.g., described in more detail in embodiments below, or there may, e.g., be one subtractor per column of the pixel array 34.

[0141] As mentioned above, the difference parameter D may, e.g., be equal to or proportional to the difference II - 10, i.e., it may be an equally-weighted difference where II and 10 have equal weights, e.g., weights being equal to 1.

[0142] Difference parameter D may, however, also be equal to a weighted difference ki ll - k2 I2. A weighted difference may, e.g., be used if the total lengths Tl, T2 of the integration periods in the first and second mode, such as (in the embodiments of Figs. 5 - 7), the integration of the first and second integrators 44-1, 44-2, are different, in which case the ratio kl :k2 may, e.g., be equal to the ratio T2:T1.

[0143] The difference parameter D may also be a function, in particular a monotonous function, of the difference or the weighted difference, such as, e.g., its logarithm.

[0144] The value(s) of the integration circuitry, e.g., the values II, 12 of the integrators, may be read out after a fixed number N of measurement cycles, such as N > 100 or even N > 1000, maybe even N > 10000. Increasing the number of N provides more accurate results, but it slows down the measurement. Also, care must be taken to prevent the integrators 44-1, 44-2 from overflowing as described in more detail below.

[0145] Alternatively, as mentioned in a section below, the number N of integration steps may be variable.

[0146] In general, however, the control unit may be adapted to cause the integration circuitry 44, over an integration cycle, to integrate over N measurement periods in the first integration mode and over M reference periods in the second integration mode, with N and M being integers larger than 1.

[0147] At the end of the integration cycle, the control unit reads out the integration circuitry 44. In other words, in each integration cycle, the integration circuitry 44 integrates over N measurement periods Pm and over M reference periods Pr before the control unit reads out the integration circuitry at the end of the integration cycle.

[0148] If N and M are different, the difference parameter may be a weighted difference as explained above. In a simple embodiment, though, M = N, in which case the difference may be an equally-weighted difference.

[0149] For the reasons above N and M may be larger than 100, in particular larger than 1000, maybe even larger than 10000.

[0150] At the end of an integration cycle, after reading out the integration circuitry 44, the integration circuitry 44 may be reset by means of signals fed to the reset inputs 50-1, 50-2. Hence, in more general terms, the integration circuitry 44, such as the integrators 44-1, 44-2, may comprise a reset circuit 52-1, 52-2 with a reset input 50-1, 50-2 adapted to reset the integration circuitry 44 upon receipt of a reset signal. The control unit may be adapted to issue the reset signal to the integration circuitry 44 after reading out the integration circuitry.

[0151] In other words, the integration circuitry 44 integrates over the integration cycle in N integration periods of the first mode and M integration periods of the second mode before it is read out and then reset by the control unit.

[0152] In an integration cycle, the integration periods may extend over a time span Ts of more than 1 second, or even more than 10 seconds. The time span Ts is the time passing between the first and the last integration period of a single integration cycle. The suppression of ambient light allows such large extensions of the integration time.

[0153] Burst Operation

[0154] In some embodiments, the camera 20, and advantageously also the light source 10, may be operated in operating cycles comprising burst phases and idle phases as illustrated in Fig. 9. Such a burst operation reduces the power consumption of the device.

[0155] Graph (9.1) shows the light control signal L as a function of time. The operating cycles, in this case, comprises a burst phase Bl, a read-out phase B2, and an idle phase B3.

[0156] In each burst phase Bl, the control unit operates light source 10 to generate the light pulses at a first repetition rate fl . Further, it operates the integration circuitry 44 to integrate the signal from photosensitive element 42 in the first integration mode during the measurement periods of the burst phase B 1. It further operates integrator circuitry 44 to integrate the signal from photosensitive element 42 in the second integration mode during the reference periods of the burst phase Bl.

[0157] In each read-out phase B2, the control unit operates the pixels 36 to perform a read-out operation. To do so, it sends read-out signals 01 and 02 to the read-out control inputs 58. In the shown embodiment, 01 indicates a read-out operation from first integrator 44-1, and 02 indicates a read-out operation from second integrator 44-2. For a differential integrator, as described below, the differential integration value may be read out in a single read-out operation.

[0158] The read-out may also occur, at least in part, during the burst phases Bl, in which case there may not be a separate read-out phase B2. However, having a separate read-out phase B2 makes it easier to maintain a high repetition rate fl as described in more detail below.

[0159] In each idle phase B3, camera 20 is idled, i.e., the integrators 44-1, 44-2 are inactive and no read-outs occur. This allows to reduce power consumption. The idle phases may B3, e.g., last at least 10 microseconds, in particular at least 0.1 seconds, in particular at least 0.5 seconds.

[0160] Hence, in some embodiments, the control unit 16 may be adapted to operate the camera in operating cycles comprising burst phases Bl and idle phases B3 wherein, with the idle phases B3 being located between burst phases Bl. In this case:

[0161] - In each burst phase Bl, the control unit 16 operates the light source 10 to generate the light pulses at a first repetition rate fl, and it operates the integration circuitry to integrate during the measurement periods Pm in the first integration mode and during the reference periods Pr in the second integration mode.

[0162] - In each idle phase B3, the control unit 16 idles the camera, i.e., it does not cause the integration circuitry 44 to integrate the signal from the photosensitive element 42 nor the pixels to be read out.

[0163] As mentioned, the length of the idle phases B3 may be at least 10 microseconds, in particular at least 0.1 s, in particular for at least 0.5 s. They may, e.g., extend over several seconds.

[0164] On the other hand, the total length of a burst cycle, i.e., the time span between two burst phases Bl, may be smaller than 4 s for quasi-continuous measurements.

[0165] The length of each burst phase Bl may be smaller than 0.3 seconds.

[0166] For a substantial reduction of the power consumption, the burst phases Bl may be shorter than the idle phases B3.

[0167] The number of measurement periods Pm per burst phase B 1 may be at least 1000. As mentioned below, the repetition rate fl of the light pulses may be between 10 kHz and 1000 MHz. Assuming it is, e.g., 1 MHz and the number of pulses per burst is 1000, the length of the burst phase may be around 1 ms. A typical range for the length of the burst phase Bl is between 0.1 ms and 500 ms.

[0168] Repetition Rate and Duty Cycle

[0169] Two further parameters of interest are the repetition rate of the light pulses and their duty cycle.

[0170] This is best understood when first considering the insight that, in a linear process (such as Raman scattering or fluorescence), and even more so in a higher-order process (such as in multi-photo interactions), a large light intensity from light source 10 on target 15 provides a better ratio between signal light (e.g., Raman- scattered or fluorescent light) and ambient light, thereby improving signal quality if the measurement period Pm overlaps strongly with the light pulse Pi in the meaning described above.

[0171] On the other hand, a large light intensity can damage target 15, in particular if target 15 is a biological target, such as the surface of a human or animal body. Also, a large light intensity can heat up target 15, which can change its spectral response in undesired manner.

[0172] To overcome this, the light should be pulsed, and the signal integration should be limited to substantially the light pulses only.

[0173] This is illustrated in Fig. 10, showing measurement results where a target of cyclohexane has been illuminated with a pulsed-narrow-band laser (wavelength 775 nm, pulse width 35 ps and a repetition rate 1 MHz) for simulating light source 10 and a broadband, continuous LED for simulating ambient light.

[0174] Plot (10.1) shows the signals recorded with a classic CCD (cooled to -60°C) continuously integrating over a large number of light pulses and the periods between the light pulses, with I being the (integrated) signal strength recorded by the pixels and R the row number (i.e., the wavelength) of the pixels.

[0175] Plot (10.2) shows the signals recorded with a camera using gated pixels of the type shown in Fig. 5 (not cooled). Here, I is the signal strength from the first integrator 44-1 (i.e. even before subtracting the signal from the second integrator 44-2, i.e. without active background correction), and R is again the row number. The integrator 44-1 was gated to integrate over 3 ns, i.e., with the measurement period Pm fully overlapping (i.e. lying within) with the illumination period Pi.

[0176] In both plots, the device has been set up such that, over the row numbers R, two distinct Raman peaks are visible. Each plot contains a first curve X that shows the signal when only the LED (ambient light) is on, a second curve Y that shows the signal when only the laser is on, and a third curve XY when both the LED and the laser are on.

[0177] As can be seen by comparing the signal curves Y (without ambient light) and XY (with ambient light), the signal XY of the first plot (10.1) with ungated integration strongly depends on the amount of ambient light while the signal XY of the second plot (10.2) depends only weakly on ambient light. (The remaining ambient light dependence can even further be reduced by subtracting the signal from second integrator 44-2 from the one of first integrator 44-1.)

[0178] Hence, using pulsed light with gated integration periods over the light pulses only provides an improved suppression of ambient light.

[0179] Further, the length of the light pulses, i.e., of the illumination periods Pi, is a relevant parameter. Short light pulses reduce the temperature rise within the target during the pulse. It has been found that this effect becomes significant for pulses shorter than 103s, and it becomes even more significant for pulses shorter than 105s, in particular shorter than 106s. On the other hand, extremely short pulses, e.g., around 1010s and less, will make the electronics for driving the light source as well as the pixel electronics and its control more challenging.

[0180] In more general terms, the length Pi of the illumination periods is best between 0.1 ns and 1 ms, in particular between 1 ns and 10 ps, even better between 1 ns and 1 ps.

[0181] Apart from the pulse length, the duty cycle D is another relevant parameter. A low duty cycle D reduces the heat-buildup in target 15 over several pulses. For example, a duty cycle of 0. 1 reduces the average thermal load at target 15 by a factor of 5 as compared to a 1 : 1 duty cycle, and a duty cycle of 0.01 reduces it by a factor 50.

[0182] In this context, the "duty cycle" D is defined as

[0183] D = Pi / C = Pi f, (1) wherein Pi is the length of the light pulses (i.e., of the illumination periods), C the length of the measurement cycle C (cf. Fig. 8), and f is the repetition rate (repetition frequency) of the light pulses.

[0184] As mentioned above, the length Pm is best between 0.1 ns and 1 ms, in particular between 1 ns and 10 ps, even better between 1 ns and 1 ps. Using Eq. (1), this leads to the following ranges for the light pulse repetition rate f: - For D = 0. 1, f should be between 100 Hz and 1 GHz, in particular between 10 kHz and 100 MHz, even better between 100 kHz and 100 MHz.

[0185] - For D = 0.01, f should be between 10 Hz and 100 MHz, in particular between 1 kHz and 10 MHz, even better between 10 kHz and 10 MHz.

[0186] On the other hand, too low repetition rates f (below 1 kHz) slow down the measurement, in particular if integration should extend over 100 pulses or more. Hence, the repetition rate f is best at least 1 kHz.

[0187] Hence, in view of the above and in order to combine fast measurement, low duty cycle, and short pule lengths, the repetition rate may be in a range between 1 kHz and 1 GHz, in articular between 10 kHz and 100 MHz.

[0188] When a burst mode is used as illustrated shown in Fig. 9, the repetition rate f and the duty cycle D are defined as the repetition rate fl of the light pulses and duty cycle during the burst phase Bl.

[0189] In-Pixel Subtraction

[0190] In some embodiments, the spectrometer device may comprise inpixel subtractors.

[0191] Fig. 11 illustrates the architecture of some possible embodiments of a pixel 36 with a subtractor 70. As described in the following, such a subtractor 70 may be used in various functions for controlling the operation of pixel 36, and it is therefore shown as an element of an in-pixel section 72 of control circuitry 38.

[0192] Subtractor 70 may be adapted to perform one or both of the following functions:

[0193] Fl) It may calculate a difference of the values II and 12 of the integrators 44-1, 44-2, i.e., it may calculate II - 12 or a weighted difference of II and 12.

[0194] F2) It may subtract an equal quantity Q from the values of both integrators 44-1, 44-2, i.e., it may reduce the value 11 of first integrator 44-1 by Q and the value 12 of second integrator 44-2 by Q.

[0195] Fig. 12 shows aspects of one of several possible analog implementations of a pixel 36 with a subtractor 70 adapted to perform function F2.

[0196] Fig. 12 is designed similar to the embodiment of Fig. 6, but here, capacitors 44-1, 44-2 are used as charge integrators to store the charges from photodiode 42, and gate circuitry 48-1, 48-2 is represented as general switches.

[0197] Pixel 36 further comprises a comparator 74 that compares the voltage in second integrator 44-2 to a threshold voltage VT. Comparator 74 may be designed as a Schmitt-trigger type of comparator with a defined hysteresis. Its output goes to 1 when the value 12 of second integrator 44-2 raises past VT + xl, and it goes back to zero when the value 12 of second integrator 44-2 falls below VT - x2. xl and x2 are non-negative values, e.g., with xl = 0 and x2 = VT.

[0198] The output of comparator 74 activates the subtractor 70 that, in the shown embodiment, comprises two constant-current drain circuits 70-1, 70-2 that can be operated to drain charges from the integrators 44-1, 44-2.

[0199] Each constant-current drain circuit 70-1, 70-2 comprises a first transistor 76 having a constant bias voltage at its gain and operating as a constant current source. First transistor 76 is connected to its respective integrator 44-1, 44-2 via a second transistor 78 that is switched on and off by the output signal of comparator 74.

[0200] Once the value 12 in second integrator 44-2 triggers comparator 74, the second transistors 78 are switched on, and the constant-current drain circuits 70-1, 70-2 start draining charges from the integrators 44-1, 44-2 with constant, equal drain currents Id. This causes the values II, 12 to drop at equal rates and by equal amounts.

[0201] Once value 12 is low enough, comparator 74 deactivates its output and the constant-current drain circuits 70-1, 70-2 are disabled.

[0202] As a result of this operation, a quantity Q = xl + x2 is subtracted from the values II and 12 of the first and second integrators.

[0203] As a result of this, each integrator44-l, 44-2 is reset at least partially. If xl = 0 and x2 = VT, second integrator 44-2 is reset fully. However, xl+x2 may also be much smaller or even substantially zero.

[0204] With a subtractor 70 performing above function F2, the value II of first integrator 44-1 will increase more slowly, and an overflow of the integrators is avoided over longer integration cycles even if the ambient light is strong,

[0205] This makes the pixel more robust during operation. It can integrate over a desired total integration rate even in bright environments without its integrators overflowing.

[0206] Fig. 13 illustrates a digital implementation of this scheme, corresponding to the circuit of Fig. 7 but comprising subtractor 70 as part of the in-pixel control circuitry 72.

[0207] To perform above function F2, control circuitry 72 may be adapted to compare the value 12 of the counter 54-2 of second integrator 44-2. If the value 12 of second integrator 44-2 exceeds a certain threshold T, subtractor 70 is operated to subtract a quantity Q from both the values II, 12 of the counters 54-1, 54-2 and to reduce the values II, 12 by Q, i.e. replacing them with II - Q and 12 - Q. For example, Q may be equal to T, in which case second counter 54-2 is reset and first counter 54-1 is set to II - 12. In the embodiments of Figs. 12 and 13, each pixel 36 comprises a threshold detector, namely the comparator 74, which detects if the second integrator 44-2 holds a value 12 exceeding a given threshold. Alternatively or in addition thereto, comparator 74 may monitor the value II of first integrator and, if the value II exceeds a given threshold, it may also trigger a quantity Q to be subtracted from both integrators 44-1, 44-2, i.e. subtractor 70 is operated to subtract a quantity Q from both the values II, 12 of the integrators 44-1, 44-2 and reduce the values II, 12 by Q, i.e. replacing them with II - Q and 12 - Q.

[0208] Hence, in more general terms, each pixel 36 may comprise a threshold detector 74 adapted to generate an overflow condition signal if one of the integrators holds a value II, 12 exceeding a given threshold, wherein the control unit is adapted to reduce the values II, 12 of both integrators 44-1, 44-2 by a quantity Q in the presence of said overflow condition signal.

[0209] Fig. 14 shows yet another digital implementation, similar to Fig. 12, but here, subtractor 70 is adapted to perform above function Fl, i.e., it calculates the difference D = Il - 12 between the values of the first and second integrators. This calculation takes place, e.g., in each of the following situations:

[0210] - when the value in second integrator 44-2 exceed a threshold T, which may, e.g., be detected by means of a comparator 74, and

[0211] - when a read-out signal is received at read-out control input 58.

[0212] Once the difference is calculated, both integrators 44-1, 44-2 are reset, and the difference D is added to an accumulator 80.

[0213] In this embodiment, accumulator 80 holds the accumulated difference between the integrators 44-1, 44-2 over the integration cycle.

[0214] When a read-out signal is received, the value of accumulator 80 is fed to read-out bus line 60, and then accumulator 80 is reset.

[0215] Hence, in more general terms, each pixel 36 may comprise an accumulator 80 adapted to accumulate differences II - 12 calculated by the subtractor 70, wherein the control unit is adapted to read out the accumulator 80 from each pixel.

[0216] Accumulator 80 may have a larger capacity than any of the integrators 44-1, 44-2 for allowing longer integration cycles.

[0217] Integrator Swapping

[0218] Fig. 15 illustrates a technique to further improve the signal quality of the spectrometry device. It is based on the understanding that the integration circuitry may be slightly asymmetric in its operating during the first and second integration periods. For example, the two integrators 44-1, 44-2 as well as the circuitry processing the signal in the first and second integration mode may be slightly asymmetric. This may be particularly true for analog implementations.

[0219] The technique illustrated in Fig. 15 is based on swapping roles between the first and second integrator 44-1, 44-2. It shows same signals as Fig. 8 but over a time range extending over several integration cycles IC1, IC2 with the integrators being swapped.

[0220] In a first integration cycle IC1, during a measurement phase M spanning several measurement cycles, the first integrator 44-1 is used to integrate the signal in the measurement periods Pm and the second integrator 44-2 is used to integrate the signal in the reference periods Pr. Then, at the end of the first integration cycle IC1, during a read-out phase R, the values of the integrators 44-1, 44-2 are read out and the integrators are reset. The value 12 from the second integrator 44-2 is subtracted from the value II of the first integrator to calculate a difference DI, with the subtraction optionally weighted as described above.

[0221] However, in the second integration cycle IC2, during a measurement phase M spanning several measurement cycles, the second integrator 44-2 is used to integrate the signal in the measurement periods Pm and the first integrator 44-

[0222] I is used to integrate the signal in the reference periods Pr. Then, at the end of the second integration cycle IC2, during a read-out phase (not shown), the values of the integrators 44-1, 44-2 are read out and the integrators are reset. In this case, the value

[0223] II from the first integrator 44-1 is subtracted from the value 12 of the second integrator to calculate a difference D2, with the subtraction optionally weighted as described above.

[0224] Ideally, the differences DI and D2 are identical. However, in the presence of asymmetries in the integration circuitry, they are not. Hence, by adding DI and D2 (and, optionally, scaling the result), these asymmetries can be removed.

[0225] The effect of this technique is illustrated in Fig. 6, where the two plots show the value of DI without integrator swapping (plot A) and the value of (DI + D2) / 2 with integrator swapping (plot B) as a function of pixel row number. As can be seen, the signal in plot B is stronger with less noise.

[0226] Hence, in more general terms, the control unit may be adapted to

[0227] - integrate, over a first integration cycle IC1 including several illumination periods Pi, the signal from the photosensitive element 42 over several measurement periods Pm by means of the first integrator 44-1 while integrating the signal from the photosensitive element 42 over several reference periods Pr by means of the second integrator and to operate the subtractor to calculate a first difference DI between values from the first and the second integrator 44-1, 44-2, - integrate, over a second integration cycle IC2 including several illumination periods Pi, the signal from the photosensitive element 42 over several measurement periods Pm by means of the second integrator 44-2 while integrating the signal from the photosensitive element 42 over several reference periods Pr by means of the first integrator 44-1 and to operate the subtractor to calculate a second difference D2 between values from the second and the first integrator 44-2, 44-1, and

[0228] - calculate a parameter indicative of a sum of the first and the second differences DI, D2.

[0229] Differential Integration

[0230] Fig. 17 shows yet another type of embodiments.

[0231] Here, integrator circuitry 44 comprises a differential integrator 86 adapted to integrate a (possibly weighted) difference between the integrals of the signal from the photosensitive element 42 in the first and second integration modes.

[0232] The figure shows a digital implementation where the photosensitive element 42 is, e.g., a SPAD with digitization circuitry 66.

[0233] The pixel 36 further comprises a gate circuit 48 with a gate input 46, which can be used to selectively forward the pulses from digitization circuit 66 to differential integrator 86, which may be a digital counter 54.

[0234] Differential integrator 86 further has an up / down input 88, which controls if the integrator input signal of the integrator 86 is to be integrated with a positive or a negative sign.

[0235] In a digital implementation, up / down input 88 may control if the counter 54 counts upwards or downwards.

[0236] Fig. 18 illustrates the operation of a device with differential integrator. It shows the signals over several measurement cycles C.

[0237] G1 and G2, together, form the integration control signal that indicates (1) when integration is to take place and if the integration is to occur with a positive sign (i.e., in the "first" integration mode) or with a negative sign (i.e., in the second integration mode).

[0238] In the shown embodiment, G1 is the signal at gate input 48, i.e., integration takes place whenever G1 is on. In other words, G1 controls the timing of the integration periods.

[0239] G2 is the signal fed to up / down input 88, i.e., G2 controls the mode of the integration periods. When it is on, for example, the integration is performed with a negative sign (i.e., in a digital implementation, counter 54 counts down), and when it is off, the integration is performed with a positive sign (i.e., in a digital implementation, counter 54 counts up).

[0240] As can be seen, the integration over the measurement phases Pm occurs in the first integration mode and the integration over the reference phases Pr occurs in the second integration mode.

[0241] At the end of an integration cycle, when the value from integration circuitry is read out, it will be equal to the difference between the integrated values over the measurement periods Pm and the reference periods Pr.

[0242] A differential integrator may also be implemented as an analog circuit. In that case, for example, it may comprise a charge integrator, such as a capacitor or pinned diode. Circuitry between photosensitive element 42 and the charge integrator is adapted to, during the integration periods, to either add charges to the charge integrator (in the first integration mode) or to remove charges from the charge integrator (in the second integration mode). The charges added or removed are proportional to the signal from photosensitive element 42.

[0243] Fig. 19 shows another type of the pixel 36 with an analog differential integrator. Here, the integrator circuitry 44 comprises a first, second, and third capacitors 54-1, 54-2, and 54x. For reasons that will become apparent below, capacitor 54x is, in the following, called the differential capacitor.

[0244] The photosensitive element 42 may be a photodiode operated in photovoltaic mode. The gate circuitry 48-1, 48-2 is adapted to selectively connect the signal from the photosensitive element 42 to the first or second capacitor 54-1 or 54- 2, similar to the embodiment of Fig. 6.

[0245] Further, a first reset switch 52-1 and a second reset switch 52-2 are provided, controlled by switch control inputs 50-1, 50-2, for resetting the capacitors 54-1, 54-2, e.g., by connecting them to ground.

[0246] Pixel 36 further comprises an amplifier 96 with two differential inputs and a current output. It generates a current at its output that is proportional to the voltage difference between its inputs. The inputs may be high-impedance inputs, i.e., i.e., the current at the output may be at least ten times larger, usually even much larger, than the current through any of the inputs.

[0247] The output of amplifier 96 is connected, via a switch 98, to differential capacitor 54x.

[0248] The voltage over differential capacitor 54x can be selectively forwarded to bus line 60 by means of read-out circuitry 56 under the control of the signal at read-out control input 58.

[0249] The operation of the pixel of Fig. 19 is described in the following. The transfer of the signal from photosensitive element 42 to first and second capacitor 54-1, 54-2 occurs in the same manner as in the embodiment of Fig. 6, i.e., in the first integration mode, during an integration period, first gate switch 48-1 connects the signal to first capacitor 54-1 and, in the second integration mode, during an integration period, second gate switch 48-2 connects the signal to second capacitor 54-2.

[0250] After one pair of integration periods of the first and second mode (or, optionally, after K such pairs, with L advantageously being smaller than 100, in particular smaller than 10, in particular K = 1, to reduce the risk of overflow), switch 98 is activated by a control signal 100, during a transfer period, with said transfer period lasting a predefined time.

[0251] The transfer period may be placed outside of any of the integration periods for more accurate signal processing (i.e., for a transfer while the charges in the capacitors 54-1, 54-2 remain unchanged).

[0252] During the transfer period, amplifier 96 charges or discharges differential capacitor 54x with an amount of charge proportional to the voltage difference over the first and second capacitor 54-1, 54-2, i.e., it adds / subtracts the difference of the signals integrated in the K integration periods in the first and second modes to / from differential capacitor 54x.

[0253] After the transfer period, the first and second capacitors 54-1, 54-2 are reset by means of the reset-switches 52-1, 52-2.

[0254] This process is repeated over the integration cycle.

[0255] At the end of the integration cycle, the charge in differential capacitor 54x is equal to the difference of the total integrals over the integration periods in the first and second integration modes.

[0256] The value (voltage) of the differential capacitor 54x can then be read out by means of the read-out circuitry 56. Then, differential capacitor 54x may be reset, either via bus line 60 or by means of a separate reset circuit (not shown).

[0257] In this way, the signal from photosensitive element 42 is integrated on differential capacitor 54x with opposite signs in the first and second integration modes. Hence, the embodiment of Fig. 19 embodies both an "In-Pixel Subtraction" as described above as well as "Differential Integration" as described in this section.

[0258] Fig. 20 shows yet another type of the pixel 36 with an analog differential integrator.

[0259] In this type of embodiment, integration directly occurs, in differential manner, on differential capacitor 54x, which obviates the need to provide dedicated first and second capacitors 54-1, 54-2. In the first integration mode, during each integration period, gate circuitry 48-1 connects the signal from photosensitive element 42 to a first side 92-1 of differential capacitor 54x. At the same time, reference switch 94-2 is enabled to connect second side 94-2 of capacitor 54 to Vr-2.

[0260] In the second integration mode, during each integration period, gate circuitry 48-2 connects the signal from photosensitive element 42 to a second side 92- 2 of differential capacitor 54x. At the same time, first switch 94-1 is enabled to connect first side 94-1 of capacitor 54 to Vr-1.

[0261] Vr-1 and Vr-2 are fixed reference voltages, such as ground or Vdd.

[0262] In Fig. 20, the read-out circuitry 56 connects, when a read-out signal is applied to read-out control input 58, one of the sides of capacitor 54 to bus line 60. At the same time, the other side of capacitor 54 is connected, e.g., either by means of switch 90-2 or by means of switch 48-2 and transistor 64, to a defined potential.

[0263] The reset circuitry in this embodiment may again comprise reset switches 52-1, 52-2 connected to the voltages Vr-1, Vr-2, respectively.

[0264] In other embodiments, the reset circuitry in this embodiment may comprise the switches 48-1, 48-2, and the reset signal is applied through the gate-inputs 46-1, 46-2.

[0265] Hence, as described, a differential integrator can be implemented as a digital integrator or as an analog integrator.

[0266] In a digital implementation, the differential integrator may, e.g., comprise a digital counter 86 adapted to count in a first direction (e.g., to in the up- direction) in the first integration mode and in a second, opposite direction (e.g., in the down-direction) in the second integration mode.

[0267] In an analog implementation, the differential integrator may, e.g., comprise a differential capacitor 54x adapted to be charged, proportionally to the signal from the photosensitive element 42, with a first sign in the first integration mode and with a second, opposite sign in the second integration mode. In this context, the "first sign" corresponds to a charging geometry that connects a first side 94-1 of the capacitor to the signal from the photosensitive element 42, and the "second sign" corresponds to a charging geometry that connects a second side 94-2 of the capacitor to the signal from the photosensitive element 42.

[0268] Notes

[0269] Any capacitors shown here may, e.g., be implemented as "floating diffusion" capacitors or as "metal-insulator-metal" capacitors as known to the skilled person. The analog examples shown here work in the voltage domain. As known to the skilled person, they may also be implemented in the charge domain, i.e., using circuit architectures as known from charge-coupled devices.

[0270] Resetting the integration circuitry in the analog embodiments may comprise discharging the capacitors or charging them to a predefined charge or voltage. Subsequent integration then adds / removes to / from this initial charge in a manner dependent on the signal from the photosensitive element 42.

[0271] As can be seen, the present technique exploits the idea of integrating light during (illuminated) measurement periods Pm and (non-illuminated) reference periods Pr using timed integration circuitry in order to perform background-corrected spectroscopy.

[0272] Similar circuitry, albeit for a very different application, has been known from time-of-flight sensing where gating is used to sample the signal from a photodetector at a large number of time windows after sending out a signal. However, in contrast to such time-of-flight sensing applications, which are geared at detecting the time of arrival or phase shift of a return signal, the present technique does not need to integrate light in the period between the measurement periods Pm and the reference periods Pr. Hence, in some embodiment, the control unit is not adapted to integrate light between a measurement period and the next reference period nor between the reference period and the next measurement period.

[0273] The present techniques improve the spectroscopy process.

[0274] For example, integration in two subtractive integration modes makes the device more robust against ambient light. The subtraction, in particular the in-pixel subtraction, reduces the data rates. Repetitive in-pixel subtractions during an integration cycle or differential integration allows to use smaller integrators while maintaining high resolution.

[0275] While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

1. Claims1. A spectrometer device comprising a light source (10), a spectrometer section (4), and a control unit (16, 38, 72), wherein the spectrometer section (4) comprises: a dispersive element (22a, 22b, 30, 32) adapted to spatially redistribute incoming light as a function of its wavelength and a camera (20) positioned to receive spatially redistributed light from the dispersive element (22a, 22b, 30, 32), wherein the camera (20) comprises several pixels (36), wherein at least some of the pixels (36) comprise- a photosensitive element (42),- integration circuitry (44, 44-1, 44-2) having an integration control input, with the integration control input adapted to identify integration periods with first and second integration modes, wherein the integration circuitry (44, 44-1, 44-2) is adapted to integrate, over the integration periods, a signal from the photosensitive element (42) i) in a first integrator (44-1) in the first integration mode and in a second integrator (44-2) in the second integration mode or ii) in a differential integrator (44) adapted to integrate a weighted difference between integrals of the signal from the photosensitive element (42) in the first and second integration modes, wherein the control unit (16, 38, 72) is adapted to- operate the light source (10) to generate light pulses defining illumination periods (Pi),- during measurement periods (Pm), with each measurement period (Pm) at least overlapping with an illumination period (Pi), apply an integration control signal to the integration control input indicating an integration period in the first integration mode,- during reference periods (Pr), with each reference period (Pr) being between the illumination periods (Pi), apply an integration control signal to the integration control input indicating an integration period in the second integration mode.

2. The spectrometer device of claim 1 wherein the at least 80%, in particular 100%, of each measurement period (Pm) overlaps with an illumination period (Pi).

3. The spectrometer device of any of the preceding claims wherein the control unit (16, 38, 72) is adapted to pulse the light source (10) with a duty cycle of less than 10%.

4. The spectrometer device of any of the preceding claims wherein the control unit (16, 38, 72) is adapted to pulse the light source (10) with a duty cycle of less than 1%.

5. The spectrometer device of any of the preceding claims wherein the light source (10) is a semiconductor light source (10).

6. The spectrometer device of any of the preceding claims wherein the control unit (16, 38, 72) is adapted to operate the integration circuitry (44, 44-1, 44-2) to integrate, over an integration cycle, in the first integration mode over N measurement periods (Pm) and the second integration mode over M reference periods (Pr), with N and M being integers larger than 1, in particular with N = M, and to read out the integration circuitry (44, 44-1, 44-2) at the end of the integration cycle.

7. The spectrometer device of claim 6 wherein N and M are larger than 100, in particular larger than 1000.

8. The spectrometer device of any of the claims 6 or 7 wherein the integration circuitry (44, 44-1, 44-2) has a reset input (50-1, 50-2; 50) adapted to reset the integration circuitry (44, 44-1, 44-2) upon receipt of a reset signal, and wherein the control unit (16, 38, 72) is adapted to issue the reset signal to the integration circuitry (44, 44-1, 44-2) after reading out the integration circuitry (44, 44-1, 44-2).

9. The spectrometer device of any of the preceding claims wherein the integration circuitry (44, 44-1, 44-2) comprises a first integrator (44-1) adapted to integrate the signal from the photosensitive element (42) in the first integration modeand a second integrator (44-2) adapted to integrate the signal from the photosensitive element (42) in the second integration mode wherein the spectrometer device further comprises at least one subtractor (68) adapted to calculate a difference parameter between values (II, 12) of the first and the second integrator (44-1, 44-2).

10. The spectrometer device of claim 9 wherein each pixel comprises a threshold detector (74) adapted to generate an overflow condition signal if one of the integrators (44-1, 44-2) holds a value (II, 12) exceeding a given threshold, wherein the control unit (16, 38, 72) is adapted to reduce the values (II, 12) of both integrators (44-1, 44-2) by a quantity (Q) in the presence of the overflow condition signal.

11. The spectrometer device of any of the claims 9 or 10 wherein each pixel (36) comprises a subtractor (70) adapted to at least one of calculate a difference (Il - 12) of values (II, 12) of the integrators, and subtract an equal quantity (Q) from values (II, 12) of both integrators.

12. The spectrometer device of claim 11 wherein each pixel (36) comprises an accumulator (80) adapted to accumulate differences (Il - 12) calculated by the subtractor (70), wherein the control unit (16, 38, 72) is adapted to read out the accumulator (80) from each pixel (36).

13. The spectrometer device of any of the claims 9 - 12 wherein the control unit (16, 38, 72) is adapted- to integrate, over a first integration cycle (IC1) including several illumination periods (Pi), the signal from the photosensitive element (42) over several measurement periods (Pm) by means of the first integrator (44-1) while integrating the signal from the photosensitive element (42) over several reference periods (Pr) by means of the second integrator (44-2) and to operate the subtractor to calculate a first difference (DI) between values from the first and the second integrator (44-1, 44-2),- to integrate, over a second integration cycle (IC2) including several illumination periods (Pi), the signal from the photosensitive element (42) over several measurement periods (Pm) by means of the second integrator (44-2) while integrating the signal from the photosensitive element (42) over several referenceperiods (Pr) by means of the first integrator (44-1) and to operate the subtractor to calculate a second difference (D2) between values from the second and the first integrator (44-2, 44-1), and- to calculate a parameter indicative of a sum of the first and the second differences (DI, D2).

14. The spectrometer device of any of the preceding claims wherein the integrator circuitry comprises a differential integrator adapted to integrate a weighted difference of integrals of the signal from the photosensitive element (42) in the first and second integration modes.

15. The spectrometer device of claim 14 wherein the differential integrator comprises a digital counter (86) adapted to count in a first direction in the first integration mode and in a second direction in the second integration mode.

16. The spectrometer device of claim 14 wherein the differential integrator comprises a differential capacitor (54x) adapted to be charged with a first sign in the first integration mode and with a second sign in the second integration mode.

17. The spectrometer device of any of the preceding claims wherein the integration circuitry (44, 44-1, 44-2) comprises at least one charge integrator.

18. The spectrometer device of any of the claims 1 to 16 wherein the photosensitive element (42) is SPAD photodiode and the integration circuitry (44, 44-1, 44-2) comprises at least one digital counter (54; 54-1, 54-2).

19. The spectrometer device of any of the preceding claims wherein the control unit (16, 38, 72) is adapted to operate the camera (20) in operating cycles comprising burst phases (Bl) and idle phases (B3) wherein, with the idle phases (B3) being located between burst phases (Bl), wherein in each burst phase (Bl), the control unit (16, 38, 72) is adapted to operate the light source (10) to generate the light pulses at a first repetition rate and at least one of the integrators to integrate during the measurement periods (Pm), the control unit (16, 38, 72) is adapted to idle the camera (20) idle phases (B3).

20. The spectrometer device of claim 19 wherein a length of the idle phases (B3) is at least 10 microseconds, in particular for at least 0.1 s, in particular at least 0.5 s.

21. The spectrometer device of any of the claims 19 or 20 wherein each burst phase (Bl) contains at least 100, in particular at least 1000, measurement periods (Pm).

22. The spectrometer of any of the claims 19 to 21 wherein a length of the burst phases (Bl) is between 0.1 ms and 500 ms.

23. The spectrometer device of any of the preceding claims wherein the control unit (16, 38, 72) is not adapted to integrate light between a measurement periods (Pm) and a next reference period (Pr) nor between a reference period (Pr) and a next measurement period (Pm).

24. The spectrometer device of any of the preceding claims wherein the dispersive element (22a, 22b, 30, 32) comprises at least one of a refractive prism (30) and a diffractive grating (22a, 22b; 32).

25. The spectrometer device of any of the preceding claims wherein the control unit (16, 38, 72) is adapted to generate the light pulses at a repetition rate between 1 kHz and 1 GHZ, in particular between 10 kHz and 100 MHz.

26. The spectrometer device of any of the preceding claims wherein the control unit (16, 38, 72) is adapted to generate the light pulses with a length between 0.1 ns and 1 ms, in particular between 1 ns and 10 ps, even better between 1 ns and 1 ps.

27. Use of the spectrometer device of any of the preceding claims for measuring at least one of a Raman spectrum and a fluorescence spectrum of a target by sending the light pulses into a target and spectrally analyzing, by means of the spectrometer section (4), a spectrum of light returning from the target.

28. A method for operating a spectrometer device, in particular the spectrometer device of any of the preceding claims, wherein the spectrometer device comprisesa light source (10), a spectrometer section (4), and a control unit (16, 38, 72), wherein the spectrometer section (4) comprises: a dispersive element (22a, 22b, 30, 32) spatially redistributing incoming light as a function of its wavelength and a camera (20) receiving spatially redistributed light from the dispersive element (22a, 22b, 30, 32), wherein the camera (20) comprises several pixels (36), wherein at least some of the pixels (36) comprise- a photosensitive element (42),- integration circuitry (44, 44-1, 44-2) having an integration control input, with the integration control input adapted to identify integration periods with first and second integration modes, wherein the integration circuitry (44, 44-1, 44-2) is adapted to integrate, over the integration periods, a signal from the photosensitive element (42) i) in a first integrator in the first integration mode and in a second integrator in the second integration mode or ii) in a differential integrator adapted to integrate a weighted difference between integrals of the signal from the photosensitive element (42) in the first and second integration modes, wherein the method comprises- operating the light source (10) to generate light pulses defining illumination periods (Pi),- during measurement periods (Pm), with each measurement period (Pm) at least overlapping with an illumination period (Pi), applying an integration control signal to the integration control inputs indicating an integration period in the first integration mode,- during reference periods (Pr), with each reference period (Pr) being between the illumination periods (Pi), applying an integration control signal to the integration control inputs indicating an integration period in the second integration mode.

29. The method of claim 28 wherein the integration circuitry (44, 44-1, 44-2) comprises a first integrator (44-1) adapted to integrate the signal from the photosensitive element (42) in the first integration mode and a second integrator (44- 2) adapted to integrate the signal from the photosensitive element (42) in the second integration mode,wherein the method comprises- calculating, by means of a subtractor (68) of the spectrometer device, a difference between values of the first and the second integrator (44-1, 44-2).

30. The method of any of the claims 28 or 29 comprising deriving a spectral composition of the incoming light from a spatial distribution of signals received by the pixels (36) of the camera (20).

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