Patent Publication Number: US-2021190585-A1

Title: Spectrometer device and system

Description:
FIELD OF THE INVENTION 
     The invention relates to a spectrometer device to a spectrometer system comprising the spectrometer device, and to various uses of the spectrometer device and the spectrometer system. Such devices and systems can, in general, be employed for investigation or monitoring purposes, in particular, in the infrared (IR) spectral region, especially in the near-infrared (NIR) and the mid infrared (MidIR) spectral regions, and for a detection of heat, flames, fire, or smoke. However, further kinds of applications are possible. 
     PRIOR ART 
     Various spectrometer devices and systems for investigations in the infrared (IR) spectral region, especially in the near-infrared (NIR) spectral region, are known. Especially, spectrometer devices which comprise a combination of a linearly variable filter (LVF) and a detector array have already been proposed. Herein, the LVF is designated for separating light captured from an object into a spectrum of constituent wavelength signals while the detector array includes a plurality of pixels, wherein each of the plurality of pixels is disposed to receive at least a portion of a plurality of the constituent wavelength signals that provides a power reading for each constituent wavelength. Typically, in order to accomplish that the incident light may impinge the LVF in a manner normal to a receiving surface of the LVF, a baffle is used for this purpose, which, however, generally results in a low light throughput and a poor signal-to-noise ratio. 
     US 2014/131578 A1 discloses a portable spectrometer device which includes an illumination source for directing at a sample as well as a tapered light pipe (TLP) for capturing the light which interacts with the sample at a first focal ratio and for delivering the light at a second focal ratio lower than the first focal ratio to the LVF. Preferably, the TLP is lensed at one end, and recessed in a protective boot with stepped inner walls. In addition, a gap between the TLP and LVF is minimized to further enhance resolution and robustness. It is emphasized here, that the TLP disclosed herein can also be denoted by the term “optical concentrator device”, wherein the optical concentrator device is operated in reverse direction for spreading out the captured light and reducing an angular spread of the captured light, wherein the optical concentrator device comprises a conical shape. 
     However, as, for example, described by S. Madala and R. F. Boehm,  Effect of reflection losses on stationary dielectric - filled non - imaging concentrators , J. Photonics for Energy 6(4), 047002, 2016, optical concentrator devices comprising a conical shape suffer from a low concentration efficiency. As further described by S. Madala et al., effects of Fresnel reflection and total internal reflection (TIR) losses on performance parameters in refractive-type non-imaging solar concentrators affect performance parameters and, thereby, energy collection. For this purpose, S. Madala et al. carried out a raytracing analysis in order to illustrate the effects of Fresnel reflection and TIR losses on four different types of stationary dielectric-filled non-imaging concentrators, including a conic concentrator (V-trough concentrator), a compound parabolic concentrator (CPC), a compound elliptical concentrator (CEC), and a compound hyperbolic concentrator (CHC). According to their findings, the refractive index (RI) of a dielectric fill material determines the acceptance angle of a solid non-imaging concentrator. Larger refractive indices yield larger acceptance angles and, thereby, larger energy collection, however, they also increase the Fresnel reflection losses. 
     Lun J. and R. Winston,  Asymmetric design for compound elliptical concentrators  ( CEC )  and its geometric flux implications , Proc. of SPIE, 9572, 2015, provide a theoretical treatise on asymmetric compound elliptical concentrators (CEC) as a further non-imaging optical element. Herein, they set out that a conventional way of understanding an ideal concentrator is based on maximizing the concentration ratio based on a uniformed acceptance angle. Although such an angle does not exist in the case of a CEC, the thermodynamic laws still hold and concentrators have been contemplated with a maximum concentration ratio allowed by them by using a string method to solve this general problem. As a result, groups of ideal concentrators using geometric flux field or a flowline method have been obtained. 
     U.S. Pat. No. 5,615,673 A discloses a compound parabolic concentrator (CPC) which is used for optically collecting Raman scattered light returning from a region of interest. Herein, the CPC is a non-imaging optical element used in a normal configuration to convert light radiated over a full hemisphere into a narrow cone which can be collected by conventional optics, including optical fibers and lenses. 
     US 2016/151009 A1 discloses a sensor for a detection of gas, in particular for detection of CO 2 , wherein a numerical aperture converter (NA converter) is arranged in a path between a wavelength filter and a radiation source. Herein, the NA converter is created as an opening in a reflective or opaque material with the shape of a compound parabolic concentrator or a compound elliptical concentrator. Alternatively, an NA filter can be used which corresponds to an NA converter, except that the NA filter confines the angular spread of the rays by absorption of high-angle rays instead of by conversion. 
     Problem Addressed by the Invention 
     Therefore, the problem addressed by the present invention is that of providing a spectrometer device and a spectrometer system which may, particularly, be suited for investigations in the infrared (IR) spectral region, especially in the near-infrared (NIR) spectral region, and which at least substantially avoid the disadvantages of known devices and systems of this type. 
     In particular, it would be desirous to have an improved simple, cost-efficient and, still, reliable spectrometer device having an optical element designed for capturing light from an object and transferring the captured light to a linearly variable filter with a higher concentration efficiency as currently available. 
     SUMMARY OF THE INVENTION 
     This problem is solved by the invention with the features of the independent patent claims. Advantageous developments of the invention, which can be implemented individually or in combination, are presented in the dependent claims and/or in the following specification and the detailed embodiments. 
     As used herein, the expressions “have”, “comprise” and “contain” as well as grammatical variations thereof are used in a non-exclusive way. Thus, the expression “A has B” as well as the expression “A comprises B” or “A contains B” may both refer to the fact that, besides B, A contains one or more further components and/or constituents, and to the case in which, besides B, no other components, constituents or elements are present in A. 
     In a first aspect of the present invention, a spectrometer device is disclosed. Accordingly, the spectrometer device comprises
         an optical element designed for receiving incident light from an object and transferring the incident light to a length variable filter, wherein the optical element comprises an optical concentrator device, wherein the optical concentrator device is operated in reverse direction, wherein the optical concentrator device has a single sidewall which is adapted for reflecting incident light, wherein the single sidewall is designed as a rounded sidewall;   the length variable filter which is designated for separating the incident light into a spectrum of constituent wavelength signals; and   a detector array comprising a plurality of pixelated sensors, wherein each of the pixelated sensors is adapted to receive at least a portion of one of the constituent wavelength signals, wherein each of the constituent wavelength signals is related to an intensity of each constituent wavelength.       

     The “object” may, generally, be an arbitrary body, chosen from a living object and a non-living object. Thus, as an example, the at least one object may comprise one or more articles and/or one or more parts of an article, wherein the at least one article or the at least one part thereof may comprise at least one component which may provide a spectrum suitable for investigations. Additionally or alternatively, the object may be or may comprise one or more living beings and/or one or more parts thereof, such as one or more body parts of a human being, e.g. a user, and/or an animal. 
     As used herein, the term “light”, generally, refers to a partition of electromagnetic radiation which is, usually, referred to as “optical spectral range” and which comprises one or more of the visible spectral range, the ultraviolet spectral range and the infrared spectral range. Herein, the term “ultraviolet spectral range”, generally, refers to electromagnetic radiation having a wavelength of 1 nm to 380 nm, preferably of 100 nm to 380 nm. Further, in partial accordance with standard ISO-21348 in a valid version at the date of this document, the term “visible spectral range”, generally, refers to a spectral range of 380 nm to 760 nm. The term “infrared spectral range” (IR) generally refers to electromagnetic radiation of 760 nm to 1000 μm, wherein the range of 760 nm to 1.5 μm is usually denominated as “near infrared spectral range” (NIR) while the range from 1.5μ to 15 μm is denoted as “mid infrared spectral range” (MidIR) and the range from 15 μm to 1000 μm as “far infrared spectral range” (FIR). Preferably, light used for the typical purposes of the present invention is light in the infrared (IR) spectral range, more preferred, in the near infrared (NIR) and the mid infrared spectral range (MidIR), especially the light having a wavelength of 1 μm to 5 μm, preferably of 1 μm to 3 μm. 
     Light emerging from the object can originate in the object itself, but can also optionally have a different origin and propagate from this origin to the object and subsequently toward the spectrometer device. The latter case can, in particular, be affected by at least one illumination source being used. Thus, the light propagating from the object to the spectrometer device may be light which may be reflected by the object and/or a reflection device connected to the object. Alternatively or in addition, the light may at least partially transmit through the object. 
     The illumination source can be embodied in various ways. Thus, the illumination source can be for example part of the spectrometer device in a housing. Alternatively or additionally, however, the at least one illumination source can also be arranged outside a housing, for example as a separate light source. The illumination source can be arranged separately from the object and illuminate the object from a distance. As indicated above, the illumination source can, alternatively or in addition, also be connected to the object or be part of the object, such that, by way of example, the electromagnetic radiation emerging from the object can also be generated directly by the illumination source. By way of example, at least one illumination source can be arranged on and/or in the object and directly generate the electromagnetic radiation. 
     The illumination source may, preferably, comprise a kind of illumination source which may be known to provide sufficient emission in the infrared (IR) spectral range, especially, in the near infrared (NIR) spectral range, in particular, an incandescent lamp. Alternatively or in addition, the illumination source may, be selected from at least one of the following illumination sources: a flame source; a heat source; a laser, in particular a laser diode, although further types of lasers can also be used; a light emitting diode; an organic light source, in particular an organic light emitting diode; a neon light; a structured light source. Alternatively or additionally, other illumination sources can be used. Herein, it may particularly be preferred when the light emitted by the object and/or by the illumination source may exhibit a spectral range which may be closely related to the spectral sensitivities of the detector array, particularly, in a manner to ensure that the detector array which may be illuminated by the respective illumination source may be capable of providing a detector signal having a high intensity, thus, enabling an evaluation of the detector signals with sufficient signal-to-noise-ratio and, concurrently, a high-resolution. 
     As generally used, the term “spectrum” refers to a partition of the optical spectral range, in particular, of the infrared (IR) spectral range, especially of the near-infrared (NIR) spectral range. Herein, each part of the spectrum is constituted by an optical signal which is defined by a signal wavelength and the corresponding signal intensity. Further, the term “spectrometer device” relates to an apparatus which is capable of recording the signal intensity with respect to the corresponding wavelength of a spectrum or a partition thereof, such as a wavelength interval, wherein the signal intensity may, preferably, be provided as an electrical signal which may be used for further evaluation. In the spectrometer device according to the present invention, a length variable filter is used for separating incident light into a spectrum of constituent wavelength signals whose respective intensities are determined by employing a detector array as described below in more detail. In addition, an optical element which is designed for receiving incident light from the object and transferring the incident light to the length variable filter is applied. As further used herein, a “spectrometer system” may, thus, refer to an apparatus which, in addition to the spectrometer device, comprises an evaluation unit which is designated for determining information related to a spectrum of an object by evaluating detector signals provided by the spectrometer device as disclosed herein. 
     Thus, according to the present invention, the spectrometer device comprises a length variable filter which is designated for separating the incident light into a spectrum of constituent wavelength signals. As generally used, the term “length variable filter” refers to an optical filter which comprises a plurality of filters, preferably a plurality of interference filters, which may, in particular, be provided in a continuous arrangement of the filters. Herein, each of the filters may form a bandpass with a variable center wavelength for each spatial position on the filter, preferably continuously, along a single dimension, which is, usually, denoted by the term “length”, on a receiving surface of the length variable filter. In a preferred example, the variable center wavelength may be a linear function of the spatial position on the filter, in which case the length variable filter is usually referred to as a “linearly variable filter” or by its abbreviation “LVF”. However, other kinds of functions may be applicable to the relationship between the variable center wavelength and the spatial position on the filter. Herein, the filters may be located on a transparent substrate which may, in particular, comprise at least one material that may show a high degree of optical transparency within in the infrared (IR) spectral range, especially, within the near-infrared (NIR) spectral range as described below in more detail, whereby varying spectral properties, especially continuously varying spectral properties, of the filter along length of the filter may be achieved. In particular, the length variable filter may be a wedge filter that may be adapted to carry at least one response coating on a transparent substrate, wherein the response coating may exhibit a spatially variable property, in particular, a spatially variable thickness. However, other kinds of length variable filters which may comprise other materials or which may exhibit a further spatially variable property may also be feasible. At a normal angle of incidence of an incident light beam, each of the filters as comprised by the length variable filter may have a bandpass width that may amount to a fraction of the center wavelength, typically to a few percent, of the particular filter. By way of example, for a length variable filter having a wavelength range from 1400 to 1700 nm and a bandpass width of 1%, the bandpass width at the normal incidence angle might vary from 14 nm to 17 nm. However, other examples may also be feasible. 
     As a result of this particular set-up of the length variable filter, only incident light having a wavelength which may, within a tolerance indicated by the bandpass width, equal the center wavelength being assigned to a particular spatial position on the filter is able to pass through the length variable filter at the particular spatial position. Thus, a “transmitting wavelength” which may be equal to the center wavelength ±½ of the bandpass width may be defined for each spatial position on the length variable filter. In other words, all light which may not pass through the length variable filter at the transmitting wavelength may be absorbed or, mostly, reflected by the receiving surface of the length variable filter. As a result, the length variable filter has a varying transmittance which may enable it for separating the incident light into a spectrum. 
     Thus, the light which may pass through the length variable filter at a particular spatial position on the length variable filter may, subsequently, impinge on a detector array. In other words, the detector array may, preferably, be placed in a manner that the light may first impinge on the length variable filter and only that the partition of the light which may pass through the particular spatial position on the length variable filter may, thereafter, be capable of impinging on a corresponding spatial position on the detector array. As a result, the length variable filter may, therefore, be used for separating the incident light by its associated wavelength or wavelengths into at least one corresponding spatial position while a particular optical sensor comprised by the detector array may, consequently, be employed for measuring an intensity of the incident light which, due to its particular wavelength, may be able to pass through the length variable filter at the corresponding spatial position and, therefore, impinge the particular optical sensors provided for determining the intensity of the incident light at the particular wavelength. In a particularly preferred embodiment, the detector array may, thus, comprise a sequence of optical sensor which may be located in form of a series of optical sensors one following the other, wherein the sequence of the optical sensors may be placed in a parallel manner with respect to the continuous arrangement of the interference filters along the length of the length variable filter. 
     In further preferred embodiment, the detector array may, preferably, be separated from the length variable filter by a transparent gap. Herein, the transparent gap may, by way of example, be obtained by using an extended transparent body having two opposing sides, wherein the plurality of the interference filters which may constitute the length variable filter may be disposed on a first side while the series of the optical sensors constituting the detector array may be placed on a second side opposing the first side. As a result, by selecting a suitable width for the transparent gap a more precise adjustment of the detector array with regard to the length variable filter can be achieved. 
     The detector array may, thus, comprise a series of optical sensors which may, preferably, be arranged in a single line as a one-dimensional matrix along the length of the length variable filter or in more than one line, especially as two, three, or four lines parallel lines, in form of a two-dimensional matrix, in particular, in order to receive most of the intensity of the incident light as possible. Thus, a number N of pixels in one direction may be higher compared to a number M of pixels in a further direction such that the one-dimensional 1×N matrix or a rectangular two-dimensional M×N matrix may be obtained, wherein M&lt;10 and N≥10, preferably N≥20, more preferred N≥50. In addition, the matrixes used herein may also be placed in a staggered arrangement. Herein, each of the optical sensors as used therein may have the same or, within a tolerance level, a similar optical sensitivity, especially for ease of manufacturing the series of the optical sensors. Alternatively, each of the optical sensors as used in the series of the optical sensors may exhibit a varying optical sensitivity that may vary in accordance with the varying transmittance properties of the length variable filter, such as by providing an increasing variation or a decreasing variation of the optical sensitivity with wavelength along the series of the optical sensors. However, other kinds of arrangements may also be feasible. 
     In particular, in order to achieve a high resolution of the spectrometer device, each of the optical sensors may, thus, be adapted to receive incident light only over a small spatial angle. This arrangement, particularly, reflects the setup of the length variable filter which is designed to generate the desired spectrum depending on the spatial position of the impingement of the incident light along the length of the filter. This particular arrangement is, according to the present invention, achieved by a detector array which, thus, comprises a plurality of pixelated sensors, wherein each of the pixelated sensors is adapted to receive at least a portion of one of the constituent wavelength signals as provided by the length variable filter. As indicated above, each of the constituent wavelength signals is, hereby, related to an intensity of each of the constituent wavelengths. As generally used, the terms “pixelated optical sensor” or, simply, “pixelated sensor” refers to an optical sensor which comprises an array of individual pixel sensors, wherein each of the individual pixel sensors has at least a photosensitive area which is adapted for generating an electrical signal depending on the intensity of the incident light, wherein the electrical signal may, in particular, be provided to an external evaluation unit for further evaluation. Herein, the photosensitive area as comprised by each of the individual pixel sensors may, especially, be a single, uniform photosensitive area which is configured for receiving the incident light which impinges on the individual pixel sensor. However, other arrangements of the pixelated sensors may also be conceivable. 
     The pixelated sensor is designed to generate signals, preferably electronic signals, associated with the intensity of the incident light which impinges on the individual pixelated sensor. The signal may be an analogue and/or a digital signal. The electronic signals for adjacent pixelated sensors can, accordingly, be generated simultaneously or else in a temporally successive manner. By way of example, during a row scan or line scan, it is possible to generate a sequence of electronic signals which correspond to the series of the individual pixel sensors which are arranged in a line. In addition, the individual pixel sensors may, preferably, be active pixel sensors which may be adapted to amplify the electronic signals prior to providing it to the external evaluation unit. For this purpose, the pixelated sensor may comprise one or more signal processing devices, such as one or more filters and/or analogue-digital-converters for processing and/or preprocessing the electronic signals. 
     The pixelated sensor may be selected from any known pixel sensor, in particular, from a pixelated organic camera element, preferably, a pixelated organic camera chip, or from a pixelated inorganic camera element, preferably, a pixelated inorganic camera chip, more preferably from a CCD chip or a CMOS chip, which are, commonly, used in various cameras nowadays. As an alternative, the pixelated sensor may be or comprise a photoconductor, in particular an inorganic photoconductor, especially PbS, PbSe, Ge, InGaAs, ext. InGaAs, InSb, or HgCdTe. As a further alternative it may comprise of pyroelectric, bolometer or thermopile detector elements. Thus, a camera chip having a matrix of 1×N pixels or of M×N pixels may be used here, wherein M&lt;10 and N≥10, preferably N≥20, more preferred N≥50. Further, a mono-chrome camera element, preferably a monochrome camera chip, may be used, wherein the monochrome camera element may be differently selected for each pixel sensor, especially, in accordance with the varying wavelength along the series of the optical sensors. 
     As a further alternative, the pixelated sensor may be based on a FiP sensor which is, among further documents, disclosed in WO 2012/110924 A1, WO 2014/097181 A1, or WO 2016/120392 A1. Herein, the term “FiP sensor” refers to a sensor in which the sensor signal, given the same total power of the illumination, is, according to the so called “FiP effect”, dependent on a geometry of the illumination of the photosensitive area, in particular on a beam cross-section of the illumination on the photosensitive area, also denoted as a “spot size”. As a result, the observable property that an electrical property of the photosensitive area depends on an extent of the illumination of the photosensitive area by incident light particularly accomplishes that two incident light beams comprising the same total power but generating different spot sizes on the photosensitive area provide different values for the electrical property of the photosensitive area and are, thus, distinguishable with respect to each other. Preferably, the photosensitive area of each of the FiP sensors may comprise a photoconductive material, especially selected from PbS, PbSe, or HgCdTe, or a solid dye sensitized solar cell (sDSC). Further, WO 2014/198625 A1 discloses a particular embodiment of a detector array which employs a M×N matrix of FiP sensors. Alternatively, further kinds of pixelated sensors may also be feasible. 
     Thus, the detector array may be adapted to provide a plurality of the electrical signals which may be generated by the photosensitive areas of the pixelated sensors comprised by the detector array. The electrical signals as provided by the detector array of the spectrometer device may, subsequently, be forwarded to an external evaluation unit, in particular, to an evaluation unit which may be comprised by a corresponding spectrometer system as described below in more detail. Herein, the term “evaluation unit” refers to an apparatus being designated for determining information related to the spectrum of the object of which a spectrum has been recorded, in particular, by using the spectrometer device as described herein, wherein the information is obtainable by evaluating the detector signals as provided by the detector array of the spectrometer device. The information may, for example, be provided electronically, visually, acoustically or in any arbitrary combination thereof. Further, the information may be stored in a data storage device of the spectrometer device, preferably of the spectrometer system, or of a separate storage device and/or may be provided via at least one interface, such as a wireless interface and/or a wire-bound interface. 
     According to the present invention, the spectrometer device further comprises an optical element which is designated for receiving incident light from the object and, simultaneously, transferring the incident light to the length variable filter, wherein the optical element comprises an optical concentrator device which is operated in reverse direction. As generally used, the term “optical concentrator” refers to a non-imaging optical element having an input, also denoted as “entrance pupil” or an “entrance aperture”, an output located oppositely to the input, wherein the output may also be denoted by one of the terms “exit pupil” or “exit aperture”, and an optically guiding structure located between the input and the output, wherein the optical concentrator is, in normal direction of operation, adapted for capturing light at the input at a large angular spread, concentrating the captured light within the optically guiding structure, and emitting the concentrated light at the output. By way of example, the optical concentrator may, therefore, be used in concentrated photovoltaics in order to allow high solar concentration under large possible entrance angles. 
     In contrast hereto, in the optical concentrator device which is used in reverse direction the previous output of the optical concentrator now serves as input for receiving incident light, while the optically guiding structure in the reverse direction, preferably, serves for spreading out the incident light, whereas the previous input now serves as output for emitting the spread light. In a preferred embodiment of the present invention the entrance pupil of the inversely-operated optical concentrator device, preferably, comprises an input angle of less than 90°, more preferably of less than 70°, in particular of less than 50°. Further in this preferred embodiment, the exit pupil of the inversely-operated optical concentrator device, preferably, comprises an output angle of not more than 30°, more preferably of not more than 15°, in particular of not more than 10°. Hereby, an angular spread of emitted light beams at the output can, simultaneously, be reduced compared to the incident light. As a result, applying the optical concentrator device in reverse direction allows capturing incident light which is emitted or reflected by the object or passes through the object in a manner that light which is emitted at the output of the inversely-operated optical concentrator device exhibits a diminished angular spread, wherein the diminished angular spread may, preferably, be restricted to an angular range of at most ±20°, preferably of at most ±10°, most preferred of at most ±5°. 
     Consequently, the optical concentrator device operated in reverse direction may, thus, be selected and arranged in a fashion that the emitted light beams can impinge on the length variable filter within the restricted angular range and, thus, predominantly in a direction normal to the receiving surface of the length variable filter, i.e. in a perpendicular manner with respect to the receiving surface of the length variable filter. As already indicated above, each of the interference filters as comprised by the length variable filter may, at such a normal angle of light incidence, have a bandpass width which may only amount to a fraction of the center wavelength of the particular interference filter. As a result, incident light may enter the length variable filter with considerably higher concentration efficiency. 
     Further according to the present invention, the inversely-operated optical concentrator device comprises a single sidewall which is adapted for reflecting incident light. This feature, which may, specifically, may allow increasing the efficiency of the spectrometer device, is in particular contrast to known inversely-operated optical concentrator devices which use non-reflective, absorbable surfaces in order to avoid under all possible circumstances that light beams may reach the length variable filter at a spatial position on the length variable filter which may not be configured for receiving the light beam having the particular wavelength as comprised by the light beam. In contrast hereto, as a result of this further preferred embodiment, the inversely-operated optical concentrator device may allow some light beams to be reflected and, still, be guided to the length variable filter. 
     Further according to the present invention, the single sidewall comprised by the inversely-operated optical concentrator device is designed as a rounded sidewall. Herein, the single rounded sidewall may, in particular, be formed as a shell surface which may, specifically, be adapted to connect the entrance aperture at the entrance pupil and the exit aperture at the exit pupil as the optically guiding structure. As a result, the single rounded sidewall may assume a shape in three-dimensional space, wherein the shape is devoid of any corners. More particular, the rounded sidewall may, advantageously, comprise a non-conical profile which may, specifically, be selected from a parabolic profile or an elliptical profile. As used herein, the profile refers to a form of a cross section of the single sidewall perpendicular to the longitudinal axis of the inversely-operated optical concentrator device which may, in particular, be or comprise a parabolic shape or an elliptical shape, respectively. In a particularly preferred embodiment, which is, specifically, adapted for further increasing the concentration efficiency of the spectrometer device, the inversely-operated optical concentrator device may, on one hand, have a round, small entrance aperture at the entrance pupil in order to be capable of catching as much light as possible and, on the other hand, have an elongated and rounded exit aperture, in particular an elliptical exit aperture, at the exit pupil, in particular in order to allow guiding as much light as possible via the length variable filter onto the detector array which may, preferably, exhibit a rectangular form. However, other shapes of the entrance aperture and/or of the exit aperture of the optical concentrator device being operated in reverse direction. As a result of the single rounded sidewall limiting the inversely-operated optical concentrator device and constituting the surface thereof more light beams are guided in a manner that they can reach the length variable filter at the desired spatial position on the length variable filter, whereby the efficiency of the spectrometer device as described herein can, further, be increased, which is in particular contrast to US 2014/131578 A1. As shown below in more detail, this kind of inversely-operated optical concentrator device may increase the efficiency of the spectrometer device by reducing an amount of reflections when transferring the incident light to a length variable filter, in particular, by avoiding that the shape of the single rounded sidewall has any corners, whereby a higher transmission efficiency of the optical concentrator may be achieved. 
     Various shapes of optical concentrator devices which are operated in normal direction have been presented before. Herein, the optical concentrator device can have a conical shape or a non-conical shape in a fashion that they may be referred to as a “compound parabolic concentrator” or “CPC” or a “compound elliptical concentrator” or “CEC” while further shapes, in particular a “compound hyperbolic concentrator” or “CHC”, may be less suited for the purposes of the present invention. According to the state of the art, the conical optical concentrator has, as generally used, been defined by two plane mirror segments which open with respect to each other in a linear manner. Further, the compound parabolic concentrator may be defined by two parabolic mirror segments which may comprise two different focal points each lying on one of the parabolic mirror segments. Hereby, surfaces of the two parabolic mirror segments may be arranged in a symmetrical manner with respect to reflection through an axis of the compound parabolic concentrator. Similarly, the compound elliptical concentrator may, thus, be defined by two elliptical mirror segments, wherein surfaces of the two elliptical mirror segments may be arranged in a symmetrical manner with respect to reflection through an axis of the compound elliptical concentrator. 
     Thus, the optical concentrator device which is operated in reverse direction, more particular the optically guiding structure which may be located between the input and the output of the optical element, may comprise a conical shape or, preferably, a non-conical shape. Herein, the optical concentrator device having the conical shape may more easily be manufactured. However, as a result of the non-conical shape, the light beams may be guided through the concentrator in a manner that the efficiency at the output of the inversely operated optical concentrator device may be increased. This effect is, especially, based on the non-conical shape of the inversely-operated optical concentrator device which allows, in particular contrast to a conically-shaped inversely-operated optical concentrator device, to guide the incident light in form of light beams through the non-conically shaped inversely-operated optical concentrator device in a manner that less light may be absorbed by the single sidewall of the inversely-operated optical concentrator device. As a result more, light beams can reach the length variable filter at the spatial position on the length variable filter which is configured for receiving the light beam with the particular wavelength as comprised by the light beam. In addition, a transfer function of a non-conical shape shows a steeper onset of light transmission with respect to an entrance angle into the inversely-operated optical concentrator device, thereby exhibiting a theoretical maximum of a completely binary transition from no light transmission to full light transmission for a compound parabolic concentrator. Such a kind of steepness of the transfer function may, thus, be advantageous in minimizing an area of spectroscopic sampling for a given light throughput, thereby providing a clear definition for the sampling spot and supporting in homogenizing a response from the sampling spot. 
     More particular, the term “conical” with regard to a optical element according to the state of the art refers to a shape of the optical element which can be described by the symmetry group C2v or higher according to the Schoenfliess notation, such as C2v, C4v, or C∞, in particular a truncated cone or a pyramidal trunk, wherein, as indicated above, manufacturing tolerances are, however, taken into account. In contrast hereto, the term “non-conical” with regard to the optical element according to the present invention refers to a shape of the optical element which can be described by a symmetry group less than C2v according to the Schoenfliess notation, such as Cs, C2, or C1 as indicated above, wherein, again, manufacturing tolerances are, however, taken into account. In other words, the optically guiding structure which is located between the input and the output of the optical element according to the present invention may, preferably, comprise a shape that may exhibit a diameter at a half distance between the input and the output of the optical element which may, for a conical shape, correspond to an arithmetic mean of a first diameter of the input of the optical element and a second diameter of the output of the optical element whereas it may deviate from the arithmetic mean of the first diameter and the second diameter by at least 10%, preferably at least 5%, mostly preferred at least 2%, especially at least 1%, for the non-conical shape. 
     In a particular embodiment, the non-conical shape of the optical concentrator device which is operated in reverse direction may, thus, preferably be selected from a parabolic shape or an elliptical shape. Consequently, the optical concentrator device may, especially, be selected from the group comprising a compound parabolic concentrator and a compound elliptical concentrator. However, other kinds of optical concentrator devices having a non-conical shape may also be feasible. 
     In particular, the parabolic shape of the optical element, especially of the optically guiding structure which is located between the input and the output of the optical element, may, leaving aside manufacturing tolerances, be described by Equation (1) as follows: 
         y=y   0   +ax   2 ,  (1)
 
     wherein the term x refers to a value along the optical axis of the optical element, in particular, of the optically guiding structure of the inversely-operated optical concentrator device, whereas the term y refers to a value perpendicular to the optical axis, wherein the terms y 0  and a provide opportunities for adjusting the parabolic shape with respect of the optical element. 
     Similarly, the elliptic shape of the optical element, especially of the optically guiding structure which is located between the input and the output of the optical element, may, leaving aside manufacturing tolerances, be described by Equation (2) as follows: 
     
       
         
           
             
               
                 
                   
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                   2 
                   ) 
                 
               
             
           
         
       
     
     wherein the term x refers to a value along the optical axis of the optical element, in particular, of the optically guiding structure of the inversely-operated optical concentrator device, whereas the term y refers to a value perpendicular to the optical axis, wherein the terms a and b which refer to semi axis of the elliptical shape, again, provide opportunities for adjusting the parabolic shape with respect of the optical element. 
     The inversely-operated optical concentrator device may be or may comprise a full body of a fully or partially optically transparent material or, as a preferred alternative, may be or may comprise a hollow body which can be, specifically fully and/or uniformly, filled with a gaseous and/or fluid and/or solid optically transparent material and which comprises the single sidewall that may assume the desired conical or non-conical shape. Herein, the at least one material that may show a high degree of optical transparency within in the infrared (IR) spectral range, especially, within the near-infrared (NIR) and mid-infrared (MidIR) spectral range, and which can be chosen for the full body of the optical concentrator device may, preferably, be selected from the group consisting of calcium fluoride (CaF 2 ), fused silica, germanium, magnesium fluoride (MgF), potassium bromide (KBr), sapphire, silicon, sodium chloride (NaCl), zinc selenide (ZnSe), zinc sulfide (ZnS), borosilicate-crown glasses, transparent conducting oxides (TCO), and transparent organic polymers, wherein silicon and germanium having high reflective indices are particularly preferred since they are capable of supporting total reflection which may occur on the single sidewall of the full body. As an alternative, the gaseous optically transparent material which may be chosen for filling the hollow body having the single rounded sidewall showing the desired conical or non-conical shape may be selected from ambient air, nitrogen gas, or carbon dioxide while the fluid optically transparent material for this purpose may be chosen from immersion oil or Canada balsam, i.e. a turpentine made from the resin of a balsam fir tree, especially from  Abies balsamea . As a further alternative, a vacuum may be present in the hollow body. 
     In a further preferred embodiment, the optical element may be arranged in a manner that the captured light is transferred to the length variable filter along a light path which is asymmetric with respect to an optical axis of the spectrometer device. As used herein, the term “optical axis” refers to an imaginary line of symmetry along which other elements of the spectrometer device apart from optical element, in particular, the length variable filter and the detector array, may be invariant with regard to a reflection or a rotation of the symmetric optical elements comprised by the spectrometer system. Transferring the captured light to the length variable filter may, thus, be achieved by using the at least one optical element, wherein at least one property of the optical element, in particular, at least one of a shape of the optical element or an arrangement of the optical element with respect to the optical axis of the spectrometer device, may result in a light path which may exhibit an asymmetric course with respect to the optical axis of the spectrometer device. 
     With regard to a body or at least one element thereof, the terms “symmetric” or “symmetrical” refer, as generally used, to a geometric property of the body having a shape or at least one element thereof that may be invariant to an application of at least one operation usually denoted as “geometric transformation” which may, in particular, include a reflection, a rotation, a translation and/or a combination thereof. In the present case, however, the preferred geometric transformations may be the reflection and the rotation. In particular, the body or the element thereof can be considered to have reflectional symmetry with respect to a line and/or to a mirror plane if the line and/or the mirror plane, respectively, can divide the shape of the body or the element thereof into two partitions which assume mirror images of each other. By way of example, the body may be considered as symmetric in an event in which it comprises the two identical mirror segments which are arranged in a manner with respect to an axis of reflection that they can be considered as mirror images of each other. Similarly, the body can be considered to have a rotational symmetry with respect to at least one fixed point and/or a mirror plane comprising the axis of rotation of the body or the element thereof if it can be rotated about the fixed point or the mirror plane, respectively, in a manner that the shape of the body or the element thereof may not change thereby. In contrast hereto, the terms “asymmetric” or “asymmetrical” may denote that at least one of the geometric properties of the body under consideration may not be invariant to at least one of the geometric transformations which may be applicable to this body. 
     More particular, one of the following symmetry groups may be applicable to the optical element. As generally used, the term “symmetry group” describes a particular symmetry of the body or the element thereof by indicating which kind of geometric transformations may be applicable to the body or the element thereof. For this consideration, a particular symmetry group may, especially, be denoted by using the so-called “Schoenfliess notation”. Herein, the following terms which are defined as follows may, preferably, be applicable to the shape of the body of the element thereof:
         C ∞  which refers to the symmetry of a truncated cone, wherein the shape of the truncated cone is preserved under a rotation about an arbitrary angle with respect to a mirror plane comprising the axis of rotation of the truncated cone;   C4v which refers to a square pyramid trunk, wherein the shape of the square pyramid trunk is only preserved under a rotation about an angle of 360°/4=90° with respect to a mirror plane comprising the axis of rotation of the square pyramid trunk;   C2v which refers to a rectangular pyramid trunk apart from a square pyramid trunk, wherein the shape of the rectangular pyramid trunk is only preserved under a rotation about an angle of 360°/2=180° with respect to a mirror plane comprising the axis of rotation of the rectangular pyramid trunk;   Cs which refers to a reflectional symmetry only, wherein the shape of the body or the element thereof is only preserved by a reflection on a mirror plane;   C2 which refers to a two-fold symmetry only, wherein the shape of the body or the element thereof may only be preserved under a rotation about an angle of 360°/2=180° with respect to a particular axis of rotation of the body or the element thereof; or   C1 which refers to an absence of any symmetry elements.       

     For further information about the Schoenflies-Notation, reference may, for example, be made to Morton Hamermesh,  Group Theory and its application to physical problems , Dover Publications, Mineola, N.Y. 1989, Chapter 2 , Symmetry Groups , pages 32-67. 
     With particular respect to the present invention, the optical element may, thus, being denoted as having an asymmetric shape in an event in which the shape of the optical element may exhibit less symmetric elements as described by the symmetry group C2v according to the Schoenfliess notation, in particular, having at least one of the symmetry groups Cs, C2, or C1. In this regard, it may, however, be emphasized that for the consideration of the symmetry of the optical element only those partitions of the optical element may be relevant which may affect the beam path of the incident light, especially, the optically guiding structure which is located between the input and the output of the optical element. In particular, other shapes of the optical element, such as an exterior shape, may, thus, be left out of consideration. In addition, for determining the symmetry of the optical element, manufacturing tolerances are taken into account. 
     In an embodiment in which the inversely-operated optical concentrator device may be arranged in an asymmetric manner with respect to the optical axis of the spectrometer device, the response of the spectrometer device can no longer be expected to be equal for small wavelengths detectable on one side of the detector array and for long wavelengths detectable on the opposing side of the detector array. However, this observation may exhibit particular advantages for the spectrometer device, in particular, when an incandescent lamp may be used as the illumination source. As generally used, the term “incandescent lamp” refers to an electric light having a heatable element, such as a wire filament heated, which may be heated to a temperature that it may emit light, especially infrared light. Since the incandescent lamp can, therefore, be considered as a thermal emitter within the infrared spectral range, an emission power of the incandescent lamp decreases with increasing wavelength. In addition, known materials which may, typically, be used for absorption within the infrared spectral range, generally, exhibit a tendency of increased absorption with increasing wavelength. Further, as already indicated above, since the bandpass width of the length variable filter, in particular of the linearly variable filter, which may, typically, assume a constant value, such as 1%, over the spectral range of the length variable filter, the resolution of the length variable filter, which is inversely proportional to the bandpass width, also decreases with increasing wavelength. Further, the resolution of the length variable filter, in general, depends on the center wavelength of the length variable filter. Combining all mentioned effects, it appears that in a symmetric spectrometer device the response of the spectrometer device decreases with increasing wavelength. 
     However, in accordance with a further preferred embodiment of the present invention, using the inversely-operated optical concentrator device having an asymmetric design or, alternatively or in addition, the inversely-operated optical concentrator device being arranged in an asymmetric manner with respect to the optical axis of the spectrometer device may allow adjusting a length of a path that a light beam may travel from the entry into the inversely-operated optical concentrator device through the inversely-operated optical concentrator device to the exit of the inversely-operated optical concentrator device. As a result, an asymmetry of the inversely-operated optical concentrator device, either in setup or arrangement, may allow to provide a shorter path for a light beam having a longer wavelength compared to a light beam having a shorter wavelength and, additionally or as an alternative, the light beam to impinge the linearly variable filter closer to normal incidence, whereby the efficiency may, further be increased. As a result, the further preferred embodiment may facilitate providing a spectrometer device which may, especially, be adapted for allowing a higher efficiency at longer wavelengths. Further, although it may appear that the efficiency at shorter wavelengths may, therefore, be diminished, in particular, due to a longer path for a light beam having a shorter wavelength compared to a light beam having a longer wavelength, this effect may, in this kind of arrangement, generally be outweighed by the above-indicated higher emission power of the incandescent lamp and the lower bandpass width at shorter wavelengths. Thus, this kind of arrangement can be used for equipping the spectrometer device with an efficiency which may be more equally distributed over the wavelength range of the spectrometer device according to the present invention compared to known spectrometer devices, especially in the infrared spectral range. 
     In this further preferred embodiment of the asymmetric arrangement according to the present invention, the optical element, in particular, the inversely-operated optical concentrator device may, therefore, be tilted with respect to a plane which is perpendicular to a receiving surface of the length variable filter as described above. As generally used, the term “tilted” refers to an inclination of a symmetry axis of the optical concentrator device with respect to the plane normal to the receiving surface of the length variable filter in a perpendicular manner. As a result of the tilted arrangement of the optical element, the entrance axis and the exit pupil axis of the inversely-operated optical concentrator device may be one of: shifted and parallel with respect to each other, shifted but not parallel with respect to each other, or not shifted and not parallel with respect to each other. More preferred, the optical concentrator device may be tilted with respect to this plane which is normal to the receiving surface of the length variable filter in a manner that the light beams may impinge the length variable filter normal to the receiving surface of the length variable filter on a spatial position on the length variable filter being designated for receiving a particular wavelength of the incident light. As an alternative, the optical concentrator device may be tilted with respect to this plane in a further manner that further light beams may impinge a further spatial position on the length variable filter being designated for receiving a further particular wavelength, wherein the further particular wavelength exceeds the wavelength of the incident light beam, i.e. exhibits a longer wavelength than the incident light which may be directed to impinge the surface of the length variable filter. 
     Therefore, the incident light beam may pass the length variable filter at a longer wavelength compared to its inherent wavelength but due to the relative arrangement between the length variable filter and the detector array, which may be separated by a gap as described elsewhere in this document in more detail, the incident light beam may, still, impinge the particular optical sensor which is provided for determining the intensity of the incident light at the particular wavelength of the incident light beam. As a result, not only light beams which impinge the length variable filter normal to the receiving surface of the length variable filter on a spatial position on the length variable filter being designated for this purpose but also light beams which impinge the length variable filter on a further spatial position being designated for receiving a longer wavelength than the wavelength of the incident light beam but, still, impinge the particular optical sensor designed for receiving the particular wavelength of the incident light beam, may contribute to the electrical signals being generated by the photosensitive area of the particular individual pixel sensor. Consequently, the efficiency of the spectrometer device can, in this manner, even further be increased. 
     In addition, the spectrometer device according to the present invention may, further, comprise at least one transfer device, which can, in particular, be arranged between the inversely-operated optical concentrator device and the length variable filter. Most preferably, the light beam which emerges from the object may, therefore, travel firstly through the inversely-operated optical concentrator device and, thereafter, at or through the transfer device until it may, subsequently, pass the length variable filter until it may, finally, impinge the detector array. As used herein, the term “transfer device” may, thus, refer to an optical component which can be configured to transfer the light beam emerging from the inversely-operated optical concentrator device to the detector array. In a particular embodiment, the transfer device can, thus, be designed to shape the light beam before it may be guided to the length variable filter. 
     Particularly, the transfer device may be selected from a group consisting of an optical lens, a curved mirror, a grating, and a diffractive optical element. More particular, the optical lens may, especially, be selected from a group consisting of a biconvex lens, a plano-convex lens, a biconcave lens, a plano-concave lens, an aspherical lens, a cylindrical lens and a meniscus lens. Hereby, the transfer element may comprise a material which may be at least partially transparent, preferably over the whole wavelength range of the length variable filter as indicated above. For this purpose, the same or similar optically transparent materials as mentioned in this respect can also be used. However, further optical elements may also be feasible. 
     In a further aspect of the present invention, a spectrometer system is disclosed. Accordingly, the spectrometer system comprises
         a spectrometer device as described above and/or below in more detail; and   an evaluation unit designated for determining information related to a spectrum of an object by evaluating detector signals provided by the spectrometer device.       

     Herein, the components of the spectrometer system as listed above may be individual components. Alternatively, two or more of the components of the spectrometer system may be integrated into a single integral component. Further, the evaluation unit may be formed as an individual evaluation unit independent from the spectrometer device but may preferably be connected to the detector array, in particular, in order to receive the detector signals as generated by the spectrometer device. Alternatively, the at least one evaluation unit may fully or partially be integrated into the at least one spectrometer device. 
     According to the present invention, the spectrometer system comprises a spectrometer device and an evaluation unit. With respect to the spectrometer device, reference may be made to the description elsewhere in this document. As further used herein, the term “evaluation unit”, generally, refers to an arbitrary device designed to generate the desired items of information, i.e. the at least one item of information related to the spectrum of the object. As an example, the evaluation unit may be or may comprise one or more integrated circuits, such as one or more application-specific integrated circuits (ASICs), and/or one or more data processing devices, such as one or more of computers, digital signal processors (DSP), field programmable gate arrays (FPGA) preferably one or more microcomputers and/or microcontrollers. Additional components may be comprised, such as one or more preprocessing devices and/or data acquisition devices, such as one or more devices for receiving and/or preprocessing of the detector signals, such as one or more AD-converters and/or one or more filters. As used herein, the detector signal is provided by the spectrometer device, in particular, by the detector array of the spectrometer device. Further, the evaluation unit may comprise one or more data storage devices. Further, the evaluation unit may comprise one or more interfaces, such as one or more wireless interfaces and/or one or more wire-bound interfaces. 
     The at least one evaluation unit may be adapted to perform at least one computer program, such as at least one computer program performing or supporting the step of generating the items of information. As an example, one or more algorithms may be implemented which, by using the sensor signals as input variables, may perform a predetermined transformation into the position of the object. For this purpose, the evaluation unit may, particularly, comprise at least one data processing device, in particular an electronic data processing device, which can be designed to generate the items of information by evaluating the detector signals. Thus, the evaluation unit is designed to use the detector signals as input variables and to generate the items of information related to the spectrum of the object by processing these input variables. The processing can be done in parallel, subsequently or even in a combined manner. The evaluation unit may use an arbitrary process for generating these items of information, such as by calculation and/or using at least one stored and/or known relationship. Besides the detector signals, one or a plurality of further parameters and/or items of information can influence said relationship, for example at least one item of information about a relative arrangement of the optical element, the length variable filter, and the detector array as comprised by the spectrometer device. The relationship can be determined or determinable empirically, analytically or else semi-empirically. Particularly preferably, the relationship comprises at least one calibration curve, at least one set of calibration curves, at least one function or a combination of the possibilities mentioned. One or a plurality of calibration curves can be stored for example in the form of a set of values and the associated function values thereof, for example in a data storage device and/or a table. Alternatively or additionally, however, the at least one calibration curve can also be stored for example in parameterized form and/or as a functional equation. Separate relationships for processing the detector signals into the items of information may be used. Alternatively, at least one combined relationship for processing the detector signals is feasible. Various possibilities are conceivable and can also be combined. 
     By way of example, the evaluation unit can be designed in terms of programming for the purpose of determining the items of information. The evaluation unit can comprise, in particular, at least one computer, for example at least one microcomputer. Furthermore, the evaluation unit can comprise one or a plurality of volatile or nonvolatile data memories. As an alternative or in addition to a data processing device, in particular at least one computer, the evaluation unit can comprise one or a plurality of further electronic components which are designed for determining the items of information, for example an electronic table and in particular at least one look-up table and/or at least one application-specific integrated circuit (ASIC). 
     Further, the evaluation unit can also be designed to completely or partially control or drive the spectrometer device or a part thereof, for example by the evaluation unit being designed to control at least one illumination source and/or to control the optical element of the spectrometer and/or to control at least one modulation device of the detector. The evaluation unit can, in particular, be designed to carry out at least one measurement cycle in which a plurality of detector signals are picked up, especially, the detector signals of successively arranged individual pixelated sensors along the length of the detector array and/or at different modulation frequencies of the illumination. Herein, acquiring the detector signals can be performed sequentially, in particular, by using a row scan and/or line scan. However, other embodiments are also possible, for example, embodiments in which especially selected individual pixel sensors are recorded simultaneously. 
     In a particular embodiment, the detector can have, thus, at least one modulation device for modulating the illumination, preferably for a periodic modulation, especially a periodic beam interrupting device. As generally used, a modulation of the illumination comprises a process in which a total power of the illumination is varied, preferably periodically, in particular with one or a plurality of modulation frequencies. In particular, a periodic modulation can be effected between a maximum value and a minimum value of the total power of the illumination. The minimum value can be 0, but can also be &gt;0, such that, by way of example, complete modulation does not have to be effected. The modulation can be effected for example in a beam path between the object and the detector array, such as by the modulation device being arranged in said beam path. Alternatively or additionally, however, the modulation can also be effected in a beam path between an optional illumination source for illuminating the object and the object, for example by the modulation device being arranged in said beam path. A combination of these possibilities is also conceivable. By way of example, the modulation device can comprise a beam chopper or some other type of periodic beam interrupting device, for example comprising at least one interrupter blade or interrupter wheel, which preferably rotates at constant speed and which can thus periodically interrupt the illumination. Alternatively or additionally, however, it is also possible to use one or a plurality of different types of modulation devices, for example modulation devices based on an electro-optical effect and/or an acousto-optical effect. Once again alternatively or additionally, the optional illumination source itself can also be designed to generate a modulated illumination, for example by said illumination source itself having a modulated intensity and/or total power, for example a periodically modulated total power, and/or by said illumination source being embodied as a pulsed illumination source, for example as a pulsed laser. Thus, by way of example, the modulation device can also be wholly or partly integrated into the illumination source. Various possibilities are conceivable. Accordingly, the detector array can be designed to detect at least two detector signals in the case of different modulations having different modulation frequencies. The evaluation device can be designed to generate the information related to the spectrum from two or more detector signals. By way of example, the detector can be designed to bring about a modulation of the illumination of the object with a frequency of 0.05 Hz to 1 MHz, such as 0.1 Hz to 10 kHz. 
     In a further aspect of the present invention, a use of a spectrometer device and a spectrometer system according to the present invention is disclosed. Therein, the use of the spectrometer device and the spectrometer system for a purpose of determining information related to a spectrum of an object is proposed. Herein, the spectrometer device and the spectrometer system may, preferably, be used for a purpose of use selected from the group consisting of: an infrared detection application; a heat-detection application; a thermometer application; a heat-seeking application; a flame-detection application; a fire-detection application; a smoke-detection application; a temperature sensing application; and a spectroscopy application. Further the spectrometer device and the spectrometer system according to the present invention can, preferably, be used to monitor exhaust gas, to monitor combustion processes, to monitor pollution, to monitor industrial processes, to monitor chemical processes, to monitor food processing processes, to assess water quality, and/or to assess air quality. Further, spectrometer devices and spectrometer systems according to the present invention may be used for quality control, temperature control, motion control, exhaust control, gas sensing, gas analytics, motion sensing, and/or chemical sensing. Further applications are feasible. 
     The above-described spectrometer device, the spectrometer system and the proposed uses have considerable advantages over the prior art. Thus, generally, a simple and, still, efficient spectrometer device and a spectrometer system for an accurate determining of information related to a spectrum of an object may be provided. Therein, as an example, an infrared spectrum of an object covering a partition of the infrared spectral range can be acquired in a fast and efficient way. As compared to devices known in the art, the spectrometer device and the spectrometer system as proposed herein provide a high degree of simplicity, specifically with regard to an optical setup of the spectrometer device. Herein, a combination of the particular optical element, the length variable filter, and the detector array as comprised by the spectrometer device may be advantageous since the particular optical element comprising an optical concentrator device having the single rounded sidewall and being operated in reverse direction is, particularly, adapted for reducing an amount of reflections when transferring the incident light to a length variable filter, in particular, by the shape of the single rounded sidewall which is devoid of any corners, whereby a higher concentration efficiency as currently available may be achieved. This high degree of simplicity, in combination with the possibility of high resolution measurements, is specifically suited for sensing, detecting and/or monitoring applications in the infrared (IR) spectral region, especially in the near-infrared (NIR) and mid-infrared (MidIR) spectral region, in particular, for sensing or detecting heat, flames, fire, or smoke, as well as monitoring exhaust gas, combustion processes, pollutions, industrial processes, chemical process, food processing processes, water quality, or air quality. Further applications are possible. 
     Summarizing, in the context of the present invention, the following embodiments are regarded as particularly preferred: 
     Embodiment 1: A spectrometer device, comprising:
         an optical element designed for receiving incident light from an object and transferring the incident light to a length variable filter, wherein the optical element comprises an optical concentrator device, wherein the optical concentrator device is operated in reverse direction, wherein the optical concentrator device has a single sidewall which is adapted for reflecting incident light, wherein the single sidewall is designed as a rounded sidewall;   the length variable filter which is designated for separating the incident light into a spectrum of constituent wavelength signals; and   a detector array comprising a plurality of pixelated sensors, wherein each of the pixelated sensors is adapted to receive at least a portion of one of the constituent wavelength signals, wherein each of the constituent wavelength signals is related to an intensity of each constituent wavelength.       

     Embodiment 2: The spectrometer device according to the preceding embodiment, wherein the optical concentrator device is operated in reverse direction for spreading out the incident light and, simultaneously, reducing an angular spread of light beams. 
     Embodiment 3: The spectrometer device according to any one of the preceding embodiments, wherein the single rounded sidewall of the optical concentrator device comprises a profile perpendicular to a longitudinal axis of the optical concentrator device, wherein the profile is, preferably, selected from a parabolic profile or an elliptical profile. 
     Embodiment 4: The spectrometer device according to any one of the preceding embodiments, wherein the optical concentrator device operated in reverse direction comprises an entrance pupil and an exit pupil. 
     Embodiment 5: The spectrometer device according to the preceding embodiments, wherein the single rounded sidewall constitutes a shell surface which is adapted to connect an entrance aperture at the entrance pupil and an exit aperture at an exit pupil as the optically guiding structure. 
     Embodiment 6: The spectrometer device according to the preceding embodiment, wherein the optical concentrator device operated in reverse direction further comprises an optically guiding structure being located between the entrance pupil and the exit pupil, wherein the entrance pupil is adapted for capturing light, wherein the optically guiding structure is adapted for concentrating the captured light, and wherein the exit pupil is adapted for emitting the concentrated light. 
     Embodiment 7: The spectrometer device according to any one of the two preceding embodiments, wherein the entrance pupil of the optical concentrator device operated in reverse direction comprises an input angle of less than 90°, preferably of less than 70°, in particular of less than 50°, and wherein the exit pupil of the optical concentrator device operated in reverse direction comprises an output angle of not more than 30°, preferably of not more than 15°, in particular of not more than 10°. 
     Embodiment 8: The spectrometer device according to any one of the three preceding embodiments, wherein the optical concentrator device operated in reverse direction has a round entrance aperture at the entrance pupil and an elongated and rounded exit aperture at the exit pupil. 
     Embodiment 9: The spectrometer device according to any one of the preceding embodiments, wherein the optical concentrator device is or comprises a full body of an optically transparent material. 
     Embodiment 10: The spectrometer device according to the preceding embodiment, wherein the optically transparent material is selected from the group consisting of calcium fluoride (CaF 2 ), fused silica, germanium, magnesium fluoride (MgF), potassium bromide (KBr), sapphire, silicon, sodium chloride (NaCl), zinc selenide (ZnSe), zinc sulfide (ZnS), borosilicate-crown glasses, transparent conducting oxides (TOO), and transparent organic polymers. 
     Embodiment 11: The spectrometer device according to any one of the preceding embodiments, wherein the optical concentrator device is or comprises a hollow body. 
     Embodiment 12: The spectrometer device according to any one the preceding embodiment, wherein a vacuum is present in the hollow body. 
     Embodiment 13: The spectrometer device according to the pre-preceding embodiment, wherein the hollow body is filled with a gaseous and/or fluid optically transparent material. 
     Embodiment 14: The spectrometer device according to the preceding embodiment, wherein the hollow body is fully and/or uniformly filled with the gaseous and/or fluid optically transparent material. 
     Embodiment 15: The spectrometer device according to any one of the two preceding embodiments, wherein the gaseous optically transparent material is selected from ambient air, nitrogen gas or carbon dioxide. 
     Embodiment 16: The spectrometer device according to any one of the three preceding embodiments, wherein the fluid optically transparent material is selected from immersion oil or Canada balsam. 
     Embodiment 17: The spectrometer device according to any one of the preceding embodiments, wherein the optical concentrator device comprises a conical shape or a non-conical shape. 
     Embodiment 18: The spectrometer device according to the preceding embodiment, wherein the non-conical shape of the optical concentrator device comprises a shape selected from a parabolic shape or an elliptical shape. 
     Embodiment 19: The spectrometer device according to the preceding embodiment, wherein the optical concentrator device is selected from the group comprising a compound parabolic concentrator and a compound elliptical concentrator. 
     Embodiment 20: The spectrometer device according to any one of the preceding embodiments, wherein the optical concentrator device has an asymmetric design with respect to an optical axis of the spectrometer device. 
     Embodiment 21: The spectrometer device according to the preceding embodiment, wherein the optical concentrator device operated in reverse direction comprises an entrance pupil and an exit pupil, wherein an entrance plane defined by the entrance pupil and an exit plane defined by the exit pupil are not parallel. 
     Embodiment 22: The spectrometer device according to any one of the two preceding embodiments, wherein the concentrator device comprises inversion symmetry and only a two-fold rotational axis. 
     Embodiment 23: The spectrometer device according to any one of the three preceding embodiments, wherein the concentrator device does not exhibit a rotational symmetry. 
     Embodiment 24: The spectrometer device according to any one of the four preceding embodiments, wherein the concentrator device comprises only a single mirror plane or no mirror plane. 
     Embodiment 25: The spectrometer device according to any one of the preceding embodiments, wherein the optical concentrator device is arranged in an asymmetric manner with respect to an optical axis of the spectrometer device. 
     Embodiment 26: The spectrometer device according to the preceding embodiment, wherein the optical concentrator device is arranged in an asymmetric manner with respect to the length variable filter. 
     Embodiment 27: The spectrometer device according to the preceding embodiment, wherein the respective arrangement of the optical concentrator device operated in reverse direction and the length variable filter comprises only a single mirror plane or no mirror plane. 
     Embodiment 28: The spectrometer device according to any one of the two preceding embodiment, wherein respective arrangement of the optical concentrator device operated in reverse direction and the length variable filter does not exhibit a rotational symmetry. 
     Embodiment 29: The spectrometer device according to any one of the three preceding embodiments, wherein the optical concentrator device is tilted with respect to a plane which is perpendicular to a receiving surface of the length variable filter. 
     Embodiment 30: The spectrometer device according to the preceding embodiment, wherein the optical concentrator device is tilted with respect to the plane perpendicular to a receiving surface of the length variable filter in a manner that the incident light which is transferred to the length variable filter impinges the length variable filter perpendicular to the receiving surface of the length variable filter on a spatial position of the length variable filter which is designed for receiving a wavelength of the incident light and/or impinges a further spatial position on the length variable filter which is designed for receiving a further wavelength which exceeds the wavelength of the incident light. 
     Embodiment 31: The spectrometer device according to any one of the preceding embodiments, wherein the detector array is separated from the length variable filter by a transparent gap. 
     Embodiment 32: The spectrometer device according to the preceding embodiment, wherein the transparent gap is obtainable by an extended transparent body having two opposing sides, wherein a plurality of interference filters which constitute the length variable filter are disposed on a first side while a series of the pixelated sensors constituting the detector array is placed on a second side opposing the first side. 
     Embodiment 33: The spectrometer device according to any one of the preceding embodiments, wherein the pixelated sensor is selected from at least one of: a pixelated organic camera element, preferably a pixelated organic camera chip; a photoconductor array, in particular an inorganic photoconductor array, especially a PbS, PbSe, Ge, InGaAs, ext. InGaAs, InSb, or HgCdTe photoconductor array; a pyroelectric, bolometer or thermopile array; a pixelated inorganic camera element, preferably a pixelated inorganic camera chip, more preferably from a CCD chip or a CMOS chip; a monochrome camera element, preferably a monochrome camera chip; a FiP sensor. 
     Embodiment 34: The spectrometer device according to any one of the preceding embodiments, wherein the incident light comprises electromagnetic radiation of 760 nm to 1000 μm (infrared spectral range). 
     Embodiment 35: The spectrometer device according to the preceding embodiment, wherein, wherein the incident light comprises electromagnetic radiation of 1 μm to 5 μm. 
     Embodiment 36: The spectrometer device according to the preceding embodiment, wherein, wherein the incident light comprises electromagnetic radiation of 1 μm to 3 μm. 
     Embodiment 37: The spectrometer device according to any one of the preceding embodiments, further comprising an illumination source adapted for illuminating the object. 
     Embodiment 38: The spectrometer device according to the preceding embodiment, wherein the illumination source is integrated or attached to the spectrometer device. 
     Embodiment 39: The spectrometer device according to any one of the two preceding embodiments, wherein the illumination source is selected from: an illumination source, which is at least partly connected to the object and/or is at least partly identical to the object; an illumination source which is designed to at least partly illuminate the object with a primary radiation. 
     Embodiment 40: The detector according to the preceding embodiment, wherein the light beam is generated by a reflection of the primary radiation on the object and/or by light emission by the object itself, stimulated by the primary radiation. 
     Embodiment 41: The detector according to the preceding embodiment, wherein the spectral sensitivities of the detector array are covered by a spectral range of the illumination source. 
     Embodiment 42: The spectrometer device according to any one of the four preceding embodiments, wherein the illumination source is selected from at least one of: an incandescent lamp; a flame source; a heat source; a laser, in particular a laser diode; a light emitting diode; an organic light source, in particular an organic light emitting diode; a neon light; a structured light source. 
     Embodiment 43: The spectrometer device according to any one of the preceding embodiments, further comprising a transfer device. 
     Embodiment 44: The spectrometer device according to the preceding embodiment, wherein the transfer device constitutes or comprises a converging optical element, wherein the converging element is at least partially optically transparent with respect to at least a partition of a wavelength range of the incident light. 
     Embodiment 45: The spectrometer device according to the preceding embodiment, wherein the converging optical element is selected from a group consisting of a converging optical lens, converging diffractive optical element and a converging curved mirror. 
     Embodiment 46: The spectrometer device according to any one of the three preceding embodiments, wherein the transfer device is located between the optical concentrator device and the length variable filter. 
     Embodiment 47: A spectrometer system, comprising
         a spectrometer device according to any one of the preceding embodiments; and   an evaluation unit designated for determining information related to a spectrum of an object by evaluating detector signals provided by the spectrometer device.       

     Embodiment 48: The spectrometer system according to the preceding embodiment, wherein the evaluation device is designed to generate the information related to the spectrum of the object from at least one predefined relationship between the location of the pixelated sensor in the detector array, the wavelength of the incident light, and the signal of the pixelated sensor. 
     Embodiment 49: The spectrometer system according to the preceding embodiment, wherein the detector signal is generated by performing at least one current-voltage measurement and/or at least one voltage-current-measurement. 
     Embodiment 50: The spectrometer system according to any one of the preceding embodiments related to the spectrometer system, further comprising an illumination source adapted for illuminating the object. 
     Embodiment 51: The spectrometer system according to the preceding embodiment, wherein the illumination source is selected from at least one of: an incandescent lamp; a flame source; a heat source; a laser, in particular a laser diode; a light emitting diode; an organic light source, in particular an organic light emitting diode; a neon light; a structured light source. 
     Embodiment 52: A use of a spectrometer device or a spectrometer system according to any one of the preceding embodiments in an infrared detection application; a heat-detection application; a thermometer application; a heat-seeking application; a flame-detection application; a fire-detection application; a smoke-detection application; a temperature sensing application; a spectroscopy application; an exhaust gas monitoring application; a combustion process monitoring application; a pollution monitoring application; an industrial process monitoring application; a chemical process monitoring application; a food processing process monitoring application; a water quality monitoring application; an air quality monitoring application; a quality control application; a temperature control application; a motion control application; an exhaust control application; a gas sensing application; a gas analytics application; a motion sensing application; a chemical sensing application. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Further optional details and features of the invention are evident from the description of preferred exemplary embodiments which follows in conjunction with the dependent claims. In this context, the particular features may be implemented alone or with features in combination. The invention is not restricted to the exemplary embodiments. The exemplary embodiments are shown schematically in the figures. Identical reference numerals in the individual figures refer to identical elements or elements with identical function, or elements which correspond to one another with regard to their functions. 
       Specifically, in the figures: 
         FIG. 1  shows an exemplary embodiment of a spectrometer system comprising a spectrometer device according to the present invention; 
         FIGS. 2A and 2B  show cross-sections through exemplary embodiments of preferred non-conical shapes of an optical concentrator device; 
         FIG. 3  shows a further cross-section through an exemplary embodiment of the optical concentrator device; 
         FIGS. 4A and 4B  show a further exemplary embodiment of the spectrometer device using an optical concentrator device in an asymmetric arrangement; and 
         FIGS. 5A and 5B  show a variation of the light transmission in a symmetric arrangement ( FIG. 5A ) and an asymmetric arrangement ( FIG. 5B ), respectively, of the optical concentrator device. 
     
    
    
     EXEMPLARY EMBODIMENTS 
       FIG. 1  illustrates, in a highly schematic fashion, an exemplary embodiment of a spectrometer system  110  which comprises a spectrometer device  112  according to the present invention. As generally used, the spectrometer device  112  is an apparatus which is capable of recording a signal intensity of incident light  114  with respect to a corresponding wavelength or a wavelength interval of the incident light  114  over a range of wavelength which is denoted as a spectrum or a partition thereof. According to the present invention, the spectrometer device  112  may, especially, be adapted for recording a spectrum in the infrared (IR) spectral region, preferably, in the near-infrared (NIR) and the mid-infrared (MidIR) spectral range, especially, wherein the incident light may have a wavelength of 1 μm to 5 μm, preferably of 1 μm to 3 μm, and can, thus, be applicable for a detection of heat, flames, fire, or smoke, wherein further applications may be feasible. Herein, the incident light  114  may be generated and/or reflected by an object  116 , which may be a living object and a non-living object, such as comprising one or more articles and/or one or more parts of an article, wherein the at least one article or the at least one part thereof may comprise at least one component which can provide a spectrum which may be suitable for investigations in the IR, especially in the NIR spectral region. 
     The exemplary spectrometer device  112  as schematically depicted in  FIG. 1  comprises a linearly variable filter  118  as a preferred example of a length variable filter. Herein, the linearly variable filter  118  is designated for separating the incident light  114  into a spectrum of constituent wavelength signals, a detector array  120  which is designed for determining respective intensities of received wavelength signals, and an optical element  122  which is designated for receiving incident light  114  from the object  116  and transferring the incident light  114  to the linearly variable filter  118 . 
     According to the present invention, the optical element  122  comprises an optical concentrator device  124 , wherein the optical concentrator device is operated in reverse direction  126 , wherein the inversely-operated optical concentrator device  124  comprises a single rounded sidewall  128 . Herein, the inversely-operated optical concentrator device  124  comprises an input  130 , an optically guiding structure  132  and an output  134 . Consequently, the incident light  114  which may be emitted or reflected by the object  116  or may have passed through the object  116  enters the inversely-operated optical concentrator device  124  at the input  130  which is designed for receiving the incident light  114 . Thereafter, the incident light  114  captured by the input  130  passes through the optically guiding structure  132  which is, preferably, designed for spreading out the incident light  114 . Finally, the incident light  114  which has been spread out in this manner is emitted by the output  134  which is being designated for this purpose. Thus, an angular spread of light beams which are emitted at the output  134  can, simultaneously, be reduced compared to the angular spread of the incident light  114 . As a result, the inversely-operated optical concentrator device  124  allows modifying the incident light  114  as provided by the object  116  in a manner that the light which is emitted at the output  134  of the inversely-operated optical concentrator device  124  exhibits a reduced angular spread. 
     Consequently, a predominant share of the light beams provided by the output  134  of the inversely-operated optical concentrator device  124  impinges the linearly variable filter  118  in a parallel manner, especially, normal to a receiving surface  136  of the linearly variable filter  118  in a perpendicular manner. As used in this exemplary embodiment, the linearly variable filter  118  is or comprises an optical filter having a plurality of interference filters which are, preferably, provided in a continuous arrangement of interference filters. Herein, each of the interference filters may form a bandpass with a variable center wavelength for each spatial position  138  on the receiving surface  136  of the linearly variable filter  118  in a manner that the variable center wavelength may be a linear function of the spatial position  138 . As exemplary shown in  FIG. 1 , the linearly variable filter  118  may, thus, be arranged, preferably continuously, along a single dimension, usually as “length” of the linearly variable filter  118 . By way of example, the linearly variable filter  118  may be a wedge filter that may carry at least one response coating  140  on a transparent substrate  142 , wherein the response coating  140  may exhibit a spatially variable property, in particular, a spatially variable thickness (not depicted here). Herein, the transparent substrate  142  may comprise at least one material that may exhibit a high degree of optical transparency in the IR spectral range which can, preferably, be selected from the group consisting of calcium fluoride (CaF 2 ), fused silica, germanium, magnesium fluoride (MgF), potassium bromide (KBr), sapphire, silicon, sodium chloride (NaCl), zinc selenide (ZnSe), zinc sulfide (ZnS), borosilicate-crown glasses, transparent conducting oxides (TOO), and transparent organic polymers, wherein CaF 2 , fused silica, MgF, KBr, sapphire, NaCl, ZnSe, ZnS, borosilicate-crown glasses, transparent conducting oxides, and selected transparent organic polymers may, especially, be applicable for the NIR spectral range. However, other embodiments of the linearly variable filter  118  may also be feasible. However, other kinds of length variable filters may also be feasible for the purposes of the present invention. 
     The linearly variable filter  118  is designated for separating the incident light  114  into a spectrum of constituent wavelength signals. For this purpose, the incident light  114  may, preferably, pass through the linearly variable filter  118  at the particular spatial position  138  which is related to the wavelength of the incident light  114 . After the incident light  114  has passed through the linearly variable filter  118  at the particular spatial position  138  related to the wavelength of the incident light  114 , it, subsequently, impinges the detector array  120 , in particular one of a plurality of pixelated sensors  144  as comprised by the detector array  120 . Thus, each of the pixelated sensors  144  receives at least a portion of one of the constituent wavelength signals as provided by the incident light  114  after having passed through the linearly variable filter  118  as described above. Moreover, each of the pixelated sensors  144  is adapted to provide a detector signal which is related to an intensity of each constituent wavelength. In other words: The spectrometer device  112  is, thus, designated to generate a plurality of detector signals based on the constituent wavelength signals, wherein each of the detector signals is related to the intensity of each constituent wavelength of the spectrum. 
     As further indicated in  FIG. 1 , the detector array  120  may, preferably, be separated from the linearly variable filter  118  by a transparent gap  146 , wherein the transparent gap  146  may, by way of example, be obtained by using the transparent substrate  142 . As a result, by selecting a suitable width for the transparent gap  146  a more precise adjustment of the detector array  120  with regard to the linearly variable filter  118  can be achieved. As indicated below in more detail, adjusting the transparent gap  146  may allow further increasing the efficiency of the spectrometer device  112 . 
     The plurality of detector signals may, as schematically depicted in  FIG. 1 , via a signal lead  148  be transmitted to an evaluation unit  150 , which may be comprised by the spectrometer system  110  in addition to the spectrometer device  110 . Herein, the evaluation unit  150  is, generally, designated for determining information related to a spectrum of the object  116  by evaluating the plurality of detector signals as provided by the detector array  120  of the spectrometer device  112 . For this purpose, the evaluation unit  150  may comprise one or more electronic devices and/or one or more software components, in order to evaluate the plurality of the detector signals, which are symbolically denoted by a signal evaluation unit  152 . Herein, the evaluation unit  150  may be adapted to determine the at least one item of information related to a spectrum of the object  116  by comparing more than one of the detector signals. 
     The incident light  114  which is received by the optical element  122  of the spectrometer device  112  may be generated by a light-emitting object  116 . Alternatively or in addition, the incident light  114  may be generated by a separate illumination source  154 , which may include an ambient light source and/or an artificial light source, in particular an incandescent lamp  156 , which may be designated for illuminating the object  116  in a manner that at least a part of the light generated by the illumination source  154  may be able to pass through the object  116  (not depicted here) and/or in a manner that the object  116  may be able to reflect at least a part of the light generated by the illumination source  154  such that the incident light  114  may be configured to be received by the optical element  122 . Herein, the illumination source  154  may be or comprise a continuously emitting light source and/or a modulated light source. As further depicted in  FIG. 1 , the illumination source  154  may be controlled by at least one illumination control unit  158  which may be adapted, if required, for providing modulated light. Herein, the illumination control unit  158  may, additionally, provide information about the illumination to the signal evaluation unit  152  and/or be controlled by the signal evaluation unit  152 , which is symbolically indicated by a connection between the illumination control unit  158  and the signal evaluation unit  152  in  FIG. 1 . Alternatively or in addition, controlling the illumination of the object  116  may be effected in a beam path between the illumination source  154  and the object  116  and/or between the object  116  and the optical element  122 . Further possibilities may be conceivable. 
     Generally, the evaluation unit  150  may be part of a data processing device  160  and/or may comprise one or more data processing devices  160 . The evaluation unit  150  may be fully or partially integrated into a housing  162  which at least comprises the spectrometer device  112  and/or may fully or partially be embodied as a separate device which may electrically be connected in a wireless or wire-bound fashion to the spectrometer device  112 . The evaluation unit  150  may further comprise one or more additional components, such as one or more electronic hardware components and/or one or more software components, such as one or more measurement units and/or one or more evaluation units and/or one or more controlling units (not depicted here). 
     As further illustrated in the exemplary embodiment of  FIG. 1 , the spectrometer device  112  comprises the optical element  122 , the linearly variable filter  118 , and the detector array  120 , which are, in this particular embodiment, arranged along an optical axis  164  of the spectrometer device  112 . Specifically, the optical axis  164  may be an axis of symmetry and/or rotation of the setup of at least one of the optical element  122 , the linearly variable filter  118 , and the detector array  120 . Especially, the optical axis  164  may, thus, be parallel to a plane which is perpendicular to the receiving surface  136  of the linearly variable filter  118 . Further, the optical element  122 , the linearly variable filter  118 , and the detector array  120  may, preferably, be located inside the housing  162  comprising at least the spectrometer device  112 . 
     In a further embodiment, at least one transfer device (not depicted here), in particular, a refractive lens, may, additionally, be placed between the optical element  122  and the linearly variable filter  118 . However, since the optical element  122  is implemented in the particular embodiment of  FIG. 1  in form of the inversely-operated optical concentrator device  124  comprising the conical shape  128 , the use of a transfer device, in particular, a refractive lens, appears to be dispensable because this implementation of the optical element  122  is capable of, concurrently, taking over the function of the transfer device, in particular, a refractive lens, especially with regard to providing a predominant share of parallel light beams which may impinge on the linearly variable filter  118  normal to the receiving surface  136  of the linearly variable filter  118  in a perpendicular manner. 
       FIGS. 2A and 2B  illustrate cross-sections through two exemplary embodiments of preferred shapes of the single rounded sidewall  128  of the inversely-operated optical concentrator device  124 .  FIG. 2A  schematically depicts a compound parabolic concentrator  166  in which the rounded shape of the single sidewall  128  of the optical concentrator device  124  comprises a parabolic shape  168  while  FIG. 2B  schematically depicts a compound elliptical concentrator  170  in which the rounded shape of the single sidewall  128  of the optical concentrator device  124  comprises an elliptical shape  172 . However, the single sidewall  128  of the inversely-operated optical concentrator device  124  may also assume a further shape which may be a single rounded shape. 
     Herein, the inversely-operated optical concentrator device  124  may be provided in form of a full body (not depicted here) of a transparent optical material having a high optical transmittance in the IR spectral range in order to enhance a reflectivity of the optical concentrator device  124 . In particular, the inversely-operated optical concentrator device  124  may be provided in form of the full body of an at least partially optically transparent material having a high degree of optical transparency in the IR spectral range which can, preferably, be selected from the group consisting of calcium fluoride (CaF 2 ), fused silica, germanium, magnesium fluoride (MgF), potassium bromide (KBr), sapphire, silicon, sodium chloride (NaCl), zinc selenide (ZnSe), zinc sulfide (ZnS), borosilicate-crown glasses, transparent conducting oxides (TOO), and transparent organic polymers, wherein CaF 2 , fused silica, MgF, KBr, sapphire, NaCl, ZnSe, ZnS, borosilicate-crown glasses, transparent conducting oxides, and selected transparent organic polymers may, especially, be applicable for the NIR spectral range, wherein silicon and germanium having high refractive indices are particularly preferred since they are capable of supporting total reflection which may occur on sidewalls of the full body. 
     However, as further illustrated in  FIGS. 2A and 2B , the inversely-operated optical concentrator device  124  may, as an alternative, be provided in form of a hollow body  174  having the single sidewall  128  which may be arranged in a manner that it constitutes the desired shape, in particular, the parabolic shape  168  of  FIG. 2A  or the elliptical shape  172  of  FIG. 2B . Again, the single sidewall  128  of the inversely-operated optical concentrator device  124  may assume a different shape  165  as long as this different shape generates the desired single sidewall  128 . For this purpose, the hollow body  174  having the single sidewall  128  may comprise a vacuum or may, preferably fully and/or uniformly, be filled with a gaseous and/or fluid optically transparent material, especially selected from ambient air, nitrogen gas, carbon dioxide, immersion oil, or Canada balsam, in order to be applicable as the optical concentrator device  124  operated in the reverse direction  126 . 
     In known optical concentrator devices the sidewalls of the hollow body  174  which constitutes the inversely-operated optical concentrator device  124  may be designed as sidewalls which are adapted to absorb such wavelengths of the incident light  114  which may deviate to a high degree from a path which can, eventually, guide the incident light to impinge the linearly variable filter  118  in a predominantly parallel manner. However, such kinds of sidewalls of the hollow body  174  may, as indicated above, diminish the efficiency of the spectrometer device  112  since an absorptive share of the incident light  114  is deterred from passing the linearly variable filter  118  and, eventually, reaching the detector array  120  and can, thus, not contribute to the detector signal. 
     Therefore, according to the present invention, the single sidewall  128  of the hollow body  174  constituting the inversely-operated optical concentrator device  124  may, as depicted in  FIGS. 1, 2A and 2B , be designated as a single reflective sidewall  128  which may be adapted for reflecting incident light  114 . As a result, the single reflective sidewall  128  may, thus, be capable of increasing the efficiency of the spectrometer device  112  by allowing additional light beams  186  to be guided, by reflection on the single reflective sidewall  128 , to the linearly variable filter  118  and, subsequently, to the detector array  120  where they, may, in addition, contribute to the detector signal. Consequently, providing the single reflective sidewall  128  which may define the hollow body  174  which implements the inversely-operated optical concentrator device  124  can, thus, further increase the efficiency of the spectrometer device  112 , in particular, by reducing the signal-to-noise ratio. 
     Further,  FIG. 3  shows a cross-section through an exemplary embodiment of a preferred inversely-operated optical concentrator device  124  according to the present invention. Herein, the single reflective sidewall  128  is generated by the hollow body  174  which may, as indicated above, comprise a vacuum or may, preferably fully and/or uniformly, be filled with a gaseous and/or fluid optically transparent material, especially selected from ambient air, nitrogen gas, carbon dioxide, immersion oil, or Canada balsam. Herein, the single reflective sidewall  128  forms a boundary which may be adapted to limit the inversely-operated optical concentrator device  124  and constitute a surface thereof. In particular, the single rounded sidewall  128  may constitute a shell surface  176  which may, specifically, be adapted to connect an entrance aperture  178  at an entrance pupil  180  and an exit aperture  182  at an exit pupil  184 . More particular, the single rounded sidewall  128  may, preferably, comprise a non-conical profile, specifically, selected from a parabolic profile or an elliptical profile. Thus, the shell surface  176  of the single rounded sidewall  128  as exemplarily illustrated in  FIG. 3  assumes a shape in three-dimensional space in a manner that the shape of the single rounded sidewall  128  is devoid of any corners. 
     In the particularly preferred embodiment as schematically depicted in  FIG. 3 , the entrance aperture  178  at the entrance pupil  180  the inversely-operated optical concentrator device  124  may, in order to be capable of catching as much light as possible, preferably comprise a small round opening having dimensions d x , d y , wherein a quotient d y /d x  may assume a value of 0.75 to 1.25, preferably of 0.9 to 1.1, in particular of 0.95 to 1.05. On the other hand, the exit aperture  182  at the entrance pupil  184  the inversely-operated optical concentrator device  124  may, in order to allow guiding as much light as possible via the length variable filter  118  onto the detector array  120  which exhibits a rectangular form in this embodiment, may comprise an elongated and rounded opening, in particular an elliptical opening having dimensions D x , D y , wherein a quotient D y /D x  may assume a value of 1.5 to 20, preferably of 2 to 10, in particular of 2.5 to 6. However, especially depending on the form of the detector array  120 , other shapes of the entrance aperture  178  and/or of the exit aperture  182  of the optical concentrator device  124  being operated in reverse direction may also be feasible. 
     Further,  FIG. 4  schematically illustrates a further exemplary embodiment of the spectrometer device  112  according to the present invention. In contrast to the exemplary embodiment of the spectrometer device  112  of  FIG. 1  which assumes a symmetric arrangement of the optical element  122 , the linearly variable filter  118 , and the detector array  120  along the optical axis  164  of the spectrometer device  112 , the optical element  122  of the spectrometer device  112  as depicted in  FIG. 4A  assumes an asymmetric arrangement  200 . 
     In addition, a transfer device (not depicted here) which may, in particular, be an optical lens, a curved mirror, a grating, or a diffractive optical element, may be arranged between the inversely-operated optical concentrator device  124  and the length linearly variable filter  118 . Preferably, the optical lens may, especially, be selected from a group consisting of a biconvex lens, a plano-convex lens, a biconcave lens, a plano-concave lens, an aspherical lens, a cylindrical lens and a meniscus lens. However, other kinds of transfer devices may also be feasible. 
     Accordingly, a symmetry axis  202  of the optical element  122  which is provided in accordance with the present invention in form of the optical concentrator device  124  arranged in reverse direction  126  and having the single rounded reflective sidewall  128  is tilted by an angle α with respect to the optical axis  164  of the spectrometer device  112  which is, as indicated above, parallel to the plane which is perpendicular to the receiving surface  136  of the linearly variable filter  118 . Herein, the angle α is chosen in view of the incident light  114  which is transferred to the linearly variable filter  118  not only impinging the linearly variable filter  118  perpendicular to the receiving surface  136  of the linearly variable filter  118  on a spatial position  138  of the linearly variable filter  118  which is designed for receiving a wavelength of the incident light  118  but, in addition, also impinging a further spatial position  138 ′ on the linearly variable filter  118  which is designed for receiving a further wavelength which exceeds the wavelength of the incident light  114 . 
     Therefore, the light beam  186  may pass the linearly variable filter  118  at a longer wavelength compared to its inherent wavelength but due to the relative arrangement between the linearly variable filter  118  and the detector array  120 , which is separated here by the transparent gap  146  as described elsewhere in this document in more detail, the light beam  186  may, still, impinge the particular pixelated sensor  144  which is provided for determining the intensity of the incident light at the particular wavelength of the incident light beam  114 . Consequently, not only light beams  186  which impinge the linearly variable filter  118  normal to the receiving surface  136  of the linearly variable filter  118  at the spatial position  138  on the linearly variable filter  118  which is designated for this purpose but also light beams  186 ′ which impinge the linearly variable filter  118  on the further spatial position  138 ′ which is designated for receiving the longer wavelength compared to the wavelength of the incident light  144 , still, impinge the same particular pixelated sensor  144  which is designed for receiving the particular wavelength of the incident light  114 . As a result, also the light beams  186 ′ may, thus, contribute to the electrical signals as generated by the particular individual pixelated sensor  144 , whereby the efficiency of the spectrometer device  112  can further be increased. 
     As schematically shown in  FIG. 4B , the asymmetric arrangement  200  of the optical element  122  within the spectrometer device  112  as depicted in  FIG. 4A  may provide additional advantages. As illustrated herein, each of three different light beams  186 ,  186 ′,  186 ″ having three different wavelengths may generate a detector signal  204 ,  204 ′,  204 ″ in the corresponding pixelated sensor  144  of the detector array  120  having, within a defined tolerance level, the same intensity irrespective of their wavelength. This advantage may even be achieved when using an incandescent lamp  156  as the illumination source  158  which can be considered as a thermal emitter within the IR spectral range and, therefore exhibits an emission power which decreases with increasing wavelength. However, this effect may be outweighed by the asymmetric arrangement  200  of the optical element  122 , wherein a longer path for the light beam  186  having the shorter wavelength compared to a light beam  186 ″ having the longer wavelength may be provided. 
     In addition, further effects which may influence the intensity of the light beam  186 ,  186 ′,  186 ″ at the pixelated sensor  144  may be outweighed in this manner. Especially, known IR absorption materials exhibit a tendency of increased absorption with increasing wavelength. Further, since a bandpass width of the linearly variable filter  118  typically, assumes a constant value, such as 1%, over the spectral range of the linearly variable filter  118 , the resolution of the linearly variable filter  118  being inversely proportional to the bandpass width, also decreases with increasing wavelength. Further, the resolution of the linearly variable filter  118 , in general, depends on the center wavelength of the linearly variable filter. However, the asymmetric arrangement  200  of the optical element  122  may facilitate the spectrometer device  112  to be more receptive for longer wavelengths while the higher emission power of the incandescent lamp  158  and the lower bandpass width at shorter wavelengths may result in an efficiency of the spectrometer device  112  being more equally distributed over its wavelength range. 
     The efficiency of the spectrometer device  112  can even further be increased since the single rounded sidewall  128  of the inversely-operated optical concentrator device is designed as a single reflective sidewall. As schematically shown in  FIG. 4B , the single reflective sidewall  128  may allow additional light beams  186 * to be guided, by reflection on the single reflective sidewall, to the linearly variable filter  118  and, subsequently, to the detector array  120  where they, may, in addition, contribute to the detector signal. 
       FIGS. 5A and 5B  each schematically show a variation of a transmission rate, which can be defined by a relative intensity I/I 0  of transmission peaks  206 , as a function of a wavelength A [nm] of the corresponding transmission peak  206  after transmission of the incident light  114  through the linearly variable filter  118  of the spectrometer device  112  as determined by a respective simulation. Herein, a symmetric arrangement  208  of the optical concentrator device  124  with respect to the optical axis  164  of the spectrometer device  112  as, for example, depicted in  FIGS. 1, 2A and 2B , has been chosen for determining the transmission peaks  206  of  FIG. 5A  whereas the asymmetric arrangement  200  of the optical concentrator device  124  as shown in  FIGS. 4A and 4B  has been used for obtaining the transmission peaks  206  as displayed in  FIG. 5B . 
     As a result of the asymmetric arrangement  200  of the optical concentrator device  124 , a shift of the maximum of the relative intensity I/I 0  of the transmission peaks  206  and of the focus of their angle α towards longer wavelengths can be observed. In addition, the transmission peaks  206  appear to have a smaller peak width  210 , in particular, a smaller peak width  210  at half-height, in the asymmetric arrangement  200  of  FIG. 5B  compared to the peak width  210 ′ at half-height in the symmetric arrangement  208  of  FIG. 5A , thus, resulting in a reduction of the bandpass width in the asymmetric arrangement  200 . By way of example, the peak width  210  at half-height and, therefore, the bandpass width can be reduced by approx. 5% for the transmission peak  212  having the longest wavelength. Further, the spectrum which can completely be acquired with the respective spectrometer device  112  can, thus, be estimated to slightly increase from a first spectral range  214  of 1270 nm to 2340 nm in the symmetric arrangement  208  as used for  FIG. 5A  to a second range  216  of 1230 nm to 2390 nm in the asymmetric arrangement  200  as used for  FIG. 5B . 
     Summarizing, the transmission rate stronger increases towards longer wavelengths and, also, stronger decreases towards smaller wavelengths in the asymmetric arrangement  208  compared to the asymmetric arrangement  200  of the optical concentrator device  124 , thus, facilitating the spectrometer device  112  to exhibit a higher reception for longer wavelengths. However, as indicated above, this effect can easily be outweighed by using an incandescent lamp  158  having a higher emission power whereby, eventually, a spectrometer device  112  having an efficiency which may be more equally distributed over its wavelength range may be provided in this manner. 
     LIST OF REFERENCE NUMBERS 
     
         
           110  spectrometer system 
           112  spectrometer device 
           114  incident light 
           116  object 
           118  linearly variable filter as a preferred example of a length variable filter 
           120  detector array 
           122  optical element 
           124  inversely-operated optical concentrator device 
           126  reverse direction 
           128  single rounded sidewall 
           130  input 
           132  guiding structure 
           134  output 
           136  receiving surface 
           138 ,  138 ′ spatial position 
           140  response coating 
           142  transparent substrate 
           144  pixelated sensor 
           146  transparent gap 
           148  signal lead 
           150  evaluation unit 
           152  signal evaluation unit 
           154  illumination source 
           156  incandescent lamp 
           158  illumination control unit 
           160  data processing device 
           162  housing 
           164  optical axis 
           166  compound parabolic concentrator 
           168  parabolic shape 
           170  compound elliptical concentrator 
           172  elliptical shape 
           174  hollow body 
           176  shell surface 
           178  entrance aperture 
           180  entrance pupil 
           182  exit aperture 
           184  exit pupil 
           186 ,  186 ′,  186 ″,  186 * light beam 
           200  asymmetric arrangement 
           202  symmetry axis 
           204 ,  204 ,  204 ″ detector signal 
           206  transmission peak 
           208  symmetric arrangement 
           210 ,  210 ′ peak width (at half-height) 
           212  transmission peak having the longest wavelength 
           214  first spectral range 
           216  second spectral range