Method for determining at least one calibrated optical property

Abstract

A method and a spectrometer device for determining at least one calibrated optical property of at least one object by using a spectrometer device are disclosed. The spectrometer device comprises: - at least one light source configured for generating illumination light; - at least one detector configured for detecting detection light; - at least one sample interface configured for allowing illumination of the object with the illumination light and configured for allowing the detection light from the object to propagate to the detector; - at least one first optical path, wherein the first optical path is configured for allowing the illumination light generated by the light source to propagate to the detector by passing the sample interface at least once; - at least one second optical path, wherein the second optical path is configured for allowing the illumination light generated by the light source to propagate to the detector without passing the sample interface, wherein the second optical path receives stray light from the first optical path; - at least one evaluation unit configured for evaluating detector signals generated by the detector. The method comprises: i. providing at least one item of reference target calibration information and at least one item of stray light correction information, wherein the item of reference target calibration information comprises a relation between a detector signal in the first optical path and a detector signal in the second optical path having at least one reference target of known reflectivity applied to the sample interface, and wherein the item of stray light correction information comprises at least one relation between a signal in the second optical path and a reflectivity of a sample applied to the sample interface; ii. illuminating the first optical path and the second optical path with the illumination light generated by the light source; iii. determining, by using the detector, at least one first detector signal from the first optical path having the at least one object applied to the sample interface; iv. determining, by using the detector, at least one second detector signal S2 from the second optical path; and v. evaluating the first detector signal and the second detector signal S2 using the item of reference target calibration information to obtain at least one first optical property estimate of the object; and vi. determining the at least one calibrated optical property of the object by using the item of reference target calibration information, the item of stray light correction information and the first reflectivity estimate of the object, wherein the calibrated optical property is corrected for stray light in the second optical path.

Description

[0001] Method for determining at least one calibrated optical property Technical FieldThe invention relates to a method for determining at least one calibrated optical property of atleast one object by using a spectrometer device and to a spectrometer device. The inventionfurther relates to a computer program and a computer-readable storage medium for performingthe method. Such devices and methods can, in general, be used for investigating or monitoringpurposes, in particular, in the infrared (IR) spectral region, especially in the near-infrared (NIR)spectral region, and in the visible (VIS) spectral region, e.g. in a spectral region allowing tomimic a human's ability of color sight. However, further applications are feasible.Background art Spectrometer devices are known to be efficient tools for obtaining information on the spectral properties of an object, when emitting, irradiating, reflecting and / or absorbing light. Spectrome- ter devices, thus, may assist in analyzing samples or other tasks in which information on thespectral properties of an object is of interest. In order to allow comparing spectra from multiplespectrometer devices, the spectrometer devices have to be calibrated. In general, spectrometerdevise require a wavelength or wavenumber calibration, e.g. a calibration of the x-axis of a rec-orded spectrum, and a calibration of the signal, reflectance, transmittance and / or absorbance, i.e. a calibration of the y-axis of the recorded spectrum. For example, calibrations of the y-axisof the recorded spectrum for reflective measurements, which is generally required in the field ofdiffusive reflective near-infrared spectroscopy, may use external reflection standards which areplaced at a sample position. Alternatively, the calibration may use internal calibration targetswhich are automatically moved into a measurement field by the spectrometer device itself tocalibrate the spectrometer´s response.Thus, typically, spectrometer devices are calibrated using reference standards. However, in or-der to make the calibration less error-prone or more user-friendly, especially in the consumermarket, it is preferred that calibrations may be performed without user interaction. For example,state of the art solutions use factory calibrations and internal reference targets for the calibrationprocess.WO 2023 / 161416 A1 discloses method of calibrating a spectral sensing device. The spectralsensing device comprises: at least one detector element configured for generating at least onedetector signal in response to an illumination of the detector element by incident light; at leastone wavelength-selective element configured for transferring incident light within at least oneselected wavelength range onto the detector element; at least one light source configured foremitting light in at least one optical spectral range; at least one sample interface configured forallowing light from the light source to illuminate at least one sample and configured for allowinglight from the sample to propagate via the wavelength-selective element to the detector ele-ment; at least one first optical path, wherein the first optical path is configured for allowing lightemitted from the light source to propagate via the wavelength-selective element to the detectorelement without passing the sample interface; at least one second optical path, wherein thesecond optical path is configured for allowing light emitted from the light source to propagate viathe wavelength-selective element to the detector element by passing the sample interface atleast once. The method comprises: illuminating the detector element via the at least one firstoptical path to obtain at least one first detector signal; illuminating the detector element via the at least one second optical path with no sample applied to the sample interface to obtain atleast one open port detector signal; illuminating the detector element via the at least one secondoptical path with at least one calibration sample applied to the sample interface to obtain at leastone calibration detector signal; and determining at least one item of calibration information byusing the first detector signal, the open port detector signal and the calibration detector signal. Further, disclosed is a method of determining at least one calibrated optical property of at least one sample, a spectral sensing device and computer programs and computer-readable storage media for performing the methods.US2020056939A1 discloses calibrating a spectrometer module by performing measurementsusing the spectrometer module to generate wavelength-versus-operating parameter calibration data for the spectrometer module, performing measurements using the spectrometer module to generate optical crosstalk and dark noise calibration data for the spectrometer module, and per- forming measurements using the spectrometer module to generate full system response cali- bration data, against a known reflectivity standard, for the spectrometer module. The method further includes storing in memory, coupled to the spectrometer module, a calibration recordthat incorporates the wavelength-versus-operating parameter calibration data, the optical cross-talk and dark noise calibration data, and the full system response calibration data, and applying the calibration record to measurements by the spectrometer module. Despite the advantages achieved by known methods and devices, several technical challengesremain. Specifically, known factory calibrations may rely on a factor between a detector signalgenerated for a second light source interacting with an internal reference target and a detectorsignal generated for a primary light source during an open port measurement. A referencestandard may be assumed for the sample plane. Such methods may not account for detectornon-linearity, specifically with respect to a dependency of a detector response to the detectedirradiance. Further, such methods may experience electrical and / or optical crosstalk betweenthe two calibration light sources. For example, such methods may assume that the detector re-sponse to the second light source does not depend on an interaction of the generated light withthe sample plane. Under realistic conditions this assumption may not be fulfilled, either due tothe setup of the spectrometer device or due to the crosstalk between the light sources, such asa direct or an indirect crosstalk, e.g. via the detector, which may depend on the sample planeand / or on the beam paths. Thus, an internal calibration with a reference light source may work ifno stray light is in the system. But due to light source crosstalk from the first optical path to the second optical path, the detector signal in the second optical path providing the internal calibra- tion may show a direct or indirect dependence of the reflectivity of the sample, which may result in a worse contrast determined from spectral information, such as absorbance or reflectance. Problem to be solvedIt is therefore desirable to provide methods and devices which at least partially address above-identified technical challenges. Specifically, a method for determining at least one calibrated op-tical property of at least one object by using a spectrometer device and a spectrometer deviceshall be proposed which at least partially account for stray light effect in the calibration paths. SummaryThis problem is addressed by a method for determining at least one calibrated optical propertyof at least one object by using a spectrometer device, a spectrometer device, a computer pro-gram and a computer-readable storage medium with the features of the independent claims.Advantageous embodiments which might be realized in an isolated fashion or in any arbitrary combinations are listed in the dependent claims as well as throughout the specification.In a first aspect of the present invention, a method for determining at least one calibrated opticalproperty of at least one object by using a spectrometer device is disclosed.The term “determining” as used herein is a broad term and is to be given its ordinary and cus- tomary meaning to a person of ordinary skill in the art and is not to be limited to a special or customized meaning. The term specifically may refer, without limitation, to a process of generat- ing at least one representative result, in particular, by evaluating the at least one measurementor detector signal as acquired the spectrometer device.The term “optical property” as used herein is a broad term and is to be given its ordinary and customary meaning to a person of ordinary skill in the art and is not to be limited to a special or customized meaning. The term specifically may refer, without limitation, to an item of infor-mation, e.g. on at least one object and / or radiation emitted by at least one object, characterizingat least one spectroscopic property of the object, more specifically at least one item of infor-mation characterizing, e.g. qualifying and / or quantifying, at least one of a transmission, an ab-sorption, a reflection and an emission of the at least one object. As an example, the at least oneoptical property may comprise at least one intensity information, e.g. information on an intensityof light being at least one of transmitted, absorbed, reflected or emitted by the object, e.g. as afunction of a wavelength or wavelength sub-range over one or more wavelengths, e.g. over a range of wavelengths. Specifically, the intensity information may correspond to or be derived from the signal intensity, specifically the electrical signal, recorded by the spectrometer devicewith respect to a wavelength or a range of wavelengths of the spectrum. As an example, the op-tical property of the object may comprise at least one of an absorbance of the object and a re- flectivity of the object. The term “calibrated optical property” as used herein is a broad term and is to be given its ordi- nary and customary meaning to a person of ordinary skill in the art and is not to be limited to a special or customized meaning. The term specifically may refer, without limitation, to an optical property being calibrated using one or more calibration processes. The calibration may specifi- cally comprise a process of correcting, adjusting and / or compensating measurement signals atthe spectrometer device. The calibration process may comprise using at least one item of cali-bration information, which may comprise at least one item of information on an envisaged resultof the calibration process, such as a calibration function, a calibration factor, a calibration matrixor the like, and may be used for transforming one or more measured values into one or morecalibrated or “true” values. The calibration may comprise at least one two-step process,wherein, in a first step, information on a relation of a measurement signal of the spectrometerdevice to a known calibration standard, specifically to an external calibration target, is deter-mined, wherein, in a second step, this information is used for correcting and / or adjusting themeasurement signal of the spectrometer device, e.g. in order to reduce, minimize and / or elimi-nate deviations of the measurement signal from the known calibration standard. Thus, the cali-bration may comprise applying the item of calibration information, for example to a measure-ment signal and / or to a measurement spectrum of the spectrometer device. The calibration ofthe spectrometer device may comprise at least one of a stray light and an intensity calibration.The term “object” as used herein is a broad term and is to be given its ordinary and customary meaning to a person of ordinary skill in the art and is not to be limited to a special or customized meaning. The term specifically may refer, without limitation, to 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 spec- trum suitable for investigations. Additionally or alternatively, the object may be or may compriseone or more living beings and / or one or more parts thereof, such as one or more body parts of ahuman being, e.g. a user, and / or an animal. The object specifically may comprise at least one sample which may fully or partially be analyzed by spectroscopic methods. As an example, the object may be or may comprise at least one of: human or animal skin; edibles, such as fruits; plastics and textile.The term “spectrometer device” as used herein is a broad term and is to be given its ordinaryand customary meaning to a person of ordinary skill in the art and is not to be limited to a spe- cial or customized meaning. The term specifically may refer, without limitation, to an optical de- vice configured for acquiring at least one item of spectral information on at least one object. Specifically, the at least one item of spectral information may refer to the at least one opticalproperty which, specifically, may be determined as a function of a wavelength, for one or moredifferent wavelengths. More specifically, the optical property or optically measurable property,as well as the at least one item of spectral information, may relate to at least one property char- acterizing at least one of a transmission, an absorption, a reflection and an emission of the atleast one object, either by itself or after illumination with external light. The at least one optical property may be determined for one or more wavelengths. The spectrometer device specificallymay form an apparatus which is capable of recording a signal intensity with respect to the corre-sponding wavelength of a spectrum or a partition thereof, such as a wavelength interval,wherein the signal intensity may, specifically, be provided as an electrical signal which may beused for further evaluation. The spectrometer device, as an example, may be or may comprise a device which allows for a measurement of at least one spectrum, e.g. for the measurement of a spectral flux, specifically as a function of a wavelength or detection wavelength. The spectrum may be acquired, as an example, in absolute units or in relative units, e.g. in relation to at least one reference measure-ment. Thus, as an example, the acquisition of the at least one spectrum specifically may be per-formed either for a measurement of the spectral flux (unit W / nm) or for a measurement of aspectrum relative to at least one reference material (unit 1), which may describe the property of a material, e.g., reflectance over wavelength. Additionally or alternatively, the reference meas-urement may be based on a reference light source, an optical reference path, a calculated refer-ence signal, e.g. a calculated reference signal from literature, and / or on a reference device.Specifically, the at least one spectrometer device may be a diffusive reflective spectrometer de-vice configured for acquiring spectral information from the light which is diffusively reflected bythe at least one object, e.g. at least one sample. Additionally or alternatively, the at least onespectrometer device may be or may comprise an absorption- and / or transmission spectrometer.In particular, measuring a spectrum with the spectrometer device may comprise measuring ab-sorption in a transmission configuration. Specifically, the spectrometer device may be config-ured for measuring absorption in a transmission configuration. However, other types of spec-trometer devices are also feasible. The at least one spectrometer device, specifically and as will be outlined in further detail below,may comprise at least one light source which, as an example, may be at least one of a tunablelight source, a light source having at least one fixed emission wavelength and a broadband light source. The spectrometer device, as will be outlined in further detail below, further comprises at least one detector device configured for detecting light, such as light which is at least one of transmitted, reflected or emitted from the at least one object. The spectrometer device further may comprise, as will be outlined in further detail below, at least one wavelength-selective ele-ment, such as at least one of a grating, a prism and a filter, e.g. a length variable filter havingvarying transmission properties over its lateral extension. The wavelength-selective element may be used for separating incident light into a spectrum of constituent wavelength signals whose respective intensities are determined by employing a detector such as a detector having a detector array as described below in more detail. The spectrometer device, specifically, may be a portable spectrometer device. The term “porta-ble” as used herein is a broad term and is to be given its ordinary and customary meaning to aperson of ordinary skill in the art and is not to be limited to a special or customized meaning. The term specifically may refer, without limitation, to the property of at least one object of being moved by human force, such as by a single user. Specifically, the object characterized by the term “portable” may have a weight not exceeding 10 kg, specifically not exceeding 5 kg, more specifically not exceeding 1 kg or even not exceeding 500 g. Additionally or alternatively, the di- mensions of the object characterized by the term “portable” may be such that the object extends by no more than 0.3 m into any dimension, specifically by no more than 0.2 m into any dimen-sion. The object, specifically, may have a volume of no more than 0.03 m3, specifically of nomore than 0.01 m3, more specifically no more than 0.001 m3or even no more than 500 mm3. In particular, as an example, the portable spectrometer device may have dimensions of e.g.10 mm by 10 mm by 5 mm. Specifically, the portable spectrometer device may be part of a mo-bile device or may be attachable to a mobile device, such as a notebook computer, a tablet, acell phone, such as a smart phone, a smartwatch and / or a wearable computer, also referred toas “wearable”, e.g. a body borne computer such as a wrist band or a watch. In particular, aweight of the spectrometer device, specifically the portable spectrometer device, may be in the range from 1 g to 100 g, more specifically in the range from 1 g to 10 g. The spectrometer device specifically may be configured for acquiring at least one spectrum or at least a part of a spectrum of detection light propagating from the object to the spectrometer. The spectrum may describe the radiometric unit of spectral flux, e.g. given in units of watt per nanometer (W / nm), or other units, e.g. as a function of the wavelength of the detection light. Thus, the spectrum may describe the optical power of light, e.g. in the NIR spectral range, in a specific wavelength band. The spectrum may contain one or more optical variables as a func- tion of the wavelength, e.g. the power spectral density, electric signals derived by optical meas-urements and the like. The spectrum may indicate, as an example, the power spectral densityand / or the spectral flux of the object, e.g. of a sample, e.g. relative to a reference sample, such as a transmittance and / or a reflectance of the object, specifically of the sample. The spectrum, as an example, may comprise at least one measurable optical variable or prop-erty of the detection light and / or of the object, specifically as a function of the illumination lightand / or the detection light. As an example, the at least one measurable optical variable or prop-erty may comprise at least one at least one radiometric quantity, such as at least one of a spec-tral density, a power spectral density, a spectral flux, a radiant flux, a radiant intensity, a spectralradiant intensity, an irradiance, a spectral irradiance. Specifically, as an example, the spectrom-eter device, specifically the detector, may measure the irradiance in Watt per square meter(W / m2), more specifically the spectral irradiance in Watt per square meter per nanometer (W / m2 / nm). Based on the measured quantity the spectral flux in Watt per nanometer (W / nm)and / or the radiant flux in Watt (W) may be determined, e.g. calculated, by taking into account anarea of the detector.The spectrometer device comprises:- at least one light source configured for generating illumination light;- at least one detector configured for detecting detection light;- at least one sample interface configured for allowing illumination of the object with the illu-mination light and configured for allowing the detection light from the object to propagateto the detector;- at least one first optical path, wherein the first optical path is configured for allowing theillumination light generated by the light source to propagate to the detector by passing the sample interface at least once;- at least one second optical path, wherein the second optical path is configured for allow-ing the illumination light generated by the light source to propagate to the detector without passing the sample interface, wherein the second optical path receives stray light from the first optical path;- at least one evaluation unit configured for evaluating detector signals generated by the de-tector.The term “light” as used herein is a broad term and is to be given its ordinary and customarymeaning to a person of ordinary skill in the art and is not to be limited to a special or customizedmeaning. The term specifically may refer, without limitation, to electromagnetic radiation in oneor more of the infrared, the visible and the ultraviolet spectral range. Herein, the term “ultravioletspectral range”, generally, refers to electromagnetic radiation having a wavelength of 1 nm to380 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”, gener-ally, refers to a spectral range of 380 nm to 760 nm. The term “infrared spectral range” (IR) gen-erally refers to electromagnetic radiation of 760 nm to 1000 μm, wherein the range of 760 nm to1.5 μm is usually denominated as “near infrared spectral range” (NIR) while the range from1.5 μm to 15 μm is denoted as “mid infrared spectral range” (MidIR) and the range from 15 μmto 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 nearinfrared (NIR) and / or the mid infrared spectral range (MidIR), especially the light having a wave-length of 1 μm to 5 μm, preferably of 1 μm to 3 μm. This is due to the fact that many materialproperties or properties on the chemical constitution of many objects may be derived from thenear infrared spectral range. It shall be noted, however, that spectroscopy in other spectralranges is also feasible and within the scope of the present invention.Consequently, the term “light source”, as used herein is a broad term and is to be given its ordi-nary and customary meaning to a person of ordinary skill in the art and is not to be limited to aspecial or customized meaning. The term specifically may refer, without limitation, to an arbi-trary device configured for generating or providing light in the sense of the above-mentioneddefinition. The light source specifically may be or may comprise at least one electrical lightsource, such as an electrically driven light source. In spectroscopy, various sources and paths of light are to be distinguished. In the context of the present invention, a nomenclature is used which, firstly, denotes light propagating from the light source to the object as “illuminating light” or “illumination light”. Secondly, light propagating from the object to the detector is denoted as “detection light”. The detection light may comprise at least one of illumination light reflected by the object, illumination light scattered by the object, illumination light transmitted by the object, luminescence light generated by the object, e.g.phosphorescence or fluorescence light generated by the object after optical, electrical or acous-tic excitation of the object by the illumination light or the like. Thus, the detection light may di-rectly or indirectly be generated through the illumination of the object by the illumination light.As an example, the light source may comprise at least one light-emitting diode (LED). Alterna- tively or additionally, the light source may comprise at least one LED for generating primary light and at least one luminescent material for light-conversion of primary light generated by the light-emitting diode, wherein, specifically, the illumination light may be a combination of the primarylight and light generated by the light-conversion by the luminescent material or light generatedby the light conversion of the luminescent material, also referred to as secondary light. Otherlight sources, such as incandescence light sources, electric discharge light sources, gas-dis-charge light sources or the like, are in principle also feasible. As further outlined above, the spectrometer device comprises the at least one detector config-ured for detecting detection light. The verb “to detect” as used herein is a broad term and is tobe given its ordinary and customary meaning to a person of ordinary skill in the art and is not tobe limited to a special or customized meaning. The term specifically may refer, without limita-tion, to the process of at least one of determining, measuring and monitoring at least one pa-rameter, qualitatively and / or quantitatively, such as at least one of a physical parameter, achemical parameter and a biological parameter. Specifically, the physical parameter may be ormay comprise an electrical parameter. Consequently, the term “detector” as used herein is abroad term and is to be given its ordinary and customary meaning to a person of ordinary skill inthe art and is not to be limited to a special or customized meaning. The term specifically mayrefer, without limitation, to an arbitrary device configured for detecting, i.e. for at least one of de-termining, measuring and monitoring, at least one parameter, qualitatively and / or quantitatively,such as at least one of a physical parameter, a chemical parameter and a biological parameter.The detector may be configured for generating at least one detector signal, more specifically atleast one electrical detector signal, such as an analogue and / or a digital detector signal, the de-tector signal providing information on the at least one parameter measured by the detector. Thedetector signal may directly or indirectly be provided by the detector to the evaluation unit, suchthat the detector and the evaluation unit may be directly or indirectly connected. The detectorsignal may be used as a “raw” detector signal and / or may be processed or preprocessed beforefurther used, e.g. by filtering and the like. Thus, the detector may comprise at least one pro-cessing device and / or at least one preprocessing device, such as at least one of an amplifier,an analogue / digital converter, an electrical filter and a Fourier transformation.In the present case, the detector is configured for detecting light propagating from the object to the spectrometer device or more specifically to the detector of the spectrometer device, which, according to the above-mentioned nomenclature, is referred to as “detection light”. Thus, specif-ically, the detector may be or may comprise at least one optical detector. The optical detectormay be configured for determining at least one optical parameter, such as an intensity and / or a power of light by which at least one sensitive area of the detector is irradiated. More specifically, the optical detector may comprise at least one photosensitive element and / or at least one opti-cal sensor, such as at least one of a photodiode, a photocell, a photosensitive resistor, a photo-transistor, a thermophile sensor, a photoacoustic sensor, a pyroelectric sensor, a photomulti-plier and a bolometer. The detector, thus, may be configured for generating at least one detec-tor signal, more specifically at least one electrical detector signal, in the above-mentionedsense, providing information on at least one optical parameter, such as the power and / or inten-sity of light by which the detector or a sensitive area of the detector is illuminated.The detector may comprise one single optically sensitive element or area or a plurality of opti- cally sensitive elements or areas. Specifically, the detector may be or may comprise at least one detector array, more specifically an array of photosensitive elements. Each of the photosen- sitive elements may comprise at least a photosensitive area which may be adapted for generat- ing an electrical signal depending on the intensity of the incident light, wherein the electrical sig-nal may, in particular, be provided to the evaluation unit, as will be outlined in further detail be-low. The photosensitive area as comprised by each of the optically sensitive elements may, es-pecially, be a single, uniform photosensitive area which is configured for receiving the incidentlight which impinges on the individual optically sensitive elements. However, other arrange- ments of the optically sensitive elements may also be conceivable.The array of optically sensitive elements may be designed to generate detector signals, prefera-bly electronic signals, associated with the intensity of the incident light which impinges on theindividual optically sensitive elements. The detector signal may be an analogue and / or a digitalsignal. The electronic signals for adjacent pixelated sensors can, accordingly, be generatedsimultaneously 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 correspondto the series of the individual optically sensitive elements which are arranged in a line. In addi-tion, the individual optically sensitive elements may, preferably, be active pixel sensors whichmay be adapted to amplify the electronic signals prior to providing it to the evaluation unit. Forthis purpose, the detector 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 elec-tronic signals.In case the detector comprises an array of optically sensitive elements, the detector, as an ex- ample, 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 alterna-tive, the detector generally may be or comprise a photoconductor, in particular an inorganicphotoconductor, especially PbS, PbSe, Ge, InGaAs, ext. InGaAs, InSb, or HgCdTe. As a further alternative it may comprise at least one of pyroelectric, bolometer or thermophile detector ele-ments. Thus, a camera chip having a matrix of 1 x N pixels or of M x N pixels may be used here,wherein, as an example, M may be < 10 and N may be in the range from 1 to 50, preferably from 2 to 20, more preferred from 5 to 10. Further, a monochrome camera element, preferably a monochrome camera chip, may be used, wherein the monochrome camera element may be dif-ferently selected for each optically sensitive element, especially, in accordance with the varyingwavelength along the series of the optical sensors. Thus, the array may be adapted to provide a plurality of the electrical signals which may be genrated by the photosensitive areas of the optically sensitive elements comprised by the array. The electrical signals as provided by the array of the spectrometer device may be forwarded tothe evaluation unit.As outlined above, the spectrometer device comprises the at least one sample interface. Theterm “sample interface” as used herein is a broad term and is to be given its ordinary and cus-tomary meaning to a person of ordinary skill in the art and is not to be limited to a special orcustomized meaning. The term specifically may refer, without limitation, to a port of the spec- trometer device through which the illumination light can leave the spectrometer device, e.g. forthe purpose of illuminating the at least one object, and / or through which the detection light canenter the spectrometer device, specifically for the purpose of the spectral sensing. The sampleinterface, as an example, may define an optical plane, e.g. a plane either material or imaginary,of the spectrometer device, through which the illumination light from the first optical path, as willbe explained in further detail below, may travel to reach the object and / or through which the de-tection light from the object may travel to reach the detector, e.g. to generate a detector signal.The sample interface may be a fictional plane The sample interface may or may not be consti-tuted by a physical element and / or barrier, such as a transparent element, e.g. a glass or quartzwindow. The sample interface may also be the sample surface itself or a plane where the sam-ple can be placed or aligned. As an example, the sample interface may be or may comprise atleast one element comprising at least one transparent material being at least partially transpar-ent in the optical spectral range, such as in at least one partition of the optical spectral range orin the full optical spectral range. The sample interface may be configured for transmitting light inthe optical spectral range. The sample interface may be arranged in an optical path of the spec-trometer device, specifically in the first optical path, to allow the illumination light emitted fromthe light source to illuminate the object placed in front of the spectrometer device, specifically infront of the sample interface. The transparent material may, as an example, comprise one ormore of a glass material, such as silica, soda lime, borosilicate or the like, and / or a polymericmaterial, such as polymethylmethacrylate or polystyrene. The spectrometer device further comprises, as outlined above, the at least one first optical path and the at least one second optical path. The term “optical path” as used herein is a broad termand is to be given its ordinary and customary meaning to a person of ordinary skill in the art andis not to be limited to a special or customized meaning. The term specifically may refer, withoutlimitation, to a trajectory of light being at least partially located in the spectrometer device. Theoptical path of light in the spectrometer device may be affected by reflection, refraction, disper-sion and / or absorption at one or more optical elements, such as lenses, prisms, mirrors, grat-ings or the like, comprised by the spectrometer device. The terms “first” and “second”, as gener-ally used herein, are used for nomenclature, only, without implying any ranking or numbering.The term “first optical path” as used herein is a broad term and is to be given its ordinary and customary meaning to a person of ordinary skill in the art and is not to be limited to a special or customized meaning. The term specifically may refer, without limitation, to an optical path with interaction of the illumination light at the object. Specifically, a detector signal obtained via the first optical path may be affected from a presence and / or an absence of the object at the spec-trometer device, specifically at the sample interface of the spectrometer device. For example, adetector signal obtained via the first optical path having the object applied to the spectrometer device may be different from a detector signal obtained via the first optical path having no object applied to the spectrometer device, specifically irrespective of constant environmental condi- tions. In particular, the first optical path may be configured for allowing the illumination lightemitted from the light source to propagate to the detector by passing the sample interface atleast once. Specifically, the first optical path may allow the illumination light emitted from thelight source to propagate to the sample interface and the detection light to propagate from the sample to the detector, optionally, via a wavelength-selective element, such as an optical filter.Via the first optical path, the illumination light emitted from the light source may be guided di-rectly or indirectly, such as by reflection, refraction and / or dispersion, to the sample interface.The first optical path may be partially arranged outside the spectrometer device, such as out-side a housing of the spectrometer device. Specifically, the illumination light in the first opticalpath may leave the spectrometer device, in particular a housing of the spectrometer device, atthe sample interface to illuminate the object arranged outside the spectrometer device. The firstoptical path may be configured for coupling the detection light reflected at the object back intothe spectrometer device. The detection light may be guided from the sample interface directly orindirectly, such as by reflection, refraction and / or dispersion, to the detector, optionally via awavelength-selective element, such as an optical filter. A reflection at the sample interface maycomprise a diffuse reflection. Specifically, the detection light in the first optical path illuminatingthe optical filter and subsequently the detector may be light diffusively reflected at the object.The detector may be configured for generating at least one detector signal in response to an il-lumination by incident light via the first optical path.The term “passing the sample interface” as used herein is a broad term and is to be given its or-dinary and customary meaning to a person of ordinary skill in the art and is not to be limited to aspecial or customized meaning. The term specifically may refer, without limitation, to one or more of propagating to the sample interface and from the sample interface, traversing the sam- ple interface, and interacting with the sample interface. The emitted light may impinge on the sample interface, e.g. under an angle of incidence. The emitted light may interact with the sam-ple interface and may leave the sample interface, e.g. under an exit angle. A point or region ofimpingement on the sample interface may be at the same side of the sample interface or on anopposing side as the point or region of exit from the sample interface. The term “second optical path” as used herein is a broad term and is to be given its ordinary and customary meaning to a person of ordinary skill in the art and is not to be limited to a spe-cial or customized meaning. The term specifically may refer, without limitation, to an optical pathwithout interaction of the illumination light at the object. Specifically, in an ideal case, a detector signal obtained via the second optical path may be unaffected from a presence and / or an ab-sence of the object at the spectrometer device. For example, a detector signal obtained via thesecond optical path having the object applied to the spectrometer device, specifically to thesample interface, may ideally be equal to a detector signal obtained via the second optical pathhaving no object applied to the spectrometer device, specifically assuming constant environ-mental conditions. However, under real conditions, stray light, such as stray light from the first optical path generated due to a presence of an object at the sample interface, may influence a detector signal obtained via the second optical path. The second optical path may be configuredfor allowing the illumination light emitted from the light source to propagate via the optical filterto the detector without passing the sample interface, specifically without being reflected at theobject. Via the second optical path, the illumination light emitted from the light source may bepassed to the detector without interacting with the object, optionally via a wavelength-selectiveelement, such as an optical filter. The second optical path may be arranged completely in thespectrometer device, such as within a housing of the spectrometer device. The illumination lightfollowing the second optical path may be emitted by the light source and may be directly or indi-rectly, such as by reflection, refraction and / or dispersion, guided to the detector, optionally via awavelength-selective element, such as an optical filter. As an example, the second optical pathmay comprise a fiber coupled optical path transferring light from the light source to the detectoror an internal calibration target reflecting the illumination light to the detector. Alternatively or ad-ditionally, the second optical path may be configured for direct illumination of the internal cali-bration target with the illumination light emitted from the light source. The detector may be con-figured for generating at least one detector signal in response to an illumination by incident lightvia the second optical path. As an example, the spectrometer device may comprise a single light source which may specifi- cally be configured for illuminating both the first optical path and the second optical path with the illumination light. In this example, the spectrometer device may comprise at least one selection element configured for selectively illuminating the first optical path and the second optical pathwith the illumination light generated by the light source, e.g. a shutter or the like. Further, option-ally, the second optical path may comprise an internal calibration target, e.g. an optical mirror or a partially reflecting surface, configured for directing the illumination light to the detector withoutpassing the sample interface. Alternatively, the spectrometer device may comprise at least onelight source in each of the first optical path and the second optical path, such as at least one first light source in the first optical path and at least one second light source in the second opti-cal path. The first light source may preferably be embodied identical to the second light source.The second light source may be arranged to illuminate the detector directly or indirectly, such asvia an internal calibration target.As outlined above, the second optical path receives stray light from the first optical path. Theterm “stray light” as used herein is a broad term and is to be given its ordinary and customary meaning to a person of ordinary skill in the art and is not to be limited to a special or customized meaning. The term specifically may refer, without limitation, to any undesired light in an optical system or path. The stray light may originate from an intended source, but follow optical pathsother than intended, and / or may originate from a light source other than that intended. Specifi-cally, the stray light may comprise any light in the second optical path which was not intended in the design of the second optical path. The stray light in the second optical path may originate from the light source following unintended optical paths in the spectrometer device and / or may originate from the first light source in the first optical path. The stray light may specifically com- prise part of the illumination light of the first optical path scattered into the second optical, e.g. being reflected at the sample interface. Additionally or alternatively, the stray light may comprise part of the detection light scattered into the second optical path.As further described above, the spectrometer device comprises the at least one evaluation unitconfigured for evaluating detector signals generated by the detector. The term “to evaluate”, asused herein, is a broad term and is to be given its ordinary and customary meaning to a personof ordinary skill in the art and is not to be limited to a special or customized meaning. The termspecifically may refer, without limitation, to the process of processing at least one first item ofinformation in order to generate at least one second item of information thereby. Consequently,the term “evaluation unit”, as used herein, is a broad term and is to be given its ordinary andcustomary meaning to a person of ordinary skill in the art and is not to be limited to a special orcustomized meaning. The term specifically may refer, without limitation, to an arbitrary device or a combination of devices configured to evaluate or process at least one first item of information, in order to generate at least one second item of information thereof. Thus, specifically, the eval-uation unit may be configured for processing at least one input signal and to generate at leastone output signal thereof. The at least one input signal, as an example, may comprise at least one detector signal provided directly or indirectly by the at least one detector and, additionally, at least one parameter, such as at least one item of calibration, e.g. provided by an internal or external storage unit. 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 microcontrol-lers. Additional components may be comprised, such as one or more preprocessing devicesand / 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. 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 method comprises the following steps that may be performed in the given order. However, a different order may also be possible. In particular, one, more than one or even all of themethod steps may be performed once or repeatedly. Further, the method steps may be per-formed successively or, alternatively, one or more of the method steps may be performed in atimely overlapping fashion or even in a parallel fashion and / or in a combined fashion. Themethod may further comprise additional method steps that are not listed.The method comprises:i. providing at least one item of reference target calibration information and at least one itemof stray light correction information, wherein the item of reference target calibration infor- mation comprises a relation between a detector signal in the first optical path and a detec- tor signal in the second optical path having at least one reference target of known reflec- tivity applied to the sample interface, and wherein the item of stray light correction infor- mation comprises at least one relation between a signal in the second optical path and a reflectivity of a sample applied to the sample interface;ii. illuminating the first optical path and the second optical path with the illumination light gen-erated by the light source;iii. determining, by using the detector, at least one first detector signal ^^ from the first opticalpath having the at least one object applied to the sample interface;iv. determining, by using the detector, at least one second detector signal ^^ from the secondoptical path; andv. evaluating the first detector signal ^^ and the second detector signal S2 using the item ofreference target calibration information to obtain at least one first optical property estimate of the object; andvi. determining the at least one calibrated optical property of the object by using the item ofreference target calibration information, the item of stray light correction information and the first reflectivity estimate of the object, wherein the calibrated optical property is cor- rected for stray light in the second optical path.The term “providing” as used herein is a broad term and is to be given its ordinary and custom-ary meaning to a person of ordinary skill in the art and is not to be limited to a special or cus-tomized meaning. The term specifically may refer, without limitation, to a process of makingavailable. The providing of items of information may specifically comprise making available theitem of information for further processing. The providing may comprise retrieving the items of information from at least one of an external storage device and an internal storage device, such as a storage device of the evaluation unit, and / or supplying the items of information to one or more processing units, such as to processing units of the evaluation unit.The term “item of reference target calibration information” as used herein is a broad term and isto be given its ordinary and customary meaning to a person of ordinary skill in the art and is not to be limited to a special or customized meaning. The term specifically may refer, without limita- tion, to data configured for calibrating detector signals to an external reference target. The item of reference target calibration information may comprise at least one relation, such as a calibra- tion factor, a calibration function or a calibration matrix, for calibration detector signals at the spectrometer device to an external reference target. The external reference target may be an eternal target of known or predetermined optical properties, e.g. of known or predetermined re- flectivity. The item of reference target calibration information may be determined in an initial cali-bration process of the spectrometer device, for example in a factory calibration process. Thus,specifically, the item of reference target calibration information may be a predetermined item ofreference target calibration information, specifically being predetermined in a factory calibration.Additionally, it may be possible to re-determine the item of reference target calibration infor-mation in-field, such as at a user site, by re-performing the steps of the initial calibration usingthe external calibration target. The item of reference target calibration information may bestored, such as in a data storage device of the spectrometer device and / or in a data storage de-vice accessible to the spectrometer device, and may be used for determining the calibrated opti- cal property of the object using the spectrometer device. Specifically, the item of reference tar- get calibration information may be applied to the detector signal of the spectrometer device,specifically in order to correct, adjust and / or compensate the detector signal, to obtain a cali-brated detector signal, such as a measurement signal being calibrated to the known externalcalibration target. The calibrated measurement signal may directly provide the calibrated opticalproperty on the object or, alternatively, may be used for further evaluation to derive the cali-brated optical property on the object.The item of reference target calibration information may comprise a relation between a detectorsignal in the first optical path and a detector signal in the second optical path having at least one reference target, specifically at least one reference target of known reflectivity, applied to the sample interface. For example, the item of reference target calibration information may com-prise at least one reference calibration factor ^ = ^^ / ^^, wherein ^^ denotes a detector signalin the first optical path having at least one reference target applied to the sample interface and ^^denotes a detector signal in the second optical path. The term “item of stray light correction information” as used herein is a broad term and is to be given its ordinary and customary meaning to a person of ordinary skill in the art and is not to be limited to a special or customized meaning. The term specifically may refer, without limitation, to data configured for quantifying stray light effects. Specifically, the item of stray light correction information may comprise correction data, such as such as a correction factor, a correction function or a correction matrix, for quantifying stray light effects in the spectrometer device. For example, the item of stray light correction information may comprise correction data quantifyingan amount of stray light, e.g. in the form of a measured intensity at the detector, in the secondoptical path when a sample of known or predetermined optical properties, e.g. of known or pre- determined reflectivity, is applied to the sample interface of the spectrometer device. The item of stray light correction information may be determined in an initial calibration process of thespectrometer device, for example in a factory calibration process. Thus, specifically, the item ofstray light correction information may be a predetermined item of stray light correction infor-mation, specifically being predetermined in a factory calibration. Additionally or alternatively, theitem of stray light correction information may comprise at least one relation between a signal inthe second optical path and a reflectivity of a sample applied to the sample interface. The term “illuminating” as used herein is a broad term and is to be given its ordinary and cus- tomary meaning to a person of ordinary skill in the art and is not to be limited to a special orcustomized meaning. The term specifically may refer, without limitation, to a process of expos-ing at least one element to light. Specifically, the illuminating may comprise exposing at leastone of the first optical path and the second optical path with the illumination light as generated by the light source. The term “first optical property estimate of the object” as used herein is a broad term and is to be given its ordinary and customary meaning to a person of ordinary skill in the art and is not to be limited to a special or customized meaning. The term specifically may refer, without limita- tion, to a preliminary optical property of the object. The first optical property estimate of the ob-ject may specifically comprise a non-calibrated or non-corrected optical property estimate of theobject. Specifically, the first optical property estimate of the object may not be calibrated or cor-rected for stray light effects, in particular for stray light in the second optical path received fromthe first optical path. As an example, the first optical property estimate of the object may takeinto account, specifically comprises, more specifically is, a first estimate of a reflectivity of theobject. The first reflectivity estimate of the object may be used for estimating the stray light in the sec- ond optical path. The estimate of the stray light may be used for correcting the at least one ofthe item of reference target calibration information for stray light effects. The corrected item ofreference target calibration information may be used for determining the calibrated optical prop-erty of the object. Herein, the following notation may be used: ^^may denote open port measurement, sometimes also referred to as background measurement, where no target or sample is placed on the sam-ple interface. ^^ may denote a reference target measurement with a 99 %-reflectance standardwith diffusive (Lambertian) reflective scattering properties. ^^ may denote object or samplemeasurements of a sample of interest, such as skin, textile, food item, being placed on the sam-ple interface of the spectrometer device. ^ may denote a temperature of the spectrometer de-vice under which a specific measurement is performed. The temperature may be directly deter-mined using a thermistor, diode or the like, and / or indirectly via a temperature-dependent prop-erty of a component of the spectrometer device, e.g. using a dark signal of a photodetector. Theindex ^ = 1,2 may denote measurements in the first optical path and the second optical path, re-spectively. For example, may denote a detector signal for an open port measurement in the first optical path in units of counts. The illumination light from in the first optical path may, inprinciple, illuminate the object. The detector signal ^^ may include a temperature dependence^^(^). Similarly, ^^ may denote a detector signal in the second optical path in units of. The sec- ond optical path may be intended to be independent of a presence or absence of an object ap- plied to the sample interface, e.g. via a separate light source an internal reference light source. ^^may include a temperature dependence ^^(^). As outlined above, the optical property of the sample may comprise at least one of an absorb- ance of the object and a reflectivity of the object. The absorbance of the object may be deter- mined according to or wherein ^ denotes the item of reference target calibration information comprising the referencecalibration factor ^ = ^^ / ^^. Alternatively or additionally, the absorbance of the object may bedetermined according to In an ideal case, the detector signals in the second optical path may be independent of an ob-ject applied to the sample interface in the first optical path ^^ = ^^ = ^^, wherein ^^ denotes thedetector signal in the second optical path with a reference 99 %-target, ^^denotes the detectorsignal in the second optical path with some reflectivity 0 < ^ < 1, and ^^ denotes the detectorsignal in the second optical path with no object applied to the sample interface, i.e. an open port measurement for the second optical path. However, in reality, stray light in the second opticalpath may lead to ^2 ≤ ^2 ≤ ^2. Additionally, ^^ and ^^ may be dependent on the reflectivity ^ ofthe object ^^ = ^^(^) and ^^ = ^^(^).The item of stray light correction information may comprise a correction curve for different re-flectivities of the object. The correction curve may comprise the open port detector signal ^^ asa starting point, one or more detector signals for standard samples with known reflectivity andthe detector signal of a 99 %-target ^^ as a final point. The correction curve may comprise apolynomial function ^(^) of nth-order fitted to the predetermined detector signals: For ^ = 0, it follows that ^ ^^ ∗ ^ = ^^ and The method may further comprise, specifically prior to step iii.:vii. determining, by using the detector, at least one first open port detector signal from thefirst optical path and at least one second open port detector signal ^^from the second op- tical path having no object applied to the sample interface. For example, the first optical property estimate may be calculated using the second detector signal ^^according to: wherein ^ denotes the item of reference target calibration information. In this example, the itemof reference target calibration information may comprise the at least one reference calibrationfactor ^ = ^^ / ^^, wherein ^^ denotes a detector signal in the first optical path having at leastone reference target applied to the sample interface and ^^denotes a detector signal in the second optical path having the at least one reference target applied to the sample interface.Further, the first optical property estimate of the object may be used for determining at least onecorrection factor according to: wherein ^ denotes the first optical property estimate and ^^ denote the item of stray light correc-tion information.The calibrated optical property of the object may be calculated according to: Thus, using the item of stray light correction information comprising above-identified correction curve, each detector signal, which differs in reflectivity from ^^can be corrected by the correc- tion function via the correction factor. Further, the absorbance can be written as a= − log ^^ − ^^^ ∗ ^^^^ ∗ ^^ ∗ ^^ − ^^. The open port detector signal in the second optical path can be written as wherein ^^and ^^denote the reflectivities of the reference target and of the object, respectively.Alternatively or additionally, the second open port detector signal may be calculated accordingto: The correction factor may be calculated according to: Further, the calibrated optical property of the object may be calculated using the second detec-tor signal ^^(^^) and the correction factor^^^^ according to Alternatively, the calibrated optical property of the object may be calculated according to: Alternatively or additionally, the calibrated optical property of the object may be determined re-cursively by using the first optical property estimate of the object as an initial value of the recur-sion. For example, the recursion may be given by:^^^ − ^^^^^=^ ∙ (^ + ^, ^(1 − ^^)) − ^^wherein ^ denotes the item of stray light correction information defined by ^^(^) = ^^ + ^ ∗ ^.In this example, the item of stray light correction information may comprise a linear correctionfunction ^^(^) = ^^ + ^ ∗ ^. Thus, the detector signal in the second optical path with the 99 %-target can be written as ^^ = ^^ + ^ = ^^ + ^(1 − ^). The item of reference target calibrationinformation may comprise the calibration factor ^ linking the detector signal in the second opti-cal path with the detector signal in the first optical path ^^ = ^^ ∗ ^. Additionally, the relation be-tween open port and reference target interaction of the second optical path may be correct us-ing the item of stray light correction information via ^^ = ^^ + ^. Thus, an actual measurementcomprising an open port measurement and a sample measurement may be performed to deter-mine the first optical property estimate according to ^^ = (^^ − ^^)⁄ (^ ∙ ^^ using the un-corrected ^^as ^^in above-identified formula of the reflectivity of the object with the calibrationfactor ^ from the factory calibration. Subsequently, the calibrated reflectivity ^^^^ may be recur-sively calculated the item of stray light correction information ^ from the factory calibration ac-cording to: ^^ − ^^^^^^=^ ∙ (^ + ^(1 − ^ ). ^^ − ^^Further, it may also be possible to calibrate an ^∗ = ^^ / ^^ in order to determine ^ = ^∗^^− ^^. This may yield a different calibration value, specifically a quotient instead of a difference be-tween the open port signal and the reference signal.Alternatively or additionally, the recursion may be given by: wherein ^ denotes the item of stray light correction information defined by ^^(^) = ^^ +∑^ ^^^ ^^ ∗ ^^and ^ = ∑^ ^^^ ^^.Thus, in this example, the relation between ^^ and ^^ may be given by ^, wherein ^ is ob-tained by ^ =(∑^ ^ ^^^^ ^ ∙ (^ = 1) ) − ^^). Thus, the detector signal in the second optical path withthe 99 %-target can be written as ^^ = ^^ + ^ = ^^ + ∑^ ^^^ ^^. The first optical property esti-mate may be determined according to ^ = (^^ − ^^)⁄ (^ ∙ ^^ − using the uncorrected ^^^^ in above-identified formula of the reflectivity of the object with the calibration factor ^ fromthe factory calibration. The polynomial correction curve ^(^) = ^ + ^ ∙ ^can be useddetermine the detector signal in the second optical path with the 99 %-target ^^ = ^^ + ^ −∙ ^^, yielding the recursive formula for the calibrated optical property of the As outlined above, the method may comprise performing an open port measurement. Alterna- tively or additionally, step i. may further comprise providing at least one item of open port cali-bration information. The term “item of open port calibration information” as used herein is abroad term and is to be given its ordinary and customary meaning to a person of ordinary skill in the art and is not to be limited to a special or customized meaning. The term specifically mayrefer, without limitation, to data configured for calibrating detector signals to an open port detec-tor signal. The item of open port calibration information may comprise at least one relation, such as a calibration factor, a calibration function or a calibration matrix, for calibration detector sig- nals at the spectrometer device to an open port detector signal. The item of open port calibra- tion information may be determined in an initial calibration process of the spectrometer device, for example in a factory calibration process. Thus, specifically, the item of open port calibration information may be a predetermined item of open port calibration information, specifically being predetermined in a factory calibration. Additionally, it may be possible to re-determine the item of open port calibration information in-field, such as at a user site, by re-performing the steps ofthe initial calibration. The item of open port calibration information may be stored, such as in adata storage device of the spectrometer device and / or in a data storage device accessible to the spectrometer device, and may be used for determining the calibrated optical property of the object using the spectrometer device. Specifically, the item of open port calibration information may be applied to the detector signal of the spectrometer device, specifically in order to correct, adjust and / or compensate the detector signal, to obtain a calibrated detector signal. The cali- brated measurement signal may directly provide the calibrated optical property on the object or, alternatively, may be used for further evaluation to derive the calibrated optical property on the object. The item of open port calibration information may specifically comprise a relation between a de- tector signal in the first optical path and a detector signal in the second optical path having noobject applied to the sample interface. For example, the open port calibration information maycomprise at least one open port calibration factor ^∗ = ^ / ^^, wherein denotes an opendetector signal in the first optical path having no object applied to the sample interface and ^^denotes a detector signal in the second optical path having no object applied to the sample in- terface. Thus, for example, the first optical property estimate may be calculated according to: wherein ^ denotes the item of reference target calibration information and ^∗denotes the itemof open port calibration information. The calibrated optical property of the object may be determined recursively by using the first op-tical property estimate of the object as an initial value of the recursion. For example, the recur-sion may be given by: wherein ^ denotes the item of stray light correction information defined by ^^(^) = ^^ + ^ ∗ ^.In this example, the relation between ^^ and ^^ may be given by ^, wherein ^ is obtained by^^ = ^^ + ^ = ^^ + ^ ∙ (1 − ^). The first optical property estimate may be determined accordingto ^ = (^ − ^∗^ ⁄ ∗^ ^ ∙ ^ ) (^ ∙ ^^ − ^ ∙ ^^) using the uncorrected ^^ as ^^ in above-identified for-mula of the reflectivity and the item of reference target calibration information ^ and the item ofopen port calibration information ^∗from the factory calibration. The calibrated optical property estimate of the object ^^^^may be determined recursively using the item of stray light correctioninformation ^ from the factory calibration: Alternatively or additionally, the recursion may be given by: wherein ^ denotes the item of stray light correction information defined by ^^(^) = ^^ + In this example, the relation between ^^ and ^^ may be given by ^, wherein ^ is obtained by^ =(∑^^^^ ^^ ∙ (^ = 1)^) − ^^). The first optical property estimate may be determined accordingto ^ = (^ − ^∗ ∙ ^ )⁄ (^ ∙ ∗^ ^ ^ ^^ − ^ ∙ ^^) using the uncorrected ^^ as ^^ in above-identified for-mula of the reflectivity and the item of reference target calibration information ^ and the item ofopen port calibration information ^∗from the factory calibration. The polynomial correction curve^(^) = ^^ + ∑^ ^ ^^^ ^^ ∙ ^can be used to determine the detector signal in the second optical pathwith the 99 %-target ^^ = ^^ + ^ − ∑^ ^^^ ^^ ∙ ^^ , yielding the recursive formula for the calibrated optical property of the object: In any of above-identified examples of a recursion, the recursion may be stopped in case a pre-determined termination criterion is fulfilled, wherein, specifically, the predetermined termination criterion may be wherein ^^ denotes the calibrated optical property in iteration step ^ and ^ denotes a number se-lected from at least one of: 10, 100, 1000.As outlined above, the second optical path may comprises at least one of a direct illumination ofthe detector with the illumination light and an indirect illumination of the detector with the illumi-nation light via at least one internal reflection target. Steps iii. and iv. may be performed in atimely overlapping fashion, specifically simultaneously, wherein the first detector signal ^^is dif- ferentiated from the second detector signal ^^using frequency division multiplexing and / or po- larization division multiplexing.The method may specifically be computer-controlled. The term “computer-controlled” as usedherein is a broad term and is to be given its ordinary and customary meaning to a person of or- dinary skill in the art and is not to be limited to a special or customized meaning. The term spe-cifically may refer, without limitation, to a way of executing a method involving at least one com-puter and / or at least one computing unit for performing and / or controlling performing the method and / or at least one method step. The computer and / or computer unit may comprise at least one processor which is configured for performing and / or controlling performing at least one of themethod steps of the method according to the present invention. Preferably, each of the methodsteps is performed and / or controlled by the computer and / or computer network. The method may be performed completely automatically, specifically without user interaction.In a further aspect of the present invention, a spectrometer device for obtaining at least one cali-brated optical property of at least one object is disclosed. The spectrometer device comprises:a. at least one light source configured for generating illumination light;b. at least one detector configured for detecting detection light;c. at least one sample interface configured for allowing illumination of the object with the illu-mination light and configured for allowing the detection light from the object to propagate to the detector;d. at least one first optical path, wherein the first optical path is configured for allowing theillumination light generated by the light source to propagate to the detector by passing the sample interface at least once;e. at least one second optical path, wherein the second optical path is configured for allow-ing the illumination light generated by the light source to propagate to the detector without passing the sample interface, wherein the second optical path receives stray light from the first optical path;f. at least one evaluation unit configured for evaluating detector signals generated by the de-tector. The spectrometer device is configured for performing the method according to the present in- vention, such as according to any one of the embodiments disclosed above and / or according to any one of the embodiments disclosed in further detail below. Thus, for definitions of termsand / or possible embodiments of the spectrometer device or any part thereof, reference is madeto the description of the method above.In a further aspect of the invention, a computer program is disclosed, comprising instructionswhich, when the program is executed by the spectrometer device according to the present in- vention, such as according to any one of the embodiments disclosed above and / or according to any one of the embodiments disclosed in further detail below, cause the spectrometer device to perform the method according to the present invention, such as according to any one of the em- bodiments disclosed above and / or according to any one of the embodiments disclosed in further detail below. Similarly, a computer-readable storage medium, specifically a non-transient computer-readablemedium, is disclosed, comprising instructions which, when the instructions are executed by thespectrometer device according to the present invention, such as according to any one of the embodiments disclosed above and / or according to any one of the embodiments disclosed in fur- ther detail below, cause the spectrometer device to perform the method according to the pre- sent invention, such as according to any one of the embodiments disclosed above and / or ac- cording to any one of the embodiments disclosed in further detail below. As used herein, the term “computer-readable storage medium” specifically may refer to non- transitory data storage means, such as a hardware storage medium having stored thereon com- puter-executable instructions. The computer-readable storage medium specifically may be or may comprise a storage medium such as a random-access memory (RAM) and / or a read-onlymemory (ROM). The computer-readable storage medium may also be referred to as “computer-readable data carrier”. Referring to the computer-implemented aspects of the invention, one or more of the method steps or even all of the method steps of the method according to one or more of the embodi-ments disclosed herein may be performed and / or controlled by using a computer or computernetwork. Thus, generally, any of the method steps including provision and / or manipulation of data may be performed and / or controlled by using a computer or computer network. Generally, these method steps may include any of the method steps, typically except for method steps re- quiring manual work, such as providing the samples and / or certain aspects of performing the actual measurements. As used herein, the terms “have”, “comprise” or “include” or any arbitrary grammatical variations thereof are used in a non-exclusive way. Thus, these terms may both refer to a situation in which, besides the feature introduced by these terms, no further features are present in the en-tity described in this context and to a situation in which one or more further features are present.As an example, the expressions “A has B”, “A comprises B” and “A includes B” may both refer to a situation in which, besides B, no other element is present in A (i.e. a situation in which A solely and exclusively consists of B) and to a situation in which, besides B, one or more further elements are present in entity A, such as element C, elements C and D or even further ele- ments.Further, it shall be noted that the terms “at least one”, “one or more” or similar expressions indi-cating that a feature or element may be present once or more than once typically are used onlyonce when introducing the respective feature or element. In most cases, when referring to therespective feature or element, the expressions “at least one” or “one or more” are not repeated,nonwithstanding the fact that the respective feature or element may be present once or more than once. Further, as used herein, the terms "preferably", "more preferably", "particularly", "more particu- larly", "specifically", "more specifically" or similar terms are used in conjunction with optional fea- tures, without restricting alternative possibilities. Thus, features introduced by these terms are optional features and are not intended to restrict the scope of the claims in any way. The inven- tion may, as the skilled person will recognize, be performed by using alternative features. Simi- larly, features introduced by "in an embodiment of the invention" or similar expressions are in-tended to be optional features, without any restriction regarding alternative embodiments of theinvention, without any restrictions regarding the scope of the invention and without any re-striction regarding the possibility of combining the features introduced in such way with otheroptional or non-optional features of the invention. The method and the spectrometer device may provide a large number of advantages over known methods and devices of similar kind. Specifically, the method and the spectrometer de- vice according to the present invention may account for stray light effect in the two optical cali- bration paths of the spectrometer device. Thus, the method and the spectrometer device ac- cording to the present invention may improve the quality of open port and internal calibration byperforming stray light correction. Further, spectral contrast from spectral measurements may beenhanced as additional interfering signals due to stray light are eliminated or at least reduced. The method can particularly miniaturized spectroscopy systems using available hardware. TheImprovement of open port and internal calibration measurements may result in both higher ac-curacy and higher usability for a customer. There method and the spectrometer device can beused for implementation in e.g. wearables or smartphones. Alternatively, the method can alsobe used for measurement systems using photoresistors that employ microprocessors as part of the readout electronics and / or other spectroscopic devices for improved sensing.A second internal optical path to the detector may be used for a calibration of the detector re-sponse. This measurement may be performed simultaneously with the sample measurement,such as via frequency division multiplexing and / or polarization division multiplexing. In the sec-ond internal optical path, a second LED, a further second light source and / or an internal reflec-tion target may be used.In the factory calibration, a calibration factor comprising a dependence of the second detectorsignal or a factor between a first detector signal and the second detector signal may be deter-mined depending on the sample interface condition, i.e. an open port measurement, a referencetarget measurement and so on.The measurement error related to the open port measurement and / or the reference targetmeasurement introduced when evaluating a detector signal related to the second optical pathby using the calibration factor may be reduced when a sample plane dependence is furthercomprised by the calibration factor. The absorbance spectrum ^ ^= log1 ^= −log ^^ − ^^^^ − ^^may be determined from a sample measurement ^^, a reference measurement ^^ and an openport measurement ^^. This may require at least three subsequent measurements with a first light source.A second optical path, e.g. a second light source, may be used for an internal calibration anddoes ideally not illuminate the sample but an internal calibration target. This internal calibration can be used to reduce the amount of measurement. Therefore, two factors may need to be cali-brated in a so-called factory calibration.One calibration factor may be the item of reference target calibration information ^ = ^^⁄ ^^ andthe further calibration factor may be the optical item of open port calibration information ^∗ =^^⁄ ^^ . Under the assumption that ^^ = ^^ = ^^ the absorbance spectrum a may be determinedvia Only one measurement may be necessary to derive the first light signal ^^ and a second lightsignal ^^. Again, the values D and D* may be fixed values known from the factory calibration.Unfortunately, the assumption from above with ^^ = ^^ = ^^ may not be exact as the measure-ment object is also illuminated by a portion of the light from the second light source due to sig-nal crosstalk. In a real scenario, the following relation may be valid: ^^ ≤ ^^ ≤ ^^.A signal crosstalk correction may be based on open port measurements and a reflection targetreflecting 99% of the incident radiation, wherein the reflection target may be approximated to re-flect 100% of the incident radiation. Since a ratio between a first signal ^^ (derived in a spectralmeasurement of an object) and a second signal ^^ (derived in a spectral measurement of a ref-erence target) may remain constant when a linear fit is applied to the first signal ^^ and the sec-ond signal ^^, a deviation from this behavior may be considered to compensate for the effect in-troduced by the signal crosstalk. The crosstalk can advantageously be subtracted per measure-ment channel via the sample measurement per wavelength channel. For each wavelengthchannel, data corresponding to a certain reflectivity may be acquired, which lies at a certain point on the linear fit curve for the signal in the first optical path. Summarizing and without excluding further possible embodiments, the following embodiments may be envisaged:Embodiment 1: A method for determining at least one calibrated optical property of at leastone object by using a spectrometer device, the spectrometer device comprising: -at least one light source configured for generating illumination light;- at least one detector configured for detecting detection light;- at least one sample interface configured for allowing illumination of the object withthe illumination light and configured for allowing the detection light from the object to propagate to the detector; -at least one first optical path, wherein the first optical path is configured for allowingthe illumination light generated by the light source to propagate to the detector by passing the sample interface at least once; -at least one second optical path, wherein the second optical path is configured forallowing the illumination light generated by the light source to propagate to the de- tector without passing the sample interface, wherein the second optical path re- ceives stray light from the first optical path; -at least one evaluation unit configured for evaluating detector signals generated bythe detector; wherein the method comprises: i. providing at least one item of reference target calibration information and at leastone item of stray light correction information; ii. illuminating the first optical path and the second optical path with the illuminationlight generated by the light source; iii. determining, by using the detector, at least one first detector signal ^^ from the firstoptical path having the at least one object applied to the sample interface; iv. determining, by using the detector, at least one second detector signal ^^ from thesecond optical path; and v. evaluating the first detector signal ^^ using the item of reference target calibrationinformation to obtain at least one first optical property estimate of the object; and vi. determining the at least one calibrated optical property of the object by using theitem of reference target calibration information, the item of stray light correction in- formation and the first reflectivity estimate of the object, wherein the calibrated opti- cal property is corrected for stray light in the second optical path.Embodiment 2: The method according to the preceding embodiment, wherein the first reflec-tivity estimate of the object is used for estimating the stray light in the second optical path, wherein the estimate of the stray light is used for correcting the at least one of the item of reference target calibration information for stray light effects.Embodiment 3: The method according to the preceding embodiment, wherein the correcteditem of reference target calibration information is used for determining the calibrated opti-cal property of the object.Embodiment 4: The method according to any one of the preceding embodiments, wherein thefirst optical property estimate of the object takes into account, specifically comprises, a first estimate of a reflectivity of the object.Embodiment 5: The method according to any one of the preceding embodiments, wherein thestray light comprises part of the detection light scattered into the second optical path.Embodiment 6: The method according to any one of the preceding embodiments, wherein theitem of reference target calibration information comprises a relation between a detector signal in the first optical path and a detector signal in the second optical path having at least one reference target, specifically at least one reference target of known reflectivity, applied to the sample interface.Embodiment 7: The method according to any one of the preceding embodiments, wherein theitem of reference target calibration information comprises at least one reference calibra- tion factor ^ = ^^ / ^^, wherein ^^ denotes a detector signal in the first optical path havingat least one reference target applied to the sample interface and ^^denotes a detector signal in the second optical path.Embodiment 8: The method according to any one of the preceding embodiments, wherein theitem of reference target calibration information is a predetermined item of reference targetcalibration information, specifically being predetermined in a factory calibration.Embodiment 9: The method according to any one of the preceding embodiments, wherein theitem of stray light correction information comprises at least one relation between a signalin the second optical path and a reflectivity of a sample applied to the sample interface.Embodiment 10: The method according to any one of the preceding embodiments, wherein theitem of stray light correction information is a predetermined item of stray light correctioninformation, specifically being predetermined in a factory calibration.Embodiment 11: The method according to any one of the preceding embodiments, wherein theoptical property of the object comprises at least one of an absorbance of the object and areflectivity of the object.Embodiment 12: The method according to any one of the preceding embodiments, wherein thesecond optical path comprises at least one of a direct illumination of the detector with theillumination light and an indirect illumination of the detector with the illumination light via at least one internal reflection target.Embodiment 13: The method according to any one of the preceding embodiments, whereinsteps iii. and iv. are performed in a timely overlapping fashion, specifically simultaneously, wherein the first detector signal ^^is differentiated from the second detector signal ^^us- ing frequency division multiplexing and / or polarization division multiplexing.Embodiment 14: The method according to any one of the preceding embodiments, wherein themethod further comprises, specifically prior to step iii.: vii. determining, by using the detector, at least one first open port detector signal from the first optical path and at least one second open port detector signal ^^from the second optical path having no object applied to the sample interface.Embodiment 15: The method according to the preceding embodiment, wherein the first opticalproperty estimate is calculated using the second detector signal ^^according to: wherein ^ denotes the item of reference target calibration information.Embodiment 16: The method according to any one of the two preceding embodiments, whereinthe first optical property estimate of the object is used for determining at least one correc- tion factor according to: wherein ^ denotes the first optical property estimate and ^^ denote the item of stray lightcorrection information.Embodiment 17: The method according to the preceding embodiment, wherein the calibratedoptical property of the object is calculated according to: Embodiment 18: The method according to any one of the three preceding embodiments,wherein the calibrated optical property of the object is determined recursively by using thefirst optical property estimate of the object as an initial value of the recursion.Embodiment 19: The method according to the preceding embodiment, wherein the recursion isgiven by: ^^ − ^^^^^=^^ ∙ (^, ^+ ^(1 − ^^)) − ^^wherein ^ denotes the item of stray light correction information defined by ^^(^) = ^^ +^ ∗ ^.Embodiment 20: The method according to any one of the two preceding embodiments, whereinthe recursion is given by: wherein ^ denotes the item of stray light correction information defined by ^^(^) = ^^ +∑^^ ∗ ^^an ∑^ ^^^ ^ d ^ = ^^^ ^^.Embodiment 21: The method according to any one of the preceding embodiments, whereinstep i. further comprises providing at least one item of open port calibration information.Embodiment 22: The method according to the preceding embodiment, wherein the item of openport calibration information comprises a relation between a detector signal in the first opti- cal path and a detector signal in the second optical path having no object applied to thesample interface.Embodiment 23: The method according to any one of the two preceding embodiments, whereinthe open port calibration information comprises at least one open port calibration factor^∗ = ^^ / ^^, wherein denotes an open port detector signal in the first optical path hav-ing no object applied to the sample interface and ^^denotes a detector signal in the sec- ond optical path having no object applied to the sample interface.Embodiment 24: The method according to any one of the three preceding embodiments,wherein the item of open port calibration information is a predetermined item of open port calibration information, specifically being predetermined in a factory calibration.Embodiment 25: The method according to any one of the four preceding embodiments, whereinthe first optical property estimate is calculated according to: wherein ^ denotes the item of reference target calibration information and ^∗denotes theitem of open port calibration information.Embodiment 26: The method according to the preceding embodiment, wherein the calibratedoptical property of the object is determined recursively by using the first optical property estimate of the object as an initial value of the recursion.Embodiment 27: The method according to the preceding embodiment, wherein the recursion isgiven by: wherein ^ denotes the item of stray light correction information defined by ^^(^) = ^^ +^ ∗ ^.Embodiment 28: The method according to any one of the two preceding embodiments, whereinthe recursion is given by: wherein ^ denotes the item of stray light correction information defined by ^^(^) = ^^ +∑^^ ∗ ^^and ^ = ∑^ ^^^ ^ ^^^ ^^.Embodiment 29: The method according to any of the preceding embodiments, wherein the re-cursion is stopped in case a predetermined termination criterion is fulfilled, wherein, spe-cifically, the predetermined termination criterion is ^^^^ − ^^ < ^^ / ^wherein ^^ denotes the calibrated optical property in iteration step ^ and ^ denotes a num-ber selected from at least one of: 10, 100, 1000.Embodiment 30: The method according to any one of the preceding embodiments, wherein themethod is computer-controlled.Embodiment 31: A spectrometer device for obtaining at least one calibrated optical property ofat least one object, the spectrometer device comprising: a. at least one light source configured for generating illumination light;b. at least one detector configured for detecting detection light;c. at least one sample interface configured for allowing illumination of the object withthe illumination light and configured for allowing the detection light from the object to propagate to the detector; d. at least one first optical path, wherein the first optical path is configured for allowingthe illumination light generated by the light source to propagate to the detector by passing the sample interface at least once; e. at least one second optical path, wherein the second optical path is configured forallowing the illumination light generated by the light source to propagate to the de- tector without passing the sample interface, wherein the second optical path re- ceives stray light from the first optical path; f. at least one evaluation unit configured for evaluating detector signals generated bythe detector, wherein the spectrometer device is configured for performing the method according to any one of the preceding embodiments.Embodiment 32: A computer program comprising instructions which, when the program is exe-cuted by the spectrometer device according to any one of the preceding embodiments re- ferring to a spectrometer device, cause the spectrometer device to perform the method according to any one of the preceding embodiments referring to a method.Embodiment 33: A computer-readable storage medium, specifically a non-transient computer-readable medium, comprising instructions which, when the instructions are executed bythe spectrometer device according to any one of the preceding embodiments referring to a spectrometer device, cause the spectrometer device to perform the method according to any one of the preceding embodiments referring to a method. Short description of the Figures Further optional features and embodiments will be disclosed in more detail in the subsequent description of embodiments, preferably in conjunction with the dependent claims. Therein, the respective optional features may be realized in an isolated fashion as well as in any arbitrary feasible combination, as the skilled person will realize. The scope of the invention is not re- stricted by the preferred embodiments. The embodiments are schematically depicted in the Fig- ures. Therein, identical reference numbers in these Figures refer to identical or functionally comparable elements. In the Figures:Figures 1 to 3 show different embodiments of a spectrometer device in schematic views;Figure 4 shows a flow chart of a first embodiment of a method for determining at leastone calibrated optical property of at least one object; andFigure 5 shows a flow chart of a second embodiment of a method for determining atleast one calibrated optical property of at least one object. Detailed description of the embodimentsFigures 1 to 3 show different embodiments of a spectrometer device 110 in schematic views.The embodiments of Figures 1 to 3 widely correspond to each other. Thus, in the following, Fig- ures 1 to 3 are described in conjunction. The spectrometer device 110 is configured for obtain- ing at least one calibrated optical property of at least one object 112. The spectrometer device 110, specifically, may be a portable spectrometer device. For example, the portable spectrome-ter device may be part of a mobile device or may be attachable to a mobile device (not shown inthe Figures), such as a notebook computer, a tablet, a cell phone, such as a smart phone, asmartwatch and / or a wearable computer.The spectrometer device 110 comprises at least one light source 114 configured for generatingillumination light 116. The light source 114 may comprise at least one light-emitting diode (LED)118. Alternatively or additionally, the light source 114 may comprise at least one LED 118 for generating primary light and at least one luminescent material (not shown in the Figures) forlight-conversion of primary light generated by the light-emitting diode, wherein, specifically, theillumination light 116 may be a combination of the primary light and light generated by the light-conversion by the luminescent material or light generated by the light conversion of the lumines-cent material, also referred to as secondary light. Other light sources, such as incandescencelight sources, electric discharge light sources, gas-discharge light sources or the like, are in principle also feasible. The spectrometer device 110 further comprises at least one detector 120 configured for detect-ing detection light 122. The detector 120 may comprise one single optically sensitive element orarea or a plurality of optically sensitive elements or areas. Specifically, the detector 120 may beor may comprise at least one detector array, more specifically an array of photosensitive ele-ments. Each of the photosensitive elements may comprise at least a photosensitive area whichmay be adapted for generating an electrical signal depending on the intensity of the incidentlight, wherein the electrical signal may, in particular, be provided to an evaluation unit, as will beoutlined in further detail below. The photosensitive area as comprised by each of the opticallysensitive elements may, especially, be a single, uniform photosensitive area which is configuredfor receiving the incident light which impinges on the individual optically sensitive elements.However, other arrangements of the optically sensitive elements may also be conceivable.The spectrometer device 110 further comprises at least one sample interface 124 configured for allowing illumination of the object 112 with the illumination light 116 and configured for allowingthe detection light 122 from the object 112 to propagate to the detector 120. For example, thesample interface 124 may be or may comprise at least one element comprising at least onetransparent material being at least partially transparent in the optical spectral range, such as inat least one partition of the optical spectral range or in the full optical spectral range, wherein,specifically, the transparent material may be arranged in a housing 126 of the spectrometer de-vice 110. The sample interface 124 may be configured for transmitting light in the optical spec-tral range. The sample interface 124 may be arranged in an optical path of the spectrometer de-vice 110, specifically in a first optical path, to allow the illumination light 116 emitted from thelight source 114 to illuminate the object 112 placed in front of the spectrometer device 110, spe-cifically in front of the sample interface 124. The transparent material may, as an example, com-prise one or more of a glass material, such as silica, soda lime, borosilicate or the like, and / or apolymeric material, such as polymethylmethacrylate or polystyrene.As shown in Figure 1, the spectrometer device 110 further comprises at least one first opticalpath 128 and at least one second optical path 130. The first optical path 128 is configured forallowing the illumination light 116 generated by the light source 114 to propagate to the detector 120 by passing the sample interface 124 at least once. The second optical path 130 is config- ured for allowing the illumination light 116 generated by the light source 114 to propagate to thedetector 120 without passing the sample interface 126, wherein the second optical path 130 re-ceives stray light from the first optical path 128.In the example of Figure 1, the spectrometer device 110 may comprise at least one light source 114 in each of the first optical path 128 and the second optical path 130, such as at least onefirst light source 132 in the first optical path 128 and at least one second light source 134 in thesecond optical path 130. The first light source 132 may preferably be embodied identical to the second light source 134. As an example, the second light source 134 may be arranged to illumi- nate the detector 120 directly via the second optical path 130. Alternatively, as shown in the exemplary embodiment of Figure 2, the second light source 134 may be arranged to illuminate the detector 120 indirectly via the second optical path 130 via aninternal calibration target 136. The internal calibration target 136 may be or may comprise anoptical mirror or a partially reflecting surface configured for directing the illumination light 116 to the detector 120 without passing the sample interface 124.As a further alternative, as can be seen in Figure 3, the spectrometer device 110 may comprisea single light source 114 which may specifically be configured for illuminating both the first opti-cal path 128 and the second optical path 130 with the illumination light 116. In this example, thespectrometer device 110 may comprise at least one selection element 138 configured for selec- tively illuminating the first optical path 128 and the second optical path 130 with the illuminationlight 116 generated by the light source 114, e.g. a shutter or the like.The spectrometer device 110 further comprises at least one evaluation unit 140 configured forevaluating detector signals generated by the detector 120. The spectrometer device 110 is con-figured for performing the method according to the present invention, such as according to any one of the embodiments described in Figures 4 and 5 and / or according to any other embodi- ment disclosed herein. Thus, for a detailed description of the method, reference is made to the description of Figures 4 and 5.Figure 4 shows a flow chart of a first embodiment of a method for determining at least one cali-brated optical property of at least one object 112. The method comprises using a spectrometerdevice 110, the spectrometer device 110 comprising:- the at least one light source 114 configured for generating illumination light 116;- the at least one detector 120 configured for detecting detection light 122;- the at least one sample interface 124 configured for allowing illumination of the object 112with the illumination light 116 and configured for allowing the detection light 128 from the object 112 to propagate to the detector 120;- the at least one first optical path 128, wherein the first optical path 128 is configured forallowing the illumination light 116 generated by the light source 114 to propagate to the detector 120 by passing the sample interface 124 at least once;- the at least one second optical path 130, wherein the second optical path 130 is config-ured for allowing the illumination light 116 generated by the light source 114 to propagate to the detector 120 without passing the sample interface 124, wherein the second opticalpath 130 receives stray light from the first optical path 128;- the at least one evaluation unit 140 configured for evaluating detector signals generatedby the detector 120. For example, the method may comprise using a spectrometer device 110 according to any one of the embodiments shown in Figures 1 to 3. The method comprises the following steps that may be performed in the given order. However, a different order may also be possible. In particular, one, more than one or even all of themethod steps may be performed once or repeatedly. Further, the method steps may be per-formed successively or, alternatively, one or more of the method steps may be performed in atimely overlapping fashion or even in a parallel fashion and / or in a combined fashion. Themethod may further comprise additional method steps that are not listed.The method comprises:i. (denoted by reference number 142) providing at least one item of reference target calibra-tion information and at least one item of stray light correction information, wherein the item of reference target calibration information comprises a relation between a detector signal in the first optical path 128 and a detector signal in the second optical path 130 having at least one reference target of known reflectivity applied to the sample interface 124, andwherein the item of stray light correction information comprises at least one relation be- tween a signal in the second optical path 130 and a reflectivity of a sample applied to the sample interface 124;ii. (denoted by reference number 144) illuminating the first optical path 128 and the secondoptical path 130 with the illumination light 116 generated by the light source 114;iii. (denoted by reference number 146) determining, by using the detector 120, at least onefirst detector signal ^^from the first optical path 128 having the at least one object 112 ap- plied to the sample interface 124;iv. (denoted by reference number 148) determining, by using the detector 120, at least onesecond detector signal ^^ from the second optical path 130; andv. (denoted by reference number 150) evaluating the first detector signal ^^ and the seconddetector signal S2 using the item of reference target calibration information to obtain at least one first optical property estimate of the object 112; andvi. (denoted by reference number 152) determining the at least one calibrated optical prop-erty of the object 112 by using the item of reference target calibration information, the item of stray light correction information and the first reflectivity estimate of the object 112,wherein the calibrated optical property is corrected for stray light in the second optical path 130.In the example of Figure 4, step i. may further comprise providing at least one item of open portcalibration information. The item of open port calibration information may be a predetermineditem of open port calibration information, specifically being predetermined in a factory calibra-tion. The item of open port calibration information may specifically comprise a relation betweena detector signal in the first optical path 128 and a detector signal in the second optical path 130having no object 112 applied to the sample interface 124. For example, the open port calibrationinformation may comprise at least one open port calibration factor ^∗ = ^^ / ^^, wherein de-notes an open port detector signal in the first optical path 128 having no object 112 applied tothe sample interface 124 and ^^ denotes a detector signal in the second optical path 130 havingno object 112 applied to the sample interface 124.Thus, for example, the first optical property estimate may be calculated according to: wherein ^ denotes the item of reference target calibration information and ^∗denotes the itemof open port calibration information.The calibrated optical property of the object 112 may be determined recursively by using thefirst optical property estimate of the object 112 as an initial value of the recursion. For example,the recursion may be given by: wherein ^ denotes the item of stray light correction information defined by ^^(^) = ^^ + ^ ∗ ^.Alternatively or additionally, the recursion may be given by: wherein ^ denotes the item of stray light correction information defined by ^^(^) = ^^ + Figure 5 shows a flow chart of a second embodiment of a method for determining at least onecalibrated optical property of at least one object 112. The embodiment of Figure 5 widely corre-sponds to the embodiment shown in Figure 4. Thus, for a detailed description of the method steps of the method shown in Figure 5, reference is made to the description of Figure 4. As can be seen in Figure 5, the method may comprise, specifically prior to step iii., the following step:viii. (determined by reference number 154) determining, by using the detector 120, at leastone first open port detector signal from the first optical path 128 and at least one sec-ond open port detector signal ^^from the second optical path 130 having no object 112 applied to the sample interface 124.For example, the first optical property estimate may be calculated using the second detectorsignal ^^according to: wherein ^ denotes the item of reference target calibration information. In this example, the itemof reference target calibration information may comprise the at least one reference calibrationfactor ^ = ^^ / ^^, wherein ^^ denotes a detector signal in the first optical path 128 having atleast one reference target applied to the sample interface 124 and ^^denotes a detector signal in the second optical path 130 having the at least one reference target applied to the sample in-terface 124. The item of reference target calibration information may be a predetermined item ofreference target calibration information, specifically being predetermined in a factory calibration. Further, the first optical property estimate of the object 112 may be used for determining at least one correction factor according to: wherein ^ denotes the first optical property estimate and ^^ denote the item of stray light correc-tion information.The calibrated optical property of the object 112 may be calculated according to: Alternatively or additionally, the calibrated optical property of the object 112 may be determinedrecursively by using the first optical property estimate of the object 112 as an initial value of therecursion. For example, the recursion may be given by:^ − ^^ =^ ^^^^ ^ ∙ (^, ^+ ^(1 − ^^)) − ^^wherein ^ denotes the item of stray light correction information defined by ^^(^) = ^^ + ^ ∗ ^.Alternatively or additionally, the recursion may be given by: wherein ^ denotes the item of stray light correction information defined by ^^(^) = ^^ +∑^^^^ ^^ ∗ ^^ and ^ = ∑^ ^^^ ^^.In any of above-identified examples of a recursion, the recursion may be stopped in case a pre-determined termination criterion is fulfilled, wherein, specifically, the predetermined termination criterion may be wherein ^^ denotes the calibrated optical property in iteration step ^ and ^ denotes a number se-lected from at least one of: 10, 100, 1000.

[0002] List of reference numbersspectrometer deviceobjectlight sourceillumination lightlight-emitting diodedetectordetection lightsample interfacehousingfirst optical pathsecond optical pathfirst light sourcesecond light sourceinternal calibration targetselection elementevaluation unitproviding at least one item of reference target calibration information and atleast one item of stray light correction informationilluminating the first optical path and the second optical pathdetermining at least one first detector signaldetermining at least one second detector signalevaluating the first detector signal and the second detector signaldetermining the at least one calibrated optical property of the objectdetermining at least one first open port detector signal and at least one sec-ond open port detector signal

Claims

1. Claims1. A method for determining at least one calibrated optical property of at least one object(112) by using a spectrometer device (110), the spectrometer device (110) comprising:- at least one light (114) source configured for generating illumination light (116);- at least one detector (120) configured for detecting detection light (122);- at least one sample interface (124) configured for allowing illumination of the object(112) with the illumination light (116) and configured for allowing the detection light (122) from the object (112) to propagate to the detector (120);- at least one first optical path (128), wherein the first optical path (128) is configuredfor allowing the illumination light (116) generated by the light source (114) to propa- gate to the detector (120) by passing the sample interface (124) at least once; -at least one second optical path (130), wherein the second optical path (130) is con-figured for allowing the illumination light (116) generated by the light source (114) to propagate to the detector (120) without passing the sample interface (124), whereinthe second optical path (130) receives stray light from the first optical path (128);- at least one evaluation unit (140) configured for evaluating detector signals gener-ated by the detector (120);wherein the method comprises: i. providing at least one item of reference target calibration information and at leastone item of stray light correction information, wherein the item of reference target calibration information comprises a relation between a detector signal in the first op- tical path(128) and a detector signal in the second optical path (130) having at least one reference target of known reflectivity applied to the sample interface (124), and wherein the item of stray light correction information comprises at least one relation between a signal in the second optical path (130) and a reflectivity of a sample ap- plied to the sample interface (124); ii. illuminating the first optical path (128) and the second optical path (130) with the illu-mination light (116) generated by the light source (114);iii. determining, by using the detector (120), at least one first detector signal ^^ from thefirst optical path (128) having the at least one object (112) applied to the sample in- terface (124);iv. determining, by using the detector (120), at least one second detector signal ^^ fromthe second optical path (130); andv. evaluating the first detector signal ^^ and the second detector signal S2 using theitem of reference target calibration information to obtain at least one first optical property estimate of the object (112); andvi. determining the at least one calibrated optical property of the object (112) by usingthe item of reference target calibration information, the item of stray light correctioninformation and the first reflectivity estimate of the object (112), wherein the cali-brated optical property is corrected for stray light in the second optical path (130).

2. The method according to the preceding claim, wherein the first reflectivity estimate of theobject (112) is used for estimating the stray light in the second optical path (130), whereinthe estimate of the stray light is used for correcting the at least one of the item of refer- ence target calibration information for stray light effects, wherein the corrected item of ref- erence target calibration information is used for determining the calibrated optical propertyof the object (112).

3. The method according to any one of the preceding claims, wherein steps iii. and iv. areperformed in a timely overlapping fashion, specifically simultaneously, wherein the first de- tector signal ^^is differentiated from the second detector signal ^^using frequency divi- sion multiplexing and / or polarization division multiplexing.

4. The method according to any one of the preceding claims, wherein the method furthercomprises, specifically prior to step iii.: vii. determining, by using the detector (120), at least one first open port detector signal^^ from the first optical path (128) and at least one second open port detector signal^^ from the second optical path (130) having no object (112) applied to the sampleinterface (124).

5. The method according to the preceding claim, wherein the first optical property estimate iscalculated using the second detector signal ^^according to:wherein ^ denotes the item of reference target calibration information.

6. The method according to any one of the two preceding claims, wherein the first opticalproperty estimate of the object (112) is used for determining at least one correction factoraccording to:wherein ^ denotes the first optical property estimate and ^^ denote the item of stray lightcorrection information, wherein the calibrated optical property of the object (112) is calcu- lated according to:

7. The method according to any one of the two preceding claims, wherein the calibrated opti-cal property of the object (112) is determined recursively by using the first optical propertyestimate of the object (112) as an initial value of the recursion, wherein the recursion isgiven by: ^^ − ^^^^^^=^ ∙ (^, ^+ ^(1 − ^^)) − ^^wherein ^ denotes the item of stray light correction information defined by ^^(^) = ^^ +^ ∗ ^.

8. The method according to any one of the three preceding claims, wherein the calibratedoptical property of the object (112) is determined recursively by using the first optical prop-erty estimate of the object (112) as an initial value of the recursion, wherein the recursionis given by:wherein ^ denotes the item of stray light correction information defined by ^^(^) = ^^ +∑^^ ∗ ^ ∑^ ^^^ ^ ^ and ^ = ^^^ ^^.

9. The method according to any one of the preceding claims, wherein step i. further com-prises providing at least one item of open port calibration information, wherein the open port calibration information comprises at least one open port calibration factor ^∗ = ^^ / ^^,whereindenotes an open port detector signal in the first optical path (128) having noobject (112) applied to the sample interface (124) and ^^ denotes a detector signal in thesecond optical path (130) having no object (112) applied to the sample interface (124),wherein the first optical property estimate is calculated according to:wherein ^ denotes the item of reference target calibration information and ^∗denotes theitem of open port calibration information.

10. The method according to the preceding claim, wherein the calibrated optical property ofthe object (112) is determined recursively by using the first optical property estimate of the object (112) as an initial value of the recursion, wherein the recursion is given by:wherein ^ denotes the item of stray light correction information defined by ^^(^) = ^^ +^ ∗ ^.

11. The method according to any one of the two preceding claims, wherein the calibrated opti-cal property of the object (112) is determined recursively by using the first optical propertyestimate of the object (112) as an initial value of the recursion, wherein the recursion is given by:wherein ^ denotes the item of stray light correction information defined by ^^(^) = ^^ +∑^^ ∗ ^^an ∑^ ^^^ ^ d ^ = ^^^ ^^.

12. The method according to any of the claims 7, 8, 10 and 11, wherein the recursion isstopped in case a predetermined termination criterion is fulfilled, wherein, specifically, thepredetermined termination criterion is ^^^^ − ^^ < ^^ / ^wherein ^^ denotes the calibrated optical property in iteration step ^ and ^ denotes a num-ber selected from at least one of: 10, 100, 1000.

13. A spectrometer device (110) for obtaining at least one calibrated optical property of atleast one object (112), the spectrometer device (110) comprising:a. at least one light source (114) configured for generating illumination light (116);b. at least one detector (120) configured for detecting detection light (122);c. at least one sample interface (124) configured for allowing illumination of the object(112) with the illumination light (116) and configured for allowing the detection light (122) from the object (112) to propagate to the detector (120);d. at least one first optical path (128), wherein the first optical path (128) is configuredfor allowing the illumination light (116) generated by the light source (114) to propa- gate to the detector (120) by passing the sample interface (124) at least once; e. at least one second optical path (130), wherein the second optical path (130) is con-figured for allowing the illumination light (116) generated by the light source (114) to propagate to the detector (120) without passing the sample interface (124), whereinthe second optical path (130) receives stray light from the first optical path (128);f. at least one evaluation unit (140) configured for evaluating detector signals gener-ated by the detector (120),wherein the spectrometer device (110) is configured for performing the method accordingto any one of the preceding claims.

14. A computer program comprising instructions which, when the program is executed by thespectrometer device (110) according to any one of the preceding claims referring to a spectrometer device (110), cause the spectrometer device (110) to perform the methodaccording to any one of the preceding claims referring to a method.

15. A computer-readable storage medium comprising instructions which, when the instruc-tions are executed by the spectrometer device (110) according to any one of the preced- ing claims referring to a spectrometer device (110), cause the spectrometer device (110)to perform the method according to any one of the preceding claims referring to a method.

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