US-12624995-B2 - Calibration light source and method of calibrating spectrometers

US12624995B2US 12624995 B2US12624995 B2US 12624995B2US-12624995-B2

Abstract

A calibration light source includes: a broad band light source emitting light in a broad spectral range; and a filter receiving light emitted by the broad band light source and providing structured light by imposing an attenuation pattern exhibiting pattern features at multiple spectral reference lines on the received light such that an emission spectrum of the structured light emitted by the calibration light source exhibits distinct, identifiable emission spectrum features corresponding to the pattern features at the multiple spectral reference lines. A method of calibrating at least one spectrometer using the calibration light source is further disclosed.

Inventors

Darren Schipper

Assignees

ENDRESS+HAUSER OPTICAL ANALYSIS, INC.

Dates

Publication Date: 20260512
Application Date: 20240130

Claims (17)

1 . A calibration light source for calibrating at least one spectrometer, the calibration light source comprising: a broad band light source operable to emit light in a broad spectral range; and a filter configured to receive the light emitted by the broad band light source and to provide structured light by imposing a predefined attenuation pattern exhibiting pattern features at multiple spectral reference lines on the received light such that an emission spectrum of the structured light emitted by the calibration light source exhibits distinct emission spectrum features corresponding to the pattern features at the multiple spectral reference lines, wherein the predefined attenuation pattern is embodied to distribute the distinct emission spectrum features across a spectral measurement range of the at least one spectrometer to improve calibration accuracy.
2 . The calibration light source according to claim 1 , wherein, at least one of: the emission spectrum features include edges, maxima, minima and/or other types of identifiable features, each occurring at one of the spectral reference lines; the emission spectrum features include at least one special feature that is distinguishable from the other emission spectrum features; and the attenuation pattern is a wavelength-dependent pattern of alternatingly increasing and decreasing attenuation or a sawtooth pattern causing the emission spectrum of the calibration light source to exhibiting a correspondingly alternatingly increasing and decreasing intensity.
3 . The calibration light source according to claim 1 , wherein the filter includes at least one filtering element or a combination of filtering elements, the filtering element(s) including at least one of: an etalon filter, a multivariate optical element filter, a jammed array wideband sawtooth filter, and an interference filter.
4 . The calibration light source according to claim 1 , wherein: the filter includes a gas exhibiting known atomic absorption lines; the gas: is a pure gas consisting of a single component or is a mixed gas comprising two or more components exhibiting different atomic absorption lines; and/or includes neon, argon, xenon, and/or at least one other component; and the light emitted by the broad band light source is transmitted through the gas, which selectively absorbs fractions of the light at the spectral reference lines corresponding to the atomic absorption lines of the gas.
5 . The calibration light source according to claim 4 , wherein: the filter includes a container configured to contain the gas; the container is a cell, a tube, a flexible tube that is at least partially arranged in a coil, a hollow core optical fiber, a hollow core optical fiber that is at least partially arranged in a coil, or another type of container; the container includes a light inlet or a transparent window closing off the container, through which light emitted by broad band light source enters the container, and the container includes a light outlet or a transparent window closing off the container, through which the structured light exits the container.
6 . The calibration light source according to claim 1 , wherein: the broad band light source includes a black-body radiator, an incandescent lamp, or a tungsten lamp; or the broad band light source includes an excitation light source operable to emit excitation light and a luminescent material emitting luminescence light in response to the luminescent material receiving the excitation light, wherein the luminescent material is a fluorescent material, a fluorescent glass, or another type of luminescent material.
7 . A method of calibrating at least one spectrometer, the method comprising: providing a calibration light source comprising: a broad band light source operable to emit light in a broad spectral range; and a filter configured to receive the light emitted by the broad band light source and to provide structured light by imposing an attenuation pattern exhibiting pattern features at multiple spectral reference lines on the received light such that an emission spectrum of the structured light emitted by the calibration light source exhibits distinct emission spectrum features corresponding to the pattern features at the multiple spectral reference lines; providing a reference characteristic of the structured light emitted by the calibration light source, the reference characteristic including at least one of: the emission spectrum of the calibration light source; and the spectral reference lines at which the emission spectrum features of the emission spectrum of the calibration light source occur; and with each respective spectrometer: determining a calibration spectrum of the structured light emitted by the calibration light source; and calibrating the respective spectrometer based on the calibration spectrum and the reference characteristic.
8 . The method according to claim 7 , wherein calibrating the respective spectrometer includes calibrating a spectral axis and/or an intensity axis of the respective spectrometer, and/or verifying a previous calibration of the respective spectrometer.
9 . The method according to claim 7 , wherein: the reference characteristic includes the spectral reference lines; and for at least one or each of the at least one spectrometer to be calibrated, calibrating the respective spectrometer includes calibrating a spectral axis of the spectrometer by: identifying calibration spectrum features included in the calibration spectrum; determining calibration spectrum lines at which the identified calibration spectrum features occur; and calibrating the spectral axis of the respective spectrometer based on the calibration spectrum lines and the corresponding spectral reference lines, at which the emission spectrum features corresponding to the calibration spectrum features occur.
10 . The method according to claim 9 , wherein: for each calibration spectrum feature, determining the emission spectrum feature corresponding to the respective calibration spectrum feature based on an order of occurrence of the calibration spectrum features along the spectral axis corresponding to an order of occurrence of the emission spectrum features along the spectral axis and on the spectral reference line corresponding to the calibration spectrum line at which the respective calibration spectrum feature occurred is given by the spectral reference line at which the corresponding emission spectrum feature occurs; and/or determining the emission spectrum features corresponding to other calibration spectrum features based on a respective order of occurrence along the spectral axis relative to reference points, wherein: the emission spectrum features include a special feature that is distinguishable from the other emission spectrum features; the calibration spectrum includes a special calibration spectrum feature corresponding to the distinguishable special feature that is distinguishable from the other calibration spectrum features; and the reference points are the special calibration spectrum feature and the corresponding special feature of the emission spectrum.
11 . The method according to claim 9 , wherein: at least one of the spectrometers includes a disperser, which disperses incident light, a detector including an array of detection elements, and a signal processor configured to determine and provide intensity spectra of the light received by the detector, wherein each detection element receives a fraction of the dispersed light and provides a detection signal corresponding to an intensity of the received fraction of the dispersed light, and wherein the signal processor determines and provides spectral intensity values of the intensity spectra based on the detection signals; and calibrating the spectral axis of the respective spectrometer includes, based on the calibration spectrum lines and the corresponding reference lines, determining and subsequently applying a mapping function correctly assigning spectral lines received by the individual detector elements to the respective detector elements such that calibration spectrum lines of a calibration spectrum determined by the respective spectrometer based on the mapping function coincide with the corresponding spectral reference lines.
12 . The method according to claim 7 , wherein: the reference characteristic includes the emission spectrum of the calibration light source; and for at least one or each of the at least one spectrometer to be calibrated, calibrating the respective spectrometer includes calibrating an intensity axis of the respective spectrometer based on spectral intensity values of the calibration spectrum and corresponding spectral intensity values the emission spectrum.
13 . The method according to claim 12 , wherein calibrating the intensity axis of the respective spectrometer includes adjusting a determination of spectral intensity values of intensity spectra performed by the respective spectrometer by determining and applying spectral correction terms, spectral correction factors, or a spectral correction function for correcting spectral intensity values determined by the respective spectrometer such that the corrected spectral intensity values of the calibration spectrum correspond to the spectral intensity values of the emission spectrum of the calibration light source.
14 . The method according to claim 7 , wherein: the reference characteristic includes the emission spectrum and the spectral reference lines; and for at least one or each of the at least one spectrometer to be calibrated, calibrating the respective spectrometer includes: identifying calibration spectrum features included the calibration spectrum; determining calibration spectrum lines at which the identified calibration spectrum features occur; calibrating the spectral axis of the respective spectrometer based on the calibration spectrum lines and the corresponding spectral reference lines at which the emission spectrum features corresponding to the calibration spectrum features occur; and subsequently calibrating an intensity axis of the respective spectrometer based on spectral intensity values of the calibration spectrum and corresponding spectral intensity values of the emission spectrum.
15 . The method according to claim 7 , wherein: the reference characteristic includes the emission spectrum; and for at least one or each spectrometer that was previously calibrated, calibrating the respective spectrometer includes, based on the calibration spectrum determined by the respective spectrometer and the emission spectrum of the calibration light source, verifying the previous calibration of the respective spectrometer and determining and providing a corresponding verification result.
16 . The method according to claim 15 , wherein: verifying the previous calibration of the respective spectrometer includes determining a degree of correspondence between the calibration spectrum determined by the respective spectrometer and the emission spectrum of the calibration light source; and determining and providing the verification result includes: providing a positive verification result indicating that the previous calibration of the respective spectrometer is valid when the degree of correspondence is larger or equal to a predetermined threshold; and/or providing a negative verification result indicating that the previous calibration of the respective spectrometer is invalid when the degree of correspondence is smaller than the predetermined threshold.
17 . The method according to claim 15 , further comprising, for at least one or each spectrometer for which a verification result indicating that the previous calibration of the respective spectrometer is invalid, re-calibrating the spectral axis and/or the intensity axis of the respective spectrometer based on the calibration spectrum determined by the respective spectrometer and the reference characteristic of the light emitted by the calibration light source.

Description

CROSS-REFERENCE TO RELATED APPLICATION The present application is related to and claims the priority benefit of U.S. Provisional Application No. 63/615,917, filed on Dec. 29, 2023, the entire contents of which are incorporated herein by reference. TECHNICAL FIELD The present disclosure relates to a calibration light source and a method of calibrating at least one spectrometer using the calibration source. BACKGROUND Spectrometers, such as dispersive spectrometers, are, e.g., included in absorption measurement devices as well as in Raman spectrometric measurement devices currently employed in a large variety of different applications including industrial applications, as well as laboratory applications to determine and to provide measurement results of various measurands of a medium. As an example, absorption spectra of a medium determined by a dispersive spectrometer of an absorption measurement device are, e.g., employed to determine and to provide measurement results of concentrations of components included in the medium, a turbidity of the medium and/or at least one other measurand. As another example, Raman spectra of a sample of the medium determined by a dispersive spectrometer of a Raman spectroscopic measurement device are, e.g., employed to determine and to provide measurement results of concentrations of components included in the medium, a pH-value of the medium, a melt index of the medium, a cell motility of the medium, and/or at least one other measurand. Dispersive spectrometers commonly include a disperser, e.g., a diffractive or holographic grating, dispersing incident light, and a detector including an array of detection elements, e.g., a CCD camera, an array of photodiodes or another type of detector array. The detection elements are arranged such that each detection element receives a different fraction of the dispersed light and determines and provides a detection signal corresponding to an intensity of the received fraction of the dispersed light. The detection signals are, e.g., provided to a signal processor determining and providing spectral intensity values of the intensity spectra of the light received by the detector based on the detection signals. Dispersive spectrometers should be calibrated, before they are put into operation. Calibration commonly includes a calibration of a spectral axis of the spectrometer(s), e.g., a wavelength axis or a frequency axis, accounting for a distribution of the spectral lines of the dispersed light across the detection elements of the respective spectrometer. As disclosed in U.S. Pat. No. 6,351,306 B1 the calibration of the wavelength axis of Raman spectrometers can e.g., be performed based on known atomic emission lines of a reference lamp, e.g., a neon or argon reference lamp. In this case, intensity spectra of light emitted by the reference lamp are determined with the Raman spectrometer to be calibrated and a mapping function correctly assigning the wavelengths or frequencies received by the individual detector elements to the respective element is determined based on the known atomic emission lines reflected in the intensity spectra. A disadvantage of this method is however, that points of reference afforded by the atomic emission lines are predetermined by the reference lamp employed and may be absent in spectral ranges of special interest for a given spectroscopic application. This results in a correspondingly poor characterization of the spectral distribution in these spectral ranges. In addition, the cost, complexity, and reliability of reference lamps exhibiting known atomic emission lines may render them undesirable for integration into, or deployment with, commercial spectrometers having expectations of low cost, low maintenance, reliable performance, and long service life. Different dispersive spectrometers may exhibit different spectral responsivities. As a result, the spectral shape and the absolute spectral intensities of intensity spectra of identical samples determined by different spectrometers may be different. Some spectroscopic applications require for these differences to be compensated for. This can be achieved by calibration methods additionally including a calibration of an intensity axis of the spectrometers in a manner accounting for the spectral responsivity of the respective spectrometer. In this context, U.S. Pat. No. 6,351,306 B1 discloses calibrating a spectral responsivity or intensity axis of a Raman spectrometer based on a known emission spectrum of a broad-band intensity calibration light source, e.g., a tungsten lamp. In this case, reference spectra of light emitted by the intensity calibration light source determined with the Raman spectrometer to be calibrated are used to determine the spectral responsivity of the respective Raman spectrometer and to subsequently adjust the determination of the spectral intensity values of the intensity spectra performed by the respective Raman spectrometer based on the ref