Device for the infrared spectrometry measurement of a polychromatic light beam and associated spectrometry measurement system
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2024-03-05
- Publication Date
- 2026-03-18
Smart Images
Figure FR2024050270_14112024_PF_FP_ABST
Abstract
Description
[0001] INFRARED SPECTROMETRIC MEASURING DEVICE FOR A POEYCHROMATIC LIGHT BEAM AND ASSOCIATED SPECTROMETRIC MEASURING SYSTEM
[0002] FIELD OF THE INVENTION
[0003] The invention relates to the field of analyzing a polychromatic light beam, for example a beam having previously interacted with a sample containing a product or a substance, by means of a spectrometric measuring device. The invention also relates to the associated spectrometric measuring system.
[0004] For the purposes of the invention, a spectrometric measurement system makes it possible to determine the chemical and / or physical composition of a sample by decomposing the wavelengths of a light source emitted or absorbed by the sample.
[0005] The invention relates more particularly to an infrared spectrometric measuring device, that is to say a device capable of analyzing the spectrum of a polychromatic light beam from an infrared light source.
[0006] The fields of application of the invention are varied, for example the fields of health, pharmacy, gas analysis, agriculture, food processing, waste sorting, such as plastic or textile waste. ..
[0007] STATE OF THE ART
[0008] Over the full mid-infrared range, i.e. between 1 and 25 micrometers, the most widely used infrared spectrometric measuring device is the FT-IR device, for Fourier Transform Infrared or "Fourier Transform InfraRed" in the English literature.
[0009] An FT-IR device typically integrates a Michelson interferometer with polychromatic illumination, a moving mirror, and a cooled single-element infrared sensor. In a Michelson interferometer with polychromatic illumination, a light source emits in a range of wavelengths. This light passes through a beam splitter, the moving mirror, which divides the beam into two parts. A sample is placed on one of the beam parts so that the light passing through the sample undergoes a phase and amplitude modification that depends on the physical or chemical properties of the sample.
[0010] Thus, each part of the beam passes through a different optical path before combining again on a detector: the cooled single-element infrared sensor. A first drawback of this FT-IR device comes from its complexity, which makes the instrument expensive and unsuitable for portable applications.
[0011] A second disadvantage comes from the movement of the movable mirror, which does not allow for an instantaneous measurement, in addition to posing problems in terms of reliability.
[0012] In order to carry out an instantaneous spectrometric measurement, it is possible to use spectrometry devices that include a dispersive optical element as well as a matrix or linear infrared sensor.
[0013] Spectrometry based on matrix or linear sensors is generally carried out on the SWIR band, or near infrared for "Short Wave InfraRed" in the English literature, characterized by the wavelength range 0.9-2.5 micrometers, or more restrictively on the wavelength range 0.9-1.7 micrometers. For example, spectrometry based on matrix sensors can use InGaAs diodes. Other sensor technologies are also possible to carry out spectrometric measurements in the SWIR band, for example a HgCdTe diode, a T2SL diode (Type-2 Super-Lattice), a SiGeSn diode, or CQD technology (for the English acronym "Colloidal Quantum Dots").
[0014] The disadvantage of SWIR spectrometry is that it is a band where only a part of the absorption lines of organic compounds accessible to FT-IR devices appear, and that these lines are generally quite non-specific. This results in a degraded spectrometric analysis. Another disadvantage of SWIR spectrometry is that it is sensitive to certain inorganic dyes, which can introduce biases into the spectrometric analysis. In particular, it is a band very sensitive to carbon black, the presence of which leads to total absorption across the entire SWIR band, making spectrometric measurement in this band impossible.
[0015] The analysis of carbon black colored samples, such as black plastics, can be done in the MWIR band, or mid-infrared for "Middle Wave InfraRed" in the English literature, which is characterized by the wavelength range of 3-5 micrometers. This is a wavelength range in which the carbon black signature is attenuated. In this MWIR band, it is possible to implement infrared spectrometry devices with HgCdTe, InSb, QWIP (for the English acronym "Quantum Well Infrared Photodetector") or T2SL diodes. A significant disadvantage associated with the use of this type of sensor is that these diodes generally require the use of a cooling device, such as a Stirling cold machine. However, the use of a cooling device poses problems of integration and reliability, which limits the lifespan of the spectrometry device.
[0016] An alternative to using photodiode sensors is the use of thermal sensors, such as pyroelectric or thermoelectric sensors. These types of sensors cover the entire mid-infrared band and can be passive, meaning they do not require a cooling device. However, they typically have limited sensitivity.
[0017] A third family of thermal sensors, microbolometer arrays, have the advantage of excellent sensitivity and can also be used passively. In addition, these uncooled thermal image sensors now exhibit technological maturity resulting from the development of thermal imagers.
[0018] As illustrated in FIG. 1 of the prior art, an uncooled thermal image sensor 100 is conventionally in the form of a hermetic cavity integrating an array of micro-bolometers 19. Each micro-bolometer 19 comprises a membrane 15 mounted in suspension on a base substrate 11 by means of anchoring nails 14. The hermetic cavity is constituted by an optical window 120 and side walls 13 fixed between the base substrate 11 and the optical window 120.
[0019] A 19 microbolometer is for example designed to have maximum sensitivity over the 7.5-14 micrometer wavelength range of interest, or more restrictively over the 8-13 micrometer band, while being relatively insensitive to radiation outside this spectral band. Thus, 19 microbolometers are typically effective in the LWIR or long infrared band for "Long Wave InfraRed" in the English literature. Intrinsically, the uncoated optical window has a transmittance wider than the wavelength range of interest.
[0020] To improve the performance of the micro-bolometers 19, this optical window is treated or structured with respect to the micro-bolometers 19. Typically, this optical window is configured to attenuate wavelengths between 2 and 8 micrometers, and to form an anti-reflection filter for wavelengths of interest between 8 and 13 micrometers. To do this, the optical window 120 may comprise a first filter 170 and a second filter 180, formed respectively on the lower face and the upper face of the optical window 120. Document US 7,135,682 and the scientific publication “Hyperspectral LWIR measurements with imaging diffraction grating spectrometer and uncooled thermal camera”, R. Olbrycht et al., in Measurement Automation Monitoring, 2015, propose two infrared spectrometric measurement devices using an uncooled thermal image sensor.
[0021] However, since uncooled thermal image sensors are sensitive in the LWIR range, these solutions do not allow the characterization of samples identifiable with the MWIR range.
[0022] To address this problem, WO 2003 / 29769 proposes to use two separate spectrometers with a first spectrometer, which uses a cooled InSb camera, operating in the MWIR band, and a second spectrometer, which uses an uncooled thermal image sensor operating in the LWIR band.
[0023] However, as with the FT-IR device, this solution is complex because it requires two sensors, one of which is cooled. Thus, this solution is also expensive and not very suitable for portable applications.
[0024] The technical problem of the invention is therefore to propose an infrared spectrometric measuring device, without moving elements, and capable of simply characterizing samples in the MWIR band and in the LWIR band.
[0025] STATEMENT OF THE INVENTION
[0026] The invention proposes to address this technical problem by using an uncooled thermal image sensor comprising an optical window designed to transmit infrared radiation in a wavelength range between 1 and 25 micrometers. With this particularly wideband optical window, the microbolometers operate in a fundamental resonance range to capture infrared radiation between 7 and 20 micrometers and in a first harmonic resonance range to capture infrared radiation between 3 and 4 micrometers. Thus, the invention does not make it possible to capture all the signatures present in the MWIR band between 3 and 5 micrometers, but simply a sub-part of this band, the sensitivity of the uncooled thermal image sensor being conventionally degraded between 4.5 and 5.5 micrometers.However, the invention stems from a first observation according to which a large number of materials can be characterized in the infrared in two distinct wavelength ranges: a first wavelength range between 7 and 20 micrometers and a second wavelength range between 3 and 4 micrometers.
[0027] This first observation is combined with a second observation that the first harmonic resonance of an uncooled thermal image sensor based on microbolometers is located in this second wavelength range between 3 and 4 micrometers.
[0028] Thus, by increasing the wavelength range transmitted by the optical window and using the fundamental resonance and the first harmonic resonance, it is possible to characterize a large number of materials with a single uncooled sensor.
[0029] This analysis is counterintuitive for a person skilled in the art who knows that the responsivity of a microbolometer drops drastically around 5 micrometers, so that the person would not use a microbolometer to capture wavelengths in the MWIR band. In addition, increasing the range of wavelengths transmitted by the optical window requires limiting its selectivity, thus degrading the response of microbolometers in the first wavelength range between 7 and 20 micrometers.
[0030] In the context of the invention, it has appeared that the degradation of the response in the first range of wavelengths in the LWIR band is advantageously compensated by the detection in the second range of wavelengths in the MWIR band.
[0031] Thus, by using an image processing unit capable of obtaining the spectrum of a polychromatic light beam based on a spectral analysis of the fundamental resonance and the first harmonic resonance, it is possible to effectively characterize a large number of materials or products with simple technical elements that can be implemented within a portable device.
[0032] More specifically, the invention relates to an infrared spectrometric measuring device for a polychromatic light beam comprising: an uncooled thermal image sensor integrating:
[0033] ■ an array of micro-bolometers; each micro-bolometer comprising a resonant optical cavity; and; ■ an optical window configured to receive the polychromatic light beam and to transmit it into the resonant optical cavity so that it is captured by each micro-bolometer; the optical window being configured to limit the range of wavelengths received by the micro-bolometers in order to maximize their response; and an image processing unit capable of obtaining a spectrometric measurement from the images captured by the uncooled thermal image sensor.
[0034] The invention is characterized in that the optical window of the uncooled thermal image sensor is designed to transmit infrared radiation in a wavelength range between 1 and 25 micrometers; and in that the resonant optical cavity of each microbolometer is designed with a fundamental resonance wavelength L) between 7 and 20 micrometers and a first harmonic resonance wavelength between 3 and 4 micrometers so that the microbolometers operate in a fundamental resonance range to capture infrared radiation between 7 and 20 micrometers and in a first harmonic resonance range to capture infrared radiation between 3 and 4 micrometers; said image processing unit being capable of obtaining a spectrometric measurement in the ranges of said fundamental resonance and said first harmonic resonance.
[0035] Preferably, the optical window of the uncooled thermal image sensor is designed to transmit infrared radiation over a wavelength range of 2.5 to 14 micrometers.
[0036] To correctly fix the fundamental resonance and the first harmonic resonance of the uncooled thermal image sensor, it can include a Fabry-Perot type resonant optical cavity with a fundamental resonance wavelength Xo of said cavity between 8 and 12 micrometers. This technological choice makes it possible in particular to optimize the absorbance of the micro-bolometers by constructive interference, and consequently the sensitivity of the sensor, around 10 micrometers, which is advantageous with respect to the transparency band of the atmosphere of the LWIR band. More precisely, a resonant optical cavity is associated with each microbolometer and this optical cavity can be produced by a substrate with reflective properties or by the use of a reflector deposited on the substrate, opposite each microbolometer.Since the spectral response of the uncooled thermal image sensor is not identical with said fundamental resonance and said first harmonic resonance, it is possible to compensate for this disparity with specific optical treatments. In one embodiment, the optical window of the uncooled thermal image sensor incorporates a thin interferometric layer designed to exhibit constructive interference in the vicinity of λo / 2.
[0037] With constructive interference in the vicinity of Xo / 2, destructive interference is generated in the vicinity of Xo / 3, and the optical window makes it possible to oppose the inhomogeneities of the spectral response of the uncooled thermal image sensor, and thus contribute to the reduction of said spectral inhomogeneities.
[0038] Alternatively or in addition, a thin interferometric layer designed to exhibit constructive interference in the vicinity of Xo / 2 is placed on a focal optic.
[0039] Indeed, the infrared spectrometric measuring device preferably comprises: an opening arranged so as to receive the polychromatic light beam; a collimation optic arranged to receive the polychromatic light beam transmitted by the opening; a passive dispersive optical element placed after the collimation optic, and configured to angularly separate the spectral components of the polychromatic light beam; and a focusing optic configured to focus the spectral components of the polychromatic light beam, coming from the passive dispersive optical element, onto the uncooled thermal image sensor.
[0040] The use of a passive dispersive optical element allows the spectral components of infrared radiation to be angularly separated without any moving mechanical elements, allowing an instantaneous measurement to be obtained. Preferably, the passive dispersive optical element is an echelle grating that maximizes the diffraction efficiency for a given order, for example diffraction order -1, and operates in a configuration close to the so-called Littrow configuration, where the grating efficiency is optimal at the desired diffraction order. Alternatively, the passive dispersive optical element is a prism.
[0041] The infrared spectrometric measuring device is preferably integrated into an opaque housing or into a hermetic vacuum enclosure. The invention also relates to an infrared spectrometric measuring system comprising: an infrared light source capable of emitting infrared radiation onto a sample; a polychromatic light beam resulting from the interaction between the infrared radiation and said sample; and an infrared spectrometric measuring device as previously defined, configured to measure the spectrum of the polychromatic light beam resulting from the interaction between the infrared radiation and said sample.
[0042] The sample can be optically connected to the infrared light source and / or the infrared spectrometric measuring device by at least one optical fiber. The advantage of such an embodiment lies in the possibility of easily moving the device outside the measurement area.
[0043] Alternatively, the system comprises: a non-imaging optic, interposed between the infrared light source and the sample, capable of controlling the numerical aperture of the infrared radiation projected onto said sample; and an objective optic, interposed between the sample and the aperture of the infrared spectrometric measuring device, capable of focusing the polychromatic beam whose spectral components are formed by the interaction between the infrared radiation and the sample on the aperture of the infrared spectrometric measuring device.
[0044] The invention thus makes it possible to integrate the infrared spectrometric measurement system into a portable device.
[0045] For example, the portable device comprises: a power source for the infrared light source and the infrared spectrometric measuring device; a measurement triggering element; a spacer capable of positioning the sample relative to the infrared radiation; and a housing integrating the power source, the triggering element, the spacer, the infrared light source and the infrared spectrometric measuring device.
[0046] The portable device may also include a human-machine interface and / or means for transmitting measurements to a remote analysis unit. BRIEF DESCRIPTION OF THE FIGURES
[0047] The invention will be better understood from reading the following description, given solely by way of example, and drawn up in relation to the appended drawings, in which identical references designate identical or similar elements, and in which:
[0048] Figure 1 illustrates a schematic sectional view of an uncooled thermal image sensor incorporating a micro-bolometer array according to the state of the art;
[0049] Figure 2 illustrates a schematic sectional view of an uncooled thermal image sensor incorporating a microbolometer array according to one embodiment of the invention; Figure 3 illustrates the characteristic wavelengths of several materials;
[0050] Figure 4 illustrates the evolution of the spectral response of the sensor of Figure 2 as a function of wavelengths; and
[0051] Figure 5 schematically illustrates an infrared spectrometric measurement device for a sample incorporating the sensor of Figure 2.
[0052] DETAILED DESCRIPTION OF THE INVENTION
[0053] As illustrated in Figure 2, an uncooled thermal image sensor 10 according to the invention is in the form of a hermetic cavity, placed under vacuum, integrating an array of microbolometers 19. Each microbolometer 19 comprises a membrane 15 mounted in suspension on a base substrate 11 by means of anchoring nails 14. The hermetic cavity is constituted by an optical window 12 and side walls 13 fixed between the base substrate 11 and the optical window 12.
[0054] The micro-bolometers 19 absorb the thermal infrared radiation via the membrane 15 and / or via a metal absorber (not shown) placed within a resonant optical cavity 50 of the Fabry-Perot type. For example, this resonant optical cavity 50 has a cavity length equal to Xo / 4, with An the fundamental resonance wavelength of said cavity 50. Preferably, the fundamental resonance wavelength of said cavity 50 is between 8 and 12 micrometers, for example 10 micrometers.
[0055] As illustrated in Figure 2, a resonant optical cavity 50 is associated with each microbolometer 19. It can be produced by a substrate with reflective properties or by the use of a reflector 51 deposited on the substrate, opposite each microbolometer 19. As illustrated in Figure 4, each microbolometer 19 has a maximum response in the LWIR band and a non-zero response for wavelengths below the LWIR band. Thus, in addition to the fundamental resonance at Xo, a first so-called harmonic resonance is present at A / 3-3.33 micrometers. This first harmonic resonance provides a second sensitivity band, which is located in the MWIR band. This first harmonic resonance band is delimited by a first destructive interference at Xo / 2 ~ 5 micrometers, and by a second destructive interference at Xo / 4 ~ 2.5 micrometers.The absorbance of the micro-bolometer 19 is degraded in the vicinity of these destructive interferences.
[0056] A second harmonic resonance is present around Xo / 6 ~ 1.67 micrometer, i.e. in the SWIR band. Although the micro-bolometer 19 is designed to have maximum sensitivity in the LWIR band, i.e. in the wavelength range of interest 7.5 - 14 micrometers, or more restrictively in the band 8 - 13 micrometers, the optical window 12 corresponds to a broadband window.
[0057] According to the invention, this optical window 12 is designed to transmit radiation over the entire mid- and long-infrared range, i.e. between 1 and 25 micrometers, or more restrictively between 2.5 and 14 micrometers. To constitute such an optical window 12, it is possible to use a silicon or germanium substrate with a first filter 17 and a second filter 18, formed respectively on the lower face and the upper face of the optical window 12. Each filter 17-18 can consist of a thin layer, or a stack of thin layers.This stack is for example an alternation of dielectric layers with a low refractive index, for example zinc sulfide, zinc selenide, ytterbium fluoride or another material with a low index transparent in the spectral range of use, and thin dielectric layers with a high refractive index, for example amorphous or crystalline layers of silicon, germanium or another material with a high index transparent in the spectral range of use. In addition, at least one filter 17-18 can be produced by means of a structuring which comprises geometric patterns of a size smaller than the wavelength of use, that is to say structurings of the “dielectric meta-surface”, “2D photonic crystal with membrane”, or even “moth-eye” type.
[0058] In a first embodiment, the transmission curve of such an optical window 12 is designed to be substantially constant over the entire spectral band of use, for example by allowing a spectral transmission inhomogeneity amplitude of less than 20%. In a second embodiment, the optical window 12 is designed with a coating capable of compensating for the spectral inhomogeneities received and detected by the micro-bolometers 19, for example by maximizing the transmission of the window in the vicinity of the destructive interference zones of the Fabry-Perot cavity 50, even if it means degrading the transmission in the vicinity of the constructive interference zones of this Fabry-Perot cavity 50.
[0059] Such a compensation coating can, for example, be implemented in one of the filters 17-18 with an interferometric thin layer, designed to exhibit constructive interference in the vicinity of Xo / 2. Alternatively, compensation for spectral inhomogeneities of the uncooled thermal image sensor 10 can be processed on the optical elements of the infrared spectrometric measuring device 20 of a polychromatic light beam.
[0060] As illustrated in FIG. 5, an infrared spectrometric measurement system 40 of a sample 26 integrates for example: an infrared light source 21 capable of emitting infrared radiation; a non-imaging optic L3, placed after the infrared light source 21, capable of controlling the numerical aperture of the infrared radiation projected onto the sample 26; an objective optic L4 capable of focusing the polychromatic beam whose spectral components are formed by the interaction between the infrared radiation and the sample 26 onto an aperture 28 of an infrared spectrometric measurement device 20; and an analysis unit 35 intended to determine the characteristics of the sample 26 as a function of the spectral measurement of the images captured by the uncooled thermal image sensor 10.
[0061] For example, the infrared light source 21 comprises at least one high luminance emissive element over at least the spectral band of use of the infrared spectrometric measuring device 20. This infrared light source 21 may be a thermal source, such as a halogen source, or a so-called “Globar” source. A Globar source comprises a silicon carbide rod heated to a high temperature, typically of the order of 1500K, so as to emit infrared radiation which is similar to black body radiation.
[0062] Alternatively, the infrared light source 21 may be a semiconductor-based source, such as a light-emitting diode (LED), a super-luminescent diode (SLED), or a plurality of lasers, for example QCLs (Quantum Cascade Lasers), ICLs (Interband Cascade Lasers), or a broadband source resulting from a nonlinear optical phenomenon of super-continuum generation, for example using the Kerr effect. Preferably, the luminance of the infrared light source 21 is stabilized, for example in temperature, using a feedback loop on the electric current injected through the infrared light source 21, or using a Pelltier module in thermal contact with the infrared light source 21. The infrared light source 21 may also be provided with a radiator, making it possible to evacuate the heat generated by the infrared light source 21 when it is switched on.
[0063] The infrared light source 21 can be directly applied to the sample 26, for example by using such a source associated with a non-imaging optic L3. A non-imaging optic L3 consists of at least one refractive lens, for example an aspherical condenser or a Fresnel lens.
[0064] Alternatively, the non-imaging optics L3 may consist of a parabolic mirror whose focus is in the vicinity of the infrared light source 21, or an elliptical mirror whose first focus is in the vicinity of the infrared light source 21 and a second focus is in the vicinity of the sample 26.
[0065] After interaction between the infrared radiation and the sample 26, an objective optic L4 can be used to focus the spectral components onto an aperture 28 of an infrared spectrometric measuring device 20.
[0066] In specular mode, the objective optics L4 projects the beam formed by the interaction between the infrared radiation and the sample 26, at low numerical aperture, with the plane of the aperture 28 of the infrared spectrometric measuring device 20 so as to transmit a collimated beam through the aperture 28. The diffused portion of this beam does not reach the aperture 28.
[0067] In diffuse mode, the non-imaging optics L3 focuses the infrared radiation onto an area of the sample 26, preferably with a high numerical aperture. The objective optics L4 then focuses this illuminated area of the sample 26 with the plane of the aperture 28. This mode of interaction makes it possible to send the radiation scattered by the sample 26 to the infrared spectrometric measuring device 20, with a numerical aperture which corresponds to the numerical aperture of the objective optics L4.
[0068] In addition to transmission or scattering measurement, it is also possible to use multi-pass transmission, reflection, transflection, ATR interaction mode, evanescent wave attenuation interaction mode or any other known interaction mode between light and a sample 26. Preferably, the sample 26 is placed within a sample holder 25. A sample holder 25 comprises, for example, a base on which the sample 26 is placed. Alternatively, the sample 26 is placed on a block that is substantially transparent in the spectral range of use. Alternatively, the sample 26 in liquid or gaseous form is projected through the beam, for example at the outlet of a conveyor or by means of forced convection.The transflection mode consists of placing the sample 26 on a reflective surface of known / calibrated reflectance, and measuring the combination of a reflection on the sample 26 as well as a double transmission of the sample 26. The multi-pass transmission interaction mode consists of placing the sample 26 in a non-resonant multi-pass optical cavity, making it possible to virtually multiply the length of the sample 26. Such an interaction mode is suitable for samples 26 which have low attenuation. For example, this is a mode suitable for gas analysis, within a gas cell which includes internal mirrors, transparent windows in the spectral range of use making it possible to ensure the sealing of the cell, as well as gas inlets and outlets.
[0069] The reflection interaction mode makes it possible to evaluate the light-sample interaction in the vicinity of the surface of the sample 26, over a thickness called "skin thickness" of the order of magnitude of the penetration depth of the evanescent wave. The penetration depth of the evanescent wave is proportional to the wavelength, which is particularly interesting for measurements in the LWIR band compared to the SWIR band.
[0070] A sample holder 25 suitable for reflection measurement is, for example, a base on which the sample 26 is placed on a surface opposite the measurement surface. Alternatively, the sample 26 is placed on a moving conveyor. Preferably, the reflectance spectrum of the base / conveyor is known / calibrated. In the case where the device operates in diffuse mode, the optics L3, L4 are focused in the vicinity of the measurement surface of the sample 26, or in the vicinity of the surface of the base. Alternatively, the base is a spacer on which the sample rests at the measurement surface, and which makes it possible to control the distance between the sample 26 and the optics L3, L4, and thus guarantee good quality focusing of the infrared radiation.
[0071] The ATR interaction mode, for "Attenuated Total Reflectance" in the English literature, evaluates in particular the attenuation of an evanescent wave in the case of total internal reflection within a refractive material. To do this, a Dove prism is conventionally used. In this interaction mode, the sample 26 is positioned on the facet where the total internal reflection occurs and the attenuation of the evanescent wave is measured. The interaction mode by attenuation of the evanescent wave is also carried out in the vicinity of a waveguide, such as for example an optical fiber, a waveguide on a substrate, or a planar guide. This waveguide can be single-mode, or multi-mode. The interaction is made possible when the cladding of the guide is released, to position the sample 26 there.The advantage of such an interaction mode comes from the fact that it is easy to inject the light at the input of the waveguide, and to optically couple the output of the waveguide to the spectrometer. To optimize the interaction of the evanescent wave with the sample 26, it is possible to increase the length of the released guide, for example by making it form a spiral, or to reduce the confinement of the guided mode, for example by reducing the section of the core of the guide.
[0072] After passing through the sample 26, the infrared radiation formed by the interaction between the infrared radiation and the sample 26 is captured by the uncooled thermal image sensor 10. Preferably, this infrared spectrometric measuring device 20 is integrated into an opaque housing 30, optically open only at an opening 28, and configured to limit the impact of external thermal radiation sources on the spectrometric measurement.
[0073] The opening 28 has a controlled or variable size so as to receive and spatially select the infrared polychromatic light beam to be analyzed. This opening 28 may be circular, for example an iris, or rectangular, for example a slit.
[0074] The opening 28 can alternatively be replaced by a female fiber optic connector, for example a connector chosen from the FP / PC standards, for "Fiat Polish / Physical Contact" in the English literature, or SMA, for "SubMiniature version A" in the English literature. In addition, an optical fiber can also be used to optically connect the infrared light source 21 to the sample 26.
[0075] In the infrared spectrometric measuring device 20, numerous optical elements may be used between the aperture 28 and the uncooled thermal image sensor 10. For example, a collimating optic L1 may be used to receive the polychromatic light beam transmitted through the aperture 28. This collimating optic L1 may consist of a refractive lens, a group of refractive lenses or a concave mirror, such as a parabolic mirror.
[0076] A refractive lens is formed from a material transparent in the spectral range of use, which may be selected from the following exemplary materials: zinc sulfide, zinc selenide, chalcogenide glass, barium fluoride, calcium fluoride, silicon, germanium, or a semiconductor alloy of silicon and germanium. Preferably, at least one optical passage surface of the lens is coated with an anti-reflective coating, designed according to substantially the same methods as those implemented for the design of the coatings of the optical window 12. A concave mirror is either formed from a material reflective in the spectral range of use, such as a metal, or the concave mirror is provided with a coating reflective in the spectral range of use, such as a metal coating.
[0077] The passive dispersive optical element 24 makes it possible to angularly separate the spectral components of a polychromatic infrared beam. The passive dispersive optical element 24 is characterized in particular by its deviation parameter dfUdX, given in rad / nm, which determines its capacity to angularly separate the spectral components. This dispersive element is chosen from a prism or a diffraction grating. It is also possible to combine the properties of prisms and gratings. A dispersive prism is a refractive element consisting of a single block, and which has at least one substantially planar input facet and a substantially planar output facet. The prism can alternatively have at least one facet for internal reflection of the light beam passing through it. The dispersive prism is characterized by a non-zero angle between the input facet and the virtual output facet, derived from the tunnel diagram of the prism.The polychromatic beam enters the prism through the entrance facet, and exits through the exit facet with angular deviation of the spectral components of the exit beam.
[0078] The prism is formed from a transparent material in the spectral range of use, and which has a dispersion of refractive index n(X), empirically responding to dispersion models such as Seillmeier or Herzberger. This material is for example zinc sulfide, zinc selenide, chalcogenide glass, calcium fluoride, barium fluoride or sapphire. Preferably, the entrance facet as well as the exit facet are covered with an anti-reflective coating configured to optimize the optical transmission of the beam which passes through the prism. Such an anti-reflective coating is designed according to the same techniques used to form the coating of the optical window 12.In the case where the prism has at least one internal reflection facet, and in particular when the beam is not in conditions at an angle of incidence conducive to the phenomenon of total internal reflection, then such a reflection facet is covered with a reflective coating, such as for example a metallic coating. The geometry of the prism is for example chosen from the following geometries: triangular prism, Littrow prism, Pellin-Broca prism, Abbe prism. To obtain the desired deviation parameter dfLdX with a prism, it is possible to use the index dispersion law. A diffraction grating is an element consisting of a periodic pattern repeated linearly on a substantially flat face. The pattern, called a "line", is elongated in the direction perpendicular to the repetition direction.The pattern can be geometric, such as sinusoids, grooves, teeth, inclined facets, or a ladder grating, also called "blaze grating" in the English literature. The grating can also be formed by modulation of the refractive index. It allows the spectral components of a light beam to be angularly separated perpendicular to the axis of the lines, by the principle of diffraction. To obtain the desired deflection parameter dfLdX with a diffraction grating, it is possible to use Bragg's law.
[0079] Such a diffraction grating preferentially operates in reflection, that is to say that the incident beam is located in the same hemisphere delimited by the plane of the grating as the diffracted components of the output beam. It is formed from a reflective material in the spectral range of use, such as a metal, or is provided with a reflective coating in the spectral range of use, such as a metallic coating. Preferably, the diffraction grating is an echelle grating which makes it possible to maximize the diffraction efficiency for a given order (for example diffraction order -1), and operates in a configuration close to the so-called Littrow configuration, where the efficiency of the grating is optimal at the desired diffraction order.
[0080] Alternatively, the diffraction grating operates in transmission mode, i.e. the incident beam is not located in the same hemisphere bounded by the grating plane as the diffracted components of the output beam. It is formed from a material that is transparent in the spectral range of use. This is, for example, a transmission holographic grating.
[0081] Alternatively, the diffraction grating is formed on a concave, rather than a flat, surface, which makes it possible to fulfill the function of the at least one optic LI, L2 of the infrared spectrometric measuring device 20.
[0082] The focusing optics L2 are intended to focus said spectral components onto a focal plane, at which the uncooled thermal image sensor 10 is located. This focusing optics L2 can be designed in the same way as the collimating optics L1.
[0083] In the example of Figure 5, the polychromatic light beam is applied to the aperture 28 and it passes through the collimation optics L1, the passive dispersive optical element 24, the focusing optics L2, through the optical window 12 before being applied to the focal plane of the microbolometers 19. Optionally, the infrared spectrometric measuring device 20 can be encapsulated in a hermetic enclosure, preferably under vacuum, provided with a window transparent in the mid-infrared, and intended to limit or eliminate the impact of air absorption along the optical path. The transparent window is preferably provided with an anti-reflective coating on the spectral band of use. When the infrared spectrometric measuring device 20 is not encapsulated under vacuum, the uncooled thermal image sensor 10 must necessarily be encapsulated under vacuum to guarantee the operation of the microbolometers 19.
[0084] In the case where the system 40 is equipped with a lighting system, it is advisable to encapsulate both the infrared spectrometric measuring device 20 and the infrared light source 21 in the same housing, the latter being able to be hermetic.
[0085] The uncooled thermal image sensor 10 is provided with a circuit for reading, digital conversion and transmission of the signal received by said sensor, by means of an electrical connector. The signal from the sensor is then processed, in order to be able to provide an infrared spectrum.
[0086] To do this, a non-uniformity correction is applied to the image from the uncooled thermal image sensor 10. This non-uniformity correction is performed by subtracting a reference image that contains the information of the fixed spatial noise of the sensor. This reference image is obtained, for example, by using an image seen by the sensor when a mechanical shutter is engaged. Alternatively, this reference image is obtained, or calculated from a prior calibration of the fixed spatial noise of the uncooled thermal image sensor 10.
[0087] Preferably, the image has areas not illuminated by the useful beam, which gives a scalar reference of black. The image is optionally derived from a temporal summation of several images of the video stream, which makes it possible to reduce stochastic temporal noise. An additional step of spatial denoising of the image can be carried out, for example by means of a Gaussian blur, a median filter, a bilateral filter, etc.
[0088] A horizontal intensity profile is extracted on a line of the image. Several lines can be summed horizontally in order to reduce noise. Each of the pixels along the horizontal profile is associated with a wavelength, or with a wave number. This link between a pixel and a wavelength is achieved by a prior calibration, for example by the measurement of at least one spectrum which has marked and well-known spectral characteristics. Thus, the measured spectrum is then divided by a reference spectrum, obtained for example in the absence of the sample 26, or in reflection mode with a reference reflective sample. This operation makes it possible in particular to compensate for the inhomogeneities of the infrared light source 21, the optical elements and the uncooled thermal image sensor 10.The spectrum thus obtained is finally returned to the user, either by means of a display which takes the form of a digital screen, or by an electrical connection, or stored in a memory accessible after the measurement.
[0089] Alternatively, the system 40 integrates a classification model implemented in an analysis unit 35, intended to identify a sample class based on the analysis of the infrared spectrum. This classification model comes from prior machine learning, from a database of spectra of labeled materials. Such a model is for example a model resulting from a principal component analysis unit, also known by the acronym PCA for “Principal Component Analysis” in the English literature, followed by a support vector machine, also known by the acronym SVM for “Support Vector Machine” in the English literature.
[0090] Alternatively, such a model is for example derived from a neural network. Once the classification model is correctly trained, the infrared spectrometric measuring device 20 makes it possible to determine a class identifier from the measured spectrum.
[0091] This model is advantageously integrated into a computer embedded in the infrared spectrometric measurement system 40. This class identifier is then returned to the user by the means described above. Preferably, the class identifier is returned via a digital display. Such an infrared spectrometric measurement system 40 is thus a classification / sorting tool.
[0092] As illustrated in Figure 3, the wavelength ranges captured by the infrared spectrometric measurement system 40 can characterize a large number of materials: high-density polyethylene (HDPE), which is a type of high-density, corrosion-resistant plastic often used for water pipes and chemical storage tanks; low-density polyethylene (LDPE), which is a flexible, transparent plastic often used for plastic films and packaging bags; polyethylene terephthalate (PET), which is a type of heat- and tear-resistant plastic often used for water bottles and food packaging; polypropylene (PP), which is a heat- and chemical-resistant plastic often used for food packaging, grocery bags, and automotive parts;Polystyrene (PS) which is a lightweight, rigid plastic often used for disposable cups and food packaging; Polyvinyl chloride (PVC) which is a corrosion-resistant plastic often used for water pipes, flooring, and food packaging; Polyurethane (PU) which is a wear- and tear-resistant material often used for flooring, foams, furniture, and footwear; Polyamide (PA) which is a wear- and heat-resistant plastic often used for automotive parts, clothing, and food packaging; Polycarbonate (PC) which is a transparent, impact-resistant plastic often used for eyeglasses, computer monitors, motorcycle helmets, and cell phone cases;
[0093] 1' acrylonitrile butadiene styrene (ABS) which is an impact and scratch resistant plastic often used for automotive parts, toys, additive manufacturing materials and computer cases; polytetrafluoroethylene (PTFE) which is a heat and chemical resistant material often used for non-stick coatings and gaskets; polymethyl methacrylate (PMMA) which is a transparent plastic often used for automobile windows, television screens and optical lenses; ethylene vinyl acetate (EVA) which is a flexible and water resistant material often used for shoes and yoga mats; nitrile which is a chemical resistant synthetic rubber often used for protective gloves, latex which is a natural rubber often used for surgical gloves, condoms and balloons;Polyacrylic, which is a heat- and weather-resistant acrylic polymer often used for flooring, adhesives, and textiles; Elastane (EL), which is a polymeric textile fiber, often blended with cotton to make the textile elastic and provide soft, stretchy clothing; Wool (WO), which is a natural fiber obtained from the fleece of sheep and often used for clothing and carpets; Silk (Silk), which is a natural fiber produced by the caterpillars of certain types of butterflies and often used for clothing and bedding; Cotton (CO), which is a natural fiber, often used for clothing, bedding, and towels; Viscose (VV), which is a regenerated plant fiber often used for wash-resistant clothing;and cellulose acetate (CA) which is a regenerated plant fiber produced from cellulose and often used for clothing, curtains and carpets. In comparison, prior art devices using only an uncooled thermal image sensor 100 only capture wavelengths in the boxed area.;
[0094] In the application field of plastic waste sorting, the invention thus makes it possible to classify or quantify plastic mixtures such as PET (PolyEthylene Terephthalate), HDPE (High Density PolyEthylene), PVC (PolyVinyl Chloride), LDPE (Low Density PolyEthylene), PP (PolyPropylene), PS (PolyStyrene), ABS (Acrylonitrile Butadiene Styrene), PA (Poly Amide), PMMA (PolyMethyl MethAcrylate), PC (PolyCarbonate) as well as other types of plastics mentioned above. The device according to the invention is also capable of sorting dark plastics, colored with carbon black.
[0095] In the application field of textile waste sorting, the invention can classify or quantify mixtures of textile fibers from plant sources (cotton, linen, jute, sissal etc.), regenerated (cellulose acetate, viscose, lyocell etc.), animal (wool, silk, cashmere etc.) and synthetic (polyester, polyamide / nylon, elastane etc.).
[0096] For example, such a spectrometric measuring device 20 was tested to evaluate the spectrum of a plastic sample less than 1 millimeter thick in a specular transflectance setup, with an angle of incidence of approximately 13°.
[0097] Furthermore, the infrared spectrometric measurement system 40 can be integrated into a portable device comprising: a power supply for the infrared light source 21 and the uncooled thermal image sensor 10; a measurement triggering element; a spacer capable of positioning the sample 26 relative to the infrared radiation; and a housing integrating the power supply, the triggering element, the spacer, the infrared light source 21 and the uncooled thermal image sensor 10.
[0098] For example, the case can take an ergonomic shape with a handle in which the trigger element is arranged in the form of a trigger.
[0099] Preferably, the portable device comprises a human-machine interface and / or means for transmitting measurements to a remote unit. According to an alternative implementation of the invention, the invention can be used to form a linear scanning hyperspectral imaging system, also called "push-broom" in English literature.
[0100] The principle of such a system is to couple a two-dimensional uncooled thermal image sensor 10 with a passive dispersive optical element 24 so as to form the image of a scene line on a first dimension of the sensor, and spectral components on the second dimension of the sensor. In this embodiment, the opening 28 of the infrared spectrometric measuring device 20 is preferably a rectangular opening, for example with one or more slits. This slit is optically coupled with the objective optics L4 to a line on the object plane. In order to obtain a two-dimensional hyperspectral image of the scene, it is possible to record a sequence of images as a function of the relative rectilinear displacement (perpendicular to said line) of the imaging system with respect to the scene. This type of scene is for example a conveyor of an industrial line, or a conveyor of a sorting line, an agricultural vehicle, a drone, etc.
[0101] Said hyperspectral imager is advantageously provided with linear illumination, as well as a non-imaging optic L3, which may comprise at least one cylindrical optic, and which makes it possible to project the image of the linear illumination onto the line of the object plane evaluated by the spectrometer.
[0102] According to another implementation variant, the invention can be used to form a spectrometer associated with a microscope, making it possible to carry out a spectrometric measurement on a microscopic area of a sample 26.
[0103] The microscope may consist of a standard microscope. It comprises a revolver on which at least one microscope objective is fitted. In this embodiment, the L4 objective optics may be applied to the microscope objective, for example on a side port.
[0104] Thus, the infrared spectrometric measuring device 20 of the invention can effectively detect the spectrum of a polychromatic light beam and meet several distinct applications, in particular to form an infrared spectrometric measuring system 40.
Claims
CLAIMS 1. Infrared spectrometric measuring device (20) of a polychromatic light beam comprising: an uncooled thermal image sensor (10) integrating: ■ an array of micro-bolometers (19); each micro-bolometer (19) comprising a resonant optical cavity (50); and; ■ an optical window (12) configured to receive the polychromatic light beam and to transmit it into the resonant optical cavity (50) so that it is captured by each microbolometer (19); the optical window (12) being configured to limit the range of wavelengths received by the microbolometers (19) in order to maximize their response; and an image processing unit (29) capable of obtaining a spectrometric measurement from the images captured by the uncooled thermal image sensor (10); characterized in that the optical window (12) of the uncooled thermal image sensor (10) is designed to transmit infrared radiation in a range of wavelengths between 1 and 25 micrometers;and in that the resonant optical cavity (50) of each micro-bolometer (19) is designed with a fundamental resonance wavelength Xo of between 7 and 20 micrometers and a first harmonic resonance wavelength of between 3 and 4 micrometers so that the micro-bolometers (19) operate in a fundamental resonance range for capturing infrared radiation between 7 and 20 micrometers and in a first harmonic resonance range for capturing infrared radiation between 3 and 4 micrometers; said image processing unit (29) being capable of obtaining a spectrometric measurement in the ranges of said fundamental resonance and said first harmonic resonance.; 2. Infrared spectrometric measuring device according to claim 1, wherein the optical window (12) of the uncooled thermal image sensor (10) is designed to transmit infrared radiation over a wavelength range between 2.5 and 14 micrometers.
3. Infrared spectrometric measuring device according to claim 1 or 2, wherein the resonant optical cavity (50) is of the Fabry-Perot type with a fundamental resonance wavelength Xo of said cavity (50) between 8 and 12 micrometers.
4. Infrared spectrometric measuring device according to claim 3, wherein the optical window (12) of the uncooled thermal image sensor (10) incorporates a thin interferometric layer designed to exhibit constructive interference in the vicinity of Xo / 2.
5. Infrared spectrometric measuring device according to one of claims 1 to 4, comprising: an aperture (28) arranged to receive the polychromatic light beam; a collimation optic (L1) arranged to receive the polychromatic light beam transmitted by the aperture (28); a passive dispersive optical element (24), placed after the collimation optic (L1), and configured to angularly separate the spectral components of the polychromatic light beam; and a focusing optic (L2) configured to focus the spectral components of the polychromatic light beam, coming from the passive dispersive optical element (24), onto the uncooled thermal image sensor (10).
6. An infrared spectrometric measuring device according to claim 5, wherein the passive dispersive optical element (24) is an echelle grating or a prism.
7. Infrared spectrometric measuring device according to claim 5 or 6, wherein an interferometric thin layer designed to exhibit constructive interference in the vicinity of Ao / 2 is placed on a focal optic.
8. Infrared spectrometric measurement system (40) comprising: an infrared light source (21) configured to emit infrared radiation onto a sample (26); a polychromatic light beam being formed by the interaction between the infrared radiation and said sample (26); and an infrared spectrometric measuring device (20) according to one of claims 1 to 7, configured to measure the spectrum of the polychromatic light beam formed by the interaction between the infrared radiation and said sample (26).
9. Infrared spectrometric measuring system according to claim 8, wherein the sample (26) is optically connected to the infrared light source (21) by at least one optical fiber and / or to the infrared spectrometric measuring device (20) by at least one optical fiber.
10. Infrared spectrometric measurement system according to claim 8, wherein the system comprises: a non-imaging optic (L3), interposed between the infrared light source and the sample (26), capable of controlling the numerical aperture of the infrared radiation projected onto said sample (26); and an objective optic (L4), interposed between the sample and the aperture of the infrared spectrometric measurement device, capable of focusing the polychromatic beam whose spectral components are formed by the interaction between the infrared radiation and the sample (26) onto an aperture (28) of the infrared spectrometric measurement device (20).
11. Infrared spectrometric measurement system according to one of claims 8 to 10, integrated in a portable device comprising: an electrical power source for the infrared light source (21) and the infrared spectrometric measurement device (20); a measurement triggering element; a spacer capable of positioning the sample (26) relative to the infrared radiation; and a housing integrating the electrical power source, the triggering element, the spacer, the infrared light source (21) and the infrared spectrometric measurement device (20).
12. Infrared spectrometric measurement system according to claim 11, wherein the portable device comprises a human-machine interface and / or means for transmitting the measurements to a remote analysis unit (25).