FR2787583A1 - Infra-red thermograph electro-magnetic field sensor for viewing of such fields - Google Patents

Abstract

The sensor (1) includes a support (2) which is sensible transparent to a hyper-frequency electro-magnetic field. One of the faces of the support contains a network of periodic sensibly parallel bands (3) of a photo-thermal material which partially absorbs the energy of the electro-magnetic field. The band (3) material has at least a dielectric loss factor and are preferably made from an indium oxide and tin. The band (3) material has at least a magnetic loss factor and is preferably ferro-magnetic. The sensor has transmission factors for the electric or magnetic field components for the electro-magnetic field (E//,H//) sensibly parallel to the bands (3) which are greater than about 90 per cent. The sensor has absorption contrast between the electric and magnetic field components of the elctro-magnetic field (E//,H//) sensibly parallel to the bands (3) which is greater than 10.

Description

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  The present invention relates to the visualization of electromagnetic fields by means of electromagnetic fields.

infrared thermography.

  More particularly, the invention relates to a sensor comprising a medium sensitive to microwave radiation whose heating is

  detected to visualize the electromagnetic field.

  A brief history of the evolution of this sensor is presented at the beginning of the article "EMIR: a photothermal tool for electromagnetic phenomena characterization"

  Daniel BALAGEAS and Patrick LESVESQUE, Rev. Brig.

  Therm. (1998) 37, pp. 725-739. According to this prior art, the sensitive medium is a deposit whose electromagnetic properties and thickness are controlled to visualize with a thermal imager infrared images representative of the

electromagnetic field.

  The problems on which the technology of such a sensor known to reach the field of quantum is essentially thermal. These thermal problems are the elimination of thermal exchanges by natural convection, and the elimination of thermal losses in the substrate or support of

the absorbent layer.

  To improve this technique by eliminating the disturbing influence of the thermal effects mentioned above, the applicant has proposed and developed these solutions.

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  recent years an EMIR method presented in the aforementioned article and based jointly on: - an infrared thermography with synchronous detection and digital integration applied in conjunction with a microwave illumination modulated in amplitude, and - a thin film photothermal, less than 200 m, allowing to visualize

  the intensity of an electric or magnetic field.

  Quantitative visualization of the fields

  electromagnetic and more particularly micro-

  waves has always aroused great interest. All the methods mentioned above only provide

  the intensity of the electromagnetic field.

  A complete definition of an electromagnetic field requires knowledge of its various components. This so-called vectorial characterization is particularly difficult to obtain and is in practice only achieved by direct methods. As their name implies, these consist in directly measuring the electromagnetic field radiated by a radiating source or an electromagnetic field diffracting structure to be tested by means of an antenna or a network of point antennas for reception and reception. an electronic circuit for generating, detecting and controlling an appropriate signal (see French patents FR-2632417, FR-2663426 and FR-2674028 and patent

European EP-0355114).

  These direct methods of detection and measurement

  are not based on infrared thermography.

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  On the other hand, indirect detection and measurement methods only allow access to the field module and not to their polarization. Virtually all indirect methods are based on photon-heat conversion and dissipated powers are visualized using detectors.

  infrared devices (see US Patents US-5,417,494 and US Pat.

3693084).

  In direct methods, the visualization of electromagnetic fields is hampered by the cumbersome implementation, especially in terms of duration, as long as it is necessary to explore, by means of a probe, the surface or the volume to be measured. , under investigation. The direct methods have the following drawbacks with respect to indirect methods: low spatial resolution of the order of half-wavelength, whereas it reaches a few tens of microns by infrared thermography; relatively low bandwidth, related to the characteristic dimensions of the receiving antennas used; - Inadequacy to near field measurements: a receiving antenna placed in the vicinity of the radiating source or the diffracting structure significantly disturbs the local electromagnetic field; - impossibility to explore large volumes and to obtain electromagnetic field maps in

real time.

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  The present invention aims to provide an electromagnetic field sensor to implement an indirect method that allows a vector measurement

electromagnetic field.

  To this end, a sensor comprising a support substantially transparent to a microwave electromagnetic field, is characterized in that one of the faces of the support comprises a periodic array of substantially parallel strips of photothermal material partially absorbing the energy of said electromagnetic field. The network of bands is periodic rectilinear, which renders the sensor anisotropic so that it is sensitive to only one component of the field, incident or

local, parallel to the bands.

  An indirect method of vector measurement of electromagnetic fields by means of the sensor according to the invention has the advantages of the indirect methods compared with the drawbacks mentioned above of the direct methods: ease of visualization in the near field; - field surveys on large volumes or large areas; - field surveys in real time; - increased spatial resolution limited by step

network.

  According to first embodiments, the material of the sensor strips may have at least dielectric losses, and is, for example, indium tin oxide, or may exhibit magnetic losses, and is preferably ferrimagnetic, so that Fl r1

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  to promote the detection of electric fields or

magnetic field.

  The field component detection being performed by infrared thermography, a heating contrast between two polarizations of electric field and magnetic field parallel to the bands and therefore an absorption contrast in the sensor are sought. The sensor of the invention does not discriminate the two polarizations with respect to their transmissions as a polarizer, but with respect to their absorptions. Thus, preferably, the sensor has transmission factors that are greater than about 90%, that the electric field and magnetic field components of the electromagnetic field are substantially parallel to the bands or not, and has an absorption contrast between said electric field and field components

  magnetic which is at least greater than 10.

  High absorption contrast and good transmittivity in both polarizations are obtained only by finely tuning the intrinsic properties and geometric characteristics of the network. The pitch is preferably much smaller than the wavelength so as to disturb the electromagnetic field analyzed. Typically, the ratio of the wavelength of the electromagnetic field to the pitch of the band network is greater than about ten. In the microwave domain, the pitch of the band network is millimetric, which increases the spatial resolution compared to those of the methods.

direct known.

  As will be seen in the following, an abacus presenting the product of the transmissions in the two polarizations as a function of the absorption contrast --IF-11VIIIr

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  and doubly parameterized by a network fill rate and a square resistance of the strips makes it possible to choose the properties of the network and the material of the bands according to desired characteristics of transmission and absorption. The sensor may comprise as a thermographic imaging means, an infrared camera or an infrared detector, or a thermochromic layer on one of the faces of the support so as to directly view the heating produced by the network of bands. The thermochromic layer comprises liquid crystals encapsulated in a polymer and embedded in a solution to be deposited, for example

  spray on one side of the support.

  According to another remote-controlled embodiment, the sensor comprises an optical fiber or a bundle of optical fibers connecting a face of the support to a

means of infrared imaging.

  The invention also relates to an arrangement comprising at least two sensors having at least one of the following parameters different from one another: width of the bands, thickness of the bands, pitch of the grating, imaginary part of the dielectric permittivity of the photothermal material of the bands, imaginary part of the magnetic permeability of the photothermal material of the bands, orientation of the bands of the network. Two sensors in the arrangement may be juxtaposed or superimposed, for example with perpendicular strips in order to analyze at the

  once orthogonal field components.

  The arrangement may comprise a flexible sheet, preferably transparent, supporting a sensor or I1;

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  juxtaposed sensors. Thanks to the flexibility of the sheet, it can fit any shape or volume, which allows to analyze vectorially an electromagnetic field superficially around this shape or any volume. According to another variant, the arrangement may comprise

  pellets issued from sensors according to the invention.

  The indirect measurement method with the sensor of the invention advantageously makes it possible, with respect to known indirect methods, to measure both the modulus and the phase of electromagnetic fields using interferometric techniques. In the general case of a multi-component electromagnetic field, the aforementioned EMIR indirect method does not

  allows obtaining only a pseudo-phase scalar.

  On the other hand, the indirect method with the sensor of the invention being based on a vector measurement,

  it provides the true phase of each component.

  Finally, the indirect measurement method with the sensor of the invention offers, with respect to all the other methods, the following specific advantages: a bandwidth, typically greater than a decade in frequency, greater than that of the methods direct; a light material, a support such as a film with a deposit forming the network of strips and an infrared camera or a thermochromic layer,

easily transportable on site.

r: - TI li

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  Other features and advantages of the present invention will become more apparent to the

  reading the following description of several

  preferred embodiments of the invention with reference to the corresponding appended drawings in which: - Figure 1 shows a front view of a photothermal parallel band network in an elementary electromagnetic field sensor according to the invention; FIG. 2 shows a schematic sectional view of the elementary sensor of FIG. 1, positioned in front of an infrared camera; - Figure 3 is a sectional view on a large scale of the band network; FIG. 4 is an abacus for selecting a network of bands according to the invention; - Figure 5 is a sectional view similar to Figure 2, the sensor having a thermochromic layer replacing the infrared camera; FIG. 6 is a schematic perspective view of two superimposed elementary sensors with strips perpendicular to each other; and FIG. 7 is a sectional view of two elementary sensors superposed with networks of

  bands with widths and not different.

  As shown in FIGS. 1 and 2, an electromagnetic field vector sensor 1 according to the invention comprises a thin support 2 and a periodic network of thin parallel strips 3 on one of the

faces of the support.

  The support 2 is not very disturbing for a microwave electromagnetic field of frequency greater than about 1 GHz, that is to say is

IV: '11 1 1

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  substantially transparent to this field. The support is a priori of any size; however, it may be in the form of a rectangular strip stretched in a frame or between two rigid parallel rods in order to orient it precisely with respect to the direction of propagation of an electromagnetic field, for example by extending it transversely to a waveguide or between two antennas; it may still be in the form of a small circular pellet. The dimensions, width and length or diameter, of the support are typically between about half a centimeter and

a few meters.

  The support 2 is a plastic film, for example Kapton (registered trademark) of dielectric permittivity E2 = 3.04 + 0.013 j, which can be stretched into a flat surface, or which can adhere to a surface of any shape, even undevelopable. Kapton also has good resistance to temperature and humidity. The support preferably has a thickness e of a few

  tens of microns, for example 25 im.

  The parallel strips 3 have a thickness h of a few tens of nanometers, for example 150 nm, and have a width 1 of the order of a few hundred microns. A photothermal material is deposited on one of the faces of the support 2 to form the strips, by vaporization through a mask, or by laser ablation of an isotropic layer or by any

other way.

  The photothermal material of the strips has dielectric losses, equal to the imaginary part E "of the complex relative dielectric permittivity of the photothermal material: E3 = [F r-Iif,

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  "and / or magnetic losses equal to the imaginary part," of the complex relative magnetic permeability of the photothermal material 3 =

p, - ij i.

  A resistive photothermal material having predominant dielectric losses is, for example, indium tin oxide ITO (Indium Tin Oxide). When the resistive photothermal material constituting the grating strips 3 exhibits only magnetic losses, it may be of ferromagnetic type, or is preferably ferrimagnetic so as not to present electrical losses. Magnetic losses predominant with respect to electrical losses make it possible to detect almost only the magnetic field of an incident electromagnetic wave. A photothermal material with predominant magnetic losses is particularly interesting for non-planar waves, typically in the near field, where magnetic field and electric field are not linked.

direct way.

  In particular using photothermal material with predominant magnetic losses, the sensor can constitute a "frequency filter" which is more absorbent in a certain frequency band than in others. Several juxtaposed sensors having photothermal materials with different magnetic losses reveal the presence of certain

  specific frequencies in wave trains.

  The dielectric and / or magnetic losses are necessary in the material constituting the strips so that they absorb a small portion of the radiation energy of an electromagnetic field illuminating the sensor, having a modulus of electric field E and a magnetic field module H, and that the absorption by the material of the bands produces a heating thereof perceptible by a thermographic imaging means, such as an infrared camera. The volume density of power absorbed by the material is given by the following formula: Pabs [(O + (27.F).) E + (27.F) .H 2/2 in which a is the conductivity of the material of the bands 3 and F the frequency of the electromagnetic field Referring to FIG. 3, the strips 3 form a network of steps d deposited on the support 2 whose electrical length eJ2 is sufficiently small with respect to the wavelength X of the illuminating field

  crossing the sensor so as not to disturb the field.

  The network is considered, for the abacus calculation, as an infinite succession of strips of rectangular section of pitch d and infinite length deposited on the support. The network is also characterized by a filling ratio f equal to the ratio l / d corresponding to the linear fraction of the conductive material of the network seen in section, as shown in FIG. 3. In the case of depositing a continuous conductive strip , it can be characterized from an electromagnetic point of view by its square resistance, that is to say the electrical resistance that would meet an electric current J flowing in the thickness h of a wafer of longitudinal square section 1 x 1, the square resistance:

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R = p.l / (lxh) = 1 / (o.h).

  The square resistance R and the filling ratio f = 1 / d of the parallel strip network 3 in photothermal material are parameters which were used to search for networks having a "good" transmission factor <T> typically greater than% ( 0.45 dB) for electromagnetic field polarizations E // and H // and a <A> absorption very much less than 1%, for the electromagnetic field polarization H //. <T> and <A> denote average values for pairs {o, f} calculated with log-distributed values of C and f in two respective predetermined intervals. The polarization E // denotes the transverse component of the electric field passing through the sensor parallel to the strips 3, a monochromatic plane wave polarized rectilinearly, and the polarization H // designates the transverse component of the magnetic field passing through the sensor parallel to the strips 3d. another monochromatic wave polarized rectilinearly. By way of example, the bands 3 of the grating are oriented parallel to the electric field component of a normal incidence polarized wave in TEM, TE or TH transverse mode, and after a 90 degree rotation of the support 2 in its plane. , the bands of the network are parallel to the transverse component of the magnetic field of this wave

polarized electromagnetic.

  The discrimination between the electric field component E // and the magnetic field component

  H // is reflected by the abacus shown in Figure 4.

  By means of this abacus, bands of photopolymer bands 3 discriminating in polarization

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  and having a pitch d, such that k / d is between 15 and 300, are selected so that they have a good transmission greater than 95% for each of the H // and E // polarizations, corresponding to a product <T > H // x <T> E // greater than 90%, illustrated in Figure 4, and a good contrast between the two polarizations E // and H //, indicated by a ratio of absorption factors C = <A> E // / <A> H // at least greater than 10. A network of strips 3 is chosen as a function of the filling factor f = 1 / d and the -1 'square resistance R = (ch By way of example, an anisotropic sensor 1 according to the invention comprising an absorbent network with parallel strips 3 and a Kapton support 2 25 μm thick has been manufactured with the following characteristics: - no: d = 1 mm -1 square resistance: R = (ah) = 250 Q

- filling rate: f = 40%.

  The network of bands has been designed to have, at an electromagnetic wave frequency of F = 12 GHz, a strong absorption for the E // polarization and a virtually zero absorption

for the polarization H //.

  At 12 GHz, the network is such that S / d = 25, a value included in the experimental range [15, 300] of the X / d ratio for a grating with a pitch of d = 1 mm and a variable frequency F. In this range of frequency, the average transmission factor in the polarization E // is <T> E // = 0.5 with a relative standard deviation G <T> E // / <T> E // of 0.007%; the average value <T> E // of the transmission factor can therefore be confused with its value at 12 GHz. It is the same for the absorption and reflection factors at 12 GHz which can be respectively identified at <A> E // = 0.35

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  and <R> E // = 0.05 because the respective standard deviations are extremely small: O <A> E // / <A> E // = 0.012% and

a <R> E // / <R> E // = 0.003%.

  In the other H // polarization, the transmission factor at 12 GHz can also be confused with the mean value <T> H //, otherwise extremely high (0.993) because the relative standard deviation (<T> H // / <T> H // = 0.5% is still very low, but this approximation can no longer be used for the absorption and reflection factors because their respective standard deviations are no longer negligible at all. : <A> H // / <A> H // = 77% and <R> <R> H // <R> H // = 82% However, these significant relative fluctuations in the frequency band are to be in relation to extremely low average values of absorption and reflection factors <A> H // = 0.006 and <R> H // = 0.001 Thus, the effective value of the absorption factors at 12 GHz in the polarization H // is

  very low and between 0.012 and 0.003.

  In all cases, this sensor has good transmission at 12 GHz in both polarizations and a very important absorption contrast:

  C = 0.35 / (0.006 (177%)) or 56 <C <250.

  The resulting heating depends on the power absorbed but also the thermal properties of the photothermal material of the network of bands and the heat transfer between the latter and the external environment. The analysis of the spatial distribution and the temporal evolution of the temperature at the level of the network of bands to know a component of the local electromagnetic field makes use of the method EMIR (ElectroMagnetic and InfraRed) conceived by the applicant. The EMIR method is based on the

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  production of amplitude-modulated electromagnetic fields, analyzed and visualized with a digital synchronous demodulation IR infrared camera which is positioned facing one or the other of the faces of the sensor 1, as shown in FIG. 2. The EMIR method is notably described in the three following articles: "Contributions of infrared thermography to the measurement of electromagnetic fields" of P. LEVESQUE, M. NACITAS and D. BALAGEAS, ETTC'95, European Conference of the Tests and Telemetry, "Special Bases of Launching and Testing Centers ", Toulon (France), 30 May - 1 June 1995; "Measurement of Magnetic Fields in Amplitude and Phase by Infrared Thermography" by P. LEVESQUE, M. NACITAS and D. BALAGEAS, JINA 96 - International Nice Days on Antennae, Nice (France), 12-14 November 1996; and "EMIR: a photothermal tool for electromagnetic phenomena characterization" by Daniel BALAGEAS and Patrick

  LESVESQUE, Rev. Brig. Therm. (1998) 37, pp. 725-739.

  According to another variant shown in FIG. 5, a thermochromic layer 4 replacing, as a thermographic imaging means, the IR infrared camera is deposited on one of the faces of the support, which may be the face supporting the absorbing network of bands 3. An OBS observer in front of the thermochromic layer 4 directly observes different colors depending on the field energy

absorbed locally.

  The thermochromic layer 4 comprises, for example, cholesteric liquid crystals, protected against contamination by being enclosed in capsules of a few tens of micrometers of water.

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  diameter formed in a transparent polymer. The liquid crystals thus microencapsulated are suspended in a solution based on paint which can be sprayed on the corresponding face of the support 2. Elementary sensors, identical or different, as shown in FIG. 2 or 5 can be arranged either by juxtaposition or by superposition with a gap between them of the order of a tenth of a millimeter to a millimeter to avoid

  disruptive heating between them.

  As shown in FIG. 6, two elementary sensors 1a and 1b are superimposed with strips 2a and 2b which are arranged perpendicularly with respect to one another in order to simultaneously measure the amplitudes of the two components E // and H //, c ' that is the transverse electric field component and the transverse magnetic field component respectively parallel to the bands

  2a and 2b of a polarized electromagnetic wave.

  A microwave electromagnetic field vector measuring arrangement comprising at least two sensors of the invention may also serve to simultaneously display the same electric or magnetic field component at different frequencies, or different electric or magnetic field components at the same frequency or at different frequencies. Thus at least one of the following parameters in the sensors of the measuring arrangement are different from one another: width 1 of the strips, thickness h of the strips, pitch d of the grating, imaginary part E "of the dielectric permittivity F3 of the material photothermal strips 3, imaginary part '' of the magnetic permeability 13 of the Il.11111

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  photothermic material of the bands 3, orientation of the bands of the network. For example, as shown in FIG. 7, two elementary sensors 1c and 1d have superposed supports 2c and 2d and network bands 3c and 3d having different widths 1 and steps

d different.

  According to another embodiment, sensors of small dimensions according to the invention in the form of "lozenge" having the same support 2, or juxtaposed by gluing on a sheet of flexible plastic material or directly on the object to be analyzed, allow to determine local components of an electromagnetic field. For example, the arm of a user to a radiotelephone service can be wrapped with such a sheet of elementary sensors juxtaposed according to the invention so as to measure the distribution of the electromagnetic field radiated by a mobile radiotelephone maintained by the user's arm , or the field radiated by printed circuit conductors included in the radiotelephone or other device maintained by the user. According to another application, the components of the microwave electromagnetic field radiated in a microwave oven are analyzed by means of infrared cameras directed through screened openings of the oven structure to an object covered by an envelope supporting a plurality of elementary sensors according to the invention. 'invention,

in the form of pellets.

  According to yet another embodiment, one of the ends of an optical fiber or an optical fiber bundle is applied against one of the faces of the sensor of the invention, preferably the face of the T-i-1

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  support with the network of tapes, maintaining said end with the sensor in a mount. The optical fiber or optical fiber bundle transfers infrared radiation due to heating of the elementary sensor strip network to a remote meter, such as an infrared detector. This remote-controlled embodiment has the advantage of not disturbing the incident electromagnetic field on the

sensor.

  By way of example, the sensor according to the invention may contribute to: - the characterization of radiating systems: characterization of devices for compliance with electromagnetic field standards; measurement of the field radiated by radiotelephones; measurement of the field radiated by conductors of the printed circuit type; design of antennas and more generally of microwave transceivers; assault detection by power weapons; leak detection of microwave ovens; detections of disturbances due to high voltage lines; measurement of the absorption of electromagnetic radiation from portable radio-telephones by the human body; atraumatic control of fields used in medical hyperthermia; or - the metrology of the electromagnetic field: direct visualization of the radar signature of aircraft; visualization of nearby fields inside structures interacting with an electromagnetic wave (reactor air inlet, cockpit, drifts ...); non-destructive testing (NDT)

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  detection of objects buried in the ground; determination of radiofrequency parameters of materials such as dielectric permittivity and

magnetic permeability.

- IFr - 1 fTi

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Claims (6)

  1 - Sensor (1) comprising a support (2) substantially transparent to a microwave electromagnetic field, characterized in that one of the faces of the support comprises a periodic array of substantially parallel strips (3) of photothermal material partially absorbing energy of said electromagnetic field. 2 - Sensor according to claim 1, wherein the material of the strips (3) has at least dielectric losses, and is preferably indium and tin oxide.   3 - Sensor according to claim 1, wherein the material of the strips (3) has at least magnetic losses, and is preferably ferrimagnetic. 4 - Sensor in accordance with any of the   claims 1 to 3, having   transmission for electromagnetic field or electromagnetic field (E //, H //) components substantially parallel to the strips (3) which are greater than about 90%.   - Sensor conforming to any of the   Claims 1 to 4 having an absorption contrast   between electromagnetic field and electromagnetic field (E //, H //) components substantially parallel to the strips (3) which is less than 10. 21 2787583   6 - Sensor in accordance with any of the   Claims 1 to 5, wherein the ratio of   wavelength of electromagnetic field at pitch (d)   the band network (3) is greater than about ten.   7 - Sensor in accordance with any of the   Claims 1 to 6, comprising as a means   thermographic imaging: a thermochromic layer (4) on one of the faces of the support (2), or a camera (CIR), or an infrared detector.   8 - Sensor according to claim 7, wherein the thermochromic layer (4) comprises encapsulated liquid crystals and embedded in a solution to be deposited on one of the faces of the support (2). 9 - Sensor in accordance with any of the   Claims 1 to 6, comprising an optical fiber or   a bundle of optical fibers connecting one side of the   support (2) to infrared imaging means (IRC).   - Arrangement of at least two sensors (la, lb lc, ld) according to any one of   claims 1 to 9, having at least one of   following parameters different from each other: width (1) of the bands (3), thickness (h) of the bands, pitch (d) of the grating, imaginary part of the dielectric permittivity of the photothermal material of the bands, imaginary part of the magnetic permeability of the photothermal material of the bands, orientation of the network tapes.   11 - Arrangement comprising a flexible sheet supporting juxtaposed sensors (la, lb; lc, ld) X Il1! T Ir- 22 2787583   according to any one of claims 1 to   9. 12 - Arrangement comprising pellets from sensors conforming to any one of Claims 1 to 9. iV - IlT 1-