SYSTEM FOR GENERATING A SIGNAL REPRESENTATIVE OF THE PROFILE OF A SURFACE MOVING RELATIVE TO THE SYSTEM

Abstract

A system (1) for generating a signal from a surface (22) having a speed V in a direction U, comprising: a light source (2) emitting a Gaussian light beam along a first optical path (11); a sensor (3) able to evaluate the effects of the electromagnetic interference of the first beam; a means (2′, 4) for generating a second Gaussian light beam along a second optical path (12); a second sensor (3′) able to evaluate the effects of electromagnetic interference of the second beam; a focusing lens (5, 6) located on the first and/or the second optical path (11, 12), focusing the light beam at a distance f and defining an upstream optical path (11′, 12′); and a means (4′, 7) for routing the second beam able to redirect the second path (12′) in the direction of the first path (11′).

Claims

1.-14. (canceled)

15. A system (1) for generating at least one signal representative of the profile of an outer surface (22) of a medium (21) having a median plane (23) and having a relative speed V with respect to the system in a direction U, comprising: a first light source (2) able to emit a first light beam, the first light beam being a Gaussian beam of coherent and monochromatic light, a wavelength λ1 of which is adapted to optical absorption characteristics of the medium (21), along a first optical path (11); a first sensor (3) able to evaluate effects of electromagnetic interference between a portion of the first light beam and a portion of the first light beam backscattered from the outer surface (22) of the medium (21) of the first light beam, and delivering a first electrical signal; a means (2′, 4) for generating at least one second light beam, the at least one second light beam being a Gaussian beam of coherent and monochromatic light of wavelength λ2 adapted to the optical absorption characteristics of the medium (21) along a second optical path (12); a second sensor (3′) located on the second optical path (12) able to evaluate effects of electromagnetic interference between a portion of the second light beam and a portion of the second light beam backscattered from the outer surface (22) of the medium (21), and delivering a second electrical signal; at least one first focusing lens (5, 6) located on the first and/or the second optical path (11, 12) for focusing all or part of a corresponding light beam at a focusing distance f and defining an upstream optical path (11′, 12′); and a means (4′, 7) for routing the at least one second light beam able to redirect at least a portion of the second optical path (12′) in the same direction as a portion of the first optical path (11′), wherein the two optical paths (11′, 12′) are coplanar, wherein the wavelengths of the first and second light beams have the same sensitivity to the optical reflectivity characteristics of the outer surface (22) of the medium (21), wherein a combination of the focusing distance (f1, f2) and the optical path (11′, 12′) for each light beam defines two distinct geometric points d1 and d2 corresponding to a focal point of each light beam, wherein a distance between the geometric points d1 and d2 is greater than a greatest Rayleigh length of the first and second light beams, and wherein direction vectors of the first and second optical paths (11′, 12′) define an angle θ less than 3 degrees.

16. The system (1) for generating at least one signal representative of the profile of an outer surface (22) of a medium (21) according to claim 15, wherein the means for generating the at least one second light beam is an optical splitter element (4) situated on the first optical path (11) of the first light beam, the at least one second light beam is the other part of the light beam split by the optical element (4), and the second optical path comprises, along the second incident optical path (12), the second sensor (3′).

17. The system (1) for generating at least one signal representative of the profile of an outer surface (22) of a medium (21) according to claim 16, wherein the second optical path comprises an optical element (8) able to absorb the light on a return path upstream of the second sensor (3′).

18. The system (1) for generating at least one signal representative of the profile of an outer surface (22) of a medium (21) according to claim 15, wherein the means for generating the at least one second beam is a second light source (2′), the system (1) further comprises a second focusing lens (6) at the focusing distance f2 and the system further comprises an optical element (4′) able to merge the first and second optical paths (11, 12) such that the first and second optical paths (11, 12) are aligned.

19. The system (1) for generating at least one signal representative of the profile of an outer surface (22) of a medium (21) according to claim 18, wherein the second light source is a laser diode.

20. The system (1) for generating at least one signal representative of the profile of an outer surface (22) of a medium (21) according to claim 15, wherein the system (1) further comprises at least one electronic device (9) at an output of the electrical signal from the first and/or second sensor (3, 3′), comprising an electronic amplifier circuit.

21. The system (1) for generating at least one signal representative of the profile of an outer surface (22) of a medium (21) according to claim 15, wherein the first and second sensors (3, 3′) are selected from the group consisting of a phototransistor, a photodiode, an ammeter and a voltmeter.

22. The system (1) for generating at least one signal representative of the profile of an outer surface (22) of a medium (21) according to claim 15, wherein the wavelength of the first light beam is between 200 and 2000 nanometers.

23. The system (1) for generating at least one signal representative of the profile of an outer surface (22) of a medium (21) according to claim 15, wherein the first light source is a laser diode.

24. A static or mobile device equipped with the system (1) for generating at least one signal representative of the profile of an outer surface (22) of a medium (21) according to claim 15.

25. The system (1) for generating at least one signal representative of the profile of an outer surface (22) of a medium (21) according to claim 15, wherein, in use, with the first optical path (11′) pointing towards the outer surface (22) at a first angle of incidence θ1 with respect to a normal to the median plane (23) of the surface (22) and the second optical path (12′) pointing towards the outer surface (22) at a second angle of incidence θ2 with respect to the normal to the median plane (23) of the surface (22), the angles of incidence θ1 and θ2 are greater than 1 degree with respect to the normal to the median plane (23) of the outer surface (22) of the medium (21) and the geometric points d1 and d2 are located on either side of the median plane (23) of the outer surface (22).

26. The system (1) for generating at least one signal representative of the profile of an outer surface (22) of a medium (21) according to claim 25, wherein the first and second optical paths do not intersect before having reached the outer surface (22).

27. The system (1) for generating at least one signal representative of the profile of an outer surface (22) of a medium (21) according to claim 25, wherein a coherence length of the first light beam and/or of the second light beam is at least greater than twice a greatest length of the first (11) and second (12) incident optical paths to the outer surface (22) of the medium (21).

28. The system (1) for generating at least one signal representative of the profile of an outer surface (22) of a medium (21) according to claim 25, wherein the angles of incidence θ1 of the first light beam and θ2 of the second light beam are contained within a cone, the axis of revolution of which is the normal to the median plane (23) of the outer surface (22), and an aperture angle of the cone is less than or equal to 45 degrees.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0117] The invention will be better understood on reading the following description in the case of an application involving a fixed device and moving medium under observation. This application is given solely by way of example and with reference to the appended figures, in which:

[0118] FIG. 1 is a first example of a first embodiment of a generation system according to the invention.

[0119] FIG. 1a is a second example of the first embodiment according to the invention.

[0120] FIG. 2 is an example of the second embodiment according to the invention.

[0121] FIG. 3 is an overview of the method for evaluating the profile of the outer surface of a medium using signals coming from the generation system of the invention.

[0122] FIGS. 4a to 4f illustrate the various steps and the quality of the method for evaluating the profile of an outer surface dynamically and in real time.

DETAILED DESCRIPTION OF EMBODIMENTS

[0123] To implement the invention, it is first necessary to define an optical system that makes it possible to generate two light beams the focal point of which is located on either side of the outer surface that it is desired to observe.

[0124] FIG. 1 illustrates an example of a generation system 1 according to the first embodiment, that is to say comprising just a single light source 2 in the case of use of the system with a medium to be studied having an outer surface with a median plane. The first light beam from the first light source 2 is focused using a focusing lens 5 situated upstream of the first optical device 4. This optical device 4 also acts as a means for generating a second light beam that is Gaussian, coherent, monochromatic and, even more, focused. Thus, the second optical path 12 generated at the output of the optical device 4 corresponds to the second optical path 12′. This second light beam is due to the splitting of the light power of the first Gaussian beam through the splitter cube 4. The latter delivers a first optical path 11′ which directs a first part of the first light beam to the outer surface 22 of the medium 21 with an incidence such that the projection θ1 of the angle of incidence with respect to the normal on the median plane 23 of the outer surface 22 is greater than one degree. Thus, with this incidence, the backscattered beam is subjected to the Doppler effect provoked by the speed of movement V between the device comprising the generation system 1 and the medium 21 in the direction U.

[0125] Similarly, the second light beam which is the other portion of the first light beam follows a second optical path 12′ towards the outer surface 22 of the medium 21. Here, the means for routing this second light beam comprises a mirror 7 which redirects the second light beam to the outer surface 22. On the second optical path 12′ there is a second light sensor 3′ capable of measuring the electromagnetic interference between the second light beam emitted and the beam backscattered by the outer surface from this second light beam. In this figure, the second sensor 3′ is situated downstream of the mirror 7 along the second optical path 12′ in the incident direction. In our particular case, the projection of the angle of incidence of the second light beam with respect to the normal to the outer surface 22, denoted θ2, is close to or identical to θ1. Since the first and second light beams have the same wavelength 1, the incidences with respect to the normal to the median plane 23 of the outer surface 22 that are similar or identical, the second light sensor is essential for dissociating the two distance information items associated with the first 11′ and second 12′ optical paths.

[0126] In order for the backscattered beam from the second optical path 12′ not to disturb the electromagnetic interference between the first light beam and its backscattered beam at the first light sensor 3, it is necessary to apply an absorbent medium to the return path of the second optical path 12′. In this example, the latter is materialized by an unbroken line on the face of the mirror 7. This component absorbs the light backscattered by the outer surface 22. In another alternative, this component could be situated upstream to the splitting face of the splitter cube 4. The objective is for it not to disturb the first incident and backscattered light beams along the first optical path 11.

[0127] Obviously, the longer optical path, here the second optical path 12′, comprises a geometric point d2 situated above the outer surface 22 where the second light beam is the more focused. This point corresponds also to the maximum of light backscattered along the second optical path 12. By analogy, the first optical path 11′ is the shorter to reach the outer surface 22. Consequently, its geometric point d1 is situated on the other side of the median plane 23 and virtually inside the medium 21.

[0128] Here, the two points of impact 13 and 14 of the first 11′ and second 12′ optical paths are spaced apart by a distance X in the direction U. A time correction between the two signals from the first 3 and second 3′ light sensors will be needed to deduce therefrom the distance d from the outer surface 22 of the medium 21 and thus construct the profile of the outer surface 22. However, with the two optical paths having a similar incidence with respect to the normal to the median plane 23 in a plane defined by the direction U and the normal to the median plane 23, the correction matrix to be used on one or other of the electrical signals is easier to implement which allows a more rapid real time processing. As in FIG. 1, the sensors are, here, both linked to the electronic device comprising a signal amplifier 9 which allows a synchronization of the two measurement channels each from a light sensor 3 and 3′. It is preferably possible to use an electronic device associated with each sensor.

[0129] FIG. 1a shows another example of a generation system 1 according to the first embodiment in the case of using the system with a medium to be studied having an outer surface having a median plane. This time, the first light beam from the first light source 2 is focused using a focusing lens 5 located downstream of the first optical device 4. Once again, this optical device 4 also acts as a means for generating a second beam of Gaussian, coherent and monochromatic light. This second light beam is due to the splitting of the light power of the first Gaussian beam through the splitter cube 4. This directs a first portion of the first light beam towards the outer surface 22 of the medium 21 at an incidence such that the projection θ1 of the angle of incidence with respect to the normal along the median plane 23 of the outer surface 22 is greater than one degree. Thus, at this incidence, the backscattered beam is subject to the Doppler effect caused by the speed of movement V between the device comprising the generation system 1 and the medium 21 in the direction U.

[0130] Similarly, the second light beam, which is the other portion of the first light beam, follows a second optical path 12′ after having been focused using a second focusing lens 6 towards the outer surface 22 of the medium 21. Here, the means for routing this second light beam comprises a mirror 7 that redirects the second light beam towards the outer surface 22. On the second optical path there is a second light sensor 3′ capable of measuring the electromagnetic interference between the second light beam emitted and the beam backscattered by the outer surface 22 from this second light beam. In this figure, the second sensor 3′ is situated upstream of the mirror 7 along the second optical path 12 in the incident direction. In our particular case, the projection of the angle of incidence of the second light beam with respect to the normal to the outer surface 22, denoted θ2, is close or identical to θ1. Since the first and second light beams have the same wavelength 1, incidences with respect to the normal to the median plane 23 of the outer surface 22 that are similar or identical, the second light sensor 3′ is essential for dissociating the two distance information items associated with the first 11′ and second 12′ optical paths.

[0131] Optionally, in order for the backscattered beam from the second optical path 12′ not to disturb the electromagnetic interference between the first light beam and its beam backscattered at the first light sensor 3, it is necessary to apply a light-absorbing medium to the return path of the second optical path 12. In this example, the latter is materialized in the form of a broken line on a transition optical element 8. This component absorbs the light backscattered by the outer surface 22. In another alternative, this component could be situated upstream to the splitting face of the splitter cube 4. The objective is for it not to disturb the first incident and backscattered light beams along the first optical path 11.

[0132] It should be noted here that the first light beam has, on its path, a splitter cube which redirects an infinitesimal part of the first light beam, both incident and backscattered, to the first light sensor 3 which is therefore outside of the first optical path 11 towards the outer surface 22. Indeed, it is not necessary for the observation of the electromagnetic interference between the incident beam and the backscattered beam to be done on the optical path. The only condition is that the spatial zone of observation of this electromagnetic interference be a zone of mutual spatial coherence of the beams. Indeed, an initially coherent light beam inevitably loses its coherent nature after a certain spatial and temporal travel.

[0133] Obviously, the longer optical path, here the first optical path 11, comprises a focusing point d1 situated under the outer surface 22 where the first light beam is the more focused. This point corresponds also to the maximum of backscattered light along the first optical path 11 despite the fact that this point d1 is virtual, that is to say inside the medium 21. By analogy, the second optical path 12 is the shorter to reach the outer surface 22. Consequently, its geometric point d2 is situated on the other side of the median plane 23. The length of the optical path is here driven by the focusing distance f of the focusing lens and the routing of the optical path from the focusing lens and the outer surface 22. As in FIG. 1a, the focusing lenses 5 and 6 are at the same distance from the outer surface 22 along the routing of the light of each path, and it is the differentiation of the focusing distance of the focusing lens which generates the different lengths along the optical paths 11′ and 12′.

[0134] Here, the two points of impact 13 and 14 of the first 11′ and second 12′ optical paths are spaced apart by a distance X in the direction U. A time correction between the two signals from the first 3 and second 3′ light sensors will be needed to deduce therefrom the distance d from the outer surface 22 of the medium 21. However, with the two optical paths having a similar incidence with respect to the normal to the median plane 23 in a plane defined by the direction U and the normal to the median plane 23, the correction matrix to be used on one or other of the electrical signals is easier to implement which allows more rapid real time processing. As in FIG. 1, the sensors are, here, both linked to the electronic device comprising a signal amplifier 9 and which allows a synchronization of the two measurement channels, each from a light sensor 3 and 3′.

[0135] FIG. 2 represents a first example of a generation system 1 according to the second embodiment, that is to say comprising two light sources 2 and 2′ in the case of use of the system with a medium to be studied having an outer surface having a median plane. This time, the first light beam from the first light source 2 is focused using a focusing lens 5 situated upstream of a first optical device 4′ along the incident routing of the light. However, the second light beam is, for its part, generated by a second light source 2′ delivering also a Gaussian, coherent and monochromatic light beam. This second light beam is focused using a second focusing lens 6 situated upstream of the first optical device 4′.

[0136] This time, this optical device 4′ redirects the first focused light beam towards the outer surface 22 of the medium 21 along a first optical path 11′. This optical device 4′ also acts as a means for routing the second focused light beam by redirecting the latter towards the outer surface 22 of the medium 21 over a second optical path 12′. However, this optical device 4′ allows the two light beams to be merged into just one, which guarantees that the two optical paths 11′ and 12′ are identical and aligned after the passage of the incident beams through the optical device 4′, which is an optical cube merging the initially non-parallel beams. Thus, the first and second light beams converge towards the outer surface 22 of the medium 21 with the same incidence such that the projection θ of the angle of incidence with respect to the normal on the median plane 23 of the outer surface 22 is greater than one degree. Thus, with this incidence, the backscattered beam is subjected to the Doppler effect provoked by the speed of movement V between the device comprising the generation system 1 and the medium 21 in the direction U. However, since the geometric points d1 and d2 for each of the light beams are wanted to be situated either side of the median plane, it is sufficient for that to relatively displace the two focusing lenses 5 and 6 on their respective optical paths 11′ and 12′ for the focal distance f1 and f2 of each of the lenses to define different geometric points d1 and d2. It is also possible to use focusing lenses 5 and 6 with different focusing distances f1 and f2 so as to define the different geometric points.

[0137] Here, the generation system 1 comprises two light sensors 3 and 3′ respectively associated with the first and second optical paths. Each light sensor 3 and 3′ records the electromagnetic interference between the incident light beam and its beam backscattered by the outer surface 22 of the medium 21.

[0138] Since the light sources 2 and 2′ are physically dissociated, the light beams from one cannot be coherent with the light beams from the other which limits the interference between the first and second light beams. Thus, the electromagnetic interference measured is linked to a single light source regardless of the wavelength of the light source 2 and 2′.

[0139] The two electrical signals from each light sensor 3 and 3′ are synchronized in the electronic device comprising a signal amplifier. The signals do not require any time correction since they have the same point of impact 13 and 14 on the outer surface 22.

[0140] Here, this second embodiment is economically interesting if the light sources are conventional laser diodes having, in their amplifying cavity, an integrated photodiode which serves as light sensor 3 or 3′. The packaging is then concentrated and inexpensive allowing economical operation of the generation system 1. Indeed, when a single light source is used as in the case of the first embodiment, the use of a laser source other than a diode can be envisaged. It is also possible to use the amplifying cavity of the laser as preferred spatial zone for observation of the electromagnetic interference. The use of a light sensor in the form of a photodiode or phototransistor linked with the amplifying cavity can be envisaged as can the observation of the laser source power supply parameters using an ammeter or a voltmeter if the laser source is not equipped with electronic regulation of its power supply.

[0141] Of course, these examples of the two embodiments are specific applications of the invention, which is not intended to be limited to these examples. In particular, any combination of the characteristics of these examples is conceivable and falls within the general scope of the invention.

[0142] FIG. 3 is an overview of the method for evaluating the distance d of the outer surface from a reference potentially implementing the system for generating at least one signal representative of the profile of an outer surface of an medium moving at a relative speed V with respect to the generation system in a direction U. However, this method is not otherwise intended to be limited to signals output from this generation system.

[0143] FIG. 3 comprises three main phases. The first concerns the preparation of electrical signals, for example at the output of the system for generating a signal representative of the profile of the outer surface of the medium. The second phase concerns the implementation of these signals in order to perform the third phase, which is the actual evaluation of the distance d from the outer surface. Of course, this first phase is optional if a measurement system directly generates two signals representative of the profile of the outer surface with respect to known references for the same geometric point of a readout line of the outer surface. This system is for example the first example of the second embodiment of the measurement system of FIG. 2.

[0144] The first phase comprises a first step 100 consisting in obtaining two time signals A and B representative of the profile of the outer surface with respect to a readout line. These may for example be the output of the electronic device of the generation system according to the invention. Of course, in this step, it is not certain that the two signals are temporally and spatially phased, which means having to go through the next step 1001. For example, these two points are separated along the readout line of the outer surface by a spacing X, as in the examples of FIGS. 1, 2 and 3.

[0145] The second step 1001 corresponds to the spatio-temporal correction to be applied to one and/or the other of the time signals A and B from step 1000. For this purpose, it is necessary to know the method for obtaining the two time signals, that is to say the spatio-temporal spacing between the two measurement points each corresponding to a time signal with respect to a common reference. The spatial position may be a metric position that is obtained visually, for example. The time offset may be the date of crossing in front of a reference point serving for example as a common reference, through a clock signal with a metric for each signal. In addition, it is useful to know the scrolling speed along the readout line of the outer surface associated with each time signal. All of these data make it possible to define a correction matrix to spatio-temporally recalibrate the two signals on one and the same geometric point of the readout line. Applying this correction to the time signals from step 1000 gives the result of step 1002, which ends the signal preparation phase.

[0146] The second phase corresponds to formatting of the measured data, which are represented by the time signals obtained in step 1002 from the first phase. The principle of the method according to the invention is that the payload information of the time signals is contained in the fundamental and the harmonics of the Doppler frequency associated with the relative speed V of the medium 21 with respect to the time signal measurement system. This is independent of the physical means for measuring the signals, whether this be light, sound or any other electromagnetic wave.

[0147] The first step 2001 consists in defining the Doppler frequency associated with the relative speed V. The Doppler frequency may be determined using a mathematical formula such as, in the case of light signals, the formula linking the relative speed V, the angle of incidence with respect to the normal to the outer surface and the wavelength of the light. It may also result from analysing the signals, whether this analysis be temporal or frequency analysis. Knowing this Doppler frequency, it is necessary for the sampling frequency of the time signals to be at least twice as great as the Doppler frequency, complying with the condition of Shannon's Theorem, in order to ensure that the information of the time signals is plausible and not induced by uncertainty related to the measurement conditions, this corresponding to step 2002. Optionally, it may prove useful to filter the time signals around the Doppler frequency identified in step 2001, and this may be carried out for example over a wide band of between 0.7 and 1.3 times the Doppler frequency. Thus, depending on the mode for identifying the Doppler frequency, theoretically or through frequency analysis of the signals or through temporal analysis of the signals, and also the potential slow evolution of the relative speed V, the Doppler frequency is not necessarily determined in absolute terms, and a wide window then makes it possible to cover all of these uncertainties by isolating the usable information, this corresponding to step 2003. Of course, if the frequency interference related to the signal measurement system is low, it is entirely conceivable to take the complete signal without selective filtering and move directly to step 2004.

[0148] Step 2004 consists in focusing on the general signal carrying the information through the envelope of the payload signal. It is expected that this will be an image of the events related to the Doppler frequency associated with the relative speed V. In step 2004, the envelope of the payload time signal is determined, potentially driven by a narrow frequency band around the Doppler frequency. Of course, the envelope of the payload signal may be constructed from the minima, the maxima or the absolute value of the payload signal. The choice of method depends on the nature of the measured signals with respect to the physical quantity under observation.

[0149] Optionally, in order to statically eliminate parasitic noise on the envelope of the measured time signal, speckle cleaning is carried out in order to extract the precise information therefrom in step 2005. This makes it possible to statistically eliminate measurement randoms caused by lack of compliance with the conditions for an ideal measurement. This is carried out through a learning campaign on a known target representative of the outer surface of the medium that it is desired to observe using the envisioned measurement system. This learning phase determines a Gaussian distribution of the measurement randoms, which should be coupled with an evenly distributed noise in order to determine a speckle noise. This makes it possible to determine the time windowing of the payload signal that should be taken into account in order to clean the speckle noise by applying the identified Gaussian distribution. Step 2005 consists in removing the determined speckle noise from the envelope signal in order to obtain a cleaned envelope on each measurement channel. This step ends the second data formatting phase.

[0150] The last phase is evaluating the variation in the distance d of the outer surface from a reference, making it possible to deduce the profile of the outer surface. This comprises a first step 3001, which consists in mathematically combining the envelopes obtained in steps 2004 or 2005 so as to define a function F that is bijective. The bijectivity of the function F makes it possible to guarantee the uniqueness of the distance d from the outer surface using the information from the two envelopes. In the case of self-mixing, or optical feedback, with Gaussian and coherent light beams, defining the function F as being the difference between the envelopes expressed on a logarithmic scale ensures both monotony and good sensitivity of the function F over the distance range separating the two geometric points d1 and d2 of the generation system presented in the device invention. Precision is enhanced by taking the absolute value of the payload signal to construct the envelopes. Of course, the precision improves when taking the cleaned envelopes.

[0151] Finally, to arrive at a relative distance d between various points of the readout line of the outer surface with respect to a reference geometric position, it is necessary to establish a calibration between the result of the function F as defined in step 3001 and a target the position of which is known with respect to the geometric points of the measurement system, this corresponding to step 3002. This makes it possible to convert the response of the function F into a known metric quantity.

[0152] To this end, a calibration step should be undertaken using the measuring device, directly or indirectly delivering the time signals with respect to two different geometric points. In the case of the generation system of the device invention, the two geometric points are the points d1 and d2 where each of the light beams are focused and located on either side of the median plane of the outer surface of the medium. Here, the calibration is performed using a target the physical response of which is at least as strong as the outer surface of the medium that it is desired to observe. In the case of the generation system of the device invention, it is necessary to use a white target, that is to say having very high reflectivity with respect to the observation medium. The majority of the incident light is thus backscattered by the surface, which absorbs a very small proportion thereof. In addition, in order to observe light scattering, the target should be rough. However, in order not to be penalized by a large degree of interaction between the light from the generation system and the target, the surface roughness of the target should be greater than that of the medium under observation. It is then sufficient to calibrate the generation system by moving the target between the geometric points d1 and d2 in a known manner and to identify the value of the corresponding function F using the envelopes. This calibration will be used in step 3002 to obtain the distance d from the outer surface of the system. Here, the function F is insensitive to the backscattered power, since the function F is a relative combination of the signal envelopes, such as the linear scale ratio or the logarithmic scale difference. If the combination of the envelopes is absolute, it will be necessary to perform a more precise calibration using a target the physical properties of which are similar to those of the medium that it is desired to observe using the measuring device according to the metric used: light, sound or electromagnetic waves.

[0153] The optional speckle noise correction is also performed using the same target as in the signal calibration step. This time, the number of time measurements is increased by moving the target, knowing the result to be achieved in order to evaluate the distribution of the measurements around the reference value. For this purpose, the distribution of the measurements is evaluated in the frequency domain under diversified measurement conditions on a large time sample. This frequency distribution is modelled by a centred Gaussian. The width of this Gaussian determines the minimum size of the measurement time window, so that the Gaussian distribution is statistically representative. The speckle noise is then evaluated through the product of the Gaussian frequency distribution and a noise uniformly distributed between 0 and 1. This speckle noise is to be subtracted from the determined envelope in order to obtain a measurement that depends in the first order only on the reflectivity of the target or the outer surface of the medium under observation. Proportionality is assumed between the reflectivity of the target and that of the outer surface of the medium to be observed, which will be transparent due to the relative combination of the envelopes.

[0154] FIGS. 4a to 4e illustrate the method for measuring the profile of an outer surface of a test specimen, for which FIG. 4f shows the three-dimensional reconstruction obtained by photographic means using specific lighting. This circular test specimen has a profile, in the direction of its axis, that evolves non-monotonically as a function of the azimuth. And a proportional evolution of this profile is defined according to the radius from the centre of the circular test specimen. This is mounted on a rotary shaft rotating at an angular speed of the order of 1550 rpm. Finally, the rotary shaft is moved in a translational movement along a direction X, allowing the centre of the circular test specimen to move in translation. The surface roughness of the test specimen is of the order of millimetres with regard to the masses covering 75 percent of the test specimen. The last quarter of the test specimen resembles a smooth surface with a surface roughness of the order of around ten micrometres.

[0155] To apply the method, use was made of a preferred generation system according to the second embodiment, the principle of which is illustrated in FIG. 2. It is a generation system comprising two distinct light sources, of which the light power is merged into a single optical path destined for the outer surface of the test specimen. The angle of incidence of the first and second light beams on the outer surface of the test specimen is identical while being contained within a cone with an aperture angle of less than 30 degrees, such that the projection of these angles of incidence with respect to the normal to the median plane of the outer surface in a plane defined by this normal and the direction U of movement of the test specimen is around 5 degrees. In addition, it is ensured that the two light beams have the same point of impact on the outer surface, thereby limiting the corrections to be made to the interferometric signals on the two optical paths. In fact, the readout line on the outer surface of the test specimen is a succession of circles centred on the centre of the circular disk of the test specimen, each circle corresponding to a different translational position of the rotary shaft on which the test specimen is mounted.

[0156] The generation system comprises, as light source, a laser diode equipped with a photodiode at the entrance of the amplifying cavity of the laser diode. The laser diode emits a beam of coherent, monochromatic light at the single wavelength and the propagation of which along the direction of the beam is Gaussian. Here, the wavelength of the first laser diode is centred on 1350 nanometres. The second, meanwhile, is centred on 1500 nanometres. The photodiode associated with each laser diode records the electromagnetic interference between the incident light beam and the light beam backscattered by the outer surface of the test specimen. The electromagnetic interference of the first optical path is mainly carried by the harmonics of the first Doppler frequency related directly to the first wavelength, which is inversely proportional to the wavelength. On the other hand, the electromagnetic interference of the second optical path is carried by the harmonics of the second Doppler frequency, the Doppler frequency of which is lower than the first Doppler frequency. The differentiation of the geometric points d1 and d2 where the first and second optical paths are collimated is defined by the length of the optical paths. Here, the first optical path towards the outer surface is defined by the first focusing lens through its position on the first optical path and its focal length. The second optical path comprises a mirror, integrated into the merging optical element in order to redirect the second light beam towards the outer surface of the test specimen. The geometric point d2 is controlled directly by the positioning and the focal length of the second focusing lens of the generation system. Thus, at the output of the electronic device, two electrical signals, each associated with a photodiode are obtained, containing the payload information carried by the harmonics of each different Doppler frequency. It should be noted that, if identical laser diodes are used on the first and second light sources, the same output signals of the generation system would still be obtained because of the coherence of the light beam from each diode which limits the interference between the light beams.

[0157] The measurement is carried out by fixing the generation system on a static device located in line with the test specimen such that the geometric points d1 and d2 are located on either side of the outer surface of the test specimen. In our case, they are equidistant from the median plane of the outer surface of the test specimen, at a distance of around 5 millimetres. The spacing between the geometric points is thus of the order of a centimetre, which is less than the variations in the profile of the outer surface of the test specimen, while being greater than the Rayleigh length of the first and second Gaussian light beams.

[0158] FIG. 4a shows the temporal evolution in terms of amplitude of two signals. The first signal 101a comes from the photodiode associated with the first light source, and the second signal 102a comes from the photodiode associated with the second light source. Here, the observed interference expresses amplitude modulations of the time signal around a carrier. The succession of fronts is related directly to the interference, which evolves with the position of the points of impact of the light beams on the outer surface of the surface.

[0159] FIG. 4b is the frequency spectrum of the time signals from FIG. 4a for each of the signals. The first response spectrum 101b is mainly characterized by a mass centred on the first Doppler frequency. The second curve 102b is characterized by a succession of harmonics associated with the second Doppler frequency. The two Doppler frequencies are slightly offset in terms of frequency. Regardless of the spectral response of the signals, the fundamental frequency carries most of the energy of the signal. In addition, it is possible to note an emergence on each signal at very low frequency, which is similar to a structural mode of the device or of the generation system used. Indeed, this emergence appears on both spectra. Therefore, the temporal response is marred by the signature of the static device or of the generation system, and should be eliminated.

[0160] FIG. 4c shows time signals 101c and 102c that correspond to the signals 101a and 102a, respectively, by filtering its signals over a narrow frequency band around the fundamental Doppler frequency of each signal. These corrected time signals eliminate the vibrational contribution of the structural mode of the device or of the generation system. The frequency band is between 0.7 and 1.3 times the Doppler frequency, although a wider band could have been suitable, such as for example between 0.5 and 1.5 times the Doppler frequency. The spectral signature of each time signal with harmonics of the Doppler frequency, which are relatively unused, allows such a correction without causing a loss of information on the electromagnetic interference observed by the photodiodes. If the information is also carried by the harmonics, the harmonics should be taken into account by way of the filtering step.

[0161] FIG. 4d shows the definition of the envelopes 101d and 102d based on the filtered time signals 101c and 102c. Here, the envelopes 101d and 102d are constructed on the maxima of the time signals 101c and 102c.

[0162] Finally, a surface profile illustrated in FIG. 4e is reconstructed by combining the previously obtained envelopes 101d and 102d. Since the time signals are obtained in-phase during acquisition, no spatio-temporal correction step needs to be performed on the time signals. Here, the profile is constructed at each time sample, by taking the difference between the logarithms of the amplitudes of the envelopes 101d and 102d. Due to the spacing between the geometric points and the formation of the waists of the laser; the term “waist” is understood to mean the width of the laser beam at the focal point, at the geometric points, bijectivity of the abovementioned combination makes it possible to associate a single distance D with each combination. The distance D is measured with respect to any real or notional reference point of the measuring device. Here, for the profile, only the relative position of one sample with respect to another is of interest, regardless of the reference point. The distance D is obtained from a calibration phase of calibrating the measuring device using a white circular target the surface roughness of which is greater than the wavelengths of the first and second light beams. The cylindrical surface has a cylindrical outer surface the profile of which evolves with the radius of the cylinder and does not vary with the azimuth of the cylinder. The relative combination of the envelopes obtained using the method corresponding to the bijective function F is then compared with the altitude of the profile of the target.

[0163] FIG. 4f is the three-dimensional reconstruction of the test specimen after post-processing of the images obtained by multiple static cameras and depending on specific lighting. This reconstruction should be compared with the image in FIG. 4e. It is possible to note a similarity of the profiles between the measurement obtained using the method and the static reconstruction on the global and local level. Indeed, local imperfections may be observed in the second order, which corresponds to the spacing between two measurement circles of the test specimen. Simply smoothing the points makes it possible to overcome this problem.