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 beam of light along a first optical path (11); a sensor (3) able to evaluate the effects of the electromagnetic interference of the first beam; an optical splitter (4) located upstream of the sensor (3), generating, from the first beam of light, a second beam of light along a second optical path (12); a focusing lens (5, 6) located on the first and/or the second optical path (11, 12), focusing the beam of light at a distance f and defining an upstream optical path (11′, 12′), and a means (7) for routing the second beam, comprising a mirror redirecting the second path such that the lengths of the first (11′) and second (12′) paths are different.

Claims

1.-13. (canceled)

14. A system (1) for generating at least one signal representative of a 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 light 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 an electrical signal; at least one optical splitter element (4) located upstream of the first sensor (3), redirecting a portion of the first light beam located on the first optical path (11), the other portion of the first light beam being a second light beam following a second optical path (12); at least one focusing lens (5, 6) located upstream or downstream of the at least one optical splitter element (4) on the first and/or the second optical path (11, 12) for focusing all or part of the corresponding light beam at a focusing distance f and defining an upstream optical path (11′, 12′), and a means (7) for routing the at least one second beam of light, comprising at least one mirror able to redirect at least a portion of the second optical path in the direction of the first optical path such that the length of the first optical path (11′) is different from the length of the second optical path (12′), wherein the two optical paths (11′, 12′) are coplanar, 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 greater than or equal to 3 degrees.

15. 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 14, wherein the at least one focusing lens (5) is located upstream of the at least one optical splitter element (4).

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 14, wherein a first focusing lens (5) is located downstream of the at least one optical splitter element (4) on the first optical path (11) and a second focusing lens (6) is located downstream of the at least one optical splitter element (4) on the second optical path (12).

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 14, wherein the system (1) further comprises an electronic device (9) at an output of the electrical signal from the first sensor (3), comprising an electronic amplifier circuit.

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 14, wherein the first sensor (3) is selected from the group consisting of a phototransistor, a photodiode, an ammeter and a voltmeter.

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 14, wherein the wavelength of the first light beam is between 200 and 2000 nanometers.

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 14, wherein the first light source is a laser diode.

21. 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 14.

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 14, 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).

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 14, 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), a ratio between the angles of incidence θ1 and θ2 on the outer surface (22) of the first (11) and second (12) optical paths is between 1.2 and 1.8.

24. 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 23, wherein the first and second optical paths do not intersect before having reached the outer surface (22).

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 22, 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).

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 22, 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

[0113] 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:

[0114] FIG. 1 is a first example of a first embodiment of a generation system according to the invention.

[0115] FIG. 2 is a second, preferred example of the first embodiment according to the invention.

[0116] 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.

[0117] 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

[0118] To implement the invention, it is first necessary to define an optical system that makes it possible to generate two beams of light the focal point of which is located on either side of the outer surface that it is desired to observe.

[0119] FIG. 1 shows a first system 1 for generating a signal representative of the profile of the outer surface 22 of the medium 21 in the context of the use thereof in the presence of the surface. The outer surface 22 defines a median plane 23 for which the points of the surface are statically evenly distributed on either side of the median plane 23. This median plane 23 defines a normal direction that is characterized, in the figure, by an straight line alternating between dotted lines and dashes, located vertically in the direction W. The figure shows a sectional view of the outer surface 22 in a plane defined by the direction U of movement of the medium 21 relative to the static device on which the generation system 1 is installed. The optical paths 11′ and 12′ draw, on the outer surface 22, a readout line along the direction U.

[0120] In this FIG. 1, the generation system 1 corresponds to a generation system of the invention according to a first embodiment, in which a single light source is used. A laser source 2 emits a first Gaussian beam of monochromatic coherent light at a wavelength λ along a first optical path 11 defined by the unbroken line coming from the source 2. This first beam of light passes through a light sensor 3 in the outward direction before encountering a first focusing lens 5 with a focal length f1. At the exit of the first focusing lens 5, the light is focused at a focusing distance f1 along any optical path.

[0121] The first optical path 11′ resulting from the first beam of light having passed through the first focusing lens 5 is then divided into 2 through a splitter cube 4, for example 50/50. The light power is thus split into two equal portions, and each half-power is directed in two different directions. The splitter cube 4 is the means for generating a second beam of light from the first beam of light along a second optical path 12, which is identical to the second optical path 12′. It therefore retains its characteristics of the first beam of light. The second beam of light is thus Gaussian, coherent and monochromatic at the same wavelength as the first beam of light. Moreover, it is generated from a focused beam of light, and the second optical path 12′ is thus created. The mirror 7 here acts as a means for routing along the second optical path 12′. Its first role is to direct the second beam of light towards the outer surface 22 at an angle of incidence the angular projection of which with respect to the normal to the median plane 23 in the plane defined by the normal to the median plane 23 and the direction U is equal to θ2, which is preferably less than 45 degrees while necessarily being non-zero. The second role of the mirror 7 is to define the position of the geometric point d2 such that this point is different from the geometric point d1 where the first optical path 11′ is focused. Therefore, the geometric point d2 corresponding to the focusing distance f2 of the second beam of light is located above the median plane 23 of the outer surface 22 of the medium 21. This second beam of light impacts the outer surface 22 at the point of impact 14 at an angle of incidence the projection of which, in the plane, is equal to θ.sub.2. The angles of incidence θ.sub.1 and θ.sub.2t are 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.

[0122] The other remaining half-power follows the first optical path 11′ and impacts the outer surface 22 at the point of impact 13 at an angle of incidence the projection of which with respect to the normal to the median plane 23 in the plane defined previously is equal to θ1, which is greater than 1 degree. For this portion of the first beam of light, the focusing distance f1 and the first optical path 11′ define a first geometric point d1 located below the median plane 23 of the outer surface 22. Here, the two points of impact 13 and 14 corresponding respectively to the meeting of the first optical path 11′ and, respectively, of the second optical path 12′ with the outer surface 22 are spaced apart by a distance X in the direction U.

[0123] A portion of the first and second incident beams of light are backscattered by the outer surface 22 at each of the points of impact 13 and 14. A portion of this backscattered light follows the path opposite to the incident optical path. In particular, the two beams of light recombine after the splitter cube 4 so as to jointly continue their route towards the light source 2 and necessarily passing through the light sensor 3. The meeting of the first incident beam and the backscattered beam generates electromagnetic interference as long as these two beams are mutually coherent.

[0124] This generation system 1 according to the first embodiment, using a single light source 2, but also equipped with a single light sensor 3, delivers only a single electrical signal at the output of the light sensor 3, which it directs towards the electronic device comprising the signal amplifier 9. The light sensor 3 should be capable of translating the electromagnetic interference that arises at this light sensor 3 into an electrical signal. The location of the light sensor 3 on the first optical path is unimportant, but it is preferable to position it where the sum of the two beams of light generates the smallest beam size in order to optimize interference.

[0125] In order to dissociate, on the single electrical signal from the light sensor 3, the information resulting from the interference of the first and second beams of light, it is preferable for the information to be easily detectable. However, the payload information is carried here in particular by the fundamental of the Doppler frequency and its harmonics. The Doppler frequency is theoretically dictated by three parameters. The first corresponds to the relative speed V in the direction U between the medium 21 and the device on which the generation system 1 is mounted. The second is the angle of incidence θ between the beam of light and the normal to the outer surface 22, which will be taken as being that of the median plane 23. The third is the wavelength λ of coherent light. Here, only the angle of incidence θ of the beam of light is able to generate an effective parameter to dissociate the two Doppler frequencies. Indeed, the other two parameters are potentially identical by design of this generation system 1. Therefore, the first and second angles of incidence with respect to the normal to the median plane 23, and reference is made here to their projection in the plane defined above, are different, thereby making it possible to dissociate the Doppler frequencies from one another. This difference should be at least 3 degrees in order to dissociate the signals from each beam of light.

[0126] Finally, in order for the signals from each beam of light to be able to be used to determine the distance d from the outer surface 22, and thereby allow the reconstruction of the profile of the outer surface 22 along the readout line, it is necessary to separate the two geometric points d1 and d2 by a length greater than the greatest Rayleigh length of the two Gaussian beams of light. Indeed, the Gaussian propagation of light ensures that the amount of light backscattered is proportional to the distance between the focal point of the beam of light and the outer surface 22 where the backscattering takes place. In fact, the energy will be at a maximum if the outer surface 22 is located at the focal point. The further one moves away from it, the more the backscattered light energy decreases following a Gaussian curve, thereby providing a measurable dynamic range on electromagnetic interference. Ensuring a sufficient spacing between the two focal points ensures that a combination of the electromagnetic interference measured on each channel makes it possible to deduce the distance d from the outer surface 22.

[0127] It is possible to observe electromagnetic interference through the self-mixing phenomenon or optical feedback phenomenon, by using a laser source as light source 2, equipped for example with a photodiode as light sensor 3. The temporal record of the output signal from the photodiode is then an image of the interference of light between the incident beam and the beam backscattered by the outer surface. The temporal variations in amplitude are due both to the distance between the focal point d and the outer surface 22 and the reflectivity of the outer surface 22. The information is partly carried by the Doppler frequency related to the relative speed V between the medium 21 and the generation system 1. By ensuring two measurement channels each corresponding to a beam of light pointing to the same readout line of the outer surface 22 the focal point of which is different, the two measurement channels are each the result of the reflectivity of the outer surface 22 and the distance between the focal point and the outer surface 22. The combination of the two channels makes it possible to make the result of the two channels insensitive to the reflectivity of the outer surface 22 and to be dependent only on the distance of the two focal or geometric points from the outer surface 22.

[0128] FIG. 2 is an illustration of another example of a generation system 1 according to the first embodiment; this is the preferred example of this embodiment. The system 1 is presented here in the context of the use thereof with an outer surface 22 of a medium 21 having a median plane 23. This time, the first beam of light 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 beam of light is due to the splitting of the light power of the first Gaussian beam through the splitter cube 4. This delivers a first portion of the first beam of light 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. Prior to the point of impact 13 on the outer surface 22, the beam of light is focused by the focusing lens 5 at a focal length f1 such that the geometric point d1, which is the focal point f1 along the first optical path 11, is a virtual point located in the medium 21. Thus, at this incidence, a portion of the beam of light is subject to the Doppler effect, and the backscattered beam then carries Doppler information that will be used to determine the distance d from the outer surface.

[0129] Similarly, the second beam of light, which is the other portion of the first beam of light, follows a second optical path towards the outer surface 22 of the medium 21. Here, the means for routing this second beam of light comprises a mirror 7 that redirects the second beam of light towards the outer surface 22. On the second optical path is located a second focusing lens 6, the focal length f2 of which focuses the second beam of light at the geometric point d2 located above the median plane 23. The point of impact 14 of the second beam of light along the second optical path is identical to the first point of impact 13 of a portion of the first beam of light. Only the projection θ2 of the angle of incidence of the second beam of light on the outer surface 22 with respect to the normal to the median plane 23 differs from the projection θ1.

[0130] In this FIG. 2, the generation system 1 comprises just a single light sensor 3 located on the first optical path of the first beam of light. This is connected to a signal amplifier 9 so as to make the electrical signal delivered by the sensor 3 usable. The sensor 3 records electromagnetic interference of the first and second beam of light. These carry information about the Doppler frequencies of each beam of light. These frequencies depend on the wavelength of the monochromatic light, the speed of movement V and the angle of incidence in the direction U. Due to differences in the angle of incidence, the information carried by each of the beams of light may easily be dissociated. Indeed, the ratio of the projection angles is around 1.5. In addition, the angles of incidence are both contained within a cone the axis of revolution of which is carried by the normal to the median plane 23, the aperture angle of which is less than 30 degrees and the apex of which is located on the median plane 23 of the outer surface 22.

[0131] In order that the backscattered beam from the second optical path interferes as little as possible with the electromagnetic interference between the first beam of light and its backscattered beam at the first light sensor 3, it is preferable for the two beams of light downstream of their focusing lens, that is to say on their optical path, not to intersect before the outer surface. This is a precaution to be taken for any generation system 1 of the first embodiment having a single light source 2.

[0132] Here, the two points of impact 13 and 14 of the first 11 and second 12 optical paths are coincident. This avoids any time correction between the two coupled signals at the light sensor 3. The two signals will thus be able to be used directly in order to deduce therefrom the distance d from the outer surface 22 of the medium 21, thereby allowing faster real-time processing. In addition, the use of a single sensor does not require any synchronization of the signals, also limiting small errors, thereby reducing noise on the signals.

[0133] This is the preferred set-up of the first embodiment due to the absence of these time corrections on the signals, which speeds up the processing of the signals and limits noise on the signals. In addition, it is an inexpensive set-up since a single light source and a single sensor are used.

[0134] Of course, these examples 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.

[0135] 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.

[0136] 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 more or less complex 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 preferred example of the first embodiment of FIG. 2.

[0137] 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 example of FIG. 1.

[0138] 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.

[0139] 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.

[0140] 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.

[0141] 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.

[0142] 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.

[0143] 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 beams of light, 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.

[0144] 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.

[0145] 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 beams of light 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 integration 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.

[0146] 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.

[0147] 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.

[0148] To apply the method, use was made of a preferred generation system, the principle of which is illustrated in FIG. 2. The angle of incidence of the first and second beams of light 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 beams of light 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.

[0149] 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 beam of light and the beam of light 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 an optical splitter element in order to redirect the second beam of light 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, an electrical signal associated with a photodiode is obtained, containing the payload information carried by the harmonics of each different Doppler frequency.

[0150] 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 beams of light.

[0151] FIG. 4a shows the temporal evolution in terms of amplitude of two signals. The first signal 101a is associated with the first optical path, and the second signal 102a is associated with the second optical path. 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 beams of light on the outer surface of the surface.

[0152] 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.

[0153] 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. 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 photodiode. If the information is also carried by the harmonics, the harmonics should be taken into account by way of the filtering step.

[0154] 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.

[0155] 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 beams of light. 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.

[0156] 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.