Method for imaging by displaying variations in a speckle contrast

Abstract

The invention relates to a method for imaging by displaying variations in a speckle contrast, making it possible to more reliably reveal the movements of parts of a scattering target to be analysed (100). The method comprises using a reference target (101) to verify that an illumination laser source that is used is not undergoing operational fluctuations while the method is being executed. The use of the reference target also makes it possible to have knowledge of the maximum speckle contrast that the illumination laser source can generate. The method may also be continued by constructing a speckle contrast variation image, by subtracting each calculated contrast value for the target to be analysed from a mean speckle contrast value calculated for the reference target. Thus, speckle contrast variation images that are constructed using different laser sources, in particular with different spectral bandwidths, different image forming optics, and different image sensors, and / or during repetitions of the method that are separated in time, can be validly compared with one another. To this end, each speckle contrast image of the target to be analysed is corrected by a specific contribution to the illumination laser source and optionally also by another contribution due to image detection noise.

Description

Description IMAGING METHOD BY VISUALIZING VARIATIONS IN A SPECKLE CONTRAST Technical field

[0001] The present description relates to a method of imaging by visualizing variations in a speckle contrast. Prior art

[0002] Speckle contrast variation imaging is useful for revealing movements, or micromovements, that occur in a medium. For this, the medium must be semi-transparent or rough, and also scattering for the illumination wavelength that is used. This illumination is carried out using a laser source, to satisfy luminous coherence requirements that are necessary for this imaging technique. Due to its semi-transparent or rough nature and also scattering at the wavelength considered, the medium illuminated by the laser source produces a state of luminous interference at each point of an object plane of an imaging instrument that is focused on this medium. The object plane then contains a speckle pattern, which is made up of juxtaposed speckle grains, lighter or darker depending on the state of interference that exists at each point of the object plane.This speckle pattern can then be captured as an image using the imaging instrument which optically combines the medium to be analyzed with the photosensitive surface of a matrix image sensor. The size and contrast of the speckle grains depend on several parameters which are relative to the analyzed medium, but also on certain parameters of the imaging instrument and the laser illumination source which are used.

[0003] A method of imaging by visualizing speckle contrast variations as known prior to the present invention consists of constructing a speckle contrast image by assigning to each point of the image as captured by the imaging instrument a speckle contrast value that exists at that point in the captured image(s). This speckle contrast value can be calculated according to one of the following three modes of constructing the speckle contrast image: first mode: separately for each image of the medium which is captured using the imaging instrument, the speckle contrast value at each point of the image matrix, or at a selection of some of them, is calculated locally from the speckle pattern as captured by the matrix image sensor, by limiting this contrast calculation to a neighborhood of the point. Then the contrast values ​​which are obtained in this way for each of a plurality of successively captured images, and which are relative to the same point of the image matrix, are averaged together in order to obtain an image with enhanced contrast.This first mode therefore concerns a spatial contrast, and provides a spatial contrast image; second mode: a series of successive images of the medium is captured using the imaging instrument, for example several tens or several hundred images, and the speckle contrast value is calculated at each point of the image matrix from the intensities that have been captured for this point in all the successive images. This is then a temporal contrast; and third mode: again when several images of the medium are captured successively, but the speckle contrast that is calculated for each point of the image matrix is ​​hybrid, using a formula that combines the intensity values ​​captured within a neighborhood of this point in all the captured images. Such a contrast is said to be spatio-temporal.

[0004] When micromotions of the medium are fast enough to occur partly during the integration time that is implemented by the matrix image sensor to capture each image, the local interference state varies during this integration time. This results in a reduction of the spatial speckle contrast value that is calculated in the first mode of construction of the speckle contrast image, at the locations where the micromotions occur.

[0005] When the medium is stationary, the images that are captured successively are all almost identical, apart from detection noise, and the temporal speckle contrast that is calculated in the second mode of construction of the speckle contrast image is almost zero for all points of the image matrix. But when the medium undergoes local micromovements between two images that are captured successively, the speckle pattern varies between these images, so that the temporal speckle contrast value becomes non-zero for the locations where micromovements occur. If the micromovements accelerate while the integration time used to capture each image is kept constant, the speckles will have time to vary considerably during this integration time and each photodetector of the image sensor will integrate these variations. The reading values ​​of the image sensor for the images captured successively, at the same image point, then tend towards the same intermediate value, resulting in a reduction of their standard deviation value, and therefore of their temporal contrast value. In the absence of fixed diffusers, these temporal contrast values ​​tend towards zero if the micromovements are extremely rapid compared to the integration time. The operating regime used, corresponding either to a reduction or an increase in the temporal contrast depending on the speed of the micromovements, depends on the applications.

[0006] Thus, imaging by visualizing variations in speckle contrast is effective for detecting micromovements occurring on the surface of a rough scattering medium or in a semi-transparent scattering medium. For this reason, it is useful for many applications.

[0007] However, it is necessary to be able to attribute the speckle contrast variations that are highlighted to movements of the analyzed medium. For this, the imaging instrument is kept unchanged between images that are captured successively. But certain characteristics of the laser source that is used to illuminate the medium can vary involuntarily, causing contributions to the variations of the calculated speckle contrast values, independently of the existence of movements in the analyzed medium. It is then important to be able to attribute the speckle contrast variations that are observed to the analyzed medium, excluding that uncontrolled variations have occurred in the operation of the laser source.

[0008] In addition, it may be necessary to know precisely some of the characteristics of the laser source that is used to illuminate the medium analyzed by speckle contrast.

[0009] Some causes of uncontrolled variations that affect the operation of the laser source used to illuminate the analyzed medium are: / 1 / part of the illumination light which is emitted by the laser source can be reinjected into the laser source itself after having been backscattered by the analyzed medium; 121 a variation in power supply noise may affect the laser source; 131 the laser source may undergo temperature variations, which alter the spectral characteristics of the illuminating radiation; / 4 / the emission spectral width of the laser source may fluctuate, and its single-mode operation may be unstable. In particular, the laser source may unexpectedly switch to undesired multi-mode operation; and / 5 / other causes still exist, depending on the nature and technology of the laser source used. There are remedies that can avoid these variations in the operation of the laser source, including: for the cause of variations / 1 / : use optical isolators between the laser source and the analyzed medium; for the cause of variations 121: use a highly stabilized power supply for the laser source; for the cause of variations 13 / : implement thermal stabilization for certain components of the laser source; and for the cause of variations / 4 / : it may be necessary to implement a servocontrol of the operation of the laser source with respect to an atomic reference. However, these remedies are complex and expensive, even very expensive. Furthermore, they are not compatible with applications where the imaging instrument to be used must be light, compact, mobile and possibly manually applied to the medium to be analyzed.In addition, uncontrolled variations in the operation of the laser source may still occur, which are other than those for which remedies have been implemented.

[0010] We know the document by Colin Elise et al. “Imaging of the skin microvascularization using spatially depolarized dynamic speckle” JOURNAL OF BIOMEDICAL OPTICS, SPIE, vol. 27, no. 4, 1 er April 2022, page 46003, which describes a process of imaging the microvascularization of the skin using depolarized dynamic speckles.

[0011] EP 3 467 483 describes a pattern structure inspection device and an associated inspection method. Technical problem

[0012] From this situation, an aim of the present invention is to make it possible to attribute with certainty variations in speckle contrast to movements which occur in the analyzed medium.

[0013] Additional aims of the invention are to exclude measurement results which have been obtained during variations in the operation of the laser illumination source, without generating significant additional cost for the imaging system used, nor increasing its size, and nor reducing its ability to be moved or handled easily.

[0014] Another aim of the invention is to be compatible with speckle contrast variation imaging systems which already exist, by carrying out a retrofit adaptation of these systems.

[0015] Another object of the invention is to provide speckle contrast values ​​and images that are calibrated and free of contributions specific to the laser sources used to illuminate the medium to be analyzed, or in which such contributions due to the laser source are reduced. In other words, it is sought to have speckle contrast images that are at least partially free of artifacts caused by the laser source, in order to be able to meaningfully compare speckle contrast images captured using different laser sources.

[0016] Another object of the invention is to provide speckle contrast variation values ​​and images while minimizing the effect on these values ​​of detection noises occurring in the matrix image sensor used.

[0017] Finally, another aim of the invention is to enable the laser lighting source to be characterized in real time during its operation. Summary of the invention

[0018] To achieve at least one of these aims or another, a first aspect of the invention proposes a method of imaging by visualizing variations in a speckle contrast, according to which movements of diffusing parts of a target to be analyzed cause variations in a speckle contrast in one or more images of the target to be analyzed, the method comprising the following steps: / 1 / provide a reference target that has a static and constant scattering pattern; 121 using an imaging instrument and a laser source, capturing at least two images of the target to be analyzed at two different times, by arranging the reference target, in addition to the target to be analyzed, within a field of view of the imaging instrument which is effective for each image, such that the laser source simultaneously illuminates the target to be analyzed and the reference target while each image is captured, and such that the captured image is formed by radiation produced by the laser source and then scattered by the target to be analyzed and by the reference target; 131 separately for each image captured in step 121, calculate a speckle contrast value, denoted Cstatic, from a part of the image which corresponds to the reference target; then / 4 / validate the images entered in step 121 only if the Cstatic values ​​calculated respectively for these images have variations from any one of the images to another which are less than a threshold, otherwise repeat steps 121 to / 4 / ; and then / 5 / if the images have been validated, deduce from at least one of them information on movements of parts of the target to be analyzed, from variations in the speckle contrast which are relative to these parts of the target to be analyzed.

[0019] For the purposes of the invention, the term "target to be analyzed" means a portion of a medium to which the method of the invention is applied to visualize variations in speckle contrast, regardless of the nature of the medium. For this purpose, the target to be analyzed is at least partially diffusive for the laser radiation used. In particular, the target to be analyzed may consist of a portion of medium in which movements or micromovements are likely to occur locally or globally.

[0020] A static pattern is a pattern of which no part is in motion at a given instant or during a given duration, for example during the integration time used to capture an image, commonly also called the image accumulation time.

[0021] A constant pattern is a pattern that remains identical between two separate moments, for example between the moments at which successive images are captured.

[0022] Finally, an imaging instrument is understood to mean a system that is adapted to form and capture images, such as commonly called a camera. Such an imaging instrument comprises an image-forming optic, a matrix image sensor, electronics for reading and processing the image capture signals that are delivered by the matrix image sensor, and optionally a pupil diaphragm and a field diaphragm.

[0023] Preferably, the reference target and the laser source are selected to produce a Gaussian circular speckle, or fully developed speckle, in the portion of each image captured in step 121 that corresponds to the reference target. For this, the reference target may have a roughness height standard deviation value that is greater than one-third of a wavelength value of the laser source, preferably greater than one time this wavelength value. Furthermore, and also to obtain the Gaussian circular speckle, the reference target and the imaging instrument may be selected so that the portion of each image captured in step 121 that corresponds to the reference target incorporates radiation scattered by at least ten, preferably at least fifty, roughness reliefs separated by intermediate roughness troughs.

[0024] According to the invention, the target to be analyzed and the reference target appear simultaneously in each image that is captured. The part of this image that corresponds to the reference target, by optical conjugation through the imaging optics, is constituted by a speckle pattern that the radiation from the laser source produces by interacting with this reference target. This speckle pattern in the image of the reference target results from a combination of radiative characteristics of the laser source with characteristics of the scattering pattern of the reference target. As the latter is static and constant, a constancy or quasi-constancy of the speckle pattern that appears in the successive image parts that correspond to the reference target guarantees that the characteristics of the radiation from the laser source were identical or quasi-identical between the respective times of image capture. Another speckle pattern that appears in each image at locations of the latter corresponding to the target to be analyzed is then not affected by fluctuations in the operation of the laser source. This other speckle pattern can therefore be used validly to deduce information about the target to be analyzed.More specifically, when a single captured image is used to analyze the target of interest, the method of the invention makes it possible to state with a high level of certainty that the operation of the laser source has not varied during the integration time of this image, the other image which was captured in the method of the invention serving to verify the constancy of the operation of the laser source. When several captured images are used to analyze the target of interest, the method of the invention makes it possible to ensure that the laser source has had identical operation for all these images.

[0025] The method of the invention can be combined with each of the methods of constructing the speckle contrast image which have been recalled above.

[0026] When the first speckle contrast image construction mode is used, a spatial speckle contrast value is calculated locally in at least one of the images that have been validated in step / 4 / , for at least some of the image points thereof that correspond to the target to be analyzed. Then, a speckle contrast variation image can be constructed by assigning to each of these image points an image point value that depends on a difference result between, on the one hand, an average result <C s tatic> calculated for the speckle contrast values ​​Cstatic on all images captured according to step 121, and on the other hand the spatial speckle contrast value that was calculated for that image point.

[0027] When the second speckle contrast image construction mode is used, a series of successive images of the target to be analyzed is captured in accordance with step 121. Then, if the series of images is validated in step / 4 / , a temporal speckle contrast value is calculated separately for each of a set of image points that correspond to the target to be analyzed, from respective intensity values ​​of the images of the series at this image point. Then, a speckle contrast variation image can be constructed by assigning to each of these image points an image point value which depends on a difference result between, on the one hand, an average result <C s tatic> calculated for the speckle contrast values ​​Cstatic on all images captured according to step 121, and on the other hand the temporal speckle contrast value that was calculated for that image point.

[0028] When the third speckle contrast image construction mode is used, a series of successive images of the target to be analyzed is again captured in accordance with step 121. If this series of images is validated in step / 4 / , a speckle spatio-temporal contrast value is then calculated separately for each of a set of image points that correspond to the target to be analyzed, from respective intensity values ​​of the successive images in a neighborhood of the image point. Then, a speckle contrast variation image can be constructed by assigning to each of the image points an image point value that depends on a difference result between, on the one hand, an average result <C s tatic> calculated for the speckle contrast values ​​Cstatic on all images captured according to step 121, and on the other hand the speckle spatio-temporal contrast value that was calculated for that image point.

[0029] By means of the subtraction operation introduced according to the invention in each of the three modes of constructing the speckle contrast image, the speckle contrast variation image that is obtained for the target to be analyzed can be independent of the radiative characteristics of the laser source, or depend on them to a residual extent that is very small. In other words, the invention provides speckle contrast variation values ​​and images by minimizing the effect on these values ​​and images of the optical probe that is used and the image capture conditions. This is achieved by the subtraction operation between the speckle contrast value calculated for the reference target and each speckle contrast value relating to the target to be analyzed.Thus, the speckle contrast variation image that is obtained for the target to be analyzed directly highlights the amplitude of the movements or micromovements that occur in the target to be analyzed, independently of the laser source that is used. In other words, the invention provides an absolute, or “universal,” analysis result that allows comparison between. they are analyses carried out with different optical probes, and at separate times.

[0030] It is further possible to further reduce or eliminate the effects of image detection noise, in particular shot noise, by first correcting each of the speckle contrast values ​​relating to the target to be analyzed and each speckle contrast value Cstatic which is calculated for the reference target, for a contribution from the image detection noise fluctuations. The speckle contrast value Cstatic which is calculated from the part of each captured image corresponding to the reference target, and which is thus corrected, is essentially free from the effect of image detection noise. It then allows an exact characterization of the illumination laser source.

[0031] Generally for the invention, the reference target may be a piece of paper with an identified grain size value. It may be held by a support which is close to the object plane of the imaging system, also called the focusing plane of this system, inside the field of view, preferably close to an edge of this field of view. The reference target is thus systematically joined to the target to be analyzed for each image which is captured.

[0032] Also generally for the invention, in step / 1 / , the reference target may be preferably selected so that it has a scattering intensity level for the radiation from the laser source, such that each speckle grain in the portion of each captured image that corresponds to this reference target, is within a linear detection interval of the imaging instrument. Each speckle contrast value Cstatic that is calculated in step 131 is thus more meaningful.

[0033] Still generally for the invention, the imaging instrument may comprise: - an image forming optic; - a matrix image sensor, comprising photodetectors which are arranged at intersections of rows and columns; and - a pupillary diaphragm. Then, an aperture of the pupil diaphragm can be adjusted prior to capture images in step / 2 / , so that each speckle grain covers at least two photodetectors in the part of each captured image that corresponds to the reference target. It is thus possible to properly spatially discretize all the hues of the speckle grains in each image that is captured.

[0034] Still generally for the invention, the reference target may be selected in step / 1 / preferably to have a size and roughness characteristics such that the portion of each captured image that corresponds to this reference target contains at least fifty speckle grains. Under these conditions, each speckle contrast value Cstatic that is calculated in step 131 may have a high level of reliability.

[0035] Still generally for the invention, the method may additionally comprise characterizing a spectral distribution of the radiation which is produced by the laser source, such that this spectral distribution is effective for at least one of the captured images. This spectral distribution characterization is deduced from the speckle pattern in the part of the captured image which corresponds to the reference target. Such a spectral distribution characterization may in particular comprise determining a spectral width of the laser source used.

[0036] The method of the invention can be used for many applications.

[0037] In particular, the target to be analyzed may be a portion of a fluid that is partially transparent to the radiation from the laser source and that contains mobile scattering particles. For such applications, which may in particular relate to water purification units, the method may comprise characterizing a combination of a concentration of the particles in the fluid with a speed of movement of these particles.

[0038] Alternatively, the target to be analyzed may be a portion of a scattering solid that vibrates. Then, the method may include characterizing local vibration amplitudes that exist at distinct locations within the portion of solid.

[0039] For other applications that are also possible, the target to be analyzed may be a portion of a biological tissue. In this case, the method may include characterizing levels of movement that exist at distinct locations in the biological tissue. These movements may indicate levels of vascularization that vary between different parts of the biological tissue, or between different dates at which the method of the invention is repeated for the same location of the biological tissue.

[0040] Finally, a second aspect of the invention proposes an imaging system by visualizing variations in a speckle contrast, this system comprising: - an imaging instrument, which is adapted to capture images of a target to be analyzed; - a laser source, which is arranged to illuminate the target to be analyzed while each image is captured, so that the image is formed by radiation produced by the laser source and then diffused by the target to be analyzed; - a calculation unit, which is configured to calculate values ​​of a speckle contrast relating to the target to be analyzed, from one or more images thereof; - a display system, connected to show an image that has been constructed by the computing unit, - a reference target, which has a static and constant scattering pattern; and - support means, adapted to maintain the reference target within a field of view of the imaging instrument which is effective for each image, and so that the laser source simultaneously illuminates the target to be analyzed and the reference target while each image is captured.

[0041] According to the invention, the calculation unit is further configured to calculate, separately for each captured image, a speckle contrast value, denoted Cstatic, from a part of the image which corresponds to the reference target. It is further configured to validate several images which have been captured successively if the Cstatic values ​​which have been calculated respectively for these images have variations from any one of the images to another which are less than a threshold. The calculation unit is further configured to construct a speckle contrast variation image by assigning to each of a set of image points which correspond to the target to be analyzed, an image point value which depends on a difference result between, on the one hand, an average result <C static> calculated for the speckle contrast values ​​Cstatic on all captured images, and on the other hand a speckle contrast value that was calculated for the image point from one or more captured images and then validated.

[0042] Such an imaging system is suitable for implementing a method according to the first aspect of the invention, including its improvements and / or preferred conditions of implementation.

[0043] Optionally, the speckle contrast variation imaging system of the invention may further comprise an additional device for capturing images of the target to be analyzed, which is adapted to provide color images. Advantageously, this additional device may make it possible to illuminate the target to be analyzed successively in three separate colors, and may use the same imaging instrument to capture separate images of each color, as that used for the images captured with illumination by the laser source.

[0044] Optionally also, the imaging instrument may further comprise a linear polarizer which is oriented perpendicular to a polarization direction of the laser source. Such a linear polarizer, oriented in this way, improves speckle contrast variation images which are obtained by suppressing the portion of the illuminating radiation which is scattered by the surface of the target to be analyzed. Brief description of the figures

[0045] The characteristics and advantages of the present invention will appear more clearly in the detailed description below of non-limiting examples of implementation, with reference to the appended figures among which:

[0046] [Fig. 1] is an optical diagram of an imaging system which is in accordance with the invention;

[0047] [Fig. 2] schematically shows a content of an image as captured by the imaging system of [Fig. 1];

[0048] [Fig. 3] shows steps of an imaging method which is in accordance with the invention;

[0049] [Fig. 4a] shows an example image of organic tissue illuminated by a laser source, as captured by the imaging system of [Fig. 1];

[0050] [Fig. 4b] shows a speckle spatial contrast variation image that was constructed according to the method of the invention, from a series of successive images each captured like that of [Fig. 4a];

[0051] [Fig. 5a] shows an example of a color image of organic tissue as captured by the imaging system of [Fig. 1], using a method of obtaining color images as known in the prior art; and

[0052] [Fig. 5b] shows a temporal contrast variation image of speckle which was constructed according to the method of the invention, from a series of images captured successively with laser illumination. Detailed description of the invention

[0053] For the sake of clarity, the dimensions of the elements shown in [Fig. 1] and [Fig. 2] do not correspond to real dimensions or to real dimensional ratios. Furthermore, some of the elements shown in [Fig. 1] are only shown symbolically.

[0054] According to [Fig. 1], an imaging system comprises an optical probe 10, a computing unit 20 and a display screen 30. The computing unit 20 is denoted COMPUT. and may be a portable computer. The display screen 30, denoted DISPLAY, is preferably of a high definition type.

[0055] The optical probe 10 may be of a portable model, with a side screen (baffle) 11 and a gripping handle 12. It groups together a lighting path and a detection path, which are arranged so that the lighting path illuminates an entire field of view of the detection path. For this, a lighting direction of the lighting path may be oblique relative to an optical axis of the detection path, denoted AA, in order to juxtapose the two lighting and detection paths inside the optical probe 10. The field of view is surrounded by the side screen 11 to eliminate any stray light rays which do not come from the lighting path or the field of view.

[0056] The illumination path comprises a laser source 1 and a diverging optic 2 by which radiation which is emitted by the laser source 1 illuminates the entire field of view, or an essential part of this field of view which is useful for implementingthe invention. The laser source 1 may be of a type that produces radiation at a wavelength suitable for the intended application. For example, to observe movements of red blood cells in organic tissue, the wavelength of the laser source 1 may be in the range of 785 nm (nanometer) to 810 nm. This range is relevant because the radiation from the laser source 1 then penetrates through the skin that may be present on the surface of the organic tissue. For the detection of vibrations of metals, lower wavelength values ​​in the visible range, for example 512 nm, 630 nm, etc., are also suitable. A spectral width of the laser source 1 is preferably less than 1 nm, on the one hand to have better measurement sensitivity, and on the other hand so that the invention provides speckle contrast variation images that are independent of the laser source used to a sufficient extent.The laser source 1 may be devoid of means dedicated to stabilizing its emission operation, so that it may be inexpensive, compact and lightweight, in particular to be incorporated into the portable optical probe 10.

[0057] The detection path comprises an imaging instrument 4, which consists of an imaging optics 41, a matrix image sensor 42, a pupil diaphragm 43, also called an aperture diaphragm, an optional field diaphragm 44 and an optional linear polarizer 45. The matrix image sensor 42, denoted DETECT., preferably comprises a high density of separate photodetectors in its photosensitive surface S. A read signal output of the matrix image sensor 42 is connected to an input of the computing unit 20, and a video output of the latter is connected to the display screen 30. A sensitivity interval of the matrix image sensor 42 contains the wavelength value of the laser source 1. If desired, the field diaphragm 44 can be located against the photosensitive surface S of the matrix image sensor 42.In known manner, the photosensitive surface S is formed by the photodetectors which are arranged at intersections of rows and columns of a matrix arrangement. The integration time which is implemented by the sensor 42 to capture each image produced using illumination by the laser source 1, can be between 1 ms (millisecond) and 100 ms, for example. Since the illumination radiation which is produced by the laser source 1 is rectilinearly polarized, the linear polarizer 45 is advantageously oriented perpendicular to the polarization direction of the. radiation from the laser source 1. In this way, the polarizer 45 suppresses a part of the laser radiation which is scattered by the contents of the field of view. It consequently promotes the detection of another part of the laser radiation which undergoes multiple scatterings produced by a scattering content of the field of view, as described in FR 3 062 542 A1.

[0058] According to an optional improvement of the invention, the optical probe 10 can be completed to capture color images of the contents of the field of view. For this, the matrix image sensor 42 can be of a monochrome type sensitive over the entire visible range, and red, green and blue light sources can be added in the optical probe 10. For example, red, green and blue light-emitting diodes 3 can be juxtaposed repeatedly along a ring which surrounds the field of view at the level of the pupillary diaphragm 43, around the opening of this diaphragm as appears in [Fig. 1]. The light-emitting diodes 3 produce non-coherent illumination of the contents of the field of view for each of the blue, green and red colors.Then, three images which are captured successively with minimal intermediate durations compared to the operating cycle duration of the image sensor 42, one of the images using illumination exclusively by the blue light-emitting diodes, another image with only the green light-emitting diodes, and the last image with only the red light-emitting diodes, makes it possible to reconstruct an RGB color image using only the monochrome type image sensor 42. Thus, the illumination by the laser source 1 makes it possible to capture speckle images to implement the invention, and the manner which has just been described for acquiring RGB color images provides such color images which can be superimposed on the speckle images.

[0059] Reference 13 designates an image capture trigger. A short press on the trigger 13 by an operator triggers the capture of successive images with illumination by the laser source 1 only, followed by the calculation and real-time display of successive speckle contrast variation images, for example with a refresh rate that is between approximately 5 Hz (hertz) and approximately 25 Hz depending on a desired value for a signal-to-noise ratio of each speckle contrast variation image. The speckle contrast variation images that are displayed in this way are in accordance with the prior art as existing before the present invention, while however benefiting from the polarimetric filtering which provides better sensitivity to a part of the radiation which has penetrated more deeply into the target to be analyzed 100. A long press which is carried out alternately by the operator on the trigger 13, for example with a press duration greater than 1 s (second), triggers the capture of successive images of which the first three are captured with respective illuminations by the light-emitting diodes 3, separately and successively for the blue, green and red colors, then the following images are captured using for each one illumination by the laser source 1. These following images captured with the laser illumination are used to calculate a single speckle contrast variation image.This has a signal-to-noise ratio value that is higher than that of each speckle contrast variation image of a short press on the trigger 13, thanks to a larger number of captured images that are combined to obtain each speckle contrast variation image. Each long press on the trigger 13 thus results in an RGB color image, as commonly known, and a speckle contrast variation image with a high signal-to-noise ratio value, both of which relate to the same field of view content and are therefore superimposable. The invention that is described in detail below can be applied to the speckle contrast variation images of a short press or a long press on the trigger 13.

[0060] A target to be analyzed is designated by the reference 100. It is semi-transparent or rough, and diffusing for the radiation of the laser source 1. It can be a biological medium, such as a part of organ tissue or other. When the optical probe 10 is used to capture images of the target to be analyzed 100, the front edge of the side screen 11 can be applied to a surface of the target to be analyzed 100, around the part of the latter which is to be analyzed. The laser radiation from the source 1 penetrates into the target to be analyzed 100 to a depth which depends on the wavelength of the laser source 1 and the nature of the target to be analyzed 100. This depth of penetration of the laser radiation into the target to be analyzed can be of the order of a few tens of nanometers, in particular for rough metals which constitute the target to be analyzed, to a few millimeters, in particular for biological media, such as skin tissues.The laser radiation is backscattered by scattering centers which are present in the. target 100, and backscattered portions of the laser radiation interfere to create a speckle pattern in the focal plane of the imaging system 4. The imaging optics 41 are such that the surface of the target to be analyzed 100 is substantially optically conjugated with the photosensitive surface S of the matrix image sensor 42. Under these lighting and image capture conditions, the image that is captured by this imaging system 4 then reproduces the speckle pattern. This pattern is made up of speckle grains that are juxtaposed, being alternately lighter and darker.

[0061] To implement the invention, a reference target 101 is added in the field of view of the imaging instrument 4, so as to be illuminated by the laser source 1 at the same time as the target to be analyzed 100, and to appear with the latter in each image that is captured. Preferably, the reference target 101 is placed in the focal plane of the imaging instrument 4, or close to this focal plane. It is therefore substantially at the same level as the surface of the target to be analyzed 100 along the optical axis AA of the detection path of the imaging instrument 4. Under these conditions, the content of each image that is captured is as shown in [Fig. 2], It comprises an image portion 10T which is optically conjugated with the reference target 101 by the imaging optics 41, and the remainder of the extent of each image, designated by the reference 100', is optically conjugated with the intended portion of the target to be analyzed 100.The reference target 101 can be held fixedly at the desired position in the field of view by being attached to the side screen 11. In this case, the side screen 11 performs the additional function of support means as introduced in the general part of the present description.

[0062] The reference target 101 has a pattern that is diffusive for the radiation from the laser source 1, and which is static and constant. In particular, the reference target 101 may consist of a portion of a solid and opaque material that has a roughness on its surface. For example, it may be a piece of paper sold under the brand name CANSON®. The roughness of this reference target 101, as characterized by a standard deviation value of the surface height distribution of the target 101, is preferably greater than the wavelength value of the laser radiation that is used, and an average width of relief and hollow of the roughness is preferably such that at least ten separate reliefs of roughness, preferably fifty, are illuminated by the laser source 1 . Under these conditions, the radiation which is backscattered by the reference target 101 produces speckles which conform to circular Gaussian statistics, and which have a maximum speckle contrast. Then, the image part 10T is also constituted by a speckle pattern, which is used according to the invention on the one hand to calculate a static contrast value relative to the reference target 101 , and on the other hand to verify a posteriori the stability of the operation of the laser source 1 .Since the roughness pattern of the reference target 101 is constant and devoid of internal movements, variations in the speckle pattern in the image portion 10T, between images that have been captured successively, are necessarily caused either by excessive torsional movements of the optical probe when it is in contact with the target to be analyzed, or by variations in operation of the laser source 1, which in all cases invalidates the measurement. When several images are captured successively with illumination by the laser source 1, and these captured images have between them variations in spatial contrast of speckle in the image portion 10T which are less than a threshold, in absolute values, then the method of the invention proposes to validate these captured images by considering that the operation of the laser source 1 was constant during the integration time of each of them and was identical for all these captured images.By constant operation of the laser source 1 is meant a laser emission which is in particular: free from effects of reinjection of variable backscattered radiation into the laser source, free from effects of variations in electrical power supply noise, in amplitude and in phase, of the laser source, free from effects of thermal variations which could affect certain components of the laser source, and free from temporal fluctuations in the spectral distribution of the radiation which is emitted by the laser source, which could be due to uncontrolled transitions between a desired single-mode emission and an unwanted multi-mode emission, or which could appear in the form of temporal variations in the spectral width of the emitted laser radiation.

[0063] To implement the invention optimally, the following operating conditions can be adopted: - the reference target 101 is selected to have a scattering power at the wavelength of the laser source 101, such that the spatial variations in light intensity of the speckle pattern are within a linear detection interval of the matrix image sensor 42. When the reference target 101 is a piece of CANSON® paper, the paper color can in particular be chosen so as not to correspond to the wavelength of the laser source 101; - the aperture of the pupil diaphragm 43 is adjusted so that each speckle grain in the image portion 10T covers at least two neighboring photodetectors in the photosensitive surface S of the matrix image sensor 42; and - the reference target 101 is selected to have a size and roughness characteristics such that the image portion 101' contains at least fifty speckle grains. When the above-mentioned conditions are met, so that the speckle pattern that is captured in the 10T image part is close to a circular Gaussian pattern, as defined in the book “Speckle Phenomena in Optics, Theory and Applications”, by J. W. Goodman, and in the article entitled “Study of a circular Gaussian transition in an optical speckle field”, by I. Bergoënd, X. Orlik and Eric Lacot, Journal of the European Optical Society, Rapid Publications 3, 08028 (2008), it is possible to deduce numerical values ​​for certain characteristics of the spectral distribution of the radiation that is emitted by the laser source 1 , from the speckle pattern that is captured in the 10T image part. The possibility of such an analysis is shown in particular in the article entitled "Choosing a laser for laser speckle contrast imaging" by DD Postnov, X. Cheng, SE Erdener and DA Boas, Scientific Reports, February 22, 2019, 9(1):2542. doi: 10.1038 / s41598-019-39137-x.When the laser emission is single-mode, respective values ​​of the central emission wavelength and the spectral emission width can be obtained in this way. When such deduction of numerical values ​​which characterize the laser emission is repeated separately for several images which have been captured successively, from the 10T image parts, the results obtained characterize fluctuations in the operation of the laser source 1. These fluctuations are acceptable when the values. digital values ​​which are obtained for each characteristic considered of the spectral distribution of the radiation of the laser source 1 present variations lower than a fixed threshold. In this case, the images are validated to analyze the target 100. In the opposite case, that is to say when the laser source 1 has undergone operating fluctuations which appear too significant according to the digital values ​​obtained successively for the spectral distribution of its radiation, the images are declared unacceptable to validly analyze the target 100.

[0064] According to the invention, it is sufficient to verify that a speckle contrast within the image portion 101' varies to a sufficiently small extent between all the images in the series, to accept or reject these images for the purposes of analyzing the target 100. This speckle contrast, noted Cstatic, is calculated separately for each captured image according to the formula: Cstatic = o / <l> , où <l>and o denote the mean value and standard deviation of the intensity values ​​captured in the 10T image portion of each captured image, respectively.

[0065] With reference to [Fig. 3], the imaging method of the invention for analyzing the target 100 comprises the following steps: in step S1: a series of several successive images of the target to be analyzed 100, from two images to a few hundred images, is captured using the optical probe 10 provided with the reference target 101, with illumination by the laser source 1. This reference target 101 is assembled in the optical probe 10 identically for all the images of the series. Preferably, the reference target 101 is left in the optical probe 10 without being disassembled or moved between two successive images; in step S2: the values ​​of the speckle contrast Cstatic are calculated separately for each image of the series, from the part 10T of this image; in step S3: the existence of fluctuations in the operation of the laser probe 1, which would have occurred between some of the images of the series, is sought.For this, differences in speckle contrast values ​​Cstatic, denoted ACstatic, are calculated between two of the images in the series, and compared to a threshold ACmax, for some or all pairs of images in the series. If at least one of these differences exceeds the threshold ACmax, the series of images is rejected and the imaging process is resumed at step S1. Otherwise, the series of images is validated and the imaging process is continued with step S4;. in step S4: for each image point of the image portion 100', which was optically conjugated with the target to be analyzed 100 while each image of the series was captured, a contrast value CCibie(i, j) is calculated for this image point in accordance with the mode chosen for constructing a speckle contrast variation image, where i and j are the coordinates of the image point in the matrix of the photosensitive surface S of the matrix image sensor 2; in step S5: constructing a new image, which is the speckle contrast variation image obtained in accordance with the invention, by assigning to the image point with coordinates i and j in the image portion 100' a new image point value denoted AC(i, j) and calculated according to the following formula: AC(i, j) = <C s tatic> - Ccibie(i, j), where <C s tatic> is the average of the respective speckle contrast values ​​Cstatic obtained in step S2 for all the images in the series, for the three speckle contrast variation image construction modes indicated at the beginning of this description. The new image point values ​​which are calculated in this way, AC(i, j), are always positive; and in step S6: display of the speckle contrast variation image on the screen 30. This speckle contrast variation image highlights the parts of the target to be analyzed which have undergone movements, in particular movements on a submicron scale. It has image point values ​​which depend on the amplitude and / or speed of these movements.Furthermore, by virtue of the subtraction operation in the calculation of step S5 to obtain each image point value AC(i, j), speckle contrast variation images which are produced at different dates, or with different optical probes 10, can be compared with each other validly.

[0066] The speckle contrast value Cstatic that is calculated in step S2 includes a speckle contribution that results from the detection noise occurring in the matrix image sensor 42. This detection noise is a combination of several types of noise, the main ones being shot noise and dark noise. The dark noise is most often negligible for normal operation of the imaging system, due to the sufficient number of photons that are emitted by the laser source 1 and then backscattered by the reference target 101 to the image sensor 42. On the other hand, the shot noise, which is proportional to the square root of the captured intensity I, can contribute significantly to the standard deviation value o, and therefore intervenes in each value of speckle contrast. The intensity standard deviation value resulting from shot noise can therefore advantageously be subtracted from the intensity standard deviation value calculated for the succession of images captured in step S1 in the image parts 10T for each image point in the case of temporal contrast or for each subset of image points in the case of spatial contrast or the case of spatio-temporal contrast. This subtraction operation, to remove the contribution of shot noise, is preferably applied to the speckle contrast Cstatic for the reference target 101 and for each contrast value Ccibie(i, j) for the target to be analyzed 101.The other subtraction operation that is performed in step S5 to calculate each image point value AC(i, j) makes it possible to directly attribute, subject to the conditions mentioned above, the variations in the values ​​AC(i, j) to the movements that occur in the target to be analyzed 100, despite the use of different laser sources and different optical systems between separate executions of the method of the invention. It should be noted that the same micromovement in the target to be analyzed 100 will result in speckle contrast variation values ​​AC(i, j) that are different if the speckle contrast value Cstatic calculated for the reference target 101 is not the same for the respective laser sources of the two optical probes. Indeed, the drop in contrast does not vary linearly with the speed of the micromovements, and therefore depends on the value of the speckle contrast that would exist in the absence of the micromovements.This non-linearity is however known and published, notably in the article "Review of laser speckle contrast techniques for visualizing tissue perfusion", by Matthijs Draijer et al., Lasers Med. Sci. (2009) 24:639-651, fig. 2, p. 641, which allows a rigorous comparison of the results despite the difference in Cstatic speckle contrast value between the two laser sources used, or more generally between the two optical probes used.

[0067] Steps S2 to S5 are executed by the computing unit 20, preferably in an automated manner.

[0068] With the first speckle contrast image construction mode, spatial contrast values ​​are calculated for each image point (i, j) in the image portion 100', separately for each image that was captured in step S1. They are each calculated on a neighborhood of the image point of coordinates i and j in the image portion 100', this neighborhood being able to be a square of 5 x 5, or 7 x 7, or 9 x 9 image points centered on image point i, j. Each spatial contrast value is then equal to the quotient of the standard deviation of the intensity values ​​captured inside this neighborhood square, by the average of these same intensity values ​​captured still inside the neighborhood square. These spatial contrast values ​​are then averaged over all the captured images, separately for each image point, in order to strengthen the signal obtained. The average values ​​which are calculated in this way are those of step / 4 / noted Ccibie(i, j). For this first mode of construction of speckle contrast image, the image which is displayed in step S6 is therefore an image of variation of spatial speckle contrast.The existence of movements within the target to be analyzed 100 causes local reductions in the values ​​Ccibie(i, j), and consequently local increases in the values ​​AC(i, j) in the speckle contrast variation image which is displayed in step S6, since Cstatic corresponds to the maximum possible value for Ccibie(i, j).

[0069] With the second speckle contrast image construction mode, the contrast values ​​Ccibie(i, j) which are calculated in step S4 are temporal contrast values ​​which are calculated respectively for the image points of the image part 100'. Each contrast value Ccibie(i, j) is then equal to the quotient of the standard deviation of the intensity values ​​of all the images of the series at the image point of coordinates i and j, by an average of these same intensity values. The image which is displayed in step S6 is then a speckle contrast variation image between the spatial contrast calculated on the reference target 101 and the temporal speckle contrast calculated on the target to be analyzed 100.The existence of movements inside the target to be analyzed 100 which occur between successively captured images then causes, depending on the integration time of the matrix image sensor 42 with respect to the characteristic time which results from the speed of the movements in the target to be analyzed 100, local decreases or increases in the temporal speckle contrast which will therefore respectively cause increases or decreases in the values ​​AC(i, j), visible in the speckle contrast variation image displayed in step S6.

[0070] With the third speckle contrast image construction mode, the contrast values ​​Ccibie(i, j) that are calculated in step S4 are spatio-temporal contrast values ​​that are calculated respectively for the image points of the image portion 100'. For example, each contrast value Ccibie(i, j) can be calculated according to the same formula as previously for the temporal contrast, but where the intensity of each image point in the image part 100' is replaced by an average intensity value calculated on an environment of this image point, for example a square of dimensions 3 x 3, 5 x 5,.... The image which is displayed in step S6 is then an image of variation of spatio-temporal speckle contrast, calculated as the difference between the spatial speckle contrast of the reference target 101 and the spatio-temporal speckle contrast calculated on the target to be analyzed 100.The existence of movements inside the target to be analyzed 100 which occur between successively captured images then causes, depending on the integration time of the matrix image sensor 42 in relation to the characteristic time which results from the speed of the movements in the target to be analyzed 100, local decreases or increases in spatio-temporal contrast which will therefore respectively cause increases or decreases in the values ​​AC(i, j), as in the case of temporal contrast.

[0071] Generally, for all the modes of construction of the speckle contrast variation image, it is the variation AC(i, j) at each image point which is displayed in step S6.

[0072] For the cases of temporal contrast and spatio-temporal contrast, operating parameter values ​​for the optical probe 10 may preferably be adopted, in particular for the integration time of the matrix image sensor 42, which maximize the difference between the spatial contrast value of speckle <C s tatic> obtained for the reference target 101 and the temporal or spatio-temporal speckle contrast values ​​Ccibie(i, j) obtained for the target to be analyzed 100.

[0073] When the target to be analyzed 100 is a portion of a partially transparent fluid, the movements in it which are highlighted by the speckle contrast variation image correspond to locations where a speed and / or a concentration of mobile diffusing particles in the fluid is (are) greater.

[0074] When the target to be analyzed 100 is a portion of a vibrating scattering solid, the movements in it which are highlighted by the speckle contrast variation image correspond to locations of maximum vibration amplitude, commonly called vibration antinodes or "vibration antinodes" in English.

[0075] When the target to be analyzed 100 is a vascularized biological tissue, the wavelength of the laser source 1 is in the near infrared spectral range, and the integration time of the matrix image sensor 42 is between 1 ms and a few tens of milliseconds, the movements in it which are highlighted by the speckle contrast variation image are mainly produced by vascularization inhomogeneities. These inhomogeneities concern the speed of movement of the red blood cells and / or the concentration of mobile red blood cells.

[0076] [Fig. 4a] reproduces an image of an organic tissue, as captured by the imaging system of [Fig. 1], using illumination by the laser source 1 and an integration time of 10 ms in the matrix image sensor 42. This image has not undergone any processing, so that it corresponds to the usual designation of raw image. The image parts 100' and 10T which correspond respectively to the target to be analyzed 100 and to the reference target 101 are indicated. Each of these image parts consists of a speckle pattern, the respective origins of which have been explained above. [Fig. 4b] shows the speckle spatial contrast variation image which is constructed according to the invention from a series of successive images captured in the same way as the image of [Fig. 4a], and by applying the first mode of construction of a speckle contrast variation image.This image of spatial contrast variation of speckle reveals micromovements occurring in a network of microvascularization of the organic tissue.

[0077] [Fig. 5a] reproduces an RGB color image of the tip pulp of a human finger, as provided by the imaging system of [Fig. 1 ] using successive illumination by the blue light-emitting diodes 3, then by the green ones, then by the red ones. The color image of [Fig. 5a] was reconstructed from the three images captured separately for each color. It mainly shows the surface of the skin with the fingerprint pattern. It is intended to be superimposed on a speckle contrast variation image obtained according to the present invention. For this, a series of about a hundred successive images is then captured using illumination by the laser source 1 , without moving the optical probe 10 relative to the finger, for example at a rate of about a hundred images per second with an integration time of 10 ms for each image. [Fig.5b] shows the temporal contrast variation image of speckle which is constructed according to the invention from this series of images with laser illumination, by applying the. second mode of constructing a speckle contrast variation image. It reveals the micromovements of the subcutaneous and / or transcutaneous microvascular network. The image in [Fig. 5b] is the superposition of the temporal contrast variation image with the color image in [Fig. 5a],

[0078] Finally, and as is generally the case in dynamic speckle imaging, it is possible to use the blood flow index, or BFI for "Blood Flux Index" in English, which is equal to 1 ZCcibie(i, j) 2 to quantify the flows. It is also possible to alternatively use the VMAI parameter which is equal to 1 Z(T- Ccibie(i, j) 2 ), where T is the integration time of the matrix image sensor 42, and Ccibie(i, j) the contrast calculated at each image point i, j, whether this contrast is spatial, temporal or spatio-temporal. Other equations using speckle contrast to deduce information about the target to be analyzed are also proposed in the literature. In general, the proposed invention involves adapting these different equations to take into account not each value Ccibie(i, j) as a physical measurement but AC(i, j) = C s tatic(i, j) - <ccibie>, in order to calibrate the result.

[0079] It is understood that the invention may be reproduced by modifying secondary aspects of the embodiments which have been described in detail above, while retaining at least some of the advantages cited. In particular, the optical probe may have a constitution and an appearance which are different from those which have been described.< / ccibie> < / l> < / l>

Claims

Claims

1. A method of imaging by visualizing variations in a speckle contrast, according to which movements of diffusing parts of a target to be analyzed (100) cause variations in a speckle contrast in one or more images of the target to be analyzed, the method comprising the following steps: / 1 / provide a reference target (101) which has a static and constant scattering pattern; 121 using an imaging instrument (4) and a laser source (1), capturing at least two images of the target to be analyzed (100) at two different times, by arranging the reference target (101), in addition to the target to be analyzed, within a field of view of the imaging instrument which is effective for each image, so that the laser source simultaneously illuminates the target to be analyzed and the reference target while each image is captured, and the captured image is formed by radiation produced by the laser source then scattered by the target to be analyzed and by the reference target; 131 separately for each image captured in step 121, calculate a speckle contrast value, denoted Cstatic, from a part of the image which corresponds to the reference target (101); then / 4 / validate the images entered in step 121 only if the Cstatic values calculated respectively for said images have variations from any one of the images to another which are less than a threshold, otherwise repeat steps 121 to / 4 / ; and then / 5 / if the images have been validated, deducing from at least one of said images information on movements of parts of the target to be analyzed (100), from variations in the speckle contrast relating to said parts of the target to be analyzed.

2. A method according to claim 1, wherein the reference target (101) and the laser source (1) are selected such that said reference target has a roughness height standard deviation value that is greater than one-third of a wavelength value of the laser source, preferably greater than one-times said wavelength value, and wherein the reference target and the imaging instrument (4) are further selected such that the portion of each image captured in step 121 that corresponds to the reference target incorporates radiation scattered by at least ten, preferably at least fifty roughness reliefs separated by intermediate roughness troughs.

3. Method according to claim 1 or 2, according to which, for at least one of the images which have been validated in step / 4 / , a speckle spatial contrast value is calculated locally in the image for at least some of the image points which correspond to the target to be analyzed (100), then a speckle contrast variation image is constructed by assigning to each of said image points an image point value which depends on a difference result between, on the one hand, an average result <cstatic>calculated for the speckle contrast values Cstatic on all images captured in accordance with step 121, and on the other hand the spatial speckle contrast value calculated for the image point.

4. A method according to claim 1 or 2, wherein a series of successive images of the target to be analyzed (100) is captured in accordance with step 121 and then, if the series of images is validated in step 141, a speckle temporal contrast value is calculated separately for each of a set of image points which correspond to the target to be analyzed, from respective intensity values of the images of the series at said image point, and then a speckle contrast variation image is constructed by assigning to each of said image points an image point value which depends on a difference result between, on the one hand, an average result <C s tatic> calculated for the speckle contrast values Cstatic on all images captured according to step 121, and on the other hand the temporal speckle contrast value calculated for the image point.

5. A method according to claim 1 or 2, wherein a series of successive images of the target to be analyzed (100) is captured in accordance with step 121 and then, if the series of images is validated in step 141, a speckle spatio-temporal contrast value is calculated separately for each of a set of image points which correspond to the target to be analyzed, from respective intensity values of the successive images in a neighborhood of said image point, and then a speckle contrast variation image is constructed by assigning to each of said image points an image point value which depends on a difference result between, on the one hand, an average result <C s tatic> calculated for speckle contrast values Cstatic on all captured images according to in step / 2 / , and on the other hand the speckle spatio-temporal contrast value calculated for the image point.

6. Method according to one of the preceding claims, according to which the reference target (101) is selected in step / 1 / so that said reference target has a scattering intensity level for the radiation from the laser source (1), such that each speckle grain in the part of each captured image which corresponds to said reference target, is within a linear detection interval of the imaging instrument (4).

7. Method according to one of the preceding claims, according to which the imaging instrument (4) comprises: - an image forming optic (41); - a matrix image sensor (42), comprising photodetectors arranged at intersections of rows and columns; and - a pupil diaphragm (43), and according to which an opening of the pupil diaphragm (43) is adjusted prior to capturing the images in step 121, so that each speckle grain covers at least two photodetectors in the part of each captured image which corresponds to the reference target (101).

8. Method according to one of the preceding claims, according to which the reference target (101) is selected in step / 1 / to have a size and roughness characteristics such that the part of each captured image which corresponds to said reference target contains at least fifty speckle grains.

9. A method according to one of the preceding claims, further comprising characterizing a spectral distribution of radiation which is produced by the laser source (1), such that said spectral distribution is effective for at least one of the captured images, from the speckle pattern in the part of said captured image which corresponds to the reference target (101).

10. A method according to any preceding claim, wherein the target to be analyzed (100) is a portion of a fluid which is partially transparent to the radiation from the laser source (1) and which contains mobile scattering particles, and the method comprises characterizing a combination of a concentration of the particles in the fluid with a speed of movement of said particles; or the target to be analyzed (100) is a portion of a scattering solid which vibrates, and the method comprises characterizing local amplitudes of vibration which exist at distinct locations in the portion of solid; or the target to be analyzed (100) is a portion of a biological tissue, and the method comprises characterizing levels of movements which exist at distinct locations in the biological tissue.

11. Imaging system for visualizing variations in speckle contrast, comprising: - an imaging instrument (4), adapted to capture images of a target to be analyzed (100); - a laser source (1), arranged to illuminate the target to be analyzed (100) while each image is captured, so that the image is formed by radiation produced by the laser source then diffused by the target to be analyzed; - a calculation unit (20), configured to calculate values of a speckle contrast relating to the target to be analyzed (100), from one or more images of said target to be analyzed; and - a display system (30), connected to show an image which has been constructed by the computing unit (20), characterized in that it further comprises: - a reference target (101), which has a static and constant scattering pattern; and - support means (11), adapted to maintain the reference target (101) within a field of view of the imaging instrument (4) which is effective for each image, and so that the laser source (1) simultaneously illuminates the target to be analyzed (100) and the reference target while each image is captured, and in that the calculation unit (20) is further configured to calculate, separately for each captured image, a speckle contrast value, denoted Cstatic, from a part of the image which corresponds to the reference target (101), and further configured to validate several successively captured images if the Cstatic values which have been calculated respectively for said images have variations from any one of the images to another which are less than a threshold, and to construct a speckle contrast variation image by assigning to each of a set of image points which correspond to the target to be analyzed (100), an image point value which depends on a difference result between, on the one hand, an average result <C s tatic> calculated for the speckle contrast values Cstatic on all captured images, and on the other hand a speckle contrast value calculated for the image point from one or more captured and then validated images.< / cstatic>

Images