IMAGING METHOD BY VISUALIZING VARIATIONS OF A SPLIT CONTRAST

Abstract

A speckle contrast variation imaging method allows for the more reliable detection of motion in parts of a scattering target to be analyzed (100). The method includes using a reference target (101) to verify that a laser illumination source does not experience operating fluctuations during execution of the method. The use of the reference target also allows for determining the maximum speckle contrast that the laser illumination source can generate. The method can also be further developed by constructing a speckle contrast variation image, subtracting each calculated contrast value for the target to be analyzed from an average speckle contrast value calculated for the reference target.Thus, speckle contrast variation images constructed using different laser sources, particularly those with different spectral widths, image-forming optics, and image sensors, and / or during time-separated repetitions of the process, can be validly compared. To achieve this, each speckle contrast image of the target being analyzed is corrected for a contribution specific to the laser illumination source and possibly also for another contribution due to image detection noise. (Abstract figure: Figure 1.)

Description

Title of the invention: IMAGING METHOD BY VISUALIZING VARIATIONS OF A CONTRAST OF SCATTER technical field

[0001] The present description relates to an imaging method by visualizing variations of speckle contrast. Previous technique

[0002] Speckle contrast variation imaging is useful for revealing movements, or micromovements, occurring in a medium. For this, the medium must be semi-transparent or rough, and also diffuse at the wavelength of illumination used. This illumination is achieved using a laser source to meet the luminance coherence requirements necessary for this imaging technique. Due to its semi-transparent or rough nature and its diffusion at the considered wavelength, the medium illuminated by the laser source produces a state of light interference at each point on the object plane of an imaging instrument focused on that medium. The object plane then contains a speckle pattern, which consists of juxtaposed speckle grains, lighter or darker depending on the state of interference present at each point on the object plane.This speckling pattern can then be captured in an image using an imaging instrument that optically combines the medium to be analyzed with the photosensitive surface of a matrix image sensor. The size and contrast of the speckling grains depend on several parameters related to the analyzed medium, but also on certain parameters of the imaging instrument and the laser illumination source used.

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

[0004] When micromovements of the medium are sufficiently rapid to occur partly during the integration time 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 calculated in the first construction mode of the speckle contrast image, at the locations where the micromovements occur.

[0005] When the medium is stationary, the images captured successively are all virtually identical, apart from detection noise, and the temporal speck contrast calculated in the second construction mode of the speck contrast image is virtually zero for all points in the image matrix. However, when the medium undergoes local micromovements between two successively captured images, the speck pattern varies between these images, such that the temporal speck contrast value becomes non-zero at the locations where the micromovements occur. If the micromovements accelerate while the integration time used to capture each image is kept constant, the specks will have time to vary considerably during this integration time, and each photodetector of the image sensor will integrate these variations.The image sensor readings for successively captured images at the same image point then tend towards a common intermediate value, resulting in a reduction of their standard deviation, and therefore their temporal contrast. 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 temporal contrast depending on the speed of the micromovements, depends on the application.

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

[0007] However, it is necessary to be able to attribute the observed variations in speckle contrast to movements of the analyzed medium. To this end, the imaging instrument is kept unchanged between successively acquired images. But certain characteristics of the laser source used to illuminate the medium may vary unintentionally, contributing to the variations in the calculated speckle contrast values, independently of the existence of movements in the analyzed medium. It is therefore important to be able to attribute the observed variations in speckle contrast to the analyzed medium, ruling out the possibility of uncontrolled variations 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 which is used to illuminate the analyzed medium by speckling 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 emitted by the laser source may be reinjected into the laser source itself after being backscattered by the analyzed medium; / 2 / A variation in power supply noise can affect the laser source; / 3 / the laser source can undergo temperature variations, which alter the spectral characteristics of the lighting radiation; / 4 / The spectral emission width of the laser source can fluctuate, and its single-mode operation can be unstable. In particular, the laser source can unexpectedly switch to undesired multimode operation; and / 5 / Other causes still exist, depending on the nature and technology of the laser source used. There are remedies that can prevent these variations in the operation of the laser source, including: due to variations / 1 / : use optical isolators between the laser source and the analyzed medium; due to variations / 2 / : use a power supply for the laser source which is very stable; Due to variations / 3 / : implement thermal stabilization for certain components of the laser source; and Due to variations / 4 / : it may be necessary to implement a control system for the laser source's operation relative to an atomic reference. However, these solutions are complex and expensive, even very expensive. Furthermore, they are not compatible with applications where the imaging instrument must be lightweight, compact, mobile, and possibly manually applied to the medium being analyzed. In addition, uncontrolled variations in the laser source's operation may still occur, other than those for which solutions have been implemented. Technical problem

[0010] From this situation, one object of the present invention is to make it possible to attribute with certainty variations in speck contrast to movements which occur in the analyzed medium.

[0011] Secondary objectives of the invention are to exclude measurement results that were 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 manipulated easily.

[0012] Another object of the invention is to be compatible with existing speckle contrast variation imaging systems, by retrofitting these systems.

[0013] Another object of the invention is to provide speckle contrast values ​​and images that are calibrated and free from specific contributions 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, so as to be able to meaningfully compare speckle contrast images captured using different laser sources.

[0014] Another object of the invention is to provide speckle contrast variation values ​​and images while minimizing the effect on these values ​​of detection noise that occurs in the matrix image sensor used.

[0015] Finally, another object of the invention is to allow the laser source of illumination to be characterized in real time during its operation. Summary of the invention

[0016] To achieve at least one of these goals or another, a first aspect of the invention proposes a method of imaging by visualizing variations in a speckled contrast, according to which movements of diffusing parts of a target to be analyzed cause variations in a speckled contrast in one or more images of the target to be analyzed, the method comprising the following steps: / 1 / provide a reference target which has a static and constant scattering pattern; / 2 / using an imaging instrument and a laser source, capture at least two images of the target to be analyzed at two different times, placing 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, so that the laser source simultaneously illuminates the target to be analyzed and the reference target while each image is captured, and 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; / 3 / separately for each image captured in step / 2 / , calculate a speck contrast value, denoted Cstatic, from a portion of the image that corresponds to the reference target; then / 4 / validate the images captured in step / 2 / only if the Cstatic values ​​calculated respectively for these images have variations from any of the images to another that are less than a threshold, otherwise repeat steps / 2 / 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 speckle contrast that are relative to these parts of the target to be analyzed.

[0017] For the purposes of this invention, the target to be analyzed is understood to be 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 scattering to the laser radiation used. In particular, the target to be analyzed may consist of a portion of the medium in which movements or micromovements are likely to occur locally or globally.

[0018] A static pattern is understood to be a pattern of which no part is in motion at a given instant or during a given duration, for example during the integration time which is implemented to capture an image, commonly also called the image accumulation time.

[0019] A constant pattern is understood to be a pattern which remains identical between two separate moments, for example between the moments at which successive images are captured.

[0020] Finally, an imaging instrument is understood to mean a system that is adapted to form and capture images, such as is commonly called a camera. Such an imaging instrument comprises image-forming optics, a matrix image sensor, electronics for reading and processing the image capture signals delivered by the matrix image sensor, and optionally a pupil diaphragm and a field diaphragm.

[0021] Preferably, the reference target and the laser source are selected to produce a Gaussian circular speck, or a fully developed speck (in English, "fully developed speckle") in the portion of each image captured in step / 2 / that corresponds to the reference target. For this, the reference target may have a standard deviation value for roughness height that is greater than one-third of a wavelength value of the laser source, preferably greater than one wavelength value. Furthermore, and also to obtain the Gaussian circular speckle, the reference target and the imaging instrument may be selected such that the portion of each image captured in step / 2 / that corresponds to the reference target incorporates radiation scattered by at least ten, preferably at least fifty, roughness peaks separated by intermediate roughness troughs.

[0022] According to the invention, the target to be analyzed and the reference target appear simultaneously in each captured image. The portion of this image that corresponds to the reference target, through optical conjugation via the image-forming optics, consists of a speckle pattern produced by the laser source radiation 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. Since the latter is static and constant, a constancy or near-constancy of the speckle pattern appearing in successive image portions corresponding to the reference target ensures that the characteristics of the laser source radiation were identical or nearly identical between the respective times of image capture.Another speckling pattern that appears in each image at locations corresponding to the target being analyzed is unaffected by fluctuations in the laser source's operation. This other speckling pattern can therefore be reliably used to deduce information about the target being analyzed. More specifically, when a single captured image is used to analyze the target of interest, the method of the invention allows us to state with a high degree of certainty that the laser source's operation did not vary during the integration time of that image, the other image captured in the method of the invention serving to verify the consistency of the laser source's operation. When several captured images are used to analyze the target of interest, the method of the invention ensures that the laser source operated identically for all these images.

[0023] The method of the invention can be combined with each of the methods of constructing the speck contrast image that have been recalled above.

[0024] When the first speck contrast image construction method is used, a spatial speck contrast value is calculated locally in at least one of the images validated in step / 4 / , for at least some of its image points corresponding to the target to be analyzed. Then, a speck contrast variation image can be constructed by assigning to each of these image points an image point value that depends on a result of the difference between, on the one hand, a result of the average <cstatic>calculated for the Cstatic speck contrast values ​​on all images captured according to step / 2 / , and on the other hand the spatial speck contrast value which was calculated for this image point.

[0025] When the second speck contrast image construction method is used, a series of successive images of the target to be analyzed is captured in accordance with step / 2 / . Then, if the image series is validated in step / 4 / , a temporal speck contrast value is calculated separately for each of a set of image points that correspond to the target to be analyzed, based on the respective intensity values ​​of the images in the series at that image point. Next, a speck 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, a mean result <cstatic>calculated for the Cstatic speck contrast values ​​on all images captured in accordance with step / 2 / , and on the other hand the temporal speck contrast value which was calculated for this image point.

[0026] When the third speck contrast image construction method is used, a series of successive images of the target to be analyzed is again captured in accordance with step / 2 / . If this series of images is validated in step / 4 / , a spatiotemporal speck contrast value is then calculated separately for each of a set of image points that correspond to the target to be analyzed, based on the respective intensity values ​​of the successive images in a neighborhood of the image point. Subsequently, a speck 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, a mean result <cstatic>calculated for the Cstatic speck contrast values ​​on all images captured in accordance with step / 2 / , and on the other hand the spatio-temporal speck contrast value which was calculated for this image point.

[0027] Thanks to the subtraction operation introduced according to the invention in each of the three methods of constructing the speck contrast image, the speck contrast variation image obtained for the target to be analyzed can be independent of the radiative characteristics of the laser source, or depend on them to a very small residual extent. In other words, the invention provides speck contrast variation values ​​and images while minimizing the effect on these values ​​and images of the optical probe used and the image acquisition conditions. This is achieved by subtracting the calculated speck contrast value for the reference target from each speck contrast value relative to the target to be analyzed. Thus, the speck contrast variation image obtained for the target to be analyzed directly highlights the amplitude of the movements or micromovements that occur in the target to be analyzed, regardless of the laser source used. In other words, the invention provides an absolute, or "universal," analysis result that allows for comparison of analyses performed with different optical probes and at separate times.

[0028] It is also possible to further reduce or eliminate the effects of image detection noise, particularly shot noise, by first correcting each of the speck contrast values ​​relative to the target to be analyzed and each Cstatic speck contrast value calculated for the reference target, for a contribution from image detection noise fluctuations. The Cstatic speck contrast value calculated from the portion of each captured image corresponding to the reference target, and thus corrected, is essentially free from image detection noise. It then allows for an accurate characterization of the laser illumination source.

[0029] Generally, for the invention, the reference target can be a piece of paper with a known grain size. It can be held by a support that is close to the object plane of the imaging system, also called the focal plane of this system, within the field of view, preferably near an edge of this field of view. The reference target is thus systematically attached to the target to be analyzed for each image that is captured.

[0030] Also, generally for the invention, in step / 1 / , the reference target can preferably be selected such that it has a scattering intensity level for the laser source radiation, such that each speck in the portion of each captured image that corresponds to this reference target is within a linear detection range of the imaging instrument. Each speck contrast value Cstatic that is calculated in step / 3 / is thus more meaningful.

[0031] More generally, for the invention, the imaging instrument may include: - an image formation optics; - a matrix image sensor, comprising photodetectors arranged at row and column intersections; and - a pupillary diaphragm. Therefore, the pupil diaphragm opening can be adjusted prior to image capture in step / 2 / , so that each speck grain covers at least two photodetectors in the portion of each captured image that corresponds to the reference target. This makes it possible to accurately discretize all the speck grain shades spatially in each captured image.

[0032] More generally for the invention, the reference target can be selected in step / 1 / preferably to have a size and roughness characteristics such that the part of each captured image that corresponds to this target of The reference contains at least fifty speck grains. Under these conditions, each speck contrast value Cstatic that is calculated in step / 3 / can have a high level of reliability.

[0033] More generally, for the invention, the method may additionally include characterizing a spectral distribution of the radiation 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 portion of the captured image that corresponds to the reference target. Such a spectral distribution characterization may, in particular, include determining a spectral width of the laser source used.

[0034] The process of the invention can be used for many applications.

[0035] 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 include water purification units, the method may comprise characterizing a combination of the concentration of particles in the fluid and the velocity of the movement of these particles.

[0036] Alternatively, the target to be analyzed may be a portion of a diffusing solid that vibrates. Then, the method may include characterizing local vibration amplitudes that exist at distinct locations within the portion of the solid.

[0037] For other possible applications, the target to be analyzed may be a portion of biological tissue. In this case, the method may include characterizing levels of movement that exist in distinct locations within the biological tissue. These movements may indicate levels of vascularization that vary between different parts of the biological tissue, or between different times at which the method of the invention is repeated for the same location within the biological tissue.

[0038] Finally, a second aspect of the invention proposes an imaging system for visualizing variations in 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 scattered by the target to be analyzed; - a computing unit, which is configured to calculate values ​​of a speck contrast relative to the target to be analyzed, from one or more images of it; - 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 diffusing 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.

[0039] According to the invention, the processing unit is further configured to calculate, separately for each captured image, a speck contrast value, denoted Cstatic, from a portion of the image corresponding to the reference target. It is further configured to validate several successively captured images if the Cstatic values ​​calculated for these images vary from any one image to another below a threshold. The processing unit is further configured to construct a speck contrast variation image by assigning to each of a set of image points corresponding to the target to be analyzed, an image point value that depends on a difference between, on the one hand, a mean result <cstatic>calculated for the Cstatic speck contrast values ​​on all captured images, and on the other hand a speck contrast value that was calculated for the image point from one or more captured and then validated images.

[0040] Such an imaging system is adapted to implement a method conforming to the first aspect of the invention, including its improvements and / or preferred implementation conditions.

[0041] Optionally, the speckled 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 allow the target to be analyzed to be illuminated 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.

[0042] Optionally, the imaging instrument may also include a linear polarizer oriented perpendicular to a polarization direction of the laser source. Such a linear polarizer, oriented in this way, improves the speckle contrast variation images obtained by suppressing the portion of the illumination radiation that is scattered by the surface of the target being analyzed. Brief description of the figures

[0043] The features and advantages of the present invention will become more apparent from the following detailed description of non-limiting implementation examples, with reference to the accompanying figures, among which:

[0044] [Fig. 1] is an optical diagram of an imaging system that conforms to the invention;

[0045] [Fig.2] schematically shows the content of an image as captured by the imaging system of [Fig.1];

[0046] [Fig.3] shows steps of an imaging process which is in accordance with the invention;

[0047] [Fig.4a] shows an example of an image of organic tissue illuminated by a laser source, as captured by the imaging system of [Fig.1];

[0048] [Fig.4b] shows a spatial contrast variation image of speckling which was constructed according to the method of the invention, from a series of successive images each captured like that of [Fig.4a];

[0049] [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 color imaging as known from the prior art; and

[0050] [Fig.5b] shows a temporal contrast variation image of speckling 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

[0051] For the sake of clarity, the dimensions of the elements shown in [Fig. 1] and [Fig. 2] do not correspond to actual dimensions or actual dimension ratios. Furthermore, some of the elements shown in [Fig. 1] are shown only symbolically.

[0052] 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 laptop computer. The display screen 30, denoted DISPLAY, is preferably of a high-definition type.

[0053] The optical probe 10 may be of a portable design, with a side screen (or "baffle") 11 and a handle 12. It combines an illumination channel and a detection channel, which are arranged so that the illumination channel illuminates an entire field of view of the detection channel. To achieve this, one direction of illumination of the illumination channel may be oblique to an optical axis of the detection channel, denoted AA, in order to juxtapose the two illumination and detection channels within the optical probe 10. The field of view is surrounded by the side screen 11 to suppress any stray light rays that do not originate from the illumination channel or the field of view.

[0054] The illumination path comprises a laser source 1 and a diverging optic 2 through which radiation emitted by the laser source 1 illuminates the entire field of view, or an essential part of that field of view that is useful for implementing the invention. The laser source 1 may be of a type that produces radiation at a The wavelength is suitable for the intended application. For example, to observe the movement of red blood cells in organic tissue, the wavelength of the laser source 1 can be in the range of 785 nm (nanometers) 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 metal vibrations, shorter 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, firstly to obtain better measurement sensitivity, and secondly so that the invention provides images of speckle contrast variation that are sufficiently independent of the laser source used.The laser source 1 may lack dedicated means to stabilize its emission operation, so that it can be inexpensive, compact and lightweight, particularly for incorporation into the portable optical probe 10.

[0055] The detection path includes an imaging instrument 4, which consists of an image-forming optic 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 has a high density of separate photodetectors in its photosensitive surface S. A readout 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 range of the matrix image sensor 42 contains the wavelength value of the laser source 1. Optionally, the field diaphragm 44 can be located against the photosensitive surface S of the matrix image sensor 42.As is known, the photosensitive surface S is formed by photodetectors arranged at the intersections of rows and columns in a matrix arrangement. The integration time used by the sensor 42 to capture each image produced using illumination from the laser source 1 can be, for example, between 1 ms (millisecond) and 100 ms. Since the illumination radiation produced by the laser source 1 is linearly 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 portion of the laser radiation that is scattered by the content of the field of view. Consequently, it facilitates the detection of another portion of the laser radiation that undergoes multiple scattering caused by scattering content in the field of view, as described in FR 3 062 542 AL.

[0056] According to an optional improvement of the invention, the optical probe 10 can be supplemented to capture color images of the contents of the field of view. For this purpose, The matrix image sensor 42 can be of a monochrome type sensitive across the entire visible range, and red, green, and blue light sources can be added to the optical probe 10. For example, red, green, and blue LEDs 3 can be repeatedly juxtaposed along a ring that surrounds the field of view at the level of the pupil diaphragm 43, around the opening of this diaphragm, as shown in [Fig. 1]. The LEDs 3 produce incoherent illumination of the contents of the field of view for each of the blue, green, and red colors.Thus, three images captured successively with minimal intermediate durations relative to the operating cycle time of the image sensor 42—one image using illumination exclusively with blue LEDs, another with only green LEDs, and the last with only red LEDs—allow for the reconstruction of an RGB color image using only the monochrome image sensor 42. Therefore, illumination by the laser source 1 enables the capture of speckle images to implement the invention, and the method just described for acquiring RGB color images provides such color images that can be superimposed on the speckle images.

[0057] Reference numeral 13 designates an image capture trigger. A short press of the trigger 13 by an operator initiates the capture of successive images illuminated only by the laser source 1, followed by the calculation and real-time display of successive speck contrast variation images, for example, with a refresh rate between approximately 5 Hz and approximately 25 Hz, depending on a desired signal-to-noise ratio for each speck contrast variation image. The speck contrast variation images thus displayed conform to the prior art as existing before the present invention, while benefiting from polarimetric filtering, which provides greater sensitivity to a portion of the radiation that has penetrated deeper into the target being analyzed 100.A long press, performed alternately by the operator on trigger 13, for example with a press duration greater than 1 s (second), triggers the capture of successive images. The first three are captured with respective illumination by the light-emitting diodes 3, separately and successively for the colors blue, green, and red. Subsequent images are then captured using illumination from the laser source 1. These subsequent images captured with laser illumination are used to calculate a single speck contrast variation image. This image has a signal-to-noise ratio value that is higher than that of each speck contrast variation image obtained with a short press of trigger 13, thanks to a larger number of captured images that are combined to obtain each speck contrast variation image. Each long press on the trigger 13 thus results in an RGB color image, as commonly known, and a speckled contrast variation image with a high signal-to-noise ratio, both of which relate to the same field of view content and are therefore superimposable. The invention, which is described in detail below, can be applied to the speckled contrast variation images from a short or long press on the trigger 13.

[0058] A target to be analyzed is designated by reference numeral 100. It is semi-transparent or rough, and scatters the radiation from the laser source 1. It can be a biological medium, such as a portion of organ tissue or other material. When the optical probe 10 is used to capture images of the target 100, the leading edge of the side screen 11 can be applied to a surface of the target 100, around the portion thereof to be analyzed. The laser radiation from the source 1 penetrates the target 100 to a depth that depends on the wavelength of the laser source 1 and the nature of the target 100. This depth of penetration of the laser radiation into the target can be on the order of a few tens of nanometers, particularly for rough metals that constitute the target, to a few millimeters, particularly for biological media, such as skin tissue.The laser beam is backscattered by scattering centers present in the target 100, and backscattered portions of the laser beam interfere to create a speckle pattern in the focal plane of the imaging system 4. The image-forming optics 41 are such that the surface of the target to be analyzed 100 is substantially optically conjugate with the photosensitive surface S of the matrix image sensor 42. Under these lighting and image-capture conditions, the image captured by this imaging system 4 then reproduces the speckle pattern. This pattern consists of speckle grains that are juxtaposed, being alternately lighter and darker.

[0059] To implement the invention, a reference target 101 is added to 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 thus substantially at the same level as the surface of the target to be analyzed 100 along the optical axis AA of the detection channel of the imaging instrument 4. Under these conditions, the content of each image that is captured is as shown in [Fig. 2]. It includes an image portion 101' which is optically conjugated with the reference target 101 by the image-forming optics 41, and the remainder of the extent of each image, designated by the reference 100', is optically conjugated with the aimed portion of the target to be analyzed 100.The reference target 101 can. to be held fixedly in the desired position within 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 this description.

[0060] The reference target 101 has a pattern that diffuses the radiation from the laser source 1, and that is static and constant. In particular, the reference target 101 may consist of a portion of a solid, opaque material that has a rough surface. For example, it may be a piece of paper sold under the CANSON® brand.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 used, and the average width of the relief and pits of the roughness is preferably such that at least ten, preferably fifty, separate reliefs of roughness are illuminated by the laser source 1. Under these conditions, the radiation backscattered by the reference target 101 produces speckles that conform to Gaussian circular statistics and exhibit maximum speckle contrast. Thus, the image portion 101' also consists of a speckle pattern, which is used according to the invention, firstly, to calculate a static contrast value relative to the reference target 101, and secondly, to verify the stability of the operation of the laser source 1.Since the roughness pattern of the reference target 101 is constant and free of internal movement, variations in the speckle pattern in the image region 101' between successively acquired images are necessarily caused either by excessive torsional movement of the optical probe when in contact with the target being analyzed, or by variations in the operation of the laser source 1, which in all cases invalidates the measurement. When several images are acquired successively with illumination by the laser source 1, and these acquired images exhibit variations in spatial contrast of the speckle pattern in the image region 101' that are below a threshold in absolute values, then the method of the invention proposes to validate these acquired images by assuming that the operation of the laser source 1 was constant during the integration time of each image and was identical for all of these acquired images.Constant operation of the laser source 1 means a laser emission which is in particular: . devoid of effects from the reinjection of variable backscattered radiation into the laser source, devoid of the effects of variations in electrical power supply noise, in amplitude and phase, of the laser source, devoid of the effects of thermal variations that could affect certain components of the laser source, and devoid of temporal fluctuations in the spectral distribution of the radiation emitted by the laser source, which could be due to uncontrolled transitions between desired single-mode emission and undesired multi-mode emission, or which could appear as temporal variations in the spectral width of the emitted laser radiation.

[0061] To implement the invention optimally, the following operating conditions may 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 speck pattern are within a linear detection range 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 opening of the pupil diaphragm 43 is adjusted so that each speckled grain in the image area 101' 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 part 101' contains at least fifty speck grains. When the conditions mentioned above are met, so that the speckle pattern captured in the image part 101' is close to a circular Gaussian pattern, as defined in the book "Speckle Phenomena in Optics, Theory and Applications", by JW 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 emitted by the laser source 1, from the speckle pattern captured in the image part 101'. 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 Erdeneret DABoas, Scientific Reports, 22 February 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 spectral emission width can be obtained in this way. When such a deduction of numerical values ​​characterizing the laser emission is repeated separately for several images captured successively from image parts 101', the results obtained characterize fluctuations in the operation of the laser source 1. These fluctuations are acceptable when the numerical values ​​obtained for each considered characteristic of the spectral distribution of the . If the radiation from laser source 1 exhibits variations below a set threshold, the images are validated for analyzing target 100. Conversely, if laser source 1 has experienced operating fluctuations that appear too significant based on successive numerical values ​​obtained for the spectral distribution of its radiation, the images are declared unacceptable for validly analyzing target 100.

[0062] According to the invention, it is sufficient to verify that a speck 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 purpose of analyzing the target 100. This speck contrast, denoted Cstatic>, is calculated separately for each captured image according to the formula: Cstatic = o / , où and o respectively denote the mean value and standard deviation of the intensity values ​​captured in the image part 101' of each captured image.

[0063] With reference to [Fig.3], the imaging method of the invention for analyzing target 100 comprises the following steps: In step SI: a series of several successive images of the target to be analyzed 100, from two images to several hundred images, is captured using the optical probe 10 equipped 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 in the series. Preferably, the reference target 101 is left in the optical probe 10 without being disassembled or moved between two successive images; at step S2: the values ​​of the speck contrast Cstatic are calculated separately for each image in the series, from part 101' of that image; In step S3: the existence of fluctuations in the operation of the laser probe 1, which may have occurred between some of the images in the series, is investigated. To do 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 of the image pairs in the series. If at least one of these differences exceeds the ACmax threshold, the image series is rejected and the imaging procedure is restarted at step S1. Otherwise, the image series is validated and the imaging procedure is continued with step S4; at step S4: for each image point of the image part 100', which was optically conjugate with the target to be analyzed 100 while each image in the series was captured, a contrast value Ccibie(i, j) is calculated for this image point according to the mode chosen for the construction of 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; at step S5: construction of a new image, which is the variation image of speckle contrast obtained according to the invention, by assigning to the image point with coordinates i and j in the image region 100' a new image point value denoted AC(i, j) and calculated according to the following formula: AC(i, j) = <cstatic>- Ccibie (i, j), where <cstatic>is the average of the respective values ​​of the speck contrast Cstatic obtained in step S2 for all images in the series, for the three speck contrast variation image construction modes indicated at the beginning of this description. The new image point values ​​thus calculated, AC(i, j), are always positive; and In step S6: the speck contrast variation image is displayed on screen 30. This speck contrast variation image highlights the parts of the target being analyzed that have undergone movement, particularly submicron-scale movement. It has image point values ​​that depend on the amplitude and / or speed of this movement. Furthermore, thanks to the subtraction operation in the calculation in step S5 to obtain each image point value AC(i, j), speck contrast variation images acquired at different times, or with different optical probes 10, can be validly compared.

[0064] The speck contrast value Cstatic, which is calculated in step S2, includes a speck contribution resulting from 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. Dark noise is usually negligible for typical operation of the imaging system, due to the sufficient number of photons emitted by the laser source 1 and then backscattered by the reference target 101 to the image sensor 42. On the other hand, shot noise, which is proportional to the square root of the captured intensity I, can contribute significantly to the standard deviation value 0, and therefore plays a role in each speck contrast value.The intensity standard deviation value resulting from shot noise can therefore advantageously be subtracted from the intensity standard deviation value calculated for the sequence of images captured in step S1 in image parts 101' for each image point in the case of temporal contrast, or for each subset of image points in the case of spatial contrast or spatiotemporal contrast. This subtraction operation, to eliminate the contribution of shot noise, is preferably applied to the speck contrast Cstatic for the reference target 101 and to each contrast value Ccibie(i, j) for the target to be analyzed 101. The other subtraction operation performed in step S5 to calculate each image point value AC(i, j) allows the variations in AC(i, j) values ​​to be directly attributed, subject to the conditions mentioned above, to the movements that occur on the image. occur in the target to be analyzed 100, despite the use of different laser sources and different optical systems between separate embodiments of the method of the invention. It should be noted that the same micromovement in the target to be analyzed 100 will result in different values ​​of speckle contrast variation AC(i, j) 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 decrease 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 for a rigorous comparison of the results despite the difference in speckle contrast value Cstatic between the two laser sources used, or more generally between the two optical probes used.

[0065] Steps S2 to S5 are executed by the computing unit 20, preferably in an automated manner.

[0066] With the first speckle contrast image construction method, spatial contrast values ​​are calculated for each image point (i, j) in the image region 100', separately for each image captured in step SL. Each value is calculated on a neighborhood of the image point with coordinates i and j in the image region 100'. This neighborhood can be a 5 x 5, 7 x 7, or 9 x 9 square of image points centered on the image point i, j. Each spatial contrast value is then equal to the quotient of the standard deviation of the intensity values ​​captured within this neighborhood square, divided by the average of these same intensity values ​​captured again within the neighborhood square. These spatial contrast values ​​are then averaged over all the captured images, separately for each image point, in order to enhance the resulting signal.The average values ​​calculated in this way are those of step / 4 / , denoted Ccibie(i, j). For this first method of constructing a speck contrast image, the image displayed in step S6 is therefore a spatial speck contrast variation image. The existence of movements within the target to be analyzed causes local reductions in the Ccibie(i, j) values, and consequently local increases in the AC(i, j) values ​​in the speck contrast variation image displayed in step S6, since Cstatic corresponds to the maximum possible value for Ccibie(i, j).

[0067] With the second speck contrast image construction method, the contrast values ​​Ccibie(i, j) calculated in step S4 are temporal contrast values ​​calculated respectively for the image points of the image portion 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 in the series at the image point with coordinates i and j is averaged by averaging these same intensity values. The image 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 within the target to be analyzed 100 that occur between successively captured images then causes, depending on the integration time of the matrix image sensor 42 relative to the characteristic time resulting 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 AC(i, j) values, visible in the speckle contrast variation image displayed in step S6.

[0068] With the third speck contrast image construction method, the contrast values ​​Ccibie(i, j) calculated in step S4 are spatio-temporal contrast values ​​calculated respectively for the image points in the image region 100'. For example, each contrast value Ccibie(i, j) can be calculated using the same formula as before for temporal contrast, but where the intensity of each image point in the image region 100' is replaced by an average intensity value calculated over an environment of that image point, for example, a square with dimensions 3 x 3, 5 x 5, etc. The image displayed in step S6 is then a spatio-temporal speck contrast variation image, calculated as the difference between the spatial speck contrast of the reference target 101 and the spatio-temporal speck contrast calculated on the target to be analyzed 100.The existence of movements within 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 relative 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 cause respectively increases or decreases in the AC(i, j) values, as in the case of temporal contrast.

[0069] Generally, for all modes of constructing the speck contrast variation image, it is the AC(i, j) variation at each image point that is displayed in step S6.

[0070] For cases of temporal contrast and spatiotemporal contrast, operating parameter values ​​for the optical probe 10 can preferably be adopted, particularly for the integration time of the matrix image sensor 42, which maximize the difference between the spatial contrast value of the speckle <cstatic>obtained for the reference target 101 and the temporal or spatio-temporal contrast values ​​of speckle Ccibie(i, j) obtained for the target to be analyzed 100.

[0071] 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 places where a speed and / or a concentration of mobile diffusing particles in the fluid is (are) more important.

[0072] When the target to be analyzed 100 is a portion of a vibrating diffusing solid, the movements in it that are highlighted by the speckle contrast variation image correspond to places of maximum vibration amplitude, commonly called vibration antinodes or "vibration antinodes" in English.

[0073] 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 within this tissue that are highlighted by the speckle contrast variation image are mainly produced by vascular inhomogeneities. These inhomogeneities concern the velocity of red blood cell movement and / or the concentration of mobile red blood cells.

[0074] [Fig.4a] reproduces an image of an organic tissue, as captured by the system [Fig. 1] shows the image of [Fig. 1], using illumination from laser source 1 and an integration time of 10 ms in matrix image sensor 42. This image has not undergone any processing, and therefore corresponds to the usual designation of raw image. Image parts 100' and 101', which correspond respectively to the target to be analyzed 100 and the reference target 101, are indicated. Each of these image parts consists of a speck pattern, the respective origins of which have been explained above. [Fig. 4b] shows the spatial contrast variation image of the speck, 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 method of constructing a spatial contrast variation image of the speck. This spatial contrast variation image of scab reveals micromovements that occur in a micro-vascular network of organic tissue.

[0075] [Fig. 5a] reproduces an RGB color image of the tip pulp of a finger human, as provided by the imaging system of [Fig. 1] using successive illumination by blue LEDs, then green LEDs, then red LEDs. The color image of [Fig. 5a] was reconstructed from the three images captured separately for each color. It primarily shows the skin surface with the fingerprint pattern. It is intended to be superimposed on a speckled contrast variation image obtained according to the present invention. To this end, a series of approximately one hundred successive images is captured using illumination from the laser source 1, without moving the optical probe 10 relative to the finger, for example, at a rate of approximately one hundred images per second with an integration time of 10 ms for each image. [Fig. 5b] shows the temporal contrast variation image of the speckled skin, which is constructed according to the invention from this series of laser-illuminated images, by applying the second method of constructing a speckled 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].

[0076] Finally, and as is generally the case in dynamic speckle imaging, it is possible to use the blood flow index, or BFI, which is equal to 1 / Ccibie(i, j)2 to quantify the flows. Alternatively, the VMAI parameter, which is equal to 1 / (rCCibie(i, j)2), can be used, where r is the integration time of the matrix image sensor 42, and Ccibie(i, j) is the contrast calculated at each image point i, j, whether this contrast is spatial, temporal, or spatiotemporal. Other equations using speckle contrast to deduce information about the target to be analyzed are also proposed in the literature. Generally, the proposed invention involves adapting these different equations to take into account not each value of Ccibie(i, j) as a physical measurement, but AC(i, j) = Cstatic(i, j) - <ccibie>, in order to calibrate the result.

[0077] It is understood that the invention can be reproduced by modifying secondary aspects of the embodiments that have been described in detail above, while retaining at least some of the advantages mentioned. In particular, the optical probe may have a construction and appearance that differ from those described.< / ccibie> < / cstatic> < / cstatic> < / cstatic> < / cstatic> < / cstatic> < / cstatic> < / cstatic>

Claims

Demands

1. A method for visualizing variations in speckle contrast, wherein movements of diffusing parts of a target to be analyzed (100) cause variations in 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 diffusing pattern; / 2 / using an imaging instrument (4) and a laser source (1), capture at least two images of the target to be analyzed (100) at two different times, placing 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 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; / 3 / separately for each image captured in step / 2 / , calculate a speck contrast value, denoted Cstatic, from a portion of the image that corresponds to the reference target (101); then / 4 / validate the images entered in step / 2 / only if the Cstatic values ​​calculated for said images have variations from one image to another that are less than a threshold, otherwise repeat steps / 2 / to / 4 / ; and then / 5 / if the images have been validated, deduce from at least one of said images information on movements of parts of the target to be analyzed (100), from variations in speckle contrast relative 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 standard deviation value of roughness height that is greater than one-third of a wavelength value of the laser source, preferably greater than one 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 / 2 / that corresponds to the reference target incorporates radiation scattered by at least ten, preferably at least fifty, surface features of roughness separated by intermediate roughness hollows.

3. A method according to claim 1 or 2, wherein, for at least one of the images validated in step / 4 / , a spatial speck contrast value is calculated locally in the image for at least some of the image points corresponding to the target to be analyzed (100), and then a speck contrast variation image is constructed by assigning to each of said image points an image point value that depends on a difference result between, on the one hand, a mean result <cstatic>calculated for the Cstatic speck contrast values ​​on all images captured in accordance with step / 2 / , and on the other hand the spatial speck 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 (lOO) is captured in accordance with step / 2 / and then, if the image series is validated in step / 4 / , a temporal speckle contrast value is calculated separately for each of a set of image points corresponding to the target to be analyzed, from the respective intensity values ​​of the images in the series for said image point, and then a speckle contrast variation image is constructed by assigning to each of said image points an image point value that depends on a difference result between, on the one hand, an average result <cstatic>calculated for the Cstatic speck contrast values ​​on all images captured in accordance with step / 2 / , and on the other hand the temporal speck 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 / 2 / and then, if the image series is validated in step / 4 / , a spatiotemporal speckle contrast value is calculated separately for each of a set of image points corresponding to the target to be analyzed, from the 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 that depends on a difference result between, on the one hand, an average result <cstatic>calculated for the Cstatic speck contrast values ​​on all images captured in accordance with step / 2 / , and on the other hand the spatio-temporal speck contrast value calculated for the image point.

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

7. A method according to any one of the preceding claims, wherein the imaging instrument (4) comprises: - an image-forming optic (41); - a matrix image sensor (42), comprising photodetectors arranged at row and column intersections; and - a pupil diaphragm (43), and wherein an opening of the pupil diaphragm (43) is adjusted prior to image capture in step / 2 / , so that each speck grain covers at least two photodetectors in the portion of each captured image that corresponds to the reference target (101).

8. A method according to any one of the preceding claims, wherein the reference target (101) is selected in step / 1 / to have a size and roughness characteristics such that the portion of each captured image that corresponds to said reference target contains at least fifty speck grains.

9. A method according to any 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 speck pattern in the part of said captured image which corresponds to the reference target (101).

10. A method according to any one of the preceding claims, wherein the target to be analyzed (100) is a portion of a fluid that is partially transparent to the radiation from the laser source (1) and that contains mobile scattering particles, and the method comprises characterizing a combination of a particle concentration in the fluid with a velocity of movement of said particles; or the target to be analyzed (100) is a portion of a vibrating scattering solid, and the method comprises characterizing local vibration amplitudes that 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 The process involves characterizing levels of movement that exist in distinct locations within 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 and then scattered by the target to be analyzed; - a computing unit (20), configured to calculate speckle contrast values ​​relative 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 that has been constructed by the computing unit (20), characterized in that it further comprises: - a reference target (101), which has a static and constant diffusing 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 such 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 speck contrast value, denoted Cstatic, from a part of the image that corresponds to the reference target (101), and further configured to validate several successively captured images if the Cstatic values ​​that have been calculated respectively for said images have variations from any one of the images to another that are less than a threshold, and to construct a speckle contrast variation image by assigning to each of a set of image points corresponding to the target to be analyzed (100), an image point value that depends on a difference result between, on the one hand, a mean result <cstatic>calculated for the Cstatic speckle contrast values ​​on all captured images, and on the other hand a speckle contrast value calculated for the image point from one or more captured images then validated.< / cstatic> < / cstatic> < / cstatic> < / cstatic>

Images