Imaging apparatus and method with servo-control

Abstract

Disclosed is an imaging apparatus (1) comprising: a camera (3) having an acquisition rate greater than or equal to ten kilohertz for a length of time greater than or equal to one second; an intermittent light source (5) having a frequency greater than or equal to ten kilohertz for said length of time, the apparatus (1) being configured so that at least some of the light emitted by the light source (5) illuminates the camera (3); and a device (7) configured, for said length of time, to analyse an image produced by the camera (3) and servo-control parameters of the camera (3) and parameters of the source (5) depending on the analysis.

Description

[0001] Imaging apparatus and method with servo-control

[0002] FIELD OF THE INVENTION

[0003] The invention relates to an apparatus and method for imaging with servoing, in particular for imaging phenomena with high light intensity and / or high pressure gradients such as an explosion.

[0004] STATE OF THE ART

[0005] An imaging sensor, and in particular a pixel of a photosensitive surface of a digital device, produces a signal from incident light energy acquired by the sensor during an exposure time.

[0006] Such a sensor is preferably used in a certain light energy range where there is a proportionality between the value of the signal produced and the light energy received. This energy range can be referred to as the dynamic range of the sensor. The lower limit of this range corresponds to a threshold energy related to the sensitivity of the sensor. The upper limit of this range corresponds to a saturation energy of the sensor.

[0007] Imaging a very bright phenomenon, imaging a phenomenon that varies rapidly in light intensity, or in any case imaging a phenomenon that lasts a long time compared to a characteristic time of variation in its brightness, poses difficulties in the useful use of the sensor dynamics and in particular sensor saturation.

[0008] For example, for a very bright phenomenon, it is difficult to avoid that certain areas of the image are not saturated, that is to say that the energy received by certain pixels during the exposure time exceeds the saturation energy.

[0009] By decreasing the exposure time to avoid saturation of these brightest areas of the image, there is a risk that other areas will receive light energy below the threshold energy or so close to the threshold energy that the contrast in these other areas is not satisfactory.

[0010] These other areas correspond to details of the phenomenon that are significantly less luminous than the rest of the phenomenon, such as the shock wave front associated with a fireball. There is therefore a difficulty in obtaining an image both without saturation in the area corresponding to the fireball and with satisfactory contrast in the shock wave area. This difficulty remains even with the use of a secondary light source configured to send complementary light to the sensor, light that is added to the light of the luminous phenomenon that we wish to image.

[0011] In the case of a phenomenon which lasts a long time compared to a characteristic time of variation of its luminosity, there is a difficulty in making a satisfactory observation over its entire duration because acquisition parameters suitable for one period of the phenomenon will not be suitable for another period.

[0012] In all cases, a single acquisition system comprising a camera and possibly a light source only allows observation of a temporal fraction of the phenomenon. This is why in the prior art, recourse is had either to the use of several acquisition systems, each system being dedicated to the observation of a temporal fraction of the phenomenon, or to the repeated use of the same acquisition system, the phenomenon being reproduced several times, so that at each reproduction of the phenomenon a fraction of the phenomenon is observed.

[0013] The first case poses a difficulty because it is necessary to use more material and exploit acquisitions from different systems.

[0014] The second case poses a difficulty because it is necessary to generate the phenomenon several times and to exploit acquisitions of different phenomena.

[0015] There is therefore a need to simplify the imaging of phenomena with high light intensity or which vary rapidly in light intensity.

[0016] STATEMENT OF THE INVENTION

[0017] One aim of the invention is to improve images of high light intensity phenomena to avoid saturation in the brightest areas and to obtain satisfactory contrast in the less bright areas.

[0018] The aim is achieved within the framework of the present invention thanks to an imaging device comprising:

[0019] - a camera having an acquisition rate greater than or equal to ten kiloHertz for a duration greater than or equal to one second, the camera being configured to produce an image representing a level of light intensity as a function of two spatial coordinates,

[0020] - an intermittent light source having a frequency greater than or equal to ten kiloHertz for the duration, the apparatus being configured so that at least part of the light emitted by the light source illuminates the camera and

[0021] - a device configured to, for the duration, analyze an image produced by the camera and control camera parameters and source parameters according to the analysis.

[0022] Such a device is advantageously and optionally supplemented by the following different characteristics taken alone or in combination:

[0023] - the light source is collimated;

[0024] - a screen, the apparatus being configured so that light produced by the light source is reflected by the screen towards the camera;

[0025] - the screen is a retroreflective screen;

[0026] - the device is configured to determine in an area of ​​the image a value of a statistical parameter of a distribution of an illumination level of pixels of the area, and to control the parameters of the camera and the parameters of the source according to the determined value;

[0027] - the statistical parameter is chosen from a median, an average, a first quartile, a third quartile, a number of underexposed pixels receiving a light energy lower than an activation energy of a pixel and a number of overexposed pixels receiving a light energy greater than a saturation energy of a pixel;

[0028] - the device is configured to decrease an exposure time or a numerical aperture of the camera when the value of the statistical parameter exceeds a saturation threshold, or increase a light power emitted by the source when the value of the statistical parameter exceeds an underexposure threshold; and

[0029] - the area is a first area of ​​the image, the statistical parameter being a first statistical parameter, the device being configured to determine in a second area of ​​the image a value of a second statistical parameter of a distribution of an illumination level of the second area, the first area being more illuminated than the second area, the parameters of the camera being controlled according to the value of the first parameter and the parameters of the source being controlled according to the value of the second parameter.

[0030] The invention also relates to an imaging method in which:

[0031] - a phenomenon is illuminated with intermittent light having a frequency greater than or equal to ten kiloHertz for a duration greater than or equal to one second,

[0032] - images of the phenomenon are acquired at an acquisition rate greater than or equal to ten kiloHertz for the duration, the image representing a level of luminous intensity of a luminous phenomenon as a function of two spatial coordinates,

[0033] - during the duration the images are analyzed and intermittent light emission parameters and image acquisition parameters are controlled according to the analysis.

[0034] Such a process is advantageously and optionally supplemented by:

[0035] - a screen reflects the intermittent light towards a camera which acquires the images;

[0036] - a phenomenon triggering step, the lighting of the phenomenon and the acquisition of images being triggered according to the triggering of the phenomenon;

[0037] - the analysis of the image comprises the determination in an area of ​​the image of a value of a statistical parameter of a distribution of an illumination level of pixels of the area, the emission parameters and the acquisition parameters being controlled according to the determined value;

[0038] - the statistical parameter is chosen from a median, an average, a first quartile, a third quartile, a number of underexposed pixels receiving a light energy lower than an activation energy of a pixel and a number of overexposed pixels receiving a light energy greater than a saturation energy of a pixel;

[0039] - the reduction of an exposure time or a numerical aperture of a camera which acquires the images when the value of the parameter exceeds a saturation threshold, or the increase of an illumination power of the phenomenon when the value of the parameter exceeds an under-exposure threshold; and

[0040] - the zone is a first zone of the image, the statistical parameter being a first statistical parameter, the analysis of the image comprising the determination in a second zone of a value of a second statistical parameter of a distribution of an illumination level of the second zone, the first zone being more illuminated than the second zone, the image acquisition parameters being controlled according to the value of the first parameter and the intermittent light emission parameters being controlled according to the value of the second parameter.

[0041] DESCRIPTION OF FIGURES

[0042] Other characteristics and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and must be read in conjunction with the appended drawings in which: [Fig. 1] Figure 1 is a schematic representation of an imaging device according to one embodiment of the invention;

[0043] [Fig. 2] Figure 2 is a photograph of a fireball taken by an imaging apparatus according to one embodiment of the invention; and

[0044] [Fig. 3] Figure 3 represents the advance of a fireball photographed by an imaging device according to an embodiment of the invention.

[0045] DETAILED DESCRIPTION OF THE INVENTION

[0046] Imaging device

[0047] In relation to Figure 1, an imaging device 1 comprises a camera 3 configured to take images at an acquisition rate greater than or equal to ten kiloHertz. Advantageously, the acquisition rate may be greater than or equal to twenty kiloHertz and even one hundred kiloHertz. The camera may take images at this rate for a duration which is at least equal to one second. Advantageously, this acquisition duration may be greater than or equal to five seconds and even ten seconds. The camera may be a color camera.

[0048] An image is defined here as a two-dimensional matrix representing a level of luminous intensity of a luminous phenomenon and more generally of an object as a function of two spatial coordinates.

[0049] The imaging device 1 also comprises an intermittent light source 5 which therefore sends a pulsed light. The source 5 has a frequency greater than or equal to ten kiloHertz, that is to say that the variation of the light produced by the source has a frequency greater than or equal to ten kiloHertz. Advantageously, the frequency can be greater than or equal to twenty kiloHertz and even one hundred kiloHertz.

[0050] In terms of speed, light source 5 is advantageously as fast as camera 3.

[0051] Source 5 presents this frequency for a duration equal to the acquisition duration of the camera, that is to say a duration greater than or equal to one second, more advantageously five seconds and even more advantageously ten seconds.

[0052] The light source has a light intensity between 10 and 30 W / m 2 , and advantageously equal to 20 W / m 2at a distance of between 5 and 15 meters, and advantageously equal to 12 meters.

[0053] A discharge flash lamp can be used as a source. The light source can include one or more lamps. The different lamps can be synchronized with each other to give more freedom in the frequency of the intermittent light emitted.

[0054] The imaging apparatus 1 comprises a control device 7 configured to, during the duration, analyze an image produced by the camera and control parameters of the camera and parameters of the source according to the analysis. The device 7 can in particular analyze an image and calculate an adjustment of the parameters at a frequency greater than or equal to ten kiloHertz, or more advantageously to twenty kiloHertz or even more advantageously to one hundred kiloHertz.

[0055] In terms of speed, device 7 is advantageously as fast as camera 3.

[0056] The imaging device 1, and in particular its imaging settings, is adjustable in real time during the acquisition period. A first image produced by the camera is analyzed by the device 7. This assumes a connection 11 between the camera 3 and the device. The device 7 establishes, on the basis of this first image, that, for example, the contrast of the image can be improved by modifying a parameter of the camera or a parameter of the source.

[0057] Camera settings include frame rate, exposure time, and shutter (or aperture) settings.

[0058] Examples of source parameters include the illumination rate, the intensity per light pulse and the duration of a light pulse.

[0059] Once the device has determined which modification of which parameter of the camera or source improves the image, the camera or source is modified accordingly. This assumes a connection 13 between the device 7 and the light source 5.

[0060] The device analyzes the image produced by the camera and controls camera parameters or source parameters based on the analysis. In this way, it is possible to avoid saturation in the brightest areas and obtain satisfactory contrast in the less bright areas.

[0061] Optionally, the light source 5 of the imaging device 1 is collimated. For this purpose, the imaging device 1 may comprise a collimating lens 9 placed at the output of the light source 5. This allows a greater portion of the light emitted by the light source 5 to reach the camera 3. The margin for adjusting the images by the illumination parameters is greater.

[0062] In a particular embodiment, the imaging apparatus 1 comprises a screen 15. The apparatus 1 is configured so that light produced by the light source 5 is reflected by the screen towards the camera 3.

[0063] This screen may in particular be retroreflective, so that radiation incident on the screen is reflected in the direction opposite to the direction of incidence. In this situation illustrated in Figure 1, the light source 5 and the camera 3 are located on the same side of the screen 15 and they are both oriented towards the screen 15. The light source 5 emits light according to the beam 6 which is reflected on the screen 15 according to a beam 8 which reaches the camera 3.

[0064] A retroreflective screen can be used in particular to implement a shadow imaging process.

[0065] The screen may also be directed reflection, i.e. configured to reflect incident radiation onto the screen in a return direction defined from the direction of incidence but which is not necessarily the direction opposite to the direction of incidence. When the return direction is different from the direction opposite to the direction of incidence, the light source 5 and the camera 3 are located less close to each other than in the situation illustrated in FIG. 1. The light source 5 is located in the direction of incidence relative to the screen 15 and the camera 3 is located in the return direction relative to the screen 15.

[0066] The imaging device 1 can be used to image a phenomenon 19 such as a fireball for example. The phenomenon 19 can be triggered by a generation device 17. When the screen is retroreflective, then the generation device 17 and therefore the phenomenon 19 are located between the light source 5 and the camera 3 on the one hand and the screen 15 on the other hand.

[0067] The generating device 17 can be connected via a connection 21 to the control device 7 to transmit a signal for triggering the phenomenon 19. In this way, the light source 5 and the camera 3 can be triggered depending on the triggering of the phenomenon 19.

[0068] By using a camera to image the phenomenon, the image produced is a projection of the phenomenon in a plane orthogonal to the optical axis of the camera. It is possible to use several imaging devices of the type just described and to arrange the cameras on different axes to visualize different projections of the phenomenon and to be able to study the phenomenon by tomography.

[0069] Imaging process

[0070] An imaging device 1 as just presented makes it possible to implement a method according to the invention for imaging a phenomenon. During a first step, a phenomenon is illuminated with intermittent light having a frequency greater than or equal to ten kiloHertz for a duration greater than or equal to one second. In particular, the light source 5 is used for this purpose. The duration corresponds to the acquisition duration mentioned above, which here is a duration of study of the phenomenon.

[0071] In a second step, images of the phenomenon are acquired at an acquisition rate greater than or equal to ten kiloHertz during the duration. Camera 3 is used in particular for this purpose.

[0072] In a third step, the images are analyzed and intermittent light emission parameters and image acquisition parameters are controlled according to the analysis. In particular, the control device 7 is used for this purpose.

[0073] The first step, second step, and third step occur in a loop during the same acquisition time. The images are analyzed and the imaging device is adjusted at a high frequency so that the device adjustment time is small compared to the acquisition time.

[0074] The process allows images to be adapted in real time in relation to the dynamics of the phenomenon, i.e. the imaging system is adapted at the same time as the phenomenon unfolds and in relation to it.

[0075] This is made possible in particular because:

[0076] - the brightness level of the least illuminated pixels can be adjusted by modifying the intermittent light emission parameters, i.e. the parameters of the light source 5,

[0077] - the brightness level of the brightest pixels can be adjusted by modifying the acquisition parameters, i.e. the camera parameters.

[0078] In other words, the camera produces an image whose background light depends on the light emitted by the light source. In the statistical distribution of pixel illumination, the average level of the distribution depends on the camera parameters and the illumination level of the least illuminated pixels depends on the source parameters. These two degrees of freedom make it possible to remove pixels from saturation or underexposure or to adjust a contrast between two populations of pixels.

[0079] In this way, the full dynamic range of the camera is used in a useful way.

[0080] Such a method makes it possible to use only one imaging device comprising a single camera, so that several acquisition systems are not required. This also makes it possible to obtain images that can be used over the entire course of a single phenomenon: it is not necessary to reproduce the phenomenon several times to obtain images of all the phases of the phenomenon.

[0081] Furthermore, this process is implemented in an open field, so that there is no need for shock tube type equipment.

[0082] Figure 2 is a photograph taken by implementing the method just described in the case where the imaging device 1 comprises a retroreflective screen. The imaged phenomenon 23 is here a fireball. The retroreflective screen 15 can be seen in the background of the image. The imaging implemented here uses shadowscopy, thanks to which it is possible to distinguish the shock wave front 25 associated with the fireball. The shock wave front corresponds to a significant localized variation in pressure to which corresponds a variation in refractive index. The method makes it possible to distinguish on the same image the fireball 23 and the shock wave front 25. The fireball 23 corresponds to the brightest pixels which do not saturate the pixels because the camera parameters, and in particular the aperture and the exposure time, have been adjusted to prevent the pixels concerned from being overexposed.The shock wave front 25 corresponds to the least luminous pixels and is visible because the source parameters and in particular the emitted power have been adjusted to prevent the pixels concerned from being underexposed.

[0083] Figure 3 is a series of 4 successive photographs referenced A, B, C and D from left to right. Figure 3 was produced using the same process as for Figure 2. The phenomenon imaged 25 is here a fireball - zone 25A in photograph A, zone 25B in photograph B, zone 25C in photograph C and zone 25D in photograph D - with the associated shock wave front 27 - front 27A in photograph A, front 27B in photograph B, front 27C in photograph C and front 27D in photograph D -. In the background of the image, the retroreflective screen 15 can be seen. Since the images are taken successively, the spatial advance of the fireball can be seen from zone 25A to zone 25D and of its wavefront from front 27A to front 27D. Knowing the magnification of the image and the acquisition rate, it is thus possible to evaluate the propagation speed of the fireball on the one hand and of the shock wavefront on the other.

[0084] The analysis of an image by the control device 7 may comprise the determination in an area of ​​the image of a value of a statistical parameter of a distribution of an illumination level of pixels of the area. The emission parameters and the acquisition parameters are then controlled according to the determined value.

[0085] The image received by the control device 7 can be divided, for example, into different zones of identical surface area. For each zone, an average illumination level corresponding, for example, to a gray level can be calculated. This makes it possible to identify the most illuminated zone and the least illuminated zone. The parameters can then be controlled to prevent the most illuminated zone from being overexposed or too overexposed, or to prevent the least illuminated zone from being underexposed or too underexposed.

[0086] It is also possible to use zones of different surface areas, especially if one is interested in imaging two objects of different sizes, such as a dust cloud and a fireball. It may be possible to define zones around an object of interest, with the zones following the object's movement from one image to the next.

[0087] Within one of these areas, the distribution of the illumination level of the pixels in the area is known. It can be used to extract a statistical parameter that reflects the illumination of the object of interest located in the area. The parameters can then be controlled to ensure that each tracked object is well contrasted.

[0088] A possible statistical parameter has been cited as an average illumination, but more generally the statistical parameter can be chosen from a median, an average, a first quartile, a third quartile, a number of underexposed pixels receiving a light energy lower than an activation energy of a pixel and a number of overexposed pixels receiving a light energy higher than a saturation energy of a pixel.

[0089] The control of acquisition parameters or illumination parameters may in particular consist, once the statistical parameter has been determined, of:

[0090] - decrease an exposure time or a numerical aperture of the camera when the value of the statistical parameter exceeds a saturation threshold, or

[0091] - increase the light power emitted by the source when the value of the statistical parameter exceeds an underexposure threshold.

[0092] When the statistical parameter relates to an area that risks being overexposed, then we can define a saturation threshold beyond which we do not want the statistical parameter to take values. If this is still the case, then we must lower the illumination of the brightest areas, which can be achieved by acting on the acquisition parameters and in particular by reducing the exposure time of the camera and / or the numerical aperture of the camera (in other words its shutter).

[0093] When the statistical parameter relates to an area that risks being underexposed, then we can define an underexposure threshold below which we do not want the statistical parameter to take values. If this is still the case, then we must increase the illumination of the brightest areas, which can be achieved by acting on the illumination parameters and in particular by increasing the power emitted by the light source.

[0094] It is also possible to control the acquisition and illumination parameters based on the analysis of two areas of the image.

[0095] In this case, the mentioned area is a first area of ​​the image, and the statistical parameter is a first statistical parameter.

[0096] The control device 7 is configured to determine in a second zone of the image a value of a second statistical parameter of a distribution of an illumination level of the second zone.

[0097] For example, we choose that the first zone is more illuminated than the second zone.

[0098] The camera parameters are controlled according to the value of the first parameter, that is, they depend on the most illuminated area. This part of the control can notably consist of reducing an exposure time or a numerical aperture of the camera when the value of the first statistical parameter exceeds a saturation threshold.

[0099] The source parameters are controlled according to the value of the second parameter, that is to say, they depend on the least illuminated area. This part of the control can notably consist of increasing the light power emitted by the source when the value of the second statistical parameter exceeds an underexposure threshold.

[0100] For example, we can use for the first parameter, that is to say the parameter of the "illuminated pixels": a level N1 of illumination of the P1% of the most illuminated pixels, or of the most illuminated zone representing the percentage P1 of the image. P1% is worth for example 5%, 10%, 15%, 20%, 25% or 30%.

[0101] For the second parameter, i.e. the "dark pixels" parameter, we can use: the brightness level N2 of the P2% of the darkest pixels, or of the darkest area representing the percentage P2 of the image. P2% is for example 5%, 10%, 15%, 20%, 25% or 30%.

[0102] We can then slave the camera and the source so that N2 corresponds to P2% of the camera dynamics and N1 to 1 -P1% of the camera dynamics. In this way we ensure that:

[0103] - the illumination level of the P1% of the most illuminated pixels corresponds to the upper P1% fraction of the camera dynamics, and

[0104] - the illumination level of the P2% of the least illuminated pixels corresponds to the lower P2% fraction of the camera dynamics. The applications of the device and the method as presented so far can relate to the following phenomena: deflagration or detonation of dust, fire, deflagration of a turbulent gas jet (gas pipeline), civil pyrotechnics (airbags, fireworks, mines / quarries, space thrusters, etc.), deconfinement of pressurized bottles, mines, shock tube studies, study of blast waves (aerial, underwater, etc.), aerodynamic studies (ballistics, aeronautics, space), meteorology (lightning, hurricane, tornado, etc.), avalanches, seismology, volcanology (explosive and effusive volcanoes), offshore platform protection, embassy protection, noise barriers, cinematography, documentaries, journalism, popular science, sport, monitoring of sensitive sites, visualization of the effects of explosive charges, development of protection systems against explosions (accidental or deliberate nature) and against weapons, for goods (barriers, armor, grilles, etc.) and people (hearing protection, bulletproof vests, etc.), art & entertainment.

Claims

CLAIMS 1. Imaging apparatus (1) comprising: - a camera (3) having an acquisition rate greater than or equal to ten kiloHertz for a duration greater than or equal to one second, the camera (3) being configured to produce an image representing a level of light intensity as a function of two spatial coordinates, - an intermittent light source (5) having a frequency greater than or equal to ten kiloHertz during the duration, the apparatus (1) being configured so that at least part of the light emitted by the light source (5) illuminates the camera (3) and - a device (7) configured to, during the duration, analyze an image produced by the camera (3) and control parameters of the camera (3) and parameters of the source (5) according to the analysis.

2. Apparatus according to claim 1 wherein the light source (5) is collimated.

3. Apparatus according to any one of claims 1 or 2 further comprising a screen (15), the apparatus being configured so that light produced by the light source (5) is reflected by the screen (15) towards the camera (3).

4. Apparatus according to claim 3 wherein the screen (15) is a retroreflective screen.

5. Apparatus according to any one of claims 1 to 4 wherein the device (7) is configured to: - determine in an area of ​​the image a value of a statistical parameter of a distribution of an illumination level of pixels in the area, and - control the camera parameters (3) and the source parameters (5) according to the determined value.

6. Apparatus according to claim 5 wherein the statistical parameter is chosen from a median, an average, a first quartile, a third quartile, a number of underexposed pixels receiving light energy less than an activation energy of a pixel and a number of overexposed pixels receiving light energy greater than a saturation energy of a pixel.

7. Apparatus according to any one of claims 5 or 6 wherein the device (7) is configured to: - reduce an exposure time or a numerical aperture of the camera (3) when the value of the statistical parameter exceeds a saturation threshold, or - increase a luminous power emitted by the source (5) when the value of the statistical parameter exceeds an underexposure threshold.

8. Apparatus according to any one of claims 5 to 7 wherein the area is a first area of ​​the image, the statistical parameter being a first statistical parameter, the device (7) being configured to determine in a second area of ​​the image a value of a second statistical parameter of a distribution of an illumination level of the second area, the first area being more illuminated than the second area, the parameters of the camera (3) being controlled according to the value of the first parameter and the parameters of the source (5) being controlled according to the value of the second parameter.

9. Imaging method in which: - a phenomenon (17) is illuminated with intermittent light having a frequency greater than or equal to ten kiloHertz for a duration greater than or equal to one second, - images of the phenomenon are acquired at an acquisition rate greater than or equal to ten kiloHertz for the duration, the image representing a level of luminous intensity of the luminous phenomenon as a function of two spatial coordinates, - during the duration the images are analyzed and intermittent light emission parameters and image acquisition parameters are controlled according to the analysis.

10. Imaging method according to claim 9 wherein a screen (15) reflects the intermittent light towards a camera (3) which acquires the images.

11. Imaging method according to any one of claims 9 or 10 further comprising a step of triggering the phenomenon (17), the illumination of the phenomenon and the acquisition of the images being triggered as a function of the triggering of the phenomenon.

12. Method according to any one of claims 9 to 11 in which the analysis of the image comprises the determination in an area of ​​the image of a value of a statistical parameter of a distribution of an illumination level of pixels of the area, the emission parameters and the acquisition parameters being controlled as a function of the determined value.

13. Method according to claim 12 in which the statistical parameter is chosen from a median, an average, a first quartile, a third quartile, a number of underexposed pixels receiving light energy less than a pixel's activation energy and a number of overexposed pixels receiving light energy greater than a pixel's saturation energy.

14. Method according to any one of claims 12 to 13 comprising reducing an exposure time or a numerical aperture of a camera (3) which acquires the images when the value of the parameter exceeds a saturation threshold, or increasing an illumination power of the phenomenon when the value of the parameter exceeds an underexposure threshold.

15. Method according to any one of claims 11 to 14 in which the zone is a first zone of the image, the statistical parameter being a first statistical parameter, the analysis of the image comprising the determination in a second zone of a value of a second statistical parameter of a distribution of an illumination level of the second zone, the first zone being more illuminated than the second zone, the image acquisition parameters being controlled according to the value of the first parameter and the intermittent light emission parameters being controlled according to the value of the second parameter.

Images