Patent Application
20140061477
The present invention belongs to the field of viewing scenes. More specifically, the present invention relates to a device and method particularly suited to viewing a scene through an atmosphere loaded with aerosols, as is the case, for example, in foggy or rainy weather, in the presence of smoke, etc.
Many viewing devices are known, but these are not very suitable for viewing through an atmosphere loaded with aerosol particles. There are two categories of viewing devices: passive devices and active devices.
Passive devices just measure electromagnetic radiation emitted by the scene viewed, whereas active devices emit an electromagnetic radiation towards said scene and measure the echoes of electromagnetic radiation returned by the scene.
Passive devices generally comprise one or several cameras, which measure the electromagnetic radiation in the visible and/or infrared wavelength ranges.
However, a passive device's performance is linked to the luminance levels of any objects to be detected in the scene. Moreover, the contrast between the objects and the background of the scene is very variable, so that it is often very complicated to distinguish the background of the scene from an object to be detected. In addition, viewing at night is difficult for a passive device that is only sensitive in the visible wavelength range.
These limitations of passive devices are also magnified by foggy weather, rain, etc., insofar as the presence of aerosol particles in the atmosphere increases the attenuation to which the electromagnetic radiation emitted by the scene viewed is subjected.
Active viewing devices have the advantage of having ranges that are generally greater than those of passive devices.
It is known, particularly in the field of vehicle driving aids, to utilize active devices of the radar or lidar type.
Radars and lidars are active devices that operate in different wavelength ranges of the electromagnetic spectrum. Radars emit pulses in the radio-frequency range, while lidars emit pulses in the optical range. The optical range comprises in particular the range of infrared wavelengths and the range of visible wavelengths. The radio-frequency range corresponds to electromagnetic waves with wavelengths greater than those of the infrared range.
In practice, radars are not very sensitive to weather conditions, because the wavelengths in question are generally greater than the dimensions of the aerosol particles in the atmospheric medium when it is rainy, foggy, etc.
Nevertheless, radars have several limitations.
Firstly, radars are costly devices. Secondly, radars have difficulty detecting objects with a low RCS (Radar Cross-Section) such as, in the field of vehicle driving aids, pedestrians, two-wheeled vehicles, etc. In addition, in a complex environment such as an urban environment, the presence of multiple echoes makes the analysis of said multiple echoes very complicated.
Lidars are generally not very costly, compared to radars, and are mainly comprised of a light source, emitting a light pulse in the optical range, and a detector.
However, lidars also have limitations.
Firstly, the lidars' performance is very variable according to the LCS (Laser Cross Section) of the scene's objects. In addition, the performance of lidars is very adversely affected by certain weather conditions, in particular foggy weather.
To understand the reasons for this degradation in performance, it is necessary to analyze the propagation of a light pulse through atmosphere charged with aerosol particles.
The aerosol particles mainly have two distinct effects on the light pulse propagation. Firstly, the light pulse is attenuated in transmission. Secondly, a portion of the incident light pulse is returned by the aerosol particles towards the light source: this is the backscattering effect, which gives rise to a dazzling luminous cloud. The combination of these two effects contributes to reducing the contrast of objects in the scene viewed.
Thus, distinguishing between the information (apparent luminance of objects in the scene) and the noise (luminance backscattered by the aerosol particles) becomes very difficult.
In order to reduce the impact of backscattered photons on performance in terms of contrast, “range-gated” active viewing devices are known.
Such devices also comprise a light source and a detector, and are based on a principle of time division multiplexing of the activations of said light source and said detector. When the light source emits a light pulse, the detector is deactivated (turned off or masked by a shutter) so that the detector is not blinded by the photons backscattered by the aerosol particles in the immediate vicinity of the detector.
The detector is subsequently activated during a predefined interval of time, said time interval ending prior to the next emission of a light pulse. During this time interval, the detector measures the echoes of electromagnetic radiation returned by the scene in response to the emission by the light source.
It is understood that the glare effect is reduced, insofar as the aerosol particles in the immediate vicinity of the light source are not measured by the detector.
However, the range-gated active devices only permit the viewing of a portion of the scene located in a limited range of distances relative to the light source. Said range of distances corresponds to the distances for which a light pulse's roundtrip propagation time is within the detector's predefined activation time interval.
In addition, such range-gated active devices must comprise a complex and costly electronic control unit.
The present invention aims at proposing a solution for the active viewing of a scene, which makes it possible to have good levels of performance in a diffusing atmospheric medium, in particular foggy weather, and which is simple and not very costly to implement.
In addition, the present invention aims at proposing a solution that permits, in certain cases, a scene to be viewed, including in the immediate vicinity of a viewing device according to the invention, unlike range-gated active devices, which are limited to viewing a predefined range of distances away from these devices.
To achieve the objectives mentioned above, the present invention relates, according to a first aspect, to an active viewing device for detecting an object in a scene, comprising a light source suitable for emitting electromagnetic radiation towards the scene to be viewed. The device also comprises a detector and is configured to activate said detector so as to measure at least a portion of the electromagnetic radiation emitted by said light source and returned by the scene viewed. The electromagnetic radiation emitted by the light source is infrared radiation at least partially included within a spectral band, referred to as the “viewing band”, the wavelengths of which are between 8 micrometers and 15 micrometers, and the detector is designed to measure infrared radiation within the viewing band.
Preferably, the viewing band consists of wavelengths between 10 micrometers and 12 micrometers.
One advantage of using this viewing band lies in particular in the fact that backscattering is low for these wavelengths in foggy weather. As a result, the glare effect is significantly reduced by using wavelengths between 8 micrometers and 15 micrometers, such that the viewing device's visibility distance is much greater, in foggy weather, than the meteorological visibility distance.
Alternatively, wavelengths of the viewing band are between 2.7 micrometers and 2.9 micrometers, or between 5.8 micrometers and 6.2 micrometers.
According to particular embodiments, the viewing device comprises one or more of the following characteristics, considered either alone or in any technically possible combination:
According to a second aspect, the invention relates to using the viewing device according to the invention for viewing a scene through an atmosphere loaded with aerosol particles, more specifically for viewing a scene in foggy or rainy weather.
According to a third aspect, the invention relates to a method for viewing a scene, said method comprising steps of the light source emitting electromagnetic radiation towards the scene to be viewed, and a detector measuring at least a portion of the electromagnetic radiation emitted by said light source and returned by the scene towards said light source. The electromagnetic radiation emitted during the emission step is infrared radiation at least partially included within a spectral band, referred to as the “viewing band”, the wavelengths of which are between 8 micrometers and 15 micrometers, and, during the measurement step, the detector measures infrared radiation within said viewing band.
Preferably, the viewing band consists of wavelengths between 10 micrometers and 12 micrometers.
Alternatively, wavelengths of the viewing band are between 2.7 micrometers and 2.9 micrometers, or between 5.8 micrometers and 6.2 micrometers.
According to particular modes of implementation, the viewing method comprises one or more of the following characteristics, singly or in any technically possible combination:
The invention will be better understood in reading the following description of a non-limiting example, made with reference to the following figures, which are not to scale and which represent:
a to 7c: images obtained in a fog chamber showing the improvements brought by utilizing a viewing device according to the invention,
The light source 12 is designed to emit electromagnetic radiation towards a scene. The scene is, for example, comprised of an object 20 to be detected immersed in a diffusing medium 30 of atmospheric type loaded with aerosol particles, more specifically foggy weather. In the rest of the description, the case considered in a non-limiting way, and unless otherwise indicated, is that of viewing a scene in foggy weather.
According to the invention, the electromagnetic radiation emitted by the light source 12 is infrared radiation at least partially included within a spectral band, referred to as the “viewing band”, the wavelengths of which are between 8 micrometers (μm) and 15 μm.
The detector 14 is designed to measure infrared radiation within said viewing band. In addition, the active viewing device 10 is configured to activate the detector 14 so as to measure at least a portion of the electromagnetic radiation emitted by said light source 12 and returned by the scene viewed towards said light source.
In other words, the active viewing device 10 comprises an electronic control unit, of a type known per se and not shown in the figures, which controls the detector 14 so that it measures the infrared radiation returned by the scene in response to the emission of infrared radiation by the light source 12. Indeed, as the device 10 is an active device, the detector 14 must measure the infrared radiation in the viewing band when all or part of the scene viewed is illuminated by the light source 12.
It is therefore understood that the viewing band is within a spectral band corresponding to long wavelength infrared spectrum, known under the acronym LWIR.
It will be seen later that such a choice of wavelengths makes it possible to have good levels of performance in foggy weather. Preferably, the viewing band consists of wavelengths between 10 μm and 12 μm (it will be seen later that such a choice of wavelengths makes possible improvements in the performance of the viewing device 10).
It should be noted that, in the context of the invention, the viewing band is a spectral band common to the light source 12 and the detector 14, i.e. a spectral band in which the light source 12 is designed to emit and in which the detector 14 is designed to measure infrared radiation.
It is understood that, for the light source 12, nothing precludes emitting electromagnetic radiation in a spectral band broader than the viewing band, able to comprise wavelengths not just between 10 μm and 12 μm, or even between 8 μm and 15 μm. Similarly, for the detector 14, nothing precludes measuring electromagnetic radiation in a spectral band broader than the viewing band.
However, it is understood that for reasons of the efficiency of the viewing device 10, the spectral bands of emission (of the light source 12) and measurement (of the detector 14) are preferably substantially the same. Preferably, the emission spectral band is included within the measurement spectral band, so that all the infrared radiation emitted by the light source 12 can be measured by the detector 14. Such provisions make it possible to have a viewing device 10 with improved performance and efficiency.
An analytical model of luminance is now described, and the simulation results obtained with this analytical model are presented, which show the advantages of using the spectral band mentioned above.
The analytical model of luminance described below is based on Mie's theory of light scattering. This theory is assumed to be known to the man skilled in the art and reference may be made in particular to the following reference: Bohren C. F., Huffman D. R., “Absorption and Scattering of Light by Small Particles”, Wiley & Sons (1983).
The luminance LB backscattered by the fog and received by the receiver Rx can be approximated by the following expression:
in which:
The validity of the above approximation has been verified in particular in the following scientific publication: Taillade F., Belin E., Dumont E., “An Analytical Model for Backscattered Luminance in Fog: Comparisons with Monte Carlo Simulations and Experimental Results”, Measurement Science and Technology, (2008).
In addition, considering an object in the fog, illuminated by the emitter Tx, its luminance LO seen by the receiver Rx can be approximated by the following expression:
in which:
It should be noted that the extinction coefficient μE is approximately equal to 3/Vm, where Vm is the meteorological visibility distance. For the man skilled in the art, the meteorological visibility distance Vm is defined as the distance for which the luminance transmitted through the atmosphere is attenuated by 95%.
The signal-to-noise ratio SNR, which depends on the ratio of luminance LO of the object to the luminance LB backscattered by the fog, is for example defined according to the following expression:
SNR=10·log (LO/LB)
The visibility distance of the active viewing device, comprising the emitter Tx and the receiver Rx, is defined as the distance for which the SNR ratio is zero when expressed in decibels (i.e. when the signal is equal to the noise).
The curves shown in
Type T1 fog corresponds to a particle size distribution centered on 1 μm (i.e. aerosol particles with radius “r” equal to 1 μm are the most numerous). Type T2 fog corresponds to a particle size distribution centered on 5 μm, and type T3 fog corresponds to a particle size distribution centered on 10 μm.
The concentrations of aerosol particles are normalized so that the meteorological visibility distance Vm, in the visible spectrum for a wavelength of 0.5 μm (which corresponds approximately to the maximum sensitivity of the human eye), is substantially equal to 100 meters for each type of fog.
With type T1 fog, it can be seen that the value of the extinction coefficient μE is lower for wavelengths between 8 μm and 15 μm than for a wavelength of 0.5 μm. The extinction coefficient μE is substantially equal to 0.005 for a wavelength of 10.5 μm, i.e. a meteorological visibility distance Vm of approximately 600 meters at 10.5 μm.
As the particle size of the fog increases, the variations in the extinction coefficient μE decrease for the wavelengths shown. For example, with type T3 fog the meteorological visibility distance Vm varies little with the wavelength.
Surprisingly, the behavior of the backscattering coefficient μB with the wavelength is very different from that of the extinction coefficient μE.
Indeed, it can be seen that the values of the backscattering coefficient μB in a band of wavelengths between 8 μm and 15 μm are much lower than the values of said backscattering coefficient μB at 0.5 μm, irrespective of the type of fog considered (T1, T2 or T3).
For example, in the case where the ratio D/Vm=1 and irrespective of the type of fog considered, the utilization of an active device with wavelength 10.6 μm theoretically introduces an improvement of 40 decibels (dB) in the signal-to-noise ratio SNR, compared to an active device with wavelength 0.5 μm. In addition it can be seen that, for values of the ratio D/Vm less than 1, whereas the lowest extinction coefficient μE at 10.6 μm was obtained for type T1 fog, the signal-to-noise ratio SNR improvement is smallest with type T1 fog, utilizing the viewing device 10 according to the invention (improvement of 10 dB). In contrast, for values of the ratio D/Vm greater than 1 and for type T1 fog, the improvement in the signal-to-noise ratio SNR is greater than for the other types of fog, utilizing the viewing device 10 according to the invention. For type T1 fog, the improvement in the signal-to-noise ratio SNR at 10.6 μm is partly due to the increase in the luminance LO because the extinction coefficient μE is lower at 10.6 μm than at 0.5 μm.
It can be seen that the ratio μE/μB tends towards zero for a wavelength of 0.5 μm. The ratio μE/μB is higher in the viewing band mentioned above. It can be seen, for example, that for type T2 and T3 fogs the ratio μE/μB is greater than 50 for wavelengths between 8 μm and 15 μm, and greater than 150 for wavelengths between 10 μm and 12 μm.
The spectral band of wavelengths between 10 μm and 12 μm is also interesting in the case of type T1 fog, insofar as the ratio μE/μB shows a local maximum in this spectral band (around 11.5 μm).
The advantages of using the viewing band with wavelengths between 8 μm and 15 μm, or between 10 μm and 12 μm, can therefore be understood. Indeed, a significantly improved visibility distance in foggy weather is expected, mainly because of the very low level of backscattering at these wavelengths.
In a particular embodiment of the viewing device 10 according to the invention, the light source 12 is a CO2 laser. This example is not limiting, and it is understood that other types of light sources can be utilized to emit infrared radiation in the LWIR spectral band, such as QCL diodes.
Preferably, the viewing device 10 comprises a means of expanding a beam of the infrared radiation emitted by the light source 12, such as a diverging infrared lens 16. Such a lens 16 is shown schematically in
It is understood that utilizing such an expansion means makes it possible to increase the solid angle illuminated by the light source 12, and thus to increase the field of vision in foggy weather, provided that the detector 14 is designed to measure the infrared radiation returned by all the illuminated portion of the scene.
Preferably, the detector 14 is a matrix detector adapted to form a two-dimensional image of the scene viewed with a single measurement, such as an LWIR thermal camera. According to other examples, the detector 14 is a microbolometer, an infrared camera based on Mercury-Cadmium-Tellurium (MCT), etc.
In a particular embodiment, compatible with any one of the preceding embodiments, the viewing device 10 is configured to activate the detector 14 simultaneously with the emission of an infrared radiation by the light source 12.
In other words, the electronic control unit of the viewing device 10 controls the detector 14 so that it measures the infrared radiation of the scene, including during the emission of infrared radiation by the light source 12.
It is understood that activating the detector 14 at the same time as the light source 12 emits infrared radiation is made possible by using wavelengths within the viewing band mentioned above (between 8 μm and 15 μm, or even between 10 μm and 12 μm), since backscattering and the glare effect are very low in that range.
Such provisions make it possible to have a simple electronic control unit, insofar as the detector 14 can be activated in a continuous way during active viewing operations. In particular, the electronic control unit is simpler than in the case of range-gated active devices. In addition, activating the detector 14 continuously means that viewing the scene does not have to be restricted to a limited range of distances, as is the case for range-gated active devices.
An example of a detector operating in a continuous way is a CCD thermal camera, which produces successive images of the scene at a predefined frequency.
The light source 12 can be activated so as to illuminate the scene viewed continuously or discontinuously. Preferably, the light source 12 is activated continuously, so as to reduce the complexity of the electronic control unit.
“Illuminate discontinuously” means that the light source 12 emits pulses of light.
“Illuminate continuously” means that the light source 12 is activated permanently during active viewing operations. Nothing precludes acquiring a first image with the viewing device 10 in a passive way, i.e. with the light source 12 switched off, and then acquiring a second image in an active way with the light source 12 activated permanently during the entire acquisition of said second image.
a to 7d show images of a scene obtained experimentally in a fog chamber. Said images have been obtained under real conditions with a viewing device 10 according to a preferred embodiment, wherein said device comprises:
To obtain these results, the CO2 laser and the thermographic camera have been arranged close to each other, in this example approximately 0.5 meters apart, and such that their axes are substantially parallel (i.e. they are directed substantially towards the same area of the scene viewed). In this way, the thermographic camera is arranged so as to be able to measure at least one portion of the infrared radiation returned by the scene towards the CO2 laser.
a and 7b show images obtained with the light source 12 switched off, and
a shows an image obtained in the absence of fog. Various elements were positioned in the scene viewed, visible in
The background of the scene is at a temperature of 27° C., such that plate P1 and panel P2 have different thermal contrasts CT, respectively CT≈0.11 and CT=0 (it should be noted that the outline of panel P2 has been added so as to locate the panel in the image, said panel not being visible because its thermal contrast CT is zero).
b shows an image obtained, the light source 12 being switched off, in the presence of fog (particle size distribution centered between an aerosol particle radius of approximately 0.5 μm and 1 μm) with a meteorological visibility distance Vm of 8 meters. It can be seen that plate P1 is visible. The panel P2 is not visible.
c shows an image obtained by the detector 14 under the same conditions as
First of all, the expected absence of the glare effect can be observed. Secondly, it can be seen that the panel P2 (with thermal contrast CT zero) is visible in
The visibility distance VIR is estimated as described below.
In the total absence of fog, the light source 12 being activated, the maximum contrast C0 of the panel P2 is measured taking the temperature of the background of the scene as the reference temperature. Then, in the presence of fog and for different values of the meteorological visibility distance Vm, the apparent contrast CA of the panel P2 is measured as previously, with the light source 12 activated. The visibility distance VIR is determined according to the following expression:
in which D is the distance between the panel P2 and the viewing device 10, i.e. 25 meters. In this
From reading
In conclusion, the viewing device 10 according to the invention makes it possible to achieve a 15 to 40 fold improvement between the visibility distance VIR (obtained with said device) and the meteorological visibility distance Vm.
In addition, the viewing device 10 allows objects with a thermal contrast CT of zero to be detected, provided these objects to be detected have a non-zero albedo in the spectral band considered (between 8 μm and 15 μm).
The experimental results corroborate the results obtained by simulation, so that the approximations or any imperfections of the analytical model of luminance, used for the simulations, cannot call the invention into question, insofar as it has been verified that it actually provides the advantages identified by simulation with said analytical model of luminance.
As indicated previously, the electromagnetic radiation emitted during the emission step 52 is infrared radiation at least partially included within the viewing band (between 8 μm and 15 μm, or even between 10 μm and 12 μm) and the detector 14 measures infrared radiation in said viewing band during the measurement step 54.
Preferably, the measurement step 54 is executed simultaneously with the emission step 52. The measurement step 54 is advantageously executed continuously during active viewing operations. Preferably, the emission step 52 is also executed continuously, i.e. the light source 12 illuminates the scene viewed continuously (in contrast to illumination by pulses of light).
The present invention proposes a viewing device 10 particularly suited to viewing in foggy weather, in particular a fog with a particle size greater than 5 μm. In addition, it has also been verified that the viewing device 10 presents good levels of performance in rainy weather, in particular by simulation with the analytical model mentioned above, considering aerosol particles with a particle size distribution centered approximately on 200 μm.
It is understood, however, that the viewing device 10 can also be used in other contexts, including clear weather, in the presence of smoke, etc.
The viewing device 10 according to the invention can be used in many fields. For example, the viewing device 10 is installed in a vehicle (automobile, aircraft, boat, etc.) as a driving aid in foggy weather. According to another non-limiting example, the viewing device 10 is carried by a user to help him move around or get his bearings with respect to his surroundings in foggy weather.
More generally, from reading
It can therefore be understood that, according to the simulation results, a significantly improved visibility distance in foggy weather is also expected with an active device with a wavelength substantially equal to 2.8 μm or 6 μm. Thus, the present invention also, according to other embodiments, relates to active devices operating between 2.7 μm and 2.9 μm or between 5.8 μm and 6.2 μm. However, the viewing band with wavelengths between 8 μm and 15 μm corresponds to a preferred embodiment and mode of implementation of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10/04894 | Dec 2010 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/072826 | 12/14/2011 | WO | 00 | 11/8/2013 |
