Patent Publication Number: US-2022230455-A1

Title: Multi-exposure image capturing device and associated system for monitoring a driver

Description:
The present invention relates to an image-capturing device. It also relates to a monitoring device for monitoring a driver of a vehicle, including an image-capturing device. 
     Image-capturing devices that make it possible to view a given scene both in the visible range and in the infrared range, and to do so with one and the same image sensor, have been developed recently. 
     The image sensor of such a device, which is hybrid as it were, is sometimes called an “RGB-IR” (acronym of Red Green Blue—InfraRed) image sensor. 
     This image sensor comprises a matrix array of photosensitive pixels and an array of elementary optical filters coinciding with these various photosensitive pixels. 
     Some of these elementary optical filters are color filters. They each transmit a portion of the visible radiation received by the image sensor, this portion corresponding in practice to a red, to a green or to a blue. These various elementary color filters make it possible to acquire a color image, for example an “RGB” image. 
     The other elementary optical filters of the image sensor are at least partially transparent in the infrared range. They make it possible to acquire an image of the scene in question in the infrared range. 
     The color image and the image in the infrared range, which contain additional information, are thus obtained with one and the same sensor, this being beneficial in particular in terms of cost price and size. 
     However, the ambient brightness in the environment of an image-capturing device is often markedly different in the visible range and in the infrared range. It is therefore generally not possible with such an image sensor to obtain optimum exposure both for the color image and for the image in the infrared range. 
     In this context, the present invention proposes an image-capturing device comprising an image sensor, which includes:
         an array of optical filters receiving electromagnetic radiation and comprising first optical filters each able to transmit a first portion of the electromagnetic radiation contained in a given infrared wavelength range, and second optical filters each able to transmit at least one component of a second portion of the electromagnetic radiation located in the visible, and   a matrix array of photosensitive pixels comprising first photosensitive pixels arranged so as to receive the first portion of the electromagnetic radiation transmitted by the first optical filters, as well as second photosensitive pixels arranged so as to receive the component transmitted by the second optical filters, each of the first and second photosensitive pixels producing via the photoelectric effect an electric charge or an electric voltage depending on the power of the portion of the electromagnetic radiation that it receives,       

     According to the invention, the image-capturing device further comprises a computer programmed to: 
     a1) determine an ambient brightness in the infrared range,
 
a2) determine an ambient brightness in the visible range,
 
b1) command the image sensor to perform a first image capture, during which each of the first photosensitive pixels delivers a pixel signal representative of the electric charge or of the electric voltage that it produces, this electric charge or voltage having been accumulated by this first photosensitive pixel during a first integration time,
 
b2) command the image sensor to perform a second image capture, during which each of the second photosensitive pixels delivers a pixel signal representative of the electric charge or of the electric voltage that it produces, this electric charge or voltage having been accumulated by this second photosensitive pixel during a second integration time,
 
c1) command a preprocessing module to multiply each of the pixel signals produced in step b1) by a first gain value,
 
c2) command the preprocessing module to multiply each of the pixel signals produced in step b2) by a second gain value,
 
d1) compose a first image, from the pixel signals produced by the first photosensitive pixels in the first image capture, said pixel signals having been multiplied by the first gain value,
 
d2) compose a second image, from the pixel signals delivered by the second photosensitive pixels in the second image capture, said pixel signals having been multiplied by the second gain value,
 
the first integration time or the first gain value being determined by the computer depending on the ambient brightness in the infrared range, and
 
the second integration time or the second gain value being determined by the computer depending on the ambient brightness in the visible range.
 
     This image-capturing device makes it possible, using the same image sensor, to obtain:
         in the infrared range, a first image of a scene to be viewed, which image is suitably exposed since it is exposed depending on the ambient brightness in the infrared range, and   in the visible range, a second image of the same scene, which image is also suitably exposed since it is exposed depending on the ambient brightness in the visible range.       

     This image-capturing device moreover has the advantage of being able to be implemented by means of commercially available electronic components initially designed for high-dynamic-range (HDR) imagery, this facilitating this implementation. 
     HDR imaging may consist, for example, in:
         acquiring an image with a given exposure time, i.e. with a given integration time, allowing a suitable exposure of most of this image, then   acquiring a complementary image with another exposure time allowing a suitable, not overexposed, exposure of the high-brightness regions of this complementary image, then   combining said image and said complementary image to obtain an HDR image, in which the regions of medium brightness, and those of high brightness, are suitably exposed, avoiding an over- or under-saturation of the corresponding brightness values.       

     Electronic components for HDR imaging are therefore well suited to acquiring (or to commanding an image sensor to acquire) images in series of a plurality of images, for example in series of two images, or of four images, the various images of each series being acquired with various exposure times or various gains, or both. 
     It will therefore be understood that such electronic components make it possible to easily produce the image-capturing device according to the invention. 
     Another advantage of this image-capturing device is that it allows such HDR imaging techniques to be easily applied, when the first image is obtained in the infrared, or the second image is obtained in the visible. 
     By way of example, the infrared first image may for example be composed taking into account both:
         pixel signals produced by the first photosensitive pixels in the first image capture, and   pixel signals produced by the first photosensitive pixels in the second image capture (this second image capture initially being intended to obtain the visible second image),       this most often allowing the dynamic range of this infrared first image to be increased, without even having to carry out additional image captures since the first and second image captures are generally carried out with different integration times and/or different gain values.   

     The following are other non-limiting and advantageous characteristics of the image-capturing device according to the invention:
         the computer is programmed, steps b1), c1) and d1) having been executed beforehand, to determine said ambient brightness in the infrared range, in step a1), depending on brightness values of at least some of the image pixels of the first image produced in said previous execution of step d1);   the computer is programmed to, in step a1), determine said ambient brightness in the infrared range in such a way that it is representative of a first average-brightness level in the first image produced in said previous execution of step d1);   the computer is programmed to determine the first integration time or the first gain value depending on a first difference between, on the one hand, a first target average-brightness value, and, on the other hand, the first average-brightness level in the first image produced in the previous execution of step d1);   the computer is programmed, steps b2), c2) and d2) having been executed beforehand, to determine said ambient brightness in the visible range, in step a2), depending on brightness values of at least some of the image pixels of the second image produced in said previous execution of step d2);   the computer is programmed to, in step a2), determine said ambient brightness in the visible range in such a way that it is representative of a second average-brightness level in the second image produced in said previous execution of step d2);   the computer is programmed to determine the second integration time or the second gain value depending on a second difference between, on the one hand, a second target average-brightness value, and, on the other hand, the second average-brightness level in the first image produced in the previous execution of step d2);   the computer is further programmed to:
 
b3) command the image sensor to perform a third image capture, during which each of the first or second photosensitive pixels delivers a pixel signal representative of the electric charge or of the electric voltage that it produces, this electric charge or voltage having been accumulated by the photosensitive pixel during a third integration time,
 
c3) command the preprocessing module to multiply each of the pixel signals produced in step b3) by a third gain value,
 
the product of the third integration time multiplied by the third gain value being different from the product of the first integration time multiplied by the first gain value, or being different from the product of the second integration time multiplied by the second gain value;
   the computer is programmed to, in step d1), compose the first image while further taking into account the pixel signals produced by the first photosensitive pixels in the third image capture, these pixel signals having been multiplied by the third gain value, the product of the third integration time multiplied by the third gain value being different from the product of the first integration time multiplied by the first gain value;   in step d1), the pixel signals produced by the first photosensitive pixels in the third image capture, and which have been multiplied by the third gain value, are further multiplied by a first scaling coefficient before being taken into account to compose the first image, the first scaling coefficient depending on the ratio between the first integration time and the third integration time, or on the ratio between the first value gain and the third gain value;   the computer is programmed, steps b1), c1), b3), c3) and d1) having been executed beforehand, to determine the third integration time or the third gain value depending on brightness values of at least some of the image pixels of the first image produced in said previous execution of step d1);   the computer is programmed to determine the third integration time depending on the first integration time;   the computer is programmed to determine the third gain value depending on the first gain value;   the computer is programmed to, in step d2), compose the second image while further taking into account the pixel signals produced by the second photosensitive pixels in the third image capture, these pixel signals having been multiplied by the third gain value, the product of the third integration time multiplied by the third gain value being different from the product of the second integration time multiplied by the second gain value;   in step d2), the pixel signals produced by the second photosensitive pixels in the third image capture, and which have been multiplied by the third gain value, are further multiplied by a second scaling coefficient before being taken into account to compose the second image, the second scaling coefficient depending on the ratio between the second integration time and the third integration time, or on the ratio between the second value gain and the third gain value;   the computer is programmed, steps b2), c2), b3), c3) and d2) having been executed beforehand, to determine the third integration time or the third gain value depending on brightness values of at least some of the image pixels of the second image produced in said previous execution of step d2);   the computer is programmed to determine the third integration time depending on the second integration time;   the computer is programmed to determine the third gain value depending on the second gain value;   the computer is further programmed to:
 
b4) command the image sensor to perform a fourth image capture, during which each of the first or second photosensitive pixels delivers a pixel signal representative of the electric charge or of the electric voltage that it produces, this electric charge or voltage having been accumulated by the photosensitive pixel during a fourth integration time,
 
c4) command the preprocessing module to multiply each of the pixel signals produced in step b4) by a fourth gain value,
 
the product of the fourth integration time multiplied by the fourth gain value being different from the product of the second integration time multiplied by the second gain value,
 
the computer being programmed to, in step d2), compose the second image while further taking into account the pixel signals produced by the second photosensitive pixels in the fourth image capture, these pixel signals having been multiplied by the fourth gain value;
   the computer is programmed to, in step d1), compose the first image while further taking into account pixel signals produced by the first photosensitive pixels in the second image capture, the pixel signals produced by the first photosensitive pixels in the second image capture having been multiplied by the second gain value;   the computer is programmed to, in step d2), compose the second image while further taking into account pixel signals produced by the second photosensitive pixels in the first image capture, the pixel signals produced by the second photosensitive pixels in the first image capture having been multiplied by the first gain value.       

     Of course, the various features, variants and embodiments of the invention may be combined with one another in various combinations, as long as they are not mutually incompatible or mutually exclusive. 
     The invention also proposes a monitoring system for monitoring a driver of a vehicle, comprising an image-capturing device as described above and a processing unit programmed to determine a level of the driver&#39;s inability to drive based at least on said first image. 
     The level of inability to drive may comprise a drowsiness level or a level of distraction of the driver. 
    
    
     
       In addition, various other features of the invention will become apparent from the appended description, which is given with reference to the drawings, which illustrate non-limiting embodiments of the invention and in which: 
         FIG. 1  schematically shows, seen from the side, a motor vehicle comprising an image-capturing device implementing the teachings of the invention, 
         FIG. 2  shows, in more detail, certain elements of the image-capturing device of  FIG. 1 , 
         FIG. 3  schematically shows, seen face-on, an array of optical filters with which an image sensor of the image-capturing device of  FIG. 1  is provided, 
         FIG. 4  schematically shows, seen from the side, the same image sensor, 
         FIG. 5  schematically shows an infrared image and a color image delivered by the image-capturing device of  FIG. 1 , 
         FIG. 6  schematically shows steps of an image-capturing method according to a first embodiment implemented by an image-capturing device such as that of  FIG. 1 , 
         FIG. 7  schematically shows steps of an image-capturing method according to a second embodiment implemented by an image-capturing device such as that of  FIG. 1 , and 
         FIG. 8  schematically shows steps of an image-capturing method according to a third embodiment implemented by an image-capturing device such as that of  FIG. 1 . 
     
    
    
     I. Image-Capturing Device 
       FIG. 1  shows a vehicle  5 , here a motor vehicle, equipped with a monitoring system  2  for monitoring a driver  3  of the vehicle. This monitoring system  2  comprises an image-capturing device  1  and a processing unit  20  that is programmed to determine a level of the driver&#39;s  3  inability to drive, I L , based on one or more images delivered by the image-capturing device  1 . 
     As shown in  FIG. 1 , the image-capturing device  1  is located in a passenger compartment  7  of the vehicle, in a region close to a windshield of the vehicle. It is for example integrated into an instrument panel, into a dashboard or into a control console of the vehicle. 
     The image-capturing device  1  is oriented such that its field of view  15  covers the region usually occupied by the head of the driver  3  when said driver is seated in the driver&#39;s seat. The image-capturing device  1  is thus able to capture images of the face of the driver  3 . 
     As shown in  FIG. 2 , the image-capturing device  1  comprises:
         an image sensor  9 ,   an optical system  10  such as an objective, which forms on the image sensor  9  an image of the content of the field of view  15  of the image-capturing device  1 ,   a preprocessing module  11 , to amplify pixel signals sp 1 , sp 2  produced by the image sensor  9 , then to perform an analog-digital conversion of these signals in order to obtain a digital image signal si 1 , si 2 ,   a computer  12 , configured to receive the image signal si 1 , si 2  delivered by the preprocessing module  11  and to process this signal in order to produce one or more images representative of the content of the field of view  15  of the image-capturing device  1 .       

     Noteworthily, the image sensor  9  is in some respects a hybrid sensor, allowing the content of the field of view  15  to be viewed both in the infrared range and in the visible range. 
     To this end, the image sensor  9  is equipped with a particular array  170  of elementary optical filters ( FIG. 3 ), comprising an alternation of first optical filters  171 , which are at least partially transmissive in the infrared range, and of second optical filters  172 ,  173 ,  174 , which are at least partially transmissive in the visible range. This array  170  of optical filters makes it possible, with the same matrix array  19  of photosensitive pixels  21 , 21 ′, to obtain both a first image  31 , which is an image in the infrared range (called the “infrared image” below), and a second image  35 , which is an image in the visible range. 
     In the example described here, the second optical filters  172 ,  173 ,  174  are color filters, and the second image in question is a color image. Below, the second image  35  is moreover called the “color image  35 ”. 
     By virtue of this particular image sensor  9 , the image-capturing device  1  is able to acquire both the infrared image  31  and the color image  35 , while remaining compact. The infrared image  31  may for example be used to monitor the driver  3  and to determine his level of inability to drive I L . The color image  35  is more pleasing for the human eye to view and contains more information than an infrared image (usually displayed in grayscale). The color image may therefore be acquired for multiple purposes; for example, it may be used to communicate with a remote electronic device, for example within the context of a teleconference, or indeed be kept in memory for security reasons, or as a “souvenir photo” for commemorating a journey. 
     When the image-capturing device  1  takes a shot:
         each photosensitive pixel  21 ,  21 ′ of the matrix array  19  of the image sensor accumulates, during a given integration time, an electric charge or voltage generated via the photoelectric effect in this photosensitive pixel, and delivers a pixel signal representative of the electric charge or voltage thus accumulated, then   the preprocessing module  11  multiplies each of these pixel signals by a given gain value, then carries out analog-digital conversion of the pixel signals thus amplified (or possibly attenuated).       

     Whether such a shot results in an image (or more than one images) that is (are) suitably exposed depends on the exposure parameters with which this shot is taken. These exposure parameters comprise the integration time and gain value mentioned above, and the aperture of a diaphragm of the optical system  10  (not shown in the figures). The aperture of this diaphragm is here considered to remain constant in the series of shots described below (each series of shots being taken in a short time). Thus, the only exposure parameters considered below are the integration time and gain value in question. 
     Noteworthily, the computer  12  is programmed to command the image sensor  9  and the preprocessing module  11  in such a way as to take shots in series of a plurality of shots—for example, in pairs, or in series of 3 or 4 shots. In other words, the computer  12  is programmed so that each shot taken by the image-capturing device is necessarily accompanied by at least one other shot, associated with the previous one. These two shots are here taken in a short time, for example shorter than 0.3 seconds. 
     Each series of shots commanded by the computer  12  comprises at least:
         a first shot, taken with a first integration time ti 1  (i.e. with a first exposure time), and with a first gain value g 1 , the first integration time ti 1  or the first gain value g 1  being determined by the computer  12  depending on an ambient brightness in the infrared range, and   a second shot, taken with a second integration time ti 2  (with a second exposure time), and with a second gain value g 2 , the second integration time ti 2  or the second gain value g 2  being determined by the computer  12  depending on an ambient brightness in the visible range.       

     The computer  12  is programmed to then compose the infrared image  31  on the basis of the pixel signals sp 1  acquired during the first shot, and to compose the color image  35  on the basis of the pixel signals sp 2  acquired during the second shot. 
     By virtue of this multi-shot technique, the infrared image  31  and the color image  35  are each suitably exposed, even if the ambient brightness in the field of view  15  of the image-capturing device  1  is markedly different in the visible range and in the infrared range. Two complementary images of the content of this field of view  15 , corresponding to different wavelength bands, and each suitably exposed, are thus obtained. 
     The computer  12  is more precisely programmed to: 
     a1) determine the ambient brightness in the infrared range,
 
a2) determine the ambient brightness in the visible range,
 
b1) command the image sensor  9  to perform a first image capture, during which each of the first photosensitive pixels  21  delivers a pixel signal representative of the electric charge or electric voltage that it produces, this electric charge or voltage having been accumulated by this first photosensitive pixel  21  during the first integration time ti 1  (in the case where the image sensor is of CCD type, the pixel signal in question corresponds for example directly to the electric charge accumulated via the photoelectric effect by the photosensitive pixel in question, at the end of the integration time in question),
 
b2) command the image sensor  9  to perform a second image capture, during which each of the second photosensitive pixels  21 ′ delivers a pixel signal representative of the electric charge or of the electric voltage that it produces, this electric charge or voltage having been accumulated by this second photosensitive pixel  21 ′ during the second integration time ti 2 ,
 
c1) command the preprocessing module  11  to multiply each of the pixel signals sp 1  produced in step b1) by a first gain value g 1 ,
 
c2) command the preprocessing module to multiply each of the pixel signals sp 2  produced in step b2) by a second gain value g 2 ,
 
d1) compose the infrared mage  31 , from the pixel signals sp 1  produced by the first photosensitive pixels  21  in the first image capture, said pixel signals sp 1  having been multiplied by the first gain value g 1 ,
 
d2) compose the second image  35 , which it will be recalled is here a color image, from the pixel signals sp 2  delivered by the second photosensitive pixels  21 ′ in the second image capture, said pixel signals sp 2  having been multiplied by the second gain value g 2 .
 
     Various embodiments of the image-capturing device  1  are described here. These embodiments mainly differ from one another in the detail of the programming of the computer  12 , i.e. in the detail of the image-capturing method implemented by the image-capturing device (steps a1) to d2) of which have just been presented). In the present case, the methods schematically shown in  FIGS. 6, 7 and 8  correspond to a first, second and third embodiment of the image-capturing device  1 , respectively. 
     In the first and third embodiments ( FIGS. 6 and 8 ), the computer  12  is programmed so that each series of shots comprises two shots (in the present case the first and second shots presented above), whereas in the second embodiment ( FIG. 7 ), the computer  12  is programmed so that each series of shots comprises four shots (taken with different exposure parameters). 
     Realization of this image-capturing device  1  is facilitated by the fact that electronic components, initially designed for HDR imaging, and therefore basically configured to take multi-exposure shots, are now commercially available. Moreover, electronic components for HDR imaging are generally configured to take shots in series of two, or four shots, and are therefore particularly well suited to the implementation of the three embodiments mentioned above. 
     In the second and third embodiments (unlike the first embodiment), each of the first and second images  31 ,  35  is moreover obtained by further implementing an HDR imaging technique. 
     From one embodiment to the next, identical or corresponding elements have as much as possible been identified by the same reference signs and are not described each time. 
     The image sensor  9  is now described in more detail. The way in which the infrared image and the color image are composed, and the way in which the exposure parameters are controlled, will be presented subsequently, in the description of various embodiments of an image-capturing method implemented by the image-capturing device  1 . 
     RGB-IR Image Sensor. 
     As may be seen in  FIGS. 3 and 4 , each optical filter  171 ,  172 ,  173 ,  174  of the array  170  of optical filters of the image sensor  9  is placed facing one of the photosensitive pixels  21 ,  21 ′ of the matrix array  19 , so as to filter electromagnetic radiation coming from the field of view  15  of the image-capturing device  1  (radiation that was collected by the optical system  10 ), before this radiation reaches the photosensitive pixels  21 ,  21 ′ in question. Each photosensitive pixel  21 ,  21 ′ thus receives one portion of the electromagnetic radiation, which portion has been filtered by the optical filter  171 ,  172 ,  173 ,  174  with which it is associated (this optical filter forms a filtering element, as regards the electromagnetic radiation that it receives). 
     The first optical filters  171  are each able to transmit a first portion of the electromagnetic radiation, which portion is located in a given wavelength range of the infrared. In the embodiments described here, the first optical filters  171  mainly transmit wavelengths located in this wavelength range of the infrared. This wavelength range mainly lies beyond 700 nanometers. It may for example extend from 700 nanometers to 1100 nanometers. 
     As regards the second optical filters  172 ,  173 ,  174 , they are each able to transmit at least one component of a second portion of the electromagnetic radiation, which portion is located in the visible, between 400 nanometers and 700 nanometers. 
     In the example described here, the second optical filters  172 ,  173 ,  174 , mainly transmit wavelengths located between 400 and 700 nanometers. As a variant, however, they could transmit both wavelengths located in the visible and wavelengths located in the infrared. 
     The second optical filters  172 ,  173 ,  174  here comprise red optical filters  172 , green optical filters  173  and blue optical filters  174 . The terms “red”, “green” and “blue” are used with their common meaning. The values of the red, green and blue passbands given below are given by way of non-limiting example. 
     The red optical filters  172  here have a red passband comprised mainly between 550 nm and 700 nm. The component of the second portion of the electromagnetic radiation which they transmit therefore mainly consists of wavelengths comprised between 550 nm and 700 nm. 
     As for the green optical filters  173 , they here have a green passband mainly comprised between 450 nm and 650 nm, while the blue optical filters  174  have a blue passband mainly comprised between 400 nm and 550 nm. 
     The various optical filters  171 ,  172 ,  173 ,  174  of the array of optical filters are arranged with respect to one another so as to form a pattern  175  that is repeated regularly so as to form the array  170  of optical filters. This pattern  175  here comprises four optical filters, in the present case: one of the first optical filters  171 , one of the red optical filters  172 , one of the green optical filters  173 , and one of the blue optical filters  174 . These four adjacent optical filters form a square. As shown in the figures, the array  170  of optical filters is therefore comparable to a so-called “Bayer” array one of the green optical filters of which has been replaced with an element that is transparent in the infrared. As a variant, the pattern of optical filters, which pattern is repeated a plurality of times to form the array of optical filters, could be formed differently (for example so as to comprise more green optical filters than red or blue optical filters). 
     As regards the photosensitive pixels  21 ,  21 ′ of the matrix array  19 , they comprise:
         the first photosensitive pixels  21 , each arranged so as to capture the first portion of the electromagnetic radiation transmitted by one of the first optical filters  171 , and   the second photosensitive pixels  21 ′, each arranged so as to capture the red, green or blue component transmitted by one of the second optical filters  172 ,  173 ,  174 .       

     Each of the first and second photosensitive pixels  21 ,  21 ′ produces, via the photoelectric effect, an electric charge or an electric voltage that depends on the power of the portion of the electromagnetic radiation that it receives. 
     In the embodiment described here, the semiconductor substrate of the matrix array  19  of photosensitive pixels  21 ,  21 ′ is made of (suitably doped) silicon. The sensitivity of the photosensitive pixels in the infrared range is thus limited to the near-infrared range: the sensitivity range of the first photosensitive pixels  21 , equipped with the first optical filters  171 , is here comprised between 700 nanometers and 1100 nanometers. 
     The image sensor  9  is for example a CMOS image sensor (CMOS being the acronym of complementary metal-oxide semiconductor) or CCD image sensor (CCD being the acronym of charge-coupled device). 
     With respect now to the computer  12 , it comprises at least a processor and an electronic memory. The computer  12  here comprises:
         a processing module  13 , configured to receive the various image signals si 1 , si 2  delivered by the preprocessing module  11 , and to process these signals in order to produce the infrared image  31  and the color image  35 , and   a control module  14 , for controlling the various exposure parameters of the image-capturing device  1  and for commanding the image sensor  9  and the preprocessing module  11  to take the one or more series of shots mentioned above.       

     II. Image-Capturing Method 
     The computer  12  may be programmed so that the image-capturing device  1  implements an image-capturing method according to a first, a second or a third embodiment, corresponding to  FIG. 6 , to  FIG. 7  and to  FIG. 8 , respectively. Each of these figures shows the main steps executed by the computer  12  during the image-capturing method. 
     As explained above, whatever the embodiment in question, the method in particular comprises:
         a series of shots comprising at least the first and second shots mentioned above, these shots being tailored to the ambient brightness in the infrared and to the ambient brightness in the visible, respectively, and   composition of the infrared image  31  and of the color image  35 , on the basis of the image signals si 1 , si 2  obtained in these first and second shots.       

     The way in which this series of shots is taken will first be described. Composition of the infrared image  31  and of the color image  35  is then described, before the way in which the exposure parameters are controlled in these shots is presented. 
     Series of Shots 
     In the first and third embodiment ( FIGS. 6 and 8 ), the computer  12  commands the image sensor  9  and the preprocessing module  11  to take shots in series of two shots, each series comprising the first and second shots mentioned above. 
     The computer  12  commands the first shot in steps b1) and c1) mentioned above. This first shot comprises, in addition to the acquisition of the pixel signals sp 1  produced by the photosensitive pixels  21 , 21 ′, multiplication of these pixel signals sp 1  by the first gain value g 1  then their conversion to a digital format to obtain a digital first image signal si 1  (which collates all the pixel signals, amplified or attenuated, then converted into digital format). 
     In the same way, the computer  12  commands the second shot in steps b2) and c2). This second shot makes it possible to obtain a second image signal si 2 , which collates all the pixel signals sp 2  produced in the second image capture, after multiplication by the second gain value g 2  and conversion to digital format. 
     In the second embodiment ( FIG. 7 ), the computer  12  commands the image sensor  9  and the preprocessing module  11  to take shots in series of four shots, each series comprising the first and second shots described above, as well as a third and fourth shots. 
     The computer  12  commands the third shot in the following steps, steps b3) and c3): 
     b3) commanding the image sensor  9  to perform a third image capture, during which each of the first and second photosensitive pixels  21 ,  21 ′ delivers a pixel signal sp 3  representative of the electric charge or of the electric voltage that it produces, this electric charge or voltage having been accumulated by the photosensitive pixel  21 ,  21 ′ during a third integration time ti 3 , and
 
c3) commanding the preprocessing module to multiply each of the pixel signals sp 3  produced in step b3) by a third gain value g 3 , then to convert these (amplified or attenuated) pixel signals to digital format to obtain a digital third image signal si 3 .
 
     The third shot is taken with a set of exposure parameters that is different and complementary to that used for the first shot. Specifically, this third shot is taken with the aim of then combining the third image signal si 3  with the first image signal si 1  in order to increase the dynamic range of brightness values of the infrared image  31 , and thus of ensuring not only that the darkest regions of this image are not underexposed, but also that the brightest regions of this image are not overexposed. 
     In the present case, the third exposure time ti 3  and the third gain value g 3  are such that the product ti 3 ·g 3  (product of the third integration time ti 3  multiplied by the third gain value g 3 ), which corresponds to an effective exposure time of the third shot, is different from the product ti 1 ·g 1  (product of the first integration time ti 1  multiplied by the first gain value g 1 ). 
     Comparably, the computer  12  commands the fourth shot in the following steps, steps b4) and c4): 
     b4) commanding the image sensor  9  to perform a fourth image capture, during which each of the first and second photosensitive pixels  21 ,  21 ′ delivers a pixel signal sp 4  representative of the electric charge or of the electric voltage that it produces, this electric charge or voltage having been accumulated by the photosensitive pixel  21 ,  21 ′ during a fourth integration time ti 4 , and
 
c4) commanding the preprocessing module to multiply each of the pixel signals sp 4  produced in step b4) by a fourth gain value g 4 , then to convert these (amplified or attenuated) pixel signals to digital format to obtain a digital fourth image signal si 4 .
 
     The fourth shot is taken with a set of exposure parameters that is different and complementary to that used for the second shot. Specifically, this fourth shot is taken with the aim of combining the fourth image signal si 4  with the second image signal si 2  to increase the dynamic range of brightness values of the color image  35 , and of ensuring not only that the darkest regions of this image are not underexposed, but also that the brightest regions of this image are not overexposed. More precisely, the fourth exposure time ti 4  and the fourth gain value g 4  are such that the product ti 4 ·g 4  (product of the fourth integration time ti 4  multiplied by the fourth gain value g 4 ) is different from the product ti 2 ·g 2  (product of the second integration time ti 2  multiplied by the second gain value g 2 ). 
     Within each image signal si 1 , si 2 , si 3 , si 4 , a (numerical) brightness value is associated with each photosensitive pixel  21 ,  21 ′ of the image sensor  9 , this value being representative of the power of the portion of the electromagnetic radiation captured by this photosensitive pixel, i.e. the portion accumulated during the exposure time ti 1 , ti 2 , ti 3 , ti 4  employed in the corresponding image capture. 
     As regards the three embodiments shown in the figures, in each image capture (whether it is a question of the first, second, third or fourth), both the first photosensitive pixels  21  and the second photosensitive pixels  21 ′ accumulate the electric charges or voltages that they produce, during the same integration time ti 1 , ti 2 , ti 3 , ti 4  (during the integration time corresponding to the image capture in question), then deliver the pixel signals mentioned above. 
     In other embodiments provision could however be made for the first image capture and the second image capture to be carried out via a single exposure of all of the image sensor  9 , during which exposure the first photosensitive pixels  21  would accumulate the electric charges or voltages that they produce during a time equal to the first integration time ti 1 , while, in parallel, the second photosensitive pixels  21 ′ would accumulate the electric charges or voltages that they produce during a time equal to the second integration time ti 2  (i.e. using a technique of dual exposure of a single image capture comparable to single-shot HDR techniques). 
     In any case, when the various image captures in the series of shots are taken at different successive times, these times are preferably close to one another, for example being separated pairwise by less than 0.3 seconds, or even less than 0.1 seconds. This allows the obtained infrared image  31  and color image  35  to both correspond to the same situation and attitude of the driver, even in the event of movement of the latter. 
     Composition of the Infrared Image and of the Color Image 
     In the three embodiments described here, the infrared image  31  is formed from a matrix array of image pixels  33  ( FIG. 5 ) associated with the various photosensitive pixels  21 ,  21 ′ of the image sensor  9 . The computer  12  is programmed to implement an interpolation algorithm that makes it possible to form a “complete” infrared image  31 , even though only one photosensitive pixel  21  in four captures infrared radiation, in the image sensor  9 . In other words, the infrared image  31  here comprises as many image pixels  33  as the image sensor  9  comprises photosensitive pixels, first and second photosensitive pixels  21 ,  21 ′ taken together. 
     In step d1), the computer  12  composes the infrared image  31 , in particular from the brightness values that, in the first image signal si 1 , are associated with the first photosensitive pixels  21 . For each image pixel  33  of the infrared image  31 , the computer determines an (interpolated) brightness value, depending on the brightness values of the image signal si 1  that are associated with the first photosensitive pixels  21  that, in the image sensor  9 , are closest to a position corresponding, in this sensor, to the image pixel  33  in question. 
     The color image  35  is also formed from a matrix array of image pixels (not referenced in  FIG. 5 ) associated with the various photosensitive pixels  21 ,  21 ′ of the image sensor  9 . Just as for the infrared image, the computer  12  is here programmed to implement an interpolation algorithm that makes it possible to form a “complete” color image  35 , even though only one photosensitive pixel  21  in four captures radiation located in the aforementioned red or green or blue passband. In other words, the color image here comprises as many image pixels as the image sensor  9  comprises photosensitive pixels, first and second photosensitive pixels  21 ,  21 ′ included. 
     The color image  35  may, as here, comprise three monochromic channels, i.e. a red channel  37 , a green channel  39  and a blue channel  41 , respectively. Each of these channels is a grayscale image (each image pixel of the channel in question has a brightness value, but no hue or chrominance value) associated with the color of the channel in question and of same size as the color image  35  (i.e. comprises the same number of image pixels). 
     In step d2), the computer  12  composes:
         the red channel  37  of the color image  35  from the brightness values that, in the second image signal si 2 , are associated with the second photosensitive pixels  21 ′ located facing the red optical filters  172 ,   the green channel  39  of this image from the brightness values of the second image signal si 2  that are associated with the second photosensitive pixels  21 ′ located facing the green optical filters  173 , and   the blue channel  41  of this image from the brightness values of the second image signal si 2  that are associated with the second photosensitive pixels  21 ′ located facing the blue optical filters  174 .       

     To generate the color image  35 , the computer  12  may for example make a combination the brightness values of the red channel  37 , of the green channel  39  and of the blue channel  41 , for each image pixel, in order to obtain, for the “color” image pixel in question:
         an overall brightness value for the visible range (for example equal to the average of the luminosities of the various channels), and   two complementary chrominance values representative of the chromatic properties of the captured light, at the point in question of the image sensor  9 .       

     In other embodiments, the infrared image and/or the color image could correspond to raw images, i.e. images obtained without interpolation. In this case, the number of image pixels of the infrared image, for example, would be equal to the number of said first photosensitive pixels rather than being equal to the total number of photosensitive pixels of the image sensor. 
     In the first embodiment ( FIG. 6 ), in step d1), the computer  12  composes the infrared image  31  on the basis of the first image signal si 1 , without taking into account the second image signal si 2  or any other image signal. Furthermore, in step d2), the computer  12  composes the color image  35  only on the basis of the image signal si 2 , without taking into account the first image signal si 1  or any other image signal. 
     In the second and third embodiments, the computer also implements a HDR imaging technique. 
     In the present case, in the second embodiment ( FIG. 7 ), in step d1), the computer  12  composes the infrared image  31  taking into account both:
         brightness values that, in the first image signal si 1 , are associated with the first photosensitive pixels  21 , and   brightness values that, in the third image signal si 3 , are associated with the first photosensitive pixels  21 .       

     The brightness values of the first image signal si 1  (which correspond to the first integration time ti 1 ) are combined with those of the third image signal si 3  (corresponding to the third integration time ti 3 ) so that, in the end, the dynamic range of the brightness values of the infrared image  31  is higher than would be possible with a single shot. 
     By way of example, the third shot is considered here to correspond to an effective exposure time shorter than that used for the first shot, allowing a suitable exposure of the high-brightness regions of the image to be obtained (whereas the first shot corresponds to an intermediate effective exposure, allowing a suitable exposure of the intermediate- and low-brightness regions of the image to be obtained). In other words, the effective exposure time ti 3 ·g 3  is here considered to be shorter than the effective exposure time ti 1 ·g 1  (and, therefore, in the case where the gain values g 1  and g 3  are equal, the third integration time ti 3  is shorter than the first integration time ti 1 ). 
     Before the brightness values of the third image signal si 3  are combined with those of the first image signal si 1 , these brightness values are scaled, i.e. made comparable, in order to take into account the difference in exposure time between the first and third shots. To do this, each brightness value of the third image signal si 3  may for example be multiplied by a first scaling coefficient equal to (ti 1 ·gi 1 )/(ti 3 ·gi 3 ) (i.e. equal to the product of the first integration time ti 1  multiplied by the first gain value g 1 , divided by the product of the third integration time ti 3  multiplied by the third gain value g 3 ), in order to obtain a scaled third image signal, si 3 ′. 
     The computer  12  may then, for example:
         compose a first version of the infrared image,  31 ′, on the basis of the first image signal si 1 , as explained above (at the start of the section relative to composition of the infrared image and of the color image),   in the same way, compose a second version of the infrared image,  31 ″, on the basis of the scaled third image signal si 3 ′, and   fuse the first and second versions of the infrared image  31 ′,  31 ″ to obtain the final version of the infrared image,  31 .       

     This fusion may for example be carried out by computing, image pixel by image pixel, an average between the first version  31 ′ and the second version  31 ″ of the infrared image. 
     This fusion may also be carried out by assigning to each image pixel  33  of the infrared image  31  a brightness value:
         equal to the brightness value of the corresponding image pixel  33 ′ (i.e. of the image pixel located at the given position) of the first version of the infrared image  31 ′, when the latter brightness value is below a given brightness threshold, and   otherwise equal to the brightness value of the corresponding image pixel  33 ″ of the second version of the infrared image  31 ″.       

     This brightness threshold may for example be slightly lower than the brightness value corresponding to an over-saturation (overexposed pixel) in the first version of the infrared image  31 ′. It may for example be comprised between half of this over-saturation value, and this over-saturation value (this over-saturation value being equal for example to 255 when the brightness value of an image pixel of the first version of the infrared image is represented by an integer number coded on 8 bits). 
     The fusion of the first and second versions of the infrared image prevents, as regards the infrared image  31  obtained in the end, not only the dark regions and the regions of intermediate brightness of this image from being underexposed (i.e. these regions are not under-saturated, i.e. of a brightness value equal to 0), but also the brightest regions of this image from being overexposed (i.e. over-saturated). 
     Different ways of combining the first and third image signals si 1  and si 3  to obtain the infrared image  31  may be envisioned, in particular if the third shot corresponds to an effective exposure time ti 3 ·g 3  that is longer than the effective exposure time ti 1 ·g 1  of the first shot (instead of being shorter than the latter). 
     In this second embodiment, in step d2), the computer  12  composes the color image  35  taking into account both:
         brightness values that, in the second image signal si 2 , are associated with the second photosensitive pixels  21 ′, and   brightness values that, in the fourth image signal si 4 , are associated with the second photosensitive pixels  21 ′.       

     The color image  35  is obtained by combining the second and fourth image signals si 2 , si 4  using an HDR imaging technique comparable to the one presented above with respect to step d1). 
     By way of example the red channel  37  of the color image  35  may be obtained in an identical manner to the way presented above with respect to the infrared image  31 , but based on the brightness values that, in the image signals si 2  and si 4 , correspond to the red optical filters  172 , instead of based on the brightness values that, in the image signals si 1  and si 3 , correspond to the first optical filters  171 , which are transmissive in the infrared. Furthermore, likewise, the green channel  39  and blue channel  41  of the color image  35  may be obtained in the same way as the infrared image  31 , but based on the brightness values that, in the image signals si 2  and si 4 , correspond to the green and blue optical filters  173 ,  174 , respectively, instead of based on the brightness values that, in the image signals si 1  and si 3 , correspond to the first optical filters  171 . 
     In the third embodiment ( FIG. 8 ), the infrared image  31  and the color image  35  are obtained by implementing an HDR imaging technique, directly on the basis of the first and second image signals si 1  and si 2 , here without taking into account any other image signals. 
     This third embodiment takes advantage of the fact that the first and second shots are generally taken with different effective exposure times ti 1 ·gi 1  and ti 2 ·gi 2 , since the ambient brightness in the visible range is generally different from that in the infrared range. 
     After these two shots, the computer  12  therefore has available to it the two image signals si 1  and si 2 , which here both comprise brightness values acquired by the “infrared” first photosensitive pixels  21 , and which were obtained with two generally different effective exposure times. The two image signals si 1  and si 2  therefore together allow the infrared image  31  to be composed in such a way that it has a high dynamic range as regards brightness values, without it being necessary to take additional shots. Furthermore, these two image signals likewise allow the color image  35  to be composed in such a way that it has a high dynamic range as regards brightness values, without it being necessary to take additional shots. 
     In this third embodiment, the infrared image  31  is for example composed as was described above with respect to the second embodiment, but based on the first and second image signals si 1  and si 2 , instead of based on the first and third image signals si 1  and si 3 . Thus, before being combined with the first image signal, the second image signal si 2  is for example multiplied by the quantity (ti 1 ·g 1 )/(ti 2 ·g 2 ), so as to make its brightness values comparable to those of the first image signal si 1 . 
     Furthermore, the color image  35  is for example composed as was described above with respect to the second embodiment, but based on the second and first image signals si 2  and si 1 , instead of based on the second and fourth image signals si 2  and si 4 . 
     Control of the Exposure Parameters 
     As already indicated, whatever the embodiment in question, the computer  12  determines the first exposure time ti 1  and/or the first gain value g 1  employed in steps b1) and c1) depending on the ambient brightness in the infrared range determined in step a1). The computer  12  also determines the second exposure time ti 2  and/or the second gain value g 2  depending on the ambient brightness in the visible range determined in step a2). 
     The way in which the ambient brightness in the infrared range and in the visible range is determined is presented below first, and the way in which the exposure parameters are determined is then described. 
     Determination of the Ambient Brightness in the Infrared Range and in the Visible Range: Steps a1) and a2). 
     The ambient brightness in the infrared range is representative of the power of infrared electromagnetic radiation coming from an environment of the image-capturing device  1 , in the present case coming from the field of view  15  of this device, and received by a brightness sensor. In the three embodiments described here, this brightness sensor is the image sensor  9 . 
     Likewise, the ambient brightness in the visible range is representative of the power of visible electromagnetic radiation coming from an environment of the image-capturing device  1 , in the present case coming from the field of view  15  of this device, and received by a brightness sensor, which once again takes the form of the image sensor  9 . 
     In the three embodiments described here, the ambient brightness in the infrared range is determined, in step a1), from the infrared image  31  produced in a previous execution of step d1), and therefore corresponding to a first shot taken previously by the image sensor  9 . 
     In the present case, the computer  12  determines the ambient brightness in the infrared range depending on the brightness values of at least some of the image pixels  33  of this infrared image  31  previously acquired by the image-capturing device  1 , for example by computing an average of the brightness values of these image pixels  33 . The ambient brightness in the infrared range is then representative of an average-brightness level in this infrared image  31 , which level is called the first brightness level and denoted L IR  below. 
     The average in question may be of all the image pixels  33  of the infrared image  31 , or be of only certain of these image pixels, these image pixels being located in a region of interest of the image, for example a region corresponding to the image of the face of the driver  3 . 
     Moreover, provision may be made for this average to take into account only those brightness values of the image pixels that meet a given criterion, for example those that are below a brightness threshold value. This threshold value may for example be equal to the brightness threshold mentioned above (threshold employed in the combination of the first and third image signals si 1  and si 3 ). The first brightness level Lu is then representative of the average brightness of the regions of low brightness and of intermediate brightness of the infrared image  31 . The latter case is particularly well suited to the second embodiment, since, in the case considered above (ti 3 ·g 3 &lt;ti 1 ·g 1 ), the first shot aims to obtain an exposure suitable for dark regions and regions of intermediate brightness. 
     As regards the ambient brightness in the visible range, it is determined, in step a2), from the color image  35  produced in a previous execution of step d2), and therefore corresponding to a second shot taken previously by the image sensor  9 . 
     The computer  12  determines this ambient brightness in the visible range depending on the brightness values of at least some of the image pixels of this color image  31  previously acquired by the image-capturing device  1 , for example by computing an average of the brightness values of the image pixels of this image. The ambient brightness in the infrared range is then representative of an average brightness level in the previously acquired color image  35 , which level is called the second brightness level and denoted L V  below. Just as with the first brightness level Lm, provision may be made for the average in question to be of all the image pixels of the color image or of certain of these image pixels only and/or for this average to take into account only those brightness values of the image pixels that meet a given criterion. 
     The control of the exposure parameters as such, which is carried out on the basis of these first and second brightness levels L IR  and L V , will now be presented. 
     For the sake of the simplicity of the description, the calculator  12  is here considered to determine, or in other words adjust, the first exposure time ti 1  and the second exposure time ti 2  depending on the ambient brightness L IR  in the infrared range and on the ambient brightness L V  in the visible range, respectively, whereas the first and second gain values g 1  and g 2  are set, and for example equal to each other. It will however be understood that the computer could, comparably, be programmed to determine (to adjust) the first and second gain values g 1  and g 2  depending on the aforementioned brightness levels, while keeping the first and second integration times ti 1  and ti 2  constant (for example at values low enough that the obtained images remain sharp even in the event of movement of the head of the driver  3 ). The computer could also be programmed to determine both these gain values and these exposure times depending on the aforementioned brightness levels, which are dependent on the aforementioned ambient brightnesses. 
     In the three embodiments described here, the computer  12  determines the first integration time ti 1 , employed in the first shot, depending on a first difference el between, on the one hand:
         a first target value L IR, O  of medium brightness, and, on the other hand,   the average first brightness level L IR  in the infrared image  31  produced in the previous execution of step d1).       

     More precisely, the computer corrects, or in other words updates, the first integration time ti 1  on the basis of the first difference ε 1 . This correction is made so as to gradually bring the first brightness level La to the first target value L IR, O , during repetitions of all of steps a1), b1), c1) and d1). This correction may consist for example in adding a corrective term to a previous value of the first integration time ti 1 , this corrective term being proportional to the first difference ε 1  (proportional correction). More generally, this correction consists in automatically controlling the first brightness level L IR  to the first target value L V, O . This automatic control may in particular be proportional, proportional-integral or proportional-integral-derivative (PID). Such an automatic control of the exposure conditions is sometimes called automatic exposure control (AEC) in the specialist literature. 
     The first target value L IR, O  may correspond to the average brightness in an image considered to be suitably exposed, the first target value then for example being comprised in a given interval extending from one quarter to three quarters of the maximum brightness value that may be associated with an image pixel. 
     As regards the second integration time ti 2  employed in the second shot, it is here determined depending on a second difference ε 2  between, on the one hand:
         a second target value L V, O  of medium brightness (for example equal to the first target value L IR, O ), and, on the other hand,   the average second brightness level L V  in the color image  35  produced in the previous execution of step d2).       

     The way in which the second integration time ti 2  is determined depending on the second difference ε 2 , and the way in which the first integration time ti 1  is determined depending on the first difference ε 1  (in order to bring these differences to zero) are here identical. 
     In the case of the second embodiment ( FIG. 7 ), the computer  12  also determines the third and fourth integration times ti 3  and ti 4  and the third and fourth gain values g 3  and g 4  employed in the third and fourth shots. 
     As above, for the sake of the simplicity of the description, the computer  12  is here considered to adjust the third and fourth exposure times ti 3  and ti 4  depending on the brightness conditions in the environment of the image-capturing device  1  or depending on other shot conditions, whereas the third and fourth gain values g 3  and g 4  are set (for example equal to each other, and possibly equal to the gain values g 1  and g 2 ). It will however be understood that the computer could be programmed to also determine (to adjust) the third and fourth gain values g 3  and g 4  depending on these brightness or shot conditions, or to determine both the exposure times ti 3  and ti 4  and the gain values g 3  and g 4  depending on the conditions in question. 
     The third integration time ti 3  may be determined by the computer  12  depending on the first integration time ti 1 , for example so as to be equal to the product of the first integration time ti 1  multiplied by a constant coefficient. If the third shot is intended to obtain a non-overexposed exposure of high-brightness regions, the coefficient will be chosen to be lower than 1, and for example equal to one tenth or one quarter (the third integration time ti 3  then being equal to a fraction of the first integration time ti 1 ). The fourth integration time ti 4  may be determined depending on the second integration time ti 2 , in the same way, by multiplying the second integration time ti 2  by a constant coefficient. 
     As a variant, provision may also be made for the third integration time ti 3  to be determined, via automatic control (automatic exposure control), as explained above, so as to bring a specific brightness level, of the second version of the infrared image  31 ″ or of the final version of the infrared image  31 , to a given specific brightness value. This specific brightness level is for example equal to an average of the brightness values of the image pixels  33  of the infrared image  31  obtained in a previous execution of the step d1). Here, this average takes into account:
         only brightness values below a low brightness threshold, when the third shot is intended to obtain a suitable, non-underexposed, exposure of the low-brightness regions of the infrared image (the specific brightness level mentioned above then being representative of the average brightness of the low-brightness regions of this image), or   only the brightness values above a high brightness threshold, when the third shot is intended to obtain a suitable, non-overexposed, exposure of the high-brightness regions of the infrared image (the specific brightness level then being representative of the average brightness of the high-brightness regions of this image).       

     The fourth integration time ti 4  may be determined in a comparable way to the third integration time ti 3 , via automatic control, so as to bring a specific brightness level of the color image  35  to a specific brightness value (this specific brightness level for example being representative of the average brightness of the high-brightness regions of the color image  35 ). 
     Lastly, as may be seen in  FIGS. 6 to 8 , in these three embodiments, the set of steps comprising steps b1), b2), c1), c2), d1), d2) and possibly b3), c3), b4) and c4) (in the second embodiment), is executed a plurality of times in succession. Step a1) is for its part also executed a plurality of times in succession (each time on the basis of the result of a previous execution of step d1)), so as to continuously update the first exposure time and/or the first gain value depending on the ambient brightness in the infrared. Step a2) is for its part also executed a plurality of times in succession (each time on the basis of the result of a previous execution of step d2)), so as to continuously update the second exposure time and/or the second gain value depending on the ambient brightness in the visible. 
     III. Monitoring System for Monitoring the Driver 
     The electronic processing unit  20  of the monitoring system  2  is programmed to determine the level of the driver&#39;s  3  inability to drive I L  based on at least one of the infrared images  31  produced by the image-capturing device  1 . The level of inability to drive I L  comprises for example a drowsiness level of the driver  3  and/or a distraction level of the driver  3  (the level of inability to drive I L  may in particular be a drowsiness level of the driver  3  or a distraction level of said driver). 
     The processing unit  20  may for example be programmed so as to analyze the infrared image  31  in question, or a sequence of infrared images  31  produced by the image-capturing device  1 , in order to identify the face of the driver  3  and/or certain regions of the face of the driver  3 , in particular the regions of the infrared image  31  corresponding to the eyes of the driver  3 . The processing unit  20  may then determine the drowsiness level of the driver  3  by measuring the duration and/or the frequency of the blinks of the eyes of the driver  3 , which will have been identified beforehand in the infrared image  31 . 
     The processing unit  20  may determine the distraction level of the driver  3  on the basis of a posture of the head of the driver  3 , which is deduced from the infrared image  31 , and on the basis of the variation in this posture overtime. 
     The processing unit  20  may also evaluate (via analysis of the infrared image  31 , or of a sequence of infrared images  31 ) the gaze direction of the driver  3  or the variation in this gaze direction over time, and use this gaze direction or its variation to determine the distraction level and/or the drowsiness level of the driver. 
     The processing unit  20  may also evaluate (via analysis of the infrared image  31 , or of a sequence of infrared images  31 ) the diameter of the pupil of at least one eye of the driver  3  (and specifically variations in this diameter), and use it to determine the driver&#39;s distraction level and/or drowsiness level. 
     The processing unit  20  may be programmed, when determining the level of the driver&#39;s inability to drive I L , to also take into account one or more color images  35  delivered by the image-capturing device  1 . 
     The color image  35  may be used in other applications. 
     The computer  12  may for example transmit the color image  35 , or a sequence of color images  35 , to a telecommunications module  43  of the vehicle  5 . This telecommunications module  43  is configured so as to transmit the color image  35 , or the received sequence of color images  35 , to a remote electronic device, for example a multifunction mobile or a computer, for example via a Wi-Fi transmitter. The color image  35  or the sequence of color images  35  may then be used in the context of a teleconference, a videoconference for example. 
     The computer  12  could also transmit the color image  35 , or the sequence of color images  35 , to a memory of the vehicle  5  for it to be stored therein. 
     Various variants may be implemented in the image-capturing device or in the driver-monitoring system that have been described above. 
     For example, the second image, which is composed from the pixel signals generated by the second photosensitive pixels of the image sensor, could be a monochromic image rather than a color image. The second filter elements of the array of optical filters could moreover all be of the same type (for example all be green filter elements) rather than comprising three different types of optical filters (red, green and blue, respectively). 
     Furthermore, the various functions of the computer could be distributed differently between modules. A higher number of modules could be used, or, on the contrary, one and the same module could execute all of the operations that are executed by the computer and were described above. It should be noted that the term module may designate an electronic circuit, or a portion of an electronic circuit separate from the other modules, or a specific group of instructions stored in the memory of the computer. Moreover, the preprocessing module could be integrated directly into the image sensor. 
     The computer could also be programmed so that the series of shots taken by the image sensor contains a number of shots different from that presented above, for example so that this series contains 3 shots instead of two or four. In this case, the series of three shots would contain the first and second shots described above, and an additional shot, allowing the dynamic range of the brightness values of the infrared image and/or of the color image finally obtained to be increased.