Patent Publication Number: US-2009236505-A1

Title: Multifunctional optical sensor comprising a photodetectors matrix coupled to a microlenses matrix

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of co-pending U.S. patent application Ser. No. 11/533,089, filed Sep. 19, 2006, which claims benefit of European patent application serial number 05425654.0, filed Sep. 19, 2005. Each application is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a multifunctional optical sensor, in particular for automotive use, comprising a matrix of photodetectors of the CCD or CMOS type having a sensitive area divided into sub-areas, which, individually or combined together, are designated to specific functions of monitoring the scene or measuring environmental parameters. 
     The scene monitoring functions include monitoring the scene in front, behind or laterally to the vehicle. The frontal monitoring detects, for example, the presence of a vehicle coming from the opposite direction, the presence of a curve or the movement of the vehicle towards the longitudinal demarcation lines of the lane. The monitoring behind the vehicle can, for example, aid parking maneuvers. The lateral monitoring detects, for example, the vehicles that arrive laterally and that are not visible with the external rear-view mirror, since they are in the so-called “blind angle”. 
     The measurement of environmental parameters comprises, for example, the measurement of fog, rain, window fogging, illumination and solar irradiation conditions. 
     2. Description of the Related Art 
     The documents EP-A-1 418 089 and EP-A-1 521 226 by the same Applicant describe multifunctional optical sensors, but refer to multifunctional integration solutions on matrix of photodetectors of the CCD or CMOS type by means of single aperture optics or matrices of lenses positioned in front of the optical window of the sensor. In the document EP-A-1 521 226, each function is associated to a single lens (or to multiple lenses positioned on different matrices) and said lens is associated to a subgroup of photodetectors. 
       FIG. 1  shows a perspective view of an embodiment of the sensor according to the invention of the document EP-A-1 418 089. Use of single aperture optics limits the possibility of reducing the size of the opto-mechanical system as a whole. Moreover, the complexity of the process for the construction and assembly of the system does not allow significantly to lower the costs of the sensor for large volumes. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an optical sensor of the type defined above which enables to:
         optimise the partition of the matrix: there is more freedom in defining the shape (rectangular, trapezoidal, linear) and the co-ordinates of the sub-areas of the photodetectors matrix associated to each function, the photodetectors used solely for the separation of the sub-areas are reduced or eliminated, the entire sensitive area is used and it is possible to assign different directions and fields of view to photodetectors belonging to the same sub-area/function (as will be illustrated farther on);   miniaturise the sensor from the optics and chip viewpoint: using microlenses matrices, the typical dimensions of single aperture optical systems are eliminated, by optimising the partition of the matrix its format is reduced; high miniaturisation simplifies integration on the vehicle, enabling to insert the optical sensor in the rear-view mirrors, near the roof, in the ceiling lamp, etc.;   simplify image processing: every photodetector or group of photodetectors has its field of view and direction optimised in such a way as to achieve a sort of optical “pre-processing”;   reduce costs thanks to optimised formats of the matrix, low cost microlens fabrication processes, deposition of interference filters on the surfaces of the microlens matrices adjacent to the photodetectors.       

     According to the present invention, said object is achieved by a multifunctional optical sensor having the characteristics set out in claim  1 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention shall now be described in detail with reference to the accompanying drawings, provided purely by way of non limiting example, in which: 
         FIG. 1  shows a perspective view of an embodiment of the sensor according to the invention of the document EP-A-1 418 089; 
         FIG. 2  is a schematic view showing the principle of operation of an optical solution with single aperture; 
         FIG. 3  is a schematic view showing the principle of operation of an optical solution based on microlens matrices; 
         FIG. 4  is a schematic view showing the principle of operation of an optical system based on microlens matrices that constitutes prior art; 
         FIG. 5  is a schematic view showing the principle of operation of the optical system of the present invention according to a first embodiment; 
         FIG. 6  is a schematic view showing the principle of operation of the optical system of the present invention according to a second embodiment; 
         FIGS. 7 and 8  are schematic views illustrating the principle of operation of two variants of the optical system of  FIG. 6 ; 
         FIG. 9  is a schematic view showing the principle of operation of the optical system of the present invention according to a third embodiment; 
         FIG. 10  shows the principle of operation of a microlens matrix with high resolution, not operating with the aid of diaphragms; 
         FIG. 11  is a schematic view showing the spaces of the objects and of the images of two optical systems, constituted by a single aperture optics ( FIG. 11   a ) and by a 2D microlens matrix ( FIG. 11   b ), said optical systems being coupled to 2D photodetector matrices with the same format; 
         FIG. 12  is a variant of  FIG. 11   b;    
         FIG. 13  is a schematic view showing the spaces of the objects and of the images of an optical system constituted by a 1D microlens matrix; 
         FIG. 14  is another variant of  FIG. 11   b;    
         FIG. 15  shows an example of an optical sensor with a matrix of photodetectors with constant dimensions or pitch and microlenses with different field of view on a road scenario; 
         FIG. 16  is an example of application of the optical sensor of  FIG. 15  on a road scenario; 
         FIG. 17  is an example of a matrix of subgroups of photodetectors that frame the same portion of scene or different portions of scene; 
         FIG. 18  is a variant of the use of the diaphragms of  FIG. 17  for optical pre-processing functions; 
         FIGS. 19 and 20  show two examples of partition in sub-areas of the sensitive area of the photodetector matrix, previously described in the document EP-A-1 418 089 by the same Applicant; 
         FIG. 21  schematically shows an optical solution for detecting rain based on a matrix of microlenses with different focal length; 
         FIG. 22  shows an example of light wave guide coupled to the photodetector matrix to perform the function of monitoring the occupant of the vehicle; 
         FIG. 23  shows an example of partition into sub-areas of the sensitive area of the photodetector matrix according to a preferred characteristic of the present invention; and 
         FIG. 24  shows the possible positioning on the vehicle of multifunction sensors for the “blind angle” function according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to a multifunctional optical sensor comprising a matrix of photodetectors of the CCD or CMOS type and a matrix of microlenses, in which each microlens is coupled to a subgroup (cluster) of photodetectors (pixels) or to a single photodetector. The microlenses are grouped in subgroups, each of which, individually or combined with others, is dedicated to a specific function of monitoring the scene or measuring environmental parameters. 
     The present invention is directed, in particular, to the application on a motor vehicle of a multifunctional optical sensor of the type mentioned above, which can be positioned for example:
         in proximity to the windshield, e.g. in the interior rear-view mirror, to carry out, for example, the following functions: measurement of environmental illumination or entry into a gallery, measurement of solar irradiation, detection of the presence of raindrops on the windshield, detection of conditions of internal or external fogging of the windshield, detection of the presence of ice, detection of fog conditions and monitoring the scene in front of the vehicle (vehicle crossing); additional functions which can be integrated in addition or alternatively to the vehicle crossing function are the functions: levelling, curve or lane detection (for commanding adaptive headlights or for Lane Warning), night vision (viewing scene in the near-infrared or NIR), detection of vertical road signs, detection of pedestrians, black box (storing images relating to frontal monitoring in a circular memory buffer which can be used in case of accident);   near the rear window, to carry out the following functions: measurement of environmental illumination, rain/fogging (internal and external), ice, fog, rear monitoring (parking); additional functions which can be integrated in addition or alternatively to rear monitoring are: blind angle monitoring, levelling, lane detection, black box;   in the side mirrors to carry out the functions: measurement of environmental illumination, fog, rear monitoring (blind angle); additional functions which can be integrated in addition or alternatively to blind angle monitoring are: parking, levelling, lane detection, black box;   in the uprights of the windshield for the functions of: user identification, occupant monitoring for the air-bag system.       

     The optical sensor according to the present invention can also be used in other applications, such as: road infrastructures, robotics, domotics, agriculture, etc. 
     The present invention shall now be described in detail with reference to the operating principles of optical systems. 
       FIG. 2  shows the single aperture optical solutions whereon are based the documents EP-A-1 418 089 and EP-A-1 521 226: the lens  8  has a field of view FOV=2·arctan(d/2f) where d=n·d pixel  is the total dimension of the photodetector matrix  12 , d pixel  is the dimension or pitch of the photodetector  12  and f is the focal length of the optical system. 
     In general, to reduce the dimensions of the optical system, and in particular the focal length f by a factor n, microlenses  14  can be used, each coupled to a photodetector  12 , with linear dimension of d/n=d pixel  and focal length f 1 =f/n ( FIG. 3 ). In this case, each of the microlenses  14  has a field of view that coincides with the global field of view, FOV 1 =FOV=2·arctan(d pixel /2f 1 ). 
     To have a global field of view FOV that is the resultant of the individual fields of view of each of the microlenses  14  it is necessary to reduce the size of the active area of the photodetectors  12  in such a way that their dimensions are d pixel /n or equivalently to position diaphragms  18  having aperture with dimension d pixel /n in front of the photodetectors  12 . 
     If the distance between the centres of the diaphragms  18  is different from the dimensions of the microlenses  14 , as shown in  FIG. 4  which constitutes prior art, in front of the photodetectors matrix  12  is positioned a matrix of diaphragms  18  such that any microlens  14  has an IFOV  16  with central direction  20  and a constant angular separation αi between the central directions  20  (α1=α2=α3=α4). The sum of the individual IFOV  16  determines the global field of view FOV. Each microlens  14  can also be separated from the contiguous microlens by a “baffle”  18 ′ whose function is to prevent the radiation coming from a contiguous microlens from reaching the non corresponding photodetector. 
     If the angular separation αi between the central directions  20  is not to be constant, as contained in claim  1  and shown in  FIG. 5 , it is necessary to change the distances between the centres of the diaphragms.  FIG. 5  schematically shows a lateral view of a portion of an optical sensor  10  which comprises a photodetectors matrix  12  and a microlenses matrix  14  in which the distance between the centres of the diaphragms  18  is different from the distance between the centres of the microlenses  14 . Each microlens  14  is associated to a cluster of photodetectors  12  or to a single photodetector. In the example of  FIG. 5 , the microlenses  14  are mutually identical. Each microlens  14  is set to focalise the radiation coming from a portion of solid angle IFOV  16  on the cluster of photodetectors  12  or on the single photodetector  12  associated to the microlens  14 .  FIG. 5  shows a matrix of diaphragms  18  positioned between the photodetectors matrix  12  and the microlenses matrix  14 . The diaphragm matrix  18  enables to select for each photodetector  12  or cluster of photodetectors  12  the portion of solid angle IFOV  16 . 
     A difference with respect to the case of  FIG. 4  is that the angular separation αi between the central directions  20  of the portions of solid angle  16  subtended by the microlenses  14  is not constant. In the example of  FIG. 5 , the angles between the central directions  20  are designated α1, α2, α3, and they are such that α1≠α2≠α3. The reason for this is that the distance between the centres of the diaphragms  18  is not constant. Another difference is that the total FOV is not the sum of the IFOV  16  and therefore the FOV can be sub-sampled. To prevent the radiation coming from a contiguous microlens  14  from reaching the non corresponding photodetector  12 , each microlens  14  can be separated from the contiguous microlens  14  by a “baffle”  18 ′. 
     If the angular separation αi between the central directions  20  is not to be constant, but the distance between the diaphragms  18  is to be equal to the dimensions of the microlenses  14 , as shown in  FIG. 6 , the microlenses must be calculated adding a prismatic component to the spherical component in order to have microlenses operating with optical axis not coinciding their axis of symmetry (off-axis). The IFOV  16  of each microlens will have such central directions  20  that α1≠α 2 ≠α 3 . As in the case shown in  FIG. 6 , the total FOV can be sub-sampled. 
     In order to cover a field of view larger than what is possible with a refractive optical solution as described above, solutions with microlenses of the kind with total internal reflection, reflexive solutions and mixed solutions can be considered. For example,  FIG. 7  shows a microlens  22  operating by total internal reflection associated to refractive off-axis microlenses  14 .  FIG. 8  shows an optical sensor with a reflexive lens  24  associated to refractive off-axis microlenses  14 . 
     An additional possibility for obtaining a non constant angular separation αi between the central directions  20  is to use a matrix of micro-objectives, as shown in  FIG. 9 . The micro-objectives  15  are composed by at least two superposed micro-objectives. The doublet optical solution allows to vary effective focal length (e.f.l.) maintaining the back focal length (b.f.l.) equal for all micro-objectives. In this way, the distance between the photodetectors matrix  12  and the micro-objectives matrix  15  is constant (b.f.l.) whilst the possibility of varying e.f.l. allows to have a non constant separation between the central directions  20 , i.e. α1≠α2≠α3. In this case, the IFOV are varied at the same time. 
     For the optical systems described above, a possible alternative to the use of the diaphragms  18  consists of using a matrix with photo-detectors  12  having smaller size than the diaphragms  18 . Lastly for equal sizes of the sensitive area of the photodetectors matrix  12  it is possible to have either a high resolution photodetectors matrix  12 , or a photodetectors matrix  12  with larger size and hence smaller resolution. As shown in  FIG. 10   a , using a low resolution photodetectors matrix  12 , a diaphragms matrix  18  is used to vary the central direction  20  associated to each microlens  14  (case described in the previous optical solutions). Instead, using a high resolution photodetectors matrix  12 ,  FIG. 10   b , only some photodetectors  12 , corresponding to the central direction  20  of the microlens  14  to be obtained, are activated, and the others are rendered inactive, with no need to use a diaphragms matrix  18 . The advantages of using a high resolution matrix consist of eliminating the diaphragms  18  and being able to reconfigure (also while acquiring the images from the matrix) the active photodetectors in such a way as to change the central directions  20  of the fields of view. The disadvantages are due to the fact that the displacement of the central directions  20  occurs by discrete steps (the minimum pitch is equal to the size of the photodetector  12 ) and not continuously as when using the diaphragms  18 , and that to visualise the images requires pre-processing for addressing the active photodetectors. 
     The microlenses of the optical sensor according to the present invention can be constituted by GRIN (gradient index) material. On the lower plane of some microlens or of some subgroup of microlenses can be deposited a selective interferential coating operating as a filter to transmit only the wavelengths of interest. For some functions, for example, a NIR (near infrared) LED illuminator can be used, the related spectral band has to be selected with respect to the background. 
     The photodetectors matrix is in CCD or CMOS technology, standard or with parallel architecture (pre-processing at the photodetector level). 
     The integration of multiple functions on a photodetectors matrix coupled to a microlenses matrix according to claim  1  is in accordance with the following rules:
         each function is associated to a single microlens or to multiple microlenses, not mutually contiguous, or to a single subgroup of mutually contiguous microlenses or to multiple, not mutually contiguous subgroups of microlenses.   each microlens is associated to a single photodetector or to a subgroup (cluster) of photodetectors;   the contiguous photodetectors able to be associated to a function define a sub-area (ROI or Region Of Interest);   some photodetectors can be used only for separating the sub-areas;   the angular separation between the central directions of the fields of view (IFOV) relating to photodetectors or clusters of adjacent photodetectors is not constant within the matrix.       

     The condition whereby the angular separation between the central directions of the fields of view (IFOV) relating to adjacent photodetectors or clusters of photodetectors is not constant within the matrix occurs in the following cases:
         there are adjacent sub-areas dedicated to as many functions in which the fields of view of the sub-areas are different;   at least one of the functions integrated on the photodetectors matrix is associated to a single subgroup of microlenses which subtends a solid angle FOV, but, thanks to the fact that the central direction of the field of view IFOV of each individual microlens can be established independently, contiguous photodetectors or clusters or photodetectors, associated to the subgroup of microlenses, do not always have mutually adjacent IFOV (this case will be described and illustrated more extensively in the subsequent paragraph “Matrix shape”);   the microlenses of at least one subgroup have different and mutually contiguous fields of view, in such a way as to obtain a different resolution inside the global field of view of said subgroup (this case will be described and illustrated more extensively in the paragraph “Frontal monitoring”);   the microlenses of at least one subgroup have equal but not mutually contiguous fields of view, in such a way as to sample in non continuous fashion the global field of view of said subgroup of microlenses and therefore obtain a different resolution inside the global field of view of said subgroup of microlenses (this case will be described and illustrated more extensively in the paragraph “Frontal monitoring”);       

     The variation in angular separation between the central directions of the fields of view relating to photodetectors or clusters of photodetectors can be obtained:
         modifying the distance between the centres of the diaphragms positioned in front of the photodetector or cluster of photodetectors ( FIG. 5 );   modifying the prismatic component in the case of off-axis microlenses matrices ( FIG. 6 ).       

     The variation in the field of view of an individual microlens can be obtained:
         modifying the diameter of the diaphragms positioned in front of the photodetector or cluster of photodetectors;   using the solution with micro-objectives ( FIG. 9 ).       

     Based on the above rules, additional innovative elements can be identified, which will be described individually hereafter, relating to: 
     1. matrix shape; 
     2. frontal monitoring; 
     3. zoom; 
     4. optical pre-processing; 
     5. matrix partition. 
     Matrix Shape 
       FIG. 11  is a schematic view showing the object and image planes of two optical systems, the first one constituted by a single aperture optics  8  ( FIG. 11   a ) and the second one constituted by a 2D matrix of microlenses  14  ( FIG. 11   b ), said optical systems being coupled to matrices 2D of photodetectors  12 , said matrices having the same format m×n. 
     In  FIG. 11   a  the photodetectors  12  subtend the IFOV according to the laws of geometric optics applied to the lens  8 : adjacent portions of the plane of the objects are subtended in the image plane by mutually contiguous photodetectors  12 . 
     In  FIG. 11   b , which constitutes prior art like the previous  FIG. 11   a , the single aperture lens is replaced by a microlenses matrix  14  and the previous considerations continues to apply: adjacent portions of the object plane are subtended in the image plane by mutually contiguous photodetectors  12 . 
     However, the central direction  20  of the field of view IFOV of each individual microlens  14  can be established independently. Therefore in  FIG. 12  the microlenses  14  are so positioned that the previous rule no longer applies and thus the angular separation between the central directions  20  of the fields of view relating to adjacent photodetectors is not constant within the matrix. However, in this case a pre-processing for addressing the photodetectors is necessary in order to visualise the images. 
     The previous example can be considered as a generalisation of particular cases, two of which are illustrated below. 
     A 2D matrix of m×n photodetectors  12  having a single aperture lens  8  with field of view of x horizontal degrees and y vertical degrees ( FIG. 11   a ) can be redesigned as a 1D linear matrix of m×n photodetectors  12 , in such a way that each microlens  14  associated to the corresponding photodetector  12  has a field of view of x/m horizontal degrees and y/n vertical degrees with central direction  20  such as to cover a portion of the global field of view of x horizontal degrees and y vertical degrees ( FIG. 13 ). 
     This can be useful, for example, when it is necessary to perform the 2D monitoring of a scene and the surface available for integrating the sensor is sufficient only for a 1D linear matrix of m×n photodetectors and not for a 2D matrix of m×n photodetectors. 
     The fact that the central direction  20  of the field of view IFOV of each individual microlens  14  can be established independently, can be used also to optimise the partition of the photodetectors matrix into sub-areas dedicated to specific functions and in particular to exploit the entire sensitive area of the matrix. 
       FIG. 14  shows, by way of example, the case in which there is a need to view a portion of scene with a field of view of x horizontal degrees and y vertical degrees, where x=y, and the sub-area available on the sensor  10 , constituted by a matrix 2D of photodetectors  12 , is rectangular: if the same resolution has to be maintained along the two axes x and y of the object plane, and therefore the same field of view has to be maintained for each microlens  12 , the microlenses can be positioned on the rectangular sub-area as shown in  FIG. 14 . 
     Frontal Monitoring 
     The format of the TV camera, used in systems with single aperture lens for monitoring the scene in front of the vehicle, depends mainly on two parameters: field of view FOV and resolution R needed in the areas of the scene in which some objects have to be discriminated with higher precision (e.g., horizontal signs on the road surface). This means that in the other areas of the scene in which there are no objects of interest the previous resolution R is wholly redundant. 
     For most of the frontal monitoring functions, the format of the camera must be at least CIF (320×256 pixels) or VGA (640×480 pixels). 
     These formats are not compatible with the optical solutions based on micro-optics matrices proposed above, where the size of the photodetector is in the order of tens of microns, i.e. far larger than that of the photodetectors (less than 10 microns) of the standard matrices used today for consumer or automotive applications. The use of photodetectors, with dimensions in the order of tens of microns, combined with high resolution means excessively expanding the total area of the chip and consequently raising fabrication costs. 
     In the case of optical solutions based on micro-optics matrices, it is necessary to design the subgroup of microlenses, dedicated to the frontal monitoring function, so that the fields of view IFOV of the individual photodetectors (or clusters of photodetectors), mutually contiguous, are not kept constant for the whole field of view FOV of the microlenses subgroup, but they are defined on the basis of the resolutions actually required in the different areas of the scene as shown in  FIG. 15 . Consequently, the angular separation between the central directions of the fields of view IFOV of the individual mutually contiguous photodetectors (or clusters of photodetectors) is not constant. 
     This approach enables to define a higher resolution in the point of escape of the images relative to that of the peripheral area, as shown in  FIG. 16 . Proceeding from the point of escape towards the outer edges of the matrix, both the field of view IFOV of the individual photodetectors (or of the photodetectors clusters) and the angular separation between the central direction of the IFOV increase, or else only the angular separation between the central directions of the IFOV increases, whilst the IFOV are instead maintained constant, in order to sample non continuously the portion of the scene that requires a lower resolution. 
     Zoom 
     The frontal scene monitoring functions are manifold (vehicle crossing, Lane Warning, curve detection, vertical signs detection, pedestrian monitoring, etc.). 
     To integrate all these functions on a same photodetectors matrix, coupled with a single aperture lens, it is first of all necessary to evaluate the functional specifications in terms of field of view, minimum and maximum range, resolution of a reference obstacle at the maximum distance. Combining these specifications enables to define the format of the matrix, which will obviously be sufficient for some functions and redundant for others. With this approach, the format of the camera will definitely be greater than VGA. 
     An alternative that reduces the format of the TV camera entails the use of an optical zoom. However, the size and complexity of an optical zoom make it difficult to integrate it with other optical systems dedicated to the environmental parameters measuring functions (note the complexity of the optical sensor shown in  FIG. 1 , according to the invention of the document EP-A-1 418 089). Moreover, the optical zoom increase the fabrication costs of the sensor. 
     If a matrix of microlenses is used instead of single aperture optical systems, the sub-area dedicated to frontal scene monitoring can be optimised, increasing resolution in the areas where details need to be discriminated (horizontal signs, obstacle recognition, etc.) and reducing it in the areas where the necessary information is more qualitative (road edges, horizon, etc.). This is equivalent to processing the images with the optimal resolution, as is made possible by an optical zoom. 
     The solutions for varying resolutions have already been discussed in the previous paragraph “Frontal monitoring”. 
     Optical Pre-Processing 
     The ability to design the microlenses matrix defining the direction and amplitude of the field of view for each of them allows to simplify image processing. 
     A possible optical pre-processing function consists of applying optical filters in order to pre-transform the image for subsequent processing. With a single aperture optics, a high resolution is required even in non significant areas in order to have sufficient resolution to identify some areas of the images. Instead, using different fields of view for each microlens or subgroups of microlenses, it is possible to define the sub-areas of the matrix with appropriate resolution and field of view, in order to simplify the image processing operation. 
     With reference to  FIG. 17 , an additional possibility consists of defining on the sensitive area of the matrix k groups of j photodetectors, each able to create (by means of a single microlens or a matrix of j microlenses) the image of the same portion of scene or of different portions of scene. On each group of j photodetectors are positioned diaphragms with different shapes. When a group of photodetectors frames a portion of scene that matches the shape of the diaphragm, the signal is the highest. This approach can be used, for example, for the Lane Warning function, as shown in  FIG. 17 , in which there are k/2 groups of j photodetectors which view the left part of the road scenario (type 1 region of interest) and k/2 groups of j photodetectors which view the right part of the road scenario (type 2 region of interest). 
     The example shown in  FIG. 12  can now be analysed from a different viewpoint. As explained above, the microlenses  14  are so positioned that adjacent portions of the object plane are not subtended, in the image plane, by mutually contiguous photodetectors  12  and, therefore, the angular separation between the central directions  20  of the fields of view relating to adjacent photodetectors  12  is not constant within the matrix. Based on this general example, it is possible to design k subgroups of microlenses with such field of view as to view k portions of the scene which exactly match the shape of the diaphragms of  FIG. 17 , to select k positions of the horizontal demarcation lines. Said k subgroups of microlenses, however, are positioned horizontally on the photodetectors matrix: the first subgroup starting from the top left corner of the matrix and proceeding rightwards, the second one starting from the end of the first subgroup and proceeding rightwards and so on; reaching the right edge of the matrix, the subsequent line is started. 
       FIG. 18   a  shows the enlargement of both the subgroup of photodetectors relating to an ROI and of the photodetectors actually exposed to the radiation that passes through diaphragm.  FIG. 18   b  shows the positioning of the k th  subgroup of microlenses as described above. 
     The advantages of this solution are: the removal of the diaphragms of  FIG. 17 , the use of a matrix with smaller format, the elimination of any form of pre-processing for addressing the photodetectors, mentioned in the example of  FIG. 12 . 
     Matrix Partition 
     The partition of the sensitive area of the photodetectors matrix can have different configurations according to the number and type of integrated functions. 
       FIG. 19  shows, by way of example, a first possible partition of the sensitive area of the photodetectors matrix, already mentioned in the document EP-A-1 418 089 by the same Applicant. The same functions can be integrated differently according to the inventive elements of the present patent application. 
     The so-called “twilight” function is performed by a sub-area of the matrix that has to measure environmental illumination. The number of photodetectors dedicated to this function can even be reduced to just one and there are no constraints in terms of positioning on the sensitive area of the matrix. According to a preferred characteristic, a central photodetector (or a few photodetectors) is surrounded (are surrounded) by eight or more photodetectors that have different fields of view, the central one(s) larger and the lateral ones smaller, in order to have information both about the intensity of environmental lighting (central photodetector(s)) and about the intensity and direction of solar irradiation (lateral photodetectors). The information about environmental illumination enables automatically to turn on/off the headlights of the vehicles in conditions of poor illumination. The information on the direction of the solar illumination enables to optimise the air conditioning system of the vehicle, e.g. for activating and regulating the air flows of multi-zone air conditioning systems. According to an additional preferred characteristic, some photodetectors are oriented towards the dashboard of the vehicle to measure the radiation directed thereon (diffused radiation on the photodetectors). The photodetectors dedicated to the illumination and solar irradiation function can be positioned separately from each other, i.e. in non contiguous positions. 
     With regard to the fog detection function (based on active technique), the number of photodetectors can even be reduced to just one and there are no constraints in terms of positioning on the sensitive area of the matrix. 
     For the tunnel function, the number of photodetectors can even be reduced to just one and there are no constraints in terms of positioning on the sensitive area of the matrix. According to a preferred characteristic, a photodetector (a few photodetectors) has (have) a frontal field of view of about 20° and a second photodetector (a few photodetectors) a smaller field of view, e.g. about 10°. 
     The sub-area of the sensitive matrix marked with “frontal monitoring” performs the so-called “Lane Warning” function. Preferably, the area of interest (i.e. the area that is used effectively for image processing) is a trapezoid and therefore the number of photodetectors dedicated to this function is reduced from the one described in the document EP-A-1 418 089 by the same Applicant. Preferably, the field of view of the photodetectors are smaller (higher resolution) in the areas of the images in which the lane demarcation lines could be located. This enables to reduce the number of photodetectors dedicated to this functions. 
     The area called “frontal monitoring”, alternatively or in addition to the “Lane Warning” function, can be dedicated to the vehicle crossing detection function. According to a preferred characteristic, the area of interest (i.e. the area that is used effectively for image processing) is a trapezoid and therefore the number of photodetectors dedicated to this function is reduced from the one described in the document EP-A-1 418 089 by the same Applicant. The fields of view of the photodetectors are smaller (higher resolution) in the areas of the image where the potential indicators of the presence of headlights of a crossed vehicle or of taillights of a vehicle that precedes the reference vehicle (the one whereon the sensor is mounted) could be located. According to a preferred characteristic, two subgroups of photodetectors are provided to perform this function: the photodetectors of the first subgroup have fields of view that assure long range monitoring of the scene to detect vehicles that arrive from the opposite lane, and the photodetectors of the second subgroup have fields of view that assure short range monitoring of the scene to detect the vehicles that precede the reference vehicle. 
     According to a preferred characteristic, a set of photodetectors positioned in the unused areas of the sub-area for the frontal monitoring function can be dedicated to lateral monitoring i.e. to the detection of the presence of a curve for commanding the adaptive headlights of the vehicle. 
     The portion of sensitive area called “frontal monitoring” can serve a combination of multiple functions, e.g. Lane Warning, vehicle crossing, curve detection, etc. The sub-area dedicated to such functions is preferably constituted by photodetectors whose microlenses have optimised directions and fields of view: high resolution only in the areas where the objects of interest for the processing algorithms could be located, low resolution in non interesting areas (e.g. the horizon). The result is comparable to the one that would be obtained with an optical zoom. 
       FIG. 20  shows a second example of partition of the sensitive area of the photodetectors matrix, already described in the document EP-A-1 418 089 by the same Applicant. The same functions can be integrated according to the inventive elements of the present patent application. 
     For the rain/fogging function, there are no constraints in terms of positioning on the sensitive area of the matrix. In the solution described in the document EP-A-1 418 089, in order to have the rain drops on a same image plane, the optical axis had to be perpendicular to the windshield. According to the present invention, the use of microlenses allows to maintain the optical axis of the microlenses matrix parallel to the road plane. It is possible to compensate for the different distance between the photodetectors matrix and the windshield whereon the rain drops lie by designing microlenses with different focal length, as shown in the schematic representation of  FIG. 21 . In this figure, the windshield of the vehicle is designated by the number  30 . The number  32  schematically indicates water drops deposited on the outer surface of the windshield. The references  14 ′,  14 ″,  14 ′″ designate microlenses with different focal length positioned in such a way that the respective focalisation points fall on the plane of the photodetectors independently of the different distance of the microlenses relative to the windshield. 
     With reference to  FIG. 22 , the number  36  designates device able to perform the function of monitoring the vehicle occupant. The photodetectors matrix, designated by the reference  38 , is oriented towards the front of the vehicle. Some photodetectors  40 , e.g. positioned in the bottom left and right corner of the matrix  38 , are used to determine the position of the driver and the presence, type and position of the passenger. Since this function does not require optics capable of creating the image of the entire vehicle but only of monitoring and discriminating the presence of passengers, a number of photodetectors equal, for example, to 9 is indicated. Since the interior of the vehicle is positioned to the rear of the active side of the photodetectors matrix  38 , to view the scene an optical system  42  is used, able to receive the image positioned to the rear of the photodetectors matrix. For instance, the optical system  42  can be a wave guide element as shown in  FIG. 22 . Alternatively, the optical system  42  can comprise prismatic elements (not shown). The field of view of the optical system is designated by the reference number  44 . 
       FIG. 23  shows an additional possibility of partition of the useful surface of the photodetectors matrix. This partition enables to integrate a higher number of functions on a matrix with a reduced format (e.g., CIF). 
       FIG. 24  shows the possible arrangement of sensors  10  according to the present invention for performing the function of viewing the blind angle. According to a preferred characteristic of the invention, to perform this function each sensor  10  can have two different fields of view to cover different directions and distances, so that the arriving vehicle crosses the two beams at different times, generating a stepped signal that can be used to signal the danger.