Imaging device and associated monitoring system

Abstract

The invention relates to an imaging device (120), comprising at least one matrix image sensor (130) and an optical system (140) having an optical axis (142) contained in a plane and adapted to receive light rays from a spatial region of a scene to be imaged and having a defined field angle between the optical axis and an axis connecting the spatial region and a point of the matrix image sensor.According to the invention, the effective focal length of the optical system varies in the plane such that the imaging device exhibits an angular distribution curve of the pixels of the matrix image sensor as a function of the field angle, which includes at least one local maximum (1430) contained within an angular region of interest (150, 151), this region comprising light rays with field angles ranging from 30 degrees to 40 degrees. The at least one local maximum is such that the angular region of interest is imaged on a proportion between 8% and 11% of the pixels of the matrix image sensor arranged in the plane. Figure for the abstract: Fig. 1.

Description

Title of the invention: Imaging device and associated monitoring system. Technical field

[0001] The present invention relates generally to an imaging device.

[0002] In particular, the present invention relates to an imaging device comprising at least one matrix image detector and an optical system.

[0003] It also relates to a device for monitoring the occupants of a motor vehicle comprising such an imaging device. Technological background

[0004] To increase the safety of the occupants of a motor vehicle, it is known to equip the vehicle with an occupant monitoring system, particularly for the driver. Using sensors, placed notably at the front of the vehicle's passenger compartment, the driver's behavior can be monitored. Signs of a lack of vigilance on the part of the driver, such as signs of drowsiness or indicators of intoxication, can trigger an alert if necessary.

[0005] For example, occupant behavior can be monitored using an imaging device, which includes a camera comprising a matrix image sensor and an optical system, the optical system imaging the interior of the passenger compartment onto the matrix image sensor. The passenger compartment can also be illuminated by an illumination module included in the occupant monitoring system.

[0006] It is particularly interesting to seek to image not only the driver, but also the passenger compartment as a whole, in order to image the other occupants of the vehicle, but also to have access to visual information outside the passenger compartment (people looking into the passenger compartment, weather conditions, etc.).

[0007] To image the passenger compartment as a whole, cameras equipped with a wide-angle lens, or WFOV lenses (Wide Field of View), can be used. These cameras typically have a field of view extending over a measurement angle of between 140 degrees and 190 degrees.

[0008] However, with the same matrix image sensor, obtaining these extended fields of view comes at the expense of angular resolution at the driver level, since a wider field of view is imaged on the same number of pixels of the matrix image sensor. Image quality suffers in terms of spatial resolution. This results in a decrease in the performance of driver monitoring algorithms.

[0009] To overcome this drawback, it is known to increase the size of the pixel arrays of matrix image sensors associated with WFOV optical systems. However, this solution increases the overall cost of occupant monitoring devices, in addition to requiring an increase in the light intensity of the illumination module in order to preserve the signal-to-noise ratio.

[0010] Furthermore, increasing the size of the matrices leads to an increase in the number of pixels to be processed by the driver monitoring algorithms. One remedy for this is to increase the pixel size, but this degrades the optical resolution of the imaging device.

[0011] However, these proposed solutions negatively affect the performance of algorithms dedicated to monitoring the inhabitants of a passenger compartment. Summary of the invention

[0012] In order in particular to remedy the aforementioned drawback of the prior art, the present invention proposes to advantageously control the optical distortion of the optical system in order to specifically enlarge an area of ​​the passenger compartment of the motor vehicle, for example, an area of ​​the passenger compartment including the face of the driver.

[0013] More particularly, the invention proposes an imaging device, comprising at least one matrix image sensor and an optical system having an optical axis contained in a plane, the optical system being adapted to receive light rays from a scene in order to image the scene on the matrix image sensor, light rays from a spatial region of the scene having a field angle defined in absolute value between the optical axis and an axis connecting the spatial region and a point of the matrix image sensor in which an effective focal length of the optical system is provided to vary in the plane so that the optical imaging device presents an angular distribution curve of the pixels of the matrix image sensor as a function of the field angle, said curve comprising at least a local maximum contained in an angular region of interest, the angular region of interest comprising the light rays contained in the plane,and with a field of view ranging from 30 degrees to 40 degrees, at least one local maximum is such that the angular region of interest is imaged on a proportion between 8% and 11% of the pixels of the matrix image sensor receiving said light rays contained in the plane.

[0014] Advantageously, the invention makes it possible to use the effective focal length, and its link with the incidence position of a light ray on the plane of the matrix image sensor as a function of the field angle of the ray on the optical system, in order to dedicate a relatively larger number of pixels to a restricted part of the field of view, i.e. an angular region ranging from 30 degrees to 40 degrees.

[0015] An angular and therefore spatial resolution, for objects included in the angular region of interest is thus improved, since they are imaged on a larger number of pixels compared to the same objects located outside the angular region of interest.

[0016] Other advantageous and non-limiting (i.e., optional) features of the imaging device according to the invention, taken individually or in all technically possible combinations, are as follows: - The optical system is adapted to collect light rays with a field angle between the optical axis and a limiting field angle greater than or equal to 70 degrees. In other words, the optical system is a "wide-angle" optical system. - The optical system is adapted to collect light rays with a field angle between the optical axis and a limiting field angle greater than or equal to 80 degrees; that is to say, the optical system is a "wide-angle" system. - The proportion is between 9% and 11% of the pixels of the matrix image sensor receiving the light rays contained in the plane. - The proportion is between 9% and 10% of the pixels of the matrix image sensor receiving the light rays contained within the plane. Advantageously, the angular region is imaged on a correspondingly larger number of pixels, in order to increase the angular resolution, and therefore by extension, the spatial resolution, while maintaining sufficient spatial resolution over the rest of the scene to be imaged. The angular region of interest is imaged with a first angular resolution greater than a limit value, for example, 23.8 pixels per degree, while an angular region between the optical axis and the angular region of interest is imaged with a second angular resolution less than 85% of the limit value, for example, less than 20 pixels per degree. This angular region between the optical axis and the angular region of interest includes, for example, field angles between 0 degrees and 10 degrees. The angular distribution curve of the pixels in the matrix image sensor exhibits a unique local maximum for light rays with a field angle between 0 degrees and a limiting field angle of the optical system. This limiting angle corresponds to a field angle beyond which light rays can no longer be collected by the optical system. In other words, the local maximum within the angular region of interest is also a maximum of the curve over the angular range between the optical axis and the limiting field angle of the optical system. - The optical system does not exhibit axial symmetry and where the pixel distribution curve includes a single local maximum corresponding to a global maximum. Thus, the optical system does not exhibit symmetrical behavior with respect to a plane containing the optical axis, allowing for finer control of the incidence heights of light rays on the matrix image sensor. This enables a specific area of ​​the passenger compartment to be imaged with better optical quality than the rest of the cabin. For example, it is then possible to specifically monitor one of the occupants, particularly the driver. In another possible variant, the optical system exhibits axial symmetry; in this case, the pixel distribution curve includes two local maxima arranged symmetrically with respect to a zero field angle. Alternatively, the optical system exhibits symmetrical behavior on either side of the optical system. In this case, two areas of the passenger compartment, for example, the driver and the front passenger, are imaged with improved optical quality.- The optical system includes at least one lens shaped to have a freeform surface (i.e., a freeform lens, or "freeform" lens as it is commonly called), in order to relax the optical design constraints. - The pixel distribution curve is a polynomial curve. This advantageously allows for a continuously varying, smooth effective focal length profile, which facilitates, for example, the manufacturing stage of the optical system. - The pixel distribution curve is, for example, a polynomial curve of order 5.

[0017] The invention also relates to a monitoring system disposed in a passenger compartment of a motor vehicle, the monitoring system comprising an imaging device according to the invention, in which an area of ​​the passenger compartment included in said angular region of interest and located at a distance of between 60 centimeters and 100 centimeters from the imaging device is imaged on said matrix image sensor by the optical system with a first magnification value higher than a second magnification value of an area located outside the angular region of interest.

[0018] The monitoring system advantageously allows particular areas of the passenger compartment to be monitored using greater magnification, for example, in order to increase the performance of an image processing algorithm concerned with these areas.

[0019] The monitoring system also has the following characteristics: - a ratio between the first magnification value and the second magnification value is greater than or equal to 1.32. - the area of ​​the passenger compartment included in the angular region of interest includes a face of an occupant of the vehicle's passenger compartment. - the occupant is a driver of the vehicle.

[0020] Of course, the different features, variants and embodiments of the invention can be combined with each other in various ways insofar as they are not incompatible or mutually exclusive. Brief description of the figures

[0021] In addition, various other features of the invention become apparent from the attached description made with reference to the drawings which illustrate non-limiting embodiments of the invention and where:

[0022] [Fig.1] schematically illustrates a motor vehicle equipped with an occupant monitoring system 12 which includes in particular an imaging device.

[0023] [Fig.2] shows an example for an angular distribution curve of the pixels of the imaging device of [Fig.1].

[0024] [Fig.3] corresponds to a curve of an incidence position on a sensor of the imaging device of [Fig.1] as a function of a field angle, this curve being obtained by integration of the curve of [Fig.2].

[0025] [Fig.4] represents an image of an interior of the motor vehicle of [Fig.1], taken using a state-of-the-art imaging device.

[0026] [Fig.5] represents an image of the interior of the motor vehicle of [Fig.1], taken using the imaging device of [Fig.1].

[0027] [Fig.6] corresponds for a second embodiment to the angular distribution curve of the pixels of the imaging device of [Fig.1].

[0028] [Fig.7] illustrates for a second embodiment, the curve of an incidence position on a sensor of the imaging device of [Fig.1] as a function of a field angle.

[0029] It should be noted that in these figures the structural and / or functional elements common to the different variants may have the same references.

[0030] Various other modifications may be made to the invention within the scope of the annexed claims.

[0031] Figure [Fig. 1] shows a motor vehicle 1, for example, here, a car.

[0032] This motor vehicle 1 has a passenger compartment 10 comprising seats, here five seats, on which occupants can be installed.

[0033] By occupants, we mean for example a driver 110, a front passenger 111, and two rear passengers, or a driver 110 alone, a driver 110 and a front passenger 111, etc.

[0034] This motor vehicle 1 is equipped with an occupant monitoring system 12. The role of this monitoring system 12 is, in particular, to ensure that the occupants of the motor vehicle 1 do not engage in risky behavior.

[0035] The monitoring system 12 is located at the front of the passenger compartment 10 of the motor vehicle, the front being defined with respect to a typical direction of travel of the motor vehicle. For example, the monitoring system 12 can be located at the level of the interior rearview mirror.

[0036] This monitoring system 12 is adapted to image the passenger compartment 10, ideally in its entirety. Thus, the monitoring system 12 includes, in particular, an imaging device 120, an illumination module 122, and a computer 124.

[0037] The computer 124 is configured to analyze images acquired by the imaging device 120, these images being acquired, for example, when an imaged part of the passenger compartment 10 is illuminated by the illumination module 122. Advantageously, the illumination module 122 emits in wavelengths invisible to the human eye, in order to avoid dazzling the occupants, in particular, the driver 110. For example, the illumination module 122 may include a light source emitting in a spectral range from 700 nanometers to 2000 nanometers, for example 940 nanometers.

[0038] The imaging device 120 includes a matrix image sensor 130 and an optical system 140, the assembly being adapted to acquire images of a part of the passenger compartment 10, for example, the part of the passenger compartment 10 which is illuminated by the illumination module 122.

[0039] The matrix image sensor 130 groups together a set of photosensitive pixels arranged in an array, that is to say, a rectangular matrix. In other words, the pixels of the matrix image sensor 130 are arranged in rows along a row axis and in columns along a column axis.

[0040] For example, in a first embodiment, the pixels of the matrix image sensor 130 are arranged in 2560 columns and 1980 rows. The matrix image sensor 130 therefore comprises a total of 2560 x 1980 pixels. In practice, the pixels arranged in columns are numbered from -1280 to 1280, and the pixels arranged in rows are numbered from -990 to 990. Other dimensions of the matrix image sensor 130 are also possible, such as matrices of 2560 pixels by 2560 pixels, or matrices of smaller or larger dimensions. However, advantageously, the matrix image sensor 130 comprises a reduced number of pixels. For example, the matrix image sensor 130 here has more than 2000 pixels and / or fewer than 3000 pixels arranged in columns.

[0041] In the first embodiment, a sensor midpoint is defined by the pixel at row number 0 (zero) and column number 0 (zero).

[0042] Without limitation, the pixels in the first embodiment have a size ranging, for example, from 0.5 micrometers to 5 micrometers. For example, the pixels have a size of 2.25 micrometers.

[0043] The matrix image sensor 130 thus presents a surface, called the total surface, whose physical dimensions are given, approximately, by the product of the number of rows and the pixel size, and the product of the number of columns and the pixel size.

[0044] The pixels of the matrix image sensor 130 are sensitive to at least one spectral range. In particular, when an illumination module 122 is present, the pixels are photosensitive to a spectral range covering at least that of the illumination module. Alternatively, the pixels can be photosensitive to several distinct spectral ranges, notably through the use of optical filters adapted to select certain specific wavelength ranges. For example, the pixels of the matrix image sensor 130 are particularly sensitive to an infrared spectral range and correspond to pixels of an RGB-IR or IR matrix. In this case, the pixels of the pixel matrix are at least photosensitive to light rays emitted in the infrared range. According to a possible embodiment, the imaging system 120 is sensitive only to a spectral range located in the visible spectrum; this is then referred to as an RGB matrix image sensor.

[0045] The optical system 140, also commonly called a lens, comprises at least one optical element adapted to image incident light rays from a scene to be imaged onto the matrix image sensor 130. Here, for example, the scene corresponds to the passenger compartment 10 of the vehicle. Thus, the optical system 140 is adapted to image a region or scene of the passenger compartment 10 corresponding to the field (or field of view) of the optical system, on a surface of the matrix image sensor 130. The optical system 140 is therefore positioned at an appropriate distance from the surface of the matrix image sensor 130.

[0046] The optical system 140 has an optical axis 142, here taken in a broad sense, meaning that the optical axis 142 does not necessarily correspond to an axis of rotational symmetry of the optical system 140, for example, for an eccentric optical system 140, or for a system comprising a freeform surface optical element, or freeform lens. These optical elements are most often referred to by the English term "freeform." For example, the optical axis 142 is defined here as being perpendicular to a surface of the matrix image sensor 130, and passing through its midpoint. Thus, the optical axis 142 is defined in a common way for the imaging device 120, for the matrix image sensor 130, and for the optical system 140.

[0047] Here, for example, it is considered that a light ray incident on the optical system 140 along the optical axis 142 is not deviated from its initial trajectory.

[0048] By optical element, it is understood for example to mean spherical lenses, aspherical lenses, freeform surface lenses, mirrors, etc., that is to say elements adapted to deflect incident light rays.

[0049] The light rays incident on the optical system 140 are characterized by an angular position in the scene imaged by the optical system 140. Indeed, light rays emitted from spatial regions of the scene are then collected by the optical system 140 and imaged on a region of the matrix image sensor, in order to reproduce these spatial regions.

[0050] Thus, for a given region, light rays originating from that region of the scene are associated with an angle, called the field angle. This field angle identifies the angular position of a region in the scene and the light rays originating from it. The value of this field angle is defined, for example, as an algebraic value between the optical axis 142 and an axis connecting the spatial region and a point on the matrix image sensor 130.

[0051] For example, this point of the matrix image sensor 130 corresponds to the midpoint of the matrix image sensor 130, as defined previously. This point of the matrix image sensor 130 coincides with a point of the imaging device 120 and a point of the optical system 140, under the assumption that the imaging device 120 is considered a point with respect to the dimensions of the scene.

[0052] This field angle is defined in algebraic value with respect to the optical axis, that is to say that an angle defined in a positive direction takes positive values, while an angle defined in the opposite direction, i.e. a negative direction, takes negative values.

[0053] Similarly, the field imaged by the optical system, i.e. the scene corresponding to the region of the passenger compartment 10 that can be imaged by the optical system 140, is delimited by two limiting field angles, measured on either side of the optical axis 142. Thus, a field of view of the imaging device 120 is defined on either side of the optical axis 142, and its value extends between the two limiting field angles measured with respect to the optical axis 142.

[0054] In other words, these limiting field angles restrict the incident light rays that can be collected by the optical system 140; that is, only light rays originating from a spatial region of the scene with a field angle less than the absolute value of the limiting field angle can be collected by the optical system 140 and imaged on the sensor. Thus, the imaging device 120, comprising the combination of the optical system 140 and the matrix image sensor 130, collects and images only light rays originating from a portion of the passenger compartment 10. The field of view is usually measured on either side of the optical axis 142. Its total value is denoted Δfov-

[0055] The field of view is, for example, symmetrical on both sides of the optical axis 142, in the case of a spherical lens. In this case, the two limiting field angles take the same absolute value. On the other hand, the field of view is asymmetrical from on either side of the optical axis 142, for other optical elements not exhibiting axial symmetry.

[0056] Only light rays coming from the cabin 10 and having a field angle included in the field of view are imaged by the imaging device 120.

[0057] For example, here, the horizontal field of view is considered. This horizontal field of view is determined in a horizontal plane containing the optical axis 142. This horizontal plane is also parallel to a horizontal direction defined in the reference frame specific to the motor vehicle placed in an operating orientation.

[0058] The horizontal field of view allows observation of the passenger compartment 10 in its width (left to right direction), while the vertical field of view allows observation of the passenger compartment 10 in its height (top to bottom direction). This horizontal field of view is illustrated in [Fig. 1].

[0059] In the following description, particular attention is paid to the horizontal field of view, the value of which is delimited by a total angle &fov- Nevertheless, the properties of the optical system 140 as described below are also applicable to the vertical field of view, the diagonal field of view, etc., i.e. to the fields of view defined for a solid angle observable by the optical system 140.

[0060] In the first embodiment, the optical system 140 is considered to be a wide-angle optical system, also called a WFOV optical system (from the English term, "Wide Field of View"). The field of view then has a value ranging from 140 degrees to 190 degrees. For ease of description, it is assumed in the first embodiment that the field of view is symmetrical on both sides of the optical axis 142, that is to say, that it extends, for example, from 140 to 190 degrees. 2 “ degrees relative to the optical axis 142 on one side of the optical axis 142, and -70 degrees relative to the optical axis 142 on the other side of the optical axis 142. These sides of the field of view located on either side of the optical axis can be designated by the term "half field of view." The field of view itself measures 140 degrees in total. In other words, a first limiting field angle is equal to 70 degrees with respect to the optical axis 142, and a second limiting field angle is equal to -70 degrees with respect to the optical axis.

[0061] For example, the first side may be the one oriented towards the driver's side 110, while the second side is the one oriented towards the front passenger's side 111.

[0062] Alternatively, the field of view can extend over ^fov _ 80 degrees by 2 - relative to the optical axis on one side, and of ^fov -80 degrees relative to axis '2' optics on a second side.

[0063] The optical system 140 is adapted to receive light rays from certain spectral ranges, in particular, light rays within spectral ranges emitted by the illumination module 122. For example, it may be an optical system 140 optimized for infrared radiation, or an optical system 140 optimized for visible light, or even an optical system 140 optimized for a spectral range extending over a portion of the visible and infrared spectrum. Thus, optical properties of the optical system 140 are defined for these spectral ranges.

[0064] In particular, the optical system 140 has an effective focal length, or simply focal length, also called EFL. Here, in the first embodiment, the effective focal length is relatively short, since it is a wide-angle imaging device 120. The effective focal length is, for example, less than 50 millimeters.

[0065] In the first embodiment, the effective focal length of the optical system 140 varies within the field. In other words, when the field angle of the light rays deviates from the optical axis 142, the value of the effective focal length varies. Thus, the optical system 140 exhibits a curve showing the effective focal length as a function of the field angle of the light ray on the optical system 140, i.e., as a function of the angular position of the spatial region of the scene from which the light ray originates.

[0066] The effective focal length as a function of the field angle 0 / is denoted

[0067] A deviation of the point of impact of the light beam on the matrix image sensor 130 with respect to the middle of the matrix image sensor 130 is given by the following formula = f'^tan(0d)

[0068] With hpix the position or deviation in pixels along a horizontal axis on the matrix image sensor 130 relative to its midpoint, the effective focal length as a function of the field angle, and G has a light ray angle corresponding to the light ray with field angle 0, after being deviated following its passage through the optical system 140. This horizontal axis of the matrix image sensor 130 lies in the horizontal plane and coincides with one of the axes of the matrix image sensor 130, for example, with the column axis. It is assumed here that the light rays contained in the horizontal plane are imaged on pixels aligned with the horizontal axis.

[0069] Thus, the effective focal length, in particular the fact that its value varies in the field (i.e. as a function of the field angle), determines the position on the surface of the matrix image sensor 130 where the incident light rays strike.

[0070] This effect is usually known as optical distortion.

[0071] For the imaging device 120, a first curve 144 is defined, corresponding to a curve of the position in pixels along the horizontal plane on the matrix image sensor as a function of the field angle 0 of the light ray, that is to say, as a function of the angular positioning in the field of the optical image system. In other words, this first curve 144 gives, for a given field angle G, a position in pixels along the horizontal axis of the matrix image sensor 130.

[0072] A second curve 143 is defined for the imaging device 120, and corresponds to the angular distribution curve of the pixels of the matrix image sensor 130 as a function of the field angle G,.

[0073] This second curve 143 gives, for a given field angle G{, an angular resolution re in pixels per degree. This second curve 143 translates, for a given field angle 0 / , the number of pixels along the horizontal axis allocated per unit angle. The higher the value of the angular resolution, the better the image quality.

[0074] The first curve 144 is obtained by integrating the second curve 143.

[0075] Examples of the first curve 144 and the second curve 143 are given respectively in Figures 2 and 3.

[0076] In the first embodiment, distortion is used advantageously, for example, to increase spatial resolution on selected angular regions of interest 150, 151.

[0077] For example, in the first embodiment, the angular regions of interest 150, 151 are located on either side of the optical axis 142. In particular, in the first embodiment, the distortion of the optical system 140 is used to image two angular regions of interest 150, 151 arranged symmetrically with respect to the optical axis 142. For example, a first angular region of interest 150, 151 comprises light rays having field angles ranging from 30 degrees to 40 degrees, while a second angular region of interest 151 comprises light rays having field angles ranging from -30 degrees to -40 degrees. These two angular regions of interest 150, 151 are shown in [Fig. 1].

[0078] For light rays having field angles 3( included in the angular regions of interest 150, 151, the effective focal length varies so that the angular distribution curve of the pixels of the matrix image sensor 130 as a function of the field angle Gb ie the second curve 143, presents for the field angles included in the angular region of interest 150, 151 concerned, a local maximum 1430.

[0079] An example of such a second curve 143 is shown in [Fig. 2], in bold solid line. To improve the readability of the curves, these are given on a half of the field of view, and half of the surface of the matrix image sensor 130. It is nevertheless understood that, for the first embodiment, the second curve is symmetrical on both sides of the optical axis 142 (i.e. the field angle of 0 degrees).

[0080] Thus, in Figure 2, the second curve 143 associated with the imaging device 120 according to the invention, has a first local maximum 1430 between field angles 0,- ranging from 30 degrees to 40 degrees (first angular region of interest 150, 151), while a second local maximum 1430 between field angles di ranging from -30 degrees to -40 degrees (second angular region of interest 151) is not shown here.

[0081] Around these angular regions of interest 150, 151, the angular resolution r, as given by the second curve 143 (thick solid line), is greater than over the remaining part of the field of view. For example, in the first embodiment, the angular resolution of the imaging device 120 for field angles ranging from 25 degrees to 40 degrees is greater than 23.8 pixels per degree. In other words, a portion of the field of view, corresponding to the angular region of interest 150, 151, is imaged with an angular resolution of at least 23.8 pixels per degree. For example, the angular resolution could be equal to 25 pixels per degree. This value corresponds to a limiting value of the angular resolution.The rest of the field of view is imaged with an angular resolution of less than 23.8 pixels per degree. For example, at one edge of the field of view, i.e., for field angles between 55 and 70 degrees, the angular resolution is close to 5 pixels per degree. Thus, the edge of the field of view is imaged with an angular resolution less than 55% of the limiting angular resolution value.

[0082] Similarly, an angular region around the optical axis, for example, comprising field angles between 0 degrees and 10 degrees, is imaged with an angular resolution of less than 23.8 pixels per degree. For example, the angular resolution on the angular region around the optical axis is 20 pixels per degree, which is less than 85% of the limiting value for angular resolution. The same applies to an angular region represented by field angles between 0 degrees and -10 degrees.

[0083] Thus, the presence of at least one local maximum 1430 on the second curve 143 means that light rays with a field angle ranging from +30 degrees to +40 degrees are imaged on a proportion of between 8% and 11% of the pixels along the horizontal axis. Advantageously, light rays with a field angle G, ranging from 30 degrees to 40 degrees, are imaged on a proportion of between 9% and 11% of the pixels along the horizontal axis, or even on a proportion of between 9% and 10% of the pixels along the horizontal axis of the matrix image sensor 130. The same is true for light rays with a field angle 6, ranging from -30 degrees to -40 degrees.

[0084] By way of comparison, [Fig. 2] also shows an example of a second curve 145 drawn as a dashed line, for a typical state-of-the-art imaging device. Usually, distortion is corrected on state-of-the-art imaging devices, resulting in a relatively constant second curve 145, the average value of which corresponds to a uniform distribution of pixels per unit angle.

[0085] This average value is given by the following formula:

[0086] m _ ; where m corresponds to the average value of the angular resolution for a HAS typical state-of-the-art imaging device, &fov to the horizontal field of view, and Dy to the pixel size of the matrix image sensor 130 along the horizontal axis, for example the number of columns.

[0087] For a typical state-of-the-art imaging device with similar properties, m = 18.3 pixels per degree, as shown in Figure 2. Thus, on a typical state-of-the-art imaging device, light rays with a field angle 9, ranging from 30 degrees to 40 degrees are imaged on a proportion of about 8% of the pixels along the horizontal axis of the matrix image sensor 130. The same applies to light rays with a field angle d, ranging from -30 degrees to -40 degrees.

[0088] The imaging device 120 described herein advantageously allows for the selective improvement of angular resolution over specific angular regions, while conversely reducing angular resolution over angular regions deemed less critical. This distribution of pixels according to the field of view allows for targeted improvement of image quality, rather than having the same image quality across the entire field of view.

[0089] [Fig.3] illustrates examples of first curves corresponding to the integral of the curves of [Fig.2].

[0090] In particular, a first curve 144 drawn in bold solid lines corresponds to an imaging device 120 according to the invention. Specifically, the first curve 144 has a steeper slope in the angular region of interest 150, 151 compared to the first curve 146 (dashed lines) associated with a typical prior art imaging device. This increased slope means that, for equal parts of the field, field angles 0i delimiting this part are imaged at a greater distance from each other than for a conventional imaging device.

[0091] The first curve 144 and the second curve 143 of the imaging device 120 are, for example, optimized during the optical design phase. This optical design phase takes place upstream of the manufacture of the imaging device 120, and in particular upstream of the manufacture of the optical system 140. The properties of the latter, in particular the number of surfaces, their characteristics, etc., are, for example, optimized, as is customary, taking into account the constraints imposed by a first optimized curve 144, and a second optimized curve 143. Advantageously, the first optimized curve 144 and the second optimized curve 143 take the form of a polynomial function or curve, in order to smooth the surface characteristics of the optical system resulting from the optical design phase.

[0092] This is, for example, a polynomial curve of order 5.

[0093] These curves present numerical values ​​corresponding to the characteristics produced by the desired optical system 140.

[0094] Next, the optical system 140 can be manufactured, for example, by injection molding of an optical-grade plastic material. Such a manufacturing process offers advantages related to the cost and / or ease of manufacturing the optical system 140 as defined during the optical design phase.

[0095] Thus, advantageously, the invention proposes to selectively improve the angular resolution re on parts of the field, i.e., on the angular regions of interest 150, 151, in order to improve image quality there. This improvement in angular resolution is achieved at the expense of the rest of the field. Nevertheless, it is considered that the information contained in the rest of the field does not require as much detail for the proper functioning of the image processing algorithms implemented by the computer 124.

[0096] In particular, in the first embodiment, angular areas of interest coincide with the driver 110 and the front passenger 111, assuming, usually verified in the passenger compartments 10 of motor vehicle 1, that they are located at a distance of between 60 centimeters and 100 centimeters from the imaging device 120.

[0097] The intersection between the angular regions of interest 150, 151 and a frontal plane including the driver 110 and the front passenger 111, horizontally delimits at least one area of ​​the passenger compartment 10. Here, in the first embodiment, it is two areas of the passenger compartment 10. For example, in the present case, it is areas of the passenger compartment where the face of the driver 110 and the face of the front passenger 111 are usually located.

[0098] These areas of the passenger compartment 10 have a higher magnification than the rest of the passenger compartment 10. In particular, in the first embodiment, the areas of the passenger compartment 10 have a first magnification value at least a factor equal to the ratio of the angular resolutions between the areas of the passenger compartment 10 and the rest of the passenger compartment, for example, at least a factor of 23.8 / 18 = 1.32, compared to a second magnification value. This second magnification value is defined as the average magnification in the rest of the passenger compartment 10.

[0099] Figure 4 represents a first image II of a vehicle interior 10 automobile 1 taken using a typical state-of-the-art imaging device included in an occupant monitoring system.

[0100] Fig. 5 represents a second image 12 of the same passenger compartment 10 of a motor vehicle, under identical conditions, but acquired this time by the typical imaging device 120 as described previously.

[0101] Rectangular outlines have been drawn on both figures, around the faces of the driver 110 and the front passenger 111, and are shown on [Fig.5] for comparison.

[0102] For example, the face of driver 110 is imaged on a rectangle 212 of 434 pixels by 347 pixels by the imaging device 120 according to the invention, against a rectangle 211 of 380 pixels by 284 pixels on an imaging device according to the state of the art.

[0103] Similarly, the face of the front passenger 111 is imaged on a rectangle 221 of 585 pixels by 411 pixels, against a rectangle 222 of 500 pixels by 319 pixels.

[0104] Conversely, regions of the passenger compartment 10, deemed less interesting for the proper functioning of the algorithms implemented by the monitoring system 12, are allocated a smaller number of pixels, for example along the horizontal axis.

[0105] This is illustrated, for example, by a rectangular outline drawn in Figures 4 and 5 around the driver's side front window 110, and shown in Figure 5. A rectangle 232 measures 583 pixels by 304 pixels when imaged by the imaging device 120, compared to 448 pixels by 478 pixels for a conventional imaging device. The horizontal extent is thus reduced when the driver's side front window 110 is imaged by the imaging device 120.

[0106] Different embodiments for the imaging device 120 and the monitoring system 12 comprising such a device are also conceivable.

[0107] For example, in another embodiment, as mentioned previously, the imaging device 120 may not have a symmetrical field of view on either side of the optical axis 142. In this case, in particular if the optical system 140 includes an optical element such as, for example, a freeform surface lens, then the first curve and the second curve may not be symmetrical with respect to the optical axis 142 (field angle of 0 degrees).

[0108] This embodiment is illustrated for example in figures 6 and 7. Thus, in this second embodiment, only the face of the driver 110 is imaged with a higher angular resolution compared to the rest of the passenger compartment 10.

[0109] In this case, a part of the passenger compartment, that corresponding to field angles between -70 degrees and 0 degrees (i.e. passenger side), is imaged in such a way The first part, extending from 0 degrees to 70 degrees, is imaged using a conventional imaging device, as described previously. The second part, extending from 0 degrees to 70 degrees, is imaged using a 120-degree imaging device. Thus, the driver's side of the passenger compartment is imaged with higher angular resolution, specifically around the driver's face, while the front passenger side of the passenger compartment is imaged with uniform angular resolution across the field.

Claims

Demands

1. An imaging device (120) comprising a matrix image sensor (130) and an optical system (140) having an optical axis (142) contained in a plane, said optical system (140) being adapted to receive light rays contained in the plane and originating from a scene in order to image said scene on said matrix image sensor (130), light rays originating from a spatial region of said scene having a defined field angle between said optical axis (142) and an axis connecting said spatial region and a point of said matrix image sensor (130), said imaging device (120) being characterized in that an effective focal length of said optical system (140) varies in said plane such that said imaging device (120) exhibits an angular distribution curve of the pixels of said matrix image sensor (130) as a function of said field angle which includes at least one local maximum (1430) contained in an angular region of interest (150, 151),said angular region of interest (150, 151) comprising light rays contained in the plane and with a field angle ranging from 30 degrees to 40 degrees, at least one local maximum is such that said angular region of interest (150, 151) concerned is imaged on a proportion between 8% and 11% of the pixels of said matrix image sensor (130) receiving said light rays contained in said plane.

2. Imaging device (120) according to claim 1, wherein said optical system (140) is adapted to collect light rays having a field angle less in absolute value than an absolute value of a limiting field angle which is greater than or equal to 70 degrees.

3. Imaging device (120) according to claim 1, wherein said optical system (140) is adapted to collect light rays having a field angle less in absolute value than an absolute value of a limiting field angle which is greater than or equal to 80 degrees.

4. Imaging device (120) according to any one of claims 1 to 3, wherein said proportion is between 9% and 10% of the pixels of said matrix image sensor (130) receiving said light rays included in said plane.

5. Imaging device (120) according to any one of claims 1 to 4, wherein said angular region of interest (150, 151) is imaged with a first angular resolution greater than the limit value, while an angular region between said optical axis (142) and said angular region of interest (150, 151) is imaged with a second angular resolution less than 85% of the limit value.

6. Imaging device (120) according to any one of claims 1 to 5, wherein said angular distribution curve of pixels of said matrix image sensor (130) exhibits a unique local maximum (1430) for light rays with field angle between 0 degrees and a limiting field angle of said optical system (140).

7. Imaging device (120) according to any one of claims 1 to 6, wherein said optical system (140) does not have axial symmetry and wherein said pixel distribution curve includes a single local maximum (1430) corresponding to a global maximum.

8. Imaging device (120) according to any one of claims 1 to 7, wherein said pixel distribution curve is a polynomial curve.

9. A monitoring system (12) disposed in a passenger compartment (10) of a motor vehicle (1), said monitoring system (12) comprising an imaging device (120) as described in any one of claims 1 to 8, wherein an area of ​​the passenger compartment (10) included in said angular region of interest (150, 151) and located at a distance of between 60 centimeters and 100 centimeters from said imaging device (120) is imaged on said matrix image sensor (130) by said optical system (140) with a first magnification value higher than a second magnification value of an area located outside said angular region of interest (150, 151).

10. Monitoring system (12) according to claim 9, wherein a ratio between said first magnification value and said second magnification value is greater than or equal to 1.

32.

11. Surveillance system (12) according to claim 10, wherein said area of ​​the passenger compartment (10) included in the angular region of interest (150, 151) includes a face of an occupant of said passenger compartment (10) of said motor vehicle (1).

12. Monitoring system 12 according to claim 11, wherein said occupant corresponds to a driver (110) of said motor vehicle (1).

Images