Abstract:
An imaging mechanism provided in a forward surveillance camera can adjust, without using a mechanical constitution, light intensity brought into an image sensor by adjusting the light intensity passing through a liquid crystal shutter disposed in an adjusting part. For example, when the light intensity is great during the day, or the like, the resolution of images taken during the day can be improved by the light passing through the adjusting part being shielded and the light intensity brought into the image sensor being reduced. When there is little light intensity at night, or the like, the brightness of images taken at night can be improved by light passing through the adjusting part and the light intensity brought into the image sensor being increased.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Phase Application under 35 U.S.C. 371 of International Application No. PCT/JP2013/059088 filed on Mar. 27, 2013 and published in Japanese as WO 201 3/161495 A1 on Oct. 31, 2013. This application is based on and claims the benefit of priority from Japanese Patent Application No. 2012-103328 filed Apr. 27, 2012. The entire disclosures of all of the above applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to an imaging mechanism and a forward-monitoring camera using the imaging mechanism. 
         [0004]    2. Background Art 
         [0005]    Currently, to many vehicles, forward-monitoring cameras which monitor the conditions ahead of the vehicles are provided. 
         [0006]    For monitoring with the forward-monitoring camera, in many cases, there is adopted a method where the forward-monitoring camera is mounted to an inner surface of a windshield and monitors the conditions ahead of the vehicle through the windshield. 
         [0007]    In this case, the forward-monitoring covers a part of the windshield, and also becomes a factor narrowing the interior space of the vehicle. Accordingly, various efforts have been made for downsizing the forward-monitoring camera (PTL1). 
         [0008]    PTL1 Japanese Patent Application Publication No. 2005-112051 
       SUMMARY 
       [0009]    By the way, since the forward-monitoring camera is needed to monitor the conditions ahead of the vehicle accurately, performance depending on the accuracy is required. 
         [0010]    In order to improve the performance, it is considered to provide an aperture mechanism and so on. However, because of the aforementioned demand for downsizing and receiving shake from the vehicle, it is difficult to provide a mechanical configuration such as an aperture mechanism to the forward-monitoring camera. 
         [0011]    Accordingly, in order to secure sensitivity during the night-time, the aperture of the lens has to be fixed small. This leads to a problem where the resolution significantly lowers when the conditions ahead of the vehicle is monitored during the day-time. 
         [0012]    As the solutions for solving this problem, it is considered to combine many lenses or to use expensive lenses. However, this involves a problem of increasing the price. 
         [0013]    The present invention has as its object the provision of an imaging mechanism capable of regulating the light intensity without using a mechanical configuration and a forward-monitoring camera having the imaging mechanism. 
         [0014]    In a imaging mechanism according to an aspect of the present invention, a regulation portion is set within a light path extending toward an imaging means through the lens and along the periphery edge of the light path when its cross-section perpendicular to an optical axis of the light path is viewed, the regulation portion has a predetermined width radially extending from the periphery edge toward the optical axis, and a transmitting light intensity regulation member is disposed at the regulation portion, thereby regulating light intensity of light which the regulation portion transmits. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    In the accompanying drawings: 
           [0016]      FIG. 1  is a perspective view of a forward-monitoring camera of an embodiment of the present invention, as viewed from obliquely above; 
           [0017]      FIG. 2  shows explanatory views, on a triangular projection basis, of a bracket constituting the forward-monitoring camera of the embodiment with (a) being a plan view, (b) being a right side view and (c) being a bottom view; 
           [0018]      FIG. 3  is an exploded perspective view of the forward-monitoring camera of the embodiment; 
           [0019]      FIG. 4  is a schematic view of an imaging mechanism which the forward-monitoring camera of the embodiment has; 
           [0020]      FIG. 5  is a perspective view showing a forward-monitoring camera body of the embodiment with a hood; 
           [0021]      FIG. 6  is a flowchart of an aperture control; 
           [0022]      FIG. 7  is a chart showing light intensity at each position along a radial direction from an optical axis as a center portion with (a) being a chart when an aperture is not narrowed, (b) being a chart when the aperture is narrowed; 
           [0023]      FIG. 8  is a chart showing resolution at each position along a radial direction from an optical axis as a center portion with (a) being a chart when an aperture is not narrowed, (b) being a chart when the aperture is narrowed; 
           [0024]      FIG. 9  is a schematic view showing another imaging mechanism as a modification; 
           [0025]      FIG. 10  is a side view schematically showing the vehicle where the forward-monitoring camera of the embodiment is mounted. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    With reference to the drawings, hereinafter is described an embodiment of the present invention. 
         [0027]    A forward-monitoring camera of the present invention is used for imaging the conditions ahead of the vehicle. In the following description, a forward direction is defined to be a direction to which the forward-monitoring camera is directed in a state where the forward-monitoring camera is correctly mounted to the windshield of the vehicle at a predetermined position thereon to image the conditions ahead of the vehicle. 
         [0028]    Further, the terms rearward, rightward, leftward, upward and downward used in the following description are the directions based on triangular projection with the forward direction relative to the vehicle being a front, and thus the description referring such as to plan views or rear views is also based on triangular projection (see  FIG. 1 ). 
       General Structure 
       [0029]    As shown in  FIG. 1 , an forward-monitoring camera  1  of the present embodiment includes a bracket  3  that is attached and fixed onto a windshield from inside the vehicle, and an forward-monitoring camera body  5  that has a housing in which an imaging device and the like are accommodated. 
       Bracket 
       [0030]    As shown in  FIG. 1  and  FIG. 2  by (a), the bracket  3  includes an attachment portion  30  that is attached to a windshield. 
         [0031]    An attachment surface  30   a  of the attachment portion  30  is attached to a windshield. The attachment surface  30   a  is formed into a flat shape so as to extend along the glass surface in a portion of a windshield, to which the bracket  3  is attached. 
         [0032]    In the attachment portion  30 , a cut-out portion  30   b  is formed. The cut-out portion  30   b  is formed by cutting off a portion by an area slightly larger than the recess  50   a.  The portion that is cut off includes a portion that is opposed to a recess  50   a,  described later, when the forward-monitoring camera body  5  is mounted to the bracket  3 . 
         [0033]    The bracket  3  includes a front end portion  31  and a rear end portion  32 . The front end portion  31  is a portion bent downward from a front end of the attachment portion  30  on the right of the cut-out portion  30   b,  i.e. a portion extended downward from the front end of the attachment portion  30 . The rear end portion  32  is a portion bent downward from a rear end of the attachment portion  30 , i.e. a portion extended downward from the rear end of the attachment portion  30 . 
         [0034]    The front end portion  31  is provided with an engagement hole  31   a  into which an engagement projection  51   a,  described later, provided to a front side surface of the forward-monitoring camera body  5  is inserted and fitted. 
         [0035]    As shown in  FIG. 1  and  FIG. 2  by (b) and (c), the bracket  3  includes a pair of hook portions  33  which are arranged on right and left ends of the bracket  3 . Specifically, the hook portions  33  are each bent from the right or left side of the attachment portion  30  so as to be perpendicular thereto, and each have a hooked end extended forward along the attachment surface  30   a.  The pair of hook portions  33  are positioned slightly rearward relative to the center in a longitudinal direction. 
         [0036]    The hook portions  33  each include a base  33   a  and a hook  33   b.  The base  33   a  is projected downward from the attachment portion  30 . The hook  33   b  is extended forward along the attachment surface  30   a  from a lower end of the base  33   a  and has a length larger than that of the engagement projection  51   a,  described later. 
         [0037]    The rear end portion  32  has an inner surface (front side face) which is provided with the leaf spring  34 . 
       Forward-monitoring Camera Body 
       [0038]    As shown in  FIG. 3 , the forward-monitoring camera body  5  is formed into substantially a box-like shape with an upper surface  50  that is inclined upward in a direction from the front toward the rear. 
         [0039]    The upper surface  50  of the forward-monitoring camera body  5 , in a plan view, has the recess  50   a  in a trapezoidal shape with an end on the front left being a lower base and with a right-left width narrowing toward the rear. 
         [0040]    The recess  50   a  is a portion for preventing the field of view of the forward-monitoring camera body  5  from being blocked during imaging the conditions ahead of the vehicle through the windshield. 
         [0041]    The recess  50   a  is formed so as to be deeper toward the rear. 
         [0042]    A projection portion  59  is provided adjacent to the rear end of the recess  50   a  and on the upper surface  50  of the forward-monitoring camera body  50 . 
         [0043]    The projection portion  59  is formed to projects from the upper surface  50  upward. 
         [0044]    In this embodiment, the forward-monitoring camera body  5  is mounted to the bracket  3  in a state where the upper surface  50  of the forward-monitoring camera body  5  contacts a lower surface of the bracket  3 . The projection portion  59  is formed to have a height where the thickness of the bracket  3  is put on the upper surface  50  such that the upper end of the projection portion  59  contacts the windshield through the cut-out portion  30   b  when the forward-monitoring camera body  5  is mounted to the bracket  3 . 
         [0045]    By providing the recess  50   a  and the projection portion  59 , a wall surface  50   b  is formed at the upper base of the recess  50   a,  and a lens  50   c  for the camera is provided at an upper portion of the wall surface  50   b.    
       Imaging Mechanism 
       [0046]    Next is described an imaging mechanism provided inside the aforementioned forward-monitoring camera body  5 . 
         [0047]    The imaging mechanism  7  has the aforementioned lens  50   c,  an image sensor  70 , a lens  71  disposed between the lens  50   c  and the image sensor  70 , a liquid crystal shutter  73  and a control device  72 . The control device  72  has a computer, is electrically connected to the liquid crystal shutter  73  and the image sensor  70 , and performs an aperture control described later. Further, the control device  72  obtains an image signal from the image sensor  70  and performs predetermined image processes for recording or executing driving support. 
         [0048]    The liquid crystal shutter  73  is layered on the front surface of the lens  50   c  along the outer periphery edge. The liquid crystal shutter  73  is formed to have a ring-like shape, as shown in the front view of  FIG. 4 . 
         [0049]    The liquid crystal shutter  73  can switch between a transmissive state permeable to light and a non-transmissive state which is not permeable to light. The control for switching between the transmissive state and the non-transmissive state is performed by the aperture control executed in the control device  72 , described later. In this embodiment, the liquid crystal shutter  73  is formed as a flexible liquid crystal film which can be layered fitting the curve of surface of the lens  50   c.  The liquid crystal film has, for example, transparent and flexible substrates (polycarbonate, etc.), a liquid crystal enclosed between the substrates (polymer dispersion type crystal liquid, etc.), and transparent electrodes for an orientational control of the crystal liquid. 
         [0050]    The crystal liquid shutter  73  is formed to have a size to be disposed at least at a regulation portion. The regulation portion is within a light path  1  extending toward the image sensor  70  through the lens  50   c  and the lens  50   c,  along the periphery edge of the light path  1  when its cross-section perpendicular to the optical axis of the light path  1  is viewed, and has a predetermined width radially extending from the periphery edge toward the optical axis. The width of the regulation portion is set to each forward-monitoring camera, depending on a performance of the image sensor  70  or the lens  50   c  and the like. 
         [0051]    In this embodiment, a CMOS sensor is used as the image sensor  70 , however, it should be noted that the image sensor  70  is not limited to this. 
       Regarding Mounting Process 
       [0052]    Hereinafter is described a process of mounting the forward-monitoring camera  1  configured as described above to a windshield of a vehicle in the course of manufacturing the vehicle. 
         [0053]    The forward-monitoring camera  1  of this embodiment is used for imaging the conditions ahead of the vehicle and correctly detecting the positions of lane markers as viewed from the position of the camera and the presence/absence, for example, of lighting of the head lights of oncoming vehicles. Accordingly, the forward-monitoring camera  1  is required to be correctly attached to the windshield. 
         [0054]    Therefore, the bracket  3  is correctly attached in advance to a position that enables the detection mentioned above, at a stage where a windshield has been fabricated but is yet to be put on a vehicle assembly line. 
         [0055]    The bracket  3  is attached to the windshield by applying an adhesive to the attachment surface  30   a  of the bracket  3  and sticking the surface onto the windshield. 
         [0056]    As shown in  FIG. 5 , a hood  8  is mounted to the recess  50   a  of the forward-monitoring camera body  5 . The hood  8  is a means for blocking the light toward the lens  50   c  from directions except for the forward direction of the vehicle, in order to prevent so-called background reflections. The background reflections cause images except for the conditions ahead of the vehicle to be captured, because the light from the directions except for the forward direction of the vehicle is reflected on the windshield thereby being directed toward the lens  50   c.    
         [0057]    The forward-monitoring camera body  5  with the hood  8  is mounted to the bracket  3 . 
         [0058]    In the work of mounting the forward-monitoring camera body  5  to the bracket  3  (see  FIGS. 1 to 3 ), at first, the bosses  52   a  are firstly hooked on the respective hook portions  33 , while they are slid obliquely upward in the rear from below along the glass surface of the windshield. 
         [0059]    With the sliding, the rear surface of the forward-monitoring camera body  5  is eventually brought into contact with the leaf spring  34 , first, to push and elastically deform the leaf spring  34 . 
         [0060]    In this embodiment, the bosses  52   a  are each provided to a position that makes shorter a distance from the front end of the engagement projection  51   a  to the boss  52   a  than a distance from the base  33   a  of the corresponding hook portion  33  to the front end portion  31  in view from the front tip end of the engagement projection  51   a.  Therefore, upon contact of each of the bosses  52   a  with the base  33   a  of the corresponding one of the hook portions  33 , the forward-monitoring camera body  5  can be placed relative to the bracket  3  such that the engagement projection  51   a  is inserted and fitted into the engagement hole  31   a.    
         [0061]    When the worker&#39;s hands that have supported the forward-monitoring camera body  5  for the above work are removed after the placement, the forward-monitoring camera body  5  is slightly returned back forward by the leaf spring  34 , and along with this movement, the engagement projection  51   a  is inserted and fitted into the engagement hole  31   a.  In this case, the bosses  52   a  do not come off from the respective hook portions  33  because the hook  33   b  of each of the hook portions  33  is formed so as to have a larger length than the that of the engagement projection  51   a.    
         [0062]    In this way, the forward-monitoring camera body  5  is urged by the leaf spring  34  for the insertion of the engagement projection  51   a  into the engagement hole  31   a,  while being brought into contact with the front end portion  31 . Further, the upper surface  50  of the in-vehicle camera body  5  is also brought into contact with a rear surface of the attachment portion  30  (surface opposed to the forward-monitoring camera body  5 ) so as to press the forward-monitoring camera body  5  against the bracket  3  for fixation. 
       Aperture Control 
       [0063]    Next is described the aperture control performed in the control device  72  configuring the imaging mechanism  7  (see  FIG. 4 ). 
         [0064]    The aperture control is a control which is always performed while the forward-monitoring camera  1  monitors the conditions ahead of the vehicle, and is repeatedly performed at a given interval in this embodiment. 
         [0065]    When the aperture control starts, as shown in  FIG. 6 , at first, a process of S 70  is carried out. In this S 70 , a process where the image data of the captured image are obtained from the image sensor  70  (see  FIG. 4 ) is carried out. 
         [0066]    Next, in S 71 , the control device  72  performs, on the basis of the image data of the captured image which has been obtained in S 70 , a process where the brightness of the image is calculated. 
         [0067]    In the calculation of the brightness, the average of the brightness may be calculated from data on the brightness of all pixels configuring the image sensor  70 , or the average of the brightness may be calculated from data on the brightness of pixels at arbitrary points (for example, arbitrary pixels are chosen at a given interval from the center of the image to the edge of the image). The calculation method is arbitrary. 
         [0068]    Thereafter, in S 72 , a process where it is determined whether it is during the day-time or the night-time is carried out on the basis of the brightness calculated in S 71 . 
         [0069]    After that, if, in the determination (S 72 ), the control device  72  determines that it is during the night-time (the brightness of the image is dark, or the light intensity is small) (S 72 : NIGHT-TIME), a control is executed (S 73 ). In the control (S 73 ), the control device  72  makes the liquid crystal shutter  73  transmissive, that is, passes the light through the regulation portion to increase the light intensity captured by the image sensor  70 . On the other hand, if the control device  72  determines that it is during the day-time (the brightness of the image is brighter than that during the night-time, or the light intensity is large) (S 72 : DAY-TIME), a control is executed (S 74 ). In the control (S 74 ), the control device  72  makes the liquid crystal shutter  73  non-transmissive, that is, executes the aperture control for blocking the light from passing through the regulation portion to decrease the light intensity captured by the image sensor  70 . 
         [0070]    That is, when the S 73  and S 74  are executed, as shown in  FIG. 4 , the liquid crystal shutter  73  is opened during the night-time, and the light is captured through the maximum range (light path  1 ). On the other hand, when the liquid crystal shutter  73  is closed, the circumferential edge of the lens  50   c  is closed, thereby the light is captured through a range narrower by the closed portion (light path  2 ). 
         [0071]    Here are described effects of performing the aperture control during the day-time. 
         [0072]    As shown in  FIG. 7 , comparing the state where the liquid crystal shutter  73  is opened ((a) of  FIG. 7 ) with the state where the liquid crystal shutter  73  is closed ((b) of  FIG. 7 ), the light intensity entering into the image sensor  70  in the state where the liquid crystal shutter  73  is opened ((a) of  FIG. 7 ) is larger in the whole radial direction. 
         [0073]    However, if the liquid crystal shutter  73  is closed to narrow it down ((b) of  FIG. 7 ), the light intensity ranges substantially uniformly from the center of the image to the edge of the image, compared with when the liquid crystal shutter is opened ((a) of  FIG. 7 ). 
         [0074]    Further, as shown in  FIG. 8 , when the liquid crystal shutter  73  is closed ((b) of  FIG. 8 ), the resolution of the whole image increases, compared with when the liquid crystal shutter  73  is opened ((a) of  FIG. 8 ). 
         [0075]    Thus, during the day-time, closing the liquid crystal shutter  73  by the aforementioned aperture control enables the uniformity of the light intensity of the whole picture plane, and can increase the resolution of the image. 
         [0076]    On the other hand, during the night-time, the light intensity of the light captured by the image sensor  70  through the lens  50   c  is small. Accordingly, opening the liquid crystal shutter  73  to capture more light into the image sensor  70  causes the resolution of the image to lower ((a) of  FIG. 8 ), but makes the image bright ((a) of  FIG. 7 ), which can increases minimum sensitivity. 
         [0077]    During the night-time, only a range where the headlight lights is imaged. Because of such a limitation, the resolution only has to be secured at a level where parting lines of lanes at the vicinity of the vehicle can be recognized. Instead light intensity needed for imaging is insufficient. Accordingly, in the above aperture control, the liquid crystal shutter  73  is opened to capture most possible light in the image sensor  70 . 
         [0078]    The above-described configuration can achieve both securement of sensitivity during the night-time and good resolution during the day-time in spite of at a low price. 
         [0000]    Characteristic Functions and Effects of this Embodiment, etc. 
         [0079]    Now is described characteristic functions and effects of the aforementioned forward-monitoring camera  1  in this embodiment. 
         [0080]    In the forward-monitoring camera  1  of this embodiment, the light intensity captured in the image sensor  70  can be regulated, without using a mechanical configuration, by regulating the light intensity (further specifically, the light intensity of transmitted light per unit of area of the transmitting light intensity regulation member) passing the liquid crystal shutter  73  disposed at the regulation portion. 
         [0081]    For example, when the light intensity is large such as during the day-time, the light passing the regulation portion is blocked to use the center portion of the lens  50   c.  This can increase the resolution of the image captured during the day-time. On the other hand, when the light intensity is small such as during the night-time, the regulation portion transmits the light thereby increasing the light intensity captured in the image sensor  70 . This can also increase the brightness of the image captured during the night-time. That is, the minimum sensitivity can be increased. 
         [0082]    It can achieve both securement of sensitivity during the night-time and good resolution during the day-time in spite of at a low price. In the forward-monitoring camera  1  of this embodiment, the light except for the regulation portion can be taken in directly by the image sensor  70  through the lenses  50   c  and  71 . This enables high-performance imaging by using the minimum number of lenses  50   c,    70  and by using the image sensor  70  having basic performance. 
       Correspondence Relation 
       [0083]    The image sensor  70  of this embodiment corresponds to the imaging means of the present invention, hereinafter similarly, the liquid crystal shutter  73  corresponds to the transmitted light intensity regulation member, and the control device  72  corresponds to the transparency control means. 
       Other Modifications 
       [0084]    In the above embodiment, an example where the liquid crystal shutter  73  is layered on the lens  50   c  is described. Alternatively, the liquid crystal shutter  73  may be provided separately from the lens  50   c.  In this case, for example, as shown in  FIG. 9 , the liquid crystal shutter  73  may be provided ahead of the lens  50   c.    
         [0085]    In the above embodiment, an example where the liquid crystal shutter  73  is layered on the lens  50   c  and the transparency of the liquid crystal shutter  73  is controlled is described. Alternatively, in place of the liquid crystal shutter  73 , there may be layered a film made from a photochromic material whose transparency changes depending on light intensity (the transparency lowers when it is bright, and the transparency increases when it is dark). 
         [0086]    As the photochromic materials, there can be used materials containing an ultraviolet color-change substance which chemically changes by ultraviolet light. As the ultraviolet color-change substances, there may be used an organic photochromic material such as a spiropyran-based compound, a dihydroindolizine-based compound, a fulgide-based compound, a tetrobenzoperopyrene derivative, a dihydripyrene-based compound, a thioindigo-based compound, a acenophane derivative, a viologen or a diphenylthiocarbazone metal compound. There may be used an inorganic photochromic material such as a silver halide. 
         [0087]    It should be noted that application of the present invention is not limited to the forward-monitoring camera of the above embodiments. For example, the forward-monitoring camera of the above embodiment is attached to the upper portion of the windshield and behind the rear view mirror. However, it may be attached to the lower portion of the windshield. Alternatively, the imaging mechanism of the present invention may be used in a monitoring camera which is attached to a part of the vehicle except for the windshield or a monitoring camera used for other than the vehicle. However, it is preferred that using in the forward-monitoring camera monitoring the conditions ahead of the vehicle where it is difficult to provide a mechanical configuration because of receiving shake from the vehicle. 
         [0088]    It should be noted that the present invention may be a configuration accords with the spirit of the invention described in the claims, and is not limited to the above embodiments. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1  . . . forward-monitoring camera,  2  . . . light path,  3  . . . bracket 
           5  . . . forward-monitoring camera body,  7  . . . imaging mechanism,  8  . . . hood 
           30  . . . attachment portion,  30   a . . .  attachment surface,  30   b . . .  cut-out portion 
           31  . . . front end portion,  31   a . . . ,    32  . . . rear end portion 
           33  . . . hook portion,  33   a . . .  base,  33   b . . .  hook 
           34  . . . leaf spring,  50  . . . upper surface,  50   b . . .  wall surface 
           50   c . . .  lens,  51   a . . .  engagement projection,  52   a . . .  boss 
           59  . . . projection portion,  70  . . . image sensor,  71  . . . lens 
           72  . . . control device,  73  . . . liquid crystal shutter