Patent Publication Number: US-11645868-B2

Title: Monitoring system and monitoring method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of the prior Japanese Patent Application No. 2019-219049, filed on Dec. 3, 2019, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a monitoring system and a monitoring method. 
     BACKGROUND 
     As an example of a biometric authentication technology based on biometric images, a face authentication technique using a monitoring camera installed outside a building or at a building entrance is known in the related art. In the face authentication technique, in order to suppress the influence of backlight or oblique light, for example, an illumination device that irradiates a living body is attached to the monitoring camera to improve the image quality of a biometric image to be captured. 
     Related techniques are disclosed in, for example, International Publication Pamphlet No. WO 2016/084214. 
     SUMMARY 
     According to an aspect of the embodiments, a monitoring system includes a memory, and a processor coupled to the memory and configured to detect a face area of a subject from a captured image divided into a plurality of blocks according to a number of light emitting elements that irradiate an image capturing range, and control a light emission intensity of a light emitting element corresponding to a block including the face area among the plurality of blocks according to one of a position of the face area and a size of the face area. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a view illustrating an example of the system configuration of a monitoring system; 
         FIGS.  2 A to  2 C  are views for explaining the configuration of a monitoring device; 
         FIGS.  3 A and  3 B  are views illustrating a configuration example of a diffractive optical element; 
         FIG.  4    is a view illustrating an example of the hardware configuration of a control device; 
         FIG.  5    is a first view illustrating an example of the functional configuration of the control device; 
         FIGS.  6 A and  6 B  are first views illustrating an operation example of the monitoring system; 
         FIG.  7    is a first flow chart illustrating a flow of control process by the control device; 
         FIG.  8    is a second view illustrating an example of the functional configuration of the control device; 
         FIGS.  9 A and  9 B  are second views illustrating an operation example of the monitoring system; 
         FIG.  10    is a second flowchart illustrating a flow of control process by the control device; 
         FIG.  11    is a third view illustrating an example of the functional configuration of the control device; 
         FIG.  12    is a third flow chart illustrating a flow of control process by the control device; 
         FIG.  13    is a fourth view illustrating an example of the functional configuration of the control device; 
         FIGS.  14 A and  14 B  are third views illustrating an operation example of the monitoring system; and 
         FIG.  15    is a fourth flowchart illustrating a flow of control process by the control device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Since the brightness of an illumination light decreases in inverse proportion to the square of a distance, the face of a distant subject appears dark while the face of a nearby subject appears bright. That is, the image capturing distance at which an image may be captured with appropriate brightness is limited. 
     Hereinafter, embodiments of a technique capable of capturing a subject&#39;s face with constant brightness will be described with reference to the accompanying drawings. Throughout the present disclosure and the drawings, elements having substantially the same functional configuration are denoted by the same reference numerals, and explanation thereof will not be repeated. 
     First Embodiment 
     System Configuration of Monitoring System 
     First, the system configuration of a monitoring system will be described.  FIG.  1    is a view illustrating an example of the system configuration of a monitoring system. As illustrated in  FIG.  1   , a monitoring system  100  includes a monitoring device  110 , a control device  120 , and an authentication device  130 . 
     The monitoring device  110  is a monitoring camera with so-called spot lighting. Specifically, the monitoring device  110  has an image capturing unit  111 . The monitoring device  110  also includes a plurality of sets of LEDs (Light Emitting Diodes) which are an example of light emitting elements, and diffractive optical elements. The example of  FIG.  1    represents a case where the monitoring device  110  includes 12 sets of LEDs and diffractive optical elements (see sets  112 _ 1  to  112 _ 12 ). 
     The image capturing unit  111  captures an image of a subject (not illustrated) and transmits the captured image to the control device  120 . Each of the sets  112 _ 1  to  112 _ 12  of LEDs and diffractive optical elements operates independently based on an instruction from the control device  120 , and irradiates an image capturing range of the image capturing unit  111  with spot lighting. The light emission intensity of each of the sets  112 _ 1  to  112 _ 12  of LEDs and diffractive optical elements is controlled based on an instruction from the control device  120 . 
     A control program is installed in the control device  120 , and by executing the control program, the control device  120  functions as an image processing unit  121  and an illumination control unit  122 . 
     The image processing unit  121  detects a face area of the subject from the captured image that has been received from the image capturing unit  111 . The image processing unit  121  also determines the light emission intensity of each of the sets  112 _ 1  to  112 _ 12  of LEDs and diffractive optical elements, based on the detection result, and notifies the illumination control unit  122  of the determined light emission intensity. 
     Further, with the transmission of the determined light emission intensity to the illumination control unit  122 , the image processing unit  121  transmits the captured image that has been received from the image capturing unit  111  to the authentication device  130 , as a captured image for authentication. 
     The illumination control unit  122  evaluates the image quality of the captured image that has been received from the image capturing unit  111  at a predetermined period, and calculates the light emission intensity of each of the sets  112 _ 1  to  112 _ 12  of LEDs and diffractive optical elements which is necessary for detecting the face area of the subject. Further, the illumination control unit  122  controls each of the sets  112 _ 1  to  112 _ 12  of LEDs and diffractive optical elements to emit light with the calculated light emission intensity. 
     In addition, in order to obtain the captured image for authentication, when the light emission intensity is notified from the image processing unit  121 , the illumination control unit  122  controls the corresponding set of LED and diffractive optical element (any one of the sets  112 _ 1  to  112 _ 12 ) to emit light with the notified light emission intensity. 
     The authentication device  130  calculates biometric information of the face area of the subject based on the captured image for authentication that has been transmitted from the control device  120 , and compares the calculated biometric information with the biometric information of the face area registered in advance, so as to perform the face authentication. 
     Configuration of Monitoring Device 
     Next, the configuration of the monitoring device  110  will be described.  FIGS.  2 A to  2 C  are views illustrating the configuration of the monitoring device.  FIG.  2 A  illustrates an external appearance configuration of the monitoring device  110 . As illustrated in  FIG.  2 A , the monitoring device  110  includes a mounting jig  250  and is mounted outside a building or at a high place of building entrance. 
       FIG.  2 B  illustrates a device layout around the image capturing unit  111  when the monitoring device  110  is viewed from the front. As illustrated in  FIG.  2 B , in the case of the monitoring device  110 , the set  112 _ 1  of LED  201 _ 1  and diffractive optical element  202 _ 1 , the set  112 _ 2  of LED  201 _ 2  and diffractive optical element  202 _ 2 , and the set  112 _ 3  of LED  201 _ 3  and diffractive optical element  202 _ 3  are arranged horizontally from the left on the upper side of the image capturing unit  111  when viewed from the front. 
     Although reference numerals are omitted in  FIG.  2 B  due to space limitations, in the case of the monitoring device  110 , the set  112 _ 4  of LED  201 _ 4  and diffractive optical element  202 _ 4 , the set  112 _ 5  of LED  201 _ 5  and diffractive optical element  202 _ 5 , and the set  112 _ 6  of LED  201 _ 6  and diffractive optical element  202 _ 6  are arranged vertically from above on the right side of the image capturing unit  111  when viewed from the front. 
     Similarly, in the case of the monitoring device  110 , the set  112 _ 7  of LED  201 _ 7  and diffractive optical element  202 _ 7 , the set  112 _ 8  of LED  201 _ 8  and diffractive optical element  202 _ 8 , and the set  112 _ 9  of LED  201 _ 9  and diffractive optical element  202 _ 9  are arranged horizontally from the right on the lower side of the image capturing unit  111  when viewed from the front. 
     Similarly, in the case of the monitoring device  110 , the set  112 _ 10  of LED  201 _ 10  and diffractive optical element  202 _ 10 , the set  112 _ 11  of LED  201 _ 11  and diffractive optical element  202 _ 11 , and the set  112 _ 12  of LED  201 _ 12  and diffractive optical element  202 _ 12  are arranged vertically from below on the left side of the image capturing unit  111  when viewed from the front. 
       FIG.  2 C  illustrates the arrangement of each set from the set  112 _ 1  of LED  201 _ 1  and diffractive optical element  202 _ 1  to the set  112 _ 3  of LED  201 _ 3  and diffractive optical element  202 _ 3  when viewed from above the monitoring device  110 . As illustrated in  FIG.  2 C , the diffractive optical elements  202 _ 1  to  202 _ 3  are arranged on a diffractive optical element arrangement substrate  210 . The LEDs  201 _ 1  to  201 _ 3  are arranged on a LED mounting substrate  211  at positions corresponding to the positions where the diffractive optical elements  202 _ 1  to  202 _ 3  are arranged. 
     As a result, light emitted by the LED  201 _ 1  is converted into rectangular spot light via the diffractive optical element  202 _ 1 , and is irradiated on the image capturing range of the image capturing unit  111  as rectangular spot lighting. Similarly, light emitted by the LED  201 _ 2  is converted into rectangular spot light via the diffractive optical element  202 _ 2 , and is irradiated on the image capturing range of the image capturing unit  111  as rectangular spot lighting. Similarly, light emitted by the LED  201 _ 3  is converted into rectangular spot light via the diffractive optical element  202 _ 3 , and is irradiated on the image capturing range of the image capturing unit  111  as rectangular spot lighting. 
     It is assumed that the sets  112 _ 4  to  112 _ 12  are arranged in the same manner as in  FIG.  2 C  and their respective LEDs, and diffractive optical elements are arranged in diffractive optical element arrangement substrates  220 ,  230 , and  240  and LED mounting substrates  221 ,  231 , and  241 , respectively. 
     Example of Configuration of Diffractive Optical Element 
     Next, the configuration of a diffractive optical element (here, the diffractive optical element  202 _ 1 ) will be described.  FIGS.  3 A and  3 B  are views illustrating an example of a configuration of a diffractive optical element. As illustrated in  FIG.  3 A , the diffractive optical element  202 _ 1  is formed by arranging linear diffraction gratings (cells) two-dimensionally in the same plane. The example in  FIG.  3 A  represents a case where 50 diffraction gratings (cells) of 0.02 mm×0.02 mm are arranged in the vertical direction and 50 diffraction gratings of 0.02 mm×0.02 mm are arranged in the horizontal direction (250 in total). 
     Each of the diffraction gratings generates a plurality of dot lights when the light emitted from the LED  201 _ 1  is transmitted. In addition, since the respective diffraction gratings have different pitches and rotation angles, the generated dot lights are combined in the image capturing range of the image capturing unit  111 . Accordingly, the set  112 _ 1  of LED  201 _ 1  and diffractive optical element  202 _ 1  may irradiate the image capturing range of the image capturing unit  111  with uniform rectangular spot lighting. 
       FIG.  3 B  is a view illustrating the correspondence relationship between the image capturing range of the image capturing unit  111  which is irradiated by each of the sets of LEDs and diffractive optical elements with rectangular spot lighting, and the captured image of the image capturing unit  111 . As illustrated in  FIG.  3 B , a captured image  300  of the image capturing unit  111  is divided into a plurality of blocks according to the number of sets of LEDs and diffractive optical elements, and the corresponding set of LED and diffractive optical element irradiates the image capturing range corresponding to each block with rectangular spot lighting. 
     As described above, since 12 sets of LEDs and diffractive optical elements are arranged around the image capturing unit  111 , the captured image  300  of the image capturing unit  111  is divided into 12 blocks. In FIG.  3 B, each block is numbered, and the corresponding set of LED and diffractive optical element is indicated by a lead line. 
     For example, the set  112 _ 1  of LED and diffractive optical element irradiates the image capturing range corresponding to the block of block number=“1” with rectangular spot lighting. 
     Hardware Configuration of Control Device 
     Next, the hardware configuration of the control device  120  will be described.  FIG.  4    is a view illustrating an example of the hardware configuration of the control device. 
     As illustrated in  FIG.  4   , the control device  120  includes a CPU (Central Processing Unit)  401 , a ROM (Read Only Memory)  402 , and a RAM (Random Access Memory)  403 . The CPU  401 , the ROM  402 , and the RAM  403  make up a so-called computer. Further, the control device  120  includes an I/F (Interface) device  404 , an illumination control device  405 , and a communication device  406 . The respective units of the control device  120  are interconnected via a bus  407 . 
     The CPU  401  is an arithmetic device that executes various programs (e.g., a control program and the like) installed in the ROM  402 . The ROM  402  is a nonvolatile memory, and functions as a main memory device that stores the various programs executed by the CPU  401  and information used when the CPU  401  executes the various programs. 
     The RAM  403  is a volatile memory such as a DRAM (Dynamic Random Access Memory), a SRAM (Static Random Access Memory) or the like. The RAM  403  functions as a main memory device that provides a work area that is deployed when the various programs installed in the ROM  402  are executed by the CPU  401 . 
     The I/F device  404  is a connection device for connecting to the image capturing unit  111  of the monitoring device  110 . The illumination control device  405  is a control device for controlling the LED mounting substrates  211 ,  221 ,  231 , and  241  of the monitoring device  110 . The communication device  406  communicates with the authentication device  130 . 
     Functional Configuration of Control Device 
     Next, the functional configuration of the control device  120  will be described.  FIG.  5    is a first view illustrating an example of the functional configuration of the control device. As illustrated in  FIG.  5   , the image processing unit  121  includes an image acquisition unit  501 , a face detection unit  502 , a face position detection unit  503 , and a light emission intensity control unit  504 . 
     The image acquisition unit  501  acquires the captured image that has been transmitted from the image capturing unit  111  of the monitoring device  110 , and notifies the captured image to the face detection unit  502 . The image acquisition unit  501  also transmits the captured image that has been transmitted from the image capturing unit  111  of the monitoring device  110 , to the authentication device  130  as a captured image for authentication. 
     The face detection unit  502  is an example of a detection unit, and processes the captured image that has been notified from the image acquisition unit  501  to detect the face area of the subject. The face detection unit  502  also notifies the face position detection unit  503  of the detected face area. Further, the face detection unit  502  determines a block including the detected face area, and notifies the block to the light emission intensity output unit  507 . 
     The face position detection unit  503  is an example of a determination unit, and detects a position of the face area of the subject notified from the face detection unit  502 , that is, a predetermined position within the face area, for example, the center position of the face area, the position of a specific part (eye, nose, etc.), or the like, and determines a block to which the detected position of the face area belongs. Further, the face position detection unit  503  notifies the light emission intensity control unit  504  of the block to which the position of the face area belongs. 
     The light emission intensity control unit  504  is an example of a control unit, and based on the block notified from the face position detection unit  503 , determines the light emission intensity of a set of LED and diffractive optical element that irradiates the image capturing range corresponding to the block with spot lighting. 
     As illustrated in a table  510  of  FIG.  5   , it is assumed that the correspondence relationship between a block to which the position of the face area of the subject belongs and the light emission intensity is defined in advance. This is because the image capturing distance to the subject may be roughly estimated based on a block of the captured image to which the face area of the subject belongs, and the light emission intensity may be defined according to the estimated image capturing distance. 
     The example of the table  510  in  FIG.  5    represents that the blocks of the captured image are divided into four groups, and different light emission intensities are associated with the groups, respectively. 
     Further, as illustrated in  FIG.  5   , the illumination control unit  122  includes an image quality evaluation unit  505 , an adjustment unit  506 , and a light emission intensity output unit  507 . 
     The image quality evaluation unit  505  acquires the captured image that has been transmitted from the image capturing unit  111  of the monitoring device  110 , and evaluates the quality of the acquired captured image. Specifically, the image quality evaluation unit  505  divides the acquired captured image into a plurality of blocks, and evaluates whether or not each block has a level of image quality where a face area may be detected. The image quality evaluation unit  505  also notifies the adjustment unit  506  of the evaluation result for each block. 
     The adjustment unit  506  adjusts the light emission intensity of the set of LED and diffractive optical element that irradiate the image capturing range corresponding to each block with spot lighting according to the evaluation result for each block notified from the image quality evaluation unit  505 . Further, the adjustment unit  506  notifies the light emission intensity output unit  507  of the adjusted light emission intensity. As a result, the image capturing unit  111  may obtain a captured image that has a level of image quality where a face area may be detected. 
     The image quality evaluation unit  505  and the adjustment unit  506  operate at a predetermined period, for example, each time the surrounding environment changes, and therefore, the image capturing unit  111  may obtain a captured image that has a level of image quality where a face area may be detected even when the surrounding environment changes. 
     When the light emission intensity corresponding to each block is notified from the adjustment unit  506 , the light emission intensity output unit  507  transmits the notified light emission intensity to the LED mounting substrates  211 ,  221 ,  231 , and  241 . As a result, each set of LED and diffractive optical element that irradiates the image capturing range that corresponds to each block with spot lighting may emit light with the notified light emission intensity. 
     In addition, when the light emission intensity is notified from the light emission intensity control unit  504 , the light emission intensity output unit  507  transmits the notified light emission intensity to any of the LED mounting substrates  211 ,  221 ,  231 , and  241 . As a result, the set of LED and diffractive optical element that irradiates the image capturing range that corresponds to the block notified from the face detection unit  502  with spot lighting may emit light with the notified light emission intensity. 
     As a result, the image capturing unit  111  may obtain a captured image with the face area irradiated with spot lighting with the light emission intensity according to the image capturing distance to the subject. 
     Example of Operation of Monitoring System 
     Next, an example of the operation of the monitoring system  100  will be described.  FIGS.  6 A and  6 B  are first views illustrating an example of the operation of the monitoring system.  FIG.  6 A  illustrates a state where a subject  610  is approaching the monitoring device  110 . 
       FIG.  6 B  illustrates captured images  631 ,  632 , and  633  that are obtained by the monitoring device  110  at respective positions (positions  621 ,  622 , and  623 ) of the subject  610  illustrated in  FIG.  6 A . It is assumed that when the captured images  631 ,  632 , and  633  are obtained, the adjustment of the light emission intensity by the adjustment unit  506  has been completed, and the captured images  631 ,  632 , and  633  have a level of image quality where the face area of the subject  610  may be detected. 
     In the case of the captured image  631 , the face detection unit  502  determines that the block number of the block including the face area of the subject  610  is “2.” Further, the face position detection unit  503  determines that the block number of the block to which the detected position of the face area of the subject  610  belongs is “2.” 
     Accordingly, the light emission intensity control unit  504  determines the light emission intensity to be “I4.” Then, the light emission intensity output unit  507  controls the set  112 _ 2  of LED  201 _ 2  and diffractive optical element  202 _ 2  to emit light with the light emission intensity “I4.” 
     Similarly, in the case of the captured image  632 , the face detection unit  502  determines that the block number of the block including the face area of the subject  610  is “5.” Further, the face position detection unit  503  determines that the block number of the block to which the detected position of the face area of the subject  610  belongs is “5.” 
     Accordingly, the light emission intensity control unit  504  determines the light emission intensity to be “I3.” Then, the light emission intensity output unit  507  controls the set  112 _ 12  of LED  201 _ 12  and diffractive optical element  202 _ 12  to emit light with the light emission intensity “I3.” 
     Similarly, in the case of the captured image  633 , the face detection unit  502  determines that the block number of the block including the face area of the subject  610  is “8, 11.” Further, the face position detection unit  503  determines that the block number of the block to which the detected position of the face area of the subject  610  belongs is “I1.” 
     Accordingly, the light emission intensity control unit  504  determines the light emission intensity to be “I1”. Then, the light emission intensity output unit  507  controls the set  112 _ 6  of LED  201 _ 6  and diffractive optical element  202 _ 6  and the set  112 _ 8  of LED  201 _ 8  and diffractive optical element  202 _ 8  to emit light with the light emission intensity “I1.” 
     Flow of Control Process 
     Next, a flow of the control process by the control device  120  will be described.  FIG.  7    is a first flowchart illustrating a flow of the control process by the control device. In operation S 701 , the image acquisition unit  501  acquires a captured image from the image capturing unit  111  of the monitoring device  110 . 
     In operation S 702 , the image quality evaluation unit  505  determines whether or not each block of the acquired captured image has a level of image quality where the face area of the subject may be detected (whether the image quality is equal to or higher than a predetermined level). 
     When it is determined in operation S 702  that the image quality of any block of the captured image is lower than the predetermined level (No in operation S 702 ), the process proceeds to operation S 703 . 
     In operation S 703 , the adjustment unit  506  adjusts the light emission intensity of a set of LED and diffractive optical element that corresponds to the block whose image quality is determined to be lower than the predetermined level, and the process returns to operation S 701 . 
     Meanwhile, when it is determined in operation S 702  that the image quality of each block of the acquired captured image is equal to or higher than the predetermined level (Yes in operation S 702 ), the process proceeds to operation S 704 . It is assumed that the processes of operation operations S 702  and S 703  are executed every predetermined period. 
     In operation S 704 , the face detection unit  502  processes the acquired captured image to detect the face area of the subject. 
     In operation S 705 , the face position detection unit  503  determines a block to which the position of the detected face area of the subject belongs. 
     In operation S 706 , the light emission intensity control unit  504  determines the light emission intensity based on the block to which the position of the face area of the subject belongs. 
     In operation S 707 , the face detection unit  502  determines a block including the detected face area. Thereby, the light emission intensity control unit  504  selects a set of LED and diffractive optical element to be controlled to emit light with the determined light emission intensity. 
     In operation S 708 , the light emission intensity output unit  507  transmits the determined light emission intensity to the LED mounting substrate so that the selected set of LED and diffractive optical element emits light with the determined light emission intensity. 
     In operation S 709 , the image acquisition unit  501  acquires a captured image from the image capturing unit  111  of the monitoring device  110  and transmits the acquired captured image to the authentication device  130  as a captured image for authentication. 
     As is apparent from the above description, the monitoring system according to the first embodiment detects the face area of the subject from the captured image of the image capturing unit, which is divided into a plurality of blocks according to the number of LEDs that irradiate the image capturing range of the image capturing unit. Further, the monitoring system according to the first embodiment controls an LED corresponding to a block including the face area of the subject among the plurality of blocks to emit light with the light emission intensity predetermined according to a block to which the position of the face area of the subject belongs. 
     In this way, with the monitoring system according to the first embodiment, by controlling the light emission intensity using the relationship between the block to which the position of the face area of the subject belongs and the image capturing distance to the subject, it is possible to capture the subject&#39;s face with constant brightness, regardless of the image capturing distance to the subject. 
     In addition, the monitoring system according to the first embodiment adjusts the light emission intensity every predetermined period so as to obtain a captured image with an image quality enough to detect the face area. 
     As a result, with the monitoring system according to the first embodiment, it is possible to detect the face area to capture the face of the subject with constant brightness even when the surrounding environment changes. 
     Second Embodiment 
     The case where the light emission intensity is determined based on the block to which the position of the face area of the subject belongs has been described in the first embodiment. However, the method for determining the light emission intensity is not limited thereto, but the light emission intensity may be determined according to the size of the face area of the subject. Hereinafter, a second embodiment will be described focusing on the differences from the first embodiment. 
     Functional Configuration of Control Device 
     First, the functional configuration of the control device  120  according to the second embodiment will be described.  FIG.  8    is a second view illustrating an example of the functional configuration of the control device. A difference from  FIG.  5    lies in a face size calculation unit  801  and a light emission intensity control unit  802 . 
     The face size calculation unit  801  is an example of a calculation unit, and calculates the size of the face area of the subject notified from the face detection unit  502 . Further, the face size calculation unit  801  notifies the light emission intensity control unit  802  of the calculated size of the face area. 
     The light emission intensity control unit  802  is an example of a control unit, and determines the light emission intensity based on the size of the face area notified from the face size calculation unit  801 . In addition, as illustrated in the table  810  of  FIG.  8   , it is assumed that the correspondence relationship between the size of the face area of the subject and the light emission intensity is defined in advance. This is because the image capturing distance to the subject may be roughly estimated according to the size of the face area of the subject, and the light emission intensity may be defined according to the estimated image capturing distance. 
     The example of the table  810  in  FIG.  8    represents that the size of the face area is divided into five groups, and different light emission intensities are associated with the groups, respectively. However, instead of the table  810 , the correspondence relationship between the size of the face area and the light emission intensity may be defined by a predetermined function. 
     Example of Operation of Monitoring System 
     Next, an example of the operation of the monitoring system  100  will be described.  FIGS.  9 A and  9 B  are second views illustrating an example of the operation of the monitoring system.  FIG.  9 A  illustrates a state in which the subject  610  is approaching the monitoring device  110 . 
       FIG.  9 B  illustrates captured images  631 ,  902 , and  633  captured by the monitoring device  110  at respective positions (positions  621 ,  901 , and  623 ) of the subject  610  illustrated in  FIG.  9 A . It is assumed that when the captured images  631 ,  902 , and  633  are obtained, the adjustment of the light emission intensity by the adjustment unit  506  has been completed, and the captured images  631 ,  902 , and  633  have a level of image quality where the face area of the subject  610  may be detected. 
     In the case of the captured image  631 , the face detection unit  502  determines that the block number of the block including the face area of the subject  610  is “2.” Further, the face size calculation unit  801  determines that the size of the face area of the subject  610  is “SS.” 
     Accordingly, the light emission intensity control unit  802  determines the light emission intensity to be “I5.” Then, the light emission intensity output unit  507  controls the set  112 _ 2  of LED  201 _ 2  and diffractive optical element  202 _ 2  to emit light with the light emission intensity “I5.” 
     Similarly, in the case of the captured image  902 , the face detection unit  502  determines that the block number of the block including the face area of the subject  610  is “2.” Further, the face size calculation unit  801  determines that the size of the face area of the subject  610  is “S.” 
     Accordingly, the light emission intensity control unit  802  determines the light emission intensity to be “I4”. Then, the light emission intensity output unit  507  controls the set  112 _ 2  of LED  201 _ 2  and diffractive optical element  202 _ 2  to emit light with the light emission intensity “I4.” 
     Similarly, in the case of the captured image  633 , the face detection unit  502  determines that the block number of the block including the face area of the subject  610  is “8, 11.” The face size calculation unit  801  determines that the size of the face area of the subject  610  is “L.” 
     Accordingly, the light emission intensity control unit  802  determines the light emission intensity to be “I2.” Then, the light emission intensity output unit  507  controls the set  112 _ 6  of LED  201 _ 6  and diffractive optical element  202 _ 6  and the set  112 _ 8  of LED  201 _ 8  and diffractive optical element  202 _ 8  to emit light with the light emission intensity “I2.” 
     Flow of Control Process 
     Next, a flow of the control process by the control device  120  according to the second embodiment will be described.  FIG.  10    is a second flowchart illustrating a flow of the control process by the control device. The difference from the first flow chart illustrated in  FIG.  7    lies in operations S 1001  and S 1002 . 
     In operation S 1001 , the face size calculation unit  801  calculates the size of the face area of the subject notified from the face detection unit  502 . 
     In operation S 1002 , the light emission intensity control unit  802  determines the light emission intensity based on the size of the face area calculated by the face size calculation unit  801 . 
     As is apparent from the above description, the monitoring system according to the second embodiment detects the face area of the subject from the captured image of the image capturing unit, which is divided into a plurality of blocks according to the number of LEDs that irradiate the image capturing range of the image capturing unit. Further, the monitoring system according to the second embodiment controls an LED corresponding to a block including the face area of the subject among the plurality of blocks to emit light with the light emission intensity predetermined according to the size of the face area of the subject. 
     In this way, with the monitoring system according to the second embodiment, by controlling the light emission intensity using the relationship between the size of the face area of the subject and the image capturing distance to the subject, it is possible to capture the subject&#39;s face with constant brightness, regardless of the image capturing distance to the subject. 
     Third Embodiment 
     The case where the light emission intensity is determined based on the block to which the position of the face area of the subject belongs has been described in the first embodiment. However, the method for determining the light emission intensity is not limited thereto, but the light emission intensity may be determined according to the image quality of the block including the face area of the subject. Hereinafter, a third embodiment will be described focusing on the differences from the first embodiment. 
     Functional Configuration of Control Device 
     First, the functional configuration of the control device  120  according to the third embodiment will be described.  FIG.  11    is a third view illustrating an example of the functional configuration of the control device. The difference from  FIG.  5    is a block image quality evaluation unit  1101  and a light emission intensity control unit  1102 . 
     The block image quality evaluation unit  1101  is an example of an evaluation unit, and evaluates the image quality of a block including the face area of the subject notified from the face detection unit  502 . Specifically, the block image quality evaluation unit  1101  evaluates whether or not the block including the face area of the subject has a level of image quality where the authentication device  130  may execute a face authentication process. Further, the block image quality evaluation unit  1101  notifies the light emission intensity control unit  1102  of the evaluation result. 
     The light emission intensity control unit  1102  is an example of a control unit, and determines the light emission intensity based on the evaluation result notified from the block image quality evaluation unit  1101 . Specifically, the light emission intensity control unit  1102  determines the light emission intensity of a set of LED and diffractive optical element that irradiates the corresponding image capturing range with spot lighting, so that the authentication device  130  may obtain an image quality necessary to execute the face authentication process. 
     Flow of Control Process 
     Next, a flow of the control process by the control device  120  according to the third embodiment will be described.  FIG.  12    is a third flowchart illustrating a flow of the control process by the control device. The difference from the first flow chart illustrated in  FIG.  7    lies in operations S 1201  to S 1205 . 
     In operation S 1201 , the block image quality evaluation unit  1101  evaluates the image quality of a block including the face area of the subject notified from the face detection unit  502 . 
     In operation S 1202 , the block image quality evaluation unit  1101  determines whether or not each block including the face area has a level of image quality where the face authentication process may be executed (whether the image quality is in a level where a face authentication is possible). When it is determined in operation S 1202  that the block does not have a level of image quality where the face authentication is possible (No in operation S 1202 ), the process proceeds to operation S 1203 . 
     In operation S 1203 , the light emission intensity control unit  1102  determines the light emission intensity based on the evaluation result. 
     In operation S 1204 , the light emission intensity output unit  507  transmits the determined light emission intensity to the LED mounting substrate so that a set of LED and diffractive optical element that irradiates the image capturing range corresponding to each block including the face area may emit light with the determined light emission intensity. 
     In operation S 1205 , the image acquisition unit  501  acquires the captured image from the monitoring device  110 , and the process returns to operation S 704   
     Meanwhile, when it is determined in operation S 1202  that the image quality is in a level where the face authentication is possible (Yes in operation S 1202 ), the process proceeds to operation S 709 . 
     In operation S 709 , the image acquisition unit  501  acquires the captured image from the image capturing unit  111  of the monitoring device  110  and transmits the acquired captured image to the authentication device  130  as a captured image for authentication. 
     As is apparent from the above description, the monitoring system according to the third embodiment detects the face area of the subject from the captured image of the image capturing unit, which is divided into a plurality of blocks according to the number of LEDs that irradiate the image capturing range of the image capturing unit. Further, the monitoring system according to the third embodiment controls an LED corresponding to a block including the face area of the subject among the plurality of blocks to emit light with the light emission intensity according to the evaluation result of the image quality of the block including the face area of the subject. 
     In this way, with the monitoring system according to the third embodiment, by controlling the light emission intensity using the evaluation result of the image quality of the block in which the face area of the subject is captured, it is possible to capture the subject&#39;s face with constant brightness, regardless of the image capturing distance to the subject and the surroundings. 
     Fourth Embodiment 
     The case where there is one subject appearing in the captured image transmitted from the image capturing unit  111  has been described in the first embodiment. However, the number of subjects appearing in the captured image transmitted from the image capturing unit  111  is not limited to one. In a fourth embodiment, a case will be described in which a plurality of subjects appear in the captured image transmitted from the image capturing unit  111 . The description will be focused on the differences from the first embodiment. 
     Functional Configuration of Control Device 
     First, the functional configuration of the control device  120  according to the fourth embodiment will be described.  FIG.  13    is a fourth view illustrating an example of the functional configuration of the control device. The difference from  FIG.  5    lies in a face area number detection unit  1301 . 
     The face area number detection unit  1301  counts the number of subject&#39;s face areas notified from the face detection unit  502 , and notifies the face position detection unit  503  of the counted number of subject&#39;s face areas together with the face areas. Thereby, the face position detection unit  503  detects face area positions for the subject&#39;s face areas that correspond to the notified number of subject&#39;s face areas, and determines a block to which each detected face area position belongs. Further, the light emission intensity control unit  504  refers to the table  510  to determine the light emission intensity of a set of LED and diffractive optical element that irradiates the image capturing range corresponding to each block with spot lighting. 
     Example of Operation of Monitoring System 
     Next, an example of the operation of the monitoring system  100  including the control device  120  according to the fourth embodiment will be described.  FIGS.  14 A and  14 B  are third views illustrating an example of the operation of the monitoring system.  FIG.  14 A  illustrates a state in which the subject  610  is approaching the monitoring device  110 . 
     While  FIG.  14 A  does not illustrate subjects other than the subject  610  due to space limitations, it is assumed that, at the point of time when the subject  610  is walking at the position  621 , another subject  1410  is walking at a position closer to the monitoring device  110  than the subject  610 . Further, it is assumed that, at the point of time when the subject  610  is walking at the position  622 , another subject  1410  is walking at a position closer to the monitoring device  110  than the subject  610 . Furthermore, it is assumed that, at the point of time when the subject  610  is walking at the position  623 , another subject  1420  is walking at a position farther from the monitoring device  110  than the subject  610 . 
       FIG.  14 B  illustrates captured images  1401 ,  1402 , and  1403  obtained by the monitoring device  110  at respective positions (positions  621 ,  622 , and  623 ) of the subject  610  illustrated in  FIG.  14 A . It is assumed that when the captured images  1401 ,  1402 , and  1403  are obtained, the adjustment of the light emission intensity by the adjustment unit  506  has been completed, and the captured images  1401 ,  1402 , and  1403  have a level of image quality where the face areas of the subjects  610 ,  1410 , and  1420  may be detected. 
     In the case of the captured image  1401 , the face detection unit  502  determines that the block numbers of the blocks including the face areas of the subjects  610  and  1410  are “2” and “6.” Further, the face area number detection unit  1301  determines that the number of face areas is “2.” Further, the face position detection unit  503  determines that the block number of the block to which the detected face area position of the subject  610  belongs is “2,” and the block number of the block to which the detected face area position of the subject  1410  belongs is “6.” 
     Accordingly, the light emission intensity control unit  504  determines the light emission intensity to be “I4” or “I3.” Then, the light emission intensity output unit  507  controls the set  112 _ 2  of LED  201 _ 2  and diffractive optical element  202 _ 2  to emit light with the light emission intensity “I4,” and controls the set  112 _ 4  of LED  201 _ 4  and diffractive optical element  202 _ 4  to emit light with the light emission intensity “I3.” 
     Similarly, in the case of the captured image  1402 , the face detection unit  502  determines that the block numbers of the blocks including the face areas of the subjects  610  and  1410  are “5” and “9, 12.” Further, the face area number detection unit  1301  determines that the number of face areas is “2.” Further, the face position detection unit  503  determines that the block number of the block to which the detected face area position of the subject  610  belongs is “5,” and the block number of the block to which the detected face area position of the subject  1410  belongs is “9.” 
     Accordingly, the light emission intensity control unit  504  determines the light emission intensity to be “I3” or “I2.” Then, the light emission intensity output unit  507  controls the set  112 _ 12  of LED  201 _ 12  and diffractive optical element  202 _ 12  to emit light with the light emission intensity “I3.” Further, the light emission intensity output unit  507  controls the set  112 _ 5  of LED  201 _ 5  and diffractive optical element  202 _ 5  and the set  112 _ 7  of LED  201 _ 7  and diffractive optical element  202 _ 7  to emit light with the light emission intensity “I2.” 
     Similarly, in the case of the captured image  1403 , the face detection unit  502  determines that the block numbers of the blocks including the face areas of the subjects  610  and  1420  are “8, 11” and “1.” Further, the face area number detection unit  1301  determines that the number of face areas is “2.” Further, the face position detection unit  503  determines that the block number of the block to which the detected face area position of the subject  610  belongs is “11,” and the block number of the block to which the detected face area position of the subject  1420  belongs is “1.” 
     Accordingly, the light emission intensity control unit  504  determines the light emission intensity to be “I1” or “I4.” Then, the light emission intensity output unit  507  controls the set  112 _ 6  of LED  201 _ 6  and diffractive optical element  202 _ 6  and the set  112 _ 8  of LED  201 _ 8  and diffractive optical element  202 _ 8  to emit light with the light emission intensity “IL” Further, the light emission intensity output unit  507  controls the set  112 _ 1  of LED  201 _ 1  and diffractive optical element  202 _ 1  to emit light with the light emission intensity “I4.” 
     Flow of Control Process 
     Next, a flow of the control process by the control device  120  according to the fourth embodiment will be described.  FIG.  15    is a fourth flow chart illustrating a flow of the control process by the control device. The difference from the flowchart illustrated in  FIG.  7    lies in operation S 1501 . 
     In operation S 1501 , the face area number detection unit  1301  counts the number of subject&#39;s face areas detected in operation S 704 . Thereby, in operation S 705 , the face position detection unit  503  detects face area positions for the notified number of subject&#39;s face areas, and determines a block to which each detected face area position belongs. In operation S 706 , the light emission intensity control unit  504  determines the light emission intensity corresponding to each block. Further, in operation S 707 , the face detection unit  502  determines a block including each of the detected face areas, and the light emission intensity control unit  504  selects a set of LED and diffractive optical element, which controls to emit light with the determined emission intensity. 
     As is apparent from the above description, the monitoring system according to the fourth embodiment performs the same process as in the first embodiment on the face area of each of a plurality of face areas of the subjects appearing in the captured image transmitted from the image capturing unit  111 . 
     As a result, the monitoring system according to the fourth embodiment may obtain the same effects as the first embodiment. 
     OTHER EMBODIMENTS 
     In the above-described first and second embodiments, the light emission intensity control units  504  and  802  have been described as having the tables  510  and  810 , respectively. However, each of the light emission intensity control units  504  and  802  may have a plurality of tables  510  and  810  in order to cope with a change in the surrounding environment such as the weather, the time zone and the like. Alternatively, instead of the tables  510  and  810 , the light emission intensity control units  504  and  802  may perform a machine learning in advance on the correspondence relationship between the block to which the face area position belongs along with the surrounding environment, and the light emission intensity of the set of LED and diffractive optical element, and may use the learning result at that time. 
     In addition, in the control device  120 , for example, the image quality evaluation unit  505  evaluates the quality of the captured image to specify the surrounding environment. Further, the light emission intensity control unit  504  determines the light emission intensity by referring to the table corresponding to the surrounding environment specified by the image quality evaluation unit  505 , among the plurality of tables. Alternatively, the light emission intensity control unit  504  may determine the light emission intensity by inputting the block to which the subject&#39; face area position belongs and the specified surrounding environment into the learning result. 
     In addition, in the third embodiment, the correspondence relationship between the image quality of the block including the face area and the light emission intensity of a set of LED and diffractive optical element is not mentioned, but the correspondence relationship may be, for example, defined by a table. Alternatively, the correspondence relationship may be defined based on machine learning. 
     In addition, in each of the above-described embodiments, the monitoring device  110  in which the image capturing unit  111  and the sets  112 _ 1  to  112 _ 12  of LEDs and diffractive optical elements are integrated has been described. However, the image capturing unit  111  and the set  112 _ 1  to  112 _ 12  of LEDs and diffractive optical elements may be separate devices. 
     In addition, in each of the above-described embodiments, the monitoring device  110  and the control device  120  are separate devices. However, the monitoring device  110  and the control device  120  may be an integrated device. Alternatively, some of the functions of the control device  120  may be implemented in the monitoring device  110 . 
     The present disclosure is not limited to the elements described herein, and for example, the elements described in the foregoing embodiments may be combined with other elements. In view of this point, the present disclosure may be changed without departing from the gist of the present disclosure, and may be appropriately determined according to the application form. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to an illustrating of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.