Patent Publication Number: US-9413460-B2

Title: Communication method

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 14/260,351 filed on Apr. 24, 2014, which claims the benefit of U.S. Provisional Patent Application No. 61/921,132 filed on Dec. 27, 2013. The entire disclosure of the above-identified application, including the specification, drawings and claims is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The present invention relates to a method of communication between a mobile terminal such as a smartphone, a tablet terminal, or a mobile phone and a home electric appliance such as an air conditioner, a lighting device, or a rice cooker. 
     BACKGROUND 
     In recent years, a home-electric-appliance cooperation function has been introduced for a home network, with which various home electric appliances are connected to a network by a home energy management system (HEMS) having a function of managing power usage for addressing an environmental issue, turning power on/off from outside a house, and the like, in addition to cooperation of AV home electric appliances by internet protocol (IP) connection using Ethernet® or wireless local area network (LAN). However, there are home electric appliances whose computational performance is insufficient to have a communication function, and home electric appliances which do not have a communication function due to a matter of cost. 
     CITATION LIST 
     Patent Literature 
     Japanese Unexamined Patent Application Publication No. 2002-290335 
     SUMMARY 
     Technical Problem 
     However, the conventional method is limited to a case in which a device to which the method is applied has three color light sources such as an illuminator. The present invention solves this problem, and provides an information communication method that enables communication between various devices including a device with low computational performance. 
     Solution to Problem 
     In accordance with an aspect of the present disclosure, there is provided a communication method used in a terminal that includes a front camera as a first image capturing unit and a back camera as a second image capturing unit, the communication method including: switching between the front camera and the back camera according to an orientation of the terminal; and performing visible light communication by using the front camera or the back camera which is selected in the switching. 
     General and specific aspect(s) disclosed above may be implemented using a system, a method, an integrated circuit, a computer program, or a computer-readable recording medium such as a CD-ROM, or any combination of systems, methods, integrated circuits, computer programs, or computer-readable recording media. 
     Additional benefits and advantages of the disclosed embodiments will be apparent from the Specification and Drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the Specification and Drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages. 
     Advantageous Effects 
     An information communication method disclosed herein enables communication between various devices including a device with low computational performance. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other objects, advantages and features of the disclosure will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate a specific embodiment of the present disclosure. 
         FIG. 1  is a diagram illustrating an example of a situation of using a mobile terminal according to Embodiment 1. 
         FIG. 2  is a schematic diagram illustrating an example of the mobile terminal according to Embodiment 1. 
         FIG. 3  is a diagram illustrating an example of a situation of holding the mobile terminal to be horizontal according to Embodiment 1. 
         FIG. 4  is a diagram illustrating an example of a situation of holding the mobile terminal to be vertical according to Embodiment 1. 
         FIG. 5  is a schematic diagram illustrating examples of a shop map presented by the mobile terminal according to Embodiment 1. 
         FIG. 6  is a diagram illustrating an example of a product UI presented by the mobile terminal according to Embodiment 1. 
         FIG. 7  is a diagram illustrating an example of a situation where the product UI presented by the mobile terminal is operated, according to Embodiment 1. 
         FIG. 8  is a diagram illustrating user&#39;s actions of moving the mobile terminal in a direction from right to left according to Embodiment 1. 
         FIG. 9  is a diagram illustrating an example of a situation of using a watch-type device according to Embodiment 1. 
         FIG. 10  is a diagram illustrating a whole configuration of a communication system according to Embodiment 1. 
         FIG. 11  illustrates an example of a structure of pieces of information stored in a product information storage unit according to Embodiment 1. 
         FIG. 12  is a diagram illustrating an example of a layout of the product UI presented by the mobile terminal according to Embodiment 1. 
         FIG. 13  illustrates an example of a structure of pieces of information stored in a map information storage unit according to Embodiment 1. 
         FIG. 14  illustrates an example of a structure of pieces of information managed by a state management unit according to Embodiment 1. 
         FIG. 15  is a flowchart of processing performed by a lighting device according to Embodiment 1. 
         FIG. 16  is a flowchart of processing performed by the mobile terminal according to Embodiment 1. 
         FIG. 17  is a flowchart of ceiling light-related processing according to Embodiment 1. 
         FIG. 18  is a flowchart of base light-related processing according to Embodiment 1. 
         FIG. 19  is a flowchart of UI-related processing according to Embodiment 1. 
         FIG. 20  is a flowchart of UI processing for map information according to Embodiment 1. 
         FIG. 21  is a flowchart of UI processing for product information according to Embodiment 1. 
         FIG. 22  is a flowchart of entire display processing according to Embodiment 1. 
         FIG. 23  is a flowchart of preparation for display updating according to Embodiment 1. 
         FIG. 24  is a flowchart of display updating according to Embodiment 1. 
         FIG. 25  is a diagram illustrating a detailed structure of a light receiving control unit according to Embodiment 2. 
         FIG. 26  is a flowchart of illuminance pattern detection performed by a light receiving control unit according to Embodiment 2. 
         FIG. 27  is a diagram illustrating a detailed structure of a light receiving control unit according to Embodiment 3. 
         FIG. 28  is a flowchart of illuminance pattern detection performed by a light receiving control unit according to Embodiment 3. 
         FIG. 29  is a diagram illustrating an example of a situation of using a mobile terminal for detecting a movement of a line of sight according to Embodiment 4. 
         FIG. 30  is a diagram illustrating an example of a structure of the mobile terminal according to Embodiment 4. 
         FIG. 31  is a flowchart performed when making inquiry to a server according to Embodiment 4. 
         FIG. 32  illustrates an example of a structure of pieces of information managed by a database (DB) of identifiers according to Embodiment 4. 
         FIG. 33  is a diagram illustrating a detailed structure of a light receiving control unit according to Embodiment 5. 
         FIG. 34  is a diagram illustrating a detailed structure of a light receiving control unit according to Embodiment 6. 
         FIG. 35  is a diagram illustrating an example of a situation where imaging elements arranged in a line are exposed simultaneously with an exposure start time being shifted in order of lines according to Embodiment 7. 
         FIG. 36  is a diagram illustrating a situation where, after exposure of one exposure line ends, exposure of a next exposure line starts according to Embodiment 7. 
         FIG. 37  is a diagram illustrating a situation where, after exposure of one exposure line ends, exposure of a next exposure line starts according to Embodiment 7. 
         FIG. 38  is a diagram illustrating a situation where, before exposure of one exposure line ends, exposure of a next exposure line starts according to Embodiment 7. 
         FIG. 39  is a diagram illustrating influence of a difference in an exposure time in the case where an exposure start time of each exposure line is the same, according to Embodiment 7. 
         FIG. 40  is a diagram illustrating influence of a difference in an exposure time of exposure lines in the case where each exposure time is the same, according to Embodiment 7. 
         FIG. 41  is a diagram illustrating advantages of using a short exposure time in the case where each exposure line does not overlap in an exposure time, according to Embodiment 7. 
         FIG. 42  is a diagram illustrating a relation between a minimum change time of light source luminance, an exposure time, a time difference between exposure start times of the exposure lines, and a captured image, according to Embodiment 7. 
         FIG. 43  is a diagram illustrating a relation between a transition time of light source luminance and a time difference between exposure start times of the exposure lines, according to Embodiment 7. 
         FIG. 44  illustrates a relation between a high frequency noise of light source luminance and an exposure time, according to Embodiment 7. 
         FIG. 45  is a graph representing a relation between an exposure time and a magnitude of high frequency noise when the high frequency noise of the light source luminance is 20 microseconds, according to Embodiment 7. 
         FIG. 46  is a diagram illustrating a relation between an exposure time and a recognition success rate according to Embodiment 7. 
         FIG. 47  is a diagram illustrating an example of a method for observing luminance by a light emitting unit according to Embodiment 7. 
         FIG. 48  is a diagram illustrating an example of each mode of a receiving device according to Embodiment 7. 
         FIG. 49  is a diagram illustrating a service provision system according to Embodiment 8. 
         FIG. 50  is a flowchart of service provision according to Embodiment 8. 
         FIG. 51  is a flowchart of service provision according to a variation of Embodiment 8. 
         FIG. 52  is a flowchart of service provision according to another variation of Embodiment 8. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In accordance with an aspect of the present invention, there is provided a communication method used in a terminal that includes a front camera as a first image capturing unit and a back camera as a second image capturing unit, the communication method including: switching between the front camera and the back camera according to an orientation of the terminal; and performing visible light communication by using the front camera or the back camera which is selected in the switching. 
     In accordance with another aspect of the present invention, there is provided a communication method used in a terminal that includes a front camera as a first image capturing unit and a back camera as a second image capturing unit, the communication method including: switching between the front camera and the back camera at predetermined intervals; and performing visible light communication by using the front camera or the back camera which is selected in the switching. 
     For example, it is also possible that when the orientation of the terminal is substantially perpendicular to ground, the performing of visible light communication includes: performing, by using the back camera selected in the switching, visible light communication with a first device which is located in front of a user and emits visible light, thereby obtaining product information of a product rack provided with the first device; and displaying the product information on a display unit of the terminal, and when the orientation of the terminal is substantially parallel to ground, the performing of visible light communication includes: performing, by using the front camera selected in the switching, visible light communication with a second device which is located above the user and emits visible light, thereby obtaining position information of the second device; and displaying, on the display unit of the terminal, the position information as a current position of the user together with map information of an area where the user is present. 
     For example, it is further possible that the terminal is included in a watch, the front camera is provided on a front surface of the watch, and the back camera is provided on a side surface of the watch, the front surface having a display unit that displays a time of the watch, and when the display unit of the watch is substantially parallel to ground, the performing of visible light communication includes: performing, by using the front camera, visible light communication with a second device which is located above the user and emits visible light, thereby obtaining position information of the second device; performing, by using the back camera, visible light communication with a first device which is located in front of the user and emits visible light, thereby obtaining product information of a product rack provided with the first device; and displaying one of the position information and the product information on the display unit of the terminal according to selection of the user, the position information being displayed as a current position of the user together with map information of an area where the user is present. 
     For example, it is still further possible that the performing of visible light communication includes inquiring a server about an identifier uniquely assigned to a device that emits visible light, thereby obtaining information related to the device corresponding to the identifier, the identifier being obtained through visible light communication, and when no information corresponds to the identifier as a result of the inquiring the server, the performing of visible light communication further includes displaying, on the display unit of the terminal, an error notification indicating that no information is related to the device. 
     For example, it is still further possible that the communication method further includes changing the product information displayed on the display unit of the terminal, wherein in the changing, the product information to be displayed on the display unit is changed by user&#39;s flicking on the display unit. 
     For example, it is still further possible that the communication method further includes changing the map information displayed on the display unit of the terminal, wherein in the changing, the map information to be displayed on the display unit is changed by user&#39;s flicking on the display unit. 
     For example, it is still further possible that the performing of visible light communication includes initializing an offset assigned to the map information, when the product information of the product rack provided with the first device is obtained by using the back camera selected in the switching after displaying the map information on the display unit of the terminal, and the performing of visible light communication includes initializing an offset assigned to the product information, when the position information is obtained as the current position of the user by using the front camera selected in the switching after displaying the product information on the display unit of the terminal. 
     For example, it is still further possible that the communication method further includes detecting a line of sight of the user, wherein in the switching, the back camera is selected when the orientation of the terminal is substantially perpendicular to ground, in the detecting, the line of the sight of the user is detected by using the front camera, and the performing of visible light communication includes: performing, by using the back camera selected in the switching, visible light communication with the first device that is located in front of the user and emits visible light, thereby obtaining product information of a first product rack provided with the first device; and, when a second product rack in the line of the sight detected in the detecting is different from the first product rack, displaying product information of the second product rack instead of the product information of the first product rack. 
     For example, it is still further possible that the terminal includes an image sensor having a plurality of exposure lines, and the communication method further includes: obtaining image data by sequentially starting exposure of the exposure lines of the image sensor at different times and capturing a subject in an exposure time of 1/480 seconds or less to partially and temporally overlap exposure times of adjacent exposure lines among the exposure lines; obtaining identification information of the subject by demodulating a bright line pattern which appears in the image data and corresponds to the exposure lines; and providing a user of the terminal with service information associated with the identification information of the subject. 
     For example, it is still further possible that the communication method further includes: receiving a request for distribution of information from a requester; distributing the information requested from the requester as information corresponding to data obtained by the visible light; and performing information processing for charging the requester according to the distribution of the information. 
     In accordance with still another aspect of the present invention, there is provided a communication device that includes a front camera as a first image capturing unit and a back camera as a second image capturing unit, the communication device including: a switching unit configured to switch between the front camera and the back camera according to an orientation of a terminal; and a visible light communication unit configured to perform visible light communication by using the front camera or the back camera which is selected by the switching unit. 
     These general and specific aspects may be implemented to a system, a method, an integrated circuit, a computer program, and a computer-readable recording medium, such as a Compact Disc-Read Only Memory (CD-ROM), and may be implemented also to a desired combination of them. 
     Hereinafter, embodiments are specifically described with reference to the Drawings. However, the description in details beyond necessity may be omitted. For example, detailed explanation of well-known matters and repeated explanation of the same structure may be omitted. This is because unnecessarily redundant explanation is avoided below to facilitate understanding of those skilled in the art. 
     Furthermore, each of the embodiments described below is a general or specific example. The numerical values, shapes, materials, structural elements, the arrangement and connection of the structural elements, steps, the processing order of the steps etc. shown in the following embodiments are mere examples, and therefore do not limit the scope of the present invention. Therefore, among the structural elements in the following embodiments, structural elements not recited in any one of the independent claims representing the broadest concepts are described as arbitrary structural elements. 
     Embodiment 1 
     Embodiment 1 describes a communication device that switches between light receiving units for visible light communication. 
     The communication device according to the present embodiment is assumed to be included in a mobile terminal, such as a smartphone. 
     The following describes a situation of using a mobile terminal including the communication device according to the present embodiment. 
       FIG. 1  is a diagram illustrating an example of a situation of using the mobile terminal according to Embodiment 1.  FIG. 2  is a schematic diagram illustrating an example of the mobile terminal according to Embodiment 1. 
     The communication device used by a user performs visible light communication with a ceiling light A 1101  provided on a ceiling so as to receive an ID transmitted from the ceiling light A 1101 . Furthermore, the communication device performs visible light communication with a base light (rack light) A 1103  in front of the user so as to receive an ID transmitted from the base light A 1103 . 
     The communication device is characterized by (i) including a plurality of light receiving units (image capturing units) each receiving visible light, and (ii) by selecting a suitable light receiving unit from among the light receiving units according to an attitude of the mobile terminal in order to receive a plurality of IDs through visible light communication with a plurality of devices provided at different positions, such as the ceiling light A 1101  and the base light A 1103 . 
     In the present embodiment, the communication device includes a front camera that is the first image capturing unit and a back camera that is the second image capturing unit. More specifically, for example, as illustrated in  FIG. 2 , a mobile terminal A 1201  including the communication device has a front camera A 1202  (first image capturing unit) on the front, a back camera A 1203  (second image capturing unit) on the back, and a touch panel unit A 1204  on the front. The communication device switches between the front camera A 1202  (first image capturing unit) and the back camera A 1203  (second image capturing unit) according to an orientation of the terminal (mobile terminal) including the communication device, and thereby performs visible light communication using the switched camera. 
     The following describes a situation where visible light communication is performed by switching between the image capturing units according to an orientation of the mobile terminal including the communication device. 
       FIG. 3  is a diagram illustrating an example of a situation of holding the mobile terminal to be horizontal according to Embodiment 1.  FIG. 4  is a diagram illustrating an example of a situation of holding the mobile terminal to be vertical according to Embodiment 1. 
     For example, as illustrated in  FIG. 3 , when the user holds the mobile terminal A 1201  such that a display surface of the mobile terminal A 1201  faces a ceiling (in other words, the display surface is substantially parallel to the ground), the mobile terminal A 1201  determines by using an acceleration sensor in the mobile terminal A 1201  that the mobile terminal A 1201  is parallel to the ceiling (the ground), and therefore operates the front camera A 1202 . Here, operation of the back camera A 1203  may be stopped for saving power consumption. 
     While the ceiling light A 1101  is lighted, a light amount of the lighting is varied with a specific pattern (light emitting pattern), thereby performing visible light communication. For example, as illustrated in a region A 1301 , when the front camera A 1202  currently operating detects light emitted from the ceiling light A 1101 , the front camera A 1202  captures the light to perform the visible light communication. In other words, the mobile terminal A 1201  obtains an ID associated with the light emitting pattern of the ceiling light A 1101  through the visible light communication with the ceiling light A 1101 , and thereby changes actions according to a type of the ID. 
     Furthermore, for example, as illustrated in  FIG. 4 , when the user holds the mobile terminal A 1201  to be perpendicular to the ground, the mobile terminal A 1201  determines by using the included acceleration sensor that the mobile terminal A 1201  does not face the ceiling light on the ceiling but faces the base light A 1103  provided on a wall or a rack A 1102 , and thereby operates the back camera A 1203 . Here, operation of the front camera A 1202  may be stopped for saving power consumption. 
     While the base light A 1103  is lighted, a light amount of the lighting is varied with a specific pattern (light emitting pattern), thereby performing visible light communication. When the back camera A 1203  currently operating detects the light emitted from the base light A 1103 , the back camera A 1203  captures the light to perform the visible light communication. In other words, the mobile terminal A 1201  obtains an ID associated with the light emitting pattern of the base light A 1103  through the visible light communication with the base light A 1103 , and thereby changes actions according to a type of the ID. 
     Next, an example of the action according to a type of an obtained ID is described. 
       FIG. 5  is a diagram illustrating examples of a shop map presented by the mobile terminal according to Embodiment 1. 
     As illustrated in  FIG. 3 , if the mobile terminal A 1201  determines that the mobile terminal A 1201  is parallel to the ceiling (ground), the mobile terminal A 1201  transmits, to a server, an ID associated with a light emitting pattern obtained by visible light communication performed using the front camera A 1202 . The server searches out position information corresponding to the ID, and notifies the position information to the mobile terminal A 1201 . 
     When the position information is obtained from the server, the mobile terminal A 1201  presents the user with a map (map information) showing a user&#39;s current position, such as a shop map as illustrated in  FIG. 5 , in which the obtained position information is presented as the user&#39;s current position. 
     As described above, if the orientation of the mobile terminal A 1201  is substantially parallel to the ceiling (the ground), the front camera A 1202  is used to perform visible light communication with the ceiling light A 1101  (second device) that is located above the user and emits visible light, so as to obtain position information of the ceiling light A 1101  (second device). The mobile terminal A 1201  displays, on a display unit of the mobile terminal A 1201 , the obtained position information together with map information of an area where the user is present. 
     It should be noted that the map information displayed on the display unit of the mobile terminal A 1201  may be changed by flicking. For example, if the user flicks on the display unit of the mobile terminal A 1201 , an offset position of the map information displayed on the display unit is changed. 
       FIG. 6  is a diagram illustrating an example of a product user interface (UI) presented by the mobile terminal according to Embodiment 1.  FIG. 7  is a diagram illustrating an example of the situation where the product UI presented by the mobile terminal is operated, according to Embodiment 1. 
     If it is determined that the mobile terminal A 1201  is perpendicular to the ground as illustrated in  FIG. 4 , the mobile terminal A 1201  transmits, to the server, an ID which is obtained through visible light communication using the front camera A 1202 . The server searches a database for a product type and stock of a product rack associated with the ID, and generates an image showing a stock amount of products according to the found product type and stock. Here, for example, as illustrated in  FIG. 6 , the server generates an image in which an image of products in stock is superimposed on an image of products arranged on the product rack. Then, the server transmits the generated image to the mobile terminal A 1201 . 
     The mobile terminal A 1201  presents, on the display unit, the image received from the server, namely, the image showing a stock amount of the products. 
     As described above, when the orientation of the mobile terminal A 1201  is substantially perpendicular to the ground, the back camera A 1203  is used to perform visible light communication with the base light A 1103  (first device) that is located in front of the user and emits visible light, so as to obtain product information of the product rack provided with the base light A 1103  (first device). The mobile terminal A 1201  displays the obtained product information on the display unit of the mobile terminal A 1201 . 
     It should be noted that the image displayed on the display unit of the mobile terminal A 1201  to show the stock amount of the products may be changed by flicking. For example, if the user flicks on the display unit of the mobile terminal A 1201 , an offset of the image displayed on the display unit to show the stock amount of the products is changed, or the image itself is changed, thereby changing the product information displayed on the display unit. 
     When the server transmits the generated image (image data) to the mobile terminal A 1201 , it is also possible to transmit a plurality of images (plural pieces of image data) to the mobile terminal A 1201 . For example, as illustrated in  FIG. 7 , the server sequentially transmits an image A 1701  of a rack with a small stock amount and an image A 1703  of a rack with a large stock amount, and the mobile terminal A 1201  therefore displays the image A 1701  of the rack with the small stock amount. According to a screen operation of the user, such as flicking, the mobile terminal A 1201  displays sequentially the image A 1701  of the rack with the small stock amount, then an image A 1702  showing a scrolling transition, and finally the image A 1703  of the rack with the large stock amount. As a result, the user can check product stocks more instinctly and more easily. 
       FIG. 8  is a diagram illustrating user&#39;s actions of moving the mobile terminal in a direction from right to left according to Embodiment 1. 
     As described above, the mobile terminal A 1201  can switch presented details according to a received ID. Therefore, for example, as illustrated in  FIG. 8 , it is possible to switch pieces of product information of a rack A 1801  and a rack A 1802  which the user has viewed. In other words, when the user causes the mobile terminal A 1201  to face the rack A 1801  and then face the rack A 1802 , the mobile terminal A 1201  sequentially receives an ID from a base light A 1803  and an ID from a base light A 1804 . Therefore, the mobile terminal A 1201  can sequentially display product information corresponding to the ID received from the base light A 1803  and then product information corresponding to the ID received from the base light A 1804 . 
     As described above, the mobile terminal A 1201  can present the user with pieces of information corresponding to an ID obtained by visible light communication with a device selected according to an attitude of the user. 
     It should be noted that the terminal including the communication device according to the present embodiment is not limited to the mobile terminal A 1201  having image capturing units on the front and on the back. The terminal may be a watch. The following describes an example of the situation where the terminal including the communication terminal according to the present embodiment is a watch (watch-type device) with reference to  FIG. 9 . 
       FIG. 9  is a diagram illustrating an example of the situation of using a watch-type device according to Embodiment 1. 
     The watch-type device A 1901  illustrated in  FIG. 9  includes a light receiving unit A 1902  on the front part and a light receiving unit A 1903  on the side part. The light receiving unit A 1902  corresponds to the above-described front camera A 1202  (first image capturing unit), and the light receiving unit A 1903  corresponds to the above-described back camera A 1203  (second image capturing unit). 
     The watch-type device A 1901  also includes an acceleration sensor. 
     If, for example, as illustrated in  FIG. 9 , the front part of the watch-type device A 1901  faces a direction parallel to the ground, the watch-type device A 1901  operates the light receiving unit A 1902  and the light receiving unit A 1903 . The light receiving unit A 1902  receives light emitted from a ceiling light A 1904 , and the light receiving unit A 1903  receives light emitted from a base light A 1905 . 
     More specifically, the watch-type device A 1901  uses the light receiving unit A 1902  to perform visible light communication with the ceiling light A 1904  (second device) that is located above the user and emits visible light, so as to obtain position information of the ceiling light A 1904  (second device). On the other hand, the watch-type device A 1901  uses the light receiving unit A 1903  to perform visible light communication with the base light A 1905  (first devices) that is located in front of the user and emits visible light, so as to obtain product information of the product rack provided with the base light A 1905  (first device). Then, according to the user&#39;s selection, the obtained position information is displayed as a current position of the user on the display unit together with map information of an area where the user is present, or the obtained product information is displayed on the display unit. 
     It is also possible that the watch-type device A 1901  may switch between the light receiving unit A 1902  and the light receiving unit A 1903  at predetermined intervals, and the switched light receiving unit A 1902  or the light receiving unit A 1903  is used to perform visible light communication. 
     As described above, the mobile terminal or the watch including the communication device according to the present embodiment is used. 
     The following describes the structure of the communication device according to the present embodiment. 
       FIG. 10  is a diagram illustrating a whole configuration of a communication system according to Embodiment 1. 
     The communication system according to the present embodiment includes at least a mobile terminal A 11001 , a lighting device A 11002 , a lighting device A 11003 , and a server device A 11015 . 
     [Structure of Lighting Device A 11002 ] 
     The lighting device A 11002  includes an ID storage unit A 11004 , an encoding unit A 11005 , an illuminance pattern storage unit A 11006 , and a light emitting unit A 11007 . The lighting device A 11002  corresponds to the second device, such as the ceiling light A 1101  illustrated in  FIG. 1  or the ceiling light A 1904  illustrated in  FIG. 9 . 
     The ID storage unit A 11004  is a memory region. In the ID storage unit A 11004 , identifiers each uniquely identifying an individual, such as media access control (MAC) addresses, are previously written. 
     When the lighting device A 11002  is supplied with power, the encoding unit A 11005  reads an ID from the ID storage unit A 11004 , and converts the ID to an illuminance pattern of the lighting, in other words, an illuminance of the lighting at corresponding timing, and stores the illuminance pattern to the illuminance pattern storage unit A 11006 . 
     When the above-described start phase is completed, the light emitting unit A 11007  controls light emission by updating illuminance at a high speed according to the pattern stored in the illuminance pattern storage unit A 11006 . The lighting device A 11002  thereby performs illuminance control (visible light communication) based on the ID. 
     [Structure of Lighting Device A 11003 ] 
     Like the lighting device A 11002 , the lighting device A 11003  includes an ID storage unit A 11018 , an encoding unit A 11009 , an illuminance pattern storage unit A 11010 , and a light emitting unit A 11011 . The lighting device A 11003  corresponds to the first device, such as the base light A 1103  illustrated in  FIG. 1  and the base light A 1905  illustrated in  FIG. 9 . 
     The ID storage unit A 11008  is a memory region. In the ID storage unit A 11008 , identifiers each uniquely identifying an individual, such as MAC addresses, are previously written. 
     When the lighting device A 11003  is supplied with power, the encoding unit A 11019  reads an ID from the ID storage unit A 11008 , and converts the ID to an illuminance pattern of the lighting, in other words, an illuminance of the lighting at corresponding timing, and stores the illuminance pattern to the illuminance pattern storage unit A 11010 . 
     When the above-described start phase is completed, the light emitting unit A 11011  controls light emission by updating illuminance at a high speed according to a pattern stored in the illuminance pattern storage unit A 11010 . The lighting device A 11003  thereby performs illuminance control (visible light communication) based on the ID. 
     [Structure of Mobile Terminal A 11001 ] 
     Subsequently, the structure of the mobile terminal A 11001  is described. 
     The mobile terminal A 11001  includes a light receiving control unit A 11012 , an ID storage unit A 11013 , a DB management unit A 11014 , a product information storage unit A 11016 , a map information storage unit A 11017 , a display unit A 11018 , a state management unit A 11019 , and an UI unit A 11020 . The mobile terminal A 11001  corresponds to the mobile terminal A 1201  illustrated in  FIG. 2  or the watch-type device illustrated in  FIG. 9 . 
     [Light Receiving Control Unit A 11012 ] 
     The light receiving control unit A 11012  detects an illuminance pattern of the light emitting unit A 11007  and an illuminance pattern of the light emitting unit A 11011 , then converts the detected patterns to IDs, and stores the IDs to the ID storage unit A 11013 . In the case of the present embodiment, since an illuminance pattern of the light emitting unit A 11007  or the light emitting unit A 11011  is detected, the ID stored in the ID storage unit A 11013  is identical to either an ID stored in the ID storage unit A 11004  or an ID stored in the ID storage unit A 11008 . 
     [Product Information Storage Unit A 11016 ] 
     Next, the product information storage unit A 11016  is described. 
       FIG. 11  illustrates an example of a structure of pieces of information stored in a product information storage unit according to Embodiment 1.  FIG. 12  is a diagram illustrating an example of a layout of the product UI presented by the mobile terminal according to Embodiment 1. 
     For example, as illustrated in  FIG. 11 , in the product information storage unit A 11016 , there are stored a validity flag A 11101 , rack exterior A 11102 , rack interior A 11103 , product type information A 11104 , product stock information A 11105  are stored. 
     The validity flag A 11101  indicates whether or not to render product stock information. The validity flag A 11101  has a value indicating TRUE or a value indicating FALSE. If the value is TRUE, the display unit A 11018  renders the product stock information. 
     The rack exterior A 11102  indicates an exterior of a rack to be drawn. As the rack exterior A 11102 , a height and a width of a plain scale of the rack are recorded. Based on the plain scale information, the display unit A 11018  performs the rendering to draw the rack within the image. For example, the display unit A 11018  renders an outer line of the rack as a layout illustrated in  FIG. 12 . 
     The rack interior A 11103  indicates the number of shelves in the rack and a height ratio of each of the shelves to the rack. Based on these pieces of information, the display unit A 11018  renders the internal boards of the shelves with intervals as illustrated in  FIG. 12 . 
     The product type information A 11104  stores the number of product variations and pieces of image data of respective products. For example, when the format of the image data received from the server device A 11015  is PNG data, the PNG data is directly arranged in the product type image data corresponding to the product type information A 11104 . Then, the display unit A 11018  develops the PNG data to a cache region (not illustrated) immediately prior to the rendering, and renders details of the cache region to present the user with the image of the products. 
     The product stock information A 11105  indicates (a) the number of stocked products, and (b) a type and coordinate information of the stocked products to be rendered. Among pieces of coordinate information in  FIG. 11 , x-coordinates indicate “where a target product is in a product rack when it is counted from the left”, y-coordinate indicates “where a shelf with a target product is in a product rack as being counted from the top”, and z-coordinate indicates “where a rack with a target product is among the racks arranged to the depth direction as being counted from the front.” The display unit A 11018  renders the product image identified by a product type number on the coordinates matching these pieces of coordinate information. 
     [Map Information Storage Unit A 11017 ] 
     Next, the map information storage unit A 11017  is described. 
       FIG. 13  illustrates an example of a structure of pieces of information stored in the map information storage unit A 11017  according to Embodiment 1. 
     For example, as illustrated in  FIG. 13 , the map information storage unit A 11017  stores a validity flag A 11301 , a data length of a map image (map image data length) A 11302 , and a map image A 11303 . 
     The validity flag A 11301  indicates whether or not to render the map information. The validity flag A 11301  has a value indicating TRUE or a value indicating FALSE. If the value is TRUE, the display unit A 11018  renders the map information. 
     The map image A 11303  stores information of image data itself of a map. For example, if a format of image data received from the server device A 11015  is PNG data, the map image A 11303  stores the PNG data directly. 
     The map image data length A 11302  indicates a data length of the map image A 11303 . For example, if a format of image data received from the server device A 11015  is PNG data, the display unit A 11018  develops the PNG data to a cache region (not illustrated) immediately prior to rendering, and renders details of the cache region to present the user with the image of the products. 
     [State Management Unit A 11019 ] 
     Next, the state management unit A 11019  is described. 
       FIG. 14  illustrates an example of a structure of pieces of information managed by the state management unit according to Embodiment 1. 
     As illustrated in  FIG. 14 , the state management unit A 11019  includes an error flag A 11601 , a previous frame drawing type A 11602 , a display rack depth A 11603 , a map offset X A 11604 , a map offset Y A 11605 , a previously-obtained ID A 11606 . 
     An initial value of the previously-obtained ID A 11606  is an invalid value. According to the present embodiment, it is assumed that the invalid value=−1. 
     The error flag A 11601  has a value indicating TRUE or a value indicating FALSE. The state management unit A 11019  sets the error flag A 11601  to TRUE, if, for example, an ID transmitted to the server device A 11015  is an invalid ID that is not related to any of the lighting devices (lighting device A 11002 , lighting device A 11003 ). Furthermore, for example, if the error flag A 11601  is TRUE, the display unit A 11018  notifies the user that receiving is failed. 
     Values of the display rack depth A 11603 , the map offset X A 11604 , and the map offset Y A 11605  are updated by the UI unit A 11020 , when the user operates the touch panel unit A 1204 . 
     [DB Management Unit A 11014 ] 
     Next, the DB management unit A 11014  is described. 
     The DB management unit A 11014  reads an ID from the ID storage unit A 11013 , and then inquires the server device A 11015  whether or not the readout ID matches the recorded ID. 
     If it is determined that the readout ID matches, for example, the ID of the ceiling light on the ceiling, the DB management unit A 11014  requests the server device A 11015  to transmit the map information as illustrated in  FIG. 5 , namely, map image showing the ceiling light and its periphery. As a result of the request, the DB management unit A 11014  obtains the map image (map information) as illustrated in  FIG. 5  and stores it to the map information storage unit A 11017 . Then, the DB management unit A 11014  sets the validity flag A 11301  of the map information storage unit A 11017  to TRUE, sets the validity flag of the product information storage unit A 11016  to FALSE, and sets the error flag A 11601  to FALSE. 
     On the other hand, if it is determined that the readout ID is, for example, an ID of any of the base lights provided on product racks, the DB management unit A 11014  requests the server device A 11015  to transmit the product information illustrated in  FIG. 11 . As a result of the request, the DB management unit A 11014  stores the obtained product information to the product information storage unit A 11016 . Then, the DB management unit A 11014  sets the validity flag A 11301  of the map information storage unit A 11017  to FALSE, sets the validity flag of the product information storage unit A 11016  to TRUE, and sets the error flag A 11601  to FALSE. 
     It should be noted that, if the ID transmitted to the server device A 11015  is an invalid ID that is not related to any of the ceiling light and the base lights, the DB management unit A 11014  sets the validity flag A 11301  of the map information storage unit A 11017  to FALSE, sets the validity flag of the product information storage unit A 11016  to FALSE, and sets the error flag A 11601  to TRUE. 
     [UI Unit A 11020 ] 
     Subsequently, the UI unit A 11020  is described. 
     The UI unit A 11020  is cooperated with the touch panel unit A 1204 . 
     If, for example, the validity flag A 11101  of the product information storage unit A 11016  is TRUE and the user flicks on the touch panel unit A 1204 , the UI unit A 11020  updates the values of the map offset X A 11604  and the map offset Y A 11605 . For example, if the user flicks to the right on the touch panel unit A 1204 , the UI unit A 11020  increases the value of the map offset X A 11604  by an amount of the flicking. If the user flicks to the left on the touch panel unit A 1204 , the UI unit A 11020  decreases the value of the map offset X A 11604  by an amount of the flicking. Furthermore, for example, if the user flicks upwards on the touch panel unit A 1204 , the UI unit A 11020  decreases the value of the map offset Y A 11605 . If the user flicks downwards on the touch panel unit A 1204 , the UI unit A 11020  increases the value of the map offset Y A 11605 . 
     On the other hand, for example, if the validity flag A 11301  of the map information storage unit A 11017  is TRUE, the UI unit A 11020  updates the value of the display rack depth A 11603 . For example, if the user flicks to the right on the touch panel unit A 1204 , the UI unit A 11020  increases the value of the display rack depth A 11603 . If the user flicks to the left on the touch panel unit A 1204 , the UI unit A 11020  decreases the value of the display rack depth A 11603 . 
     When the value of the display rack depth A 11603  exceeds a maximum value of z coordinates of a product of the product stock information A 11105 , the UI unit A 11020  resets the value of the display rack depth A 11603  to zero. When the value of the display rack depth A 11603  becomes smaller than zero, the UI unit A 11020  sets the value of the display rack depth A 11650  to the maximum value of the z coordinates of the product of the product stock information A 11105 . 
     [Display Unit A 11018 ] 
     The display unit A 11018  renders the above-described map information or product information to be displayed. In addition, the display unit A 11018  updates displayed details according to the above-described processing. 
     When processing for updating the displayed details (updating) is to be performed, the display unit A 11018  first performs preparation for display updating which is described below. 
     If the validity flag A 11101  of the product information storage unit A 11016  is TRUE and the previous frame drawing type A 11602  is not product information, the display unit A 11018  determines that the state of the rack to be displayed is to be initialized, and therefore resets the value of the display rack depth A 11603  to zero. 
     In other words, if, after displaying product information on the display unit A 11018  of the mobile terminal A 11001 , a newly switched first image capturing unit (for example, front camera A 1202 ) is used to obtain position information as a current position of the user, an offset assigned to the product information is initialized. 
     Likewise, if the validity flag A 11301  of the map information storage unit A 11017  is TRUE and the previous frame drawing type A 11602  is not map information, the display unit A 11018  determines that the state of the map to be displayed is to be initialized, and therefore resets the values of the map offset X A 11604  and the map offset Y A 11605  to zero. In other words, if, after displaying map information on the display unit A 11018  of the mobile terminal A 11001 , a newly switched second image capturing unit (for example, back camera A 1203 ) is used to obtain product information of a product rack provided with the first device (lighting device A 11003 ), an offset assigned to the map information is initialized. 
     It is also possible that, if the previously-obtained ID in the state management unit A 11019  and the ID in the ID storage unit A 11013  have different values, the display unit A 11018  determines that the display state is to be initialized, and therefore resets the values of the map offset X A 11604 , the map offset Y A 11605 , and the display rack depth A 11603  to zero. In this case, data of the ID storage unit A 11013  is copied to the previously-obtained ID in the state management unit A 11019 . 
     Furthermore, for the preparation for display updating, in the state management unit A 11019 , if the validity flag A 11101  of the product information storage unit A 11016  is TRUE, the previous frame drawing type A 11602  is set to “product information”, and if the validity flag A 11301  of the map information storage unit A 11017  is TRUE, the previous frame drawing type A 11602  is set to “map information”. If the error flag A 11601  is TRUE, the previous frame drawing type A 11602  is set to “error”. 
     It is assumed that the above-described preparation for display updating is completed and therefore either the validity flag A 11301  of the map information storage unit A 11017  and the validity flag of the product information storage unit A 11016  is TRUE. 
     In this case, if the validity flag A 11301  of the map information storage unit A 11017  is TRUE, the display unit A 11018  performs rendering to display the map information as illustrated in  FIG. 5 . Furthermore, if the validity flag of the product information storage unit A 11016  is TRUE, the display unit A 11018  performs rendering to display a product stock state as illustrated in  FIG. 12 . On the other hand, if the error flag A 11601  is TRUE, the display unit A 11018  updates displayed details to notify that an unauthorized ID is received. 
     [Processing Flow of Communication System] 
     Sequentially, the processing flow of the communication system according to the present embodiment is described. 
     [Processing Flow of Lighting Device] 
       FIG. 15  is a flowchart of processing performed by the lighting device according to Embodiment 1. 
     The following describes the processing flow of the lighting device A 11002  with reference to  FIG. 15 . 
     First, the lighting device A 11002  is supplied with power, and thereby the processing flow starts (SA 11401 ). 
     Next, the encoding unit A 11005  reads an ID from the ID storage unit A 11004  (SA 11402 ). 
     Next, the encoding unit A 11005  performs encoding to convert the ID to an illuminance pattern of lighting, namely, an illuminance of lighting at each timing (SA 11403 ). 
     Next, the encoding unit A 11005  stores the illuminance pattern to the first illuminance pattern storage unit A 11006  (SA 11404 ). 
     Next, because the above start phase is completed, the light emitting unit A 11007  reads the illuminance pattern from the illuminance pattern storage unit A 11006  (SA 11405 ), and controls light emission by updating illuminance at a high speed according to the pattern indicated by the illuminance pattern (SA 11406 ). 
     By the above processing flow, the lighting device A 11002  performs illuminance control (visible light communication) based on the ID. 
     Next, the lighting device A 11003  is also described. 
     First, the lighting device A 11003  is supplied with power, and thereby a processing flow starts (SA 11401 ). The encoding unit A 11009  reads an ID from the ID storage unit A 11008  (SA 11402 ), performs encoding to convert the ID to an illuminance pattern of lighting, namely, an illuminance of lighting at each timing (SA 11403 ), and stores the illuminance pattern to the illuminance pattern storage unit A 11010  (SA 11404 ). 
     When the above start phase is completed, the light emitting unit A 11011  reads the illuminance pattern from the illuminance pattern storage unit A 11010  (SA 11405 ), and controls light emission by updating illuminance at a high speed according to the pattern indicated by the illuminance pattern (SA 11406 ). 
     By the above processing flow, the lighting device A 11003  performs illuminance control based on the ID. 
     [Processing Flow of Mobile Terminal A 11001 ] 
     Subsequently, the flow of the mobile terminal A 11001  is described with reference to  FIG. 16 . 
       FIG. 16  is a flowchart of processing performed by the mobile terminal according to Embodiment 1. 
     First, the light receiving control unit A 11012  switches a shutter speed to be high (SA 11500 ). 
     Next, the light receiving control unit A 11012  detects an illuminance pattern of the light emitting unit A 11007  or the light emitting unit A 11011 , then decodes the detected illuminance pattern to convert the pattern to an ID (SA 11501 ), and stores the ID to the ID storage unit A 11013  (SA 11502 ). 
     Next, the DB management unit A 11014  reads the ID from the ID storage unit A 11013  and makes inquiry to the server device A 11015  (SA 11503 ), and determines whether or not the readout ID matches the recorded ID (SA 11504 , SA 11506 ). 
     If it is determined at SA 11504  that the readout ID matches an ID of any of ceiling lights on the ceiling (Yes at SA 11504 ), then the DB management unit A 11014  performs processing related to ceiling light (SA 11505 ). 
     On the other hand, if it is determined at SA 11506  that the ID matches an ID of any of the base lights provided on product racks (Yes at SA 11506 ), the DB management unit A 11014  performs processing related to base light (SA 11507 ). 
     It should be noted that, if the ID transmitted to the server device A 11015  is an invalid ID that is not related to any of the ceiling light and the base lights, the DB management unit A 11014  sets the validity flag A 11301  of the map information storage unit A 11017  to FALSE, sets the validity flag of the product information storage unit A 11016  to FALSE, and sets the error flag A 11601  to TRUE. The display unit A 11018  displays an error notification indicating that there is no information related to lighting device (SA 11508 ). In other words, the mobile terminal A 11001  inquires the server device A 11015  about an identifier uniquely assigned to the lighting device which is obtained by the visible light communication with the lighting device emitting visible light, thereby obtaining information of the lighting device which is associated with the identifier. However, if the mobile terminal A 11001  makes inquiry to the server device A 11015  but there is no information associated with the identifier, the mobile terminal A 11001  displays, on the display unit A 11018 , an error notification indicating that there is no information related to the lighting device. 
     [Flow of Processing Related to Ceiling Light] 
     Next, the ceiling light-related processing (SA 11505 ) is described. 
       FIG. 17  is a flowchart of processing related to the ceiling light according to Embodiment 1. 
     First, as illustrated in  FIG. 5 , the DB management unit A 11014  transmits, to the server device A 11015 , a request for transmission of a map image showing a target ceiling light and its periphery (SA 11701 ). 
     Next, the DB management unit A 11014  obtains the map image illustrated in  FIG. 5  and stores it to the map information storage unit A 11017  (SA 11702 ). 
     Then, the DB management unit A 11014  sets the validity flag A 11301  of the map information storage unit A 11017  to TRUE, and sets the validity flag of the product information storage unit A 11016  to FALSE (SA 11703 ). 
     [Flow of Processing Related to Base Light] 
     Next, the base light-related processing (SA 11507 ) is described. 
       FIG. 18  is a flowchart of processing related to the base light according to Embodiment 1. 
     First, the DB management unit A 11014  requests the server device A 11015  for the product information illustrated in  FIG. 11  (SA 11801 ). 
     Next, the DB management unit A 11014  stores the obtained product information to the product information storage unit A 11016  (SA 11802 ). 
     Then, the DB management unit A 11014  sets the validity flag A 11301  of the map information storage unit A 11017  to FALSE, and sets the validity flag of the product information storage unit A 11016  to TRUE (SA 11803 ). 
     [Processing Flow of UI Unit A 11020 ] 
     Subsequently, the UI unit A 11020  is described with reference to  FIG. 19 . 
       FIG. 19  is a flowchart of processing related to UI according to Embodiment 1. 
     The UI unit A 11020  is cooperated with the touch panel unit A 1204 , as described previously. 
     If, at SA 119001 , the validity flag A 11301  of the map information storage unit A 11017  is TRUE (yes at S 11901 ), the UI unit A 11020  performs UI processing for map information (S 11902 ). 
     On the other hand, if at SA 119003 , the validity flag A 11101  of the product information storage unit A 11016  is TRUE (yes at S 11903 ), then the UI unit A 11020  performs UI processing for product information (S 11904 ). 
     [Flow of UI Processing for Map Information] 
     Subsequently, the UI processing for map information (S 11802 ) is described. 
       FIG. 20  is a flowchart of UI processing for map information according to Embodiment 1. 
     First, when the UI processing for map information starts (SA 11902 ), the UI unit A 11020  determines whether or not the user flicks to the right on the touch panel unit A 1204  (SA 12001 ). 
     At SA 12001 , if the user flicks to the right on the touch panel unit A 1204  (yes at SA 12001 ), the UI unit A 11020  increases the value of the map offset X A 11604  by an amount of the flicking (SA 12002 ). 
     At SA 12003 , if the user flicks to the left on the touch panel unit A 1204  (yes at SA 12003 ), the UI unit A 11020  decreases the value of the map offset X A 11604  (SA 12004 ). 
     At SA 12005 , if the user flicks upwards on the touch panel unit A 1204  (yes at SA 12005 ), the UI unit A 11020  decreases the value of the map offset Y A 11605  (SA 12006 ). 
     At SA 12007 , if the user flicks downwards on the touch panel unit A 1204  (yes at SA 12007 ), the UI unit A 11020  increases the value of the map offset Y A 11605  (SA 12008 ). 
     As described above, when the user flicks on the touch panel unit A 1204 , the UI unit A 11020  updates the values of the map offset X A 11604  and the map offset Y A 11605 . 
     [Flow of UI Processing for Product Information] 
     Subsequently, the UI processing for product information (S 11804 ) is described. 
       FIG. 21  is a flowchart of UI processing for product information according to Embodiment 1. 
     First, when the UI processing for product information starts (SA 11904 ), the UI unit A 11020  determines whether or not the user flicks to the right on the touch panel unit A 1204  (SA 12101 ). 
     At SA 12101 , if the user flicks to the right on the touch panel unit A 1204  (yes at SA 12101 ), the UI unit A 11020  increases the value of the display rack depth A 11603  (SA 12102 ). 
     At SA 12103 , if the user flicks to the left on the touch panel unit A 1204  (yes at SA 12103 ), the UI unit A 11020  decreases the value of the display rack depth A 11603  (SA 12104 ). 
     At SA 12105 , when the value of the display rack depth A 11603  exceeds a maximum value of z coordinates of a product of the product stock information A 11105 , the UI unit A 11020  resets the value of the display rack depth A 11603  to zero (SA 12106 ). 
     On the other hand, at SA 12107 , when the value of the display rack depth A 11603  becomes smaller than zero (yes at SA 12107 ), the UI unit A 11020  sets the value of the display rack depth A 11603  to the maximum value of the z coordinates of the product of the product stock information A 11105  (SA 12108 ). 
     As described above, the UI unit A 11020  updates the display rack depth A 11603 . 
     [Processing Flow of Display Unit A 11018 ] 
     Subsequently, the processing flow of the display unit A 11018  is described. 
       FIG. 22  is a flowchart of entire display processing according to Embodiment 1.  FIG. 23  is a flowchart of preparation for display updating according to Embodiment 1.  FIG. 24  is a flowchart of the display updating according to Embodiment 1. 
     As illustrated in  FIG. 22 , when display starts, the display unit A 11018  performs preparation for display updating before performing the display updating (SA 12201 ). Then, the display unit A 11018  performs the display updating after performing the preparation (SA 12202 ). 
     Here, the preparation for display updating (SA 12201 ) is described in more detail with reference to  FIG. 23 . 
     When the preparation for display updating starts, the display unit A 11018  confirms whether or not product information is valid as seen in  FIG. 23  (SA 12301 ). 
     If the validity flag A 11101  of the product information storage unit A 11016  is TRUE (yes at SA 12301 ) and the previous frame drawing type A 11602  is not product information (no at SA 12302 ), the display unit A 11018  determines that the state of the rack to be displayed is to be initialized and therefore resets the value of the display rack depth A 11603  to zero (SA 12303 ). 
     Next, at SA 12304 , the display unit A 11018  confirms whether or not map information is valid (SA 12304 ). 
     If the validity flag A 11301  of the map information storage unit A 11017  is TRUE (yes at SA 12304 ) and the previous frame drawing type A 11602  is not map information (no at SA 12305 ), the display unit A 11018  determines that the state of the map to be displayed is to be initialized, and therefore resets the values of the map offset X A 11604  and the map offset Y A 11605  to zero (SA 12306 ). 
     Next, at SA 12307 , the display unit A 11018  confirms whether or not an ID is updated (SA 12307 ). 
     If the previously-obtained ID in the state management unit A 11019  and the ID in the ID storage unit A 11013  have different values (yes at SA 12307 ), the display unit A 11018  determines that the display state is to be initialized, and therefore resets the values of the map offset X A 11604 , the map offset Y A 11605 , and the display rack depth A 11603  to zero (SA 12308 ). 
     Next, at SA 12309 , the display unit A 11018  copies data of the ID storage unit A 11013  to the previously-obtained ID of the state management unit A 11019 , thereby updating the previous ID (SA 12309 ). 
     Next, the display unit A 11018  updates the previous frame drawing type (SA 12310 ). More specifically, if the validity flag A 11101  of the product information storage unit A 11016  is TRUE, the display unit A 11018  sets the previous frame drawing type A 11602  to “product information”. If the validity flag A 11301  of the map information storage unit A 11017  is TRUE, the display unit A 11018  sets the previous frame drawing type A 11602  to “map information”. If the error flag A 11601  is TRUE, the display unit A 11018  sets the previous frame drawing type A 11602  to “error”. 
     Subsequently, the display updating (SA 12202 ) is described with reference to  FIG. 24 . 
     When the display updating starts, the display unit A 11018  confirms whether or not the map information is valid as illustrated in  FIG. 24  (SA 12301 ). 
     If the validity flag A 11301  of the map information storage unit A 11017  is TRUE (yes at SA 12401 ), the display unit A 11018  reads the value of the map offset X A 11604  and the value of the map offset Y A 11606  (SA 12402 ), and renders the map information as illustrated in  FIG. 5  by shifting it by the values of the offsets (SA 12403 ). 
     At S 12401 , if the validity flag of the map information is invalid, the display unit A 11018  confirms whether or not the product information is valid (SA 12304 ). 
     If the validity flag of the product information storage unit A 11016  is TRUE (yes at SA 12404 ), the display unit A 11018  reads the value of the display rack depth A 11603  (SA 12404 ), and renders the product stock state as illustrated in  FIG. 12  to present the image of the rack indicated by the display rack depth A 11603  (SA 12406 ). 
     At S 12404 , if the validity flag of the product information is invalid (no at SA 12404 ) and the error flag A 11601  is TRUE, then the display unit A 11018  updates the displayed details to notify that an unauthorized ID is received (SA 12407 ). 
     Embodiment 2 
     In Embodiment 2, a structure and a flow of a light receiving control unit A 11012  are described in detail. The other structure is the same as described in Embodiment 1 and therefore not described again below. 
     [Detailed Structure of Light Receiving Control Unit A 11012 ] 
     First, the detailed structure of the light receiving control unit A 11012  according to Embodiment 2 is described. 
       FIG. 25  is a diagram illustrating the detailed structure of the light receiving control unit according to Embodiment 2. 
     As illustrated in  FIG. 25 , the light receiving control unit A 11012  according to Embodiment 2 includes a light receiving unit A 12501 , a light receiving unit A 12502 , a selector A 12503 , an acceleration detection unit A 12504 , a light receiving signal storage unit A 12505 , a decoding unit A 12506 , and a threshold storage unit A 12507 . 
     The light receiving unit A 12501  corresponds to, for example, the front camera A 1202  (first image capturing unit) according to Embodiment 1, and the light receiving unit A 12502  corresponds to, for example, the back camera A 1203  (second image capturing unit) according to Embodiment 1. 
     The selector A 12503  selects one of the two light receiving units to be operated. 
     The acceleration detection unit A 12504  detects an acceleration of the mobile terminal A 11011  to provide the selector A 12503  with information necessary in the selection. 
     The light receiving signal storage unit A 12505  stores light receiving signal obtained from the light receiving unit A 12501  or the light receiving unit A 12502  via the selector A 12503 . 
     The decoding unit A 12506  reads the light receiving signal from the light receiving signal storage unit A 12505 , then decodes the light receiving signal to an ID and stores the resulting ID to the ID storage unit A 11013 . 
     The threshold storage unit A 12507  previously holds a first threshold and a second threshold. These thresholds are used in a determination as to, based on an acceleration detected by the selector A 12503 , whether or not the mobile terminal A 11011  including the light receiving control unit A 11012  is parallel to the ground. 
     [Processing of Light Receiving Control Unit] 
     Subsequently, the processing flow of the light receiving control unit A 11012  according to Embodiment 2 is described. 
       FIG. 26  is a flowchart of illuminance pattern detection performed by the light receiving control unit according to Embodiment 2. 
     First, after starting (SA 11501 ), the light receiving control unit A 11012  detects, by using the acceleration detection unit A 12504 , an acceleration in a back surface direction of the mobile terminal A 11011  (SA 12601 ). 
     Next, the light receiving control unit A 11012  compares the detected acceleration to the thresholds stored in the threshold storage unit A 12507 , thereby determining whether or not the light receiving unit A 12501  (front camera) faces upwards (SA 12602 ). 
     At SA 12602 , if the acceleration is greater than the first threshold, the light receiving control unit A 11012  determines that the light receiving unit A 12501  (front camera) faces upwards in respect with the ground (yes at SA 12602 ) and therefore selects the light receiving unit A 12501  (front camera) (SA 12603 ). More specifically, if the acceleration is greater than the first threshold, this means that a greater acceleration, in other words, a gravity acceleration occurs in the back surface direction of the mobile terminal A 11011 . Therefore, the light receiving control unit A 11012  can determine that the mobile terminal A 11011  is parallel to the ground and the light receiving unit A 12501  (front camera) faces upwards in respect with the ground. Then, the light receiving control unit A 11012  selects or operates the light receiving unit A 12501 . 
     On the other hand, at SA 12602 , even if the acceleration is smaller than or equal to the second threshold, the light receiving control unit A 11012  determines that the light receiving unit A 12501  (front camera) does not face upwards in respect with the ground (no at SA 12602 ), and selects the light receiving unit A 12502  (back camera) (SA 12604 ). More specifically, if the acceleration is smaller than or equal to the second threshold, this means that the gravity acceleration hardly occurs in the back surface direction of the mobile terminal A 11011 . Therefore, the light receiving control unit A 11012  determines that the mobile terminal A 11011  is not parallel to the ground and the light receiving unit A 12502  (back camera) faces a wall or a rack. Then, the light receiving control unit A 11012  selects or operates the light receiving unit A 12502  (back camera). 
     Next, the light receiving control unit A 11012  obtains still pictures (SA 12605 ). More specifically, in the above-described processing, the light receiving control unit A 11012  obtains images captured by the light receiving unit A 12501  (front camera) and the light receiving unit A 12502  (back camera). 
     Next, the light receiving control unit A 11012  obtains an illuminance change of lighting from the captured images, in other words, performs decoding (SA 12606 ). 
     Next, the light receiving control unit A 11012  stores the ID obtained by the decoding, to the ID storage unit (SA 12607 ). 
     As described above, the light receiving control unit A 11012  can select a light receiving unit (image capturing unit) according to an attitude of the mobile terminal A 11011 . It is therefore possible to select the most appropriate light receiving unit (front camera) for a lighting device that looks at the mobile terminal from the above (in other words, looks down at a floor plan of a building), or select the most appropriate light receiving unit (back camera) for a lighting device that looks at a rack and other subjects in front in a horizontal direction. 
     Embodiment 3 
     The details of the structure and the flow of the light receiving control unit A 11012  are not limited to those described in Embodiment 2. In Embodiment 3, a structure and a flow of the light receiving control unit A 11012  which are different from those described in Embodiment 2 are described. The other structure is the same as described in Embodiment 1 and therefore not described again below. 
     [Detailed Structure of Light Receiving Control Unit A 11012 ] 
     First, a detailed structure of the light receiving control unit A 11012  according to Embodiment 3 is described. 
       FIG. 27  is a diagram illustrating the detailed structure of the light receiving control unit according to Embodiment 3. 
     The light receiving control unit A 11012  according to Embodiment 3 includes a first light receiving unit A 12701 , a second light receiving unit A 12702 , a selector A 12703 , a timer control unit A 12704 , a light receiving signal storage unit A 12705 , a decoding unit A 12706 , and a threshold storage unit A 12707 . 
     The light receiving unit A 12701  corresponds to, for example, the front camera A 1202  (first image capturing unit) according to Embodiment 1, and the light receiving unit A 12702  corresponds to, for example, the back camera A 1203  (second image capturing unit) according to Embodiment 1. 
     The selector A 12703  selects one of these two light receiving units to be operated. 
     The timer control unit A 12704  provides the selector A 12703  with information necessary in the selection. 
     The light receiving signal storage unit A 12705  stores light receiving signal obtained from the first light receiving unit A 12701  or the second light receiving unit A 12702  via the selector A 12703 . 
     The decoding unit A 12706  reads the light receiving signal from the light receiving signal storage unit A 12705 , then decodes the light receiving signal to an ID and stores the resulting ID to the ID storage unit A 11013 . 
     The threshold storage unit A 12707  previously holds a threshold  1  for a time period required from when the selector A 12703  selects the first light receiving unit A 12701  to start receiving light to when the selector A 12703  switches to the second light receiving unit A 12702  to receive light. The threshold storage unit A 12507  previously holds a threshold  2  for a time period required from when the selector A 12703  selects the second light receiving unit A 12702  to start receiving light to when the selector A 12703  switches to the first light receiving unit A 12701  to receive light. 
     [Processing of Light Receiving Control Unit] 
     Subsequently, the processing flow of the light receiving control unit A 11012  according to Embodiment 3 is described. 
       FIG. 28  is a flowchart of illuminance pattern detection performed by the light receiving control unit according to Embodiment 3. 
     First, after starting (SA 11501 ), the light receiving control unit A 11012  confirms whether or not the timer control unit A 12704  is stopped (SA 12801 ). 
     At SA 12801 , if the timer control unit A 12704  is not operated (yes at SA 12801 ), the light receiving control unit A 11012  operates the timer control unit A 12704  and the first light receiving unit A 12701  (SA 12802 ). 
     Next, at SA 12803 , if the first light receiving unit A 12701  is operating (yes at SA 12803 ) and a time period of the first threshold has passed since the first light receiving unit A 12701  starts the operation (yes at SA 12804 ), the light receiving control unit A 11012  stops the first light receiving unit A 12701  and starts the second light receiving unit A 12702  for a balanced use of these light receiving units (SA 12806 ). 
     On the other hand, if the first light receiving unit A 12701  is stopped and therefore the second light receiving unit A 12702  is operating (no at SA 12803 ), and a time period of the second threshold has passed since the second light receiving unit A 12702  starts the operation (yes at SA 12805 ), then the second light receiving unit A 12702  is stopped and the first light receiving unit A 12701  is operated for a balanced use of these light receiving units (SA 12807 ). 
     Next, the light receiving control unit A 11012  captures still pictures (SA 12608 ). More specifically, in the above-described processing, the light receiving control unit A 11012  obtains images captured by the first light receiving unit A 12501  or the second light receiving unit A 12502 . 
     Next, the light receiving control unit A 11012  obtains an illuminance change of lighting from the captured images, in other words, performs decoding (SA 12609 ). 
     Next, the light receiving control unit A 11012  stores the ID obtained by the decoding, to the ID storage unit A 11013  (SA 12610 ). 
     As described above, the light receiving control unit A 11012  selects a light receiving unit (image capturing unit) at predetermined intervals. As a result, for example, in the case where a watch-type device as illustrated in  FIG. 9  is horizontally held, it is possible to obtain, in a balanced manner, an ID from the ceiling light A 1904  on the ceiling and an ID from the base light A 1905  provided on a rack. 
     According to the present embodiment, the light receiving control unit A 11012  may further include an acceleration detection unit. In this case, if it is expected that the display surface of the watch-type device is not parallel to the ground, in other words, the user does not look at the display surface of the watch-type device, the light receiving control unit A 11012  may stop the first light receiving unit A 12501  and the second light receiving unit A 12502  to save power consumption. 
     Embodiment 4 
     According to Embodiment 4, the description is given for an example where a technique of detecting a line of sight is applied to Embodiment 1 to more appropriately reflect user&#39;s intention to the display unit A 11018 . 
       FIG. 29  is a diagram illustrating an example of a situation of using a mobile terminal for detecting a movement of a line of sight according to Embodiment 4. 
     As illustrated in  FIG. 29 , when a user holds a mobile terminal A 12901  by a hand and causes the back side of the mobile terminal A 12901  to face a rack A 12903 , a display unit of the mobile terminal A 12901  displays information of the rack A 12903 . According to the present embodiment, if the user&#39;s line of sight looks at the right of the rack A 12901  in the above situation, the mobile terminal A 12901  can detect the user&#39;s line of sight by image recognition. Then, it is determined that “what the user looks at is not the rack A 12901  but a rack A 12905  located on the right side of the rack A 12901 ”, and therefore information related to the rack A 12905  is displayed on the display unit of the mobile terminal A 12901 . 
     As described above, in the present embodiment, an object to be displayed is selected in consideration of not only a lighting device in front of the mobile terminal A 12901  but also a user&#39;s line of sight. As a result, it is possible to examine information to which user&#39;s intention is more appropriately reflected. 
     [Structure of Mobile Terminal A 12901 ] 
     Subsequently, the structure of the mobile terminal A 12901  according to Embodiment 4 is described. 
       FIG. 30  is a diagram illustrating an example of a structure of a mobile terminal according to Embodiment 4. It should be noted that, in  FIG. 30 , structural elements overlapping in Embodiment 1, in other words, the same structural elements as illustrated in  FIG. 10 , are assigned with the same reference numerals. 
     A mobile terminal A 12901  illustrated in  FIG. 30  includes a light receiving control unit A 13002 , an ID storage unit A 11013 , a DB management unit A 13003 , a product information storage unit A 11016 , a map information storage unit A 11017 , a display unit A 11018 , a state management unit A 11019 , an UI unit A 11020 , and a line-of-sight information storage unit A 13001 . Furthermore, the mobile terminal A 12901  is connected to the server device A 13004  by wireless communication. 
     The light receiving control unit A 13002  detects an illuminance pattern of the light emitting unit A 11007  and an illuminance pattern of the light emitting unit A 11011 , then converts the detected patterns to IDs, and stores the IDs to the ID storage unit A 11013 . In the case of the present embodiment, since the light receiving control unit A 13002  detects an illuminance pattern of the light emitting unit A 11007  or the light emitting unit A 11011 , the ID stored in the ID storage unit A 11013  is identical to either an ID stored in the ID storage unit A 11004  or an ID stored in the ID storage unit A 11008 . 
     Furthermore, the light receiving control unit A 13002  detects, by an internal light receiving function in the light receiving control unit A 13002 , line-of-sight classification information indicating to which the user&#39;s line of sight is classified among “front, right, and left”, and eventually stores the detected classification result to the line-of-sight information storage unit A 13001 . 
     The other structure is the same as described in Embodiment 10, so that it is not described in detail again. 
     [Operation Flow of DB Management Unit A 13003 ] 
     Next, a flow of a DB management unit A 13003  is described. In the first embodiment, SA 11801  in  FIG. 18  according to Embodiment 1 is described in more detail. Since the other steps are the same as described previously, the following mainly describes a detailed flow of SA 11801 . 
       FIG. 31  is a flowchart of inquiry performed by the DB management unit according to Embodiment 4.  FIG. 32  illustrates an example of a structure of pieces of information managed by the DB in the server device according to Embodiment 4. 
     First, the DB management unit A 13003  starts processing for requesting the server device A 11015  to transmit a product image (SA 11801 ). 
     Then, the DB management unit A 13003  inquires the server device about an ID and line-of-sight classification information (SA 13201 ). More specifically, the DB management unit A 13003  reads the ID from the ID storage unit A 11013 , and reads the line-of-sight classification information from the line-of-sight information storage unit A 13001 . Furthermore, the DB management unit A 13003  requests the server device A 11015  for product information. In this case, the ID of the base light is transmitted to the server device A 11015 , and the line-of-sight classification information stored in the line-of-sight information storage unit A 13001  is also transmitted to the server device A 11015 . 
     Next, the server device A 11015  searches the held DB for the ID (SA 13202 ). More specifically, the server device A 11015  holds a DB as illustrated in  FIG. 32 , and searches a column A 13102  of the DB for the received ID. 
     At SA 13202 , if the ID is found in the DB (yes at SA 13203 ), then the server device A 11015  searches a row of the DB according to the ID and the line-of-sight classification information (SA 13205 ), and confirms whether or not there is a rack identifier corresponding to the line-of-sight classification information (SA 13206 ). More specifically, if the corresponding row is found in the column A 13102  of the DB, the server device A 11015  extracts a rack identifier from the row. Here, if the line-of-sight classification information indicates “front”, a rack identifier is obtained with reference to a column A 13101  in the corresponding row. If the line-of-sight classification information indicates “left”, a rack identifier is obtained with reference to a column A 13103  in the corresponding row. If the line-of-sight classification information indicates “right”, a rack identifier is obtained with reference to a column A 13104  in the corresponding row. 
     Next, the server device A 11015  searches for a row corresponding to the obtained rack identifier (SA 13208 ), and notifies rack information indicated in the rack information column in the corresponding row (SA 13209 ). More specifically, the server device A 11015  searches the A 13101  column for the obtained rack identifier, and notifies rack information A 13105  in the corresponding row to the DB management unit A 13003  in the mobile terminal A 12901 . For example, if an ID having a value of 200 and line-of-sight classification information indicating “right” are received and the ID having a value of 200 is searched in lighting ID A 13102 , the ID is in the first row. Since a rack identifier A 13104  on the right side of the first row is 101, rack information A 13105  in a row in which a rack identifier A 13101  is 101, namely, rack information  2 , is transmitted to the mobile terminal A 12901 . 
     Then, the DB management unit A 13003  of the mobile terminal A 12901  stores the obtained product information to the product information storage unit A 11016 , and sets the validity flag A 11301  of the map information storage unit A 11017  to FALSE, sets the validity flag of the product information storage unit A 11016  to TRUE, and sets the error flag A 11601  to FALSE. 
     Here, at SA 13206 , if no rack identifier is extracted based on the received ID and the line-of-sight classification information although the ID is detected from the column A 13102  (no at SA 13206 ), then the server device A 11015  notifies the rack information A 13105  of the row in which the received ID is found, to the DB management unit A 13003  in the mobile terminal A 12901  (SA 13207 ). 
     On the other hand, at SA 13023 , if the server device A 11015  cannot detect the received ID from the column A 13102  (no at SA 13203 ), the server device A 11015  notifies the DB management unit A 13003  in the mobile terminal A 12901  of that there is no corresponding ID (SA 13204 ). It should be noted that, if, for example, the ID transmitted to the server device A 11015  is an invalid ID that is not related to any of the ceiling light and the base lights, the DB management unit A 13003  sets the validity flag A 11301  of the map information storage unit A 11017  to FALSE, sets the validity flag of the product information storage unit A 11016  to FALSE, and sets the error flag A 11601  to TRUE. 
     As described above, according to the present embodiment, the mobile terminal including the communication device is further capable of detecting a user&#39;s line of sight. More specifically, if an orientation of the mobile terminal is substantially perpendicular to the ground, the back camera is selected to perform visible light communication with the first device that emits visible light, and the front camera is used to detect the user&#39;s line of sight. 
     As a result, the back camera is used to perform visible light communication with the first device that is in front of the user and emits visible light, the product information of the first product rack to which the first device is provided is obtained. If the second product rack in the detected user&#39;s line of sight is not the first product rack, it is possible to display product information of the second product rack instead of the product information of the first product rack. 
     Embodiment 5 
     Embodiment 5 is based on Embodiment 4 to more clarify the structure of the light receiving control unit A 11012 . 
       FIG. 33  is a diagram illustrating the detailed structure of the light receiving control unit according to Embodiment 5. 
     As illustrated in  FIG. 33 , the light receiving control unit A 11012  according to Embodiment 5 includes a light receiving unit A 12501 , a light receiving unit A 12502 , a selector A 12503 , an acceleration detection unit A 12504 , a light receiving signal storage unit A 12505 , a decoding unit A 12506 , a threshold storage unit A 12507 , and a line-of-sight detection unit A 13301 . It should be noted that the same reference numerals in  FIG. 25  are assigned to the identical structural elements, and therefore the detailed description of the identical structural elements are not given below. 
     The light receiving unit A 12501  corresponds to, for example, the front camera A 1202  (first image capturing unit) according to Embodiment 1, and the light receiving unit A 12502  corresponds to, for example, the back camera A 1203  (second image capturing unit) according to Embodiment 1. It should be noted that, according to the present embodiment, the light receiving unit A 12501  is a video imaging device such as a camera device. 
     The selector A 12503  selects one of the two light receiving units to be operated. 
     The acceleration detection unit A 12504  detects an acceleration of the mobile terminal A 12901  to provide the selector A 12503  with information necessary in the selection. 
     The light receiving signal storage unit A 12505  stores light receiving signal obtained from the light receiving unit A 12501  or the light receiving unit A 12502  via the selector A 12503 . 
     The decoding unit A 12506  reads the light receiving signal from the light receiving signal storage unit A 12505 , then decodes the light receiving signal to an ID and stores the resulting ID to the ID storage unit A 11013 . 
     The threshold storage unit A 12507  previously holds a first threshold and a second threshold. These thresholds are used to determine, based on an acceleration detected by the selector A 12503 , whether or not the mobile terminal A 12901  including the light receiving control unit A 11012  is parallel to the ground. 
     The line-of-sight detection unit A 13301  receives a video signal from the light receiving unit A 12501 , and detects a position of a user&#39;s line of sight and stores line-of-sight classification information to the line-of-sight information storage unit A 13001 . 
     Embodiment 6 
     Embodiment 6 is based on Embodiment 4 to more clarify the structure of the light receiving control unit A 11012 . 
       FIG. 34  is a diagram illustrating the detailed structure of the light receiving control unit according to Embodiment 6. 
     As illustrated in  FIG. 34 , the light receiving control unit A 11012  according to Embodiment 6 includes a light receiving unit A 12501 , a light receiving unit A 12502 , a selector A 12503 , an acceleration detection unit A 12504 , a light receiving signal storage unit A 12505 , a decoding unit A 12506 , a threshold storage unit A 12507 , an image capturing unit A 13401 , and a line-of-sight detection unit A 13402 . It should be noted that the same reference numerals in  FIG. 25  are assigned to the identical structural elements, and therefore the detailed description of the identical structural elements are not given below. 
     According to the present embodiment, the light receiving unit A 12501  is assumed to be a sensor device such as an illuminance sensor. The light receiving unit A 12502  corresponds to, for example, the out-camera A 1203  (second image capturing unit) according to Embodiment 1. 
     The selector A 12503  selects one of these two light receiving units to be operated. 
     The acceleration detection unit A 12504  detects an acceleration of the mobile terminal A 12901  to provide the selector A 12503  with information necessary in the selection. 
     The light receiving signal storage unit A 12505  stores light receiving signal obtained from the light receiving unit A 12501  or the light receiving unit A 12502  via the selector A 12503 . 
     The decoding unit A 12506  reads the light receiving signal from the light receiving signal storage unit A 12505 , then decodes the light receiving signal to an ID and stores the resulting ID to the ID storage unit A 11013 . 
     The threshold storage unit A 12507  previously holds a first threshold and a second threshold. These thresholds are used to determine, based on an acceleration detected by the selector A 12503 , whether or not the mobile terminal A 12901  including the light receiving control unit A 11012  is parallel to the ground. 
     The image capturing unit A 13401  is a video imaging device such as a camera device. 
     The line-of-sight detection unit A 13402  receives a video signal from the light receiving unit A 13401 , and detects a position of a user&#39;s line of sight and stores resulting line-of-sight classification information to the line-of-sight information storage unit A 13001 . 
     As described above, according to the present embodiment, it is possible to provide a communication terminal and a communication method which are capable of presenting information to which a current position and intension of a user are more appropriately reflected according to an attitude of a mobile terminal. 
     Embodiment 7 
     The following describes Embodiment 7. 
     (Observation of Luminance of Light Emitting Unit) 
     The following proposes an imaging method in which, when capturing one image, all imaging elements are not exposed simultaneously but the times of starting and ending the exposure differ between the imaging elements.  FIG. 35  illustrates an example of imaging where imaging elements arranged in a line are exposed simultaneously, with the exposure start time being shifted in order of lines. Here, the simultaneously exposed imaging elements are referred to as “exposure line”, and the line of pixels in the image corresponding to the imaging elements is referred to as “bright line”. 
     In the case of capturing a blinking light source shown on the entire imaging elements using this imaging method, bright lines (lines of brightness in pixel value) along exposure lines appear in the captured image as illustrated in  FIG. 36 . By recognizing this bright line pattern, the luminance change of the light source at a speed higher than the imaging frame rate can be estimated. Hence, transmitting a signal as the luminance change of the light source enables communication at a speed not less than the imaging frame rate. In the case where the light source takes two luminance values to express a signal, the lower luminance value is referred to as “low” (LO), and the higher luminance value is referred to as “high” (HI). The low may be a state in which the light source emits no light, or a state in which the light source emits weaker light than in the high. 
     By this method, information transmission is performed at a speed higher than the imaging frame rate. 
     In the case where the number of exposure lines whose exposure times do not overlap each other is 20 in one captured image and the imaging frame rate is 30 fps, it is possible to recognize a luminance change in a period of 1.67 millisecond. In the case where the number of exposure lines whose exposure times do not overlap each other is 1000, it is possible to recognize a luminance change in a period of 1/30000 second (about 33 microseconds). Note that the exposure time is set to less than 10 milliseconds, for example. 
       FIG. 36  illustrates a situation where, after the exposure of one exposure line ends, the exposure of the next exposure line starts. 
     In this situation, when transmitting information based on whether or not each exposure line receives at least a predetermined amount of light, information transmission at a speed of fl bits per second at the maximum can be realized where f is the number of frames per second (frame rate) and l is the number of exposure lines constituting one image. 
     Note that faster communication is possible in the case of performing time-difference exposure not on a line basis but on a pixel basis. 
     In such a case, when transmitting information based on whether or not each pixel receives at least a predetermined amount of light, the transmission speed is flm bits per second at the maximum, where m is the number of pixels per exposure line. 
     If the exposure state of each exposure line caused by the light emission of the light emitting unit is recognizable in a plurality of levels as illustrated in  FIG. 37 , more information can be transmitted by controlling the light emission time of the light emitting unit in a shorter unit of time than the exposure time of each exposure line. 
     In the case where the exposure state is recognizable in Elv levels, information can be transmitted at a speed of flElv bits per second at the maximum. 
     Moreover, a fundamental period of transmission can be recognized by causing the light emitting unit to emit light with a timing slightly different from the timing of exposure of each exposure line. 
       FIG. 38  illustrates a situation where, before the exposure of one exposure line ends, the exposure of the next exposure line starts. That is, the exposure times of adjacent exposure lines partially overlap each other. This structure has the feature (1): the number of samples in a predetermined time can be increased as compared with the case where, after the exposure of one exposure line ends, the exposure of the next exposure line starts. The increase of the number of samples in the predetermined time leads to more appropriate detection of the light signal emitted from the light transmitter which is the subject. In other words, the error rate when detecting the light signal can be reduced. The structure also has the feature (2): the exposure time of each exposure line can be increased as compared with the case where, after the exposure of one exposure line ends, the exposure of the next exposure line starts. Accordingly, even in the case where the subject is dark, a brighter image can be obtained, i.e. the S/N ratio can be improved. Here, the structure in which the exposure times of adjacent exposure lines partially overlap each other does not need to be applied to all exposure lines, and part of the exposure lines may not have the structure of partially overlapping in exposure time. By keeping part of the exposure lines from partially overlapping in exposure time, the occurrence of an intermediate color caused by exposure time overlap is suppressed on the imaging screen, as a result of which bright lines can be detected more appropriately. 
     In this situation, the exposure time is calculated from the brightness of each exposure line, to recognize the light emission state of the light emitting unit. 
     Note that, in the case of determining the brightness of each exposure line in a binary fashion of whether or not the luminance is greater than or equal to a threshold, it is necessary for the light emitting unit to continue the state of emitting no light for at least the exposure time of each line, to enable the no light emission state to be recognized. 
       FIG. 39  illustrates the influence of the difference in exposure time in the case where the exposure start time of each exposure line is the same. In  7500   a , the exposure end time of one exposure line and the exposure start time of the next exposure line are the same. In  7500   b , the exposure time is longer than that in  7500   a . The structure in which the exposure times of adjacent exposure lines partially overlap each other as in  7500   b  allows a longer exposure time to be used. That is, more light enters the imaging element, so that a brighter image can be obtained. In addition, since the imaging sensitivity for capturing an image of the same brightness can be reduced, an image with less noise can be obtained. Communication errors are prevented in this way. 
       FIG. 40  illustrates the influence of the difference in exposure start time of each exposure line in the case where the exposure time is the same. In  7501   a , the exposure end time of one exposure line and the exposure start time of the next exposure line are the same. In  7501   b , the exposure of one exposure line ends after the exposure of the next exposure line starts. The structure in which the exposure times of adjacent exposure lines partially overlap each other as in  7501   b  allows more lines to be exposed per unit time. This increases the resolution, so that more information can be obtained. Since the sample interval (i.e. the difference in exposure start time) is shorter, the luminance change of the light source can be estimated more accurately, contributing to a lower error rate. Moreover, the luminance change of the light source in a shorter time can be recognized. By exposure time overlap, light source blinking shorter than the exposure time can be recognized using the difference of the amount of exposure between adjacent exposure lines. 
     As described with reference to  FIGS. 39 and 40 , in the structure in which each exposure line is sequentially exposed so that the exposure times of adjacent exposure lines partially overlap each other, the communication speed can be dramatically improved by using, for signal transmission, the bright line pattern generated by setting the exposure time shorter than in the normal imaging mode. Setting the exposure time in visible light communication to less than or equal to 1/480 second enables an appropriate bright line pattern to be generated. Here, it is necessary to set (exposure time)&lt;⅛×f, where f is the frame frequency. Blanking during imaging is half of one frame at the maximum. That is, the blanking time is less than or equal to half of the imaging time. The actual imaging time is therefore ½f at the shortest. Besides, since 4-value information needs to be received within the time of ½f, it is necessary to at least set the exposure time to less than 1/(2f×4). Given that the normal frame rate is less than or equal to 60 frames per second, by setting the exposure time to less than or equal to 1/480 second, an appropriate bright line pattern is generated in the image data and thus fast signal transmission is achieved. 
       FIG. 41  illustrates the advantage of using a short exposure time in the case where each exposure line does not overlap in exposure time. In the case where the exposure time is long, even when the light source changes in luminance in a binary fashion as in  7502   a , an intermediate-color part tends to appear in the captured image as in  7502   e , making it difficult to recognize the luminance change of the light source. By providing a predetermined non-exposure vacant time (predetermined wait time) t D2  from when the exposure of one exposure line ends to when the exposure of the next exposure line starts as in  7502   d , however, the luminance change of the light source can be recognized more easily. That is, a more appropriate bright line pattern can be detected as in  7502   f . The provision of the predetermined non-exposure vacant time is possible by setting a shorter exposure time t E  than the time difference t D  between the exposure start times of the exposure lines, as in  7502   d . In the case where the exposure times of adjacent exposure lines partially overlap each other in the normal imaging mode, the exposure time is shortened from the normal imaging mode so as to provide the predetermined non-exposure vacant time. In the case where the exposure times of adjacent exposure lines partially overlap each other in the normal imaging mode, the exposure time is shortened from the normal imaging mode so as to provide the predetermined non-exposure vacant time. In the case where the exposure end time of one exposure line and the exposure start time of the next exposure line are the same in the normal imaging mode, too, the exposure time is shortened so as to provide the predetermined non-exposure time. Alternatively, the predetermined non-exposure vacant time (predetermined wait time) t D2  from when the exposure of one exposure line ends to when the exposure of the next exposure line starts may be provided by increasing the interval t D  between the exposure start times of the exposure lines, as in  7502   g . This structure allows a longer exposure time to be used, so that a brighter image can be captured. Moreover, a reduction in noise contributes to higher error tolerance. Meanwhile, this structure is disadvantageous in that the number of samples is small as in  7502   h , because fewer exposure lines can be exposed in a predetermined time. Accordingly, it is desirable to use these structures depending on circumstances. For example, the estimation error of the luminance change of the light source can be reduced by using the former structure in the case where the imaging object is bright and using the latter structure in the case where the imaging object is dark. 
     Here, the structure in which the exposure times of adjacent exposure lines partially overlap each other does not need to be applied to all exposure lines, and part of the exposure lines may not have the structure of partially overlapping in exposure time. Moreover, the structure in which the predetermined non-exposure vacant time (predetermined wait time) is provided from when the exposure of one exposure line ends to when the exposure of the next exposure line starts does not need to be applied to all exposure lines, and part of the exposure lines may have the structure of partially overlapping in exposure time. This makes it possible to take advantage of each of the structures. Furthermore, the same reading method or circuit may be used to read a signal in the normal imaging mode in which imaging is performed at the normal frame rate (30 fps, 60 fps) and the visible light communication mode in which imaging is performed with the exposure time less than or equal to 1/480 second for visible light communication. The use of the same reading method or circuit to read a signal eliminates the need to employ separate circuits for the normal imaging mode and the visible light communication mode. The circuit size can be reduced in this way. 
       FIG. 42  illustrates the relation between the minimum change time t S  of light source luminance, the exposure time t E , the time difference t D  between the exposure start times of the exposure lines, and the captured image. In the case where t E +t D &lt;t S , imaging is always performed in a state where the light source does not change from the start to end of the exposure of at least one exposure line. As a result, an image with clear luminance is obtained as in  7503   d , from which the luminance change of the light source is easily recognizable. In the case where 2t E &gt;t S , a bright line pattern different from the luminance change of the light source might be obtained, making it difficult to recognize the luminance change of the light source from the captured image. 
       FIG. 43  illustrates the relation between the transition time t T  of light source luminance and the time difference t D  between the exposure start times of the exposure lines. When t D  is large as compared with t T , fewer exposure lines are in the intermediate color, which facilitates estimation of light source luminance. It is desirable that t D &gt;t T , because the number of exposure lines in the intermediate color is two or less consecutively. Since t T  is less than or equal to 1 microsecond in the case where the light source is an LED and about 5 microseconds in the case where the light source is an organic EL device, setting t D  to greater than or equal to 5 microseconds facilitates estimation of light source luminance. 
       FIG. 44  illustrates the relation between the high frequency noise t HT  of light source luminance and the exposure time t E . When t E  is large as compared with t HT , the captured image is less influenced by high frequency noise, which facilitates estimation of light source luminance. When t E  is an integral multiple of t HT , there is no influence of high frequency noise, and estimation of light source luminance is easiest. For estimation of light source luminance, it is desirable that t E &gt;t HT . High frequency noise is mainly caused by a switching power supply circuit. Since t HT  is less than or equal to 20 microseconds in many switching power supplies for lightings, setting t E  to greater than or equal to 20 microseconds facilitates estimation of light source luminance. 
       FIG. 45  is a graph representing the relation between the exposure time t E  and the magnitude of high frequency noise when t HT  is 20 microseconds. Given that t HT  varies depending on the light source, the graph demonstrates that it is efficient to set t E  to greater than or equal to 15 microseconds, greater than or equal to 35 microseconds, greater than or equal to 54 microseconds, or greater than or equal to 74 microseconds, each of which is a value equal to the value when the amount of noise is at the maximum. Though t E  is desirably larger in terms of high frequency noise reduction, there is also the above-mentioned property that, when t E  is smaller, an intermediate-color part is less likely to occur and estimation of light source luminance is easier. Therefore, t E  may be set to greater than or equal to 15 microseconds when the light source luminance change period is 15 to 35 microseconds, to greater than or equal to 35 microseconds when the light source luminance change period is 35 to 54 microseconds, to greater than or equal to 54 microseconds when the light source luminance change period is 54 to 74 microseconds, and to greater than or equal to 74 microseconds when the light source luminance change period is greater than or equal to 74 microseconds. 
       FIG. 46  illustrates the relation between the exposure time t E  and the recognition success rate. Since the exposure time t E  is relative to the time during which the light source luminance is constant, the horizontal axis represents the value (relative exposure time) obtained by dividing the light source luminance change period t S  by the exposure time t E . It can be understood from the graph that the recognition success rate of approximately 100% can be attained by setting the relative exposure time to less than or equal to 1.2. For example, the exposure time may be set to less than or equal to approximately 0.83 millisecond in the case where the transmission signal is 1 kHz. Likewise, the recognition success rate greater than or equal to 95% can be attained by setting the relative exposure time to less than or equal to 1.25, and the recognition success rate greater than or equal to 80% can be attained by setting the relative exposure time to less than or equal to 1.4. Moreover, since the recognition success rate sharply decreases when the relative exposure time is about 1.5 and becomes roughly 0% when the relative exposure time is 1.6, it is necessary to set the relative exposure time not to exceed 1.5. After the recognition rate becomes 0% at  7507   c , it increases again at  7507   d ,  7507   e , and  7507   f . Accordingly, for example to capture a bright image with a longer exposure time, the exposure time may be set so that the relative exposure time is 1.9 to 2.2, 2.4 to 2.6, or 2.8 to 3.0. Such an exposure time may be used, for instance, as an intermediate mode in  FIG. 48 . 
     Depending on imaging devices, there is a time (blanking) during which no exposure is performed, as illustrated in  FIG. 47 . 
     In the case where there is blanking, the luminance of the light emitting unit during the time cannot be observed. 
     A transmission loss caused by blanking can be prevented by the light emitting unit repeatedly transmitting the same signal two or more times or adding error correcting code. 
     To prevent the same signal from being transmitted during blanking every time, the light emitting unit transmits the signal in a period that is relatively prime to the period of image capture or a period that is shorter than the period of image capture. 
     Embodiment 8 
       FIG. 49  is a diagram illustrating a service provision system using the reception method described in any of the foregoing embodiments. 
     First, a company A ex 8000  managing a server ex 8002  is requested to distribute information to a mobile terminal, by another company B or individual ex 8001 . For example, the distribution of detailed advertisement information, coupon information, map information, or the like to the mobile terminal that performs visible light communication with a signage is requested. The company A ex 8000  managing the server manages information distributed to the mobile terminal in association with arbitrary ID information. A mobile terminal ex 8003  obtains ID information from a subject ex 8004  by visible light communication, and transmits the obtained ID information to the server ex 8002 . The server ex 8002  transmits the information corresponding to the ID information to the mobile terminal, and counts the number of times the information corresponding to the ID information is transmitted. The company A ex 8000  managing the server charges the fee corresponding to the count, to the requesting company B or individual ex 8001 . For example, a larger fee is charged when the count is larger. 
       FIG. 50  is a flowchart illustrating service provision flow. 
     In Step ex 8000 , the company A managing the server receives the request for information distribution from another company B. In Step ex 8001 , the information requested to be distributed is managed in association with the specific ID information in the server managed by the company A. In Step ex 8002 , the mobile terminal receives the specific ID information from the subject by visible light communication, and transmits it to the server managed by the company A. The visible light communication method has already been described in detail in the other embodiments, and so its description is omitted here. The server transmits the information corresponding to the specific ID information received from the mobile terminal, to the mobile terminal. In Step ex 8003 , the number of times the information is distributed is counted in the server. Lastly, in Step ex 8004 , the fee corresponding to the information distribution count is charged to the company B. By such charging according to the count, the appropriate fee corresponding to the advertising effect of the information distribution can be charged to the company B. 
       FIG. 51  is a flowchart illustrating service provision in another example. The description of the same steps as those in  FIG. 50  is omitted here. 
     In Step ex 8008 , whether or not a predetermined time has elapsed from the start of the information distribution is determined. In the case of determining that the predetermined time has not elapsed, no fee is charged to the company B in Step ex 8011 . In the case of determining that the predetermined time has elapsed, the number of times the information is distributed is counted in Step ex 8009 . In Step ex 8010 , the fee corresponding to the information distribution count is charged to the company B. Since the information distribution is performed free of charge within the predetermined time, the company B can receive the accounting service after checking the advertising effect and the like. 
       FIG. 52  is a flowchart illustrating service provision in another example. The description of the same steps as those in  FIG. 51  is omitted here. 
     In Step ex 8014 , the number of times the information is distributed is counted. In the case of determining that the predetermined time has not elapsed from the start of the information distribution in Step ex 8015 , no fee is charged in Step ex 8016 . In the case of determining that the predetermined time has elapsed, on the other hand, whether or not the number of times the information is distributed is greater than or equal to a predetermined number is determined in Step ex 8017 . In the case where the number of times the information is distributed is less than the predetermined number, the count is reset, and the number of times the information is distributed is counted again. In this case, no fee is charged to the company B regarding the predetermined time during which the number of times the information is distributed is less than the predetermined number. In the case where the count is greater than or equal to the predetermined number in Step ex 8017 , the count is reset and started again in Step ex 8018 . In Step ex 8019 , the fee corresponding to the count is charged to the company B. Thus, in the case where the count during the free distribution time is small, the free distribution time is provided again. This enables the company B to receive the accounting service at an appropriate time. Moreover, in the case where the count is small, the company A can analyze the information and, for example when the information is out of season, suggest the change of the information to the company B. In the case where the free distribution time is provided again, the time may be shorter than the predetermined time provided first. The shorter time than the predetermined time provided first reduces the burden on the company A. Further, the free distribution time may be provided again after a fixed time period. For instance, if the information is influenced by seasonality, the free distribution time is provided again after the fixed time period until the new season begins. 
     Note that the charge fee may be changed according to the amount of data, regardless of the number of times the information is distributed. Distribution of a predetermined amount of data or more may be charged, while distribution is free of charge within the predetermined amount of data. The charge fee may be increased with the increase of the amount of data. Moreover, when managing the information in association with the specific ID information, a management fee may be charged. By charging the management fee, it is possible to determine the fee upon requesting the information distribution. 
     INDUSTRIAL APPLICABILITY 
     The communication method and the like according to the present invention are capable of securely and actively obtaining information except image. Therefore, they are suitable for various uses, such as transfer of information with image and information transmission, in any situations, since the active obtaining allows not only apparatuses in home, such as television sets, PCs, and tablet terminals, but also signage, information terminals, and information display apparatuses outside home to securely obtain necessary information by a necessary amount.