DESCRIPTION TITLE OF THE INVENTION: INFORMATION DISPLAY DEVICE TECHNICAL FIELD [0001] The present invention relates to an information display device that provides various categories of information to passengers in a vehicle such as a railroad vehicle. BACKGROUND ART [0002] As services to passengers of railroads or the like, various categories of information such as operating conditions, current location, or the like are provided by using information display devices installed in vehicles. An information display device is installed, for example, in space above an entrance door, the space above a passage, or the like, and displays information by using a display device such as a Light Emitting Diode (LED) or a Liquid Crystal Display (LCD) . Display contents and their switching timing are provided separately from a control device installed in a train, and delivered to each information display device. [0003] Conventionally, this delivery from the control unit is performed through a network that uses wires. As a result, installation of information display devices in an existing vehicle requires burdensome wiring work. Further, installation of equipment such as network equipment is required also. [0004] To simplify installation work in an existing vehicle, for example the system described in Patent reference 1 uses a wireless Local Area Network (LAN). This system simplifies installation by partially using a wireless LAN for delivering information to information display devices. PRIOR ART REFERENCE PATENT REFERENCE [0005] PATENT REFERENCE 1: Japanese Patent Publication No. 54 80771 (Paragraphs 0012 - 0035, Fig. 1) SUMMARY OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION [0006] However, a wireless system using radio wave has difficulty in communication between vehicles, and thus wiring work is still required at key portions of a LAN that connect vehicles with each other. In addition, work is required for installing network control equipment such as a wireless LAN access point or a DHCP server. Further, it is necessary to install RFID tags for identifying installation positions in vehicles, and this requires time and effort for installation work of RFID tags and for issuing and managing RFI tags, for example. Accordingly, a system using a wireless LAN can have only a partial effect in simplification from a viewpoint of the whole installation work. [0007] Further, a currently-prevailing wireless LAN system uses a radio wave of the ISM band. The ISM band is originally a band for a radio source of non-communication use. Accordingly, it should be taken as the premise that stable communication is not possible due to interference of another radio source when a wireless LAN system is used for a communication purpose. [0008] Further, since a radio wave can be used without a license when some requirements are satisfied, the ISM band is also used for various communication purposes. In particular, owing to the widespread use of smartphones and tablet terminals having a plurality of wireless communication functions in addition to a wireless function for original telephone communication, there are a lot of radio waves of a wireless LAN, Bluetooth, and the like within a train. Thus it is possible that these interfere with the wireless LAN, to make communication unstable. [0009] In addition, considering along a railroad, vehicles themselves that a train or a passing train has become sources of radio disturbance; and facilities such as overhead wires or power lines, and the like become sources of radio disturbance. Accordingly, it is difficult to use stably a wireless LAN that uses a radio wave. [0010] Thus, an object of the present invention is to provide an information display device that is easy in installation work and hardly affected by radio wave interference. MEANS OF SOLVING THE PROBLEM [0011] One mode of the present invention provides an information ? display device for transmitting and receiving data to and from ;. oneanother, comprising: adisplaypanel thatdisplays information; [ and a plurality of optical communication modules that perform transmission and reception of the data including the information I to be displayed on the display panel by wireless using light; wherein, j the plurality of optical communication modules include a first * optical communication module directed in a first direction and | a second optical communication module directed in a different direction from the first direction; and the data received by the first optical communication module is transmitted from the second i optical communication module. i EFFECTS OF THE INVENTION [0012] i According to the present invention, by using optical : communication, it is possible to provide an information display ". device that is easy in installation work and hardly affected by radio wave interference. BRIEF DESCRIPTION OF THE DRAWINGS [0013] Fig. 1 is a block diagram showing schematically a configuration of an information display device according to a first and a second embodiments; Fig, 2 is a schematic diagram showing an example of a hardware configuration of the information display device according to the first and the second embodiments; Fig. 3 is a schematic view showing an example of an outer configuration of the information display device of the first embodiment when the information display device is made to be a so-called hanging type device that is placed in space above a passage; Fig. 4 is a schematic diagram showing an example in which the information display devices of the first embodiment are placed in vehicles; Fig. 5 is a schematic diagram for explaining a communicable angle of an optical communication part in the information display device of the first embodiment; Fig. 6 is a schematic diagram for explaining a communicable angle of an optical communication part in the information display device of the first embodiment; Fig. 1 is a flowchart showing data transmission and reception control of the information display device of the first embodiment; Fig. 8 is a flowchart showing data transmission and reception control of the information display device of the second embodiment; Fig. 9 is a block diagram showing schematically a configuration of an information display device according to a third - a fifth embodiments; Fig. 10 is a schematic diagram showing an example of a hardware configuration of the information display device of the first - the third embodiments; Fig. 11 is a schematic view showing an example of an outer configuration of the information display device of the third embodiment when the information display device is made to be a so-called above-entrance-door installation type device that is placed above an entrance door; Fig. 12 is a schematic diagram showing an example in which the information display devices of the third embodiment are placed in vehicles; Fig. 13 is a flowchart showing data transmission and reception control of the information display device of the third embodiment; Fig. 14 is a diagram showing an example of connection information in the third embodiment; Fig. 15 is a diagram showing an example of update of the connection information in the third embodiment; Fig. 16 is a flowchart showing connection information transmission and reception control for detecting an installation position in the third embodiment; Fig. 17 is a schematic view showing an example of an outer configuration of the information display devices of the fourth embodiment when the - information display devices are made to be so-called above-gangway installation type devices that are placed above a gangway between vehicles; and Fig. 18 is a schematic diagram showing arrangement of the information display devices of the fourth embodiment in a train. MODE FOR CARRYING OUT THE INVENTION [0014] FIRST EMBODIMENT Fig. 1 is a block diagram showing schematically a configuration of an information display device 100 as an information display device according to a first embodiment. Reference characters shown in parentheses indicate components in a second embodiment. [0015] The information display device 100 has one or more display parts 101; a display data switching part 104; a control part 105; one or more optical communication parts 106; and a storage part 107. The storage part 107 has a data accumulation part 102; and a data storage part 103. A plurality of information display devices 100 are installed in a vehicle. The display data switching part 104 is an example of a display control part. [0016] The display part 101 displays various categories of information. The information includes character information, image information, or the like. [0017] The data accumulation part 102 stores data for displaying information on the display part 101, data for identifying position and the like of the information display device 100, and the like. [0018] The data storage part 103 stores data for processing in the information display device 100. For example, the data stored in the data storage part 103 are information such as temporary data used by the control part 105, data transmitted and received by the information display device 100, or intermediate values of arithmetic operation. [0019] The display data switching part 104 causes the display part 101 to display information, based on the data stored in the data accumulation part 102. Further, the display data switching part 104 switches information displayed on the display part 101. [0020] The control part 105 controls processing in the information display device 100. For example, the control part 105 controls communication performed by the optical communication part 106 with another information display device 100, data accumulation performed by the storage part 107, and the like. Further, displaying of information on the display part 101, switching of display depending on the location and conditions of the train, and the like, which are performed by the display data switching part 104 , are controlled by the control part 105. [0021] The optical communication part 106 performs wireless communication using light. For example, the optical communication part 106 is installed to face another information display device 100. The optical communication part 106 performs optical space communication or optical wireless communication. The "optical space communication" is optical communication utilizing space propagation. The "optical wireless communication" is communication using a light ray, which is one type of wireless communication. In other words, the optical communication part 106 performs communication without using a communication path such as optical fiber. Here, the "communication path" means a medium for transferring information from a transmitter of the information to a recipient. [0022] Typically, the optical communication part 106 has a light emitting element and a light receiving element. A pair of optical communication parts 106 positioned to face each other perform one-to-one communication. However, many other variations can be considered. [0023] The storage part 107 has the data accumulation part 102 and the data storage part 103 as components. [0024] Fig. 2 is a schematic diagram showing an example of a hardware configuration of each information display device 100. [0025] Each information display device 100 has one or more display panels 11; a CPU 15; one or more optical communication modules 16; and a memory 17. The memory 17 includes a nonvolatile memory 12 and a volatile memory 13. In Fig. 2, reference characters shown in parentheses indicate components in the second embodiment, [0026] The CPU 15 functions as a processor. The processor is hardware for executing instruction sets described in software programs. Although one CPU is used in Fig. 2, a plurality of CPUs 15 may be employed to perform processing. [0027] For example, the display part 101 can be substantialized when the CPU 15 uses the display panel 11. For example, the display part 101 is substantialized when the CPU 15 takes out image data from the nonvolatile memory 12 and renders the image data on the display panel 11. [0028] The data accumulation part 102 of Fig. 1 can be substantialized when the CPU 15 uses the nonvolatile memory 12. That is to say, by storing and reading data to and from the nonvolatile memory 12 by the CPU 15, the data accumulation part 102 can store and read display data, installation information, or information such as order of data transfer. [0029] The data storage part 103 of Fig. 1 can be substantialized when the CPU 15 uses the volatile memory 13. That is to say, by storing and reading data to and from the volatile memory 13 by the CPU 15, the data storage part 13 can store communication data or information such as intermediate values of arithmetic operation. [0030] The display data switching part 104 and the control part 105 of Fig. 1 can be substantialized when the CPU 15 reads programs stored in the nonvolatile memory 12 into the volatile memory 13 and executes those programs. That is to say, the CPU 15 selects image data to be displayed on the display panel 11 depending on a lapse of time, a traveling location of the vehicle, conditions of the vehicle, and renders the selected image data on the display panel 11. In other words, the CPU 15 of Fig. 2 has the functions of the display data switching part 104 and the control part 105 of Fig. 1. [0031] The optical communication part 106 of Fig. 1 can be substantialized when the CPU 15 uses the optical communicat ion module 16. That is to say, the CPU 15 sends data, which the CPU 15 desires to transmit, to the optical communication module 16. Further, the CPU 15 receives data received by the optical communication module 16. By these operations, the optical communication part 106 can perform wireless communication using light. At that time, depending on the type of the optical communication module 16, there is either a case where the CPU 15 sends and receives information as data to and from the optical communication module 16, or a case where the CPU 15 performs conversion into the level of a blinking signal so as to control the optical communication module 16 directly. [0032] in the case where the CPU 15 sends and receives information as data to and from the optical module 16, the CPU 15 delivers data to be transmitted in the form of character data or numerical data to the optical communication module 16. Then, the optical communication module 16 converts the data into a blinking light of a prescribed format to transmit the converted data. At the time of receiving data, the optical communication module 16 receives blinking of light, and measures the blinking pattern. When the received blinking signal coincides with a blinking signal of a prescribed format, the received data is interpreted as character data or numerical data. Then, the data whose interpretation has been finished by the optical communication module 16 is received by the CPU 15. [0033] In the case where the optical communication module 16 is directly cont rolled by converting the data into the level of a blinking signal, usually the optical module 16 has simply a light emitting element and a light receiving element. The CPU 15 converts the data to be transmitted into an ON signal or an OFF signal of a prescribed format. Further, the CPU 15 adds a control signal for the optical communication module 16, and outputs the signal to a control terminal of the light emitting element to cause the light emitting element to blink directly. [0034] At the time of receiving data, for example, an electric signal outputted by the light receiving element is measured and the times of turning-on and turning-off of the received light are recorded. The CPU 15 finds out a pattern of the ON signal or OFF signal of the prescribed format from the record of the times of turning-on and turning-off, to interpret the pattern as data. Such a method is usually used in the case where a relatively simple optical signal format such as a format in an infrared remote controller of a home electric appliance is used. [0035] Although an example of a blinking light signal has been described, it can be considered to use another method utilizing change of color, change of the number of light emitting points, or a configuration of light emitting points, for example. [0036] Referring to Fig. 2, the description has been given taking an example where the display data switching part 104 and the control part 105 are substantialized when the CPU 15 executes the programs. However, the embodiment is not limited to this example. [0037] For example, all or a part of the functions of the display data switching part 104 and the control part 105 maybe substantialized by hardware using an integrated logic IC such as Application Specific IntegratedCircuits (ASIC) or a Field Programmable Gate Array (FPGA), based on instructions of the CPU 15. Alternatively, all or a part of the functions of the display data switching part 104 and the control part 105 may be substantialized by software used by a Digital Signal Processor (DSP), based on instructions of the CPU 15. [0038] Fig. 3 is a schematic view showing an example of an outer configuration of the information display device 100. Fig. 3 shows a hanging type information display device 100. Here, the "hanging type" means hanging of a poster or the like in space above a passage of a train or a bus, for example. In Fig. 3, the direction Df indicates the front side of a vehicle, and the direction Db the rear side of the vehicle. [0039] In this example, the information display device 100 is installed in space above a passage of a vehicle. Further, the information display device 100 has a display part 101a in the front surface of the information display and a display part 101b in the rear surface of the information display device 100, so that it is easy to see the display parts from both of the front side and the rear side. [0040] Further, other information display devices 100 are placed on the front and rear sides of this information display device 100. Accordingly, . optical communication parts 106a and 106b are positioned to face forward and backward, respectively. In other words, the optical communication parts 106a and 106b are positioned to face in the different directions from each other. Here, the optical communication parts 106a and 106b are positioned to face opposite directions from.each other. That is to say, the optical communication part 106 faces in the direction Df, and the optical communication part 106b in the direction Db. [0041] In Fig. 3, the optical communication parts 106a and 106b are positioned in the upper parts of the information display device 100. This is for the purpose of positioning an optical communication path near the ceiling, so that communication is not prevented by passengers or their luggage. Here, the "optical communication path" means space through which light used for optical communication passes. [0042] In the case where a structural object or the like exists on the ceiling and becomes a blockage or where optical communication is performed through a gangway that connects vehicles with each other, the optical communication parts 106 can be positioned in the lower parts of the information display device 100. Alternatively, the optical communication parts 106 may be positioned to project from the information display device 100. The "gangway" means a passage provided in a connection part of a train, through which passengers can move between vehicles. [0043] Fig. 4 is a schematic diagram showing an example of arrangement of the information display devices 100 shown in Fig. 3, which are installed in vehicles 901a and 901b. Fig. 4 is a schematic view of a train 900 seen in the lateral direction. [0044] The train 900 is usually operated with a plurality of vehicles connected. For example, in Fig. 4, the train 900 has the vehicle 901a and the vehicle 901b that are connected with each other by a coupler (not shown} . When it is not necessary to differentiate the vehicle 901a and the vehicle 901b, they are referred to as vehicle (s) 901. Some trains 900 are of a small scale having only one vehicle 901, while other trains 900 are of a large scale having dozens of vehicles 901. [0045] For the sake of explanation, Fig. 4 shows an example where the train 900 has two vehicles 901. In Fig. 4, reference characters in parentheses indicate examples in the second embodiment. [0046] The information display devices 100 are arranged along the passages of the vehicles 901. Further, the information display devices 100 are arranged dispersedly in the longitudinal direction of the vehicles 901. In other words, the information display devices 100 are arranged at intervals in the longitudinal direction of the vehicles 901. [0047] Further, the information display devices 100 are installed so that display surfaces of the display parts 101 face each other, for example. Here, three information display devices 100 are installed for each vehicle 901. In the train 900 as a whole, six information display devices 100a, 100b, 100c, lOOd, lOOe, and XOOf are installed. [0048] Further, in the head or the tail of the train 900, a train operation system 910 is placed. The train operation system 910 functions as an information processing device. [0049] The train operation system 910 transmits data to the information display device 100a placed in the neighborhood by using optical communication or another interface {not shown). Alternatively, it is also possible that an operator 930 has a terminal device 920 as an information processing device, and transmits data from the terminal device 920 to the information display device 100a by using optical communication or another interface (not shown). [0050] The "interface" is what determines a connection form for connecting a plurality of devices to perform communication. Here, the interface is a form of communication between the information display device 100a and the train operation system 910 or the terminal device 920. The above-mentioned "another interface" means wired or wireless communication, for example. [0051] In the example shown in Fig. 4, the information display device 100c and the information display device lOOd communicate with each other through the connection part between the vehicles. In this case, an optical communication part 106 of the information display device 100c and an optical communication part 106 of the information display device lOOdare positioned so that each optical communication part can see the other optical communication part. In other words, nothing interrupts light used for communication between the optical communication part 106 of the information display device 100c and the optical communication part 106 of the information display device . lOOd. Accordingly, light emitted from the optical communication i part 106 of the information display device 100c can reach the optical [ communication part 10 6 of the information display device lOOd. Which allows optical communication. ; [0052] Optical communication through a connection part of vehicles is also affected by conditions of the train 900. Fig. 5 and Fig. I 6 are schematic diagrams for explaining a communicable angle of an optical communication part 106 of the information display device 100. Both Fig. 5 and Fig. 6 are schematic diagrams of vehicle (s) seen from above. Description will be given referring to Fig. 5 and Fig. 6. [0053] The information display device 100c is mounted in the end area in the longitudinal direction of the vehicle 901a. The information display device 100c has the optical communication part 106b. The optical communication part 106b is directed toward the adjacent vehicle 901b. [0054] Further, the information display device lOOd is mounted in the end area in the longitudinal direction of the vehicle 901b. The information display device lOOd has the optical communication part 106a. The optical communication part 106a is directed toward the adjacent vehicle 901a. [0055] The optical communication part 106b can emit light: used for communication in the range of an effective angle ANG. In other words, the effective angle ANG is an angle that allows the optical communication part 106 to perform communication. [0056] As shown in Fig. 6, the optical communication part 106b of the information display device 100c emits light toward the optical communication part 106a of the information display device lOOd. Further, the optical communication part 106a of the information display device lOOd emits light toward the optical communication part 106b of the information display device 100c. [0057] When the optical communication part 106a of the information display device lOOd goes out of this range (effective angle ANG) from the position where the optical communication part 106a faces the optical communication part 106b of the information display device 100c, communication between them stops . Accordingly, for example, to perform communication during operation of the train 900, it is necessary to set the effective angle ANG so that the optical communication part 106a remains in the range allowing communication even if the angle of the vehicle 901b to the vehicle 901a becomes the assumed maximum value while the train 900 is moving. [0058] When the train 900 is in a curved section of a railroad, vehicles 901 adjacent to each other in the longitudinal direction of the train 900 are not aligned in a line, but have a positional relationship of forming an angle with oach other. [0059] The angle between the vehicles 901 when the train 900 is in a curved section of a railroad is determined by the vehicle length, the curvature radius of the railroad, and the inclination amounts of cant and the vehicles 901 themselves. The vehicle length is the length of a vehicle and represents a distance between the coupling surfaces of one vehicle 901. The curvature radius represents a degree of curving of the railroad. The cant represents inward inclination of the railroad surface. [0060] In fact, in the operating section, the angle becomes maximum in a low-speed section having a small curvature radius. In that section, the inclination amounts of cant and the vehicles themselves are small. Accordingly, it is possible to calculate the angle between vehicles 901 approximately from the vehicle length and the curvature radius. [0061] A vehicle length and a minimum curvature radius that are standard are determined by each railroad company for each line. Thus, the maximum value of the angle between vehicles 901 (maximum angle between vehicles) in a train 900 traveling a specific line is expressed approximately by the equation (1}. [Equation 1] VchiclslenPth *r 2 Maximumanglebetweenvehicles « 2 x tan -———2— — Q) Mifi irn umcurva tureradius [0062] It is possible that the angle between vehicles 901 swings in a horizontal direction. In other words, the train 900 may turn to the right or to the left. Accordingly, the optical communication part 106b requires that the effective angle ANG of optical communication is more than or equal to twice the maximum value of the angle between vehicles. [0063] That is to say, when the effective angle ANG of optical communication for the optical communication part 106b satisfies the equation (2), the optical communication part 106 can perform communication even if the train 900 is passing through a curved railroad section. in the equation (2), "Optical communication effective angle" represents the effective angle ANG of optical communication. [Equation 2] i VehicleleYi£th~^~ 2 Opticalcommunication effective angle> 4 x tan" — — (2) Minimum atrvatweradius [0064] For example, in the case where the standards for the target line are the vehicle length of 20 m, the minimum curvature radius of 400 m in a high-speed traveling section, and the minimum curvature radius of 100 m in a low-speed traveling section, the effective angle ANG of optical communication can be calculated by using the minimum curvature radius of 100 m in a low-speed traveling section of a large curvature. By applying the equation (2), it is found that the effective angle ANG of optical communication requires 28 degrees ormore. Depending on conditions, forexample, itispossible touse, inpart, wired communication or wireless systemfor connecting the information display devices 100 with each other. Wired communication uses optical fiber, or electric wire, for example. Wireless system uses radio wave, for example. [0065] Fig. 7 is a flowchart showing data transmission and reception control of the information display device 100 in the first embodiment. [0066] First, in the step S10, the information display device 100 is in a reception standby state. When one of the optical communication parts 106 receives data, the processing proceeds to the step Sll. For example, the information display device 100 shown in Fig. 3 has two optical communication parts 106a and 106b. [0067] In the step Sll, the control part 105 causes the data storage part 103 to store optical communication part identification information Di for identifying the optical communication part 106 that has received data Do- Here, as the optical communication part identification information Di, information on the position of the optical communication part 106 arranged in the information display device 100 can be used. [0068] Next, in the step S12, the control part 105 causes the data accumulation part 102 to accumulate the received data Do. [0069] Next, in the step S13, the control part 105 selects an optical communication part 10 6 (transmission destination) to which the received data Do is to be transmitted. Here, forexample, the control part 105 performs transmission from all the optical communication parts 106 that have not been used for receiving the data Do. Taking the information display device 100 shown in Fig. 3 as an example, in the case where the information display device 100 has received the data by the optical communication part 106a, transmission is performed from the optical communication part 106b. [0070] In the step S14, the control part 105 sends the received data Do to the selected optical communication part 106, and causes the selected optical communication part 106 to transmit the data Do. [0071] | Then, after completion of the transmission of the data Do, | the control part 105 returns to the reception standby state (S10) ; again. [0072] Application of the control shown in Fig. 7 to the information display device 100 shown in Figs. 1 and 3 will be described. [0073] The information display device 100 in the reception standby state (S10) receives data Do by the optical communication part 106a. [0074] On receiving the data Do, the control part 105 causes.the data storage part 103 to store optical communication part identification information Di of the optical communication part 106a that has received the data Do (Sll). [0075] Then, the control part 105 causes the data accumulation part 102 to accumulate the received data Do (S12). [0076] Further, referring to the optical communication part identification information Di stored in the data storage part 103, the control part 105 selects the optical communication part 106b that has not received the data Do (S13). [0077] Then, the control part 105 causes the optical communication part 106b, which is mounted on the back side, to transmit the received data Do 4 x tan" (1) Minimumcurvatureradius 3. The information display device according to claim 1 or 2, wherein: when the received data includes first position identification information that can identify a position of its own information display device, a second position identification information that can identify a position of another information display device is added to the received data before transmission of the received data. 4. T.he information display device according to claim 3, wherein: the first position identification information indicates a first count value, and the second position identification information indicates a second count value that is the first count value changed by a predetermined value. 5. The information display device according to claim 3 or 4, wherein: information corresponding to the first position identification information included in the received data is displayed on the display panel. 6. The information display device according to claim 1 or 2, wherein: connection information includes an individual identifier for identifying its own information display device, a direction of an optical communication module that has been used for transmission of the data, a connection destination of the optical communication module that has been used for transmission of the data, or a type identifier for identifying a type of its own information display device; and when the optical communication module receives the connection information of other information display devices, its own connection information and the connection information of the other information display devices are transmitted from the optical communication module. 7. The information display device according to claim 6, wherein; the information display device specifies its own installation position based on its own connection information and the connection information of the other information display devices. 8. The information display device according to claim 6 or 7, wherein: the optical communication module identifies a source of light that includes the data; and based on an incident direction of the light, whether the source is in the vehicle or in a different vehicle is determined.