Patent Publication Number: US-9906298-B2

Title: Visible light transmitter, visible light receiver, visible light communication system, and visible light communication method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY 
     The present application is a continuation of U.S. patent application No. 12/450,592, filed on Sep. 30, 2009, which claims priority under 35 U.S.C. §365 to International Patent Application No. PCT/JP2007/074907 filed Dec. 26, 2007, entitled “VISIBLE LIGHT TRANSMITTER, VISIBLE LIGHT RECEIVER, VISIBLE LIGHT COMMUNICTAION SYSTEM, AND VISIBLE LIGHT COMMUNICATION METHOD”. International Patent Application No. PCT/JP2007/074907 claims priority under 35 U.S.C. §365 and/or 35 U.S.C. §119(a) to Japanese Patent Application No. 2007-091744 filed Mar. 30, 2007 and which are incorporated herein by reference into the present disclosure as if fully set forth herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a visible light transmission apparatus, a visible light reception apparatus, a visible light communication system, and a visible light communication method. 
     BACKGROUND ART 
     Optical communication technologies using light of a visible light range have recently been attracting much interest. Along with the rapid proliferation of lighting devices using light emitting devices such as Light Emitting Diodes (LEDs) or the like, techniques for realizing more convenient, higher-rate data communication using infrastructures including indoor and outdoor lightings are under development. 
     Although LEDs are most promising for light emission in high-speed optical data transmission in terms of influence on the human body and medical equipment, other semiconductor light emitting devices are also available, inclusive of Laser Diodes (LDs) or Super Luminescent Diodes (SLDs) that offer a faster response performance. Also, data rate is dependent on the response speed of a light emitting device in optical communication. In this context, a technique for transmitting a large amount of data stably in one signal of light emission of a light emitting device is needed. 
     In relation to the above problem, for example, Japanese Laid-Open Patent Application Publication No. 2004-147063 (Patent Document 1) discloses a structure for adjusting a control voltage to change the intensity of light emitted from a light source according to a frequency-multiplexed signal. With the use of this structure, it is possible to change the light intensity of a light source or transmit multiplexed data under on/off control. Compared to non-multiplexing, a higher data rate can be achieved. Japanese Laid-Open Patent Application Publication No. 2006-325085 (Patent Document 2) discloses a structure for transmitting different data by controlling light emission of a plurality of light sources having different spectrums. To be more specific, Patent Document 2 discloses a structure for transmitting data by mapping digital values (1, 0) to phase changes at a transition timing of a Pulse Width Modulation (PWM) signal. 
     DISCLOSURE 
     Technical Problem 
     However, the transmission technique of Patent Document 1 increases a transmission error rate due to a decrease in light intensity on a transmission path because multiplexed data is transmitted by modulating a light intensity. Especially in visible light communication, if a light source as a transmission apparatus is a lighting device, its light intensity is decreased due to effects of shielding. Consequently, an increased transmission error rate decreases a data rate. Even though the transmission technique of Patent Document 1 is implemented, it is not easy to increase a data rate up to the response limit of the light source. 
     Meanwhile, no more than a few-fold increase in data rate is expected from the transmission technique of Patent Document 2 because the multiplicity of data is equal to or less than the number of light sources having different emission spectrums. Moreover, the multiplicity is further decreased, except when a light source emits white light. 
     The present invention is intended to address the above problems. Accordingly, an object of the present invention is to provide a visible light transmission apparatus, a visible light reception apparatus, a visible light communication system, and a visible light communication method, for enabling transmission of data modulated to a multiple-value number including as many values as or values more than number of light sources and at the same time, suppressing an increase in a transmission error rate, caused by a decrease in light intensity on a transmission path. 
     Technical Solution 
     To achieve the above object, in one aspect of the present invention, there is provided a visible light communication system comprising a transmission apparatus and a reception apparatus that conduct visible light communication. The transmission apparatus includes a plurality of light emitters for emitting light in different colors, a chromaticity coordinates calculator for mapping a digital value to a chromaticity coordinates value, and a light intensity controller for controlling a light intensity of each of the light emitters based on the chromaticity coordinates value corresponding to the digital value. The reception apparatus includes a plurality of light receivers having different spectral characteristics, a chromaticity coordinates calculator for calculating a chromaticity coordinates value according to a received light intensity detected by each of the light receivers, and a demodulator for demodulating the digital value from the chromaticity coordinates value. 
     In another aspect of the present invention, there is provided a visible light transmission apparatus including a plurality of light emitters for emitting light in different colors, a chromaticity coordinates calculator for mapping a digital value to a chromaticity coordinates value, and a light intensity controller for controlling a light intensity of each of the light emitters based on the chromaticity coordinates value corresponding to the digital value 
     The plurality of light emitters may be grouped into a plurality of light emitter groups each group having a plurality of light emitters. 
     The light intensity controller may control the plurality of light emitters to emit light periodically with light intensities corresponding to a predetermined chromaticity coordinates value being a reference point. 
     The light intensity controller may control part of the plurality of light emitters to emit light always with light intensities corresponding to a predetermined chromaticity coordinates value being a reference point. 
     The chromaticity coordinates calculator may map a DC-free encoded transmission signal to the chromaticity coordinates value. 
     The light intensity controller may control the light intensity of each of the light emitters so that a total light intensity ratio of the light emitters in one PWM period correspond to the chromaticity coordinates value corresponding to the digital value. 
     In another aspect of the present invention, there is provided a visible light reception apparatus for demodulating a digital value corresponding to a chromaticity coordinates value, including a plurality of light receivers having different spectral characteristics, a chromaticity coordinates calculator for calculating the chromaticity coordinates value according to a received light intensity detected by each of the light receivers, and a demodulator for demodulating the digital value from the chromaticity coordinates value. 
     The visible light reception apparatus may further include a chromaticity coordinates corrector for calculating a vector having elements being the received light intensities detected by the light receivers for a linear matrix for calibrating color reproduction and compensating the received light intensities detected by the light receivers. 
     The chromaticity coordinates corrector may calculate each coefficient of the linear matrix based on received light intensities of the light receivers detected from received light corresponding to a predetermined chromaticity coordinates value being a reference point and light intensities of the light receivers calculated from the predetermined chromaticity coordinates value being the reference point. 
     The chromaticity coordinates calculator may calculate the chromaticity coordinates value based on a total received light intensity detected by each of the light receivers in one PWM period. 
     In another aspect of the present invention, there is provided a visible light communication method in a visible light communication system having a transmission apparatus with a plurality of light emitters for emitting light in different colors and a reception apparatus with a plurality of light receivers having different spectral characteristics, including mapping a digital value to a chromaticity coordinates value by the transmission apparatus, controlling a light intensity of each of the light emitters based on the chromaticity coordinates value corresponding to the digital value by the transmission apparatus, calculating a chromaticity coordinates value according to a received light intensity detected by each of the light receivers by the reception apparatus, and demodulating the digital value from the chromaticity coordinates value by the reception apparatus. 
     In another aspect of the present invention, there is provided a visible light transmission apparatus with a plurality of light emitters for emitting light in different colors, including mapping a digital value to a chromaticity coordinates value, and controlling a light intensity of each of the light emitters based on the chromaticity coordinates value corresponding to the digital value. 
     In a further aspect of the present invention, there is provided a visible light communication method in a visible light reception apparatus having a plurality of light receivers having different spectral characteristics, for demodulating a digital value corresponding to a chromaticity coordinates value, including calculating the chromaticity coordinates value according to a received light intensity detected by each of the light receivers, and demodulating the digital value from the chromaticity coordinates value. 
     When the above-described configuration is applied, a digital value can be mapped to a chromaticity coordinates value, for transmission. Therefore, the multi-value transmission increases a data rate. Also, since the data transmission is not affected by light intensity attenuation on a transmission path, a high data rate is realized stably. 
     Advantageous Effects 
     According to the present invention as described above, data modulated to a multiple-value number including values more than the number of light sources can be transmitted and, at the same time, an increase in a transmission error rate caused by a decreased light intensity on a transmission path can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
         FIG. 1  illustrates a configuration of a visible light communication system according to an embodiment of the present disclosure; 
         FIG. 2  illustrates a functional structure of a visible light transmission apparatus according to an embodiment of the present disclosure; 
         FIG. 3  illustrates a functional configuration of the visible light reception apparatus according to an embodiment of the present disclosure; 
         FIG. 4A  is a diagram illustrating a predetermined chromaticity point layout defined in a Yxy color system according to an embodiment of the present disclosure; 
         FIG. 4B  illustrates a table including information about the chromaticity point layout of the color system illustrated in  FIG. 4A  according to an embodiment of the present disclosure; and 
         FIG. 5  illustrates a structure for transmitting a reference signal from some light source group that enables calibration of a linear matrix using a signal corresponding to a reference point according to an embodiment of the present disclosure. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Now a detailed description will be made of preferred embodiments of the present invention with reference to the attached drawings. Through the specification and drawings, the same drawing reference numerals will be understood to refer to elements having substantially the same functions and configuration, to avoid redundancy in the description. 
     &lt;Embodiment of the Invention&gt; 
     A visible light communication system  1000  according to an embodiment of the present invention will first be described below. The visible light communication system  1000  according to the embodiment of the present invention includes a structure for mapping a multiplexed digital value to a chromaticity coordinates value in a predetermined color system, a structure for controlling a plurality of light emitters to emit light at a chromaticity indicated by the chromaticity coordinates value, a structure for detecting the chromaticity coordinates value according to the intensity ratio of light received at each chromaticity, and a structure for demodulating an original digital value from the chromaticity coordinates. This will be described in more detail with reference to the drawings. 
     [Configuration of Visible Light Communication System  1000 ] 
     With reference to  FIG. 1 , the visible light communication system  1000  according to the embodiment of the present invention will be described.  FIG. 1  illustrates the configuration of the visible light communication system  1000  according to the embodiment of the present invention. 
     As illustrated in  FIG. 1 , the visible light communication system  1000  mainly includes a visible light transmission apparatus  100  and a visible light reception apparatus  200 . The visible light transmission apparatus  100  mainly includes a plurality of light emitters  110  and a multiple-value modulation/emission controller  120 . Meanwhile, the visible light reception apparatus  200  mainly includes a plurality of light receivers  210  and a chromaticity detector/demodulator  220 . While not shown, the visible light transmission apparatus  100  may include, for example, a serial/parallel converter for converting digital values to parallel values on a bit basis and multiplexing the parallel values. Meanwhile, the visible light reception apparatus  200  may include, for example, a parallel/serial converter for converting a plurality of bits to a serial digital value. 
     (Visible Light Transmission Apparatus  100 ) 
     In the visible light transmission apparatus  100 , the multi-value modulation/emission controller  120  multiplexes input digital values and maps the multiplexed digital value to a chromaticity coordinates value in a predetermined color system. A plurality of chromaticity points are placed in the color system according to a modulation multiplicity. The chromaticity points correspond accurately to signal points on a constellation used in multi-value communication of a propagation area. Notably, the chromaticity coordinates of the color system are obviously different from the signal points in that the chromaticity coordinates represent colors and chromas. Accordingly, the multiplicity of a digital value may be increased by as much as chroma representations. 
     To emit light from the plurality of light emitters  110  at the chromaticity indicated by the chromaticity point corresponding to the multiplexed digital value, the multi-value modulation/emission controller  120  controls the light intensity of each of the light emitters  110 . After setting the sum of light intensities of all the light emitters  110  to a predetermined value, the multi-value modulation/emission controller  120  may determine the light intensity ratio of each light emitter  110 . That is, the multi-value modulation/emission controller  120  may apply the light intensity ratios of the light emitters  110  to the modulation signal corresponding to the multiplexed digital value. As a consequence, the problem of a decrease in a transmission error rate caused by a decrease in light intensity on a transmission path can be avoided. 
     (Visible Light Reception Apparatus  200 ) 
     In the visible light reception apparatus  200 , the plurality of light receivers  210 , which have light reception sensitivities (spectral sensitivities) to different wavelengths, receive light corresponding to the chromaticity point and measure the intensity of the received light at each wavelength (color). The chromaticity detector/demodulator  220  calculates three stimulation values from the digitally modulated light intensity of the received light at each wavelength and detects a chromaticity point using the stimulation values using a color matching function. Also, the chromaticity detector/demodulator  220  demodulates a digital value corresponding to the chromaticity coordinates value of the chromaticity point. Herein, the chromaticity detector/demodulator  220  demodulates the digital value based on the same color system as used in the multi-value modulation/emission controller  120 . It is assumed that the chromaticity detector/demodulator  220  already has information about the positions of color points placed in the predetermined color system. 
     The configuration of the visible light communication system  1000  has been described in brief. According to the configuration, for example, as illustrated in  FIG. 1 , information about a chromaticity point corresponding to multi-value signals S 2 , S 8  and S 6  can be transmitted at the ratios of the intensities of light emitted from the light emitters  110 . As a result, the multiplicity may be increased to or above the number of the light emitters  110  and the problem of a decrease in a transmission error rate caused by a decrease in light intensity on a transmission path may be avoided. With reference to  FIGS. 2 and 3 , functional structures of the visible light transmission apparatus  100  and the visible light reception apparatus  200  will be described below in detail. 
     [Functional Structure of Visible Light Transmission Apparatus  100 ] 
     Referring to  FIG. 2 , a functional structure of the visible light transmission apparatus  100  according to the embodiment of the present invention will be described.  FIG. 2  illustrates the functional structure of the visible light transmission apparatus  100  according to the embodiment of the present invention. 
     As illustrated in  FIG. 2 , the visible light transmission apparatus  100  mainly includes a chromaticity coordinates calculator  122 , a light intensity controller  124 , light source driving circuits  112 , and light sources  114 . The chromaticity coordinates calculator  122  and the light intensity controller  124  form the multi-value modulation/emission controller  120 . The light source driving circuits  122  and the light sources  114  form the light emitters  110 . While not shown, the visible light transmission apparatus  100  may further include the serial/parallel converter. 
     (Chromaticity Coordinate Calculator  122 ) 
     The chromaticity coordinate calculator  122  maps a received digital value a to a chromaticity point. That is, the chromaticity coordinate calculator  122  maps each digital value to the chromaticity coordinates value of a chromaticity point corresponding to the digital value according to a layout of chromaticity points having predetermined multiple-value numbers. The chromaticity coordinate calculator  122  may use any of color systems including, for example, a Munsell color system and an Ostwald color system as well as a CIE color system (RGB, XYZ(Yxy), LxUxV, Lxaxb, etc) adopted by the Commission Internationale de l&#39;Eclairage (CIE), as the color system having chromaticity points. This is because the visible light communication system  1000  according to the embodiment of the present invention can perform chromaticity detection by a scheme such as spectrophotometric colorimetry, as described later. 
     With reference to  FIG. 4 , a specific function of the chromaticity coordinate calculator  122  will be described in brief.  FIG. 4  is a diagram illustrating a predetermined chromaticity point layout defined in a Yxy color system. In  FIG. 4 , a 2-degree visual field (a solid line) and a 10-degree visual field (a dotted line) are illustrated. 
     As illustrated in  FIG. 4 , for example, a predetermined chromaticity point layout s is preliminarily set in the color system. Information about the type of the color system and the chromaticity point layout is shared between the visible light transmission apparatus  100  and the visible light reception apparatus  200  in advance, or notified by a predetermined method during data transmission. In the example of  FIG. 4 , the chromaticity point layout s is for the case of 4-bit multiplexing of digital values (modulation multiplicity=16). Chromaticity points S 0  to S 15  are mapped to, for example, digital values illustrated in  FIG. 4(B) . The chromaticity coordinate calculator  122  selects a chromaticity point corresponding to the input digital value a from among the chromaticity points S 0  to S 15  and determines the chromaticity coordinates value (x, y) of the selected chromaticity point. 
     For example, if the digital value α=0×7, the chromaticity coordinate calculator  122  selects the chromaticity point S 7  and transmits the chromaticity coordinates value (x, y) of the chromaticity point to the light intensity controller  124 . As noted from the above example, as the multiplicity is expressed as 2 M  (M is an integer), data transmission can be carried out by mapping an M-bit digital value to one chromaticity point. Also, information about the chromaticity point layout of the color system illustrated in  FIG. 4(A)  may be tabulated as illustrated in  FIG. 4(B)  and shared between the visible light transmission apparatus  100  and the visible light reception apparatus  200 . 
     (Light Intensity Controller  124 ) 
       FIG. 2  will be referred to again. The light intensity controller  124  determines the light intensities of the light emitters  110  at respective wavelengths of light to emit light with the chromaticity indicated by the chromaticity coordinates value (x, y) determined by the chromaticity coordinate calculator  122 . Especially, the light intensity controller  124  calculates a light intensity ratio for each wavelength to realize the chromaticity corresponding to the chromaticity coordinates value (x, y). For example, the light intensity controller  124  calculates a mixing ratio of Red (R), Green (G) and Blue (B) to realize the chromaticity corresponding to the chromaticity coordinates value (x, y) and calculates driving voltages for the light emitters  110  according to the light intensities of the colors. 
     The light intensity controller  124  also determines the light intensity of each color so that the total light intensity of the light emitters  110  is a predetermined value and calculates a driving voltage corresponding to the light intensity of each color. This is because considering that chromaticity coordinate values are usually standardized as luminances, it is preferable to control the light intensities of the light emitters  110  according to the total light intensity. The light intensity controller  124  transmits a driving circuit control signal to each light source driving circuit  112  in order to emit light from each light source  114  at the driving voltage calculated for each color, as described later. The driving circuit control signal is used to control the light intensity of each light source  114 . In the example of  FIG. 2 , the light intensity controller  124  transmits three driving circuit control signals a, b and c to light source driving circuits (A), (B) and (C), respectively. 
     Also, the light intensity controller  124  may represent a chromaticity by controlling the total light intensity of the light sources  114  for a predetermined time. That is, the light intensity controller  124  multiplies the intensity of light that each light source  114  emits for a predetermined time, for each color and calculates a chromaticity coordinates value using the product value of each color as the received light intensity of each color. Then the light intensity controller  124  determines the driving voltage of each light source  114  as represented by the chromaticity coordinates value (x, y). For instance, if the visible light transmission apparatus  100  is a lighting device that performs PWM control, the influence of the PWM control may be eliminated by mapping an accumulated light intensity value to a chromaticity coordinates value (x, y) according to a PWM period. 
     Further, the light intensity controller  124  controls the light intensity of each light source  114  according to a chromaticity corresponding to a predetermined chromaticity coordinates point (hereinafter, referred to as a reference point or a predetermined pattern) during initialization or periodically. As described later, the visible light reception apparatus  200  uses a linear matrix that calibrates the difference between the light emission spectrum of the transmitting side and the spectral sensitivity of the receiving side, for calibration of color reproduction. Accordingly, the visible light transmission apparatus  100  may transmit the reference signal for calibration of a linear matrix value by emitting light with a chromaticity corresponding to the reference point at a predetermined timing or periodically in order to cope with a change in the light emission spectrum of the transmitting side or a change in the spectral sensitivity of the receiving side. 
     (Light Source Driving Circuits  112 ) 
     The light source circuits  112  control the light intensities of the later-described light sources  114  according to the driving circuit control signals received from the emission controller  124 . For example, a light source (A) driving circuit  112  connected to the light source (A)  114  receives the driving circuit control signal a and drives the light source (A)  114  to emit light at a driving voltage indicated by the driving circuit control signal a. Similarly, a light source (B) driving circuit  112  and a light source (C) driving circuit  112  also drive the light source (B)  114  and the light source (C)  114  according to the driving circuit control signals b and c, respectively. 
     (Light Sources  114 ) 
     The light sources  114  emit light according to the driving voltages supplied from the light source driving circuits  112 . Although the light sources  114  are configured with semiconductor light emitting devices such as LEDs, LDs or SLDs, they may be configured with light emitting devices included in fluorescent lights, CRT displays, Plasma Display Panel (PDP) devices, Electro Luminescence (EL) displays, or LCDs. 
     As described later in detail, the visible light communication system  100  according to the embodiment of the present invention transmits data by mapping a digital value to chromaticity coordinates. The data transmission is different from data transmission based on wavelength modulation or the like and is performed to avoid influence on the spectral distribution of the light sources  114 . For example, when the spectrum of white light emitted from the light sources  114  using LEDs is detected, peaks appear at the wavelengths of R, G and B. However, the spectrum of white light emitted by display devices described above has small peaks at the wavelengths of R, G and B and the spectrum is wide. If a structure for detecting a signal based on a spectrum shape or a signal strength is used, a difference from the spectrum shape is represented as a difference from demodulated data. Therefore, the type of a device used as a light source needs to be limited. In contrast, if data is transmitted using chromaticity coordinates as in the embodiment of the present invention, the signal may be received using the visible light reception apparatus  200  irrespective of the type of the light sources  114 . As a result, light sources  114  having a variety of light emitting devices can be handled. 
     As illustrated in  FIG. 4 , the visible light transmission apparatus  100  has a plurality of light sources  114 . The light sources  114  have their unique spectral distributions and may emit lights of different colors (La, Lb and Lc). Also, each light source  114  is not confined to a single-mode oscillator. It may be possible to group the plurality of light resources  114  into a plurality of groups each for emitting light of the same color and to control light intensity on a group basis. In this case, for example, the light source driving circuits  112  are configured so as to supply driving voltages to the light sources  114  on a group basis or the light intensity controller  124  is configured so as to transmit the same driving circuit control signal to light source driving circuits  112  connected to the same group of light sources  114 . Alternatively, the chromaticity of light may be adjusted by controlling the number of light emissions of the light sources  114  in each group through on/off control of all or part of the light sources  114  in the group. 
     The functional configuration of the visible light transmission apparatus  100  according to the embodiment of the present invention has been described so far. If the functional configuration is applied, the chromaticity coordinates-based data transmission enables modulation of data to a multi-value number having as many values as or values more than the number of the light sources  114  in the visible light transmission apparatus  100 , increases the amount of data transmittable in one pulse, and achieves higher-rate data transmission. Also, as information is carried at the light intensity ratio of each color, the resulting little influence of light intensity attenuation on the transmission path may lead to a decreased transmission error rate. Since data transmission is based on chromaticities, it is possible to achieve high speed by increasing the number of multiple values by the number of chromas included in the chromatcities and the type of the light sources  114  is not selected. 
     (Functional Configuration of Visible Light Reception Apparatus  200 ) 
     With reference to  FIG. 3 , a functional configuration of the visible light reception apparatus  200  according to the embodiment of the present invention will be described.  FIG. 3  illustrates the functional configuration of the visible light reception apparatus  200  according to the embodiment of the present invention. 
     As illustrated in  FIG. 3 , the visible light reception apparatus  200  mainly includes filters  212 , opto-electric converters  214 , AD conversion circuits  216 , a chromaticity coordinates corrector  222 , a chromaticity coordinates value detector  224 , and a demodulator  226 . The filters  212 , the opto-electric converters  214 , and AD conversion circuits  216  form the light receivers  210 . The chromaticity coordinates corrector  222 , the chromaticity coordinates value detector  224 , and the demodulator  226  form the chromaticity detector/demodulator  220 . 
     (Filters  212  and Opto-Electric Converters  214 ) 
     The filters  212  are optical filters having predetermined spectral sensitivity characteristics. To be more specific, the filters  212  are color filters for separating light of each color. When light is incident through the filters  212 , the opto-electric converters  214  output power according to their received light intensities. Hence, the received light intensity of each color may be detected by combining the filters  212  and the opto-electric converters  214 . 
     The opto-electric converters  214  are configured with, for example, Photo Diodes (PDs) (pn-type PDs, pin-type PDs, or Avalanche Photo Diodes (APDs)) and output currents of amounts proportional to the received light intensities. Thus, the opto-electric converters  214  may output signals according to the magnitudes of the output currents. Meanwhile, since color reproduction is calibrated using a linear matrix at a rear end in the visible light communication system  1000  according to the embodiment of the present invention, a chromaticity coordinates value can be detected even though the spectral sensitivity of the visible light reception apparatus  200  does not perfectly match to the spectral distribution of the visible light transmission apparatus  100 . 
     (AD Conversion Circuits  216 ) 
     The AD conversion circuits  216  are A/D converters for converting analog signals to digital signals. The A/D conversion circuits  216  convert signals of respective colors (analog signals) received from the opto-electric converters  214  to digital signals. For instance, in the example of  FIG. 3 , light incident on an opto-electric converter  214  through a filter (A′)  212  is converted to an analog electric signal by opto-electric conversion, converted to a digital signal a′ by an AD conversion circuit  216 , and then provided to the chromaticity coordinates corrector  222 . In the same manner, digital signals b′ and c′ corresponding to light incident through a filter (B′)  212  and a filter (C′)  212  are provided to the chromaticity coordinates corrector  222 . 
     (Chromaticity Coordinate Corrector  222 ) 
     The chromaticity coordinates corrector  222  calibrates color reproduction using a linear matrix scheme. The linear matrix scheme will be described briefly. 
     A color calibration technology called a linear matrix scheme is used to calibrate color reproduction in a photographing device such as a digital camera or a video camera. This photographing device functions to determine components of a linear matrix by comparing the color reproduction of sample images (predetermined color patterns, etc.) captured under a predetermined condition (e.g. a constant lighting condition, etc.) with the color reproduction of sample images captured under an actual condition. Specifically, the photographing device captures a sample image and determines matrix components that convert the RGB data (a received light intensity measured for each color) of the actually captured sample image to the RGB data of a preserved sample image. 
     For an image other than the sample image, a linear matrix is applied to captured RGB data and the RGB data is converted to RGB data with no change in filter characteristics or received light sensitivity characteristics. Consequently, despite a change in filter characteristics or received light sensitivity characteristics, a linear matrix can be calibrated by use of a predetermined sample image. Therefore, an output result based on constant filter characteristics or received light sensitivity characteristics can be achieved. 
     As described above, when multiplexed data is optically transmitted using color coordinates, a decrease in the spectral sensitivity of the receiving side leads to an increase in a demodulation error rate, compared to a general wavelength multiplexing scheme in which light with a separated emission spectrum is received and demodulated. Moreover, since the spectral sensitivity of the receiving side is dependent on the spectral sensitivity of the opto-electric converters  214  and the color separation filters  212 , for example, a change in the spectral sensitivities affects color reproduction. 
     In this context, the linear matrix scheme used for a video camera or a digital video camera is adopted. According to the linear matrix scheme, reproduction-calibrated light intensities R′, G′ and B′ of the respective colors are calculated by operating a predetermined matrix (a linear matrix) with the received light intensities of colors R, G and B, as by equation (1). For a video camera or the like, the linear matrix scheme is used to make color reproduction approximate to an ideal color reproduction defined as a standard of a video signal. 
     
       
         
           
             
               
                 
                   
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                   ) 
                 
               
             
           
         
       
     
     The elements of the matrix, a 11 -a 33  are coefficients determined according to the spectral sensitivities of the filters  212 , the opto-electric converters  214 , etc. Each coefficient of the linear matrix may be calculated using at least three sets of data with respect to the light intensities (R, G, B) and the post-calibration light intensities (R′, G′, B′). Thus, the visible light transmission apparatus  100  mixes a transmission signal with a reference signal corresponding to a predetermined chromaticity coordinates value (a reference point) and transmits the mixed signal periodically or at a predetermined timing. The visible light reception apparatus  200  may calculate each coefficient of the linear matrix at the chromaticity coordinates corrector  222  after receiving the reference signal through the light receivers  210 . Obviously, values of post-calibration received light intensities (R′, G′ and B′) corresponding to the reference point are preserved in advance in the chromaticity coordinates corrector  222 . Also, the chromaticity coordinates corrector  222  may calculate each coefficient of the linear matrix using three or more sets of data by a least square method. According to the least square method, more reliable coefficients may be calculated. 
     The chromaticity coordinates corrector  222  calculates equation (1) using the linear matrix acquired by the above method. That is, the chromaticity coordinates corrector  222  calibrates color reproduction so that the color coordinates of the transmitting side are identical to those of the receiving side. Consequently, the color coordinates of both the transmitting and receiving sides can be rendered identical through linear matrix conversion, even though the emission spectrum distribution of the visible light transmission apparatus  100  does not perfectly match to the spectral sensitivity of the visible light reception apparatus  200 . For example, even though the color combination of the light sources  114  in the visible light transmission apparatus  100  is changed, a digital value corresponding to a chromaticity point can be decoded because the spacing between the chromaticity points S 0  to S 5  and their relative position relationship are known. 
     When there is light incident on the light receivers  210  of the visible light reception apparatus  200  from a device other than the visible light transmission apparatus  100 , an addition term is added during converting the light intensities (R, G and B) to the post-calibration light intensities (R′, G′ and B′). Thus, the chromaticity coordinates corrector  222  calculates the following equation (2). 
     
       
         
           
             
               
                 
                   
                     ( 
                     
                       
                         
                           
                             R 
                             ′ 
                           
                         
                       
                       
                         
                           
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                             ′ 
                           
                         
                       
                       
                         
                           
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                     ) 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             
                               
                                 a 
                                 11 
                               
                             
                             
                               
                                 a 
                                 12 
                               
                             
                             
                               
                                 a 
                                 13 
                               
                             
                           
                           
                             
                               
                                 a 
                                 21 
                               
                             
                             
                               
                                 a 
                                 22 
                               
                             
                             
                               
                                 a 
                                 23 
                               
                             
                           
                           
                             
                               
                                 a 
                                 31 
                               
                             
                             
                               
                                 a 
                                 32 
                               
                             
                             
                               
                                 a 
                                 33 
                               
                             
                           
                         
                         ) 
                       
                       ⁢ 
                       
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                     + 
                     
                       ( 
                       
                         
                           
                             
                               R 
                               0 
                             
                           
                         
                         
                           
                             
                               G 
                               0 
                             
                           
                         
                         
                           
                             
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                               0 
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The chromaticity coordinates corrector  222  compensates digital signals a′, b′ and c′ corresponding to the received light intensities of the respective colors by calculating equation (1) or equation (2) and provides the resulting compensated digital signals a, b and c to the chromaticity coordinates value detector  224 . 
     (Chromaticity Coordinates Value Detector  224 ) 
     The chromaticity coordinates value detector  224  detects a chromaticity coordinates value (x, y) based on the digital signals a, b, and c compensated by the chromaticity coordinates corrector  222 . Herein, the chromaticity coordinates value detector  224  detects the chromaticity coordinates value (x, y) based on the color system used in the chromaticity coordinate calculator  122  of the visible light transmission apparatus  100 . For example, in case of the Yxy color system illustrated in  FIG. 4 , the chromaticity coordinates value detector  224  calculates three stimulation values (X, Y, Z) from the digital signals (a, b, c) using a color matching function representing a spectral sensitivity corresponding to the human eye and derives the chromaticity coordinates value (x, y) from the calculation result. Meanwhile, the chromaticity coordinates value (x, y) may be derived from the digital signals (a, b, c) as done in spectrophotometric colorimetry. The chromaticity coordinates value detector  224  transmits the derived chromaticity coordinates value (x, y) to the demodulator  226 . 
     In the case where the light intensity controller  124  of the visible light transmission apparatus  100  drives the light sources  114 , taking into account PWM control, a signal is transmitted according to a PWM period. Therefore, the chromaticity coordinates value detector  224  multiplies the digital signals (a, b, c) for the respective colors in one PWM period or over a predetermined time period and derives the chromaticity coordinates value (x, y) according to the resulting products (A, B, C). 
     In a general lighting device, a light intensity perceivable to a human being can be adjusted by prolonging or shortening a light emission time through light intensity control by PWM driving of a light source. According to the above configuration, since the chromaticity coordinates value (x, y) is measured using the light intensity multiplied over at least one PWM period, there is little influence of the PWM control. One thing to note here is that PWM periods should be synchronized among the light sources  114 . 
     (Demodulator  226 ) 
     The demodulator  226  demodulates the original digital value a based on the chromaticity coordinates value (x, y). The demodulator  226  may detect the digital value a corresponding to a chromaticity point at the chromaticity coordinates value (x, y) in the chromaticity point layout (refer to  FIG. 4 ) of the color system used in the chromaticity coordinates calculator  122 . Notably, it is assumed that the demodulator  226  preserves information about the chromaticity point layout. 
     The functional configuration of the visible light reception apparatus  200  according to the embodiment of the present invention has been described above. According to the configuration, the chromaticity coordinates value corresponding to the received light intensities of the respective colors is calculated and the original digital value a is demodulated based on the chromaticity coordinates value (x, y). Therefore, an increase in a transmission error rate caused by attenuation of light intensity on a transmission path can be suppressed. Also, since color reproduction is calibrated using the linear matrix scheme, the original digital value a can be demodulated even though the emission spectrum of the transmitting side does not perfectly matches the light reception sensitivity of the receiving side. Now a detailed description will be made of effects that can be achieved from applying the visible light communication system  1000  according to the embodiment of the present invention. 
     [Effects of the Embodiment] 
     Effects that can be achieved by applying the configuration of the visible light communication system  1000  according to the embodiment of the present invention will be described in detail. First, a visible light communication technology is investigated from a different viewpoint from the embodiment of the present invention and then effects of the embodiment of the present invention will be described in contrast. 
     (Consideration 1) 
     A structure for carrying out visible light communication by on/off control of three types of (R, G, B) light sources will be investigated. For example, in a structure for independent on/off control of a light source R, a light source G, and a light source B, 1-bit data can be transmitted by each of R, G and B and thus 3-bit data can be transmitted by their combination. While octal numbers may be mapped to RGB combinations, for transmission, the transmission rate of data may not be increased because the number of bits of data transmittable at one time is less than that of light sources. In this context, the visible light communication system  1000  according to the embodiment of the present invention may offer special effects in that multiplexing of as many data as or data more than the number of light sources is possible, compared to the simple RGB on/off structure, because multiplexed digital values are mapped to multiple-value chromaticity points at chromaticity coordinates. 
     (Consideration 2) 
     A structure for mapping data to the absolute values of the intensities of light emitted from three types of (R, G, B) light sources and transmitting the absolute values will be considered. For example, a structure for emitting light from light sources by controlling their light intensities in the transmitting side, and measuring the absolute values of received light intensities of R, G, and B and demodulating data corresponding to the received light intensities in the receiving side may be considered. This structure increases an error rate during demodulation as light intensity attenuates on a transmission path. Particularly, when the structure is applied to a lighting device or the like, the light intensity is readily attenuated due to the influence of shielding, etc. As a result, it is difficult to increase a data rate. In this respect, the visible light communication system  1000  according to the embodiment of the present invention offers special effects in that a change in light intensity seldom affects an error rate during demodulation because a digital value is mapped to a chromaticity coordinates value, for transmission and thus the chromaticity coordinates value is detected according to a ratio (a relative value) of the received light intensity measured for each color, even though the light intensity is changed on the transmission path. 
     (Consideration 3) 
     Now a structure for mapping a digital value to a chrominance, for transmission may be considered. A chrominance may be expressed as an angle of a color circle. For example, a structure for defining data  1  as light emission in color corresponding to  10 -degree clockwise movement of the color circle from red at the next timing after light emission in color at the position of red may be contemplated. With the use of chrominance, the attenuation of light intensity on a transmission path does not affect a transmission error rate much as in the embodiment of the present invention. However, if data  1  is mapped to the chrominance resulting from the 10-degree clockwise movement, successive transmission of data  1  changes color significantly in a particular direction. Hence, the color of a lighting device changes considerably and the types of available lighting devices are limited. Moreover, when data is mapped to a small chrominance, a signal identification level drops and thus a transmission error rate during demodulation is increased. Consequently, a data rate is decreased. In this respect, the visible light communication system  1000  according to the embodiment of the present invention offers the special effects that successive transmission data are not followed, multiplexing using a predetermined color range is possible, and the colors of lighting devices are not changed much (the type and usage of a light source are not selected) because a digital value is modulated to a multiple-value number according to the chromaticity point layout of a color system. Compared to modulation to a chrominance, since as much multi-value modulation as chromas included in chromaticities is enabled, higher-rate data transmission is possible. 
     As described above, the visible light communication system  1000  according to the embodiment of the present invention achieves the foregoing effects in view of the basic characteristic of mapping digital values to color coordinates, for transmission. However, the effects that the visible light communication system  1000  can achieve are not limited to the foregoing effects. 
     For example, an LCD or PDP that has a wider 3-color (R, G, B) light emission spectrum than an LED can be used as a light source. The chromaticity coordinates define color matching function-based responses, rather than they correspond to the spectrum of light. Therefore, the same chromaticity coordinates may result, despite different spectrums. For instance, white of a fluorescent light with a wide light emission spectrum and white having a strong peak in R, G, B (e.g. 623 nm, 523 nm, and 456 nm) expressed as R, G, B of an LED are at the same chromaticity coordinates. Therefore, compared to a structure for identifying a signal by the shape of a spectrum, a special effect is achieved in that signal identification is possible irrespective of the spectral distribution of a light source. 
     In another example, there is the effect of obviating the need for using three primary R, G, B colors. Since the receiving side has only to detect the chromaticity coordinates value of a chromaticity point, if the spacing between chromaticity points is set to be larger than noise, data can be transmitted even by light other than white light. For example, multiplexing is possible using blue (B) and green (G) light sources only. 
     In a further example, even though the colors of the light sources of the visible light transmission apparatus  100  are changed, data transmission is possible without changing the configuration of the visible light reception apparatus  200 . This is based on a structure for transmitting a predetermined pattern at a chromaticity corresponding to a reference point and calibrating a linear matrix using the predetermined pattern. With the use of the structure for calibrating the linear matrix using the reference signal transmitted at a predetermined timing or periodically, the chromaticity coordinates of the transmitting side can be matched to those of the receiving side based on the spacing between chromaticity points and the relative position relationship of the chromaticity points. Therefore, it is possible to detect a chromaticity coordinates point according to the received light intensity of each color that is converted by the linear matrix. 
     Further, in still another example, since a clock signal may be generated from transmitted data, many transmission rates may be handled. 
     So far, the effects of the visible light communication system  1000  according to the embodiment of the present invention have been described in detail. As stated before, only a relatively low-rate modulation is achieved in a typical lighting LED or the like. Hence, it is important to increase a data rate in optical communications based on the relatively low-rate modulation. In that aspect, the use of the above-described configuration enables high-speed data transmission using a lighting device with a low modulation speed. In the mean time, the upper limit of multiplexing is determined according to the distance between chromaticities, noise, a mismatch width in light reception sensitivity, and an error correction capability, which also determine the upper limit of a data rate in the above configuration. 
     While the preferred embodiment of the present invention has been described with reference to the attached drawings, it is obvious that the present invention is not limited to related examples. Those skilled in the art will understand that various modifications or changes can be made within the spirit scope of the present invention as defined by the appended claims and their equivalents and they also fall into the technical scope of the present invention. 
     For example, while transmission paths have not been addressed in the above description of the embodiment of the present invention, the technology according to the embodiment of the present invention is applicable to an optical fiber transmission line as well as a wireless transmission line. Especially, it is possible to realize high-speed fiber optic communication using a multiplexed signal due to strong robustness against light intensity attenuation on a transmission path. 
     While a structure for spatially multiplexing a plurality of light sources is not described in the description of the embodiment of the present invention, it is possible to place a plurality of light source groups of the same configuration spatially (two-dimensionally), for multiplexing. In this case, data can be transmitted at as high a speed as spatial multiplexing. Also, as illustrated in  FIG. 5 , a structure for transmitting a reference signal from some light source group always enables calibration of a linear matrix using a signal corresponding to a reference point. Thus, even when light is incident from other light sources, the original state may be recovered fast. When this structure is employed, it may be set whether to perform the linear matrix calibration always or at a predetermined timing in the receiving side. 
     While the received light intensity of each color is measured using a plurality of light receivers with different spectral characteristics at different colors in the description of the embodiment of the present invention, the spectrum decomposition of light may be carried out by using one spectral sensor or one detector, and one spectrum analyzer in combination. Also, the transmitting side may be configured so as to use a DC-free coded signal and make the DC-free signal appear as a single illuminated light to the human eye. Herein, the DC-free code is a code including the same number of 1s and 0s.