embodiments of the invention will be explained with reference to the accompanying drawings . a pulse wave measuring apparatus 10 in a first embodiment of the invention will be hereinafter explained with reference to fig1 to 7 . fig1 is a block diagram showing a structure of the pulse wave measuring apparatus 10 . fig2 , 3 , and 4 are diagrams showing example of arrangement of plural light - emitting elements l and plural light - receiving elements p in a sensor module 11 . as shown in fig1 , the pulse wave measuring apparatus 10 amplifies a pulse wave signal , which is obtained from the sensor module 11 , with amplifiers 12 to 14 and , then , converts the pulse wave signal into a digital signal with an a / d converter 15 . thereafter , the pulse wave measuring apparatus 10 processes the digital signal with a cpu 15 and displays a result of the processing on a display 17 . electric power for the pulse wave measuring device 10 is supplied from a battery 18 . infrared leds are used for the light - emitting elements l of the sensor module 11 and photodiodes are used for the light - receiving elements p . note that a program for realizing a processing method for measurement of a pulse wave described below is stored in the pulse wave measuring device 10 and is processed by the cpu 15 . a structure of the sensor module 11 will be explained . the plural light - emitting elements l and the plural light - receiving elements p are provided on an array substrate 9 of the sensor module 11 . the light - emitting elements l and the light - receiving elements p are attached to a wrist or the like . fig2 is a diagram of a first example of arrangement of the light - emitting elements l and the light - receiving elements p on the array substrate 9 . this is standard arrangement of the light - emitting elements l and the light - receiving elements p in the sensor module 11 . light - emitting elements l 1 to l 5 are arranged one after another in a row in the horizontal direction and light - receiving elements p 1 to p 5 are arranged one after another in a row in the horizontal direction in parallel to the light - emitting elements l 1 to l 5 . the light - emitting element l 1 and the light - receiving element p 1 are in a one - to - one relation . the same holds true for the other light - emitting elements l 2 to l 5 and the other light - receiving elements p 2 to p 5 . when the sensor module 11 is attached to a wrist 19 , the plural light - emitting elements l and the plural light - receiving elements p are wound along the periphery of the wrist 19 . fig3 is a diagram of a second example of the attachment , in which two light - emitting elements l are associated with one light - receiving element p . the light - emitting elements l 1 to l 5 are arranged one after another in a row in the horizontal direction and the light - receiving elements p 1 to p 5 are arranged one after another in a row in the horizontal direction such that the light - emitting elements l deviate in the horizontal direction with respect to the light - receiving elements p . with the arrangement shown in fig3 , the light emitting elements l and the light - receiving elements p are excellent in detection of a position when the sensor module 11 is attached . fig4 is a third example of the arrangement , in which the arrangement in fig3 is further expanded . the light - emitting elements l and the light - receiving elements p are also arranged in the vertical direction to aim at further improvement of detection accuracy . the light - emitting elements l and the light - receiving elements p are arranged alternately in a row in the horizontal direction . the row in the horizontal direction , in which the light - emitting elements l and the light - receiving elements p are arranged in this way , are arranged in plural stages in the vertical direction . ( 2 ) processing method of the pulse wave measuring apparatus 10 a processing method of the pulse wave measuring apparatus 10 will be explained using the sensor module 11 shown in fig2 with reference to a flowchart in fig6 . first , the sensor module 11 is attached on an artery 20 in a wrist 19 of a patient as shown in fig5 . in step 1 , the pulse wave measuring apparatus 10 sets processing to be performed for all combinations of the light - emitting elements l and the light - receiving elements p . in step 2 , the pulse wave measuring apparatus 10 causes the light - emitting element l 1 to emit light . in step 3 , the pulse wave measuring apparatus 10 receives reflection intensity of the light from the light - emitting element l 1 in the light - receiving elements p 1 to p 5 and stores a pulse wave signal of a light - receiving element with strongest signal intensity . for example , the pulse wave signal is assumed to be a pulse wave signal y ( t ) of the light - receiving element p 3 . in step 4 , the pulse wave measuring device 10 slices a pulse waveform for one heart beat that is obtained when the pulse wave signal y ( t ) is measured for a fixed period . the fixed period is 1 . 0 to 1 . 5 seconds . in step 5 , the pulse wave measuring apparatus 10 compares a sliced pulse waveform h for one heart beat and the pulse wave signal y ( t ), which is continuously inputted , to obtain an autocorrelation s ( t ). for example , the autocorrelation s ( t ) is equal to or higher than − 1 and equal to or lower than 1 . the autocorrelation s ( t ) is the highest at 1 . in step 6 , the pulse wave measuring apparatus 10 calculates an autocorrelation value s 0 that is a value obtained by averaging the autocorrelation s ( t ) in a predetermined time ( e . g ., five seconds ). if a value of the autocorrelation s ( t ) in a fixed time is constant , the autocorrelation value s 0 is high ( e . g ., s 0 & gt ; 0 . 8 ). a signal intensity of a pulse wave signal from the light - receiving element is stably high and it can be judged that the pulse wave signal is a pulse wave . if the signal intensity is unstable , it can be judged that the pulse wave signal is noise . in step 7 , the calculated autocorrelation value s 0 is an autocorrelation value measured for the first time , the pulse wave measuring apparatus 10 proceeds to step 8 . if the calculated autocorrelation value s 0 is the autocorrelation value s 0 calculated for the second or subsequent time , the pulse wave measuring apparatus 10 proceeds to step 9 . in other words , when combinations of the light - emitting element l 1 and the light - receiving elements p 1 to p 5 are processed first , the autocorrelation value s 0 highest in the combinations is set as the autocorrelation value s 0 at the first time . in step 8 , the pulse wave measuring apparatus 10 stores the calculated autocorrelation value s 0 at the first time as an initial value . in step 9 , if the calculated autocorrelation value s 0 is high compared with the initial value stored in step 8 in advance or the autocorrelation value s 0 updated in processing in step 9 of the last time , the pulse wave measuring apparatus 10 updates the initial value or the updated autocorrelation value s 0 to the calculated autocorrelation value s 0 and stores the calculated autocorrelation value s 0 . if the calculated autocorrelation value s 0 is low compared with the initial value stored in step 8 in advance or the autocorrelation value s 0 updated in processing in step 9 of the last time , the pulse wave measuring apparatus 10 does not update the initial value or the updated autocorrelation value s 0 . in step s 10 , the pulse wave measuring apparatus 10 repeats the processing from step 1 until processing for all combinations of the light - emitting elements l and the light - receiving elements p ends . when the processing for all the combinations ends , the pulse wave measuring apparatus 10 proceeds to step 11 . in step 11 , the pulse wave measuring apparatus 10 determines a combination of the light - emitting element l and the light - receiving element p having the highest autocorrelation value . for example , when it is assumed that the combination is a combination of the light - emitting element l 4 and the light - receiving element p 1 , the combination is as shown in fig7 . with the processing described above , although the pulse wave measuring apparatus 10 in this embodiment has the plural light - emitting elements l and the plural light - receiving elements p , the pulse wave measuring apparatus 10 can identify an optimum position . thus , it is possible to realize the wave pulse measuring apparatus that copes with a positional deviation of the sensor module 11 and a change in a position of the artery 20 on a real time basis and is robust against disturbances . a second embodiment of the invention will be explained with reference to fig8 to 11 . in this embodiment , a method of selecting an optimum combination of the light - emitting element l and the light - receiving element p , which occurs when a position of the blood vessel ( the artery ) 20 changes in the pulse wave measuring device 10 , will be explained . fig8 and 10 are sectional views showing positions of the sensor module 11 and the artery 20 of the wrist 19 . fig9 and 11 are schematic diagrams of the sensor module 11 and the artery 20 viewed from a surface of an organism . in fig9 and 11 , positions of the sensor module 11 and the artery 20 before and after a change of a position of the artery 20 are shown , respectively . as a characteristic of the light - emitting elements l used in the sensor module 11 in this embodiment , when light is made incident in an organism and the light is reflected to return to the outside of the organism , a distance between the light - emitting elements l and the light - receiving elements p is proportional to a depth of the light made incident in the organism . as the distance of the light - emitting elements l and the light - receiving elements p are larger , the depth of the light made incident in the organism detected by the light - receiving elements p is larger . in fig8 and 9 , the artery 20 is close to the surface of the organism . since the artery 20 is close to the surface of the organism , an interval between the light - emitting elements l and the light - receiving elements p is preferably small . a combination of the light - emitting element l 4 and the light - receiving element p 2 shown in fig9 is appropriate . in fig1 and 11 , the artery 20 is far from the surface of the organism . since the artery 20 is far from the surface of the organism , an interval between the light - emitting elements l and the light - receiving elements p is preferably large . a combination of the light - emitting element l 5 and the light - receiving element p 1 shown in fig1 is appropriate . a third embodiment of the invention will be explained with reference to fig1 . in this embodiment , assuming that an optimum combination of a pair of the light - emitting element l and the light - receiving element p has already been acquired by the method explained in the first embodiment , a method of causing the light - emitting elements p to emit light selectively in selecting an optimum combination again will be explained . when a currently selected combination of a pair of the light - emitting element l and the light - receiving element p is as shown in fig2 , if intensity of a pulse wave signal weakens in the combination because of influence of movement , the pulse wave measuring apparatus 10 tries to search for an optimum combination again . in this search , the pulse wave measuring apparatus does not cause the light - emitting elements l to emit light in order from the light - emitting element l 1 . instead , as indicated by steps 20 to 22 in a flowchart in fig1 , the pulse wave measuring apparatus 10 causes the light - emitting elements l to emit light in order from the light - emitting element l near the currently selected light - emitting element l . this makes it possible to select an optimum combination by performing light emission as a small number of times as possible . in that case , as indicated by step 21 , if a pulse wave signal has intensity equal to or higher than a certain threshold value , which allows the pulse wave signal to be used as a pulse wave , the pulse wave measuring apparatus 10 stops the search for an optimum combination at that time . note that steps 23 to 32 are the same as the processing in steps 2 to 11 . a fourth embodiment of the invention will be explained with reference to fig1 . apart from a combination of the light - emitting element l and the light - receiving element p with a high autocorrelation value calculated according to autocorrelation , intensity of a pulse wave signal may be equal to or lower than a predetermined threshold value . in this embodiment , as shown in fig1 , when an amplitude of the pulse wave signal y ( t ) falls to be lower than the predetermined threshold value , the pulse wave measuring apparatus 10 detects a combination of the light - emitting element land the light - receiving element p , which has the pulse wave signal y ( t ) with signal intensity equal to or higher than the threshold value , again . a fifth embodiment of the invention will be explained with reference to fig1 . in the first embodiment , the sensor module 11 is attached to a site near a radial artery or an ulnar artery of the wrist 19 . however , in this embodiment , as shown in fig1 , the same effect as the first embodiment is obtained by attaching the sensor module 11 to a carotid artery of a neck 21 . a sixth embodiment of the invention will be explained with reference to fig1 and 16 . in this embodiment , unlike the first and the sixth embodiments , the sensor module 11 is attached to a planta artery near an ankle 22 as shown in fig1 . however , since a surface of the skin near the ankle 22 is extremely rough because of a melleolus , a pulse wave is detected by curving the sensor module 11 to stick firmly to the skin as shown in fig1 . the invention is not limited to the embodiments described above and can be modified in various ways as long as modifications do not depart from the spirit of the invention . the invention is suitably applied to measurement of a pulse wave for various purposes such as autonomous nervous system measurement , prevention of life - style related diseases , and sleep state measurement .
a pulse wave measuring apparatus can determine an optimum combination of a pair of light - emitting element and light - receiving element out of plural light - emitting elements and plural light - receiving elements on a real time basis according to a difference of signal intensity of a pulse wave signal from the light - receiving element and always measure an stable pulse wave of a wrist artery robustly .