Patent Publication Number: US-11653846-B2

Title: Device and method suitable for monitoring arterial blood in a body part

Description:
FIELD OF THE INVENTION 
     The current invention relates to a device suitable for monitoring blood in a body part. In particular, the current invention relates to a device suitable for monitoring blood in a body part for determining heart rate or oxygen level during exercise. 
     BACKGROUND OF THE INVENTION 
     A type of heart rate monitors measures heartbeat based on the absorption or transmission of infrared light projected through a limb or digit of a person, or animal. The heart rate monitor typically comprises an emitter and a sensor. The emitter emits infrared light into the limb towards the sensor. Skin, tissues, venous blood and arterial blood absorb and reflect parts of this infrared light. However, the volume of arterial blood periodically increases and decreases with heartbeat. This causes the absorption and reflection of the infrared light to fluctuate with the heartbeat, which is detected by the sensors as periodic fluctuations of infrared transmission. This can be distinguished from the relatively constant effects of skin, tissue and venous blood on infrared light transmission. 
     There are generally two methods of measuring infrared light projected into a limb. In the first method, the emitter and the sensor are placed on somewhat opposite sides of the limb, while avoiding any bone within the limb, so that the infrared is transmitted from the emitter to the sensor through the limb. In the other method, the emitter and the sensor are placed somewhat on the same side of the limb, so that a portion of the infrared light from the emitter projected into the limb is dispersed by the layers of tissues in the limb to arrive at the sensor. 
     Unfortunately, the accuracy of such heart rate monitors is affected by wearer&#39;s movements which introduce noise into the infrared transmission detected by the sensor. This is due in part to relative dislocation of the emitter and the sensor as the wearer moves, and in part to the flexing of the limb during movements which increase or decrease the transmission path length between the emitter and the sensor. That is, the skin and soft tissues of the limb is capable of wobbling and affecting the length of the transmission path. 
     A heart rate monitor in the form of an arm band arranged with three pairs of emitter and sensor has been proposed. The pairs are positioned on the arm band in such a way that noise caused by movements of the wearer of the arm band and detected by the three sensors are observed in different directions and angles, and are therefore mutually out-of-phase. In this way, the three pairs of emitter and sensor provide three sets of observations which can be used to remove noise components without requiring any external sensors to create a motion reference, as is required in many other earlier heart rate monitors of similar technology. However, this heart rate monitor requires three independent observations which have to be obtained by the same number of emitter and sensor pairs; a single pair of emitter and sensor is not enough for provide a sufficient number of independent observations. Unfortunately, the three emitter and sensor pairs compromise the robustness of the heart rate monitor, as the heart rate monitor will fail to work as soon as any one of the three sensors or three emitters fails to work. Also, manufacture and repair of this heart rate monitor is costly since so many emitters and sensors are required. 
     Accordingly, it is proposed to provide a heart rate monitor which is at least as accurate in determining heart rate with more robust resistance to malfunction, and preferably providing the possibility of using less hardware while achieving the same or better performance. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the invention proposes a device suitable for monitoring blood in a body part, the device being suitable for wearing on the body part, and the device comprising: a plurality of light emitters at least one sensor, the plurality of light emitters arranged such that light from the plurality of light emitters is capable of passing through the body part to arrive at the least one sensor, wherein: the plurality of light emitters emit light in consecutive order to be detected by the at least one sensor. 
     The device can be used as a heart rate monitor. Alternatively, the device can be used as part of an oximeter. 
     The invention provides an advantageous possibility that only one sensor is required to obtain a plurality of signal observations by detecting transmissions from different emitters. This reduces the number of sensors required to obtain an equal number of observations. This also reduces the amount of hardware and allows the device to be made smaller, lighter and cheaper. 
     In a certain embodiment, the at least one sensor is a plurality of sensors. The plurality of sensors and the plurality of emitters are capable of being defined as a plurality of sensor and emitter pairs, wherein the emitter in a first pair of sensor and emitter is capable of emitting to the sensors of at least two other pairs, and the sensor in the first pair is capable of detecting light from the emitter in each of the at least two other pairs. 
     Optionally, two sensors are used to obtain at least four observations of light transmissions or, as the case may be, just three observations may be selected for use out of the four observations. This allows that the number of sensors used is less than the equivalent number of observations obtainable, effectively reducing the amount of hardware required for obtaining that number of observations. As the skilled man knows, having a plurality of observations is useful for minimising noise to signal ratio. 
     More preferably, three sensors are used to obtain at least three observations of light transmissions. Typically, three sensors can be used with two emitters to provide six observations. Therefore, in the event that anyone of the three sensors fails to work, there will still be at least two sensors working with the two emitters to provide four observations. The four observations may all be used to monitor heart rate but it is possible, as a matter of choice, that only three out of the four observations may be used. In other words, if not more than three or four observations are required to produce a stable monitoring of heart rate, the use of three sensors with two emitters provides redundancy of three or two observations respectively without requiring additional hardware. This provides back up or redundant observations in the event one of the sensors fails to work. In another situation, one emitter may fail to work but the device may still monitor heart rate based on three data observations obtained from the remaining three sensors and one emitter. 
     Monitoring heart rate based on four observations of data is already superior to most prior art devices, which typically use three observations only. Furthermore, the prior art provides that each sensor is dedicated to only one emitter and three sensor and emitter pairs are used to provide only three observations; the number of observations is the same as the number of sensors. In other words, there is no redundancy of observations in such prior art. 
     In one embodiment, the device is a circular support capable of being attached to the body part, the at least one sensor is a plurality of sensors, the plurality of sensors being evenly distributed about the circular support. This avoids exposing all the plurality of sensors to ambient light from any one direction at the same time; the distribution about a circular support prevents strong ambient light coming from one direction and striking on a sensor from striking the other sensors too. Coupled with the advantage of redundant observations, particularly where the device comprises at least three sensors coupled with at least two emitters taking turns to emit light, this provides the possibility that the device is able to monitor heart rate with sufficient number of data observations even when the ambient light has crippled one of the sensors. The capacity to avoid breakdown when one of the sensors fail to work provides a possible advantage of lowering hardware cost by reducing the light detection range required for each sensor. Preferably, the circular support is in the form of a ring wearable on finger. 
     In a more specific preferred embodiment, the device comprises three of the at least one sensor are arranged to detect light transmission from three light emitters, each of the three sensors is arranged to detect light transmission from at least two of the light emitters so as to detect at least six observations of light transmissions by the three sensors, the device comprises a support capable of being attached to the body part, the support having a curved surface, the plurality of sensors is distributed along the curvature of the curved surface of the support so as to reduce the likelihood of uniform exposure of the plurality of sensors to ambient light from any one direction at the same time, the device is configured to monitor blood in a body part using only four observations such that two of the at least six observations are redundant observations, and the device is configured to disregard the two observations of any of the sensors which the device detects as failing to work properly due to saturation by ambient light while regarding the four observations of the remaining working sensors to monitor blood. 
     Preferably, the plurality of sensors is arranged in mutually different positions. Optionally, different positions refers to different mutual distances from the at least one light emitter. Alternatively, different positions refers to different directions to the at least one light emitter. Having different directions or distances promotes diversity in noise data, increasing the likelihood that noise sensed by each sensor is different from or is out of phase with those detected by the other sensors. This allows noise to be eliminated more easily. In contrast, heartbeat signals detectable by all the sensors are synchronous and in-phase, and may be extracted from the noise. 
     In a second aspect, the invention proposes a method of obtaining observations of light transmission to monitor blood in a body part, comprising the steps of: providing at least one sensor at a side of the body part; providing a first emitter and second emitter at different sides of the body part such that light emitted from the first emitter and second emitter transmits through the body part to arrive at the at least one sensor; causing the first emitter and second emitter to emit light one after the other, and the at least one sensor to detect light from the first emitter and second emitter in accordance to the order in which the first emitter and second emitter emit light to obtain a first observation and a second observation. 
     Preferably, the method comprises the further steps of: providing a further sensor at a further side of the body part; and causing the further sensor to detect light from either the first emitter or the second emitter to obtain a third observation. Preferably, the method also comprises the further step of: causing the further sensor to detect light from either the first emitter or the second emitter to obtain a fourth observation. 
     Preferably, the method comprises the further steps of: determining any one of the sensors or emitters as failing to work properly; disregarding the observations made with the one of the sensors or emitters failing to work properly and regarding the readings of the remaining working sensors to monitor blood. Typically, the sensor fails to work properly due to saturation by over exposure to ambient light. 
     The ability to disregard any of the sensor or emitter which fails to work properly provides the device with adaptability to different ambient conditions. 
     In a further aspect, the invention proposes a device suitable for monitoring blood in a body part, comprising a substrate suitable for adhering onto the skin of a person; the substrate attached with at least one light emitter at least one sensor; the at least one light emitter arranged such that light from at least one light emitter is capable of diffusing through the body part to arrive at the least one sensor, wherein: the substrate is capable of substantially holding the emitter and sensor in a plane. 
     in a further aspect, the invention proposes a device suitable for monitoring blood in a body part, comprising a substrate suitable for being placed on the body part the substrate attached with at least two light emitters, and at least two light sensors; the at least two light emitters arranged such that light from each of the at least two light emitters is capable of diffusing through the body part to arrive at each of the at least two light sensors; the substrate being capable of substantially holding the emitter and sensor in a plane; wherein the at least two light emitters operate sequentially to emit light to be detected by the sensors; and the at least two light sensors are positioned such that each of the at least two light sensors detects light from every one of the at least two light emitters in a different direction. 
     Preferably, the at least two sensors operate to sequentially detect light. 
     Preferably, the at least two emitters operate to sequentially emit light. 
     A planar arrangement is able to prevent inconvenience of arranged the device around a body part such as a limb. While it may be advantageous to secure a device to the limb by tying the device around the limb, it is also advantageous in the alternative to reduce the likelihood of any part of the device being wedged between the limb and another body part, such as the inner part of the arm and the rib cage. This reduces the chance of damaging any part of the device from being battered by the body parts, and also reduces the chance of body parts from abrasion by protruding parts of the sensors and emitters. 
     Optionally, the substrate is a flexible fabric, and the substrate is capable of being configured to arrange the emitter and sensor in the plane. 
     Alternatively, the substrate is a flexible plastic, and the substrate is capable of being configured to arrange the emitter and sensor in the plane. 
     Alternatively, the substrate comprises an inflexible material. 
     Preferably, the substrate is provided with an adhesive surface for adhering to the skin of the wearer. 
     Preferably, the device comprises at least one light emitter and at least two sensors. More preferably, the at least two sensors are arranged in different illuminating directions to the at least one light emitter. 
     Alternatively, the device comprises at least one sensor and at least two light emitters. More preferably, the at least two emitters are arranged in different illuminating directions to the at least one sensor. 
     Different illuminating direction can he created by physical different location of sensor/emitter or by making using of some optical light guide design. 
     In a further aspect, the invention proposes a device suitable for monitoring blood in a body part, comprising at least two light emitters arranged such that light from the at least two light emitters is capable of diffusing through the body part to arrive at the at least two sensors, the light emitted from each of the at least two light emitters being more intense along a first planar axis, one of the at least two sensors being arranged to detect light projected along said first planar axis of the each of the emitters, the other one of the at least two sensors being arranged to detect light projected along a second planar axis which is substantially orthogonal to said planar axis of the each of the least two light emitters, and the light emitted from each of the at least two light emitters being less intense along the second planar axis, wherein the least two light emitters emit light sequentially to the two sensors. 
     In yet a further aspect, the invention proposes a device suitable for monitoring blood in a body part, comprising at least two light emitters arranged such that light from the at least two light emitters is capable of diffusing through the body part to arrive at the at least two sensors, light emitted from each of the at least two light emitters being more intense along a respective first planar axis, light emitted from each of the at least two light emitters being less intense along a respective second planar axis, each of the at least two sensors being arranged to detect light projected along the first planar axis of one of the two emitters, and to detect light projected along the second planar axis of the other one of the two emitters, Wherein the least two light emitters emit light sequentially to the two sensors. Typically, the second axis is orthogonal or is in a substantially different direction to that of the first axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention, in which like integers refer to like parts. Other embodiments of the invention are possible, and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention. 
         FIG.  1    is an illustration of a first embodiment of the invention; 
         FIG.  2    is a schematic diagram of the internal arrangement of the embodiment of  FIG.  1   ; 
         FIG.  3    is a schematic diagram of the internal arrangement of the embodiment of  FIG.  1   ; 
         FIG.  4    is a chart showing noise signals which can be treated by the embodiment of  FIG.  1   ; 
         FIG.  5    is a diagram explaining the effects of noise components which can be treated by the embodiment of  FIG.  1   ; 
         FIG.  6    is a chart showing heart rate and noise motions which can be treated by the embodiment of  FIG.  1   ; 
         FIG.  7    further explains the noise component discussed for  FIG.  5   ; 
         FIG.  8    shows a second embodiment to the invention; 
         FIG.  9    further shows the second embodiment of  FIG.  8   ; 
         FIG.  10    further shows the second embodiment of  FIG.  8   ; 
         FIG.  11    further shows the second embodiment of  FIG.  8   ; 
         FIG.  12    explains the workings of the embodiment of  FIG.  8   ; 
         FIG.  13    explains an advantage in the embodiment of  FIG.  8   ; 
         FIG.  14    shows yet a further embodiment of the invention; 
         FIG.  15    shows yet a further embodiment of the invention; 
         FIG.  16    further shows the second embodiment of  FIG.  15   ; 
         FIG.  17    shows yet a further embodiment of the invention; 
         FIG.  18    shows yet a further embodiment of the invention; 
         FIG.  19    shows yet a further embodiment of the invention; 
         FIG.  20    shows yet a further embodiment of the invention; 
         FIG.  21    shows yet a further embodiment of the invention; 
         FIG.  22    shows the embodiment of  FIG.  21    in use; 
         FIG.  23    shows how the embodiment of  FIG.  21    may be made; 
         FIG.  24    shows how the embodiment of  FIG.  21    may operate; 
         FIG.  25    shows how the embodiment of  FIG.  21    may operate; 
         FIG.  26    shows how the embodiment of  FIG.  21    may operate; 
         FIG.  27    a variation of the embodiment of  FIG.  21   ; and 
         FIG.  28    a variation of the embodiment of  FIG.  21   ; 
         FIG.  29    shows yet a further embodiment of the invention; 
         FIG.  30    shows how the emitter in the embodiment of  FIG.  29    operates; 
         FIG.  31    also shows how the emitter in the embodiment of  FIG.  29    operates; 
         FIG.  32    also shows how the emitter in the embodiment of  FIG.  29    operates; 
         FIG.  33    also shows how the emitter in the embodiment of  FIG.  29    operates; and 
         FIG.  34    illustrates a heartbeat signal read by the embodiment of  FIG.  29   . 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG.  1    shows a first embodiment  100 , comprising a ring  102  which is a heart rate monitor. The ring can be worn on the finger of a person whose heart rate is to be monitored during exercise. The finger is not illustrated in the drawing. 
     The ring  102  is installed with two emitter-and-sensor  106  pairs. Therefore, there are two emitters  104  and two sensors  106  in total. A suitable housing  108  is attached to the ring  102  for containing a microprocessor and memory required for operating the emitters  104  and sensors  106 , and for manipulating infrared signal data as detected by the sensors  106 . 
     Typically, the emitters  104  are Light Emitting Diodes or LEDs which emit light that is absorbable by blood, such as infrared. In other embodiments however, any other suitable frequency can be used including visible red, green or blue light, or any combination thereof. 
     The two emitters  104  are provided in the ring  102  such that they are at opposite sides of the wearer&#39;s finger when the ring  102  is worn. The two sensors  106  are also provided such that they are at opposite sides of the finger when the ring is worn but also to be at about 90 degrees to an imaginary line drawn through the emitters  104 . The positions of both sensors  106  allows each sensor  106  to be capable of detecting light projected into the finger by both emitters  104 . 
       FIG.  2    is a schematic diagram of the arrangement of the two emitters  104  and two sensors  106  in the ring  102 . Due to the small radius of the ring  102 , light emitted by the emitters  104  need not be projected directly at the sensors  106 . Instead, the sensors  106  merely needs to detect infrared light scattered through or reflected by tissues and blood in the finger. Other ways of arranging the emitters  104  and the sensors  106  are possible as long as each emitter  104  is placed a distance away from both of the sensors  106 , such that a sufficiently long transmission path through the finger is provided for absorption of infrared light by blood. 
     The microprocessor operates the emitters  104  such that they emit infrared light one after the other. Thus, both sensors  106  first detect infrared light scattered through the finger from one emitter  104 , and then detect infrared light scattered through the finger from the other emitter  104 . 
     As illustrated in  FIG.  2    and  FIG.  3   , transmission paths P 11  and P 12  are observed by sensors S 1  and S 2  respectively when emitter L 1  emits infrared light into the finger. The infrared signals observed by both sensors S 1  and S 2  are then recorded for processing by the microprocessor. Subsequently, the microprocessor instructs emitter L 1  to stop emitting infrared light and instructs emitter L 2  to start emitting infrared light. Both sensors S 1  and S 2  then detect the infrared signals via transmission paths P 21  and P 22 , respectively. 
     Accordingly, four transmission paths through the finger P 11 , P 12 , P 21 , P 22  are monitored in this embodiment. Four sets of data are observed using only two sensors  106  and two emitters  104 . In contrast, the prior art requires three emitters and three sensors to obtain just three observations. Advantageously, the present embodiment requires less hardware while making a greater number of observations than devices of the prior art. 
     Typically, the sensors can detect fluctuating infrared transmissions through the finger which are attributed to heartbeat. Skin, tissue, venous blood, and the blood are all capable of absorbing infrared light. However, the volume of arteries periodically increases and decreases with the pumping of the heart, giving rise to these fluctuating transmissions. 
       FIG.  4    explains how these fluctuating transmissions come about. The vertical axis of the chart in  FIG.  4    shows absorption of the infrared light. Together, the troughs form the base  302  of the waveform which is indicative of the amount of light absorbed by skin, tissues, venous blood, which are relatively constant, and when artery blood volume is smallest. The magnitude  300  between the troughs to the peaks is attributed to an increase of infrared absorption when the artery volume increases and is filled with blood. 
     However, the infrared transmission signals detected by the sensor are subjected to noise when the wearer exercises. For example, the wearer&#39;s movements can cause continual small physical displacements of emitter and sensor positions. Furthermore, the cross-sectional area of the finger on which the ring is worn varies easily when the finger is flexed during exercise. All these movements vary the distance of the transmission paths through the finger, which introduce variations into the transmission signals as unwanted noise. 
     In practice, only transmission path changes in the plane defined by the sensors  106  contribute noise.  FIG.  5    illustrates the plane as an x-y plane. Muscle or tissue volume changes in the q-axis perpendicular to the x-y plane are relevant only if they cause the finger to expand or contract in the x-y plane, or if any vertical movements cause the ring  102  to slip along the finger which effectively moves the x-y plane along the finger length. 
       FIG.  6    is a chart showing how the heart rate of a wearer is detected as a periodic signal and how the periodic heart rate signal can be overwhelmed by noise due to movements of the wearer. In about the first 10 seconds of the chart the wearer stays stationary and the signals representing heart rate are low in peak-to-peak amplitude because volumetric changes in the arteries tend to be relatively small. 
     All periodic fluctuations in the infrared transmission signal can be attributed to arterial pulsation, and one set of observation obtained using one emitter and one sensor may even be enough to monitor the heart rate. The DC component in these 10 seconds is largely contributed by tissues, venous blood and other stable components in the wearer&#39;s finger, while the AC component is contributed by heartbeat. 
     At 15 seconds in the chart, however, when the wearer starts to run, jump and or move his finger, the movements easily overshadow the heart rate signals with noise. That is, the AC component now include noise and fluctuates much higher and lower about the DC level and overshadows the AC component contributed by the heartbeat. 
     The amount of light emitted by one of the emitters  104  and detected at any one of the sensors  106  can be approximately modelled as follows:
 
 m ( t )= L I   0 ( t )(1 +γ hb ( t ))(1 +N   s ( t )+ N   f ( t )+ z ( t ))
         where:
           m(t) is the signal received at any one of the IR sensors  106     L is constant gain of the IR sensor   I 0 (t) is the transmitted signal to the IR emitter   hb(t) is the heart rate signal   γ is coupling coefficients of the heart rate signal hb(t)   N s (t) is slow varying noise in the detected signals   N f (t) is are typical additive thermal noise in the detected signals, and   z(t) is noise signals due to movement caused by flexing of the body part.   
               

     If N s (t)=0, N f(t)= 0, z(t)=0, the infrared signals are proportional to periodic pumping of blood by the heart, i.e.
 
 m ( t )= L I   0 ( t )(1 +γ hb ( t ))
 
     If there is no noise in the infrared signals, the peaks in the waveform can be directly counted to obtain the heart rate of the wearer. However, if there is a lot of noise from wearer movement and z(t) becomes significant, then the infrared signals have to be mathematically treated to extract the heart rate signal from the noisy signal. 
       FIG.  7    illustrates how movement signals z(t) in the x-y plane defined by the location of the sensors  106  can be re-written as:
 
 z ( t )=ε[ h ( t )cos(θ)+ v ( t )sin(θ)]
         where
           h(t) is the movement signal caused by flexing the finger in the wearer&#39;s horizontal direction;   v(t) is movement signal caused by flexing the finger in the wearer&#39;s relative vertical direction;   the direction of sensor k is θ from the horizontal direction; and   ε is the coupling coefficient for the movement signal to the sensor.   
               

     Movements affecting the infrared signals are mathematically determined for their effects within the x-y plane only. The x-y plane is defined by the sensors  106  and need not necessarily be ‘horizontal’ or parallel to the ground. 
     In this embodiment, the four observations obtained from the two sensors  106  can be modelled as follows:
 
 m   1 ( t )= L   1    I   01 ( t )(1+γ 1    hb ( t ))(1 +N   s1 ( t )+ N   f1 ( t )+ z   1 ( t ))  (1)
 
 m   2 ( t )= L   2    I   02 ( t )(1+γ 2    hb ( t ))(1 +N   s2 ( t )+ N   f2 ( t )+ z   2 ( t ))  (2)
 
 m   3 ( t )= L   3    I   03 ( t )(1+γ 3    hb ( t ))(1 +N   s3 ( t )+ N   f3 ( t )+ z   3 ( t ))  (3)
 
 m   4 ( t )= L   4    I   04 ( t )(1+γ 4    hb ( t ))(1 +N   s4 ( t )+ N   f4 ( t )+ z   4 ( t ))  (4)
         Where:
           m(t), m 2 (t), m 3 (t), m 4 (t) are the signal received at the 4 sensors  106  respectively   L 1 , L 2 , L 3 , L 4  are constant gain of each IR sensors  106     I 01 (t), I 02 (t), I 03 (t), I 04 (t) are the transmitted signal to the IR LED emitters  104  respectively   hb(t) is the heart rate signal   γ 1 , γ 2 , γ 3 , γ 4  are coupling coefficients of the heart rate signal hb(t)   N s1 (t), N s2 (t), N s3 (t), N s4 (t) are slow varying noise in the detected signals   N f1 (t), N f2 (t), N f3 (t), N f4 (t) are typical additive thermal noise in the detected signals, and   z 1 (t), z 2 (t), z 3 (t), z 4 (t) are noise signals due to movement.   
               

     The movement noise signals z 1 (t), z 2 (t), z 3 (t), z 4 (t) can be re-written as:
 
 z   k ( t )=ε k ( h ( t )cos(θ k )+ v ( t )sin(θ k ))
         where
           h(t) is the movement signal in the horizontal;   v(t) is movement signal in the relative vertical direction;   the direction of sensor  106  k is θ k  from the horizontal direction; and   ε k  are coupling coefficients for the movement signal to the sensors  106 .   
               

     Assuming that both γ k , ε k  are much smaller than 1, the infrared signals at each sensor can be represented as being composed of both DC and AC components, (m ack (t), m dck (t)). 
     When the wearer first puts on the ring  102 , he is requested by the microprocessor via a display (not shown) in the housing  108  to stay stationary without moving. At this stage, the infrared signals detected by the sensors  106  can be attributed to heart rate only. The raw data from each of the sensors  106  is firstly treated with a simple Finite Input Response (FIR) low pass filter to remove all high frequency signals. Subsequently, the slow drifting DC offset is removed using a filter or a moving window to extract the DC offset and subtract it from the signals. At this stage, if the microprocessor detects that the infrared signals read by the different sensors  106  differ greatly in amplitude, the gain of each of the four sensors  106  is adjusted until the difference in the amplitudes of the transmission signals fall within a pre-determined deviation. By this, the gain of each of the sensors are normalised, and equations (1) to (4) can then be approximated as:
 
 m   ac1 ( t )= hb ( t )+ N′   s1 ( t )+ N′   f1 ( t )+ z   1 ′( t )  (1b)
 
 m   ac2 ( t )= hb ( t )+ N′   s2 ( t )+ N′   f2 ( t )+ z   2 ′( t )  (2b)
 
 m   ac3 ( t )= hb ( t )+ N′   s3 ( t )+ N′   f3 ( t )+ z   3 ′( t )  (3b)
 
 m   ac4 ( t )= hb ( t )+ N′   s4 ( t )+ N′   f4 ( t )+ z   4 ′( t )  (4b)
         where N′ sk (t), N′ fk (t), z k ′(t) are scaled versions of the original noise signals.       

     After normalisation, the ring  102  can now be used to monitor heart rate. When there is no movement or a very small amount of movements, the deviation of the amplitudes of signals detected by the sensors remains at the normalised level. The maximum signal to noise ratio (SNR) of the heart rate signal can be obtained by adding up the normalised AC component input signal, i.e.
 
 y ( t )= m   ac1 ( t )+ m   ac2 ( t )+ m   ac3 ( t )+ m   ac4 ( t )
 
     Effectively, the noise will be reduced as the signal is accentuated by the summation of the independent observations of each sensor. 
     However, when the wearer exercises, noise signals z 1 ′(t), z 2 ′(t), z 3 ′(t), z 4 ′(t) dominate the signals detected by the sensors  106 . The noise can then be treated by finding the column vector ŵ=[w 1  w 2  w 3  w 4 ] T  where 
     
       
         
           
             
               
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                         ] 
                       
                     
                   
                 
                 ] 
               
             
             ; 
             and 
           
         
       
       
         
           
             
               y 
               ^ 
             
             = 
             
               [ 
               
                 
                   y 
                   ⁡ 
                   
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                     0 
                     ] 
                   
                 
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                   y 
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                   y 
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     and ŷ is a linear combination of input signal which maximizes: 
     
       
         
           
             
               
                 
                   w 
                   ^ 
                 
                 T 
               
               ⁢ 
               
                 s 
                 ^ 
               
               ⁢ 
               
                 
                   s 
                   ^ 
                 
                 T 
               
               ⁢ 
               
                 w 
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                   w 
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                   m 
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                   m 
                 
                 
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                   ℜ 
                 
               
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                 w 
                 ^ 
               
             
           
         
       
     
     where
             mm  is the cross correlation matrix of the 4 signals from movement.   ŝ=[s 1  s 2  s 3  s 4 ] T  the corresponding gain of the heart rate signal, in this case when all the 4 input channels are normalized       

     ŝ=[1 1 1 1] T  and    mm =MM T −σ 2 ŝŝ T , where σ 2  is the variance of the heart rate signal. 
     As    mm  is positively defined, it can be written that 
     
       
         
           
             
               ℜ 
               
                 m 
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             = 
             
               
                 R 
                 
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               · 
               
                 R 
                 
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     and it can be written that 
     
       
         
           
             
               u 
               ^ 
             
             = 
             
               
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           and 
         
       
       
         
           
             
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               ^ 
             
             = 
             
               
                 R 
                 
                   - 
                   
                     1 
                     2 
                   
                 
               
               ⁢ 
               
                 u 
                 ^ 
               
             
           
         
       
     
     Accordingly, the problem to be solved becomes: 
     
       
         
           
             
               max 
               
                 | 
                 | 
                 
                   u 
                   ^ 
                 
                 | 
                 | 
               
             
             ⁢ 
             
               
                 
                   u 
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                 · 
                 
                   
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                 R 
                 
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                 u 
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           or 
         
       
       
         
           
             
               max 
               
                 | 
                 | 
                 
                   u 
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                 | 
               
             
             ⁢ 
             
               
                 ( 
                 
                   
                     
                       u 
                       ^ 
                     
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                     R 
                     
                       - 
                       
                         1 
                         2 
                       
                     
                   
                   ⁢ 
                   
                     s 
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                 ) 
               
               2 
             
           
         
       
     
     The expression is maximum when: 
     
       
         
           
             
               u 
               ^ 
             
             = 
             
               
                 
                   
                     R 
                     
                       - 
                       
                         1 
                         2 
                       
                     
                   
                   ⁢ 
                   
                     s 
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                 ⁢ 
                 
                   
 
                 
                 ∴ 
                 
                   w 
                   ^ 
                 
               
               = 
               
                 
                   
                     R 
                     
                       - 
                       
                         1 
                         2 
                       
                     
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         R 
                         
                           - 
                           
                             1 
                             2 
                           
                         
                       
                       ⁢ 
                       
                         s 
                         ^ 
                       
                     
                     ) 
                   
                 
                 = 
                 
                   
                     ℜ 
                     
                       m 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       m 
                     
                     
                       - 
                       1 
                     
                   
                   ⁢ 
                   
                     s 
                     ^ 
                   
                 
               
             
           
         
       
       
         
           
             
               where 
               ⁢ 
               
                   
               
               ⁢ 
               
                 ℜ 
                 
                   m 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   m 
                 
               
             
             = 
             
               
                 MM 
                 T 
               
               - 
               
                 
                   σ 
                   2 
                 
                 ⁢ 
                 
                   s 
                   ^ 
                 
                 ⁢ 
                 
                   
                     s 
                     ^ 
                   
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     To remove movement noise, the four sets of observations obtained from the two sensors  106  are used. If the four signals detected in the four transmission paths are synchronous and in-phase, and if the amplitudes of all the four signals differ only within a signal standard deviation of σ, it may be taken that heartbeat signals dominate the infrared signals with little noise. The signals can simply be added up and the peaks in the signal waveform counted to determine heart rate. 
     On the other hand, if noise dominates the infrared signals, the infrared signals will not be in phase and the amplitudes of all the four signals will differ beyond the standard deviation σ. In this case, to extract the heart rate signal, the correlation index across the four signals is calculated. That is, the covariance matrix of the signals calculated:
 
   mm   =MM   T −σ 2   ŝŝ   T .
 
     As mentioned earlier σ 2 ŝŝ T  was obtained by calculating the standard deviation of the four input signals when there was no wearer movement. The four input signals are then normalised to standard deviations of σ, and σ 2 ŝŝ T  becomes 
     
       
         
           
             σ2 
             = 
             
               [ 
               
                 
                   
                     
                       1 
                       ⁢ 
                       1 
                       ⁢ 
                       1 
                       ⁢ 
                       1 
                     
                   
                 
                 
                   
                     
                       1 
                       ⁢ 
                       1 
                       ⁢ 
                       1 
                       ⁢ 
                       1 
                     
                   
                 
                 
                   
                     
                       1 
                       ⁢ 
                       1 
                       ⁢ 
                       1 
                       ⁢ 
                       1 
                     
                   
                 
                 
                   
                     
                       1 
                       ⁢ 
                       1 
                       ⁢ 
                       1 
                       ⁢ 
                       1 
                     
                   
                 
               
               ] 
             
           
         
       
     
     The vector can be calculated, where
 
w=   mm   −1 ŝ
         where   w is a 4×1 column vector: ŷ=ŵ T M   y[n] is a linear combination of the 4 input signals       

     At ŝ=[1 1 1 1] T  all four channels would be normalized. 
     The above mathematical treatment normalises the signals received by the sensor such that the combined signals have the lowest total energy. Having the lowest total energy implies that the total amount of noise has been adjusted to be at the lowest and least influential. 
     Furthermore, the independent observations of transmission signals due to heartbeat will be in phase with each other, differing only by a scaling factor. Therefore, if the infrared signals are summed up with some specific weight, the noise signals can be minimized and hence increasing the signal to noise ratio. 
     Typically, the wearer&#39;s movements can be quite periodic when he is performing a repetitive exercise such as running. Nevertheless, these periodic exercise movements do not impart an identical and periodic noise to the sensor readings. This is because the several transmission paths through the finger between each pair of emitter and sensor are different, and it may be expected that unique noise is imposed on each sensor by the different local layers of wobbly tissue and other bodily components, even if the wearer&#39;s movements is periodic and applied to the device  100  as a whole. Accordingly, summation of the infrared signals detected by the different sensors will not add up to accentuate any identical periodic noise signals. 
     To further improve signal to noise ratio, it is desirable to have more independent observations to increase observation diversity.  FIG.  8    shows a second embodiment to this effect.  FIG.  9    to  FIG.  11    are schematics diagrams of the second embodiment of  FIG.  8   . The second embodiment comprises a ring  102  installed with three emitter-and-sensor pairs, instead of two. Each emitter  104  is placed immediately next to one of the sensors  106  and away from the other two sensors  106 . In this way, each emitter  104  is able to project light to the two sensors  106  across the ring  102  and through the finger. Likewise, the sensor  106  immediately next to an emitter  104  is able to receive light from the two emitters  104  across the ring  102 . 
     In operation, the emitters L 1 , L 2 , L 3  are switched on in consecutive order, one after another. When emitter L 1  is ON, both sensors S 1  and S 2  detect light from emitter L 1  in respective transmission paths P 11  and P 12 . When L 2  is ON, sensors S 2  and S 3  detect light from emitter L 2  in respective transmission paths P 22  and P 23 . When L 3  is ON, sensors S 1  and S 3  detect light from emitter L 2  in respective transmission paths P 31  and P 33 . 
     As with the first embodiment, in order to remove noise from wearer movements, the amplitudes of the six observations are normalized by calculating their variance or standard deviation. The detected signals are modelled as follows:
 
 m   1 ( t )= L   1    I   01 ( t )(1+γ 1    hb ( t ))(1 +N   s1 ( t )+ N   f1 ( t )+ z   1 ( t ))  (1a)
 
 m   2 ( t )= L   2    I   02 ( t )(1+γ 2    hb ( t ))(1 +N   s2 ( t )+ N   f2 ( t )+ z   2 ( t ))  (2a)
 
 m   3 ( t )= L   3    I   03 ( t )(1+γ 3    hb ( t ))(1 +N   s3 ( t )+ N   f3 ( t )+ z   3 ( t ))  (3a)
 
 m   4 ( t )= L   4    I   04 ( t )(1+γy 4    hb ( t ))(1 +N   s4 ( t )+ N   f4 ( t )+ z   4 ( t ))  (4a)
 
 m   5 ( t )= L   5    I   05 ( t )(1+γ 5    hb ( t ))(1 +N   s5 ( t )+ N   f5 ( t )+ z   5 ( t ))  (5a)
 
 m   6 ( t )= L   6    I   06 ( t )(1+γ 6    hb ( t ))(1 +N   s6 ( t )+ N   f6 ( t )+ z   6 ( t ))  (6a)
         Where:
           m(t), m 2 (t), m 3 (t), m 4 (t), m 5 (t), m 6 (t) are the signal received at the 6 sensors  106  respectively   I 01 (t), I 02 (t), I 03 (t), I 04 (t), I 05 (t), I 06 (t) are the transmitted signal to the IR LED emitters  104  respectively   L 1 , L 2 , L 3 , L 4 , L 5 , L 6  are constant gain of each IR sensors  106     hb(t) is the heart rate signal   γ 1 , γ 2 , γ 3 , γ 4 , γ 5 , γ 6  are coupling coefficients of the heart rate signal hb(t)   N s1 (t), N s2 (t), N s3 (t), N s4 (t), N s5 (t), N s6 (t) are slow varying noise in the detected signals   N f1 (t), N f2 (t), N f3 (t), N f4 (t), N f5 (t), N f6 (t) are typical additive thermal noise in the detected signals, and   z 1 (t), z 2 (t), z 3 (t), z 4 (t), z 5 (t), z 6 (t) are noise signals due to movement.   
               

     The subsequent mathematical treatment for six sensors  106  is the same as that described for the first embodiment. 
       FIG.  12    shows why although transmission paths P 12  and P 33  are practically the same physical path but the noise imposed onto the signals transmitted in paths P 12  and P 33  are not identical. This is because P 12  and P 33  are transmissions in opposite directions although the transmission transmits in virtually the same path  802  between the two sensor S 2  and sensor S 3 . The random folds of skin on the finger and the different tissues under the skin provide an asymmetrical transmission path between sensor S 2  and sensor S 3 . Therefore, the angle of infrared light incident on the skin from emitter L 1  towards S 2  is likely to be different from the angle of infrared light incident on the skin from emitter L 3  towards S 3 . Similarly, the extent of reflection and penetration of the incident infrared light into the skin surface is different in the two different directions, as indicated by the arrows in  FIG.  12   . Thus, noise signals imposed on the sensors S 2  and S 3  due to movement and flexing of the finger are not identical in the different transmission directions. This explains why using the same physical transmission path for two sensors in different directions does not amount to replicating a same signal. This also explains why the wearer&#39;s periodic exercise movements do not impose periodic and identical noise signals on all the sensors even if wearer movements are periodic, as the different transmission directions ensures that the noise is unique in each sensor. 
     In both embodiments, the use of one sensor  106  with two different emitters  104  increases the number of independent observations made with each sensor. In the first embodiment, only two emitters  104  and two sensors  106  are required for obtaining four observations. In the second embodiment, three sensors  106  and three emitters  104  provide six observations. This is advantageous over the prior art as less sensors or emitters is required for obtaining a greater number of observations. 
     More advantageously, in the second embodiment, any one of the emitters  104  or the sensors  106  may break down and four observations are nevertheless obtainable. For example, temporarily failure situations may happen when ambient light shines directly into one of the sensors, saturating the sensor  106 .  FIG.  13    shows the ring of the second embodiment having three sensor and emitter pairs. Strong ambient light represented by the block arrow shines onto sensor S 1  but is blocked by the wearer&#39;s finger over which the ring is worn from shining onto S 2  and S 3 . When the sensor S 1  is so exposed to ambient light, sensor S 1  becomes saturated and unusable for detecting infrared transmission signals. However, as sensors S 3  and S 2  are blocked from the ambient light, sensors S 3  and S 2  remain functional. Thus, sensor S 3  remains capable of reading infrared transmission signals from L 2  and L 3 , and sensor S 2  remains capable of reading infrared transmission signals from L 1  and L 2 . The microprocessor can detect that sensor S 1  is saturated and disregard the sensor S 1 , and use the remaining four observations provided by sensors S 2  and S 3  to determine the heart rate. As described, L 1  and L 2  takes turns to emit to the sensors S 2  and S 3  to provide the four observations. In contrast, an ambient light saturation of a sensor in a prior art device which uses three sensors to detect light from three respective, separate emitters will compromise the accuracy of the device because one observation becomes unusable. 
     As a matter of choice, in the first embodiment, only three observations may be used to monitor heart rate from the two emitters  104  and two sensors  106 , even though four observations are obtainable. Similarly, in the second embodiment, only three observations may be used even though the three sensors  106  and three emitters  104  provide the possibility of six observations, particularly where any two of such three observations is obtained using the same sensor  106 . 
     Accordingly, the embodiments described includes a device suitable for monitoring blood in a body part  100  comprising: a plurality of light emitters  104  at least one sensor  106 , the plurality of light emitters  104  arranged such that light from the plurality of light emitters  104  is capable of passing through a body part to arrive at the least one sensor  106 , wherein: the plurality of light emitters  104  emits light in consecutive order to be detected by the at least one sensor  106 . The device for monitoring blood  100  has been described as a heart rate monitor. 
     Furthermore, the embodiments described includes a device suitable for monitoring blood in a body part  100  comprising at least one light emitter  104 , a plurality of sensors  106 , the at least one light emitter  104  arranged such that light from the at least one light emitter  104  is capable of passing through the body part to arrive at the plurality of sensors  106 . 
     Furthermore, the embodiments described includes a method of obtaining observations of light transmission to monitor heart rate comprising the steps of: providing at least two sensors  106  for detecting light, placing the two sensors  106  at different sides of a body part, providing a first light emitter  104  at another side of the body part, such that light emitted from emitter  104  transmits through the body part to arrive at the at least two sensors  106 , the transmission path to one of the sensors  106  providing a first observation, and the transmission path to the other of the sensors  106  providing a second observation. 
     Furthermore, the embodiments described includes a method of obtaining observations of light transmission to monitor heart rate comprising the steps of: providing at least two light emitters  104 , providing a sensor  106  for detecting light, placing the at least two emitters  104  at different positions on a side of the body part, providing the sensor  104  on another side of the body part, such that light emitted from the emitters  104  is capable of transmitting through the body part to arrive at the sensor  106 , and operating the emitters  104  one after another to obtain different observations at the sensor  106 . 
     While there has been described in the foregoing description preferred embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design, construction or operation may be made without departing from the scope of the present invention as claimed. 
     For example, although the sensors  106  have been described to detect light from the emitters  104  transmitted through the body part, it is possible that the sensors  106  can be arranged in other embodiment to detect light from the emitters  104  by reflection or dispersion from the body part. In this case, the sensors  106  would be placed next to but a distance away from the emitters  104 . 
       FIG.  14    shows a further embodiment which is much simpler, comprising only one emitter  104  and two sensors  106  placed across the emitter. Only two observations can be made from the two sensors. This embodiment can be used in situations where noise from movements is very unlikely, such for monitoring the heart rate of an infant, but use of only two sensors and one emitter instead of two sensors and two emitters allow a much lighter, smaller and cheaper device  100 . 
       FIG.  15    and  FIG.  16    illustrate yet another embodiment showing how the emitters  104  can be placed adjacent each other on one side of the ring  102  and the sensors  106  placed adjacent each other but on the other side of the ring  102 , instead of the configuration in the first embodiment.  FIG.  15    shows emitter L 2  lighting up first to transmit towards sensor S 1  and sensor  2 , and  FIG.  16    shows emitter L 1  lighting up subsequently to transmit towards sensor S 1  and sensor S 2 . 
     In other embodiments, as shown in  FIG.  17   , a single emitter  104  is used to project infrared light to be detected by three or more sensors  106 . This allows three observations to be made with only one emitter  104  and three sensors  106 , instead of thee emitter and three sensors as in the prior art. 
       FIG.  18    shows how three sensors  106  are used to provide six observations with only two emitters  104 . When the emitters  104  emit in consecutive order, the sensors  106  are able to detect six observations. This also provides the advantage that if one of the sensors  106  fail to work, the device  100  as a whole is still functional with two sensors  106  and two emitters  104  providing four observations, which is superior to the use of three observations typical in prior art. In the prior art, each sensor is dedicated to only one emitter and there is therefore no redundancy of observations; the number of observations is the same as the number of sensors  106 . Alternatively, in a variation of this embodiment, three emitters  104  and two sensors  106  may be used. In general, use of less emitters  104  with more sensors  106  leads to faster operation than more emitters  104  with less sensors  106 , as there is less need of time to wait for each emitter  104  to take its turn. 
     Although the device  100  has been described as a ring  102 , the device may be provided in other forms such as an ear plug  102   a,  as shown in  FIG.  19   . The ear plug typically has emitters  104  arranged on the surface of the ear plug to emit light into the ear of the wearer. The tissues surrounding the ear plug reflects the light back to the ear plug. The ear plug also has sensors  106  arranged on the ear plug surface to detect the reflected light. A drawing of an ear plug is shown in  FIG.  19    without illustrating the positions of the emitters and sensors. By the term ear plug, the skilled reader should note that it includes any sufficiently stable attachment to the ear that monitors blood, including ear plug which are insertable into the ear hole, or any attachment which can sit just outside the ear hole in the outer ear such as ear phones or any device that can clip to the ear in any way to read light transmission or dispersion in any part of the ear. 
     Alternatively, the device  100  can be in the form of an arm band  102   b  instead of a ring  102 , as shown in  FIG.  20   , in which emitters and sensors are installed for projecting and detecting infrared or other light into the arm of the wear for monitoring heartbeat. 
     Although infrared light has been described in the embodiments, other wavelengths are possible, as it has been found that blood is capable of absorbing other wavelengths including red, green and blue light. 
     The embodiments can be implemented in oximeters for detecting oxygen level in blood, which operates by contrasting the ratio of transmission of visible red light to transmission of infrared light. The ratio of the amount of absorbed red light to the amount of absorbed infrared light indicates the amount of oxygen in the blood. Therefore, in embodiments having oximeters functions, there may be an emitter emitting red light to the sensor, and an emitter emitting infrared light to the same sensor which is capable of detecting in both ranges of wavelengths. In this case, the red light emitter and the infrared light emitter take turns to emit light. Alternatively, the emitter is capable of emitting red light and infrared light at the same time, but a separate red light sensor and a separate infrared light sensor detects the different wavelengths selectively and at the same time. Alternatively, the oximeters comprises a set of emitters and sensors as described for emitting and sensing only in red light, and comprises a separate set of emitters and sensors as described for emitting and sensing only in infrared only. 
     Although a digital signal processing method has been described for treating the infrared signals mathematically, other processing methods or other mathematical treatment is possible. Also an analogue treatment for removing the noise instead of the digital ways described is also possible. 
     It should be noted that the meaning of a ‘pair’ of emitter and sensor is merely functionally defined, and each pair of emitter and sensor may be placed immediately next to each other, such as emitter L 1  and sensor S 3 , or emitter L 3  and sensor S 2  or emitter L 2  and sensor S 1  of  FIG.  10   . Alternatively, in other configurations, any pair of emitter and sensor may be placed apart from each other such as the configuration in the embodiment of  FIGS.  1 ,  2  and  3   . 
       FIG.  21    shows yet another embodiment, which comprises a piece of planar substrate  2101  embedded with two sensors  106  and two emitters  104 . The substrate has a generally planar surface for application onto the wearer&#39;s body part, i.e. the embodiment may be placed onto a human part with a generally planar surface. Preferably, the substrate comprises a piece of flexible woolen or cotton fabric, or a piece of flexible plastic sheet such as polyvinylchloride. However, any other designs which provides a planar application surface is within the intention of this invention. 
     The two light emitters  104  and two sensors  106  are embedded into the substrate in such a way that when the substrate is applied onto the wearer&#39;s body part, the emitters  104  and the sensors  106  are pressed snugly against the wearer&#39;s skin. A suitable housing  108  is also embedded into the substrate which contains the microprocessor and memory required for operating the emitters and sensors, and for recording data as detected by the sensors. The microprocessor, memory, light emitters and sensors, as well as the wiring between them, are not illustrated but the skilled reader will understand that all manner of suitable wiring or printed circuitry may be used to connect microprocessor, memory, light emitter and two sensors one to another to provide the required functions to the embodiment  100 . 
     Preferably, the surface of the substrate to he applied onto a body part of the user is provided with adhesive for securing to the skin of the person, such as those used to stick medical plaster to body parts.  FIG.  22    shows how the generally planar substrate  2101  can he plastered or adhered to the arm. The generally planar substrate  2101  can also be adhered to body parts such as the chest or back, or wherever body part is large enough to be adhered with the substrate  2101  without requiring the substrate  2101  to fold significantly or be creased. 
     Preferably, the adhesive is of a type which is re-useable, such that the substrate  2101  may he peeled off the wearer&#39;s body part and be re-applied. onto another body part of the same wearer or another wearer. There are many suitable adhesives that may he re-applied onto a target surface repeatedly. Alternatively, a vacuum application may be used, such as a planar substrate which has small suction cups for sucking onto the wearer&#39;s body part. All these technology are known in the art and requires no elaboration here. It is also possible that the planar substrate may he provided with a belt which is tied around the arm, instead of using adhesive or suction cups. The belt in this case is merely an accessory and does not contribute to the arrangement, or the possibility of different arrangements, of the sensors and emitters. 
       FIG.  23    and  FIG.  24    show one possible way in which the sensors  106  and the emitters  104  may be embedded into a substrate  2101 . The substrate  2101  is shown in the sidewise cross-section, and comprises a backing layer  2301  and an application layer  2303  which are pressed together. For ease of illustration, two emitters and one sensor is shown in  FIG.  23    and  FIG.  34   , while the other sensor is not shown. It suffices that in  FIG.  21   , the emitters and sensors are shown to be arranged in such a way that no subset of any two emitters and one sensor, or subset of any two emitters and one sensor, forms a straight line, which prevents the two sensors from detecting the same noise caused by the same wearer movements. 
     The application layer  2303  has suitable holes cut out to allow the operational parts of the emitters and sensors to peek out of the substrate  2101 , which is to say, the emitters are able to emit light from the substrate  2101  onto the wearer, and the sensors are able to detect light from the surroundings through the holes. The backing layer  2301  and the application layer  2303  generally enclose the emitters and sensors like a sandwich. The housing  108  containing the microprocessor and memory, however, may be completely encased within the backing layer and the application layer without need of being accessible to the wearer, which serves to protect the housing from damage. 
       FIG.  24    shows how the emitters  104  and sensors  106  are operated to detect heart rate of the wearer on whose skin is the substrate  2101  applied. The emitters emit light into the skin of the wearer. The light penetrates into the wearer&#39;s skin and tissue to be diffused and scattered  2401 . Some of the scattered light is reflected towards the sensors  106 . However, a portion of the light is absorbed by blood. Blood content in the skin is not constant but changes with the beating of the heart. Therefore, light scattered back towards the sensors has a pulsating intensity. By monitoring this pulsating intensity, the sensors are able to monitor pulsating blood flow and hence monitor heart rate of the wearer. 
     To ensure that the heart rate signals detected by each sensor have different noise components despite being caused by the same wearer movements, the two emitters are placed as far apart from each other as possible and preferable at a respectively different angle to each sensor within the plane of the substrate  2101 . That is, the emitters do not form a straight line with each sensor, and each emitter emits light to the same sensor in a different direction to that of the other emitter. in this way, the influence of motion on light transmitted from each of the emitters reaching the same sensor is different, and may possibly be cancelled out by summing the sensor readings of the light transmission from two emitters. 
     By comparison, if the emitters form a straight line with a sensor, movement vectors of the wearer along the line might be undetected, or even accentuated if the transmissions of two emitters are added together. 
     In order that the sensor is able to detect light more sensitively from both emitters  104 , the microprocessor operates the emitters  104  such that they emit infrared light sequentially. Thus, the sensor S 1  first detects infrared light scattered through skin and tissue from the one emitter L 1  ( FIG.  25   ) via path P 11 . At the same time, the sensor S 2  detects infrared light scattered through skin and tissue from one emitter L 1  ( FIG.  25   ) via path P 12 . The infrared signals observed by the sensor S 1  and by the sensor S 2  are recorded for processing by the microprocessor. Subsequently, the microprocessor instructs emitter L 1  to stop emitting infrared light and instructs emitter L 2  to start emitting infrared light. The sensor S 1  then detects the infrared signals via transmission path P 21  and sensor S 2  detects the infrared signals via transmission path P 22 . in this way, the embodiment in  FIG.  21    to  FIG.  26    provides a planar or generally planar device which can accurately determine heart rate of the wearer, with noise components which is caused by wearer movements easily cancelled out in four observations of transmissions i.e. P 11 , P 12 , P 21  and P 22 , using only two emitters and two sensors. 
     In a variation of this embodiment, the substrate  2101  is a non-flexible, stiff, substrate  2101  having a generally planar but curved surface conforming to the contours of the arm or other body part intended for wearing the embodiment. Such slight curvature is within the meaning of ‘planar’ here, and does not encircle around the entire body part, i.e. different from an arm band or ring. For example, the top of a helmet may be installed with the proposed substrate  2101 , which can sit on the head of the wearer to monitor heart rate. The hard substrate  2101  can be made of hard plastic, clay or stiffened leather and so on. 
       FIG.  27    shows a planar device having three emitters and one centrally placed sensor, in which the emitters take turns to transmit light through the skin and tissue of the wearer to the sensor, each from a different direction. Preferably, the emitters are arranged space apart from each other by 120 degrees. However, this is not necessary as long as the emitters do not experience the same motion artefacts, which is provided by pointing towards the sensor from different directions. Being in different positions, the transmissions do not acquire the same noise despite being caused by the same wearer movements. Hence the common signal read from the three emitters is the heart rate of the wearer which may be amplified by summing the signals together.  FIG.  28    shows a reverse configuration of the embodiment of  FIG.  27    in which a single emitter transmit light to three sensors sequentially, each sensor arranged in a different direction to the single emitter. In this case, the light transmission may be continuous and the sensors operate sequentially, one after another. Alternatively, however, the sensors are also in operation continuously. 
       FIG.  29   ,  FIG.  30   ,  FIG.  31   ,  FIG.  32    and  FIG.  33    illustrate another planar embodiment.  FIG.  29    is the plane view of the embodiment showing two emitters L 1 , L 2  arranged with two sensors, S 1 , S 2  embedded into a planar substrate  2101 , such that each emitter is able to project to the two sensors. The two sensors are each placed in a different angle to each of the emitters, which ensures that the transmission from each emitter reaches either sensor in a different angle. 
     The light emitter is preferably an LED which is selected to project a narrow beam. The beam is illustrated in  FIG.  30   . The origin of the beam is the LED and is represented by the letter o. The centre arrow represents the direction to which the LED is pointed and is the main incident ray. The LED is also selected such that the narrow beam projects an elongate, oval beam spot. The spot is longer and more powerful along line rq, and narrower and weaker along line sp. The cause of this oval beam spot is due to the typical housing of an LED, giving a non-circular beam spot as a natural product imperfection. Therefore, there is no need for an LED to be specifically manufactured to provide an oval beam spot, under normal circumstances. 
     The light rays along rq penetrate relatively deeper into the tissue of the wearer of the embodiment. The light rays along sp penetrate to a relatively lesser extent into the tissue of the wearer of the embodiment. This is illustrated schematically in  FIG.  32    and  FIG.  33   . 
     Strictly, scattering of light happens at every layer of skin and tissue. However, the intensity of light being scattered is different at different layers of the skin ad tissue. The reason for ‘deeper penetration’ of light rays along pq is because stronger intensity allows the ray of light to be scattered at every layer of the skin and tissue while yet having enough intensity to reach the deeper layers before the rebounding light is too weak be detected by sensors at the skin surface. If strong enough to be detected when it reaches the sensors, light rebounded from deeper layers carries information about these deeper layers. 
     For the weaker light intensity in the sp direction, much of the ray of light would have dissipated before it reaches very deeply into the skin and tissue. Any tiny bit of this ray of light which manages to reach deep into the skin and tissue is unlikely to be rebounded towards the sensors in detectable intensity. Hence, scattered light of this weaker intensity is mostly rebounded at the shallower layers of skin and tissue, and carries more information of these shallower layers than of deeper layers. 
       FIG.  32    shows the case Where an LED is provided as the ‘first’ emitter L 1  (the one on the bottom left of  FIG.  29   ) arranged such that the axis rq of its beam spot is aligned with axis-b. The penetration of the LED&#39;s light in this axis is along the length of the oval shape of the beam spot, is relatively deep and passes through some deeper blood vessels  3201  along rq. As explained, light is scattered by tissue and blood at every layer in the tissue but a good portion of intense light is scattered or rebounded only after it has penetrated deeply. Scattering of the light in the deeper tissue is less prone to the influence of motions of the wearer on the embodiment. A ‘first’ sensor S 1 , i.e. the one shown in the bottom right of  FIG.  29   , is arranged to sense scattered light which originates along axis-b. Scattered light carrying information on pulsating blood flow received by the ‘first’ sensor S 1  is less influenced by movement artefacts because of the deeper light penetration and scattering. 
       FIG.  33    shows the case where the rays along sp axis of the beam spot from the same ‘first’ emitter L 1  are aligned with axis-a, that is, the rays along the breath of the oval shape of the beam spot. The scattered rays along axis-a reaches the second sensor S 2  (the one provided on the upper left of  FIG.  29   ). In other words, both the first sensor S 1  and second sensor S 2  receive light transmissions from the same first emitter L 1 . However, light penetration is shallower along axis-a. Scattering or rebounding of light therefore takes place largely in the shallower layers of skin and tissue. When light from the LED is scattered nearer to the surface of the wearer&#39;s body part, it is more likely that motion artefacts are present as noise in the signal generated by scattering of the light. Therefore, scattered light carrying information on pulsating blood flow received by the ‘second’ sensor S 2  along axis-a, which is the sensor shown in the one in the upper left corner in  FIG.  29   , is more influenced by movement artefacts and has more noise. However, the noise is made up of different noise components from those in the scattered light detected by S 1 . 
     Accordingly, the light transmissions detected by both sensors S 1  and S 2  comprise respectively different noise signals. Their noise signals are distinct one from the other due to both the sensors&#39; different positions to the same emitter L 1 , and also by the different depths of tissue penetration and scattering of light. The different directions of the sensors to the same emitter L 1  ensure that wearer movements in the three-dimension impart different noise signals onto the readings of the sensors S 1 , S 2 . The only common, identical signal component in the readings of both sensors S 1 , S 2  is therefore the heart rate signal, caused by the pulsation of blood in the tissue. The greater the difference between the noise signals in the readings of both sensors, the easier it is to cancel out the noise and to amplify the heart rate signal by merely adding the signals of the two sensors S 1 , S 2 . 
     Once the sensors S 1  and S 2  have read the light transmission from the first emitter L 1 , the first emitter L 1  is switched off and the second emitter L 2  is switched on. The frequency of switching over between emitter L 1  and emitter L 2  may be in periods of milli-seconds to a few seconds, as long as heart rate may be represented or reconstructed from the signals detected by the sensors S 1  and S 2  (that is, a single heartbeat may be read in discrete portions by the sensors alternating in periods of milli-seconds and concatenated to produce the complete heartbeat signal, as illustrated in  FIG.  34   . Preferably, the period between switching over from S 1  to S 2 , or vice versa, is 2 milli-seconds to 200 milli-seconds). The converse then happens: an LED provided as the ‘second’ emitter L 2  (the one on the top right of  FIG.  29    is arranged such the respective axis rq of this second emitter L 2  is aligned with axis-b. The penetration of the light of the second emitter L 2  in this axis is along the length of the oval shape of the beam spot, is relatively deep and passes through some deeper blood vessels. The ‘second’ sensor S 2  is able to sense scattered light which originates along axis-b from L 2 . Again, scattered light carrying information on pulsating blood flow received by the ‘second’ sensor S 2  is less influenced by movement artefacts because of the deeper light penetration and scattering. Furthermore, the rays along the respective axis sp from the same ‘second’ emitter L 2  are aligned with axis-a, that is, the rays along the breath of the oval shape of the beam spot. The scattered rays along axis-a reach the first sensor S 1 . In other words, subsequent to receiving light emitted from emitter L 1 , both the first sensor S 1  and second sensor S 2  then receive light transmissions from the second emitter L 2 . Light penetration is shallower along axis-a. Therefore, scattered light carrying information on pulsating blood flow received by the first sensor S 1  along axis-a from L 2  is more influenced by movement artefacts and has more noise. The only common, identical signal component in the readings of both sensors S 1 , S 2  is the heart rate signal, caused by the pulsation of blood in the tissue. The noise is eliminated and the heart rate is amplified by merely adding the signals of the two sensors S 1 , S 2 . 
     Therefore, when the first emitter L 1  is switched on, S 2  reads a signal along the path P 12 , and S 1  reads a signal along the path P 11 . The signal, in this embodiment, along P 11  penetrates deeper than the signal P 12 . Both S 1  and S 2  read their signals from L 1  at the same time. When the second emitter L 2  is switched on, S 2  reads a signal along the path P 22 , and S 1  reads a signal along the path P 21 . The signal, in this embodiment, along P 22  penetrates deeper than the signal P 21 . Both S 1  and S 2  read their signals from L 2  at the same time. Accordingly, the four observations obtained from the two sensors S 1 , S 2  can be modelled as equations (1), (2), (3) and (4) as discussed above. The noise components in these four observations are even more distinct from that in each other because of the different depth penetration of the LED light emitted in different directions. 
     In a variation of the embodiment, each of the sensors S 1  and S 2  takes turns to read from the first emitter L 1 , and then each of the sensors S 1  and S 2  takes turns to read from the second emitter L 2 . This makes the four observations of the sensors even more independent from each other. 
     In general, the embodiment provides a different transmission route between each permutation of emitter and sensor pair, such that the same movement of the wearer will end up causing as varied a noise in the readings of each emitter and sensor pair as possible. The variation can be provided by different direction between each emitter and sensor pair, or by different depth of skin and tissue penetration between each emitter and sensor pair or both. The greater the variation and randomness between the noise in the readings of each emitter and sensor pair, the more likely those noise can be eliminated to retrieve the wearer&#39;s heart rate signal which is the common component in the signal of each emitter and sensor pair.