Patent Publication Number: US-2015065889-A1

Title: Bodily worn multiple optical sensors heart rate measuring device and method

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of bodily worn heart rate sensors, and in particular, to such sensors that are based on Photoplethysmography. 
     BACKGROUND OF THE INVENTION 
     Prior to a short discussion of the related art being set forth, it may be helpful to set forth definitions of certain terms that will be used hereinafter. 
     The term “light source” as used herein may include any component capable of emitting light in the desirable intensity and wavelength, such as a light emitting diode (LED) and the light detector may include any component capable of detecting and measuring the light emitted by the light source, such as a photodiode or phototransistor. Typically, the desirable wavelength of the light source would be within the range of 350-1100 nm. 
     The term “Photoplethysmography” or PPG as used herein is defined as the use of light traces transmitted through organ tissues in order to analyze physiologic parameters of the organ. 
     The term “Reflectance Photoplethysmography” or “Reflectance PPG” as used herein is defined as PPG based on measurement of the intensity of light passed through the tissue and reflected back to the same side of the tissue as the light source. 
     Photoplethysmography is known in the art to be used in measuring heart rate. Heart rate may be detected by analyzing the transmitted light in transmittance PPG or the reflected light in reflectance PPG. Changes of the blood volume in the tissue modify the absorption, reflection or scattering of the light, so the measured reflected or transmitted light varies with the heart cycle. Thus, heart rate may be derived from the measured reflected or transmitted light by means of signal analysis. 
     The penetration depth of light in biological tissues is typically limited. Therefore, transmittance PPG is typically designed to operate at relatively thin parts of the human body such as the fingertip or the ear lobe. This drawback limits the application of transmittance PPG for heart rate measurements for sport activities. Reflected PPG measurement is not limited in this way and theoretically can be taken at any skin surface at any part of the body. 
     BRIEF SUMMARY OF EMBODIMENTS OF THE INVENTION 
     The present invention, in embodiments thereof, addresses the sensitivity of PPG sensors to movements which may cause undesired noise and inaccurate heart rate measurement. Embodiments of the present invention provide a multi sensor approach that together with a validation process that takes into account the ratio of the incoming signals provides a far more robust PPG sensor for heart rate measurement purposes than PPG based sensors that are currently available. 
     According to some embodiments, an accelerometer may be further used herein for enhancing the quality and the correctness of the heart rate measuring of the aforementioned heart rate measuring device. The use of an accelerometer may be advantageous in at least three of the following manners: to measure degree of activity of the person wearing the sensing device, to detect a rate of change in that activity and to derive a transfer function of the person wearing the heart rate measuring device. A processor may then use the data collected by the accelerometer and correct the optical measurement accordingly. 
     According to some embodiments of the present invention, a Photoplethysmography-based sensor for measuring heart rate is provided herein. The sensor takes advantage of two or more light sources and two or more light detectors, wherein a processor analyzed the cross measurements and ratios between the different light and the corresponding reflections. More specifically, the sensor may include a first light source and a second light source configured to illuminate a body tissue by a first light and a second light respectively; and a first and a second light detectors, each configured to detect light comprising portions of said first light and of said second light, transferred through the body tissue; and a processor with an analog measurement part configured to: receive readings of any combination of light intensity as sensed by both sensors and coming from both sources; and calculate a measure of tissue absorption based on ratios of light portions transmitted by each one of both sources and measured by each one of both detectors 
     These additional, and/or other aspects and/or advantages of the present invention are set forth in the detailed description which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention and in order to show how it may be implemented, references are made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding elements or sections. In the accompanying drawings: 
         FIG. 1  is a schematic illustration of a heart rate sensing device according to some embodiments of the present invention; 
         FIG. 2  is a schematic block diagram of a system for receiving, processing and presenting to a user heart rate readings based on signals received from heart rate sensing device according to some embodiments of the present invention; 
         FIGS. 3A and 3B  are schematic illustrations of an attachment unit for attaching sensing device to an examined tissue in schematic partial section view and in schematic partial isometric view, respectively, according to some embodiments of the present invention; 
         FIG. 4  is a schematic illustration of a helmet system comprising heart rate sensor and processing unit according to some embodiments of the present invention; 
         FIG. 5  is a schematic illustration of a sunglass system for providing heart rate sensor and signals processing and information display management unit according to some embodiments of the present invention; 
         FIG. 6  is a schematic illustration of a swimming goggles system for providing heart rate sensor and signals processing and information display management unit according to some embodiments of the present invention; 
         FIG. 7  is a schematic illustration of a heart rate measurement and display system according to some embodiments of the present invention; and 
         FIG. 8  is a schematic illustration of a heart rate measurement and display system according to some embodiments of the present invention. 
     
    
    
     The drawings together with the following detailed description make the embodiments of the invention apparent to those skilled in the art. 
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     With specific reference now to the drawings in detail, it is stressed that the particulars shown are for the purpose of example and solely for discussing the preferred embodiments of the present invention, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention. The description taken with the drawings makes apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
     Before explaining the embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following descriptions or illustrated in the drawings. The invention is applicable to other embodiments and may be practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
     Heart rate measurement devices using reflected PPG are known in the art. A measuring device using reflected PPG as known in the art is usually based on the measurement of the intensity of light passing through the skin of a living tissue from one light source. Current devices do not present a configuration of multiple light sources coupled with multiple sensors for heart rate measurement. 
     A severe disadvantage of using only one light source and only one light detector is the high sensitivity to artifacts stemming from the relative movement of the measurement device with respect to the measured tissue. The intensity of the light I PH  transmitted by a light source which is received by the light detector after passing through an inspected material/tissue may be defined by expression (1) set forth below: 
         I   PH ( t )= I   LED   ×K   LEDcpt   ×K   skinL ( t )× K   PHcpl   (1)
 
     Wherein: 
     
         
         
           
             I LED —light source Intensity; 
             K LEDcpl —light source optical coupling coefficient to the inspected material/tissue, which determines the intensity attenuation of the light entering into the material/tissue from the light source; 
             K PHcpl —Light detector optical coupling coefficient to the inspected material/tissue, which determines the intensity attenuation of the light entering into the Light detector from the material/tissue; 
             K skin L (t)—Absorption value of light passing distance L within an examined tissue, such as skin; 
             L—Distance from the light source to the light detector; 
             t—Time. 
           
         
       
    
     As may be seen from equation (1), when the light source intensity and optical coupling coefficients are stable, the I PH (t) represents a stable PPG signal that is proportional to K skin L (t), with the proportionality constant being I LED ×K LEDcpt ×K PHcpl . This is not true if moving artifacts are present. 
     The optical coupling coefficients of the light source and the light detector are extremely sensitive to the pressure values in the coupling zone and their values change due to the pressure changes (due to moving artifacts influence). The frequency range of these changes usually coincides with, or in close vicinity to, that of the heart rate. The extent of these changes can be greater than the relative changes of the clean PPG signal. Pressure stabilization can improve the PPG measurement quality. 
     The coupling coefficients values strongly depend on the magnitude of the pressure at the coupling zone. In the case of low pressures, changes caused by the moving artifact lead to strong changes of the coupling coefficients as a result of the presence of air layer in the coupling zone. When the coupling pressure is high, the sensitivity of the coupling coefficients to the moving artifacts is much lower. This may be because of two different reasons. First reason is—the relative change of the coupling coefficients is defined by the relative pressure change. The relative pressure p=P/P 0  change depends on the moving artifact&#39;s acceleration a as in expression (2) set forth below: 
         p =( P   0   ±m×a )/ a   0   (2)
 
     Wherein: 
     
         
         
           
             P 0 —nominal pressure value; 
             m—mass of the moving part. 
           
         
       
    
     As follows from (2), the greater the pressure P 0 , the less the p change. 
     A second reason is that preferably there is no air in the coupling zone between the device and the inspected tissue. 
     Unfortunately, applying high pressure value to the coupling zone is inconvenient to the user. Moreover, too high pressure may severely interrupt with the blood current in the measurement zone or even completely block it, especially for people with low systolic pressure. 
     A two light detectors sensing scheme, according to some embodiments of the present invention, which may also be considered as a scheme with a reference light detector, is more stable with respect to artifacts influence. This scheme includes two light detectors and one light source. Light emitted from the light source may reach light detector 1 and light detector 2 after passing through an examined tissue, such as a skin 
     The intensity of the light I PH1  received by the light detector 1 is defined by the expression (3) as set forth below: 
         I   PH1 ( t )= I   LED   ×K   LEDcpl   ×K   skin L1 ( t )× K   PH1cpl   (3)
 
     The intensity of the light I PH2  received by light detector 2 is as set forth below in expression (4): 
         I   PH2 ( t )= I   LED   ×K   LEDcpl   ×K   skin L2 ( t )× K   PH2cpl   (4)
 
     Wherein: 
     
         
         
           
             K skin L1 , K skin L2 —Absorption value of light passing distance L1 and L2, respectively, within an examined tissue, such as skin; 
             L1, L2—Distances from light source to light detector 1 and 2, respectively; 
           
         
       
    
     From (3) and (4) follows expression (5) below: 
         K   skin L2 ( t )÷ K   skin L1 ( t )=( I   PH2 ( t )/ I   PH1 ( t ))× K   PH1cpl ( t )/ K   PH2cpl ( t ))  (5)
 
     As follows from (5), the two light detector scheme eliminates or substantially reduces the influence of the changes of optical coupling of the light source, previously denoted K LEDcpl . But this scheme does not eliminate influence of changes of the optical coupling of the light detector. 
     Reference is made to  FIG. 1 , which is a schematic illustration of heart rate sensing device  100 , according to certain embodiments of the present invention. Sensing device  100  may include at least two light sources  102 ,  108 , such as LED type diodes. Sensing device  100  may further include at least two light detectors  104 ,  106  such as photodiodes. Light sources  102  and  108  may illuminate light in response to light excitation signals LT 1  and LT 2 , respectively. Light sources  102 ,  108  may be configured to illuminate at two distinguishable lights. For example, light sources  102 ,  108  may be configured to illuminate at different wavelengths and/or light sources  102 ,  108  may be configured to alternately illuminate at different time slots. According to some embodiments, light sources  102 ,  108  may be configured to illuminate light having wavelengths within the range of 350-1100 nm. 
     Light sources  102 ,  108  may be arranged distal from each other leaving distance between them for placing light detectors  104 ,  106  substantially between them, so that a potential path PT 12  of a light ray from light source  102  to light detector  106  passes substantially opposite of light detector  104  and a potential path PT 21  of a light ray from light source  108  to light detector  104  passes substantially opposite of light detector  106 . According to some embodiments of the present invention, light sources  102 ,  108  and light detectors  104 ,  106  may be arranged in a row, with light sources  102 ,  108  at the extremities of the row and light detectors  104 ,  106  in between. According to some embodiments of the present invention, light source  102  may be placed at a distance of 3-10 mm from light detector  104 , light source  108  may be placed at a distance of 3-10 mm from light detector  106 , light source  108  may be placed at a distance of 15 mm from light detector  104 , and light source  102  may be placed at a distance of 3-10 mm from light detector  106 . 
     Potential path of a light ray from illumination source  102  to light detector  104  will be denoted PT 11 , and potential path of a light ray from light source  108  to light detector  106  will be denoted PT 22 . According to some embodiments of the present invention, when sensing device  100  is placed abutting examined article  150 , such as a living tissue, light rays along paths PT 11 , PT 12 , PT 21  and PT 22  may pass through, or be reflected from, the outer layers of the tissue of examined article  150  onto light detectors  104 ,  106 . The light received by light detectors  104 ,  106  may be transmitted by signals SG 1 , SG 2 , respectively. Signals SG 1 , SG 2  may be analog or digital signals. 
     The intensity of the light I PH11 (t) detected by light detector  104  from light source  102  along path PT 11  is as in expression (6) set forth below: 
         I   PH11 ( t )= I   LED1   ×K   LED1cpl   ×K   skin L11 ( t )× K   PH11cpl   (6)
 
     The intensity of the light I PH21 (t) detected by light detector  106  from light source  102  along path PT 12  is as set forth below in expression (7). 
         I   PH21 ( t )= I   LED1   ×K   LED1cpl   ×K   skin L12 ( t )× K   PH21cpl   (7)
 
     The intensity of the light I PH12 (t) detected by light detector  104  from light source  108  along path PT 21  is as in expression (8) below. 
         I   PH12 ( t )= I   LED2   ×K   LED2cpl   ×K   skin L21 ( t )× K   PH12cpl   (8)
 
     The intensity of the light I PH22 (t) detected by light detector  106  from light source  108  along path PT 22  is as set forth below in expression (9). 
         I   PH22 ( t )= I   LED2   ×K   LED2cpl   ×K   skin L22 ( t )× K   PH22cpl   (9)
 
     Wherein: 
     
         
         
           
             I LED1 , I LED2 —Intensities of light source  102  and light source  108 , respectively; 
             K LED1cpl , K LED2cpl —optical coupling coefficients of light source  102  and light source  108 , respectively; 
             K PH11cpl , K PH12cpl —optical coupling coefficients, when it is illuminated by light source  102  or by light source  108 , respectively; 
             K PH21cpl , K PH22cpl —Light detector  106  optical coupling coefficients, when it is illuminated by light source  102  or by light source  108 , respectively; 
             K skin L11 , K skin L12 , K skin L21 , K skin L22 —skin absorption values along paths PT 11 , PT 12 , PT 21  and PT 22 , respectively. 
           
         
       
    
     According to some embodiments of the present invention, a measure of tissue absorption is calculated based on a ratio of the intensity of light detected by light detector  106  from the light source  102  and the intensity of light detected by the light detector  104  from the light source  102 , and a ratio of the intensity of light detected by the light detector  104  from the light source  108  and the intensity of light detected light detector  106  from the light source  108 . For example, a measure of tissue absorption is calculated by multiplying the ratio of the intensity of light detected by light detector  106  from the light source  102  and the intensity of light detected by the light detector  104  from the light source  102 , by the ratio of the intensity of light detected by the light detector  104  from the light source  108  and the intensity of light detected light detector  106  from the light source  108 . 
     Specifically, from (6)-(9) follows expressions (10) and (11) set forth below: 
         K   skin L12 ( t )/ K   skin L11 ( t )= K   skin Δ1 =( I   PH21 ( t )/ I   PH11 ( t ))×( K   PH11cpl   /K   PH21cpl )  (10)
 
         K   skin L21 ( t )/ K   skin L22 ( t )= K   skin Δ2 =( I   PH12 ( t )/ I   PH22 ( t )×( K   PH22cpl   /K   PH12cpl )  (11)
 
     Wherein: 
     
         
         
           
             K skin Δ1 , K skin Δ2 —skin absorption values for Δ distances, when it is measured in condition of light source 1 ( 102 ) or light source 2 ( 108 ) illuminating.
 
and as in expression (12) below:
 
           
         
       
    
     
       
         
           
             
               
                 
                   
                     
                       
                         K 
                         
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     The propagation of light in the skin is well described by a diffusion theory. The intensity of the passing light has an exponential decay dependence on the distance from the light source, and its exponential form does not depend on the direction of the light propagation. If photo diodes are pressed to the skin and the transmitted light intensity change law is independent of the direction, then 
     
       
         
           
             
               
                 
                   
                     
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     And light detector  104  may measure: 
     
       
         
           
             
               
                 
                   
                     
                       
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     As is seen from equation 14, the expression K skin Δ1 (t)×K skin Δ2 (t) is independent of all coupling coefficients. In actual measurement conditions, there is some non-uniformity of blood concentration in the skin. Therefore, coupling coefficients do not possess exactly the same values and their ratio is not exactly equivalent to one. But this ratio is much less sensitive to the pressure changes than the light detector coupling coefficient. K skin Δ1 (t)×K skin Δ2 (t) is a measure of tissue absorption, from which heart rate, and other physiological parameters related to the blood pulse, such as oxygen saturation and arterial stiffness, may be calculated using signal analysis methods. 
     The low dependence of heart rate sensing device according to the present invention, such as sensing device  100 , to changes in the coupling coefficients of the device to the examined article may be used for embedding it in various devices and elements which are worn any way by people active in sport activities, thus eliminating the unpleasant burden of wearing chest strap, as is known in the art. 
     According to some embodiments of the present invention, sensing device  100  may offer a good ambient light resistance. The system which includes sensing device  100  and signal interface unit  240  may be equipped with one or more current drivers for the light illumination sources, a photocurrent or trans-impedance amplifier(s), an analog to digital convertor(s). The system may further include other units such as: a light illumination current driver(s) controller which changes the current in compliance with the skin absorption and ambient light, an automatic gain control circuit, an ambient light photocurrent compensation controller. Moreover, photo receivers  104  and  106  may include optical filters for an ambient light protection. 
     According to some embodiments of the present invention, sensing device  100  may offer a mechanical solution to support optics geometry for implementing the aforementioned optical sensing architecture. The sensing device  100  may have an unlimited angle of view of the photo receivers  104  and  106  and so it is very sensitive to small changes of a distance between the sensor and the skin. These changes greatly alter the effective distance Δ between the photo receivers, notably for small Δ values. Therefore, it is desirable that the sensing device  100  may be equipped with elements that may limit the angle of view of the photo receivers  104  and  106 . This limiting may be obtained by deepening the photo receivers as shown on  FIG. 1  or by an optical guide or by a lens system or by others methods. For stable operation under varying distances between the sensor and the skin it is also desirable to limit the illuminating angles of the light sources. The limitation stabilizes the distances. 
     Reference is made now to  FIG. 2 , which is a schematic block diagram of system  200  for receiving, processing and presenting to a user heart rate readings based on signals received from heart rate sensing device according to some embodiments of the present invention. Heart rate monitoring system  200  may comprise optical sensing unit  215 , accelerometer  216 , signals processing and display management unit  210  and readings display unit  260 . Unit  210  may comprise processing unit  220 , memory storage means  230 , signal interface (I/F) unit  240 , communication unit  246  and power supply unit  250 . Sensing unit  215  may be similar or equal to sensing device  100  of  FIG. 1 . Signals sent to sensing unit  215  to invoke light emitting patterns by light illumination sources, such as light illumination sources  102  and  108  ( FIG. 1 ) and signals received from light detectors such as light detectors  104  and  106  ( FIG. 1 ) may be transmitted between sensing unit  215  and unit  210  via I/F unit  240  and communication channel  218 . Communication channel  218  may be embodied via wires or wireless channel. In low power consumption embodiments, as is typical with sport related embodiments, performing channel  218  by wires is preferred, however in some embodiments, where wiring the sensing unit to the processing and presenting unit is impossible, short range wireless solutions may be used, such as Bluetooth (BT) wireless communication. Storage means  230  may be any non-transitory storage means known in the art, such as ROM, PROM. EPROM, EEPROM, DRAM, SDRAM and the like. Storage unit  230  may store data, parameters and program code which when executed by processor unit  220  perform the operations, commands and calculations described throughout this description. It is understood however that processing unit  220  may be implemented as analog circuits and is not limited to digital electronics circuits. 
     Processor unit  220  may be any suitable processor, processing unit, programmable logical computer (PLC) computer, etc. Typically, the selected processing unit will be as small and light as possible, to allow its embedding in the intended sport related devices and accessories. Processor unit  220  may be adapted to perform program code stored in storage unit  230 , to receive signals from sensing unit  215  via I/F unit  240  and to invoke illumination control signals toward sensing unit  215  via I/F unit  240 . Unit  210  may be adapted to store user—specific parameters, either entered manually or stored during use and processed to represent the user&#39;s specifics in order to provide more accurate readings of the heart rate. Unit  210  may also be adapted to store parameters specific to the sport branch taken by the user and may further be adapted to process the heart rate signals in accordance with these sport specific parameters in order to provide more accurate heart rate readings. Display unit  260  may be any low power, short focus length and light weight display, either containing the display surface as part of it or, according to other embodiments, screening the visual information on a visor surface being an integral part of a hat, glasses, sun glasses or the like. In some embodiments, display unit  260  may be packed together with unit  210  and in other embodiments it may depart from unit  210 , for example in order to enable convenient location with respect to the eye of the user. When display unit  260  is located away from unit  210  it may be in active communication with unit  210  via communication channel  219 , being wired or wireless channel as may be required. 
     In order to improve the quality of the heart rate signal picked by a sensor, according to some embodiments of the present invention, such as sensing device  100 , proper mechanical installation need to be provided. Most common phenomenon which may induce noise into the heart rate signal picked by a sensor according to some embodiments of the present invention are the movements of the user when in a sportive activity such as walking, running, swimming, riding bicycles and the like, which may cause movements of the sensing device relative to the examined tissue. One of the harshest movements is incurred during running; however, other sportive activities may also induce noticeable relative movements that may deteriorate the quality of the heart rate signal picked by the sensing device. According to some embodiments of the preset invention, movements artifacts may be filtered from the detected signal based on measurements of these movements, for example, by accelerometer  216 . Still, there is a need for a mechanical attachment unit that will provide sufficient attachment pressure to sufficiently attach the sensing device to the examined tissue and will allow maximal mechanical detachment of the sensing device from the accessory it is attached to, so as to minimize influence of movements of the accessory relative to the body organ it relates to on the attachment of the sensing device to a tissue of that body organ. 
     According to some embodiments, accelerometer  216  may be used herein for enhancing the quality and correctness of the heart rate measuring of measuring device  100 . The use of an accelerometer may be in at least three manners: to measure degree of activity of the person wearing the sensing device, to detect a rate of change in that activity and to derive a transfer function of the person wearing the heart rate measuring device. Processor unit  210  may then use the data collected by the accelerometer and correct the optical measurement accordingly. 
     Reference is made to  FIGS. 3A and 3B , which schematically illustrate attachment unit  300  for attaching a sensing device to an examined tissue  350  in schematic partial section view and in schematic partial isometric view, respectively, according to some embodiments of the present invention. Attachment unit  300  may comprise a wearable or other attachable accessory  310  in which sensor  320 , built and operative according to some embodiments of the present invention, such as sensing device  100  ( FIG. 1 ) may be embedded, for example in recess  340  made in attachable accessory  310  so as to include sensor  320  in it and to enable the face  320 A of sensor  320  to attach or about the outer face of examined tissue  350 . As seen in  FIG. 3B , recess  340  may be made to allow sufficient freedom for movements of sensor  320  within recess  340  along axes X and Y, which define a plane that is parallel to the face  320 A of sensor  320 . Sensor  320  may be supported by support element  330  which may be formed to provide sufficient attaching force along axis Z substantially perpendicular to face  320 A. The attaching force along axis Z provided by support element  330  should be substantially constant or kept within a desired range, for example, support element  330  may be formed to provide pressure of 30-40 mmHg between sensor  320  and the outer face of examined tissue  350 . Support element  330  may be flexible enough to allow sensor  340  to conform to the different surfaces of the skin. Support element  330  may concurrently provide sufficient freedom for sensor  340  to move along axes X and Y, to allow for small high-frequency relative movements between sensor  340  and the outer face of examined tissue  350 . This arrangement may ensure sufficient attachment of sensor  320  in a direction perpendicular to the adjacent surface of examined tissue  350  while providing mechanical disengagement of sensor  320  from relative movements of attachment accessory  310  with respect to examined tissue  350 , thus allowing sensor  320  to provide signal with better S/N ratio. Attachable accessory  310  may include a shell  360  (shown only in  FIG. 3A  for clarity) to optically isolate the optical sensor from ambient light. Advantageously, the aforementioned structure may guarantee meeting the constant or sufficient pressure requirement applied to the skin by the optical sensor. 
     Attachable accessory  310  may be any sportive accessory, such as safety helmet, sun glasses, swimming goggles, etc. in each such accessory a respective location for sensor  320  may be selected, to ensure good attachment of sensor  320  to the surface of examined tissue  350 . Support element  330  may be implemented in many ways, as is known in the art. For example, support element  330  may be implemented by a thin membrane, made from an elastic fabric or elastomer. Sensor  320  may be attached to the middle of the membrane and the surrounding edge may be connected to attachable accessory  310 . According to some embodiments, support element  330  may be implemented by an elastic layer of sponge, made of any applicable material such as silicone or Urethane. The sponge may be placed between sensor  320  and attachable accessory  310 . In addition, support element  330  may be implemented by a spring system, connecting sensor  320  to attachable accessory  310 . 
     According to some embodiments of the present invention, a heart rate sensing device such as sensing device  100 , may be embedded in a safety helmet, such the helmet of a bicycle rider. Reference is made to  FIG. 4 , which schematically illustrates helmet system  400  comprising sensor  410  according to some embodiments of the present invention. Helmet system  400  may comprise helmet  405 , such as helmet used for riding bikes, in which sensor  410  is installed, for example in the forward portion  405 A of the head&#39;s cushioning belt of helmet  405  so as to enable sensor  410  to be pressed against and abutting the forehead of the bike rider with sufficient sideways movement freedom, as described with respect to drawings  3 A and  3 B. Signals processing and information display management unit  420 , similar to unit  210  of  FIG. 2 , may be embedded, for example, in the cushioning portion  405 B of the scruff. Display of the heart rate and potentially other data may be implemented in several ways, as is discussed herein below. 
     According to some embodiments of the present invention, sensor  410  and display management unit  420  may be embedded into helmet system  400 , or be designed as a standalone heart rate measurement system, adapted to be attachable to standard helmet systems. A standalone heart rate measurement system including sensor  410  and display management unit  420  according to some embodiments of the present invention is advantageous since a user may fit such system to practically any safety helmet. Similarly, the heart rate measurement system may be easily implemented and inserted within a head band. 
     Reference is made now to  FIG. 5 , which schematically illustrates sunglass system  500  for providing sensor  510 ,  510 A  510 B (near the nasal bridge) and signal processing and information display management unit  520  according to some embodiments of the present invention. Sunglass system  500  may include sportive, or other type of sunglass  505  to which heart rate sensor  510 , or  510 A which are similar to sensor  100  ( FIG. 1 ) may be attached, for example to one of the sunglass&#39;s bars so as to place sensor  510 , or in a different location sensor  510 A, close to the wearer skin and provide a required pressure of sensor  510 ,  510 A to that skin. Signals processing and information display management unit  520  may be located, for example, on the other bar of sunglasses  505 . In system  500 , sensor  510  or  510 A may be connected to unit  520  by wires. 
     Reference is made now to  FIG. 6 , which schematically illustrates swimming goggles system  600  for providing sensor  610 ,  610 A and signals processing and information display management unit  620 , according to some embodiments of the present invention. Swimming goggles system  600  may comprise sportive or other type of swimming goggles  605  to which heart rate sensors  610   610 A,  610 B, and  610 C which are similar to sensor  100  ( FIG. 1 ) may be attached, for example to one of the goggles&#39; flexible strap so as to place sensor  610 , or in a different location sensor  610 A  610 B, and  610 C, close to the wearer&#39;s skin and provide a required pressure of sensor  610 ,  610 A  610 B, and  610 C to that skin. Signals processing and information display management unit  620  may be located, for example, on another portion of the flexible strap of goggles  605 . In system  600 , sensors  610  or  610 A  610 B, and  610 C may be connected to unit  620  by wires. 
     Reference is made now to  FIG. 7 , which schematically illustrates heart rate measurement and display system  700  according to some embodiments of the present invention. System  700  may comprise sunglasses  705  and heart rate sensor such as sensor  100  and signals processing and information display management unit such as unit  620  both are not shown in this drawing so as to not obscure the drawing. System  700  may further comprise mini-display device  720  attached on one of the glasses of sunglasses  705  placed and oriented so as to enable the respective eye of the sunglasses wearer to conveniently watch images displayed on the inner side of display element  720 A. 
     Reference is made now to  FIG. 8 , which schematically illustrates heart rate measurement and display system  800  according to some embodiments of the present invention. System  800  may comprise eye-shade  805  and heart rate sensor such as sensor  100  and signals processing and information display management unit such as unit  620  both are not shown in this drawing so as to not obscure the drawing. System  800  may further comprise mini-display device  820  attached on one side of the eye-shade  805  placed and oriented so as to enable the respective eye of the eye-shade wearer to conveniently watch images displayed on the inner side of display element  820 . 
     According to some embodiments of the present invention, each of the heart rate measurement and display systems described hereinabove, such as systems  400 ,  500 ,  600 ,  700 ,  800 , may include more than one heart rate sensor such as sensor  100 , located, for example, in different parts of the system. Obtaining readings from more than one sensor may enable the processing and information display management units such as unit  420 ,  520 ,  620  to produce more accurate results by integrating readings from the more than one sensor, for example by averaging heart rate readings or by disregarding measurements with poor signal quality and relaying of readings with better signal quality. Additionally, each of the heart rate measurement and display systems described hereinabove, such as systems  400 ,  500 ,  600 ,  700 ,  800 , may include an accelerometer to measure accelerations of the user, and use that data to filter movement artifacts from the optical signal. 
     According to some embodiments, an algorithm that may be implemented in the aforementioned device is presented hereinafter. The aim of the algorithm is to robustly calculate heart-rate from pulse and 3D acceleration signals while the monitored person is non-stationary (e.g., running, cycling, swimming). The algorithm is carried out in real-time and automatically quantifies the quality of the current signal, enhances it by removing motion artifacts, and continuously calculates and tracks the heart-rate. 
     The algorithm may include five sub-modules as follows: artifacts removal; pulse enhancement; noise cancellation; frequency estimation; and frequency tracking. The artifacts removal module may receive the pulse and acceleration signals in real-time and removes the movement&#39;s artifacts from the optical signal using adaptive filters. Then, the pulse enhancement module may emphasize the pulsatile component of the signal and reduce transient noise components. The noise cancellation may automatically identify legal and illegal pulses in a pulse window and pass on only the legal areas for further processing. The frequency estimation module may be applied on windows of data and estimate the dominant frequency in it in several ways (e.g., spectral domain and time domain) and pass the estimated frequencies to the tracking module. The frequency tracking module may be based on a physiological model that allows the heart-rate frequency to change in a realistic way. The predicted frequency may be given back to the frequency estimation module as a feedback in order to enhance the next estimated frequency. 
     As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method or an apparatus. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit, “module” or “system.” 
     The aforementioned flowchart and block diagrams illustrate the architecture, functionality, and operation of possible implementations of systems and methods according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
     In the above description, an embodiment is an example or implementation of the inventions. The various appearances of “one embodiment, “an embodiment” or “some embodiments” do not necessarily all refer to the same embodiments. 
     Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment. 
     Reference in the specification to “some embodiments”, “an embodiment”, “one embodiment” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the inventions. 
     It is to be understood that the phraseology and terminology employed herein is not to be construed as limiting and are for descriptive purpose only. 
     The principles and uses of the teachings of the present invention may be better understood with reference to the accompanying description, figures and examples. 
     It is to be understood that the details set forth herein do not construe a limitation to an application of the invention. 
     Furthermore, it is to be understood that the invention can be carried out or practiced in various ways and that the invention can be implemented in embodiments other than the ones outlined in the description above. 
     It is to be understood that the terms “including”, “comprising”, “consisting” and grammatical variants thereof do not preclude the addition of one or more components, features, steps, or integers or groups thereof and that the terms are to be construed as specifying components, features, steps or integers. 
     If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element. 
     It is to be understood that where the claims or specification refer to “a” or “an” element, such reference is not be construed that there is only one of that element. 
     It is to be understood that where the specification states that a component, feature, structure, or characteristic “may”, “might”, “can” or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. 
     Where applicable, although state diagrams, flow diagrams or both may be used to describe embodiments, the invention is not limited to those diagrams or to the corresponding descriptions. For example, flow need not move through each illustrated box or state, or in exactly the same order as illustrated and described. 
     Methods of the present invention may be implemented by performing or completing manually, automatically, or a combination thereof, selected steps or tasks. 
     The term “method” may refer to manners, means, techniques and procedures for accomplishing a given task including, but not limited to, those manners, means, techniques and procedures either known to, or readily developed from known manners, means, techniques and procedures by practitioners of the art to which the invention belongs. 
     The descriptions, examples, methods and materials presented in the claims and the specification are not to be construed as limiting but rather as illustrative only. 
     Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the invention belongs, unless otherwise defined. 
     The present invention may be implemented in the testing or practice with methods and materials equivalent or similar to those described herein. 
     While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention. Accordingly, the scope of the invention should not be limited by what has thus far been described, but by the appended claims and their legal equivalents.