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Report on the topic Wearable Photoplethysmographic Sensors.

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  1. 1. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS Seminar Report
  2. 2. ABSTRACT Photoplethysmography (PPG) technology has been used to develop small, wearable, pulse rate sensors. These devices, consisting of infrared light-emitting diodes (LEDs) and photodetectors, offer a simple, reliable, low-cost means of monitoring the pulse rate noninvasively. Recent advances in optical technology have facilitated the use of high-intensity green LEDs for PPG, increasing the adoption of this measurement technique. In this review, we briefly present the history of PPG and recent developments in wearable pulse rate sensors with green LEDs. The application of wearable pulse rate monitors is discussed. The principle behind PPG sensors is optical detection of blood volume changes in the microvascular bed of the tissue. The sensor system consists of a light source and a detector, with red and infrared (IR) light-emitting diodes (LEDs) commonly used as the light source. The PPG sensor monitors changes in the light intensity via reflection from or transmission through the tissue. The changes in light intensity are associated with small variations in blood perfusion of the tissue and provide information on the cardiovascular system, in particular, the pulse rate. i
  3. 3. CONTENTS CHAPTER TITLE PAGE NO: 1. INTRODUCTION 1 2. PLETHYSMOGRAPHY 3 2.1 DEFINITION 3 2.2 TYPES OF PLETHYSMOGRAPHY 4 2.2.1 Air-Displacement plethysmography 4 2.2.2 Photo plethysmography 5 2.2.3 Strain gauge plethysmography 5 2.2.4 Impedance plethysmography 5 3. PHOTOPLETHYSMOGRAPHY 6 3.1 PRINCIPLE 6 3.2 LIGHT WAVELENGTH 6 3.2.1 The optical water window 7 3.2.2 Isobestic wavelength 7 3.2.3 Tissue penetration depth 8 3.3 DIFFERENT MODES OF PPG 9 3.3.1 Transmission Mode 9 3.3.2 Reflectance Mode 11 ii
  4. 4. 3.4 PLETHYSMOGRAPHIC WAVEFORM 12 3.5 PHOTOPLETHYSMOGRAPHIC DEVICES 13 3.5.1 Earphone earbud PPG sensors 13 3.5.2 PPG ring sensor 14 3.5.3 Wristwatch-type sensors 15 3.5.4 Forehead sensors 16 3.6 ADVANTAGES 17 4. CONCLUSION 18 REFERENCE 19 iii
  5. 5. LIST OF FIGURES FIGURE NO. TITLE PAGE NO: 3.1 TRANSMISSION MODE PPG 10 3.2 REFLECTIVE MODE PPG 11 3.3 PPG WAVEFORM 12 3.4 EAR PIECE PPG SENSOR 13 3.5 PPG RING SENSOR 15 3.6 WRIST WATCH TYPE SENSOR 16 3.7 FOREHEAD SENSORS 16 iv
  6. 6. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS CHAPTER 1 INTRODUCTION It is important to monitor the perfusion of the circulation. The most important cardiopulmonary parameter is blood pressure, but monitoring it is complicated. A second important parameter is blood flow, which is related to blood pressure. We can monitor the blood perfusion in large vessels using ultrasound devices, but it is not practical to use these routinely. Several devices for monitoring blood perfusion have been developed but unfortunately, it is difficult to find a practical device. However, the perfusion of blood flow and blood pressure can be determined easily using a pulse rate monitor. Wearable pulse rate sensors based on Photoplethysmography (PPG) have become increasingly popular, with more than ten companies producing these sensors commercially. The principle behind PPG sensors is optical detection of blood volume changes in the microvascular bed of the tissue. The sensor system consists of a light source and a detector, with red and infrared (IR) light-emitting diodes (LEDs) commonly used as the light source. The PPG sensor monitors changes in the light intensity via reflection from or transmission through the tissue. The changes in light intensity are associated with small variations in blood perfusion of the tissue and provide information on the cardiovascular system, in particular, the pulse rate. Due to the simplicity of this device, wearable PPG pulse rate sensors have been developed. This review describes the basic principles of PPG, previous and current developments in wearable pulse rate monitors with a light source, and the elimination of motion artifacts. Arterial blood pressure (ABP) is one of the most important hemodynamic characteristics of the cardiovascular system. It not only changes with the heart pulsation, but also varies naturally throughout the day as part of the circadian rhythm. In addition, ABP also changes in response to stress, drugs or diseases. Thus, Dept. of ECE, LMCST 1
  7. 7. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS the development of an accurate and reliable method for continuous ABP measurement has attracted much research effort. Although both invasive and non-invasive methods have been developed, the latter are more desirable for replacing the invasive method that is currently used in clinical practice. Photoplethysmography (PPG) technology has been used to develop small, wearable, pulse rate sensors. These devices, consisting of infrared light-emitting diodes (LEDs) and photodetectors, offer a simple, reliable, low-cost means of monitoring the pulse rate noninvasively. Recent advances in optical technology have facilitated the use of high-intensity green LEDs for PPG, increasing the adoption of this measurement technique. In this review, we briefly present the history of PPG and recent developments in wearable pulse rate sensors with green LEDs. The application of wearable pulse rate monitors is discussed. Dept. of ECE, LMCST 2
  8. 8. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS CHAPTER 2 PLETHYSMOGRAPHY 2.1 DEFINITION Plethysmography is a non-invasive diagnostic treatment used for screening and patient follow-ups with various arterial and venous pathologies. This treatment is concerned with the measurement of volume and volume displacement of blood. The screening provides a circulatory assessment via a waveform representation of pulsatile peripheral blood flow. Instrumentation providing blood volume parameters exists but nothing to measure volume directly. An example of this instrumentation is the use of an ultrasound. While ultrasound provides hemodynamic (hemodynamic refers to the forces generated by the heart and the motion of blood through the cardiovascular system) data on vein segments, plethysmography provides information that is indirectly related to venous volume changes. The data obtained is not specific to venous function because limb volume changes may be caused by several factors. Rapid changes are typically associated with changes in blood volume or movement artifact. If movement is controlled, information specific to blood volume can be obtained. Further separation of arterial and venous flow effects can be observed through electronic filtration. Venous flow changes typically involve long transient time constants with duration of seconds or minutes. Venous displacement measurements are typically associated with shifts in body position and limb compressions which allow measurements of magnitude and duration. Dept. of ECE, LMCST 3
  9. 9. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS 2.2 TYPES OF PLETHYSMOGRAPHY The different types of plethysmography is classified according to the basis of the source used. Four main types of plethysmography exist. All of them are used in clinical applications inorder to monitor the patients at different parts of the body. They are: 1. Air-Displacement 2. Photo 3. Strain gauge 4. Impedance 2.2.1 Air-Displacement plethysmography It is mainly done for the whole body. With air-displacement plethysmography, the volume of an object is measured indirectly by determining the volume of air it displaces inside an enclosed chamber (plethysmograph). Thus, human body volume is measured when a subject sits inside the chamber and displaces a volume of air equal to his or her body volume. Body volume is calculated indirectly by subtracting the volume of air remaining inside the chamber when the subject is inside from the volume of air in the chamber when it is empty. The volume of air inside the chamber is calculated by slightly changing the size of the chamber (e.g. by moving a diaphragm in one of the walls) and applying relevant physical gas laws to determine the total volume from the changing air pressure within the chamber as its size is altered. By subtracting the remaining volume of air inside the chamber when the patient is inside from the volume of air in the chamber empty, you get the body volume. Dept. of ECE, LMCST 4
  10. 10. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS 2.2.2 Photo plethysmography They are mainly used for monitoring blood perfusion in different parts of the body. It is the simplest way of monitoring the blood perfusion non-invasively. Photoelectric plethysmography is concerned with assessment based on cutaneous blood volume. An electrode consisting of an infrared LED and a photosensor is attached to the skin. Light transmitted into the skin is scattered and absorbed by tissue in the illuminated field. Blood attenuates the reflected light and intensity of reflected light changes with blood tissue density. The voltage signal generated by the photosensor is amplified by a DC circuit. Low frequencies are passed which produces relatively stable tracing. This corresponds to blood density in the underlying tissue. 2.2.3 Strain gauge plethysmography Strain gauge plethysmography uses a transducer filled with mercury or indium- gallium metal alloy conductor. Stretching the strain gauge causes a decrease is diameter causing an increase in voltage. When wrapped around a limb segment, the gauge provides a circumferential measurement that can be used to compute area. The “slice volume” of the limb segment changes as the limb volume expands and contracts. The mercury gauge is a very sensitive indicator of changes in the digital volume and permits measurement of systolic blood pressure at any level of the extremity. 2.2.4 Impedance plethysmography The final type of plethysmography is impedance plethysmography. A weak current is passed through a limb and the electrical resistance to current flow is measured. Four conductive bands are taped around the limb as outer and inner pairs of electrodes. The inner pair is then used to measure electrical resistance. Dept. of ECE, LMCST 5
  11. 11. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS CHAPTER 3 PHOTOPLETHYSMOGRAPHY 3.1 PRINCIPLE The principle of PPG has been reviewed previously, and is explained briefly here. Light travelling though biological tissue can be absorbed by different substances, including pigments in the skin, bone, and arterial and venous blood. Most changes in blood flow occur mainly in the arteries and arterioles (but not in the veins). For example, arteries contain more blood volume during the systolic phase of the cardiac cycle than during the diastolic phase. PPG sensors optically detect changes in the blood flow volume (i.e., changes in the detected light intensity) in the microvascular bed of tissue via reflection from or transmission through the tissue. 3.2 LIGHT WAVELENGTH The interaction of light with biological tissue is complex and includes the optical processes of (multiple) scattering, absorption, reflection, transmission and fluorescence (Anderson and Parrish 1981). Several researchers have investigated the optical processes in relation to PPG measurements. Researchers have highlighted the key factors that can affect the amount of light received by the photodetector; the blood volume, blood vessel wall movement and the orientation of red blood cells (RBC). The orientation effect has been demonstrated by recording pulsatile waveforms from dental pulp and in a glass tube where volumetric changes should not be possible, and more recently by N¨aslund et al (2006) who detected pulsatile waveforms in bone. The recorded pulses do bear a direct relationship with perfusion, and the greater the blood volume the more the light source is attenuated. However, attempts at pulse amplitude quantification (‘calibration’) have been largely unsuccessful. Dept. of ECE, LMCST 6
  12. 12. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS The interaction of light with biological tissue can be quite complex and may involve scattering, absorption and/or reflection. Anderson and Parrish examined the optical characteristics and penetration depth of light in human skin. The wavelength of optical radiation is also important in light–tissue interactions (Cui et al 1990), and for three main reasons: 1. The optical water window 2. Isobestic wavelength 3. Tissue penetration depth 3.2.1. The optical water window The main constituent of tissue is water that absorbs light very strongly in the ultraviolet and the longer infrared wavelengths. The shorter wavelengths of light are also strongly absorbed by melanin. There is, however, a window in the absorption spectra of water that allows visible (red) and near infrared light to pass more easily, thereby facilitating the measurement of blood flow or volume at these wavelengths. Thus, the red or near infrared wavelengths are often chosen for the PPG light source (Jones 1987). 3.2.2. Isobestic wavelength Significant differences exist in absorption between oxyhaemoglobin (HbO2) and reduced hemoglobin (HB) except at the isobestic wavelengths (Gordy and Drabkin 1957). For measurements performed at an isobestic wavelength (i.e. close to 805 nm, for near infrared range) the signal should be largely unaffected by changes in blood oxygen saturation. Dept. of ECE, LMCST 7
  13. 13. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS 3.2.3. Tissue penetration depth The depth to which light penetrates the tissue for a given intensity of optical radiation depends on the operating wavelength (Murray and Marjanovic 1997). In PPG the catchment (study) volume, depending on the probe design, can be of the order of 1 cm3 for transmission mode systems. PPG can provide information about capillary nutritional blood flow and the thermoregulatory blood flow through arterio-venous anastomosis shunt vessels. Within the visible region, the dominant absorption peak corresponded to the blue region of the spectrum, followed by the green-yellow region (between 500 and 600 nm) corresponding to red blood cells. The shorter wavelengths of light are strongly absorbed by melanin. Water absorbs light in the ultraviolet and longer IR regime; however, red and near-IR light pass easily. As a result, IR wavelengths have been used as a light source in PPG sensors. Blood absorbs more light than the surrounding tissue. Therefore, a reduction in the amount of blood is detected as an increase in the intensity of the detected light. The wavelength and distance between the light source and photodetector (PD) determine the penetration depth of the light. Green light is suitable for the measurement of superficial blood flow in skin. Light with wavelengths between 500 and 600 nm (the green-yellow region of the visible spectrum) exhibits the largest modulation depth with pulsatile blood absorption. IR or near-IR wavelengths are better for measurement of deep-tissue blood flow (e.g., blood flow in muscles). Thus, IR light has been used in PPG devices for some time. However, green-wavelength PPG devices are becoming increasingly popular due to the large intensity variations in modulation observed during the cardiac cycle for these wavelengths. A green LED has much greater absorptivity for both oxyhaemoglobin and deoxyhaemoglobin compared to infrared light. Dept. of ECE, LMCST 8
  14. 14. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS Therefore, the change in reflected green light is greater than that in reflected infrared light when blood pulses through the skin, resulting in a better signal-to- noise ratio for the green light source. Several green-light-based photoplethysmographs are available commercially. For example, MIO Global has developed the MIO Alpha in cooperation with Philips; this measures the electrocardiogram (ECG) with 99% accuracy, even while cycling at speeds of up to 24 kmph. For daily use, Omron has developed a green light pulse rate monitor (HR-500U, OMRON, Muko, Japan). Furthermore, the use of video cameras using the signal based on the red green blue (RGB) colour space has been considered, as shown in Section 3.3. The green signal was found to provide the strongest plethysmographic signal among camera RGB signals. Haemoglobin absorbs green light better than red and green light penetrates tissue to a deeper level than blue light. Therefore, the green signal contains the strongest plethysmographic signal. 3.3 DIFFERENT MODES OF PPG The different modes in PPG is categorized according to the positions in placement of the light source and the photodetector. According to this way there are two modes of operation of PPG. They are: 1. Transmission 2. Reflectance 3.3.1 Transmission Mode In transmission mode, the light transmitted through the medium is detected by a PD opposite the LED source. The transmission mode is capable of obtaining a relatively good signal, but the measurement site may be limited. To be Dept. of ECE, LMCST 9
  15. 15. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS effective, the sensor must be located on the body at a site where transmitted light can be readily detected, such as the fingertip, nasal septum, cheek, tongue, or earlobe. Sensor placement on the nasal septum, cheek or tongue is only effective under anesthesia. In transmission mode, when the IR LED illuminates, the light is forced to fall on the part of the body on to which the device is kept. The light start transmitting through the body part. In this case some of the light is absorbed by the capillary bed, some will be reflected back from the capillary bed, some transmits through and reach the photodiode kept at the opposite end of the source and the rest reflect back at the surface itself. The light transmitted through the body part and reaching the photodiode is noted and the waveform is noted as the plethysmographic waveform. Fig 3.1 Transmission mode PPG The fingertip and earlobe are the preferred monitoring positions; however, these sites have limited blood perfusion. In addition, the fingertip and earlobe are more susceptible to environmental extremes, such as low ambient temperatures (e.g., for military personnel or athletes in training). The greatest disadvantage is that the fingertip sensor interferes with daily activates. Dept. of ECE, LMCST 10
  16. 16. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS 3.3.2 Reflectance Mode Reflectance mode eliminates the problems associated with sensor placement, and a variety of measurement sites can be used (as discussed in the following section). However, reflection-mode PPG is affected by motion artifacts and pressure disturbances. Any movement, such as physical activity, may lead to motion artifacts that corrupt the PPG signal and limit the measurement accuracy of physiological parameters. Pressure disturbances acting on the probe, such as the contact force between the PPG sensor and measurement site, can deform the arterial geometry by compression. Thus, in the reflected PPG signal, the AC amplitude may be influenced by the pressure exerted on the skin. Fig 3.2 Reflectance mode PPG In this case also as the light is illuminated, some light will be transmitted through, some will be reflected back at the surface itself, some are absorbed by the capillary bed. Here the reflected signals are captured by the photodiode kept adjacent the source and the waveform is noted according to the amount of light falling on the photodiode after reflection. Dept. of ECE, LMCST 11
  17. 17. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS 3.4 PLETHYSMOGRAPHIC WAVEFORM PPG sensors optically detect changes in the blood flow volume (i.e., changes in the detected light intensity) in the microvascular bed of tissue via reflection from or transmission through the tissue. Photoplethysmographic waveform, consisting of direct current (DC) and alternating current (AC) components. The DC component of the PPG waveform corresponds to the detected transmitted or reflected optical signal from the tissue, venous blood, non-pulsatile component of artery blood and depends on the structure of the tissue and the average blood volume of both arterial and venous blood. Note that the DC component changes slowly with respiration. Fig 3.3 PPG waveform The AC component shows changes in the blood volume that occurs between the systolic and diastolic phases of the cardiac cycle. The fundamental frequency of the AC component depends on the heart rate and is superimposed onto the DC component. The AC current shows the pulsatile component of the artery blood. The variation of blood flow represented by AC component is the flow during the systolic phase of the cardiac cycle. Dept. of ECE, LMCST 12
  18. 18. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS 3.5 PHOTOPLETHYSMOGRAPHIC DEVICES Wearable pulse rate sensors based on photoplethysmography (PPG) have become increasingly popular in recent years with the technology advancing day by day. Nowadays more than ten companies producing these sensors commercially. The principle of PPG have become more popular as it is non-invasive. Due to the simplicity of this device, wearable PPG pulse rate sensors have been developed. Some of the mostly used devices are: 3.5.1 Earphone earbud PPG sensors Fig 3.4 Earpiece PPG sensor Dept. of ECE, LMCST 13
  19. 19. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS Earphone/earbud PPG sensors are also available and provide greater comfort for the user. In this design, a reflective photosensor is embedded into each earbud, as shown in Figure 3. The sensor earbuds are inserted into the ear and positioned against the inner side of the tragus to detect the amount of light reflected from the subcutaneous blood vessels in the region. The PPG sensor earbuds look and work like a regular pair of earphones, requiring no special training for use. A headset with an ear-clip, transmission-type PPG sensor allows continuous, real-time monitoring of heart rate while listening to music during daily activities. In addition, the proposed headset is equipped with a triaxial accelerometer, which enables the measurement of calorie consumption and step-counting. However, over the course of a variety of daily activities (e.g., walking, jogging, and sleeping), the PPG sensor signal may become contaminated with motion artifacts. 3.5.2 PPG ring sensor The most common commercially available PPG sensor is based on finger measurement sites. The transmission mode PPG sensors are commonly used for this operation. Finger sites are easily accessed and provide good signal for PPG sensor probes. For example, a ring sensor can be attached to the base of the finger for monitoring beat-to-beat pulsations. Data from the ring sensor are sent to a computer via a radiofrequency transmitter, as shown in Figure. To minimize motion artifacts, a double ring design was developed to reduce the influence of external forces, acceleration and ambient light, and to hold the sensor gently and securely to the skin, so that the blood circulation in the finger remained unobstructed. Experiments have verified the resistance of the ring sensor to interfering forces and acceleration acting on the ring body. Benchmark testing with FDA-approved PPG and ECG sensors revealed that the ring sensor is comparable in the detection of beat-to-beat pulsations, despite disturbances. Dept. of ECE, LMCST 14
  20. 20. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS Fig 3.5 PPG ring sensor 3.5.3 Wristwatch-type sensors Wristwatch-type sensors have been developed and commercialized by several companies. These devices, although much easier to wear, are not usually used in clinical settings, due to several technical issues. However, a novel PPG array sensor module with a wristwatch-type design has been developed. The proposed module measures the PPG signal from the radial artery and the ulnar artery of the wrist, whereas previous methods obtained signals from the capillaries in the skin. Phototransistors and IR-emitting diodes were placed in an array format to improve the PPG signal sensitivity and level of accuracy. Various arrays were considered for optimization. A conductive fiber wristband was used to reduce external noise. In the experiments, the proposed module was assessed and compared with the commercially available product produced by BIOPAC. A reflective brachial PPG sensor has also been examined. Although the pulse amplitude is lower than those from the finger and earlobe, the PPG pulse waveforms from regions in the vicinity of a human artery could be detected and measured easily. Dept. of ECE, LMCST 15
  21. 21. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS Fig 3.6 Wristwatch-type sensors 3.5.4 Forehead sensors Fig 3.7 Forehead sensors Dept. of ECE, LMCST 16
  22. 22. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS Forehead sensors have shown greater sensitivity to pulsatile signal changes under low perfusion conditions, compared with other peripheral body locations [31]. The thin-skin layer of the forehead, coupled with a prominent bone structure, helps to direct light back to the PD. Sensor placement on the forehead has been shown to result in decreased motion artifacts during certain types of physical activity. 3.6 ADVANTAGES PPG sensors has provided many advantages over conventional techniques. Some of the major advantages of this technology is: 1. PPG is inexpensive and cheap. 2. Since it consumes very less power, it is an ideal ambulatory device. 3. Does not need special training or guidance. 4. A range of clinically relevant parameters can be obtained from PPG signal. 5. They offer a simple, reliable, low-cost means of monitoring pulse rate non-invasively. Dept. of ECE, LMCST 17
  23. 23. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS CHAPTER 4 CONCLUSION Wearable PPG sensors have become very popular. Although a great deal of progress has been made in the hardware and signal processing, an acceptable wearable PPG sensor device has yet to be developed. Green light sources in PPG sensors minimize motion artifacts. Several filters and algorithms have been examined to mimic daily activities on limited time scales. However, better accuracy and reproducibility of real environments are required to eliminate motion artifacts. Further research is needed for the development of practical wearable PPG pulse rate monitors and pulse oximeters. The calculation of blood pressure and pulse rate become very common in clinical applications inorder to check the patients’ condition. Several devices for monitoring blood perfusion have been developed but unfortunately, it is difficult to find a practical device. However, the perfusion of blood flow and blood pressure can be determined easily using a pulse rate monitor. PPG technology has provided an easy method in analyzing the technical measurement within the blood and blood volume changes associated with it. They are especially used to develop small wearable pulse rate sensors which can be easily used by patients itself. No further knowledge of using the device is required in using these devices. They can be used without much knowledge. Wireless wearable sensors have become more familiar with the new incoming technologies where the doctor doesn’t need much guidance. The technology has become popular much due to its non-invasive nature. Dept. of ECE, LMCST 18
  24. 24. WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS REFERENCE 1. Togawa, T.; Tamura, T.; Öberg, P.Å. Biomedical Sensors and Instruments, 2nd ed.; CRC Press: New York, NY, USA, 2011; pp. 19–190. 2. Challoner, A.V.J . Photoelectric plethysmography for estimating cutaneous blood flow. In Non-invasive Physiological Measurement; Rolfe, P., Ed.; Academic Press: Oxford, UK, 1979; Volume 1, pp. 127–151. 3. Kamal, A.A.R.; Harness, J.B.; Irving, G.; Mearns, A.J. Skin photoplethysmography—A review. Comput. Methods Programs Biomed. 1989, 28, 257–269. 4. Alen.J . Ph o t o p l e t h y smo g r a p h y and i t s a p p l i c a t i o n i n c l i n i c a l p h y s i o l o g i c a l me a s u r eme n t . Physiol. Meas. 2007, 28, R1–R39. 5. Anderson, R.R.; Parris, E.D. The optics of human skin. J. Invest. Dermatol. 1981, 77, 13–19. 6. Giltvedt, J.; Sita, A.; Helme, P. Pulsed multifrequency photoplethysmograph. Med. Biol. Eng. Comput. 1984, 22, 212–215. 7. Cui, W.; Ostrander, L.E.; Lee, B.Y. In vivo reflectance of blood and tissue as a function of light wavelength. IEEE Trans. Biomed. Eng. 1990, 37, 632–639. 8. Z i j l s t r a , W.G. ; Bu u r sma , A. ; Me e u w s e n - v a n d e r R o e s t , W. P . A b s o r p t i o n s p e c t r a o f h uma n f e t a l a n d a d u l t o x y h emo g l o b i n , d e - o x y h emo g l o b i n , c a r b o x y h emo g l o b i n , a n d Me t h emo g l o b i n . Cl i n . C h em . 1 9 9 1 , 3 7 , 1 6 3 3 – 1 6 3 8 . 9. Meada, Y.; Sekine, M.; Tamura, T. The advantage of green reflected J. Med. Syst. 2011, 35, 829–834. Dept. of ECE, LMCST 19

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