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WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS
Seminar Report

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

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

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

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

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

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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

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.

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.

Fig 3.6 Wristwatch-type sensors
3.5.4 Forehead sensors
Fig 3.7 Forehead 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.

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.

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