IEEE Circuits and Systems Magazine - Q3 2018 - 10

To minimize the loss in thick tissues, the tissue thickness
at the PPG sensor position should not be greater than
the penetration depth of the light wavelength.

B. Light Wavelength
The light penetration depth is inversely related to its absorption coefficient (a) . The greater the absorption, the
smaller the light penetration depth in tissue. To minimize
the light loss in thick tissues, the tissue thickness at the
PPG sensor position should not be greater than the penetration depth of the used light wavelength. For visible
light, the penetration depth through tissue under the skin
surface ranges from 100 nm to 1200 nm [19]. A reflectance
PPG sensor located in thin tissue regions like fingertips,
earlobe, and the forehead results in stronger signals than
in thicker tissue regions such as the chest. Red and IR
light can penetrate deeper into the tissue than green light
wavelengths can; these wavelengths can thus give better
results for deep-tissue blood flow in the muscles. In addition, the photodetector responsivity of the red and IR
wavelengths is greater than that of the green light. Green
light, on the other hand, is absorbed near the surface,
so it is suitable for the shallow blood flow directly under
the skin. Short wavelengths like blue and green (405 to
520 nm) have stronger blood absorption than red and IR
light (650 to 850 nm). Therefore, the signal-to-noise ratio
(SNR) for a green light source is better than that of an IR
source when used for superficial blood flow in the skin.
The received signal at a certain light penetration depth
can be controlled by adjusting the distance between the
LED and the photodiode. By increasing the distance between the photodiode (PD) and the LED, the PD will detect the light rays with deeper penetration. The closer the

Circuit
Board (CPU,
RF Transmitter,
etc.)

Flexible Wires
Connecting the
Inner Ring and
the Outer Ring
Figure 5. PPG ring sensor [23].

IEEE CIrCUITs AND sYsTEMs MAGAzINE

Outer Ring
(Housing)

Inner Ring
(With Optical
Sensor Unit)

PD and the LED are, the better shallow surface light can
be detected. The practical distance between the PD and
LED is from 4 to 6 mm [18]. Green light has better immunity to MAs than red and IR lights, as the IR light penetrates
more deeply and is affected by all surface and deep tissue
movements [20]. The PPG signal from the green light is reduced sharply by increasing contact pressure compared
with the IR PPG signal. This is an indication of the occlusions of superficial skin blood vessels. The PPG secondary peak and dicrotic notch are more structured with red
than with green light (Fig. 4) [19].
C. PPG Sensor Position
There are different possible sites for PPG sensor placement, such as the fingertip, forehead, earlobe, chin, and
wrist. Fingertips give the strongest PPG signal, the forehead and earlobe have good signal strength, and the
wrist has a low signal amplitude. Although the fingertip
produces the strongest PPG signal, the MAs associated
with routine activities produce a great deal of noise,
which reduces the signal quality. Wrist and earlobe locations also suffer from MAs due to hand and neck movements, respectively. The forehead has a flat and large
region for sensor placement, easy attachment by using a
head bandage, and is a stable region with low motion artifact effects. It also produces a strong PPG signal. However, as forehead PPG sensors work only in reflectance
mode, dirt and sweat may affect measurement accuracy.
The chin region has a fair PPG signal, but it suffers from
growing hairs, dirt and sweat [18], [21], [22]. The work in
[18] studied the different locations with respect to PPG
signal strength, attachment, artifact, and portability.
They introduced a decision matrix to determine the preferred sensor locations, and the forehead received the
highest score.
D. PPG Sensor Structures
A PPG sensor position and structure should be comfortable enough for use during daily activities. Fingers can
be easily accessed, and they provide a strong PPG signal. One of the finger-based structures is a ring sensor,
which can be attached to the finger base, as shown in
Fig. 5 [23]. To reduce the MAs, a double ring structure
was developed to reduce the effect of outside forces.
A wristwatch-type PPG sensor was introduced in
[24]. This design collects the PPG signal from both the
ThIrD qUArTEr 2018

Table of Contents for the Digital Edition of IEEE Circuits and Systems Magazine - Q3 2018