180
N. Blanik
movements of the measurement object. Since I 0 is given, and ξ T −PPG is constant,
the only unknown modulation of I T −PPG is caused by time-varying change of E.
Although the Beer-Lambert-Law requires isotropy, it allows an approximation of
tissue behavior [3]:
E(t) = e
−ε(λ)c(t)d
(11.3)
In the above equation, ε is the mean extinction coefficient of the tissue dependent
on the wavelength of light λ; c is the time-dependent material concentration of tissue
(blood volume among others), and d the mean path length of the photons through
the tissue.
In the case of reflection mode (Fig. 11.2b), additionally, light paths directed toward
the sensor via bloodless epidermal layers have to be considered. The reason for this is
that, because these regions are not modulated by blood volume changes, they reduce
the percentage of the detected signal which contains vital rhythms:
I R−PPG (t) = ξ R−PPG · I 0 · E(t) + ξ epidermis · I 0 · E const + I err
(11.4)
As a result, PPG sensors utilize a single-point illumination, which is the most
common reflective mode. Distinct light paths that originate at the illumination point
and head towards the measured area become relevant for the detected PPG signal.
Direct cross talk by reflections at the skin surface is prevented by an optical barrier.
In PPGI (see Fig. 11.2c), the detected light intensity of a single pixel complies
with the light intensity of the corresponding skin area, emitted toward the camera.
The signal is composed as follows:
I PPGI (t) = I 0 · R +
x,y
(I R−PPG (t, x, y)) + I err
(11.5)
In the absence of an optical barrier, large portions of the detected light arise from
direct skin surface reflections (with skin reflection factor R), and so do not contribute
modulated information of vital parameters. Only minor quantities of the light get
backscattered from deeper tissue layers. In contrast to the reflective mode, PGGI
signal parts compound information from all the possible paths in the illuminated
surroundings of the field under measurement, and do not restrict themselves to a
single LED position (x, y). Sources of errors can be summarized to I err again.
The requirements for a PPGI measurement setup, particularly the PPG camera,
are defined by the fact that the static part (DC) of the PPGI signals is large compared
to the alternating component (AC). The ability to register small AC signal parts is
one of the most important requirements, as also camera speed and cost-effectiveness.
The most crucial prerequisites may be listed as follows:
• Dynamic range of the camera
• Spatial resolution
• Intrinsic sensor noise (dark current)
N. Blanik
movements of the measurement object. Since I 0 is given, and ξ T −PPG is constant,
the only unknown modulation of I T −PPG is caused by time-varying change of E.
Although the Beer-Lambert-Law requires isotropy, it allows an approximation of
tissue behavior [3]:
E(t) = e
−ε(λ)c(t)d
(11.3)
In the above equation, ε is the mean extinction coefficient of the tissue dependent
on the wavelength of light λ; c is the time-dependent material concentration of tissue
(blood volume among others), and d the mean path length of the photons through
the tissue.
In the case of reflection mode (Fig. 11.2b), additionally, light paths directed toward
the sensor via bloodless epidermal layers have to be considered. The reason for this is
that, because these regions are not modulated by blood volume changes, they reduce
the percentage of the detected signal which contains vital rhythms:
I R−PPG (t) = ξ R−PPG · I 0 · E(t) + ξ epidermis · I 0 · E const + I err
(11.4)
As a result, PPG sensors utilize a single-point illumination, which is the most
common reflective mode. Distinct light paths that originate at the illumination point
and head towards the measured area become relevant for the detected PPG signal.
Direct cross talk by reflections at the skin surface is prevented by an optical barrier.
In PPGI (see Fig. 11.2c), the detected light intensity of a single pixel complies
with the light intensity of the corresponding skin area, emitted toward the camera.
The signal is composed as follows:
I PPGI (t) = I 0 · R +
x,y
(I R−PPG (t, x, y)) + I err
(11.5)
In the absence of an optical barrier, large portions of the detected light arise from
direct skin surface reflections (with skin reflection factor R), and so do not contribute
modulated information of vital parameters. Only minor quantities of the light get
backscattered from deeper tissue layers. In contrast to the reflective mode, PGGI
signal parts compound information from all the possible paths in the illuminated
surroundings of the field under measurement, and do not restrict themselves to a
single LED position (x, y). Sources of errors can be summarized to I err again.
The requirements for a PPGI measurement setup, particularly the PPG camera,
are defined by the fact that the static part (DC) of the PPGI signals is large compared
to the alternating component (AC). The ability to register small AC signal parts is
one of the most important requirements, as also camera speed and cost-effectiveness.
The most crucial prerequisites may be listed as follows:
• Dynamic range of the camera
• Spatial resolution
• Intrinsic sensor noise (dark current)