User manual

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consists of the direct component DC (absorption of light via a tissue with fixed amount of
venous and arterial blood), and of the alternating component AC (absorption of light via
pulsating arterial blood). Due to the fact that the photosensor (photodiode) sensing light
transmitted through both the LED does not have the same sensitivity for both the
wavelengths, we observe the alternating component of the light for both the wavelengths via
the direct component. The ratio R of the standardized light components is expressed via the
following equation:
DCAC
DCAC
IRIR
RR
R
/
/
=
, (4.2)
R - standardized ratio of the light intensities, R
AC
and R
DC
- alternating and direct components
of red light, IR
AC
and IR
DC
- alternating and direct components of infrared radiation.
To re-calculate the R ratio into O
2
saturation in arterial blood (4.3), it is possible to use the
equation (4.2) and the modified Lambert-Beer law equation (4.1).
%100
)].()([)()(
).()(
22
2
x
R
R
OS
IRHbIRHbORHbORHb
IRHbRHb
a
λελελελε
λ
ε
λ
ε
−+−
−
= , (4.3)
S
a
O
2
- oxygen saturation of arterial blood, )(),(
IRHbRHb
λ
ε
λ
ε
- deoxy-Hb absorption coefficient
for red and infrared radiation (light),
)(),(
22
IRHbORHbO
λ
ε
λ
ε
-oxy-Hb absorption coefficient for
red and infrared radiation (light), R - standardized ratio of radiation (light) intensities.
Fig. 4.4: Conversion curve of R ratio and O
2
saturation.
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