 Pulse oximetry is the noninvasive measurement of
arterial blood oxygen saturation and heart rate .
 The Pulse Ox use a spectrophotometric probe .
 The pulse oximeter probe contains two light emitting
diodes (LEDs) and a photodetector .
 the pulse oximeter probe is attached to a highly
vascularized tissue, such as a finger or earlobe .
 The LEDs emit red and infrared (IR) light .
 The signal emitted by the photodetector which convert
the red and IR light to volt signal and move it to
processor .
 The processor compute the percent of blood oxygen
saturation (SpO2) according to red and IR absorption
and calculate the heart rate .
 Finally move the result to display unit .
 When the light emit to finger it will absorption from :
 Tissue .
 Bones .
 Blood (venous , arterial ) .
 After we read the result light signal we can divide it to
DC current and AC current as we see in figure
 SpO2 is a measure of the number of oxygenated
hemoglobin molecules in the blood stream relative to
the number of molecules of deoxygenated hemoglobin.
 Red light is absorbed much by oxygenated hemoglobin
than deoxygenated hemoglobin.
 IR light is absorbed much by deoxygenated hemoglobin
than oxygenated hemoglobin.
 The difference in absorption of the two types of light is
shown in the figure
 The AC-signal is isolated by dividing the maximum
absorption by the minimum (or DC-signal) value.
 the blood oxygen saturation value is a function of the
natural logarithm of the maximum and minimum of
the red and infrared (IR) signals.
 In picture, red refers to the AC-component produced by
the pulsing arterial blood, and IR refers to the
background DC-component. Max refers to the peak
value (amplitude) of the signal, while min refers to the
trough (reference) of the signal.
 “max” and “min” values in Equation are voltage readings
from the photodetector in the pulse oximeter probe.
 The exact wavelengths of the LEDs in the pulse
oximeter probe vary among the different probe models
and manufacturers. For red LEDs, the wavelength is
660 nm. For infrared LEDs, the wavelengths vary
between 880 nm and 950 nm, but 905 nm and 940 nm
LEDs are the most common. The probe LEDs emit
alternate flashes of red and infrared light at a
frequency approximately equal to 400 Hz5. Figure 5
shows a sample output waveform from the LEDs of a
pulse oximeter probe.
Nonin 8600 Pulse Oximeter Waveform
0.7
0.6
Voltage (V)
0.5
0.4
0.3
0.2
0.1
0
0.E+00
-0.1
2.E-03
4.E-03
6.E-03
8.E-03
1.E-02
1.E-02
1.E-02
Time (s)
Output from Nonin Model 8600 Pulse Oximeter
2.E-02
2.E-02
 The photodetector in the pulse oximeter probe is typically a
photodiode, which transforms an input signal into a current. The
current is then amplified, converted to a voltage, and analyzed by
circuitry within the pulse oximeter itself. As the LEDs send alternating
pulses of red and infrared light through the tissue and into the
photodetector, the photodetector records the corresponding voltages as
discrete values and sends them to the pulse oximeter. The pulse
oximeter analyzes and compiles these discrete red and infrared voltage
values into arrays representing the red and infrared waveforms. The
photodetector, therefore, effectively samples the red and infrared
waveforms by detecting the alternating red and infrared signals and
recording their respective voltage values at a sampling frequency of
about 400Hz. Because the signals emitted by the LED are transformed
by pulsatile blood flow through the tissue before they are received by
the photodetector, the waveforms produced by the voltage
measurements are periodic, with a frequency dependent upon heart
rate and an amplitude dependent upon oxygen saturation.
