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Pulse oximetry is nothing more than a specialized version of spectrometry / absorption spectroscopy. In spectrometry if the wrong wavelengths are used the results will not be accurate. This is no different for pulse oximetry.

The two main differences between pulse oximetry and spectroscopy are:-
  1. Usually when spectroscopy technology is used for analysis the sample to be analyzed is contained in a vial and the path length of the light through the sample to be analyzed is known. In pulse oximetry the path length of the light is not known, for example the thickness of fingers can vary considerably.
  2. In spectroscopy the sample is usually prepared in some way so that it is homogenous. In pulse oximetry the sample, such as a finger tip is not homogenous. There are unknown quantities of skin, bone, muscle, venous blood etc in the light path.
Pulse oximetry overcomes these obstacles by using the ratio of ratio technique where the relative sizes of the pulsatile signals generated by the red and infra red sources are compared, this allows analysis of specifically arterial blood. The pulse oximetry signal is dependant on the oxygenation state of the haemoglobin in the arteries.
  1. The pulse oximetry signal is also dependant on the wavelengths of the red and infra red light sources in the sensor.
  2. It is assumed that the wavelengths of the light sources in replacement pulse oximeter sensors are identical to the original specification.
  3. If replacement sensors have incorrect wavelengths the consequent system bias increases predictably as the patient's oxygen level falls.