2004B Question 11

Explain how a pulse oximeter works. Do NOT discuss the potential sources of error.

Examiner Report

29.9% of candidates achieved a pass in this question.

The major domains assessed in this question were:

  • System components
  • Beer-Lambert law
  • Differential absorption
  • Isolation of the pulsatile component
  • Converting ratio R to SpO2

Common problems with the answers included:

  • Incorrect use of the terms “SaO2”, “SpO2”, and “PaO2”
  • The misconception that only infrared light is used in pulse oximetry
  • Limited or no application of the Beer-Lambert law to pulse oximetry
  • Incorrect or contradictory statements about the absorption spectra for oxyhaemoglobin and deoxyhaemoglobin
  • Absent or insufficient explanation of the pulsatile or “alternating current” (AC) compartment.

Model Answer


  • Physics
  • Device


Property Mechanism
Beer-Lambert principle

- of the absorbing substance in the medium (Beer’s law)

- (Lambert’s law)

- Describes the basis for, but not the functioning of, the pulse oximeter

Known absorption spectra

- 940nm: O2Hb > HHb

- 660nm: O2Hb < HHb


Property Mechanism

- Probe on the finger, nose, ear

- Two monochromatic LEDs on one side, rapid on and off

- Photodetector on the other side

- Electrical cable

- Processor

- Display: SPO2 and plethysmograph


1. Pulsatile signal isolated

- Excludes venous and tissue absorption

2. Ratio of absorbances calculated

3. Ratio correlated with SaO2 derived from experiments

- R 0.4 → 100%

- R 1 → 85%

- R 2 → 50%

Last updated 2021-08-23

results matching ""

    No results matching ""