How does a pulse oximeter distinguish between oxygenated and deoxygenated hemoglobin to determine SpO2?

A transmittance pulse oximeter contains two small light-emitting diodes (LED) which each emit a specific wavelength of light, 660 nm (red) and 940 nm (near infrared). Oxygenated Hemoglobin (O2Hb), absorbs a greater amount of 940 nm wavelength light and a lower amount of 660 nm wavelength light than does deoxygenated hemoglobin (HHb).  Opposite from the LED, a transmittance pulse oximeter utilizes a photodiode to measure the amount of 940 nm and 660 nm wavelength light that is transmitted through the body part between the LED and photodiode.  A microprocessor then calculates SpO2 using the ratio of the measured absorbances over a series of pulses based on individual device calibration to determine the modulation ratio and its corresponding SpO2.

Figure: Jubran, Critical Care, 2015

References: Chan et al, Respiratory Medicine 2013 Lifebox Pulse Oximetry Learning Module; WHO Using Pulse Oximeters Jubran, Critical Care, 2015 How Equipment Works – Pulse Oximeter 

Keywords: wavelengths, IR, infrared, red, LED, photodiode, hemoglobin