The effect of varying LED intensity on pulse oximeter accuracy

Most pulse oximeters automatically alter the intensity of their light-emitting diodes (LEDs) according to the absorption of the finger, toe or earlobe to which they are attached. This paper investigates the effect of changing LED intensity on pulse oximeter accuracy. Our results show that the peak wavelength of a red LED typically increases by 8 nm as its intensity is increased ten-fold. To determine whether this shift introduces a significant error, a simple theoretical model based on the Beer-Lambert law is used. The model predicts that a 10:1 change in LED intensity results in a 2.5% error at 50% arterial oxygen saturation (SpO2). At high saturations (SpO2 greater than or equal to 85%) the model predicts little loss of accuracy and thus any effect due to changes in LED intensity will be apparent only at low saturations.