An example method for performing pulse oximetry can commence with receiving at least three light signals of three different wavelengths reflected from a human tissue. The human tissue includes a pulsatile tissue and a non-pulsatile tissue. Based on the three light signals, values of at least three functions are determined. The three functions are invariant to an oxygen saturation in the pulsatile tissue and depend on location of a sensor operable to detect the three light signals and pressure of the sensor on the human tissue. Based on the values of the three functions, non-pulsatile components are analyzed for intensities of a red light signal and infrared light signal reflected from the human tissue. The non-pulsated components are removed from the intensities to allow correct estimates of a ratio of the absorption coefficients, with the ratio being used to determine the oxygen saturation in the pulsatile tissue.
