The phase shift error (PSE) for DS detection is analyzed using frequency domain measure theory of DS. Two factors
lead to the difference among different pulse waves, and introduce PSE into DS. Firstly, the period, amplitude and the
base line of pulse wave are unstable. Secondly, the phases of pulse waves under different wavelengths are different. The
PSE of transmitted and reflected pulse waves are both discussed quantitatively. The results showed that the PSE is
correlated with the position of rectangular intercept window that intercepts pulses from the waves. It can be minimized
into about 10% by selecting the start points of the windows. Because of the sensor contact pressure, the shapes of
transmitted and reflected pulse waves are different. Thus the right intercept locations are at the start of ascending limb
and the dicroticpulse respectively according to minimum PSE rule.