Abstract
The stability of undulator light source will influence the accuracy of the experimental results. Researchers of synchrotron radiationsource in worldwide were aim to develop precise, non-destructive measurements to diagnose the beam position.Recently, at Synchrotron Radiation Research Center (SRRC), has developed photon beam position monitor (photon BPM) as adetector. However, it suffers from the fact that they fail to offer correct information due to detect a pseudo-signal from upstream and thedownstream bending magnet synchrotron radiation, especially when the undulator gap was adjusting.Based on the above fact, this study propose a modify version of Lloyd mirror interference method to distinguish the radiation frombending and undulator. We obtain the relationship between interference fringes and beam position. As our results, using Lloyd mirrorinterference method is capable of obtaining the information of beam position without blocking the main undulator light beam.