  A high-precision hypernuclear experiment has been performed at the Mainz
Microtron (MAMI) to determine the hypertriton {\Lambda} binding energy via
decay-pion spectroscopy. A key element of this measurement is an accurate
calibration of the magnetic spectrometers with the MAMI beam. For such an
absolute calibration with small statistical and systematic uncertainties the
undulator light interference method will be applied. In this contribution the
basic principle of this method is discussed and the analysis status of the
measured synchrotron radiation spectra is presented
