Although many samples in electron microscopy are phase objects, as in the case of optical microscopy, we cannot directly measure phase modulation by microscopy. Dennis Gabor proposed a technique called in-line holography to record both amplitude and phase information at the rather early stage of electron microscopy (in 1947). With the development of highly coherent field emission electron sources, another type of holography, off-axis holography, became available for electrons. However, holography cannot be applied to general cases, since there is a requirement for a vacuum region where the reference wave passes through.