The exit electron wave from transmission electron microscopy (TEM) specimens contains both amplitude and phase information. In routine TEM imaging, only amplitude information is recorded on the recording devices (film or CCD camera) and phase information of the electron wave function normally is canceled out.
In 1947, Dennis Gabor proposed off-axis electron holography, a method of interference imaging in which the phase and amplitude components of the electron beam are obtained to correct spherical aberration of the transmission electron microscope to improve spatial resolution. In that process, the electron beam is split into the two beams: the un-scattered electron beam (i.e. the reference wave) and the image beam (or object wave) diffracted by the specimen and exiting the bottom of the specimen surface.