Electron holography has been successfully used in the observation of phase objects such as magnetic fields and electrostatic potentials. Electron holograms are constructed by the interference of a modulated object wave and a well-defined reference wave and in most cases the reference wave is required to be a plane wave. Frequently, however, the magnetic or electric field extends beyond the interference region, resulting in a distorted reference wave. The reconstructed wave from a hologram made with a distorted reference wave does not accurately express the object wave but the difference from the reference wave.

Differential interferometry is a useful technique in the absence of a well-defined reference wave. Some electron holography techniques using two-beam illumination for differential interferometry have been proposed. We have also proposed a new technique for differential microscopy by conventional electron off-axis holography, which utilizes an electron biprism to shear the object wave. Shearing of the object wave is essential for differential interferometry.