Image-plane off-axis electron holography: Low-magnification arrangements

Measurements of the deflection angle at the electron biprism indicate that the biprism fibre is charged positively by the imaging electron beam. The charging depends on the accelerating voltage and on the biprism bias. Four basic ray diagrams with the biprism above or below the imaging lens and biased positively or negatively are discussed and applied to low-magnification arrangements. The dependences of the interference pattern and the spacing of its fringes on the voltage on the biprism fibre and the geometrical parameters given by the lens-current settings are evaluated.