In order to form an electron hologram some requirements should be satisfied.
First, the electron beam should be coherent. Although the source is never perfectly coherent, the degree of coherence of the electron beam must be such that an interference pattern of sufficient contrast and intensity can be recorded. Because of the high temporal and spatial coherences of cold field-emission (CFE) gun, so far CFE gun is the best choice for any holography experiment.
Second, the specimen and electron drifts should be small enough to have sufficient contrast and intensity to be recorded.
Furthermore, depending on the geometrical details of the holographic setup, the recorded phase of the object wave can be more or less accurately in a hologram.