Persistent misconceptions about incoherence in electron microscopy

Incoherence in electron microscopic imaging occurs when during the observation the microscope and the object are subject to fluctuations. In order to speed up the computer simulation of the images, approximations are used that are considered as valid. In this paper we will question the validity of these approximations and show that in specific cases they can lead to erroneous results.It is shown in particular in the case of one single vibrating atom that the thermal diffuse scattering that causes the signal in HAADF STEM is not only dependent on Z but also on the mean square displacement of the atom so that it can even be large for light atoms in soft matter, provided the right HAADF aperture is used.In HREM imaging the diffuse scattering leaks out of the coherent (elastic) wave and is redistributed in the background. This might explain the mismatch in elastic contrast (Stobbs factor) especially for crystals with a thickness beyond the extinction distance, where also the HAADF signal saturates and the elastic (coherent) component vanishes.

Research highlights
► The approximations that are usually adopted to handle incoherence in electron imaging are not always valid and may lead to erroneous results.
► The HAADF STEM signal which is caused by thermal diffuse scattering is not only proportional to Z2 but also to mean square displacement of the atom so that it can be important even for light atoms in soft matter.
► In a HREM most of the thermal diffuse scattering leaks out of the coherent wave into the background which might explain the Stobbs factor.