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The temporal coherence is the degree to which the wave can be approximated as monochromatic.
In EMs (especially in TEMs), the temporal coherency effects comes from the small instabilities in the accelerating voltage and electron gun emission over time, which will give the illumination a small energy spread, and from variations in the lens currents, which induces focus variation with time. Because the electrons with different energy are focused at different planes, the total effect of the temporal coherency variations is to create a defocus spread Δ (usually in the order of a few nm) due to the chromatic aberration. Therefore, the temporal coherence affects the overall envelope function.
FEG-TEMs with field-emission electron sources [1 - 2] offering much better spatial and temporal coherence than those offered by CTEMs with LaB6 and W (tungsten) electron guns. Therefore, FEG-TEMs have allowed images to contain much higher spatial frequencies [3 - 4].
Table 3722. Types of coherencies in EM (electron microscope) measurements.
Types of coherencies |
Mechanisms |
| Temporal Coherency |
All
the electrons have the same wavelength, just like monochromatic light. |
| Spatial Coherency |
Spatial coherency is related to the size of the electron source. Perfect
spatial coherence would imply that the electrons were all
emanating from the same point at the source. Therefore, source size
governs spatial coherence and smaller source size gives better
coherency. |
[1] M.T. Otten, W.M.J. Coene, Ultramicroscopy 48 (1–2)
(1993) 77–91.
[2] T. Honda, T. Tomita, T. Kaneyama, Y. Ishida, Ultramicroscopy
54 (2–4) (1994) 132–144.
[3] W. Coene, G. Janssen, M. Op de Beeck, D. van Dyck,
Phys. Rev. Lett. 69 (1992) 3743–3747.
[4] M. Op de Beeck, D. van Dyck, W. Coene, Ultramicroscopy
64 (1–4) (1996) 167–183.
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