Editing Electron microscope (section)

=== Diffraction contrast imaging ===
Diffraction contrast uses the variation in either or both the direction of diffracted electrons or their amplitude as a function of position as the contrast mechanism. It is one of the simplest ways to image in a transmission electron microscope, and widely used.

The idea is to use an objective aperture below the sample and select only one or a range of different diffracted directions, then use these to form an image. When the aperture includes the incident beam direction the images are called ''bright field'', since in the absence of any sample the field of view would be uniformly bright. When the aperture excludes the incident beam the images are called ''dark field'', since similarly without a sample the image would be uniformly dark.<ref name="HirschEtAl">{{Cite book |last1=Hirsch |first1=P. B. |url=https://www.worldcat.org/oclc/2365578 |title=Electron microscopy of thin crystals |last2=Howie |first2=A. |last3=Nicholson |first3=R. B. |last4=Pashley |first4=D. W. |last5=Whelan |first5=M. J. |date=1965 |publisher=Butterworths |isbn=0-408-18550-3 |location=London |oclc=2365578}}</ref><ref>{{Cite journal |last=Reimer |first=Ludwig |date=1997 |title=Transmission Electron Microscopy |url=https://doi.org/10.1007/978-3-662-14824-2 |journal=Springer Series in Optical Sciences |volume=36 |doi=10.1007/978-3-662-14824-2 |isbn=978-3-662-14826-6 |issn=0342-4111}}</ref> One variant of this is called [[weak-beam dark-field microscopy]], and can be used to obtain high resolution images of defects such as dislocations.<ref>{{Cite journal |last1=Cockayne |first1=D. J. H. |last2=Ray |first2=I. L. F. |last3=Whelan |first3=M. J. |date=1969-12-01 |title=Investigations of dislocation strain fields using weak beams |url=https://www.tandfonline.com/doi/abs/10.1080/14786436908228210 |journal=The Philosophical Magazine |volume=20 |issue=168 |pages=1265–1270 |doi=10.1080/14786436908228210 |bibcode=1969PMag...20.1265C |issn=0031-8086}}</ref>