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Diffraction and Imaging. Topics. Objective aperture selection BF/DF imaging Relationship of DP to image. BFP. stronger lens - shorter focal length. Image vs. Diffraction. Objective aperture selection. The BFP contains the DP (shows reciprocal space) and the objective aperture.

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Presentation Transcript
  • Objective aperture selection
  • BF/DF imaging
  • Relationship of DP to image
image vs diffraction


stronger lens - shorter focal length

Image vs. Diffraction
objective aperture selection
Objective aperture selection
  • The BFP contains the DP (shows reciprocal space) and the objective aperture.
  • Remember, small objects in real space become large in reciprocal space.
  • Larger objective apertures allow finer detail.
  • … but by allowing more of the scattered electrons to contribute to the image it has lower contrast.
objective aperture size

No scattered electrons from hole

Large aperture – lower contrast

Small aperture – higher contrast

Objective Aperture Size
bf df aperture position
BF/DF Aperture Position

Dark Field

Bright Field

tilt to g

Not in Bragg condition!

Tilt to +g

If you move diffracted spot into center, it will disappear.

tilt to g1

Still in Bragg condition!

Tilt to -g

If you move direct beam to +g spot, -g will light up.

df imaging
DF imaging

Dark Field

df controls

BF/DF selector

Beam Tilts in DF mode

DF controls
crystallographic imaging
Crystallographic Imaging
  • TEM can give both image and crystallographic (diffraction) information.
  • To be useful, we need to relate the image to the diffraction pattern.
  • We can then mark crystallographic directions on our images.
image rotation
Image Rotation
  • Lorentz force causes the electron to spiral through the lens.
  • The amount of spiral varies with the magnetic field.
  • Changing the field (lens strength) rotates the image.
dp under over focus


1st intermediate image

DP under/over focus
rotation angle1
Rotation Angle
  • We usually work at fixed CL,  measure from DP to image
  • Note sense of rotation (CW/CCW)
  • Work with film emulsion up. Be consistent.
lab 5
Lab 5
  • Sample: MoO3 crystals on lacy carbon
  • Tasks: BF/DF imaging, Rotation calibration
  • Investigate effects of obj. aperture size, on-axis vs. off-axis DF imaging, rotation of image w.r.t. DP