Aberrations and Optical Defects
- Sources of optical defects
- intrinsic lens defects
- geometric aberrations
- distortion
- extrinsic defects
- Geometric aberrations
- Spherical aberration
- focus depends where e- enters lens field
- e- converge more strongly near periphery
- ds = ks ´ f ´ ao3
- highly dependent on aperture size
- Chromatic aberration
- focus depends on speed of e- in field
- slower e-s converge more strongly
- differences in speed (if > 1:100,000)
- voltage instability
- accelerating voltage
- electron excitation
- dcv = kc ´ f ´ ao ´ dV/V
- lens current-instability
- dci = 2kc ´ f ´ ao ´ dI/I
- dependent on microscope design
- weakly dependent on aperture size
- Distortion
- astigmatism
- symmetrical axial aberration
- focal length dependent on lens sector
- stronger field is at 90° to weaker
- directional smearing of image
- changes axis 90° through focus
- resolution sharply decreases
- counter-balanced by stigmator
- azimuth = rotation of field
- amplitude = strength of field
- barrel distortion
- pincushion distortion
- Extrinsic defects
- misalignment
- lenses, apertures, current center
- contamination
- column, apertures, pole pieces
- specimens
- Other optical phenomena
- contrast can be improved by:
- smaller apertures
- lower accelerating voltage
- increasing section thickness
- increasing staining
- resolution can be improved by:
- use of optimum aperture (balancing diffraction and aberrations)
- higher accelerating voltage
- thinner specimen thickness