optical diffraction method

A method used to examine the performance of the objective lens in a TEM. In the "optical diffraction method," an HREM image of a very-thin amorphous specimen is taken and is illuminated with a laser beam to obtain an optical diffraction pattern. Using the obtained optical diffraction pattern, corrections of spherical aberration, coma axis and axial astigmatism of the objective lens are performed. The radius of the diffraction pattern (the most distant visible spot) reveals the maximum spatial frequency or the resolution of the HREM image. If the diffraction pattern is perfectly circular, the astigmatism of the objective lens is fully corrected. Recently, instead of the laser beam, computer processing is used. A computer processed aberration correction is effectively utilized for the alignment of a Cs corrector.

Related Term(s)