Abstract
For the high resolution imaging of thin specimens, it has been shown by Cowley and Au that the bright-field image intensity may be written asHere s(r) and c(r) are the two-dimensional Fourier transforms of the imaginary and real parts of the transfer function, A(u)sinx(u) and A(u)COSX(u), where A(u) is the aperture function, x(u) is the phase factor usually writtenand u = (2sinθ)/λ is the magnitude of the two dimensional reciprocal space vector u;Δf and Cs are the defocus and spherical aberration constants of the objective lens and the * symbol indicates a convolution integral. In deriving equation (1) we have assumed the validity of the phase object approximation but not the usual weak phase object approximation (WPOA) which applies only for the weak-scattering case that σφ(r)<<l, where σ is the interaction constant (=п/λE) and ϕ(xy) is the projected potential of the object. The WPOA is known to have severely restricted validity for crystalline specimens or for specimens containing heavy atoms such as negatively stained or positively stained biological material.
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