Mass-thickness contrast (p.95 in the textbook) Primary beam C=1- Imin Sample Imax Scattered beams Objective lens Only electrons near the direct Objective aperture beam are allowed to pass through objective aperture to form image. I(x,y)=1o-Is(x,y) Additional magnifying lenses Is(x,y)fs(e) A B Fluorescent screen Intensity profile Bright on the screen Dark Atomic scattering factor,average Mechanism of mass-thickness contrast over different atoms in specimen 8Mass-thickness contrast Mechanism of mass 8 -thickness contrast Only electrons near the direct beam are allowed to pass through objective aperture to form image. 𝐶 = 1 − 𝐼𝑚𝑖𝑛 𝐼𝑚𝑎𝑥 𝐼 𝑥, 𝑦 = 𝐼0 − 𝐼𝑠(𝑥, 𝑦) 𝐼𝑠(𝑥, 𝑦)~𝑓𝑠(𝜃) Atomic scattering factor, average over different atoms in specimen (p.95 in the textbook)