Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2017 / Paper 1 / Section B / 8 / i / Solution

In transmission electron microscopy, the electron objective lens forms an intermediate real-space image and intermediate/projector lenses enlarge it onto a screen or camera. The total optical magnification is the product of the relevant stage magnifications. The user changes imaging-lens excitations to select magnification, while separately controlling illumination and focus. Display resizing introduces an additional enlargement, so the recorded pixel calibration matters.

In scanning transmission electron microscopy, a focused probe is rastered across the specimen and a detector signal is assigned to each probe position. The displayed magnification is display width divided by scanned specimen width. The user increases magnification by reducing raster-deflection amplitude, independently of choosing a smaller incident probe with the probe-forming lenses. TEM changes image-forming lens magnification; STEM changes the raster field of view. A smaller field or more pixels does not create finer physical resolution than the probe, specimen scattering and detection permit. A calibrated real-space scale is more transferable than the apparent size on a monitor.^[ref]