Editing Scanning electron microscope (section)

===Photometric 3D SEM reconstruction from a four-quadrant detector by "shape from shading"===
This method typically uses a four-quadrant BSE detector (alternatively for one manufacturer, a 3-segment detector). The microscope produces four images of the same specimen at the same time, so no tilt of the sample is required. The method gives metrological 3D dimensions as far as the slope of the specimen remains reasonable.<ref name="Mignot-2018"/> Most SEM manufacturers now (2018) offer such a built-in or optional four-quadrant BSE detector, together with proprietary software to calculate a 3D image in real time.<ref>{{cite journal|last1=Butterfield|first1=Nicholas|last2=Rowe|first2=Penny M.|last3=Stewart|first3=Emily|last4=Roesel|first4=David|last5=Neshyba|first5=Steven|title=Quantitative three-dimensional ice roughness from scanning electron microscopy|journal=Journal of Geophysical Research: Atmospheres|date=16 March 2017|volume=122|issue=5|pages=3023–3025|doi=10.1002/2016JD026094|ref=Butterfield2017|bibcode=2017JGRD..122.3023B|doi-access=free}}</ref>

Other approaches use more sophisticated (and sometimes GPU-intensive) methods like the [[optimal estimation]] algorithm and offer much better results<ref>{{cite journal|last1=Butterfield|first1=Nicholas|last2=Rowe|first2=Penny M.|last3=Stewart|first3=Emily|last4=Roesel|first4=David|last5=Neshyba|first5=Steven|title=Quantitative three-dimensional ice roughness from scanning electron microscopy|journal=Journal of Geophysical Research: Atmospheres|date=16 March 2017|volume=122|issue=5|pages=3025–3041|doi=10.1002/2016JD026094|ref=Butterfield2017|bibcode=2017JGRD..122.3023B|doi-access=free}}</ref> at the cost of high demands on computing power.

In all instances, this approach works by integration of the slope, so vertical slopes and overhangs are ignored; for instance, if an entire sphere lies on a flat, little more than the upper hemisphere is seen emerging above the flat, resulting in wrong altitude of the sphere apex. The prominence of this effect depends on the angle of the BSE detectors with respect to the sample, but these detectors are usually situated around (and close to) the electron beam, so this effect is very common.