= Physical section thickness
{title2=$t$}
<Physical section thickness> is the distance through a recovered physical section, measured normal to its faces. For a flat section with parallel faces, write this distance as $t$; a nonuniform section instead requires a local thickness or a stated averaging convention. It is distinct from the <nominal section feed> commanded to the <microtome>, the spacing of sampled planes in an <image stack>, and the <spatial resolution> of the resulting image.
In <ultramicrotomy>, a controlled advance presents material to a <diamond knife> on each cutting stroke. JEOL describes preset feed increments for thin sections and notes that actual thickness depends on the material and temperature.<Ultramicrotomy by JEOL>[Glossary, preset specimen feed and following paragraph on actual section thickness]{ref} <Section compression> and other <deformation (mechanics)> can further complicate the relation between the machine setting and the recovered section. A feed setting alone is not an independent thickness measurement.
An original geometric example shows why finite thickness matters. Consider a straight thin feature whose direction makes angle $\theta$ with the section's <normal vector>. Choose coordinates so the section occupies $0\leq z\leq t$ and the feature lies along
$$
x(z)=x_0+z\tan\theta,\qquad y(z)=0.
$$
An image combining contributions through the section can therefore contain feature positions spanning the lateral distance
$$
\Delta x=t|\tan\theta|.
$$
For stipulated $t=100\,\mathrm{nm}$ and $\theta=30^\circ$, this range is $57.7\,\mathrm{nm}$. It is a geometric range, not a universal formula for the measured image width: <image contrast>, the imaging response and the feature's own width also matter. Replacing the slab by one mathematical <plane> discards this depth variation.
For an ideal undeformed series of contiguous $100\,\mathrm{nm}$ sections, imaging every fifth section gives a centre-to-centre sampling interval of $500\,\mathrm{nm}$, while each imaged section remains $100\,\mathrm{nm}$ thick. Record omitted sections and the <image stack axis calibration> separately. To test a feed setting experimentally, measure recovered thickness with an independent method and stated <measurement uncertainty>, rather than using the same feed display as both command and verification.
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