Interactive Guide · TEM Series

What causes TEM image contrast

Contrast is not a vibe, it is a mechanism (usually three of them at once).

A TEM image is not a photograph. Nothing in the microscope "shades" the picture the way light shades the world. In conventional TEM, nearly every dark patch traces back to one of three workhorse mechanisms, often more than one at once, and reading an image correctly starts with knowing which one you're looking at.

Mass–thickness

Thicker or heavier regions scatter more electrons out of the beam → they appear darker. The "intuitive" contrast.

Diffraction

Crystals aligned to a Bragg condition redirect electrons into diffracted beams → they go dark in bright-field. Orientation-dependent.

Phase

Electron waves interfere. At high magnification, contrast comes from phase shifts, and flips with focus. The subtle one.

1 · Mass–thickness contrast: the honest one

Electrons passing through more material, or heavier atoms, get scattered to higher angles and stopped by the objective aperture. Fewer electrons reach the screen, so the region looks dark. It behaves like an X-ray in reverse and mostly "means what it says." It dominates for amorphous and biological samples.

Bright-field view of four particles of increasing atomic number sitting on a support film.

Sample thickness:

Test particle Z:

Try it: raise the atomic number of the rightmost particle from carbon (Z=6) to gold (Z=79). Heavier scatters harder, so it darkens. This is exactly why heavy-metal stains and Au nanoparticles show up so well.

2 · Diffraction contrast: the crystal's mood ring

In a crystalline sample the story changes completely. A grain that happens to satisfy a Bragg condition diverts a big share of the beam into a diffracted spot. In bright-field (aperture around the direct beam) that grain goes dark: not because it's thicker or heavier, but purely because of its orientation. Tilt the sample a fraction of a degree and the darkness jumps to a different grain.

A polycrystalline foil. Every grain has the same composition and thickness; only orientation differs.

Specimen tilt:

Imaging mode

Try it: sweep the tilt slowly and watch grains blink dark one by one as each hits its Bragg condition. Then switch to dark field: the aperture now sits on a diffracted beam, so the exact grains that were dark light up. Same sample, same instant, inverted logic.

This is also where bend contours and thickness fringes come from: in a bent or wedge-shaped foil, the Bragg condition is met only along certain bands, which appear as sweeping dark curves. They move when you tilt: a dead giveaway that you're seeing diffraction contrast, not a real feature.

3 · Phase contrast: interference, not shadows

Zoom in far enough (high-resolution TEM), and the picture is formed by interference between the direct beam and diffracted beams. The lattice "fringes" you see are an interference pattern, and their contrast depends on defocus. Change focus slightly and atomic columns can flip from dark to bright, or vanish entirely. An HRTEM image is a fingerprint of the lattice, not a portrait of it.

Simulated lattice image of the same crystal at different defocus values.

Defocus Δf:

Try it: sweep the defocus through zero. Watch contrast fade to almost nothing, then reappear reversed. This is why "are the atoms the black dots or the white dots?" has no universal answer: it depends on defocus (and on the aberrations of your particular microscope; an image corrector like the one on a Spectra 300 tames them but doesn't remove the physics).

Which contrast am I looking at?

A practical field guide: if the feature persists when you tilt and scales with how thick or heavy the region is, it's mass–thickness. If it moves, blinks, or sweeps when you tilt (grains flickering, contours crawling), it's diffraction contrast. If it's fine periodic fringes at high magnification that change character when you touch focus, it's phase contrast. When in doubt at the microscope: wobble the tilt and wobble the focus, and see what responds.

Key takeaways

Dark in a TEM image means "electrons were removed from the direct beam here": by absorption-like scattering, by diffraction, or by interference.
Mass–thickness contrast is the steady one when it dominates (amorphous or weakly diffracting material): heavier/thicker → darker in bright field. In a strongly diffracting crystal, diffraction can override it.
Diffraction contrast depends on crystal orientation; a region feeding the selected reflection typically shows up dark in BF and bright in that DF (not a strict pixel-for-pixel inversion), and the contrast shifts as you tilt.
Phase contrast is an interference effect: fringe contrast and even its sign depend on defocus and aberrations.
The tilt-and-focus wobble test tells you which mechanism you're seeing before you interpret anything.

For the physics-curious: a little more depth

Mass–thickness: a simplified attenuation model. Intensity follows roughly I = I₀e^(−t/Λ), where the mean free path Λ shrinks with atomic number (elastic scattering cross-section rises steeply with Z), hence darker for thick or heavy. Treat it as a first-order trend for amorphous or weakly diffracting material, not an image-formation law for crystalline foils.

Diffraction: in a two-beam condition the direct and diffracted intensities trade off as I_g = sin²(πt s_eff)/(ξ_g s_eff)²-type expressions, with extinction distance ξ_g and excitation error s. Thickness fringes are the sin² oscillation in t; bend contours are the locus of s ≈ 0 in a curved foil.

Phase: the lens transfers each spatial frequency g with a contrast transfer function CTF(g) = sinχ(g), χ(g) = πλΔf g² + ½πC_sλ³g⁴. (Defocus sign conventions differ between texts and software; this site takes underfocus as negative Δf.) When sinχ crosses zero, that spacing disappears from the image; when it changes sign, contrast reverses. The demo above literally multiplies the lattice by sinχ.

Cite this page: Tripathy, Manisha. “The Three Contrasts of TEM.” untethered atom, 2026, https://untetheredatom.com/tem/tem-contrast-guide.

BibTeX

@misc{tripathy2026thethreecontrastsoftem,
  author = {Tripathy, Manisha},
  title  = {The Three Contrasts of TEM},
  year   = {2026},
  howpublished = {\url{https://untetheredatom.com/tem/tem-contrast-guide}},
  note   = {Interactive teaching resource}
}

Last updated 25 August 2026.