---
title: "TEM Bright Field, Dark Field & SAED Interactive Lab | Sandbox Physics"
description: "In a TEM, which aperture selects one beam and which selects one region of the specimen? To pick a beam, use the objective aperture: it sits in the back focal plane, where electrons are sorted by scattering angle. To pick a region, use the selected-area aperture: it sits in an image plane, where position matches position on the specimen. The intermediate lens then decides what the detector records: focused on the image plane, an image; on the back focal plane, a diffraction pattern."
canonical_url: "https://sandboxphysics.com/labs/tem-mode-selector/"
language: "en"
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last_modified: "2026-10-25"
---

E27 · Electron acquisition / diffraction

# TEM Bright Field, Dark Field & SAED

**In a TEM, which aperture selects one beam and which selects one region of the specimen?** To pick a beam, use the objective aperture: it sits in the back focal plane, where electrons are sorted by scattering angle. To pick a region, use the selected-area aperture: it sits in an image plane, where position matches position on the specimen. The intermediate lens then decides what the detector records: focused on the image plane, an image; on the back focal plane, a diffraction pattern.

The specimen has two grains with different orientations. Move the objective aperture across the back focal plane until one grain lights up in dark field. Then switch the relay lens to diffraction focus and move the selected-area aperture in the first image plane to isolate the diffraction pattern of that grain.

> Interactive simulation: open this page in a web browser to change the parameters and run the model.

Model assumptions

Tilting changes only how strongly the reflections are excited; the projected layout of the grains stays fixed. The beam is fixed at 200 keV, the lattice is monatomic simple cubic with a 0.4 nm parameter, and the two grain orientations differ by 35 degrees. Intensities use a weak single-scattering approximation for thicknesses of 1–5 nm, and the direct beam loses exactly the modeled diffracted fraction. The grain images add intensities incoherently, so phase interference and dynamical diffraction are left out. Each acquisition saves a seeded electron-count array together with its settings. Images are sampled at 1 nm per pixel, and the selected-area signal scales with the illuminated area and the fluence. The diffraction spot spread is set to 0.35 mrad and each detector pixel holds at most 65535 electrons; saturated pixels are marked orange. Counts are Poisson below a mean of 40 and use a rounded Gaussian approximation above it. Detector efficiency, charge sharing, read noise, beam damage and camera calibration are not modeled. Images are displayed with a gamma of 0.4, and SAED patterns on a log scale. Display gain changes brightness but not counts. Dark field is made by moving the objective aperture, not by tilting the incident beam. Ideal paraxial objective and relay lenses separate the focal and image planes; the drawn ray slopes are enlarged 22 times. In SAED the objective aperture is withdrawn automatically, and changing the selected region changes the recorded diffraction pattern.

Physics tutorial

## In a TEM, which aperture selects one beam and which selects one region of the specimen?

**Background** The specimen has two grains with different orientations. You move the objective aperture across the back focal plane until one grain lights up in dark field. Then you switch the relay lens to diffraction focus and move the selected-area aperture in the first image plane to isolate the diffraction pattern of that grain.

**Why it matters** Before you interpret the specimen, you need to know how the acquisition and the choice of aperture change the data.

Start with the essentials

- **Focus question**: In a TEM, which aperture selects one beam and which selects one region of the specimen?
- **One-sentence intuition**: The two apertures sit in different conjugate planes. The objective aperture, in the back focal plane, picks scattering angles; the selected-area aperture, in the image plane, picks positions on the specimen. The relay focus decides whether the detector records an image or a diffraction pattern.

### Core mathematical model

#### Angle and position separate in different planes

$$
y_{\mathrm{BFP}}=f_o\theta,\quad y_{\mathrm{image}}=-\frac vu y_0,\quad \frac1{f_o}=\frac1u+\frac1v
$$

In the ideal objective shown, the object and image distances are both 90 schematic units and the focal length is 45. The back focal plane sorts rays by angle, and the image plane sorts them by position.

#### Which waves the objective aperture passes

$$
I_{A}=P_0A(0)+\sum_g P_gA(\boldsymbol\theta_g),\quad P_0=1-\sum_gP_g
$$

In bright field the aperture passes the direct wave. In dark field the aperture is moved off axis to pass chosen diffracted waves. The formula predicts the mean intensity. Dividing the counts in an image region by the incident fluence estimates the passed fraction, and sampling noise can push that estimate above the prediction. Display gamma and gain do not affect the count statistics.

#### The area aperture chooses which region contributes

$$
I_{\mathrm{SAED}}(\boldsymbol\theta)=\sum_{j\in\{0,A,B\}}w_jI_j(\boldsymbol\theta),\quad \sum_jw_j=1
$$

The weights come from the specimen pixels that fall inside the projected area aperture. In diffraction mode the objective aperture is withdrawn, and the relay images the back focal plane with its coordinates inverted.

#### Area, counts and detector limits

$$
N_{\mathrm{in}}=\Phi_e A_s,\quad N_i\sim\operatorname{Poisson}(\mu_i),\quad C_i=\min(N_i,65535),\quad \frac{\sigma_N}{N}\simeq\frac1{\sqrt N}
$$

At a fixed fluence, a smaller selected area receives fewer electrons. The noise estimate assumes the counts are independent and unsaturated; clipped pixels lose information. The detector spread and capacity are set for teaching and do not describe a real camera.

### Common difficulties

#### Objective aperture versus selected-area aperture

**Typical misconception** The selected-area aperture and objective aperture are interchangeable.

**Better mental model** The two apertures sit in different conjugate planes. The objective aperture, in the back focal plane, picks scattering angles; the selected-area aperture, in the image plane, picks positions on the specimen. The relay focus decides whether the detector records an image or a diffraction pattern.

### Run the experiment

1. 01

   #### Select a wave

   Compare Bright field with Dark field: grain A and then Dark field: grain B. Watch the aperture move in the back focal plane.

   **What to observe:** The selected grain turns bright, and the other grain loses its diffracted intensity. The specimen itself has not changed.

2. 02

   #### Select a region

   Switch to SAED: grain A, then SAED: both grains. Move the area center toward grain B.

   **What to observe:** The image-plane selector changes which crystal contributes to the recorded diffraction pattern; it does not pick a single diffraction spot.

3. 03

   #### Check both targets

   Use Check target on the grain A dark-field task and on the grain A selected-area task. Then tilt the specimen away and compare how much intensity is lost.

   **What to observe:** Acquire at each setting before you check. Grain fractions and noise readouts come from the saved linear counts. The low-count and saturated examples fail even with the apertures in the right place, and raising the display gain fixes neither problem.

Physical conclusion

The two apertures sit in different conjugate planes. The objective aperture, in the back focal plane, picks scattering angles; the selected-area aperture, in the image plane, picks positions on the specimen. The relay focus decides whether the detector records an image or a diffraction pattern.

Starter path · Step 5 of 6

## From a field to an electron microscope

[Next step → **SEM Secondary-Electron Topography** Rotate the side detector around the same surface. Does a brighter region mean a taller one? Transfer · 20 min](https://sandboxphysics.com/labs/sem-topography/)

1. [1 Make an Electric Field Visible](https://sandboxphysics.com/labs/electric-field/)
2. [2 A Charge Bending through Fields](https://sandboxphysics.com/labs/charged-particle/)
3. [3 Electron Wavelength & Voltage](https://sandboxphysics.com/labs/electron-wavelength/)
4. [4 Electron Diffraction & Reciprocal Space](https://sandboxphysics.com/labs/electron-diffraction/)
5. [5 TEM Bright Field, Dark Field & SAED](https://sandboxphysics.com/labs/tem-mode-selector/)
6. [6 SEM Secondary-Electron Topography](https://sandboxphysics.com/labs/sem-topography/)

[View the full path →](https://sandboxphysics.com/topics/fields-and-matter/#starter-path)