A selected area aperture is inserted into the back focal plane of the objective lens to select the required beam. When the direct beam is selected, a bright field image is formed, and when the diffracted beam is selected, a dark field image is formed. Bright field image The given micrograph shows a bright field image of a MgO crystal. The crystal specimen appears dark with a bright background. The background appears bright because only the direct beam of transmitted electrons is selected and let to pass through the aperture. The surface topology and the raised texture on top of the crystal are clearly observable. This kind of image is obtained by placing the objective diaphragm or the selected area aperture in the back focal plane of the objective lens. The aperture allows only direct beam to pass through while blocking the diffracted beam. The direct beam appears as a bright central spot. The aperture also maintains the collection angle. As seen in the ray diagram below, the objective aperture blocks the diffracted beam, allowing only the transmitted beam to reach the image plane. Darkfield image The given micrograph shows a dark field image of a MgO crystal. The crystal specimen appears lighter than the background. The background is dark. The edges of the crystal are highly pronounced.
In case of the dark field imaging, also called as the central dark field operation, the selected area aperture is not shifted, but the incident beam is tilted to allow the scattered electrons in the diffracted beam to pass through the objective aperture. A collective ray diagram for both bright field and dark field imaging is given below: Selected Area Diffraction Pattern The given micrograph shows the selected area diffraction pattern of a MgO crystal. The lattice structure of the crystal is easily decipherable from the given SAED pattern. Diffraction from a single crystal in a polycrystalline sample can be captured if the aperture is small enough and the crystal is large enough. To obtain such a pattern, the selected area aperture is placed in the image plane of the objective lens and used to select only one part of the image. Using projector lenses to focus on electron beams to obtain small spots on the object surface, the diffraction patterns can be obtained. Using this pattern, the lattice of crystals can be easily studied and it is also possible to determine the orientation relationships between grains or even different phases. 2. (a) From a lattice image obtained from a single crystal of BaZrO3 (Fig. 4) determine the magnification. Compare this with the magnification obtained using the scale bar. Calculate the length the scale bar should be. To calculate the magnification from the lattice image given, the following formula for magnification is used: Magnification (M) = A stepwise solution for the given problem is presented below: Step 1 Calculation of pixel to cm ratio: The size of the image is measured in pixels and centimeters and found to be: Height in pixels= 556 pixels= 14.7cm