Coils of copper wire produce magnetic fields. These fields are shaped by the surrounding iron fixture into geometries producing focusing actions. By adjusting the currents supplied to the coils, the focal lengths of the lenses can be changed. A SEM electron beam scans the surface of sample line by line, from top to bottom, similar to the method by which an electron beam scans a tv phosphorus screen. Two sets of scan coils control the scanning of the beam across the sample.
Magnification is the ratio of the sample scan length to the display length on the CRT monitor.
By shortening the sample scan length, the magnification is increased. A sample can be continually magnified, however it is useless unless the image is in focus. For SEM, the magnification does not depend on the focal length or the strength of the lenses. The ASM handbook refers to the beam diameter on the sample as spot size and this term will be used for discussion in this lab. The smaller the spot size, the higher the resolution and the more likely the image will be in focus at high magnifications.