...what every true artist wants, really wants, is to be paid. Terry Pratchett, 1995 (Soul Music)
The Scanning Electron Microscope
How does a scanning electron microscope, or SEM, work? Very much like an older, tube-type television set.
The ‘scanning’ in an SEM refers to an electron beam, which is swept from side to side, line by line, until it has covered a rectangular area. Then it does the same thing again, time after time. This same process is used in the television tube to form an image, line by line, again and again, very quickly---the entire rectangular tube surface is scanned 30 times a second. The image on the television screen is formed as the electron beam shifts intensity very rapidly, to make points on the screen bright, dark, or in between (color television is more complicated).
In the SEM the electron beam stays the same brightness as it scans across an object, and an electron detector---off to one side---records electrons that reach it indirectly. The electrons of the beam are mostly absorbed by the part of the object being scanned, but a few ‘bounce’ off in all directions. In general, the detector ‘sees’ more reflected electrons from surfaces tilted toward it, and very few from surfaces tilted away, or at the bottom of deep holes. The number of electrons reaching the detector at any moment is then translated into a measure of brightness.
We see the recorded image in a simple television tube, whose scan has been synchronized to the scan in the microscope. Such images appear to be formed by light coming from the direction of the detector (relative to the object), as surfaces on the object facing the detector will be brightest, and deep holes and cracks the darkest. Flat, polished objects will not yield good SEM images by this method (there are other methods); an irregular surface works best.
The great power of the SEM lies in the ability to change the size of the area being scanned, from as large as a kernel of corn to as small as a single bacterium. This is similar to the ability of a broadcast television camera to zoom from an entire side of a football stadium to a single face in the crowd, but of course at a very different range of sizes.
The range of sizes dealt with by the SEM includes areas smaller than the wavelengths of visible light, so we are dealing with a world without color. And even for the larger areas, the electrons reaching the detector carry no information regarding color, so the images formed are always grayscale, or ‘black & white’.
Modern technology has produced even more powerful microscopes, such as the atomic force microscope, which can image the separate atoms---or at least their energy fields---on a very flat surface. But it seems unlikely that we will develop anything soon that will produce images as fascinating, and as artistic, as those made possible by the scanning electron microscope.
