When an electron hits this coating, it causes the coating to glow. The image created on the plate is really a highly detailed shadow of the specimen that is in the beam. Below the viewing screen is a camera chamber, holding a sheet of film. In a light microscope, the image is seen in color, and contrast is achieved by staining the specimen with different dyes. In the TEM, since there is no light, and since the electron beam has only a single wavelength, color has no meaning.
Contrast is achieved instead by introducing variable density in the different parts of the specimen.
All shades of gray are represented by partial transmission. Although the theoretical principles were quite well understood more than 50 years ago, the realization of the SEM as a practical device required advances in electronics far beyond the frontiers of the early 1930's. The Second World War gave a considerable impetus to the development of related technology, as did the massive expansion of the television industry in the post-war period. Today they find their greatest use in the manufacture of microchips. The SEM is unmatchable in its utility for examining surface structure, which is why chip producers use them.