Closed-Circuit Television in Electron Microscopy

Closed-circuit television adapts well to the educational aspects of electron microscopy in terms of extending immediate student participation. The advantages and some examples of the dual role in education and research of such a system are presented. A research electron microscope is invariably called upon to flex its educational muscles. One which does so regularly is operated by the Iowa State University Engineering Research Institute. Undergraduate and graduate courses in electron microscopy of inorganic materials are offered on a regular basis by the Department of Ceramic Engineering at Iowa State University. An electron microscope suffers from the same educational drawback as its light counterpart; namely, direct viewing of the electron image on the fluorescent screen is limited to a few observers. This situation ideally lends itself to the use of closed-circuit television. Experience has shown that research dividends are also realized. The microscope at Iowa State, a Siemens ELMISKOP I, was purchased in 1962 and has been updated with accessories from time to time. A part of the electron microscope fluorescent screen is imaged through suitable light optics onto the target of a Vidicon. This image is converted into an electrical signal, amplified, and reconstructed into an optical image on a television screen. The light optics consist of a tandem lens, which includes a 70 mm f 1.4 Heligon and a 140 mm f 2.3 Lumar. The Heligon f stop is variable from 1.4 to 8.0. Tandem optics are preferable to single optics since the light intensity falling on the Vidicon plate is increased by a factor of 2.25. The Vidicon camera tube, also known in Europe as a Resistron, is extremely sensitive to light and can pick up objects under conditions where a lm."IDeter held facing the object would register about 30 lux. In low light applications the Vidicon can have a better signal-to-noise ratio than an Orthicon. A tremendous advantage of the Vidicon is that it does not retain the optical image even after the object has been removed. The tandem lens and Vidicon tube are housed in a conveniently small unit which also contains the associated deflection circuit, the video preamplifier and a transistorized combined protection and blanking circuit. The complete camera circuitry is transistorized except the amplifier input. A separate control unit generates and shapes the required waveforms, amplifies the video signal from the camera, and mixes it 1 Department of Ceramic Engineering and Engineering Research Institute, Iowa State University, Ames, Iowa.