Theoretical and practical considerations affecting high resolution HVEM of organic materials

Radiation damage, which results from inelastic electron scattering and ionization, is perhaps the most important obstacle at the present time for attaining high resolution electron micrographs with biological specimens. The choice of accelerating voltage influences the radiation damage problem in a number of different ways. The best known effect is that the specimen lifetime increases with increasing voltage. The voltage dependence of the image detector sensitivity in also an easily recognized problem. Perhaps the most important factor is the voltage dependence of image contrast. The voltage dependence of contrast is also, by far, the most complicated matter to consider since contrast considerations depend also upon the specimen thickness, the type of object, the optical conditions employed, the instrumental resolving power, and the question of whether the image can be interpreted in terms of the original object structure