Many of the TEM studies of radiation damage in crystalline materials have been directed toward illucidating the nature, number density, and size distributions of the primary structural defects resulting from the displacement of atoms from their normal lattice sites, i.e., "black spots." The "black spots" have been identified as either nonresolvable dislocation loops or planar clusters of self interstitials or vacancies by TEM techniques and diffraction contrast theories that are described in the book by Hirsch, et al. and the reviews by Ruhle and Eyre. An indispensable part of the studies of point defect clusters is the identification of their character (vacancy or interstitial).