The energy of the electrons in the TEM determines the relative degree of penetration of electrons in a specific sample, or alternatively, influence the thickness of material from which useful information may be obtained. Thus a 400 kV TEM not only provides the highest resolution but also allows for the observation of relatively thick samples (eg. less than 0.2 micrometers) when compared with the more conventional 100 kV or 200 kV instruments. Because of the high spatial resolution obtained, TEMs are often employed to determine the detailed crystallography of fine-grained, or rare, materials. Thus, for the physical and biological sciences, TEM is a complementary tool to conventional crystallographic methods such as X-ray diffraction.