The soft x-ray region covers wavelengths
longer than those of medical x rays, where
vacuum is required for propagation (A ^ 5 A).
Absorption of all materials increases with
increasing wavelength in this region, and the
absorption length of, the most transparent
specimens is around 1 pm for A « 50 A.
Conventional lens and mirror systems cannot
be built for soft x rays (due to absorption
in lenses and low reflectivity of normal incidence
mirrors), and only three types of imaging
systems remain possible for soft x rays:
grazing incidence reflectors, multilayer mirrors,
and diffractive elements such as zone
plates. Soft x-ray optics has advanced dramatically
during the last decade.