Everyone knows holograms from their everyday life, for instance the ones
applied to credit cards as security indicators. Unlike a photography of an object
which only records the amplitude of the light wave coming from the object, the
hologram also includes local information about the light wave's phase.
In appropriate lighting, the initial wave front is reconstructed in proper phase
and the spectator has a three dimensional sensation of the object.
But it is not this characteristic of holography that is central when it comes
to the imaging of small structures, but the fact that for the recording of a
hologram no lenses are required at all. In order to conduct research on nanometer
scaled objects, light of an equally small wave length is needed (soft X-rays).
The only lenses working in this wave spectrum (so-called Fresnel zone plates)
are very sophisticated in design and still yield a quality of imaging one scale
inferior then lenses for visible light. The modus operandi of recording holograms
without the use of lenses is to superimpose the light wave having radiated the
object at the time of recording with a reference wave having a known and stable
(coherent) phase.
