A key towards understanding the properties of high-temperature superconductors is knowledge about the distribution of the valence electrons and holes. Despite their complicated crystal structure, electronic structure of many cuprates has been calculated. The most successful calculation method is based on the theorem that the ground state energy is a unique functional of the electron density. Thus, experimental techniques that give information about the valence electron density are essential to test the calculations and to gain further insight into these important materials.

To move beyond locating the atoms in the crystal unit cell towards addressing the details of the electron distribution requires in general extremely accurate diffraction experiments. Highly perfect crystals with small unit cells have been approached. It has been known for several decades that in electron diffraction the amplitudes of those reflections at small scattering angles that are present in crystals with large unit cells, are strongly influenced by the valence electron distribution.