 The most direct experimental technique to resolve the electronic structure of crystals in the momentum-energy space (see reciprocal lattice), and, consequently, the Fermi surface, is the angle resolved photoemission spectroscopy (ARPES).An example of the Fermi surface of superconducting cuprates measured by ARPES is shown in the figure 3.With positron annihilation it is also possible to determine the Fermi surface as the annihilation process conserves the momentum of the initial particle.