  The spectacular physical properties of quantum materials based on transition
metal, rare earth, and actinide elements continue to challenge our
comprehension of solid state physics and chemistry. The electronic states of
these materials are dominated by the $d$ and $f$ wave functions intertwined
with the strong band formation of the solid. In order to estimate which wave
functions contribute to the ground state formation, we have had to rely, until
now, on theoretical calculations combined with spectroscopy. Here we show that
$s$-core-level non-resonant inelastic x-ray scattering ($s$-NIXS) can directly
image the active orbital in real space, without the necessity of any modeling.
The power and accuracy of this new technique is shown using the text-book
example, x$^2$-y$^2$/3$z^2$-r$^2$ orbital of the Ni$^{2+}$ ion in NiO single
crystal.
