  Substituting $Eu$ by $Ca$ in ferromagnetic $EuB_6$ leads to a percolation
limited magnetic ordering. We present and discuss magneto-optical data of the
$Eu_{1-x}Ca_{x}B_6$ series, based on measurements of the reflectivity
$R(\omega)$ from the far infrared up to the ultraviolet, as a function of
temperature and magnetic field. Via the Kramers-Kronig transformation of
$R(\omega)$ we extract the complete absorption spectra of samples with
different values of $x$. The change of the spectral weight in the Drude
component by increasing the magnetic field agrees with a scenario based on the
double exchange model, and suggests a crossover from a ferromagnetic metal to a
ferromagnetic Anderson insulator upon increasing $Ca$-content at low
temperatures.
