  We show that the isotropic discrepancy of a lattice point set can be bounded
from below and from above in terms of the spectral test of the corresponding
integration lattice. From this we deduce that the isotropic discrepancy of any
$N$-element lattice point set in $[0,1)^d$ is at least of order $N^{-1/d}$.
This order of magnitude is best possible for lattice point sets in dimension
$d$.
