  We outline a general perturbative method of evaluating scattering features of
finite-range complex potentials and use it to examine complex perturbations of
a rectangular barrier potential. In optics, these correspond to modulated
refractive index profiles of the form $n(x)=n_0+f(x)$, where $n_0$ is real,
$f(x)$ is complex-valued, and $|f(x)|\ll1\leq n_0$. We give a comprehensive
description of the phenomenon of unidirectional invisibility for such media,
proving five general theorems on its realization in ${\cal PT}$-symmetric and
non-${\cal PT}$-symmetric material. In particular, we establish the
impossibility of unidirectional invisibility for ${\cal PT}$-symmetric samples
whose refractive index has a constant real part and show how a simple scaling
transformation of a unidirectionally invisible ${\cal PT}$-symmetric index
profile with $n_0=1$ may be used to generate a hierarchy of unidirectionally
invisible ${\cal PT}$-symmetric index profiles with $n_0>1$. The results
pertaining unidirectional invisibility for $n_0>1$ open up the way for the
experimental studies of this phenomenon in a variety of active material. As an
application of our general results, we show that a medium with $n(x)=n_0+\zeta
e^{iK x}$, $\zeta$ and $K$ real, and $|\zeta|\ll 1$ can support unidirectional
invisibility only for $n_0=1$. We then construct unidirectionally invisible
index profiles of the form $n(x)=n_0+\sum_\ell z_\ell e^{iK_\ell x}$, with
$z_\ell$ complex, $K_\ell$ real, $|z_\ell|\ll 1$, and $n_0>1$.
