  Let $\Phi_1$ and $\Phi_2$ be nondecreasing functions from
$\mathbb{R_+}=(0,\infty)$ onto itself. For $i=1,2$ and $\gamma \in \mathbb{R}$,
define the Orlicz class $L_{\Phi_{i}}(\mathbb{R_+})$ to be the set of
Lebesgue-measurable functions $f$ on $\mathbb{R_+}$ such that \begin{equation*}
\int_{\mathbb{R_+}} \Phi_{i} \left( k|(Tf)(t)| \right) t^{\gamma}dt < \infty
\end{equation*} for some $k>0$.
  Our goal in this paper is to find conditions on $\Phi_1$, $\Phi_2$, $\gamma$
and an operator $T$ so that the assertions \begin{equation} T :
L_{\Phi_2,t^{\gamma}}(\mathbb{R_+}) \rightarrow
L_{\Phi_1,t^{\gamma}}(\mathbb{R_+}), \tag{I} \end{equation} and
\begin{equation}\label{modularA} \int_{\mathbb{R_+}} \Phi_1 \left( |(Tf)(t)|
\right)t^{\gamma}dt \leq K \int_{\mathbb{R_+}} \Phi_2 \left( K|f(s)|
\right)s^{\gamma}ds, \tag{M} \end{equation} in which $K>0$ is independent of
$f$, say, simple on $\mathbb{R_+}$, are equivalent and to then find necessary
and sufficient conditions in order that (\ref{modularA}) holds.
