\begin{figure}[htp]
\centering
\includegraphics[width=90mm]{Interference}
\caption{Contour Plot of Substrate}
\label{Ref}
\end{figure}
\begin{figure}[htp]
\centering
\includegraphics[width=90mm]{Interference2}
\caption{3D Plot of Substrate}
\label{Ref}
\end{figure}
And for comparison from the Lloyd's Mirror Powerpoint
\begin{figure}[htp]
\centering
\includegraphics[width=90mm]{Lloyd}
\caption{Llyod's Miror}
\label{Ref}
\end{figure}