\documentclass{minimal}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}[scale=2]
    \pgfmathsetseed{5}

  \foreach \i in {1,2,...,150}{
    \pgfmathsetmacro{\x}{(rand*0.5 + 1)*4 - 4.0}
    \pgfmathsetmacro{\y}{(rand*0.5 + 1)*4 - 4.0}
    \pgfmathsetmacro{\opacVal}{rand*0+1}
    \shade [ball color = green, opacity = \opacVal] (\x,\y) circle (0.05);
  }

  \draw (0,0) circle [radius=2.75];

\end{tikzpicture}
\end{document}
