function popupWindow( window_name, pop_width, pop_height, page_url ) {
  var win_width = pop_width + 40;
  var win_height = pop_height + 90;
  var win_top = ( screen.height - win_height ) / 2 - 4; // window borders approx.
  var win_left = ( screen.width - win_width ) / 2 - 4;
  win = window.open( page_url, window_name, "toolbar=no, directories=no, location=no, status=yes, menubar=no, resizable=yes, scrollbars=no, width=" + win_width + ", height=" + win_height + ", left=" + win_left + ", top=" + win_top );
  
  win.document.close();
  
  return false;
}