function ouvrir_popup(lien) {  
  var width;
  var height;  
  if (screen.width <= 800)
  {
    width = 780;
    height = 530;
    var options="enubar=no,scrollbars=yes,statusbar=no";
    var top=(screen.height-height)/2-30;
    var left=(screen.width-width)/2;
  }
  else
  {
    width = 950;
    height = 700;
    var options="enubar=no,scrollbars=no,statusbar=no";
    var top=(screen.height-height)/2-30;
    var left=(screen.width-width)/2;
  }
  
  window.open(lien,"","top="+top+",left="+left+",width="+width+",height="+height+","+options);
}