var venster = '';
function popup(url,breedte,hoogte){
	if(!venster.closed && venster.location){
    	venster.close();
    }

   	var LeftPosition=(screen.width)?(screen.width-breedte)/2:100;
   	var TopPosition=(screen.height)?(screen.height-hoogte)/2:100;

	settings = 'toolbar=0, location=0, directories=0, statusbar=0, menubar=0, scrollbars=1, resizable=1, width='+breedte+', height='+hoogte+', top='+TopPosition+',left='+LeftPosition;

	venster = window.open(url, 'popupje', settings);
	venster.focus();
}