function OpenWin(mypage,myname,w,h,features) {
     if(screen.width){
     var winl = (screen.width-w)/2;
     var wint = (screen.height-h)/2;
     }else{winl = 0;wint =0;}
     if (winl < 0) winl = 0;
     if (wint < 0) wint = 0;
     var settings = 'height=' + h + ',';
     settings += 'width=' + w + ',';
     settings += 'top=' + wint + ',';
     settings += 'left=' + winl + ',';
     settings += features;
     win = window.open(mypage,myname,settings);
//             win = window.open(mypage,myname,"");
     win.window.focus();

  }

function checkIfEmpty(field) {
	if (document.getElementById(field).value != "") {
   		return true;
	} else {
   		alert(field + " is verplicht");
	    document.getElementById(field).focus();
	   return false;
 	}
}