Magento: Login required for Add Review

If you want to restriction for adding review than add below condition to form which is located at.
app\design\frontend\base\default\template\review\form.phtml
add below code to before form start

if ((boolean) $this->helper('customer')->isLoggedIn() )
{ ?>

add below code to after form end and before script start

else //customer is NOT logged in
{
        echo $this->__('To prevent SPAM we ask our customers to login before posting a review. Thank you for your understanding!'); 
        echo 'Please Sign In to post review ';
} 
?>