Rule 36.
A witness has a privilege to refuse to disclose the identity of a person who has furnished information purporting to disclose a violation of a provision of the laws of this State or of the United States to a representative of the State or the United States or a governmental division thereof, charged with the duty of enforcing that provision, and evidence thereof is inadmissible, unless the judge finds that (a) the identity of the person furnishing the information has already been otherwise disclosed or (b) disclosure of his identity is essential to assure a fair determination of the issues.