The in camera proceeding is to be only a preliminary inquiry into the question of disclosure. As the court explained in People v. Superior Court (1971) 19 Cal. App. 3d 522 [97 Cal. Rptr. 118], the in camera hearing provided by section 915, subdivision (b) "offers the judge a guarded look into the government's secrets as a prelude to a more extended inquiry. Although it may furnish a fairly accurate measure of the government's claim, it supplies no more than a tentative, strictly provisional notion of the defendant's need. It does not place the court in readiness to rule on the claim of privilege. The court should continue its inquiry in an adversary setting, probing the information's relevance to the defense, exploring with counsel the availability of other alternatives and, if necessary, hearing testimony voir dire. Only at the conclusion of an adversary inquiry is the court in a position to assess the counter-balancing weight of the defendant's need, to appraise the possibility of reasonable alternatives and to determine what cost shall be exacted of the prosecution. Only at the conclusion of an adversary inquiry is the court qualified to rule for or against the government's claim of privilege." (19 Cal. App. 3d at p. 531; see also 1042, subd. (d).)