Cal. EVID § 915

(a) Subject to subdivision (b), the presiding officer may not require disclosure of information claimed to be privileged under this division or attorney work product under subdivision (a) of Section 2018.030 of the Code of Civil Procedure in order to rule on the claim of privilege; provided, however, that in any hearing conducted pursuant to subdivision (c) of Section 1524 of the Penal Code in which a claim of privilege is made and the court determines that there is no other feasible means to rule on the validity of the claim other than to require disclosure, the court shall proceed in accordance with subdivision (b).

(b) When a court is ruling on a claim of privilege under Article 9 (commencing with Section 1040) of Chapter 4 (official information and identity of informer) or under Section 1060 (trade secret) or under subdivision (b) of Section 2018.030 of the Code of Civil Procedure (attorney work product) and is unable to do so without requiring disclosure of the information claimed to be privileged, the court may require the person from whom disclosure is sought or the person authorized to claim the privilege, or both, to disclose the information in chambers out of the presence and hearing of all persons except the person authorized to claim the privilege and any other persons as the person authorized to claim the privilege is willing to have present. If the judge determines that the information is privileged, neither the judge nor any other person may ever disclose, without the consent of a person authorized to permit disclosure, what was disclosed in the course of the proceedings in chambers.