What is the test for pre-trial access to unprivileged evidence in criminal discovery?

The exercise of these powers is consistent with "the fundamental proposition that [the accused] is entitled to a fair trial and an intelligent defense in light of all relevant and reasonably accessible information." (Pitchess v. Superior Court (1974) 11 Cal.3d 531, 535, 113 Cal.Rptr. 897, 522 P.2d 305.) As Chief Justice Traynor once noted, "in the absence of a countervailing showing by the prosecution that the information may be used for an improper purpose, discovery is available not merely in the discretion of the trial court, but as a matter of right." (Traynor, Ground Lost and Found in Criminal Discovery (1964) 39 N.Y.U.L.Rev. 228, 244 [hereafter Traynor ].) Thus, it is established that an accused is entitled to any " 'pretrial knowledge of any unprivileged evidence, or information that might lead to the discovery of evidence, if it appears reasonable that such knowledge will assist him in preparing his defense....' " (Ballard v. Superior Court (1966) 64 Cal.2d 159, 167, 49 Cal.Rptr. 302, 410 P.2d 838, quoting Traynor, op. cit. supra, 39 N.Y.U.L.Rev. at p. 244.)