What is the legal test for instructing a jury on a defense?

A trial court has a sua sponte duty to instruct on a defense if it is supported by substantial evidence and it is not inconsistent with the defendant's theory of the case. (People v. Salas (2006) 37 Cal.4th 967, 982.) In deciding whether there is substantial evidence to support the instruction, "the trial court does not determine the credibility of the defense evidence, but only whether 'there was evidence which, if believed by the jury, was sufficient to raise a reasonable doubt....' " (Ibid.)