What is the test for instructing a jury in a criminal case regarding affirmative defense?

If substantial evidence supports an affirmative defense, a criminal defendant has a right to have the jury instructed regarding that defense. (People v. Salas (2006) 37 Cal.4th 967, 982.) "In determining whether the evidence is sufficient to warrant a jury 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.)