What is the "good faith exception" to the exclusionary rule of search warrants?

The "good faith exception" exists in recognition of the underlying purpose of the exclusionary rule. " 'If the purpose . . . is to deter unlawful police conduct, the evidence obtained from a search should be suppressed only if it can be said that the law enforcement officer had knowledge, or may properly be charged with knowledge, that the search was unconstitutional under the Fourth Amendment.' " (United States v. Leon (1984) 468 U.S. 897, 919 (Leon).) "This is particularly true . . . when an officer acting with objective good faith has obtained a search warrant from a judge or magistrate and acted within its scope. In most such cases, there is no police illegality and thus nothing to deter. It is the magistrate's responsibility to determine whether the officer's allegations establish probable cause and, if so, to issue a warrant comporting in form with the requirements of the Fourth Amendment. In the ordinary case, an officer cannot be expected to question the magistrate's probable-cause determination or his judgment that the form of the warrant is technically sufficient." (Id. at pp. 920-921, fn. omitted.)