Police agencies must disclose to the prosecution, who must disclose to the defence, any exculpatory or impeachment evidence that demonstrates that a witness is lying about specific facts in a case, or is generally unworthy of belief. Evidence of this nature often is referred to as Brady material.
Explanation:
When a witness is suspected or is known by the defendant of lying and manipulating the facts of the case, he can request exculpatory evidence in the possession of the government to be presented in the court.
This evidence often serves in favour of the accused. It can not only clear the accused of the charges on him but can also raise questions on the integrity of the government's witness.