What is the role of an expert in a court system consistent with Professional standards (codes of ethics)?

Role of an Expert in a Court System

Experts must earnestly try to be impartial in identifying and interpreting complicated data thrown up by the complexity of modern science and  technology  to  help  the courts

Ideally, if courts pay the expert witness, the expert will become totally unbiased.

But it is a very costly issue parties to the dispute are called upon to pay and hire them on both sides and also allow them to be cross examined by both sides