The phrase "impartial judge" means someone who is fair and unbiased when making decisions or judgments. They do not favor one side over the other and consider all facts and evidence before coming to a conclusion. Full definition
Finally, we incur expenses exhibiting our best breed - stock before impartial judges, so we can make the best decision about which are the finest to carry on their lines. (beechcreekbostonterriers.com)
And the reason is simple: we tend to trust those whom we consider impartial judges. (petbusiness.com)
Critics are observing that Mr Cameron may not be the most impartial judge of the situation, after the PM made clear he believed Mr Hunt had not breached the ministerial code. (politics.co.uk)