Megrahi has been granted a second appeal against conviction
The man convicted of the Lockerbie bombing has called in an American psychologist to help clear his name.
Abdelbasset al Megrahi, 56, claims an identity parade line-up was unfair and has challenged the way photos were shown to potential witnesses.
Prof Steve Clark, of the University of California, is said to be an expert on identification procedures.
At Edinburgh's Court of Criminal Appeal, Megrahi's defence team sought the right to show photos to Prof Clark.
Margaret Scott QC said it was "unprecedented" that she should have to ask judges to order the handover of the original photos.
Appeal judges are expected to hear further legal argument on the question next month.
Megrahi is currently serving life in jail for the 1988 atrocity, in which 270 people died.
He lost one appeal against conviction but the Scottish Criminal Cases Review Commission, which investigates possible miscarriages of justice, told the courts to look at the case again.
Megrahi's second attempt to overturn his conviction has led to a number of legal wrangles - including a long-running row about whether defence lawyers should be allowed access to secret documents.