Image caption The famed fresco on the surface was a known fake by the collector or one of his proteges

A technique based on the same kind of technology used in airport scanners has revealed images beneath a fresco held at the Louvre museum in Paris.

Trois Hommes Armes de Lances was known to be a fresco forged by Giampetro Campana on a wall from Roman times.

The new research suggests that under that forgery lies a real Roman fresco.

The discovery was announced at the American Chemical Society meeting by Bianca Jackson of the University of Rochester in the US.

Terahertz waves are known for their ability to penetrate materials without damaging them, and have in recent years been added to the suite of tools used to examine items of cultural heritage.

These tools span much of the electromagnetic spectrum from X-rays to ultraviolet to the infrared - and of course microscopy with visible light.

Safe application

Terahertz light - which lies between infrared light like that used by remote controls and the microwaves in the appliance of the same name - has become popular in scanning technology at airports and museums' back rooms because it can extract information without risk of damage.

"It's very desirable for cultural heritage conservation because with a lot of other techniques like X-ray or ultraviolet, there is some molecular breakdown in the materials," Dr Jackson told the meeting.

"So even though you're using the equipment to get information to conserve it, you're at the same time risking some deterioration of the object."

Giampetro Campana was a renowned collector from the mid-19th Century who specialised in Roman artefacts. But late in his career he took to restoring - or outright creating - Roman-style works and passing them off as genuine.

Previous studies of Trois Hommes Armes de Lances had used X-ray fluorescence - which yields a list of all the atoms within an object - but showed that there were atoms present in the work that were not present on the surface.

Dr Jackson and her colleagues were called in to apply terahertz imaging to find out what lay beneath.

"After quite a bit of data processing, we were able to pull out some signs that there is a figure beneath… what looked like two eyes, eyebrows, nose, mouth, a shadow for a chin," she said.

It remains to be proven that the image beneath is of Roman origin, but the collector's history seems to suggest it. The original may simply have been of poor quality.

"If you go on Ebay and you can get a Roman coin from 200BC for 25 cents, there's a reason - it's not high quality," Dr Jackson told BBC News.

"So he probably painted over it because he could get more credit if he had a nice painting."