The US Patent and Trademark Office is to re-examine an image compression patent which has raked in licence fees of $105m and sparked 30 active patent trials.

The patent in question is commonly referred to as the JPEG-patent because it covers the compression technology used in the hugely popular image format that is extensively used for digital images.

A re-evaluation was requested by the Public Patent Foundation, a not-for-profit legal group which aims to protect the public from harmful patent claims that are the result of unsound patent policy. 

The group has ties with activists in the open source movement and the anti-patent lobby.

It claims that the JPEG-patent should be invalidated because of prior art, a legal term indicating that somebody else invented the technology covered in the patent before the patent holder.

Patent holding company Forgent acquired the JPEG patents in 1997 from the original inventor, and started seeking licence fees from software developers and makers of portable devices.

Forgent has collected more than $105m from over 50 companies including Sony and Research In Motion.

A group of about 30 companies, including Microsoft, is fighting the patent claim in a California court case.

If invalidated, the pending cases are likely to be dismissed, but Forgent would be allowed to keep the licence fees that it has already collected.

Forgent chief executive Richard Snyder said that he was confident that the patent would be upheld.

"We have not found any convincing arguments of invalidity, including the recent claims, and we are confident in the patent and look forward to an efficient re-examination."