(Adds Limelight decision, detail on Nautilus, quotes from experts)
WASHINGTON, June 2 (Reuters) - The U.S. Supreme Court threw out two patent decisions on Monday, one related to heart rate monitors and the other on management of web images, rulings that could make it harder for patent trolls to win infringement cases.
The court tossed out an appeals court decision in a patent fight between exercise equipment maker Nautilus Inc and Biosig Instruments over heart rate monitors, a decision that raises the bar on how clearly patents must be written.
The Supreme Court also ruled in favor of Limelight Networks over claims by Akamai Technologies that it infringed on patented technology for managing web images and video. The decision overturned an appeals court decision that had made it easier to prove that a company is liable based on the induced infringement theory.
Both decisions were unanimous, and the cases were remanded for reconsideration.
In the Limelight decision, the justices said Limelight cannot be held liable for inducing patent infringement when no party has directly infringed on the patent in question.
The court, in an opinion by Justice Samuel Alito, reversed a August 2012 ruling by a divided U.S. Court of Appeals for the Federal Circuit in Akamai's favor, which effectively made it easier to prove that a company is liable based on the induced infringement theory.
Under induced infringement, one company may perform some of the steps of a patent while a second may perform other steps. With Monday's decision, the patent holder will have to show one company has some sort of control over the other, perhaps in the form of a contract, in order to show induced infringement, said Darryl Woo, a patent expert with Fenwick & West LLP.
In the Nautilus case, Biosig Instruments Inc had accused Nautilus of infringing on its patented technology for heart rate monitors built into fitness machines like treadmills. The U.S. Court of Appeals for the Federal Circuit had ruled that the patent was valid.
But the Supreme Court said the appeals court had set the bar too low in allowing patents to be written vaguely by allowing patents that were not "insolubly ambiguous."
"In place of the 'insolubly ambiguous' standard, we hold that a patent is invalid for indefiniteness if its claims, read in light of the specification delineating the patent, and the prosecution history, fail to inform, with reasonable certainty, those skilled in the art about the scope of the invention," the court said.
"Historically speaking, the patents that are often asserted by patent trolls do tend to have ambiguous claims," said James Barney, a patent expert with Finnegan, Henderson, Farabow, Garrett & Dunner, LLP.
"This case will have an impact on those types of claims. Claims that did not have indefiniteness problems before probably will not have them now," Barney said.
Companies that hold patents only for the purpose of suing other companies seeking to develop new patents are considered patent trolls.
The cases at the Supreme Court are Nautilus v Biosig Instruments, Inc, No. 13-369 and Limelight Networks, Inc v Akamai Technologies, Inc, No 12-786.
(Reporting by Diane Bartz and Lawrence Hurley; Editing by Ros Krasny, Doina Chiacu and Leslie Adler)