What constitutes infringement of a patent?

To decide whether an inventor is violating (infringing) a patent, it is necessary to carefully examine the patent's "claims." (Claims are terse statements of the scope of the invention, and most patents contain more than one of them.) The elements of each claim must be compared with the elements of the accused infringer's invention (usually a device or process).

If the elements of a patent claim match the elements of the device or process (called "reading on" or "teaching" the device or process), an infringement has occurred. Even if the claims don't literally match the elements of the infringing device, it is possible that a court would find an infringement by applying what's known as the "doctrine of equivalents"; that is, the patented invention and the allegedly infringing device or process are sufficiently equivalent in what they do and how they do it to warrant a finding of infringement.

For example, Steve invents a tennis racket with a score keeper embedded in the racket handle's end. The invention is claimed as a tennis racket handle that combines grasping and score keeping functions. Steve receives a patent on this invention. Later, Megan invents and sells a tennis racket with a transparent handle that provides a more sophisticated score keeping device than Steve's racket. Even though Megan's invention improves on Steve's invention in certain respects, it will most likely be held to be an infringement of Steve's invention, for one of two reasons:

• Megan's invention teaches the same elements as those claimed in Steve's patent (a tennis racket handle with two functions) or
• When considering what it is and how it works, Megan's invention is the substantial equivalent of Steve's invention (using the doctrine of equivalents).

In 2002, the Supreme Court handed down a ruling that makes it harder for patent owners who amended their patent claims to assert that others are infringing their patent. In essence, patent owners can use the doctrine of equivalents only if they can show that their amendment did not "surrender" the equivalents at issue.

Here are the details. In 2000, a federal appeals court ruled that a patent owner could not assert any element of a patent claim in an infringement lawsuit if that element was amended (including voluntary amendments) during the patent prosecution process. In other words, the appeals court ruling, if upheld, would have barred the use of the doctrine of equivalents for any amended patent claim. In 2002, the U.S. Supreme Court struck down this absolute bar to the doctrine of equivalents and replaced it with a less arbitrary standard. Under the Supreme Court's standard, all amended claims are presumed to be narrowed so as to bar the doctrine of equivalents. But this presumption can be rebutted if a patent owner can demonstrate that the amendment did not surrender the equivalent at issue. In that case, the patent owner can use the doctrine of equivalents. In summary, patent owners who amended their claims prior to or after the Supreme Court's decision can still use the doctrine of equivalents if they can overcome the presumption that the amendment surrendered the equivalents at issue. Festo Corp. v. Shoketsu Kinzoku Kabushiki Co. Ltd., 535 U.S. 722 (2002).

If a case is brought to enforce a patent on a method or process invention, the defendant can escape liability if the defendant was using the invention more than one year prior to the patent application date.

What remedies are available for patent infringement?

A patent owner may enforce his patent by bringing a patent infringement action (lawsuit) in federal court against anyone who uses his invention without permission. If the lawsuit is successful, the court will take one of two approaches. It may issue a court order (called an injunction) preventing the infringer from any further use or sale of the infringing device, and award damages to the patent owner. Or, the court may work with the parties to hammer out an agreement under which the infringing party will pay the patent owner royalties in exchange for permission to use the infringing device.

Bringing a patent infringement action can be tricky, because it is possible for the alleged infringer to defend herself by proving to the court that the patent is really invalid (most often by showing that the U.S. Patent and Trademark Office (USPTO) made a mistake in issuing the patent in the first place). In a substantial number of patent infringement cases, the patent is found invalid and the lawsuit dismissed, leaving the patent owner in a worse position than before the lawsuit.