A means by which a patentee may raise a claim of infringement even though each and every element of the patented invention is not identically present in the allegedly infringing product. The purpose of the doctrine is to prevent an infringer from stealing the benefit of a patented invention by changing only minor or insubstantial details of the claimed invention while retaining the same functionality. The essential inquiry in determining equivalency is whether the accused product or process contains elements identical or equivalent to each claimed element of the patented invention.
In Graver Tank & Mfg. Co. v. Linde Air Prods., Inc., the U.S. Supreme Court held that a patentee may invoke this doctrine to proceed against the producer of a device if it performs substantially the same function in substantially the same way to obtain the same result. See Graver Tank, 339 U.S. 605 (1950). This is often referred to as the Graver Tank "triple identity" test for equivalence.
The Supreme Court enunciated the "all elements" test for equivalence in Warner-Jenkinson v. Hilton Davis Chemical Co., 520 U.S. 17 (1997). Under the "all elements" rule, the doctrine of equivalents must be applied to each individual element of a claim, not to the invention as a whole. It is necessary to show that every element of the patented invention, or its substantial equivalent, is present in the accused product or process.
To determine whether a substitute element is the substantial equivalent of an element in the patented invention, it is necessary to analyze the role played by each element in the context of the specific patent claim. To ba a substantial equivalent, the substitute element must match the function, way, and result of the claimed element. A missing claim element is considered equivalently present if only insubstantial differences distinguish that element from the corresponding aspects present in the accused device or process. See Id.
The proper time for evaluating whether an accused element is equivalent to a claimed element is at the time of infringement, not at the time the patent was issued. Subsequent changes in the state of the art, such as technologies developed after the date of invention, will often challenge the definition of a previously essential element or limitation present in the original invention. See Sage Products, Inc. v. Devon Industries, Inc., 126 F.3d 1420 (Fed. Cir. 1997).