A determination of infringement by a court in the United States is a two-step process:

Literal infringement exists if an accused device falls directly within the scope of properly interpreted claims.

Even though a claim is not literally infringed under any section of 35 U.S.C. §271, infringement can still be found if the accused product and the patented product are substantial equivalents of each other.

Here, there is no literal infringement because the accused product does not meet each of the terms of the patent claim. Namely, the average particle size in the accused product is 1.1 µm, whereas the patent claim requires an average particle size of less than 1.0µm. However, a court could still find infringement if the accused product and the patented product are substantial equivalents of each other.

On the one hand, the claims must particularly point out and distinctly claim the invention (35 U.S.C. §112) so that the public has fair notice of what infringes and what is outside the scope of the claimed invention.

On the other hand, the doctrine of equivalents protects the patentee from competitors who appropriate the essence of the invention while barely avoiding the literal language of the claims.

The Supreme Court in the case of Graver Tank & Manufacturing Co. v. Linde Air Products Co., 85 USPQ 328 (1966) formulated a three-pronged test for equivalence. Infringement may be found if the accused product and the claimed invention:

Lack of one of these (must often the "way") was often found to remove the accused product from infringement.

The specification describes that the slipper sock keeps the shoe "in tune" with the wearer's foot during vigorous activity by "moving with the foot and stretching momentarily away from the shoe as necessary, but almost immediately bringing the shoe into total conformity with the foot". There is no literal infringement because the "inner sleeve" is attached to the upper and is therefore not "independent of the upper" as required by the claim. Even assuming that the slipper sock in the claimed shoe and the inner sleeve of the accused shoe perform the same function, namely, keeping the shoe "in tune" with the wearer's foot, there is no infringement under the doctrine of equivalents. This is because way in which the claimed shoe performs that function (by allowing the wearer's foot to move independently of the shoe's upper) is not substantially the same as the way in which the accused shoe performs that function (by securing the foot to the shoe's upper).

Recently, the Federal Circuit in the Hilton Davis decision announced the "insubstantial differences" test for infringement under the doctrine of equivalents. Namely,

Even though a claim may not be literally infringed, infringement can still be found under the doctrine of equivalents if the differences between the claimed and accused products are insubstantial. Often, the Graver Tank function-way-result test will suffice to show the extent of the differences.

(1) Known interchangeability of the accused and claimed elements by persons skilled in the art;

(2) whether there is evidence of intentional copying; and

(3) whether there is evidence of an attempt to design around the patent

may be relevant as to whether or not the differences are insubstantial.

Subsequent decisions of the Federal Circuit emphasized that equivalence must exist both for the claimed invention as a whole and for each individual claim element. Most recently, the Supreme Court held that the doctrine of equivalents must be applied on an element by element basis. The Court=s view is that by applying the doctrine of equivalents on an element by element basis, the patent is not enlarged beyond the scope of its claims (which inform the public as to what is covered and what is not covered by the patent). If a claim limitation or its substantial equivalent is not present, there can be no infringement. More particularly, the essential inquiry is as follows:

The specification teaches that the polymer serves as a binder, and that the wax overcomes the affinity of the polymer for the dye so that the dye can easily transfer from the toner to a copying surface. There was no literal infringement because the accused toner did not contain four distinct components as claimed. The accused toner also did not infringe under the doctrine of equivalents because the patent owner was unable to show that the accused toner contained a polymer or an equivalent thereof. The patent owner argued that the binding function of the polymer is performed by the wax in the accused toner. The court found that although the function (dyeing) and the result (dyed material) of the patented and accused toners were broadly the same, the way in which that function is performed was entirely distinct. Namely, in the patented invention, the polymer binds the dye and wax facilitates release of the dye from the polymer. However, there is no polymer binding action or release function by the wax in the accused toner. Thus, the release function of the wax is entirely absent from the accused toner, and the way in which the dyeing function is performed by the accused toner is not substantially the same as the way in which the dyeing function is performed by the claimed toner.

Infringement under the doctrine of equivalents does not require a showing that the alleged infringer acted in bad faith or pirated the invention.

The proper time frame for evaluating equivalency, and thus knowledge of interchangeability between elements, is at the time of infringement and not the time that the patent was issued.

The plaintiff has the burden of proving that the differences between the claimed invention and the accused product are insubstantial.

A finding of equivalence is a determination of fact that is made by the jury (the Hilton Davis decision). Proof may be in the form of expert testimony, by texts and treatises, and by the disclosure of the prior art. A final determination requires a balancing of the evidence.

Prosecution history estoppel prevents the patent owner from extending the range of equivalents so far as to resurrect subject matter given up during prosecution to obtain allowance of the patent.

For example, the Hilton Davis case concerned a filtering process for the purification of a dye solution. During prosecution, the patentee amended its claims to recite that the dye solution is filtered at a pH of from 6.0 to 9.0 to overcome a rejection over prior art which disclosed a similar filtering process operating at pH's above 9. The accused process (Warner-Jenkinson) operated at a pH of 5.0. There was no literal infringement. However, Hilton Davis charged infringement under the doctrine of equivalents.

Clearly, Hilton Davis surrendered pH's above 9 in order to obtain allowance of the patent.

Prior to the Supreme Court's decision in Hilton Davis, the Federal Circuit had consistently held that a close examination must be made as to not only what was surrendered, but also the reason for such a surrender. Thus, where a patentee's amendments were not required in response to an examiner's rejection over prior art or critical to allowance of the claims, no estoppel has been found.

Thus, in the above situation, the patent owner surrendered an operating pH range above 9. However, the Federal Circuit held that this did not bar Hilton Davis from asserting equivalency of the accused process operating at a pH below 6.

The trial court found that Warner-Jenkinson infringed under the doctrine of equivalents. The Federal Circuit affirmed, holding that substantial evidence supported the jury finding that the difference in pH was insubstantial.

On appeal, the Supreme Court reversed and remanded because Hilton Davis was unable to establish the reason for adding the lower pH limit. The Court then proceeded to add a new aspect to the infringement analysis under the doctrine of equivalents and the application of prosecution history estoppel as follows.

There is no infringement under the doctrine of equivalents if the asserted scope of equivalence would encompass (i.e., either read on or be obvious over) the prior art.

For example, suppose that a patent claim was directed to an alloy containing:

The prior art discloses the same Fe-base alloys but containing 10 to 20% Cr. In that case, the patent owner could not be accorded a range of equivalents that reached alloy compositions containing Cr in an amount of 10% or more. Otherwise, the claim would be invalid as reading on the prior art. However, the patent owner could be accorded at least some range of equivalents between a Cr content of something more than 6% to something less than 10%.

Suppose that the accused alloy contained Cr in an amount of 9%. The patent owner charges infringement under the doctrine of equivalents. The prior art at a Cr content of 10% would prevent the patent owner from extending the range of equivalents to encompass a Cr content of 9% if the alloy containing Cr in an amount of 9% would have been obvious over the prior art alloy containing Cr in an amount of 10%. Namely, the patent owner should not be able to obtain, under the doctrine of equivalents, coverage which he could not lawfully have obtained from the PTO with claims which literally cover the asserted range of equivalents.

In order to determine whether or not a patent owner may be accorded a range of equivalents wide enough to encompass the accused product in view of the prior art, the court will often employ what is called a "hypothetical claim analysis".

This is done by formulating a hypothetical claim similar to the patented claim, but just sufficient in scope to literally cover the accused product. The pertinent question then becomes whether the hypothetical claim would have been allowed by the PTO over the prior art (that is, unobvious over the prior art). If not, then it would be improper to permit the patentee to obtain that coverage in an infringement suit under the doctrine of equivalents. If the hypothetical claim could have been allowed, then prior art is not a bar to infringement under the doctrine of equivalents.

As applied to the above, the hypothetical claim might be:

If the above hypothetical claim could have been allowed over the prior art (Fe-based alloy containing Cr in an amount of from 10 to 20%), then prior art would not limit the doctrine of equivalents.

The judge interprets the patent claims based on the disclosure of the specification and the file history (intrinsic evidence). Extrinsic evidence is only considered if the meaning of the claim terms is ambiguous. Also, the outcome of infringement cases most often turns on the claim interpretation. Therefore, in order to allow the judge to properly interpret the patent, the claim terms should be clearly defined in the specification with examples.

It is now important to particularly point out in the record the reasons for any and all clarifying amendments or other claim amendments unrelated to patentability over the prior art (e.g., an amendment designed to remove a §112 indefiniteness rejection). In this manner, the patentee should be able to rebut any presumption that prosecution history estoppel should apply to the amended claim language or the new claim element.

As an example, suppose that the applicant claims:

During prosecution, the examiner cites a prior reference which teaches a paint composition containing TiO₂ in an amount of 50 wt% or more. The applicants specification recites that in a preferred embodiment, the paint composition contains TiO₂ in an amount of from 10 to 30 wt%.

The applicant then amends the claim as follows in order to distinguish over the prior art and obtain allowance.

If the applicant says nothing in the Remarks portion of the amendment, then the court is to presume that the amendment (both lower and upper limits) where made to distinguish over the prior art and that prosecution history estoppel is to apply. Namely, that the applicant "surrendered" paint compositions containing less than 10 wt% TiO₂ and more than 30 wt% TiO₂.

However, if the applicant in the record states that the lower limit of 10 wt% is in the claim to comply with the written description support requirement, then the applicant is not precluded from asserting equivalency of an accused infringer which manufactures a product containing less than 10 wt% TiO₂. Also, the applicant should state that the upper limit of 30 wt% was added to distinguish over the prior art which teaches 50 wt% or more, but that the applicant expects that the effects of the invention are also obtained for compositions containing in excess of 30 wt% TiO₂. In this manner, the applicant may be able to extend the range of equivalents to include compositions containing somewhat more than 30 wt% TiO₂ but something less than 50 wt% TiO₂.