Case Name: HILTON DAVIS CHEMICAL CO., Plaintiff-Appellee, v. WARNER-JENKINSON COMPANY, INC., Defendant-Appellant
Court: United States Court of Appeals for the Federal Circuit
Jurisdiction: United States
Decision Date: 1995-08-08
Citations: 62 F.3d 1512
Docket Number: No. 93-1088
Parties: HILTON DAVIS CHEMICAL CO., Plaintiff-Appellee, v. WARNER-JENKINSON COMPANY, INC., Defendant-Appellant.
Judges: Before ARCHER, Chief Judge, RICH, Circuit Judge, COWEN, Senior Circuit Judge, NIES, NEWMAN, MAYER, MICHEL, PLAGER, LOURIE, CLEVENGER, RADER, and SCHALL, Circuit Judges.
Reporter: Federal Reporter 3d Series
Volume: 62
Pages: 1512–1583

Head Matter:
HILTON DAVIS CHEMICAL CO., Plaintiff-Appellee, v. WARNER-JENKINSON COMPANY, INC., Defendant-Appellant.
No. 93-1088.
United States Court of Appeals, Federal Circuit.
Aug. 8, 1995.
David E. Schmit, Frost & Jacobs, Cincinnati, OH, argued, for plaintiff-appellee. Of counsel were Martin J. Miller and Jayadeep R. Deshmukh.
J. Robert Chambers, Wood, Herron & Evans, Cincinnati, OH, argued, for defendant-appellant.
Roger W. Parkhurst, Parkhurst, Wendel & Rossi, Alexandria, VA, Harold C. Wegner, Wegner, Cantor, Mueller & Player, Washington, DC, Nancy J. Linck, Cushman, Darby & Cushman, Washington, DC, Kenneth E. Kro-sin, Lowe, Price, LeBlanc & Becker, Alexandria, VA, and Gary L. Newtson, President, American -Intellectual Property Law Ass’n, Arlington, VA, were on the brief, for amicus curiae, American Intellectual Property Law Ass’n.
Stanley L. Amberg, Davis Hoxie Faithfull & Hapgood, New York City, was on the brief, for amicus curiae, Stanley L. Amberg.
Donald Chisum, Morrison & Foerster, Seattle, WA, and William Alsup, Morrison & Foerster, San Francisco, CA, were on the brief, for amicus curiae, Acusón Corp.
R. Carl Moy, Asst. Professor, William Mitchell College of Law, St. Paul, MN, and Roy E. Hofer, President, Federal Circuit Bar Ass’n, of Washington, DC, were on the brief, for amicus curiae, Federal Circuit Bar Ass’n.
R. William Ide, III, President, American Bar Ass’n, Chicago, IL, was on the brief, for amicus curiae, American Bar Ass’n. Of counsel were Douglas W. Wyatt, Stanley L. Amberg, Tom Arnold, Frederick J. Dorchak, Donald R. Dunner, J. Michael Jakes, Joseph R. Re, William C. Rooklidge and Harrie R. Samaras.
Edmund J. Sease, Zarley, McKee, Thomte, Voorhees & Sease, Des Moines, IA, was on the brief, for amicus curiae, Iowa State Bar Ass’n.
Timothy B. Dyk, Jones, Day, Reavis & Pogue, Washington, DC, was on the brief, for amicus curiae, California Ass’n for the Advancement of Technology and Invention.
Michael F. Heim and Leslie V. Payne, Conley, Rose & Tayon, Houston, TX, were on the brief, for amicus curiae, The Houston Intellectual Property Law Ass’n. Also on the brief was Jeffrey W. Tayon, President, Houston Intellectual Property Law Ass’n, Houston, TX, of counsel.
Donald J. Harrington and Frank A. Angi-leri, Brooks & Kushman, P.C., Southfield, MI, were on the brief, for amicus curiae, Intellectual Property Law Institute.
Before ARCHER, Chief Judge, RICH, Circuit Judge, COWEN, Senior Circuit Judge, NIES, NEWMAN, MAYER, MICHEL, PLAGER, LOURIE, CLEVENGER, RADER, and SCHALL, Circuit Judges.
Circuit Judge Archer assumed the position of Chief Judge on March 18, 1994, replacing Circuit Judge Nies.
Circuit Judge Bryson, who entered on duty on October 7, 1994, has not participated in the disposition of this case.
Carver v. Hyde, 41 U.S. 513, 16 Peters, 513, 10 L.Ed. 1051; Brooks v. Fiske, 56 U.S. 212, 15 Howard, 212, 14 L.Ed. 665; Stimpson v. Railroad, 10 Id. 329; Prouty v. Draper, Ruggles & Co., 41 U.S. 336, 16 Peters, 341, 10 L.Ed. 985.

Opinion:
Opinion of the court filed PER CURIAM. Concurring opinion filed by Circuit Judge NEWMAN. Dissenting opinion filed by Circuit Judge PLAGER, in which Chief Judge ARCHER and Circuit Judges RICH and LOURIE join. Dissenting opinion filed by Circuit Judge LOURIE, in which Circuit Judges RICH and PLAGER join.
Dissenting opinion filed by Circuit Judge NIES, in which Chief Judge ARCHER joins in part.
PER CURIAM.
Hilton Davis Chemical Co. sued Warner-Jenkinson Co., Inc. for infringement of U.S. Patent No. 4,560,746 (the '746 patent). The jury found that the '746 patent was not invalid and that Warner-Jenkinson infringed under the doctrine of equivalents. The trial court entered judgment on the jury verdict. Hilton Davis Chem. Co. v. Warner-Jenkinson Co., No. C-1-91-218 (S.D.Ohio June 22, 1992). Because substantial evidence supports the jury verdict of infringement, the court en banc affirms.
BACKGROUND
Hilton Davis and Warner-Jenkinson manufacture dyes, including FD & C (food, drug, and cosmetic) Red Dye # 40 and Yellow Dye # 6. The process of making these dyes yields impurities as byproducts. Manufacturers must remove these impurities from the dyes to meet stringent governmental requirements for food and drug purity. Historically, Hilton Davis and Warner-Jenkin-son used an expensive and wasteful process known as "salting out" to purify the dyes. The '746 patent, assigned to Hilton Davis, discloses an improved purification process that replaces salting out with "ultrafiltration." Ultrafiltration uses osmosis to separate components of a solution by drawing some of the components, but not others, through a membrane. Thus, the '746 process filters impure dye solution through a membrane at certain pressures, pHs, and pore diameters. Impurities, but not dye molecules, pass through the membrane, leaving a high purity dye product.
Hilton Davis began its search for an alternative to the salting out process in 1982. The co-inventors of the '746 process, Drs. Cook and Rebhahn, led this project for Hilton Davis. The inventors decided to investigate a membrane separation process. Dr. Cook then hired Osmonies, Inc., a filtration equipment manufacturer, to test the process on a Hilton Davis Red Dye #40 solution which had been disclosed to Osmonies under a secrecy agreement. The first test, in August 1982, did not succeed. Dr. Cook then instructed Osmonies to perform a second Red Dye # 40 test with specified changes in the test membrane and filtration procedures. This second test, in October 1982, succeeded. Osmonies successfully purified Hilton Davis Yellow Dye # 6 under Dr. Cook's instructions in January 1983.
The inventors filed them initial patent application based on the October 1982 and January 1983 test results. After further in-house testing by Dr. Cook, the inventors filed a continuation-in-part application claiming a broader range of membrane pore sizes. The '746 patent issued in 1985.
The '746 patent claims a process for purifying commercial dyes including Red Dye # 40 and Yellow Dye # 6. Claim 1, the only independent claim at issue, appears in Jep-son form. See In re Jepson, 1917 C.D. 62, 243 O.G. 525 (Ass't Comm'r Patents 1917). Claim 1 recites:
In a process for the purification of a dye selected from [a group including Red Dye # 40 and Yellow Dye #6] . the improvement which comprises: subjecting an aqueous solution . to ultrafiltration through a membrane having a nominal pore diameter of 5-15 Angstroms under a hydrostatic pressure of approximately 200 to 100 p.s.ig., at a pH from approximately 6.0 to 9.0, to thereby cause separation of said impurities from said dye, said impurities of a molecular size smaller than the nominal pore diameter passing [through] said membrane and said dye remaining in the concentrate, and when substantially all said impurities have been removed from said concentrate . recovering said dye, in approximately 90% purity from said concentrate by evaporation of said concentrate to dryness.
(Emphasis added.) The inventors added the phrase "at a pH from approximately 6.0 to 9.0" during prosecution to distinguish U.S. Patent 4,189,380 to Booth et al. (the Booth patent). The Booth patent discloses an ul-trafiltration process that, among other differ.ences from the '746 process, operates at a pH above 9 and preferably between 11 and 13.
Warner-Jenkinson developed its accused ultrafiltration process for Red Dye # 40 and Yellow Dye # 6 in 1986. Like the '746 process, Warner-Jenkinson's accused process included ultrafiltration through a membrane. At trial, Hilton Davis showed that Warner-Jenkinson's process operated at pressures somewhere in a range of 200 to nearly 500 p.s.i.g. and a pH of 5. While Hilton Davis did not present actual pore size measurements for Warner-Jenkinson's membrane, several experts testified that a membrane collecting Red Dye # 40 and Yellow Dye # 6 would have a nominal pore diameter of 5 to 15 Angstroms.
In 1982, Warner-Jenkinson had tested a membrane separation process on a dye solution that had already been salted out. Warner-Jenkinson, like Hilton Davis, hired Os-monics under a secrecy agreement to perform its test. Osmonics performed the Warner-Jenkinson test in August 1982, one week before it performed the first Hilton Davis test. The Warner-Jenkinson test was not successful, however, because it did not produce a sufficiently pure dye. After the unsuccessful test, Warner-Jenkinson ceased work on filtration of Red Dye # 40 and Yellow Dye # 6 until 1986.
Warner-Jenkinson did not learn of the '746 patent until October 1986, after it had begun commercial use of its ultrafiltration process to purify Red Dye # 40. Hilton Davis learned of Warner-Jenkinson's process in 1989 and sued Warner-Jenkinson for patent infringement in 1991.
After considering extensive evidence offered over nine days, the jury found that the '746 patent was not invalid and that Warner-Jenkinson infringed under the doctrine of equivalents. The jury found that Warner-Jenkinson did not willfully infringe, however, and awarded only 20% of Hilton Davis' request in damages. The district court then denied Warner-Jenkinson's post-trial motions, and entered a permanent injunction prohibiting Warner-Jenkinson from practicing ultrafiltration except at pressures above 500 p.s.i.g. and pHs above 9.01.
Warner-Jenkinson appealed the infringement and validity findings. After a panel of this court heard oral argument on July 9, 1993, the court en banc decided to rehear the appeal to consider the important issues raised concerning the doctrine of equivalents. This court asked the parties to brief three questions:
1. Does a finding of infringement under the doctrine of equivalents require anything in addition to proof of the facts that there are the same or substantially the same (a) function, (b) way, and (c) result, the so-called triple identity test of Graver Tank [ & Manufacturing Co.] v. Linde Air Products Co., 339 U.S. 605 [70 S.Ct. 854, 94 L.Ed. 1097] (1950), and cases relied on therein? If yes, what?
2. Is the issue of infringement under the doctrine of equivalents an equitable remedy to be decided by the court, or is it, like literal infringement, an issue of fact to be submitted to the jury in a jury case?
3. Is application of the doctrine of equivalents by the trial court to find infringement of the patentee's right to exclude, when there is no literal infringement of the claim, discretionary in accordance with the circumstances of the case?
Oral argument directed to the en banc questions occurred on March 7, 1994.
DISCUSSION
I.
This case presents an opportunity to restate — not to revise — the test for infringement under the doctrine of equivalents. Courts have applied the doctrine of equivalents to protect the substance of the paten-tee's right to exclude since the first few decades after enactment of the Patent Act of 1790, ch. 7, 1 Stat. 109. Sitting as Circuit Justice, Justice Story, the leading intellectual property scholar of that era, stated:
Mere colorable differences, or slight improvements, cannot shake the right of the original inventor.
Odiorne v. Winkley, 18 F.Cas. 581, 582 (C.C.D.Mass.1814) (No. 10,432) (Story, C.J.). Indeed, the Supreme Court has consistently recognized the doctrine of equivalents as a protection for patent owners. See, e.g., Winans v. Denmead, 56 U.S. (15 How.) 330, 343, 14 L.Ed. 717 (1854); Sewall v. Jones, 91 U.S. 171, 184, 23 L.Ed. 275 (1875); Sanitary Refrigerator Co. v. Winters, 280 U.S. 30, 41-42, 50 S.Ct. 9, 12-13, 74 L.Ed. 147 (1929). Often the Supreme Court noted that an accused product or process, to avoid infringement under the doctrine, must include "substantial and not merely colorable" differences from the patent claims. Singer Mfg. Co. v. Cramer, 192 U.S. 265, 286, 24 S.Ct. 291, 299, 48 L.Ed. 437 (1904); see also McCormick v. Talcott, 61 U.S. (20 How.) 402, 405, 15 L.Ed. 930 (1858); Duff v. Sterling Pump Co., 107 U.S. 636, 639, 2 S.Ct. 487, 490, 27 L.Ed. 517 (1883).
The Supreme Court mapped the modern contours of the doctrine of equivalents in its landmark Graver Tank decision. Graver Tank & Mfg. Co. v. Linde Air Prods. Co., 339 U.S. 605, 70 S.Ct. 854, 94 L.Ed. 1097 (1950). In Graver Tank, the Court addressed whether Graver Tank's use of a particular electric welding flux infringed Linde Company's patent. The patent claimed "essentially a combination of alkaline earth metal silicate and calcium fluoride." Id. at 610, 70 S.Ct. at 857. Graver Tank's accused flux substituted "silicates of calcium and manganese — the latter not an alkaline earth metal- — for silicates of calcium and magnesium." Id. Because only exact duplicates literally infringe, id. at 607, 70 S.Ct. at 855-56, the Court recognized that infringement depended on the long-standing-doctrine of equivalents. Id. at 608, 70 S.Ct. at 856.
In explaining the bases for the doctrine, the Supreme Court observed that limiting enforcement of exclusive patent rights to literal infringement "would place the inventor at the mercy of verbalism and would be subordinating substance to form." Graver Tank, 339 U.S. at 607, 70 S.Ct. at 856. Such a limitation, the Court reasoned,, might even encourage infringers "to make unimportant and insubstantial changes and substitutions in the patent which, though adding nothing, would be enough . [to evade] the reach of law." Id.
Based on these predicates, the Supreme Court concluded, the doctrine applies if, and only if, the differences between the claimed and accused products or processes are insubstantial. Graver Tank, 339 U.S. at 610, 70 S.Ct. at 857. The Court expressed the doctrine in the following terms for the Graver Tank case:
The question which thus emerges is whether the substitution of the manganese which is not an alkaline earth metal for the magnesium which is, under the circumstances of this case, and in view of the technology and prior art, is a change of such substance as to make the doctrine of equivalents inapplicable; or conversely, whether under the circumstances the change was so insubstantial that the trial court's invocation of the doctrine of equivalents was justified.
Id. The Court defined the doctrine of equivalents in terms of the substantiality of the differences between the claimed and accused products or processes. The Supreme Court in Graver Tank thus made insubstantial differences the necessary predicate for infringement under the doctrine of equivalents.
In recent decisions, this court has also stressed the significance of this "insubstantial differences" standard. Valmont Indus., Inc. v. Reinke Mfg. Co., 983 F.2d 1039, 1043, 25 USPQ2d 1451, 1454 (Fed.Cir.1993); Charles Greiner & Co. v. Mari-Med Mfg., Inc., 962 F.2d 1031, 1036, 22 USPQ2d 1526, 1529 (Fed.Cir.1992); London v. Carson Pirie Scott & Co., 946 F.2d 1534, 1538, 20 USPQ2d 1456, 1458 (Fed.Cir.1991); Slimfold Mfg. Co. v. Kinkead Indus., Inc., 932 F.2d 1453, 1457, 18 USPQ2d 1842, 1846 (Fed.Cir.1991); Moleculon Research Corp. v. CBS, Inc., 872 F.2d 407, 409, 10 USPQ2d 1390, 1392 (Fed.Cir.1989); Pennwalt Corp. v. Durand-Wayland, Inc., 833 F.2d 931, 935, 4 USPQ2d 1737, 1739 (Fed.Cir.1987) (en banc), cert. denied, 485 U.S. 961, 108 S.Ct. 1226, 99 L.Ed.2d 426, and cert. denied, 485 U.S. 1009, 108 S.Ct. 1474, 99 L.Ed.2d 703 (1988); Per kin-Elmer Corp. v. Westinghouse Elec. Corp., 822 F.2d 1528, 1532, 3 USPQ2d 1321, 1324 (Fed.Cir.1987). In many of these cases, this court also relied on the so-called triple identity, or function-way-result, test to measure the substantiality of the differences. Valmont, 983 F.2d at 1043; London, 946 F.2d at 1538; Slimfold, 932 F.2d at 1457; Moleculon, 872 F.2d at 410; Pennwalt, 833 F.2d at 934-35. With this case, this court explicitly holds that the application of the doctrine of equivalents rests on the substan-tiality of the differences between the claimed and accused products or processes, assessed according to an objective standard.
II.
In applying the doctrine of equivalents, it is often enough to assess whether the claimed and accused products or processes include substantially the same function, way, and result. Courts recognized this principle as early as 1817, when Justice Bushrod Washington, riding circuit, instructed a jury that "[wjhere the [claimed and accused] machines are substantially the same, and operate in the same manner, to produce the same result, they must be in principle the same." Gray v. James, 10 F.Cas. 1015, 1016 (C.C.D.Pa.1817) (No. 5,718); see also Sanitary Refrigerator, 280 U.S. at 42, 50 S.Ct. at 13 ("[Gjenerally speaking, one device is an infringement of another 'if it performs substantially the same function in substantially the same way to obtain the same result.' . [The patent claim] is nevertheless infringed by a device in which there is no substantial departure from the description in the patent, but a mere colorable departure therefrom.") (quoting Machine Co. v. Murphy, 97 U.S. 120, 125, 24 L.Ed. 935 (1877)) (emphasis added). Because examination of function, way, and result often discloses the substantiality of the differences between the accused and claimed products or processes, many courts, including the Supreme Court itself, have used this formulation to describe the doctrine of equivalents. See, e.g., Graver Tank, 339 U.S. at 608, 70 S.Ct. at 856 (quoting Sanitary Refrigerator, 280 U.S. at 42, 50 S.Ct. at 13); Machine Co. v. Murphy, 97 U.S. at 125, 24 L.Ed. 935.
It goes too far, however, to describe the function-way-result test as "the" test for equivalency announced by Graver Tank. The function-way-result test often suffices to assess equivalency because similarity of function, way, and result leaves little room for doubt that only insubstantial differences distinguish the accused product or process from the claims. But evaluation of function, way, and result does not necessarily end the inquiry. Indeed, the Supreme Court explained that the function-way-result test arose in an era characterized by relatively simple mechanical technology. Graver Tank, 339 U.S. at 609, 70 S.Ct. at 856-57 (reciting history of doctrine). As technology becomes more sophisticated, and the innovative process more complex, the function-way-result test may not invariably suffice to show the substantiality of the differences.
Thus evidence beyond function, way, and result is also relevant to the doctrine of equivalents. In Graver Tank, the Supreme Court identified and relied on factors in addition to similarity of function, way, and result. The Court considered that persons reasonably skilled in the art knew that the manganese in the accused flux was interchangeable for the magnesium in the claimed flux. 339 U.S. at 609, 612, 70 S.Ct. at 856-57, 858. The Court also permitted the fact-finder to infer infringing "imitation" from the lack of evidence that the accused infringer independently developed its flux. Id. at 612, 70 S.Ct. at 858.
The Supreme Court's reliance in Graver Tank on factors other than function, way, and result endorses consideration of all evidence relevant to the substantiality of the differences. Because "[equivalence, in the patent law, is not the prisoner of a formula," id. at 609, 70 S.Ct. at 856, the available relevant evidence may vary from case to case. When a trial record presents only evidence of function, way, and result, then application of the doctrine will necessarily rest on function, way, and result alone. When a record presents other evidence relevant to the substantiality of the differences, however, the fact-finder must consider it.
In either event, the vantage point of one of ordinary skill in the relevant art provides the perspective for assessing the sub-stantiality of the differences. Valmont, 983 F.2d at 1043. The test is objective, with proof of the substantiality of the differences resting on objective evidence rather than unexplained subjective conclusions, whether offered by an expert witness or otherwise.
According to the Supreme Court, "[a]n important factor" to be considered, quite apart from function, way, and result, "is whether persons reasonably skilled in the art would have known of the interchangeability of an ingredient not contained in the patent with one that was." Graver Tank, 339 U.S. at 609, 70 S.Ct. at 857. The precedent of this court has also stressed the importance of evidence of known interchangeability to show infringement under the doctrine. See Corning Glass Works v. Sumitomo Elec. U.S.A., Inc., 868 F.2d 1251, 1261, 9 USPQ2d 1962, 1969 (Fed.Cir.1989); Thomas & Betts Corp. v. Litton Sys., Inc., 720 F.2d 1572, 1579, 220 USPQ 1, 6 (Fed.Cir.1983). The known interchangeability of the accused and claimed elements is potent evidence that one of ordinary skill in the relevant art would have considered the change insubstantial. Without such evidence, the patentee will need other objective technological evidence demonstrating that the substitute nevertheless represents a change that the ordinary artisan would have considered insubstantial at-the time of infringement.
Evidence of copying is also relevant to infringement under the doctrine of equivalents, see Graver Tank, 339 U.S. at 612, 70 S.Ct. at 858, not because the doctrine of equivalents rests on the subjective awareness or motivation of the accused infringer, but rather because copying suggests that the differences between the claimed and accused products or processes—measured objectively—are insubstantial. When an attempt to copy occurs, the fact-finder may infer that the copyist, presumably one of some skill in the art, has made a fair copy, with only insubstantial changes. Such an inference, of course, would not dominate the doctrine of equivalents analysis. Instead, where the inference arises, it must be weighed together with the other evidence relevant to the sub-stantiality of the differences.
By considering evidence of copying, however, the Supreme Court did not imply that infringement under the doctrine requires bad faith or some other subjective component. Intent is not an element of infringement. See, e.g., Kewanee Oil Co. v. Bicron Corp., 416 U.S. 470, 478, 94 S.Ct. 1879, 1884, 40 L.Ed.2d 315 (1974). A patent owner may exclude others from practicing the claimed invention, regardless of whether infringers even know of the patent:
This question [of infringement] is one irrespective of motive. The defendant may have infringed without intending, or even knowing it; but he is not, on that account, the less an infringer. His motives and knowledge may affect the question of damages, to swell or reduce them; but the immediate question is the simple one, has he infringed?
Parker v. Hulme, 18 F.Cas. 1138, 1143 (C.C.E.D.Pa.1849) (No. 10,740); see also Kewanee, 416 U.S. at 478, 94 S.Ct. at 1884; Intel Corp. v. United States Int'l Trade Comm'n, 946 F.2d 821, 832, 20 USPQ2d 1161, 1171 (Fed.Cir.1991). The Supreme Court, in the exegesis of the doctrine in Graver Tank, notably does not mention a threshold showing of bad faith or evil intent. Proof of bad faith by an infringer may entitle the patent owner to enhanced damages and attorney fees for willful infringement under 35 U.S.C. § 284-285 (1988). Evidence of culpable conduct, however, is not a prerequisite nor necessary for application of the doctrine.
The Supreme Court applied the doctrine of equivalents in Graver Tank to prevent "fraud on a patent," 339 U.S. at 608, 70 S.Ct. at 856, not fraud by the accused in-fringer. As Graver Tank demonstrates, preventing "fraud on a patent" involves an objective assessment of the substantiality of the differences between the claimed and accused products or processes. The doctrine of equivalents does not rely on the subjective awareness or intent of the accused infringer. While the doctrine discourages the "unscrupulous copyist," 339 U.S. at 607, 70 S.Ct. at 856, its reach is not so limited. Thus, lack of substantial differences, not the accused in-fringer's motives or intent, triggers application of the doctrine of equivalents.
Evidence of "designing around" the patent claims is also relevant to the question of infringement under the doctrine. The ability of the public successfully to design around — to use the patent disclosure to design a product or process that does not infringe, but like the claimed invention, is an improvement over the prior art — is one of the important public benefits that justify awarding the patent owner exclusive rights to his invention. Designing around "is the stuff of which competition is made and is supposed to benefit the consumer." State Indus., Inc. v. A.O. Smith Corp., 751 F.2d 1226, 1236, 224 USPQ 418, 424 (Fed.Cir.1985). When a competitor becomes aware of a patent, and attempts to design around its claims, the fact-finder may infer that the competitor, presumably one of skill in the art, has designed substantial changes into the new product to avoid infringement. Again, the strength of this inference may vary from case to case. Evidence of designing around therefore weighs against finding infringement under the doctrine of equivalents.
Evidence that the accused infringer developed its product or process through independent research is not directly relevant to the question of infringement under the doctrine of equivalents. Independent development is not designing around. Independent development means that the accused infringer had no knowledge of the patented invention when it developed its product or process. Without knowledge, the independent developer could not have set out to make its product or process either similar to or different from the claimed invention. Unlike copying or designing around, therefore, independent development itself provides no information about the substantiality of the differences. Furthermore, because intent is not an element of infringement, independent development does not excuse infringement of the patent owner's right to exclude. See Parker v. Hulme, 18 F.Cas. at 1143. An independently developed product or process that falls within the patent claims or includes only insubstantial differences nevertheless infringes. See Kewanee, 416 U.S. at 478, 94 S.Ct. at 1884. In sum, those who make only insubstantial changes to a patented product or process are liable for infringement, regardless of their awareness of the patent and its disclosure.
Evidence of independent development is highly relevant, however, to refute a patent owner's contention that the doctrine of equivalents applies because the accused infringer copied, that is, intentionally appropriated the substance of the claimed invention. In Graver Tank, the Supreme Court linked the fact-finder's inference of copying to independent research. 339 U.S. at 612, 70 S.Ct. at 858. Because the record lacked evidence of independent development, the fact-finder could infer copying or "imitation." Id. When the patent owner asserts copying, evidence that the accused infringer developed its product or process without knowing of the patent becomes relevant in rebuttal. For this reason, the fact-finder must consider any evidence of independent development in a case where the patent owner alleges copying as probative of infringement under the doctrine of equivalents.
III.
Infringement, whether literal or under the doctrine of equivalents, is a question of fact. Winans v. Denmead, 56 U.S. (15 How.) at 338, 14 L.Ed. 717; SRI Int'l v. Matsushita Elec. Corp., 775 F.2d 1107, 1125, 227 USPQ 577, 589 (Fed.Cir.1985) (era banc). The Supreme Court made this abundantly clear in Graver Tank:
A finding of equivalence is a determination of fact. Proof can be made in any form: through testimony of experts or others versed in the technology; by documents, including texts and treatises; and, of course, by the disclosures of the prior art. Like any other issue of fact, final determination requires a balancing of credibility, persuasiveness and weight of evidence. [When tried to the trial court, it] is to be decided by the trial court and that court's decision, under general principles of appellate review, should not be disturbed unless clearly erroneous. Particularly is this so in a field where so much depends upon familiarity with specific scientific problems and principles not usually contained in the general storehouse of knowledge and experience.
339 U.S. at 609-10, 70 S.Ct. at 857. The Supreme Court thus reemphasized that infringement under the doctrine of equivalents is an issue of fact. When infringement is tried to the court, as in Graver Tank, an appellate court reviews the trial court's infringement finding for clear error. When tried to a jury, an appellate court reviews the jury verdict for lack of substantial evidence. See, e.g., Genentech, Inc. v. Wellcome Found. Ltd., 29 F.3d 1555, 1565, 31 USPQ2d 1161, 1168-69 (Fed.Cir.1994).
In several recent opinions, this court has referred to the doctrine of equivalents as "equitable." The term "equitable" can have many meanings. The Supreme Court explained in Graver Tank that the doctrine prevents the unfairness of depriving the patent owner of effective protection of its invention, 339 U.S. at 607, 70 S.Ct. at 855-56, thereby achieving a fair or "equitable" result. Thus, in doctrine of equivalents eases, this court's allusions to equity invoke equity in its broadest sense — equity as general fairness. While recognizing the equity, or fairness, promoted by the doctrine of equivalents, furthermore, the Supreme Court stated unequivocally that application of the doctrine is a question of fact. Id. at 609, 70 S.Ct. at 856-57. This court has followed. SRI, 775 F.2d at 1118 ("It is settled that the question of infringement (literal or by equivalents) is factual. Graver Tank.").
By referring to the doctrine as a doctrine of fairness, neither the Supreme Court nor this court has invoked the myriad implications of an alternative to legal remedies. In addition, neither the Supreme Court nor this court has invoked equity in the technical sense of a set of principles originating in England to compensate for the historically harsh rules of common law. Graver Tank does not discuss any of the principles commonly attending the chancellor's invocation of equitable power, such as the "clean hands" doctrine, the elevated burden of proof, the abuse of discretion standard of review, or the mandatory balancing of the equities. Indeed, the Supreme Court has more than once stated that every patent owner is entitled to invoke the doctrine of equivalents — a proposition inimical to the hypothesis that the doctrine is equitable. See Sanitary Refrigerator, 280 U.S. at 42, 50 S.Ct. at 13; Seymour v. Osborne, 78 U.S. (11 Wall.) 516, 556, 20 L.Ed. 33 (1871) ("Paten-tees . are entitled in all cases to invoke to some extent the doctrine of equivalents . "). Moreover, the Supreme Court in Graver Tank credited the origin of the doctrine of equivalents to its own decision in Winans v. Denmead — a case at law, not equity. See Graver Tank, 339 U.S. at 608, 70 S.Ct. at 856; Winans v. Denmead, 56 U.S. (15 How.) at 338, 14 L.Ed. 717. Therefore, by its terms, Graver Tank did not impliedly transform a legal basis for recovery into an equitable one. In short, the Supreme Court's cases on the doctrine of equivalents foreclose a holding that the doctrine is a matter of equity to be applied at the court's discretion.
IV.
In answer to the first question posed by this court en banc, a finding of infringement under the doctrine of equivalents requires proof of insubstantial differences be tween the claimed and accused products or processes. Often the function-way-result test will suffice to show the extent of the differences. In such cases, the parties will understandably focus on the evidence of function, way, and result, and the fact-finder will apply the doctrine based on that evidence. Other factors, however, such as evidence of copying or designing around, may also inform the test for infringement under the doctrine of equivalents. No judge can anticipate whether such other factors will arise in a given case. Instead, the presence of such factors will depend on the way the parties frame their arguments. Neither the Supreme Court nor this court limits the types of evidence that either party may proffer in support of a factor it considers probative of infringement under the doctrine. 'The trial judge, however, has a duty to decide whether the proffered evidence is relevant. This duty to assess relevance is no different in a doctrine of equivalents ease than in any other type of case. Relevance will be self-evident to the judge in a case tried to the bench. In a jury trial, however, the judge must admit only relevant evidence, and instruct the jury to consider only the admitted evidence in reaching its decision.
In answer to the second question posed by this court en banc, infringement under the doctrine of equivalents is an issue of fact to be submitted to the jury in a jury trial with proper instructions, and to be decided by the judge in a bench trial. The answer to the third question posed by this court en banc necessarily flows from the answer to the second question. The trial judge does not have discretion to choose whether to apply the doctrine of equivalents when the record shows no literal infringement.
V.
This court reviews a jury verdict on the fact question of infringement under the doctrine of equivalents for prejudicial error in the jury instructions, Biodex Corp. v. Loredan Biomedical, Inc., 946 F.2d 850, 854, 20 USPQ2d 1252, 1254 (Fed.Cir.1991), cert. denied, 504 U.S. 980, 112 S.Ct. 2957, 119 L.Ed.2d 579 (1992), and lack of substantial evidence supporting the verdict, see Genentech, 29 F.3d at 1565.
While jury instructions must correctly state the law, they need only be sufficiently comprehensive to address the issues of material fact raised by the record evidence. Biodex, 946 F.2d at 854. Indeed, "[j]ury instructions must be confined to the issues as presented by the pleadings and evidence." American Hoist & Derrick Co. v. Sowa & Sons, Inc., 725 F.2d 1350, 1363, 220 USPQ 763, 773 (Fed.Cir.), cert. denied, 469 U.S. 821, 105 S.Ct. 95, 83 L.Ed.2d 41 (1984).
The sufficiency of jury instructions is not tested in a vacuum. "It is well established that [each] instruction 'may not be judged in artificial isolation,' but must be considered in the context of the . trial record." Estelle v. McGuire, 502 U.S. 62, 72, 112 S.Ct. 475, 482, 116 L.Ed.2d 385 (1991) (quoting Cupp v. Naughten, 414 U.S. 141, 147, 94 S.Ct. 396, 400, 38 L.Ed.2d 368 (1973)); see also Biodex, 946 F.2d at 862. The words of the instructions take on meaning from the context of what happened at trial, including how the parties tried the case and their arguments to the jury. Accordingly, we must examine the trial context in which the jury instructions were given by the trial court.
Hilton Davis offered evidence of and argued function, way, and result in asserting the doctrine of equivalents. Warner-Jenkin-son responded in kind. Warner-Jenkinson also offered evidence of independent development, contending repeatedly that its lack of knowledge of the '746 patent when it developed its ultrafiltration process excused it from infringement under the doctrine. In its appeal brief, Warner-Jenkinson summarized its interpretation of the doctrine of equivalents:
[The doctrine of equivalents] is an equitable remedy available only upon a suitable showing of the equities. To demonstrate that the equities favor application of the doctrine, the patentee must put forth proof of the equities, and the trial court must find the type of conduct which triggers its application. London v. Carson Pirie Scott & Co., [946 F.2d 1534, 1538, 20 USPQ2d 1456, 1458 (Fed.Cir.1991) ]; Charles Greiner & Co. v. Mari-Med Mfg., Inc., 962 F.2d 1031, 1036-36, 22. [USPQ2d] 1526, 1529 (Fed.Cir.1992). As an equitable remedy, the doctrine is an issue of law for the court, not the jury.
B. Here, no equitable basis exists for the application of the doctrine. Warner-Jenkinson's processes were developed in cooperation with knowledgeable and experienced vendors in the field including Os-monics- There was no copying or piracy.
As noted, however, the doctrine of equivalents is not an equitable remedy available only on a showing of the equities. Lack of awareness of the patent or its disclosure does not excuse infringement. Parker v. Hulme, 18 F.Cas. at 1143. Accordingly, Warner-Jenkinson could not succeed in erecting an equitable defense to infringement.
In this context, the trial court instructed the jury about the doctrine of equivalents in terms of function, way, and result:
You may find infringement under the doctrine of equivalents when the accused process and the claimed invention perform substantially the same function in substantially the same way to yield substantially the same result even though the processes differ in name, form or shape.
(Emphasis added.) This formulation correctly stated that the jury "may" rely on the function-way-result test, recognizing that this test is often enough to show infringement under the doctrine. In particular, the trial court tailored its function-way-result instruction to the parties' reliance on evidence of function, way, and result in this case.
Moreover, the trial court's instructions correctly resisted Warner-Jenkinson's effort to erect an equitable threshold for application of the doctrine of equivalents. The doctrine of equivalents has no equitable or subjective component. The eases Warner-Jenkinson cites to the contrary — London, 946 F.2d at 1538, and Charles Greiner, 962 F.2d at 1035-36 — reaffirm that the Graver Tank objective criteria, as limited by prosecution history and prior art, confine the range of equivalents. These cases do not condition effective patent protection on proof of bad faith. Accidental or "innocent" infringement is still infringement. Intent becomes a requirement only if and when the patent owner seeks enhanced damages-or attorney fees for willful infringement under 35 U.S.C. § 284-285. In this case, for example, the jury took Warner-Jenkinson's lack of intent into account when it found that Warner-Jenkinson did not willfully infringe. Therefore, under the trial court's instructions, Warner-Jenkinson's evidence of independent development played its proper role: it shielded Warner-Jenkinson from an enhanced damages award, but did not provide a basis for avoiding application of the doctrine. The evidence of independent development in this ease, directed as it is only to the issue of whether the accused infringer acted with knowledge of the patent, is irrelevant to showing substantial differences. This court cannot fault the absence of any reference to such evidence in the jury instructions.
Under these circumstances, the trial court's function-way-result instruction correctly guided the jury to consider the relevant evidence. In a perfect world, the trial court might have provided further guidance — by instructing the jury, for instance, that Warner-Jenkinson's independent development was not an excuse to infringement. Nonetheless, the trial court's function-way-result instruction directed the jury to consider, in assessing infringement, the only evidence relevant to the substantiality of the differences that was offered at trial. Significantly, Warner-Jenkinson did not object to this instruction. Rather, Warner-Jenkinson objected to sending the doctrine of equivalents question to the jury at all, contending that independent development excused it from infringing.
The trial context left the jury to consider the evidence of function, way, and result presented by both parties, the only available evidence going to the substantiality of the differences. In the context of this trial, the instructions properly focused the jury on the evidence relevant to the doctrine of equivalents.
When a jury makes a finding on infringement under the doctrine of equivalents, this court will uphold that finding if supported by substantial evidence. Genentech, 29 F.3d at 1565. "It is not for this Court to even essay an independent evaluation of this evidence. This is the function of the trial court." Graver Tank, 339 U.S. at 611, 70 S.Ct. at 857. Thus, the next question is the sufficiency of the evidence to support the jury verdict.
The '746 claims recite a pH "from approximately 6.0 to 9.0." Warner-Jenkinson at times used a lower pH of 5.0. The claims also recite a pressure of "approximately 200 to 400 p.s.i.g." Warner-Jenkinson used a pressure somewhere in a range of 200 to nearly 500 p.s.i.g.
Substantial evidence supports the jury finding that Warner-Jenkinson's pH variation from the claimed approximate range was insubstantial. The claimed pH limitation prevents damage to the membrane and produces a neutral final dye product. Dr. Cook, one of the inventors, testified that a pH of 5 would have the same effect as a pH of 6, as would any pH above 2. Even Warner-Jenkinson's expert agreed that Hilton Davis' process would operate at a pH of 5. The record contains substantial evidence that one of skill in the art would know that performing ultrafiltration at a pH of 5 would allow the membrane to perform the same function, in an equivalent way, to achieve the same result as at a pH of approximately 6 to 9.
Substantial evidence also supports the jury finding that Warner-Jenkinson's pressure for some of its membranes was in the claimed range of approximately 200 to 400 p.s.i.g. Warner-Jenkinson argues for pressure measurement at the high pressure pump instead of at the membrane. Warner-Jenkinson's pressure at the high pressure pump may have been as high as nearly 500 p.s.i.g. The specification, however, defines the pressure as "applied to the upstream side of the membrane." The '746 patent, col. 6, lines 20-21. In any event, the record contains substantial evidence that Warner-Jen-kinson's pressure performed the same function — forcing the solution through the membrane — -in an equivalent way, to achieve the same result.
As for pore size, the trial record details the difficulty of measuring a membrane's exact pore size in light of variations in fluid and pressure conditions. The record contains considerable evidence, however, about the pore size necessary to separate dye molecules from smaller molecular impurities. The record thus includes substantial evidence that Warner-Jenkinson necessarily used membranes of the claimed "nominal pore diameter of 5 to 15 Angstroms" to accomplish ultrafiltration.
Nor does prosecution history estoppel preclude application of the doctrine of equivalents in this case. "Whenever prosecution history estoppel is invoked as a limitation to infringement under the doctrine of equivalents, 'a close examination must be made as to, not only what was surrendered, but also the reason for such a surrender.' " Insta-Foam Prods., Inc. v. Universal Foam Sys., Inc., 906 F.2d 698, 703, 15 USPQ2d 1295, 1298 (Fed.Cir.1990) (quoting Bayer AG v. Duphar Int'l Research B.V., 738 F.2d 1237, 1243, 222 USPQ 649, 653 (Fed.Cir.1984)). The inventors amended the '746 claims to recite "a pH from approximately 6.0 to 9.0" to avoid the disclosure in the Booth patent of an ultrafiltration process operating at a pH higher than 9. This amendment surrendered pHs above 9, but does not bar Hilton Davis from asserting equivalency to processes such as Warner-Jenkinson's operating sometimes at a pH below 6.
Warner-Jenkinson performed the process disclosed in the '746 patent — purifying dye by collecting relatively large dye molecules on the concentrate side of a membrane. Warner-Jenkinson did not use precisely the claimed process parameters, but the jury found the differences between Warner-Jen-kinson's and the claimed processes insubstantial. The record contains substantial evidence supporting this finding of infringement under the doctrine of equivalents.
VI.
While agreeing that the substantiality of the differences between the claimed and accused products or processes is the ultimate question under the doctrine of equivalents, one dissent contends that "[t]he authority to exercise the unique remedy which is the doctrine of equivalents lies exclusively in courts of equity." This dissent argues that, because the Patent Act of 1952 does not expressly provide a general remedy for infringement under the doctrine of equivalents, the doctrine must be, like other judge-created extensions of inadequate legal remedies, purely equitable. This reasoning, however, is a sharp departure from a century of Supreme Court decisions issued under a stable statutory regime.
The Supreme Court has long held that the question of infringement, whether literal or by equivalents, is a question of fact for the jury if properly demanded. For example, in Coupe v. Royer, 155 U.S. 565, 15 S.Ct. 199, 39 L.Ed. 263 (1895), the Court faced an accused infringer's contention that the trial court had improperly instructed the jury to the plaintiffs advantage regarding the question of infringement under the doctrine of equivalents. Remanding the case for a new trial, the Court held that, despite the lack of a genuine issue of fact regarding either the proper construction of the patent claim or the nature of the accused device, the question of infringement under the doctrine presented a question requiring trial to the jury. Id. at 579-80, 15 S.Ct. at 205. In so holding, the Court merely followed its decision in Winans v. Denmead: "whether, in point of fact, the defendant's [devices] did copy the plaintiffs invention, in the sense above explained [i.e., by an equivalent], is a question for the jury." Winans v. Denmead, 56 U.S. (15 How.) at 344, 14 L.Ed. 717. Citations for this principle can, of course, be multiplied. See, e.g., Royer v. Schultz Belting Co., 135 U.S. 319, 325, 10 S.Ct. 833, 835, 34 L.Ed. 214 (1890); Tyler v. Boston, 74 U.S. (7 Wall.) 327, 330-31, 19 L.Ed. 93 (1869).
This dissent also contends that the claiming requirement compels limiting the doctrine of equivalents by equitable principles. The claiming requirement, however, was contained in the Patent Act of 1870, eighty years before Graver Tank. See Patent Act of 1870, ch. 230, § 26, 16 Stat. 198, 201 ("[B]efore any inventor or discoverer shall receive a patent . he shall particularly point out and distinctly claim the part, improvement, or combination which he claims as his invention or discovery...."); see also Patent Act of 1836, ch. 357, § 6, 5 Stat. 117,119 (requiring inventor to "particularly specify and point out the part . which he claims"). "Moreover the Supreme Court in Graver Tank reached its decision over Justice Black's dissent which also invoked the statutory claiming requirement against application of the doctrine. See Graver Tank, 339 U.S. at 613-14, 70 S.Ct. at 859 (Black, J., dissenting).
The Supreme Court explained that the doctrine is not inconsistent with the requirement for explicit claims. According to Graver Tank, the "theory on which [the doctrine of equivalents] is founded is that 'if two devices do the same work in substantially the same way, and accomplish substantially the same result, they are the same, even though they differ in name, form, or shape.' " 339 U.S. at 608, 70 S.Ct. at 856 (quoting Machine Co. v. Murphy, 97 U.S. at 125, 24 L.Ed. 935) (emphasis added); see also Sanitary Refrigerator, 280 U.S. at 41-42, 50 S.Ct. at 12-13 ("There is a substantial identity, constituting infringement, where a device is a copy of the thing described by the patentee, 'either without variation, or with such variations as are consistent with its being in substance the same thing.' "; quoting Burr v. Duryee, 68 U.S. (1 Wall.) 531, 573, 17 L.Ed. 650 (1864)); Machine Co. v. Murphy, 97 U.S. at 125, 24 L.Ed. 935 ("Authorities concur that the substantial equivalent of a thing, in the sense of the patent law, is the same as the thing itself.... "). In other words, equivalency, like exact copying, gives rise to infringement liability because it too is a relationship of identity, a proposition quite consistent with the requirement that the patent claim "particularly point out" and thereby circumscribe the protected invention. See, e.g., 3 William C. Robinson, The Law of Patents for Useful Inventions § 894 (1890) (discussing the identity theory underlying infringement by equivalents). In addition, by citing Machine Co. v. Murphy for the concept of identity that undergirds infringement by equivalents, the Court in Graver Tank, decided eighty years after the advent of peripheral claiming, relied on a case involving a patent granted in 1859, 97 U.S. at 121, 24 L.Ed. 935, eleven years before the advent of peripheral claiming. Because the statutory definition of infringement has, as will be demonstrated below, remained virtually unchanged since 1790, the Court could rely on this pre-1870 case for the doctrine of equivalents. The dissent's equity model — and its underlying premise that the advent of peripheral claiming in the Patent Act of 1870 dramatically altered the legitimacy or basis of the doctrine of equivalents— would leave the Court's reliance on Machine Co. v. Murphy in Graver Tank utterly inexplicable.
The dissent also suggests that Congress could have, but has not, included the doctrine of equivalents in the Patent Act of 1952, 35 U.S.C. § 1-376 (1988 & Supp. V 1993). According to this dissent, "[i]f Congress wanted to provide for [infringement by] equivalents to what is claimed, it knew how to do it." This argument suffers at least three fatal flaws. First, the statutory definition of infringement, which appears in 35 U.S.C. § 271(a), makes no reference to claims at all. Thus, the definition of infringement is at best equivocal on the question of infringement under the doctrine of equivalents. Second, the Supreme Court itself has noted that "§ 271(a) of the new Patent Code [of 1952], which defines 'infringement,' left intact the entire body of case law on direct infringement." Aro Mfg. Co. v. Convertible Top Replacement Co., 365 U.S. 336, 342, 81 S.Ct. 599, 602, 5 L.Ed.2d 592 (1961) (emphasis added). Thus, contrary to the dissent's inference that the Patent Act of 1952's silence repealed Graver Tank and its forebears, the Supreme Court in Aro states that section 271(a) left intact the doctrine of equivalents.
Third, and perhaps most importantly, infringement was defined and understood long before 1952. According to section 4 of the Patent Act of 1790, anyone who without authority "devise[d], ma[d]e, construct[ed], use[d], employ[ed], or vend[ed]" a patented invention was liable to the patentee for "such damages as shall be assessed by a jury." Patent Act of 1790, § 4,1 Stat. at.111. Similarly, according to section 5 of the Patent Act of 1793, anyone who "ma[d]e, devise[d], and use[d], or s[old]" a patented invention was liable to the patentee for a sum "at least equal to three times the price, for which the patentee has usually sold or licensed to other persons, the use of said invention," a liability to be adjudicated in a legal, not equitable, "action on the ease." Patent Act of 1793, ch. 11, § 5, 1 Stat. 318, 322. Section 14 of the Patent Act of 1836 simply provided for a legal "action for damages for making, using, or selling" a patented invention without authority. Patent Act of 1836, § 14, 5 Stat. at 123. Finally, section 59 of the Patent Act of 1870 simply provided "that damages for the infringement of any patent may be recovered by action on the case." Patent Act of 1870, § 59, 16 Stat. at 207. Against this stable backdrop of statutory definitions of infringement, definitions which are virtually indistinguishable from the current 35 U.S.C. § 271(a), the Supreme Court has recognized actions at law for recovery for infringement under the doctrine of equivalents since its Winans v. Denmead decision in 1854. In light of this authority, this court detects no basis to hold that the doctrine of equivalents was rendered extrastatutory and thus equitable by virtue of the silent repeal that the dissent finds in the Patent Act of 1952.
A separate dissent concurs in our conclusion that the function-way-result test is riot "the" test for equivalents. This dissent, however, would prefer to treat the substantiality of the differences as "only one of the factors according to Graver, arguably the most important factor, which a court should consider in deciding whether to apply the" doctrine of equivalents. The Supreme Court, in our view, made the fundamental question "whether under the circumstances the change [in the accused product or process] was so insubstantial that . invocation of the doctrine of equivalents [is] justified." Graver Tank, 339 U.S. at 610, 70 S.Ct. at 857.
In addition, this separate dissent argues that the accused infringer's intent is relevant to the question whether the accused product or process infringes under the doctrine. As clarified above, however, intent is not an element of direct infringement, whether literal or'by equivalents. Neither Graver Tank nor any other authority supports the proposition that preventing "fraud on a patent," 339 U.S. at 608, 70 S.Ct. at 856, turns on the subjective awareness or intent of the accused infringer. Moreover,' neither evidence of copying, independent development, nor "designing around" owes its relevance in a doctrine of equivalents analysis to the state of mind of the accused infringer. Infringement is, and should remain, a strict liability offense.
Finally, this dissent departs from our basic disposition of the case, arguing that our basic holding as to the status of the function-way-result test renders the trial court's instructions in this case plainly erroneous. As has been demonstrated, however, the trial court's instructions were sufficiently comprehensive to direct the jury to consider, in assessing infringement under the doctrine of equivalents, the only evidence relevant to the sub-stantiality of the differences that the parties offered. No more is required. See Biodex, 946 F.2d at 853-54.
A third dissent contends that "because we know what the claim means and we know what process parameters Warner-Jenkinson uses, the issue of infringement under the doctrine [of equivalents] . resolves itself into one of law," reviewable de novo on appeal. The Supreme Court squarely renounced arrogation of the fact-finding function in Graver Tank, however, recognizing that an appellate court is not best qualified to assess the facts in a doctrine of equivalents case:
A finding of equivalence is one of fact.... Like any other issue of fact, final determination requires a balancing of credibility, persuasiveness and weight of evidence. It is to be decided by the trial court [in a bench trial] and that court's decision, under general principles of appellate review, should not be disturbed unless clearly erroneous. Particularly is this so in a field where so much depends upon familiarity with specific scientific problems and principles not usually contained in the general storehouse of knowledge and experience.
Graver Tank, 339 U.S. at 609-10, 70 S.Ct. at 857 (emphasis added). In short, the Supreme Court settled the question of the standard of review in doctrine of equivalents cases, foreclosing the dissent's contention.
This dissent also argues for adoption of a new "legal limitation" on the doctrine of equivalents prohibiting "enlargement of the claim." The rule against enlargement of claim scope during claim construction is well settled. See, e.g., Keystone Bridge Co. v. Phoenix Iron Co., 95 U.S. 274, 278-79, 24 L.Ed. 344 (1877). This dissent errs, however, in arguing that application of the doctrine of equivalents enlarges the claim scope. Instead the doctrine of equivalents provides the same protection to the substance of the claim scope provided by the doctrine of literal infringement. As explained in Graver Tank, when there are no substantial differences between the claimed and accused products or processes, "they are the same" in the eyes of the patent law. Graver Tank, 339 U.S. at 608, 70 S.Ct. at 856 (quoting Machine Co. v. Murphy, 97 U.S. at 125, 24 L.Ed. 935). The rule against enlargement, although an essential tenet of claim construction, thus contributes nothing to the inquiry into infringement under the doctrine.
This dissent also purports to discern in some of the Supreme Court's decisions a second "legal limitation" on the doctrine of equivalents limiting "the range of infringing substitutions to those in which components were substituted which were known to be equivalents" when the patent issued. The dissent's analysis is flawed, however. The Court has recognized that post-issuance improvements can infringe under the doctrine. See Sanitary Refrigerator, 280 U.S. at 40-43, 50 S.Ct. at 12-13. Our cases have followed the same course. See, e.g., Moleculon, 872 F.2d at 409; Hughes Aircraft Co. v. United States, 717 F.2d 1351, 1365, 219 USPQ 473, 483 (Fed.Cir.1983). In Graver Tank, furthermore, the Court recognized the ingenuity of would-be pirates, who may always be expected to find new ways to avoid the literal scope of a patent claim:
One who seeks to pirate an invention, like one who seeks to pirate a copyrighted book or play, may be expected to introduce minor variations to conceal and shelter the piracy. Outright and forthright duplication is a dull and very rare form of infringement.
Graver Tank, 339 U.S. at 607, 70 S.Ct. at 856. The doctrine "evolved in response to this experience," id. at 608, 70 S.Ct. at 856, and stands as a bulwark against such "insubstantial changes," id. at 607, 70 S.Ct. at 856. Limiting the range of potentially infringing substitutions to those known at the time of the patent's issuance would undermine the doctrine, denying patent owners protection of the substance of their inventions against new forms of infringement.
Finally, this third dissent takes issue with the jury's evaluation of the evidence and this court's refusal to apply prosecution history estoppel. Graver Tank bound this court only to review the jury's fact findings and not to rely on our independent views. As has been demonstrated, each of the jury's fact findings is supported by substantial evidence. Therefore this court must affirm the jury's fact findings, regardless of what result this court might have reached had it been entrusted with fact finding at trial. See Lavender v. Kurn, 327 U.S. 645, 653, 66 S.Ct. 740, 744, 90 L.Ed. 916 (1946).
As for prosecution history estoppel, as explained, this doctrine turns on the reasons for claim amendments during prosecution. Insta-Foam, 906 F.2d at 703. That the '746 inventors amended the claims to recite "a pH from approximately 6.0 to 9.0" to avoid the prior art disclosure of a process operating at a pH higher than 9 does not bar Hilton Davis from asserting equivalency to Warner-Jen-kinson's process sometimes operating at a pH below 6.
CONCLUSION
Substantial evidence supports the jury verdict of infringement under the doctrine of equivalents. The judgment of the district court is affirmed.
COSTS
Each party shall bear its own costs.
AFFIRMED
. The court took this case en banc to resolve specific questions concerning the doctrine of equivalents. Consequently, the en banc court has assigned the issue of validity of the '746 patent, raised by Warner-Jenkinson, to the panel which initially heard the appeal. The validity issue is decided in a separate panel opinion also issued today. See Hilton Davis Chem. Co. v. Warner-Jenkinson Co., No. 93-1088, 64 F.3d 675 (Fed.Cir. Aug. 8, 1995) (nonprecedential).
. See Valmont Indus., Inc. v. Reinke Mfg. Co., 983 F.2d 1039, 1043 n. 1, 25 USPQ2d 1451, 1454 n. 1 (Fed.Cir.1993) ("the doctrine 'is designed to do equity [but] it is not designed . to permit a claim expansion that would encompass more than an insubstantial change' ") (quoting Perkin-Elmer Corp. v. Westinghouse Elec. Corp., 822 F.2d 1528, 1532, 3 USPQ2d 1321, 1324 (Fed.Cir.1987)); Texas Instruments Inc. v. United States Int'l Trade Comm'n, 988 F.2d 1165, 1173, 26 USPQ2d 1018, 1024 (Fed.Cir.1993) ("the doctrine of equivalents has been 'judicially devised to do equity' ") (quoting Loctite Corp. v. Ultraseal, Ltd., 781 F.2d 861, 870, 228 USPQ 90, 96 (Fed.Cir.1985)); Charles Greiner & Co. v. Mari-Med Mfg., Inc., 962 F.2d 1031, 1036, 22 USPQ2d 1526, 1529 (Fed.Cir.1992) ("careful confinement of the doctrine of equivalents to its proper equitable role . promotes certainty and clarity in determining the scope of patent rights"); London v. Carson Pirie Scott & Co., 946 F.2d. 1534, 1538, 20 USPQ2d 1456, 1458 (Fed.Cir.1991) ("this equitable doctrine evolved from a balancing of competing policies").
. The closing arguments underscore that the jury properly focused on insubstantial differences in applying the doctrine of equivalents. Counsel for Hilton Davis extensively explained the doctrine to the jury in terms of insubstantial differences:
What you are going to hear, what you are going to hear Judge Weber tell you is he's going to tell you about the law of the doctrine of equivalents. Is one thing equivalent to another for purposes of patent infringement. And what he's going to read to you the law is, is that the doctrine of equivalents exists to prevent a fraud on the patent. It exists to keep people from tickling the patent, from making a tiny change that doesn't make any difference at all and then tiying to convince people that there really is a difference there.
. So does the acid really make any difference in this process? There is not a nickel's worth of difference.
. So docs it really make any difference? We saw from Dr. Cook's notebooks of all the tests they did there were pHs all over the place, from 2 all the way up to 8, and they adjusted these intentionally to see what would happen, and it didn't make a nickel's worth of difference. The process kept running along processing the dye.
So, tickling the patent, cosmetic changes that don't mean a nickel's worth of difference to what Hilton Davis' patent had.
What he is really meaning is Warner-Jenkin-son got caught and now they are tiying to come up with some excuses to argue their way out of it. It is different, it is different, it is different, hut there is no difference at all. Thank you, ladies and gentlemen.
(Emphasis added.) Counsel for Warner-Jenkin-son relied on the differences in function, way, and result between the claimed and accused processes and on its independent development:
In any event, what else do they do? pH. They tiy to tell you that pH doesn't matter. This is interesting the way they did this. Here's what Mr. Schmit asked Mr. Cook on direct examination. Question, "In terms of whether or not the process will work to separate the dye from the impurities, does it make a difference what pH you operate at?["] There is Mr. Cook's answer. "Not to my knowledge." Okay. And Mr. Kinman told you, "Not to my knowledge, pH doesn't make a difference." And why did they tell you that? They tell you that because they want you to think that our pH of 5 doesn't really make a difference; therefore, we are doing substantially the same thing they are doing....
So this doctrine of equivalents, the reason I'm spending so much time on it is that it has a special purpose in the law and it is to get somebody where they see someone's patent and they say, "Gee, that's a great idea." Now, let's just change this little thing and that little thing and they are outside the literal terms of the patent. That's an unscrupulous infringer. That's your fraud on the patent office. The person who does that. It is not the person who develops their own thing, who does their own thing for their own reasons and because they have their own processes....
(Emphasis added.) Thus, the jury received the case with emphasis on the substantiality of the differences supplied in closing arguments and focus on the function-way-result test supplied by jury instructions.