concurring.
I fully join the panel opinion. However, I would have gone further than the opinion in instructing the district court concerning application of the doctrine of equivalents on remand. I therefore wish to express some separate views concerning the much-troubled doctrine of equivalents.
The Supreme Court in Graver Tank & Mfg. Co. v. Linde Air Prods. Co., 339 U.S. 605, 70 S.Ct. 854, 94 L.Ed. 1097 (1950), provided today’s framework for evaluating the doctrine in 1950 when it stated that it is designed to avoid “a fraud on a patent” by discouraging “the unscrupulous copyist [from] makpng] unimportant and insubstantial changes and substitutions in the patent which, though adding nothing, *774would be enough to take the copied matter outside the claim, and hence outside the reach of law.” Id. at 607-08, 70 S.Ct. at 855-56. The Court noted that the doctrine is necessary to avoid “placpng] the inventor at the mercy of verbalism and ... subordinating substance to form.” Id. at 607, 70 S.Ct. at 855. The Court also fo-cussed on what are often referred to as the “tripartite” tests, stating that an accused device is an equivalent “ ‘if it performs substantially the same function in substantially the same way to obtain the same result.’ ” Id. at 608, 70 S.Ct. at 856 (quoting Sanitary Refrigerator Co. v. Winters, 280 U.S. 30, 42, 50 S.Ct. 9, 13, 74 L.Ed. 147 (1929)).
In recent years, we have emphasized that the doctrine is equitable in nature. See Pennwalt Corp. v. Durand-Wayland, Inc., 833 F.2d 931, 935, 4 USPQ2d 1737, 1739 (Fed.Cir.1987) (in banc); Loctite Corp. v. Ultraseal Ltd., 781 F.2d 861, 870, 228 USPQ 90, 96 (Fed.Cir.1985); Hughes Aircraft Co. v. United States, 717 F.2d 1351, 1361, 219 USPQ 473, 480 (Fed.Cir.1983). Unfortunately, however, the tripartite test has become a mechanical formula which can often be applied to accused in-fringers who are not unscrupulous copyists and who have made more than insubstantial changes, under circumstances in which it may not be equitable to do so.
The doctrine of equivalents is a sound one, important for the proper protection of patentable inventions. However, the doctrine has its limits, and I believe that a court must ensure that it achieves its intended purpose and does not simply become an automatic means for expanding the scope of a claim beyond that granted by examination in the Patent and Trademark Office. We have in fact noted that “the doctrine of equivalents is the exception, ... not the rule, for if the public comes to believe (or fear) that the language of patent claims can never be relied on, and the doctrine of equivalents is simply the second prong of every infringement charge, regularly available to extend protection beyond the scope of the claims, then claims will cease to serve their intended purpose.” London v. Carson Pirie Scott & Co., 946 F.2d 1534, 1538, 20 USPQ2d 1456, 1458-59 (Fed.Cir.1991). See also Charles Greiner & Co. v. Mari-Med Mfg. Inc., 962 F.2d 1031, 1036, 22 USPQ2d 1526, 1529 (Fed.Cir.1992).
It is important that the public be able to rely on the claims of a patent, in conjunction with the specification and file history, as indicating “the metes and bounds of the claimed invention.” London, 946 F.2d at 1538, 20 USPQ2d at 1458. Because the doctrine of equivalents runs counter to the role of claims as the sole measure of the invention, the claims may no longer adequately inform the public of the scope of protection, and uncertainty and unjustified litigation can result. The solution may be to go beyond the tripartite tests.
The Supreme Court did not make clear in Graver Tank whether the tripartite tests themselves constitute the application of equity or whether there is an equitable duty upon a court beyond the application of the tripartite tests. This court has not done so either. However, nothing in Graver Tank states that mere satisfaction of the tripartite tests itself establishes infringement by equivalence. In fact, the Court stated that “[equivalence ... is not the prisoner of a formula and is not an absolute to be considered in a vacuum.” 339 U.S. at 609, 70 S.Ct. at 856. The Court specifically distinguished non-infringing parties from infringing copyists by stating that “[wjithout some explanation or indication that [the accused product] was developed by independent research, the trial court could properly infer that the accused flux is the result of imitation rather than experimentation or invention.” Id. at 612, 70 S.Ct. at 858. Thus, the Court took account of the fact that there may be instances in which the function-way-result tests may be satisfied, but the facts may not justify the application of the doctrine of equivalents. This might occur if independent research resulted in an invention or product significantly different from what is claimed, albeit one that might perform substantially the same function in the same way to obtain the same result.
*775The tripartite tests often examine only what a device does, rather than what it is. Claims to machines, articles of manufacture, and compositions of matter generally define inventions by what they are, i.e., structurally. Thus, products may meet the tripartite tests in terms of what they do but be significantly different in terms of what they are. Processes are defined by their steps, so they may also be different in terms of what they are, even if they are the same in what they do.
Thus the Supreme Court may be considered to have articulated requirements beyond the tripartite tests to enable one to determine whether an accused device which does not literally infringe the claims of a patent infringes under the doctrine of equivalents. Prosecution history estoppel and avoidance of the prior art are of course well-established aspects of the factual evaluation of infringement under the doctrine, but the placing of an accused device on the copying-independent development spectrum should also be a critical inquiry in evaluating doctrine of equivalents questions. So should the degree of similarity of an accused invention to what is literally claimed; the closer the similarity, or the more “insubstantial” the change from what is literally claimed, the greater the argument for equivalence, independent of the tripartite tests.
Moreover, recognizing the equitable basis for the doctrine, there may be other factors that a court should consider beyond the tripartite function-way-result tests. Whether a patent claims a pioneering invention may be a factor favoring the application of the doctrine. On the other hand, an attempt at extension of coverage by means of the doctrine of equivalents to include disclosed, but unclaimed, subject matter may not be deserving of equitable consideration because it essentially permits a patentee to avoid examination by the Patent and Trademark Office. A patent applicant should not be able to present a broad disclosure in the specification of his or her patent, file narrow claims, and after allowance and grant of those narrow claims, seek to extend protection by the doctrine of equivalents to the disclosed, but unexamined, subject matter.* The district court is in the best position to determine whether there are any equities that argue for or against its application.
If there is no prosecution history estop-pel, if application of the doctrine would not encompass the prior art, and if the tripartite tests are met, I believe that a court should make a separate equitable determination, specifically including the above-noted “spectrum” and “similarity” evaluations in order to find infringement under the doctrine of equivalents. Ultimately, a court needs to balance the public’s need for certainty with the need for fairness to the patentee. The above-outlined criteria ought to enhance the accuracy of this judgment.
Patentees do have a limited two-year reissue right to enlarge the scope of granted claims. 35 U.S.C. § 251 (1988).