by Dennis Crouch
This post gets a bit into the weeds of the doctrine of equivalents — focusing on how courts identify the “function” of a claimed element when analyzing infringement under the doctrine of equivalents (DoE). In this post, I use the court’s new SMG decision to help frame the discussion. Smartrend Manufacturing Group (SMG), Inc. v. Opti-Luxx Inc., No. 2024-1616 (Fed. Cir. Nov. 13, 2025). Writing for a unanimous panel, Judge Dyk reversed a jury verdict finding that Opti-Luxx’s illuminated school bus sign infringed SMG’s U.S. Patent No. 11,348,491 under the doctrine of equivalents. The appellate court held that no reasonable jury could find the accused product’s frame performed the same function as that claimed.
The basic rule here is that the fact finder should first look to the patent’s written description and specification to determine the function of a claimed element. But, if (only if) the patent is silent or ambiguous on the function, then turn to the perspective of one of ordinary skill in the art via extrinsic evidence such as expert testimony.
Almost everyone is a bit uncomfortable with the DoE. Almost by definition, the doctrine only arises when the infringer is doing something different than what is claimed in the patent. I.e., they don’t literally infringe what has been claimed. With the DoE, they might be tagged for doing something close enough – but where is that fuzzy threshold? The discomfort with this fuzziness has led the courts to develop various mechanisms to cabin-in the DoE to avoid overreach. And, the court’s approach here in Smartrend is one such example. Most often, mechanisms operate as tools for a district court to take the decision out of the hands of the jury via summary judgment or JMOL.
The leading test for DoE requires an element-by-element function-way-result test. To infringe, the accused product must perform each and every element of the claim in substantially the same function, in substantially the same way, to achieve the same result. If any element fails this three-part test then there is no infringement.
For the first part of this test – same function – we really have a two step algorithm: (1) first figure out the function of the claimed element; then (2) determine whether the accused activity achieves that same function. Although these two steps are both questions of fact for the jury, Smartrend largely transforms step-1 into a claim construction exercise that seems to hand authority to the Judge as another tool for avoiding the jury.
SMG’s patent claims “an illuminated school bus sign” with several structural elements including opaque lettering on a translucent panel and “a frame surrounding a perimeter of the translucent panel and forming a perimeter of the sign for mounting the sign to the school bus.”
1. An illuminated school bus sign for direct mounting on a school bus, the sign comprising:
opaque lettering positioned on or over a front surface of a translucent panel;
an opaque rear panel situated opposite to the translucent panel in spaced relation therefrom to define a space therebetween;
a light source comprising a plurality of LEDs positioned in the space adjacent to the rear panel and pointing towards a front of the sign; and
[a] frame surrounding a perimeter of the translucent panel and forming a perimeter of the sign for mounting the sign to the school bus, … wherein the entire perimeter of the translucent panel is sealed….
The claim does not expressly say that the frame is separate and distinct from the translucent panel — only that it surrounds the perimeter of the panel. But, the district court construed the term to require a “separate and distinct component” from the translucent panel. The construction was based upon repeated references in the specification that refer to the mounting frame as separate and that describe functions of the frame that cannot be performed by an integrated frame. These functions include being able to replace the sign without removing the entire mount from the vehicle.
The district court ruled on summary judgment no literal infringement, but allowed the case to go to trial – with the jury finding DoE infringement. On appeal, the Federal Circuit has now reversed. (The Federal Circuit also vacated the jury’s finding of design patent infringement based on erroneous claim construction, remanding for a new trial on the design patent.)
Literal Infringement vs. Doctrine of Equivalents Literal infringement requires that the accused product contain each and every limitation recited in the patent claim. If even a single claim element is missing or materially different, there can be no literal infringement. The doctrine of equivalents provides a limited safety valve, allowing a patentee to prove infringement even when the accused product does not literally meet every claim limitation. The accused product must perform substantially the same function in substantially the same way to achieve substantially the same result as what the claim requires. The doctrine exists because strict literal interpretation could allow copyists to make minor, insubstantial changes to avoid infringement while appropriating the crux of the patented invention. But the doctrine cannot be applied so broadly that it effectively rewrites or eliminates claim limitations, which would undermine the public notice function of patent claims.
Identifying Function from Intrinsic Evidence
The court’s analysis centered on a fundamental question: what is the function of the claimed “frame” element in the ‘491 patent? Smartrend’s expert, Mr. York, testified that the accused product’s integrated frame “performs exactly the same function” as the patent’s claimed frame because it surrounds the sign and forms a perimeter for mounting to the school bus. The Federal Circuit rejected this characterization far too narrow, holding that the patent’s intrinsic evidence established a set of functions associated with the frame that were not performed by the accused product:
- Functions the accused product DID perform: Surrounding the perimeter of the sign and Forming a perimeter for mounting to the school bus. Note that these functions were both the only ones expressly stated in the claim language.
- Functions the accused product did NOT perform: Allowing signs to be swapped without removing the entire installation; Enabling manufacturers to easily customize vehicles with different signage; Allowing reuse without marring the sign with drill holes. These were touted within the specification as advantages of the disclosed invention but were not expressed directly in the claims.
These were all touted in the specification as benefits of an “illustrative embodiment” of the invention. There are several embodiments presented where the benefits were not expressly touted, but each of them appear to be designed to allow for the same benefits. The specification did include a boilerplate caveat:
Since various modifications may be made to the embodiments of the present disclosure as herein described it is intended that all matter contained herein shall be interpreted as illustrative only and not in a limiting sense
However, there appears to be no mention or discussion of any alternative embodiments that do not satisfy the discussed functions.
This was all important for the court in finding the function of the claimed frame limitation. The court outlined the process here that – if the patent is explicit about what the element is supposed to accomplish then those functions define the boundaries of equivalence. Although identifying the function is a question of fact, the court in other areas – namely claim construction – reads and interprets this sort of intrinsic evidence as requiring de novo legal analysis. And here, it appears to me that the court did just that.
Notably, the court concluded that all of the specific statements regarding functionality of the frame went into the function-way-result test — meaning that an accused equivalent frame needed to satisfy all of the functions mentioned above, and that a subset was insufficient. Drawing on precedent from Hill-Rom Co. Inc. v. Kinetic Concepts, Inc., 209 F.3d 1337 (Fed. Cir. 2000), the Federal Circuit explained that when a specification “clearly teaches” certain objectives or functions of claimed elements, those functions define what an equivalent element must do.
Crouch’s Theory: Stepping back, I think Smartrend exposes a deeper tension in Federal Circuit doctrine. Markman gave courts responsibility over claim construction because the claims are, in a real sense, the legal boundaries of the patent right. That makes them roughly analogous to statutory text, contract term, or a metes-and-bounds description in a deed. Uniformity, administrability, and the public-notice function all support treating claim meaning as a legal question resolved by judges. But Markman’s logic was always rooted in the unique status of the claims. It did not purport to transform every part of the patent document or prosecution history into a legal instrument to be construed de novo on appeal. The specification and prosecution history serve different purposes—they are disclosures, evidentiary records, and technical descriptions directed to skilled artisans. Treating them as if they were coequal legal text risks smuggling factfinding into the law category simply because the Federal Circuit prefers to resolve the issue itself and disregard our unique jury system.
When the court imports this claim-construction ethos into written-description doctrine or, as here, into the identification of “function” for the doctrine of equivalents, the methodology subtly shifts. Instead of asking what the evidence shows about how a POSITA would understand the disclosure, the court increasingly asks what are the particular words of the specification as if it were an authoritative legal document. In Smartrend, you can see that drift: the court elevates statements in the written description to dispositive status, effectively defining the “function” as a matter of law and leaving the jury no meaningful role. That may produce cleaner doctrine, but it also stretches Markman beyond its justified domain.
In my view, Smartrend is part of a larger gravitational shift. The Federal Circuit’s increasing tendency to resolve specification-based questions as matters of law reflects a broader rebalancing of adjudicative authority away from the jury and toward appellate judges. There are appealing aspects of that shift – coherence, predictability, and control. But it also places substantial interpretive weight on documents that were not drafted to operate as legal text in the way claims are. As more of these fact-laden inquiries are reframed as legal interpretation, courts risk collapsing the distinction between what the patent says and what the patent shows.