Doctrine of Equivalents under Patent Law

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A patent is a type of intellectual property that grants an inventor the right to develop something new, useful, and novel. Patent rights allow owners to prohibit others from profiting from their creation while also allowing them to receive a fair return on their time and expenditure. A patent work must be defined in “full, clear, and succinct” terms at all times. 

Copying a patent may not always be a literal imitation, but it might have the same impact. If such copying is permitted, the objective of granting the creator patent protection will be made irrelevant. It will be simple for anyone to copy the patent by making slight adjustments and reap profits greater than those received by the original inventor. The Doctrine of Equivalents was created to counteract such conduct. Even if the claims were not written in their precise form, the court ruled that infringement may occur.

Doctrine of Equivalents

Patent infringement occurs when a patent holder’s exclusive rights are violated. Anyone who exercises the patent holder’s exclusive rights without the patent owner’s permission or licence is guilty of infringement. The Indian Patent Act 1970, Sections 104 to 114, establish rules on patent infringement.

The terms “direct patent infringement” and “indirect patent infringement” (Doctrine of Equivalents) are used to describe the two types of patent infringement. The most prevalent sort of infringement is direct infringement, which occurs when a product that is similar to a patented product or innovation is utilised commercially or sold without the consent of the owner of the patented product or invention. In general, there are two types of patent infringement: literal infringement and infringement on the doctrine of equivalents. It is literal infringement when each element asserted is identical to the purportedly infringing item or procedure. 

Despite the absence of literal infringement of express provisions of a patent claim, the ‘Doctrine of Equivalents’ provides that infringement can still be shown if an aspect of an accused product or service and a claimed element of patented invention are deemed to be legally equivalent. In other words, the doctrine broadens the protection an inventor receives for his invention (based on the literal wording of a patent application’s claims), because it holds that a patentee can claim rights to minor changes to the patented item that are not covered by the original claims but could be accomplished with little effort. It “prevents a person from committing patent fraud by replacing obvious equivalents for elements in claims in order to evade their literal language.”

In an infringement suit, a patent owner can use the doctrine of equivalents if the allegedly infringing item “performs substantially the same function in substantially the same way to achieve the same outcome.” The concept also acknowledges that allowing imitation of a protected invention that does not reproduce every single detail would render the patent grant’s protection worthless and meaningless.

The doctrine of equivalents’ existence, scope, and extent constitute a balance between a fair patent scope and public notice provided by the patent, as well as a balance between incentives to innovate and the costs of uncertainty. It eliminates the unfairness that could arise from an overemphasis on the precise language of patent claims, ensuring the patent’s protection. However, guiding rules such as the all elements rule, the tri-partite test (function, way, result), the insubstantial differences test, the obviousness test, and known interchangeability tests limit the doctrine’s application. They ensure that the doctrine does not take on a “life of its own, unconstrained by the patent claim” to offer protection not covered by the patent, obviating one of patents’ most crucial functions: providing unequivocal public notice of the patentee’s exclusive rights. It’s a legal measure that achieves a proportion between the inventor’s desire to keep a temporary monopoly and the public’s desire to “pursue innovations, creations, and fresh ideas beyond the inventor’s exclusive rights.”

Scope of the Doctrine

There are two criteria that go towards determining the doctrine’s scope.

1) The equivalency test, and

2) The legal prohibitions against equivalents.

Both of these phases must be completed in order to decide if a specific element of the accused product is equal to a specific claim limitation, as a specific element may pass a test for equivalents but still not be considered equivalent if it fails one of the legal limits. 

The equivalency test:

To restrain the application of the doctrine, the

 courts have developed five legal tests:

1) All elements rule,

2) Tri-partite test: Function, way, result,

3) Insubstantial differences test,

4) Obviousness test, and

5) Known interchangeability test.

These tests have been developed by the judiciary in different cases and are now ‘crystallized’ of sorts.

All Elements Rule:

‘Infringement may be proven (but is not always) if an accused device performs essentially the same overall function or work, in substantially the same way, to achieve substantially the same overall result as the claimed invention under the doctrine of equivalents. That does not, however, imply that claim constraints can be ignored. The ingredient substituted in the accused device for the element set out in the claim must not substantially modify the way the claimed invention’s function is performed to be considered a “substantial equivalent” as held in the case of Penwalt Corp. v. Durand Wayland Inc. 

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According to the “All Elements” test, the doctrine of equivalents must be applied to all individual elements of the claims, rather than the claimed invention as a whole. Every ingredient of the claimed invention, or its significant equivalent, must be proven to be present in the accused product or process.  The doctrine of equivalents must be applied to individual aspects of the claim, not to the invention as a whole, because each element in the claim is deemed material to defining the scope of the protected invention. It is critical to ensure that the doctrine’s application, even to a single element, is not given such latitude that it effectively eliminates that element in its entirety. 

Tri-Partite Test

The most typical test for evaluating what is equivalent under the doctrine of equivalents is the tri-partite test, often known as the function-way-result test. According to the tri-partite test, an accused product infringes on the doctrine of equivalents if it performs substantially the same function in substantially the same way to get substantially the same result” as the claimed invention. 

In order for the accused product to infringe under the doctrine of equivalents, it must contain an element that performs substantially the same function in substantially the same way to obtain the same result for each claim limitation, according to a combination of the test and the all-elements rule as held in the case of Industrias Metalicas Marva, Inc. v. Lausell.

Insubstantial Differences Test 

The substantiality of the differences between the claim and accused product is the ultimate question under the doctrine of equivalents. A judgement of infringement under the theory of equivalents requires proof of insubstantial or insignificant differences between the claimed and accused products or processes, according to this rule. 

The test states that a finding of infringement under the theory of equivalents requires proof of insubstantial differences between each claim limitation and the corresponding element of the accused product or processes when combined with the all-elements rule.

Few factors related to substantiality are:

  • Whether persons skilled in the art actually know of the equivalence of the claimed and accused inventions.
  • Whether persons skilled in the art could have known of the equivalence.
  • Whether he had intended to copy, or, rather intended to design around, unintentionally arrived at the same result. 

Obviousness Test

The suggested test would entail applying the non-obviousness test to examine if the differences between the accused product and the claimed invention are substantial. If the changes are substantial and meet the non-obviousness test, the accused goods does not infringe under the doctrine of equivalents. If the changes are minor and do not fulfil the nonobviousness test, the accused product infringes under the doctrine of equivalents. This was opined in the case of Roton Barrier Inc. v. Stanley Works

Because the ultimate conclusion of obviousness is a legal matter, applying the obviousness test to identify equivalents under the theory of equivalents would acquire the accuracy and predictability benefits that come with deciding matters as legal questions rather than factual questions.

Known Interchangeability Test

The known interchangeability test states that proving equivalents under the doctrine of equivalents requires or suffices with demonstration of known interchangeability. In order to prove infringement under the doctrine of equivalents, a patent holder would have to show that the element in question was known to be interchangeable with the claim limitation as a required test.

A showing of known interchangeability would be a sufficient test to establish that the relevant ingredient and the claim limitation were equivalents, but it would not be the only way for the patent holder to prove equivalents.

Legal Bars on Application of Doctrine

Additionally, legal equivalence necessitates that the patent owner’s claim of infringement be free of the doctrine’s various legal defences. The term “legal bar” refers to an area or set of products that are not covered by a patent. They do not specify what is within the scope of a patent; the test for equivalents does that job. The courts in the US have recognized four legal bars namely, prosecution history estoppel, public dedication, afterarising equivalents for means-plus-function claims, and prior art patentability. However, Indian Judiciary has failed to take into account the legal bars to the application of the doctrine as recognized by the US Judiciary in several matters. The legal bars to the application of the doctrine ensure that the doctrine does not conflict with the fundamental principle that claims define the limits of patent protection.  

Problems Associated with the Doctrine

The doctrine of equivalents has been criticised for a variety of reasons, including the unpredictability caused by the doctrine’s lack of a precise linguistic framework, the harm that the doctrine causes to the claims’ notice function, and the unpredictable results reached when deciding the issue of equivalents. 

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The doctrine has been largely criticised on the grounds that “courts have no right to extend a patent’s scope beyond its claims.” The theory of equivalents is certainly a tool for determining non literal infringement; nevertheless, when applied too widely, it creates doubt about the scope of patent claims and interferes with the statutory claiming requirement’s public notice role.

Applicability in India

The Indian courts have also had to deal with cases involving non-literal infringement, though on a far smaller scale. To begin, it should be noted that the Indian judiciary encourages purposive construction of claims, thereby expanding the scope of claims beyond their literal meaning in such cases.

In the case of Bishwanath Prasad Radhey Shyam v. Hindustan Metal Industries, the Supreme Court stated that the proper way to interpret any specification is to read the description of the invention first, and then see what is claimed in the invention, because the patentee cannot claim more than he wishes to patent.

The Delhi High Court analysed and addressed the issue of non-literal infringement for the first time in Raj Prakash v. Mangat Ram Chowdhry and Ors., despite the fact that the Court did not use the phrase “doctrine of equivalents”, the foundation was laid. 

The Madras High Court accepted purposeful construction of claims in TVS Motor Company Limited v. Bajaj Auto Limited, where the Hon’ble High Court held that in construing an allegation of infringement, what is to be seen is whether the alleged infringement has taken the substance of the invention while ignoring the fact that certain parts were omitted or added.

The case of Ravi Kamal Bali v. Kala Tech and Ors., propelled the ideology forward in India. The plaintiff filed a patent infringement complaint, seeking an interim injunction to prevent Kala Tech, the defendant, from manufacturing, selling, or distributing tamper-proof locks/seals, as this would be infringing on his invention. He claimed that Kala Techs do the same work, in a similar manner, and produce the same results, contributing to the infringement. They requested the court to consider the Doctrine of Equivalents when deciding on patent infringement. The Court went on to say that the differences in the body’s main structure make no difference and do not suggest a new invention. Finally, the Court determined that the Defendant’s allegedly infringing product indeed infringe on the Plaintiff’s patent invention. The key reason was that there were no significant changes between the items because their construction and functionality remained the same.

Though the application of the doctrine in infringement cases is a significant step forward for the Indian judiciary, the judgement has been heavily criticised because the Hon’ble High Court failed to consider the legal barriers to the doctrine’s application as recognised by the US judiciary in several cases.


It’s important to note that obtaining a patent and disclosing the patented innovation are mutually exclusive. A patentee makes his innovation known to the public, which grants him a monopoly over it. The grant of a monopoly in favour of the patentee, on the other hand, necessitates his disclosure of the invention. This monopoly, however, is only for a limited time. Once the monopoly gets over, the invention falls in public domain so that the public could benefit from it. This is the rationale behind the law of patents. In this regard, it is opined that if the courts had accepted the proposition that the scope of a patented invention is limited to the literal interpretation of its claims, it would have not only been unfair to a patentee, but would have also deterred any inventor from revealing his invention to the world. In the absence of the doctrine, the scope of a patent would have been of no great significance. The doctrine of equivalent broadens the scope of a patent and protects it from infringement disguised as a minor change or substitution. The doctrine strikes a reasonable balance between a patent’s fair scope and the public notice it gives, as well as between incentives to develop and the costs of ambiguity.

However, it is important to look at the other side of the coin. Despite the doctrine’s existence for nearly 150 years, a logical, linguistic framework has yet to emerge. It causes ambiguity and makes application difficult. Even after looking into the exact extent of the claims and falling outside of it, a member of the public who makes an invention may later be found to have infringed the same patent under the theory of equivalents. With regard to India, it may be determined that there have been very few patent claim infringement cases till now. Despite this, given the rapid speed of technology innovation, the Indian judiciary is expected to encounter similar situations in the near future. In finding a balance between opposing interests, India can develop its own theory based on good techno-legal logic.