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This article is written by Preetish Agrawal, pursuing a Diploma in Intellectual Property, Media and Entertainment Laws from


A patent can be applied when someone creates something new, useful, and non-obvious. It thus excludes others from taking advantage of someone’s invention and form, giving a fair return on its work and investment. But sometimes, such imitation might not be an explicit imitation, but might implicitly give the same effect. If this is allowed in a country’s legal framework, the protection given to the patent-owner would simply become worthless. To prevent this from happening, the Doctrine of Equivalent Patents (also known as ‘Doctrine of Equivalence’) has been established. Under this doctrine, a patent claim would be infringed from another product or process, if ‘equivalence’ can be established between the elements in the claims of the invention and the elements of the accused product or process. In this article, we would begin by understanding the Doctrine of Equivalent Patents and trace its inception and evolution in the US Supreme Courts. We would then understand the two main tests that are used to understand and govern the doctrine. Finally, we would understand the application of the doctrine in the Indian context.

Doctrine of equivalent patents

Broadly, there are two types of patent infringements; literal infringement and non-literal infringement. For a literal infringement to occur, there needs to be a literal duplication of the patented invention. For example, in the case of Polaroid Corp v. Eastman Kodak [1986]; when Eastman Kodak directly used Polaroid’s ‘instant camera technology’, it was considered to be a literal duplication of the patented invention. An indirect infringement on the other hand suggests some amount of accidental infringement or mala fide intention. For example, in Ravi Kamal Bali v. Kala Tech and others [2008]; even when the main structure of the body of the product was different, the product was found infringing as there was no functional or constructional difference in the products. This would be explained in greater detail later in the article. The Doctrine of Equivalent Patents guides the realm of indirect infringements. The doctrine gives the patentee a means to raise a claim, even when the claim of the patented invention is not identically present in the infringing product. The rationale of the evolution of this doctrine is to prevent people from making minuscule changes to inventions while retaining the functionality of the product, and not allowing them to get benefits of the patentee’s work and investment. We will now trace the evolution of this doctrine in the US courts.

Evolution of the doctrine of equivalent patents in the US Courts

The Doctrine of Equivalent Patents was evolved in the US Supreme Court in the case Winans v. Denmead [1853]. The supreme court in this case had found patents to be valueless if they can be exploited by simply changing the form of the invention being claimed; and had ruled that infringement can occur even if the literal language of the claims is avoided. The application of the doctrine was clarified in the case Sage Products Inc v. Devon Industries Inc [1997], which held that the time of evaluation of the accused element would be of the date of infringement and not the date on which the patent was issued. This is important to ensure that various factors such as the change in technology are being taken into consideration. The doctrine was however carefully cemented in two cases. These are Graver Tank and MFG Company v. Linde Air Prods Inc [1950], and Warner-Jenkinson Co. v. Hilton Davis Chemical Company [1997]. In these cases, the ‘triple test’ and ‘all elements test’ were laid down respectively. These are the two principle tests that are used to govern and determine the Doctrine of Equivalent Patents. We would now look at these tests in greater detail.

The triple test

The triple test was laid down in the case Graver Tank and MFG Company v. Linde Air Prods Inc [1950]. Under this test, the doctrine of equivalence is held to be present when the substituted element performs ‘substantially the same function in substantially the same way to obtain the same result’. The meaning of ‘substantially the same way’ does not mean working identically. This was clarified in the case of Pennwalt Corporation v. Durand-Wayland Inc [1987]. As per the case, every element of the invention must be substituted with a substantially equivalent element. To understand whether the substituted element is substantially equivalent or not, we analyse the role played by the element in the specific patent claim. This was held in Warner-Jenkinson Co. v. Hilton Davis Chemical Company [1997]. Hence, it must be understood that the infringer’s introduction to changes would not satisfy the requirement of a different way. The role of the element must be analysed in the aspect of the claim that it fulfils. Also, as per the case Graver Tank and MFG Company v. Linde Air Prods Inc [1950], it is also important to ascertain the obviousness of the substitution as per the standards of a person who is skilled in that art.

The all elements test

The ‘all elements test’ was laid down in the case Warner-Jenkinson Co. v. Hilton Davis Chemical Company [1997]. Under this test, the doctrine of equivalent patents must be applied to each claim separately, and not to the invention as a whole. It is hence very important to prove that all the elements of a patented invention, or its substantial equivalent, are substantially present in the accused process or product. In the case, it was also highlighted that an element of a claim which is missing would be considered present if there are only insubstantial differences that differentiate the element from that which are present in the accused device or process.

Doctrine of equivalent patents in India

The Doctrine of Equivalent Patents has not been dealt with extensively in India, and hence the doctrine has not been very well developed in the country. The doctrine was first discussed in the case of Ravi Kamal Bali v. Kala Tech and others [2008] in India. However, the concept of non-literal infringement had been discussed before in a few judgements.

In the case of Raj Parkash v. Mangat Ram Chowdhry [1977], the Delhi High Court had discussed the concept of non-literal infringement. While the court had not used the words ‘Doctrine of Equivalence’ explicitly, it had found that the pith and substance of the invention must be paid heed to and that the decision shall not get lost in detailed specifications. The Delhi High Court in the case had heavily relied on the Australian case Beecham Group Limited v. Bristol Laboratories Private Limited [1968]. In this case, it was held that infringement would indeed occur if the substance of the patented article is copied; even when there is no literal infringement. The court had also stressed on the fact that someone should not exploit a patented product or process, just by making an insignificant variation in it.

Also, in cases like Bishwanath Prasad Radhe Shyam v. Hindustan Metal Industries [1978], the apex court had established a proper way of reading any specification. It had held that the description of the invention shall be read first, and only then the claims. This is necessary, as an applicant cannot claim for more than he desires to patent. This ratio is one of the useful premises used to adjudge the Doctrine of Equivalent Patents in India.

Moreover, in the case of Bajaj Auto Limited v. TVS Motor Company [2010], the purposive construction of claims was accepted by the Madras High Court. In this case as well, the court had stressed on finding the ‘substance’ of the invention, ignoring certain omission or addition of parts.

However, it was in the case of Ravi Kamal Bali v. Kala Tech and others [2008], that the first discussion of the doctrine of equivalent patents was brought in the purview of the courts. The court in their factual analysis of the case had found that the alleged product and the patented product had the same ‘usage/purpose’, the same ‘nature of material’, and also worked on the ‘same principle’. As the court found no substantial difference in the constructional and functional aspect of the product, it held that the change in the main structure of the body alone does not indicate the presence of a new invention. While the case had established the practice of the doctrine, it had not explicitly drafted the rules for interpreting the same.

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Although the Doctrine of Equivalent Patents has been developed in foreign jurisdictions for over a century now, a non-ambiguous and clear position has still not been formulated in the Indian jurisprudence. There have indeed been a few cases highlighting the presence of non-literal infringement, and discussing the doctrine of equivalence; but no concrete position has been established by the Indian courts on the interpretation of the doctrine as of now. As the doctrine has not been clearly and linguistically defined in the Indian context, it creates a great deal of inconsistency and confusion amongst the public. One of one hand, an inventor has to bear the constant terror that a party would approach the courts suing it for infringement under the doctrine of equivalence, even when it has tried and steered clear of all the literal claims of the patented products and processes. On the other hand, a victim patentee can never be sure if approaching the court of law would give him relief under the doctrine of equivalent patents, as the interpretation of the presence of the doctrine is highly subjective and unpredictable [Walter J Blenko, 1990]. The Doctrine of Equivalent Patents has been formulated to protect a patentee from non-literal infringement of its patent. However, how non-literal can this infringement get, is something that is still left for the courts to decide.

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