Since the case of Catnic Components v Hill & Smith came before the House of Lords in 1978, the principle of "purposive construction" has been established in UK law regarding interpretation of patent claims. In the Catnic case, the issue was about whether a patent claim to a lintel having a support member "extending vertically" covered an otherwise identical lintel having a support member that was not quite vertical. According to Lord Diplock's leading judgment, "A patent specification should be given a purposive construction rather than a purely literal one derived from applying to it the kind of meticulous verbal analysis in which lawyers are too often tempted by their training to indulge". This meant that the term "extending vertically" was to be purposively construed to effectively mean that the member should be sufficiently vertical for it to serve its purpose. A few degrees either way would still allow it to serve its purpose, so the term in claim 1 of the patent was not to be construed narrowly and the defendant's lintel was found to infringe.
Even though the law has since developed into what is now, following Actavis v Eli Lilly, effectively a doctrine of equivalents for patent claims, the principle of purposive construction has not gone away and is still in use when it comes to interpreting specific terms in a patent claim in a way that means they should perhaps not be construed as narrowly as they first appear.
A nice example, again with a simple mechanical invention, has recently appeared in the form of the judgment of Mr Justice Zacaroli in Heraeus Noblelight Limited v First Light Lamps Limited  EWHC 1950 (Pat), issued on 31 July 2023. The case related to Heraeus' European patent EP1598845B1, which claimed a method for forming a quartz glass lamp, the invention relating to how a seal between a metal electrode and a quartz glass tube was formed during manufacture of the lamp. The problem the invention aimed to address was the previously complicated method of manufacture that involved many different stages and sometimes still resulted in failure of the seal between the electrode and the quartz glass.
It was already known that an intermediate glass material having a thermal expansion coefficient between that of the electrode and the quartz could be used to reduce the differential stress set up on cooling the seal, but the manufacturing process was still complicated. The invention simplified this by, instead of making two seals with the intermediate glass that were subsequently joined together, forming a direct seal using a bead formed on the electrode that joined directly to the quartz tube, following which an internal pressure in the tube caused a smoothing out of any edges internally. This is best illustrated with two simple drawings from the patent, reproduced below. In the first drawing (Figure 2 of the patent), the electrode 2 has a bead 4 of intermediate glass material around its electrical feedthrough 3. This is inserted into a quartz tube 1 while heat is applied and the electrode is rotated relative to the quartz tube. After the glass bead is sealed against the quartz tube, as shown in the second drawing (Figure 1 of the patent), an internal pressure is applied to the quartz tube to smooth out the internal join with the glass bead, forming a smooth internal radius that reduces the possibility of cracking on cooling.
|Electrode (2,3) with bead (4) prior to sealing with quartz tube (1)|
|Electrode after being sealed against quartz tube.|
The patent claimed the method in the following way (with emphasis added on the key feature in question):
1. A method of forming a quartz glass laser lamp or quartz glass flash lamp containing electrodes (2) at opposite ends within a tube (1) of quartz glass, an end of said tube (1) having an internal diameter, an external diameter and an annulus, wherein said method comprises formation of a glass to metal seal at said one end of the quartz glass tube comprising the following steps:
(1) the electrode (2) or its electrical feed through (3) is surrounded with a sealing glass (4) comprising a sheath and a bead, the bead being bigger than the internal diameter of the end of the tube (1) but not bigger than the external diameter of the end of the tube (1),
(2) heating the bead to a soft state while rotating it,
(3) inserting the bead into the annulus so as to form a seal,
(4) after the insertion, heating the seal so that the sealing glass (4) wets on and fuses with said one end of the tube (1), both internally and to the end of the tube (1),
(5) after said fusing of the sealing glass (4) with said quartz tube (1), applying an internal positive pressure while the sealing glass (4) is molten causing the sealing glass (4) inside the tube (1) to move back towards the end of the tube (1) so as to form an internal radius of sealing glass (4) which is directly sealed between the electrode or its electric feed through (3) and the tube (1) of quartz glass.
It is clear that it would be necessary for the bead to be bigger than the internal diameter of the end of the quartz tube, otherwise a seal would not be made. However, did the claim really require that the bead was literally "not bigger than the external diameter of the end of the tube"? The question was important because the alleged infringing method did involve making the bead bigger, only by around 0.1-0.3 mm, but otherwise involved forming a seal in the same way as claimed in the patent.
Heraeus argued that the term "not bigger" should be interpreted purposively as "not bigger by reference to tolerances that formed part of the common general knowledge [CGK]", claiming that 0.3 mm was within such tolerances. Alternatively, the term should be interpreted to be "not bigger such that it impairs the strength or utility of the seal", encompassing oversized beads. They also argued for literal infringement on the basis that some of the beads used by the defendant would inevitably be not bigger than the quartz tube due to variations, or alternatively that according to the doctrine of equivalents a lamp manufactured with oversized beads would be an equivalent.
Having heard the arguments from both sides, including expert evidence, the judge found that the skilled person's CGK would include beads of varying sizes, including oversized beads. The question was then whether, as a matter of construction, the words "not bigger than" the outer diameter of the tube was to be read as imposing a stricter tolerance than that which the skilled person would understand to be CGK. The claimant argued that it did not, while the defendant argued it did. The judge considered that, since the requirement for the outer diameter of the finished lamp did not generally require such a tight tolerance, the protection sought in the patent was not limited to a method in which beads were precisely no bigger than the outer diameter of the tube but included variants in which the bead was not materially bigger than the outside diameter of the tube to an extent "which could have no material effect upon the way in which the invention worked" (per Lord Diplock in Catnic). Bigger in this case therefore meant not bigger such that it impaired the strength or utility of the seal, which encompassed any bead made within CGK tolerances.
For completeness, the question of infringement by equivalents resulted in the same conclusion. Although it was not necessary to consider the question of literal infringement, this was determined not to be found because the evidence did not show smaller beads actually having been used.
The patent was found to be valid over the prior art cited by the defendant (the arguments for which I will skip over here as they don't seem to me to raise anything of interest), so the patent was found to be infringed by the defendant's process on both a purposive construction of the claimed invention and according to the doctrine of equivalents.
You might think that the lesson to be learned from this would be that an overly specific patent claim can still be infringed even if some features are not literally present in the alleged infringement, and you would be correct. However, the trouble and expense caused by including such a specific feature in a patent claim can nevertheless be enormous if such a claim is litigated. Patent attorneys, being generally conservative and not wishing to cause unnecessary trouble, will most likely take a different lesson, which is that specific terms in a claim that are not strictly necessary for the invention to work or to provide a distinction over the prior art should be avoided. Following Catnic, drafting practice did not change so that specific terms such as "vertical" were used by patent attorneys, happy in the knowledge that they could be interpreted to mean "vertical enough to do the job required". Instead, the word "substantially" was used ubiquitously to provide more wriggle room, often without realising the reason behind doing it. In a similar way with this case, I would guess that any patent attorney reading it would have a more careful think about the claim in front of them and perhaps try not to use any limiting terms that really do not need to be there. I certainly will.