D.W. Windsor shines in IPEC patent judgment in the UK

An illuminating IPEC patent judgment was recently delivered by HHJ Melissa Clarke in D.W. Windsor Ltd v Urbis Schréder Ltd [2025] EWHC 563 (IPEC)  who held that D.W. Windsor Limited’s patents for lighting units were valid and infringed. The judgment analyses the law regarding the person skilled in the art (“PSA”) and provides insights into the hypothetical construct of the PSA, which this article will focus on.

Summary of the case

• D.W. Windsor Ltd, a British designer and manufacturer of lighting solutions, brought a claim for infringement against German competitor Urbis Schréder for UK patents GB 2 495 509 and GB 2 495 566. Both patents describe “A pathway lighting unit” which “relates to a pathway lighting unit for a walled pathway”. The patents also include an anti-climb measure for safety.
   
• The defendant counterclaimed for declarations of invalidity and revocation of the patents on the grounds of lack of novelty and obviousness. By the time of the trial, only the issue of validity remained (as the defendant had admitted to infringement). 
   
• The judge considered that she would “need to determine, inter alia, disputes about the identity of the person skilled in the art (“PSA”), the common general knowledge at the Priority Date, and the inventive concepts disclosed in the Patents” before deciding on validity. 
   
• The parties and experts significantly disagreed over the identity of the PSA and the common general knowledge (“CGK”) that the PSA should have, and this is addressed in depth in the judgment.
   
• Prior to trial, the defendant wrote to the claimant describing the PSA as “an individual with expertise in pedestrian safety and anti-vandalism measures in public spaces. This person would have knowledge and experience of public infrastructure projects concerning the development of bridges and walkways.” The claimant treated this letter as a formal statement from the defendant regarding the identity and expertise of the PSA. Whereas the defendant’s expert, Mr Keay, described the PSA as “someone with an interest in items (such as handrails and lights) that were suitable for safe use on publicly accessible infrastructure such as at railway stations.” The judge found both definitions too narrow and instead preferred the claimant’s definition, which stated that the PSA was a lighting support design engineer working in lighting support design. As a result, she concluded that the PSA’s CGK did not extend to safety matters relating to anti-climb/vandalism measures, nor to health and safety regulations, as the defendant had contended. 
   
• HHJ Clarke held that the key claims of the patents were valid and infringed by the defendant as admitted.
   

Who is the PSA?

The law 

The law on the PSA was summarised in Hospira UK Limited v Cubist Pharmaceuticals LLC [2016] EWHC 1285 (Pat) and again in Garmin (Europe) Ltd v Koninklijke Philips NV [2019] EWHC 107 (Ch). There was no dispute between parties as to the legal principles of the PSA. 

HHJ Melissa Clarke then considered and applied Illumina Cambridge Ltd v Latvia MGI Tech SIA [2021] EWHC 57 (Pat). The concept of the PSA applies in two distinct circumstances. The first being the person to whom the patent is addressed and whose attributes, skills and CGK are necessary to implement the patent. This will always be the appropriate PSA when addressing sufficiency, since the patentee is entitled to put together his invention by combining various skill sets. The second, refers to the PSA in relation to obviousness, and is “In nearly all cases… the same as the first kind”, but not always, see: Schlumberger Holdings Ltd v Electromagnetic Geoservices AS [2010] EWCA Civ 819). Schlumberger identified: “In the case of obviousness in view of the state of the art, the first key question is generally “what problem was the patentee trying to solve?”. HHJ Melissa Clarke considered that neither the problem nor the invention was limited to railway infrastructure, and the problem the invention sought to solve was not within the field of anti-vandalism measures. 

The hypothetical construct 

HHJ Melissa Clarke confirmed that the PSA is a hypothetical person (or team) with practical knowledge and experience of the field. The PSA will read the patent specification with the CGK of the relevant field, knowing that its purpose is to disclose and claim an invention. The PSA is uninventive and unimaginative – a fact that can be awkward when explaining this to your expert. 

Why did the definition of the PSA result in the patent being held valid?

Validity in this case hinged on whether the PSA would have needed to use an inventive step. This made establishing the correct definition of the PSA essential before assessing validity. 

Lessons learnt

HHJ Melissa Clarke agreed with the claimant that the PSA in this case was a lighting support design engineer working in the established field of lighting support design. She held that this person would not have been expected to have additional knowledge on health and safety, particularly relating to railway infrastructure. This judgment signals arguments for an overly narrow definition of the PSA (like the defendants’ expert’s attempt in this case) are unlikely to be successful and could in fact risk misidentifying the relevant field. 

HHJ Clarke’s comment that the “alternative established field” of the PSA “has not been pleaded, argued or put to the Claimant’s expert by the Defendant such that the Claimant has had an opportunity to address it” is worth noting. It confirms that arguments defining the PSA or an alternative established field (and arguably any substantive pleading) should be expressly pleaded and not hidden in the shadows. 

This article was authored by Noo Kristin-Ross, IP Paralegal Manager at Bird & Bird.