Epilepsy affects around 600,000 people in the UK. Approximately 1 in 4,000 of the general population, and up to 5% of all people with epilepsy, have photosensitive epilepsy (PSE), a form of the condition in which seizures can be directly triggered by exposure to flickering or flashing light and high-contrast visual patterns. In a workforce of any meaningful size, the probability that at least one employee has PSE is not negligible. For employers and designers responsible for the lighting environment, this creates both a duty of care and a legal obligation.
This post examines the clinical picture of photosensitive epilepsy in detail, explains the Harding guidelines that underpin broadcast and digital content standards, and sets out what responsible lighting specification and workplace management look like in practice. It is the second post in our series on Lighting, Health and the Human Mind. For a broader introduction to the science of flicker, including IEEE 1789 and the SVM metric, see Flicker in LED Lighting: The Hidden Risk Most Offices Ignore.
Photosensitive epilepsy is a condition in which the visual cortex is abnormally sensitive to certain patterns of visual stimulation. When that stimulation occurs, whether from a flickering light source, a flashing screen, a high-contrast striped pattern, or a moving visual field, it can generate abnormal electrical activity in the brain sufficient to trigger a seizure.
PSE is most commonly diagnosed in childhood and adolescence, with peak onset between the ages of 12 and 14. It is more prevalent in females. While many people with PSE see their sensitivity reduce as they reach adulthood, a significant proportion retain photosensitivity throughout their working life. It is therefore wrong to treat this as a condition that affects only children or that employers are unlikely to encounter.
Seizure types in PSE vary. Some individuals experience generalised tonic-clonic (convulsive) seizures when exposed to a trigger. Others experience absence seizures, myoclonic jerks, or other seizure types. The severity, duration, and recovery time differ significantly between individuals, which is one reason why individual risk assessment rather than blanket assumptions is the right approach in a workplace context.
Not all flickering light is equally dangerous for people with PSE. The risk is concentrated in specific frequency ranges, and understanding those ranges is essential for anyone making decisions about lighting in environments where PSE may be a factor.
The Epilepsy Society UK, in its April 2025 guidance document, identifies 3 to 30 Hz as the most common range of frequencies that trigger seizures, noting that this varies between individuals. Epilepsy Action identifies 10 to 25 flashes per second as the range most likely to trigger a seizure. The research of Professor Graham Harding of Aston University, whose work forms the basis of the broadcast industry's photosensitivity standards, established peak sensitivity at between 16 and 20 Hz, while noting that 49% of patients in his clinical studies were sensitive to 50 Hz, the frequency of the UK mains supply and the base frequency of older fluorescent and phase-control lighting systems.
This last finding is significant. A mains-frequency light source operating at 50 Hz, or a poorly driven LED producing modulation at 100 Hz, does not necessarily fall outside the risk zone for all photosensitive individuals. The commonly repeated assumption that LED lighting at 100 Hz is automatically safe for people with PSE is not supported by the clinical evidence. Safety depends on the depth of modulation (percent flicker) and the individual's specific sensitivity profile, not on frequency alone.
The Harding guidelines, formally the ITC Guidance Note for Licensees on Flashing Images and Regular Patterns, developed by Professor Graham Harding and his son Dr. Osman Harding in the 1990s and subsequently refined, represent the most widely adopted framework for managing photosensitive epilepsy risk in visual media. They were developed in response to documented incidents in which broadcast content triggered seizures in viewers with PSE, most notably a 1997 episode of a Pokémon animation that caused seizures in hundreds of viewers in Japan.
The Harding guidelines as adopted by Ofcom in the UK set out specific thresholds for broadcast content:
While the Harding guidelines were developed specifically for broadcast television and digital content rather than architectural lighting, they established the evidence base and the regulatory logic that has since informed standards in other domains. Harding's 2010 paper Photosensitive Epilepsy and Image Safety, published in the journal Applied Ergonomics, reviewed the clinical literature and confirmed that the same fundamental principles, frequency, luminance change, pattern regularity, and area of visual field affected, apply to any light stimulus capable of triggering PSE, not only screen-based media.
In 2005, Wilkins and colleagues published a paper in the journal Epilepsia, Characterising the Patterned Images that Precipitate Seizures and Optimising Guidelines to Prevent Them, that proposed further refinements to the pattern criteria. Their work identified that striped patterns lasting longer than 0.5 seconds, occupying more than a quarter of the visual field, and with bright stripes exceeding 50 cd/m² in luminance represent a risk profile analogous to that of flickering light. This is directly relevant to interior design decisions around bold striped finishes, dynamic LED feature installations, and signage in spaces where PSE may be a factor.
For lighting designers and specifiers, it is useful to identify the specific scenarios in a commercial environment that are most likely to create risk for someone with PSE. These are not hypothetical, they are real installation and product decisions that occur routinely on commercial projects.
As discussed in the previous post in this series, low-quality LED drivers can produce significant flicker at 100 Hz or at other frequencies depending on the driver topology. For most of the working population, this flicker is imperceptible and its health effects are sub-clinical. For a person with PSE whose individual sensitivity extends to higher frequencies, it represents a genuine risk. A product that passes no formal flicker testing and cannot supply IEC TR 61547-1 data offers no assurance of safety for this population.
TRIAC (phase-cut) dimming systems that were not designed for LED loads frequently introduce significant flicker at lower dim levels. At 20–50% brightness, a poorly matched dimmer can produce 100% percent flicker, meaning the LED is fully switching on and off with each half-cycle of the mains supply. This is precisely the kind of deep modulation that presents the greatest risk. Specifying DALI or 0–10V dimming systems and confirming driver-dimmer compatibility before installation are essential safeguards.
Architectural feature lighting, reception installations, retail displays, and hospitality environments increasingly use dynamic colour-changing or animated LED elements. Where these features involve rapid colour cycling, intensity variation, or pulsed effects, they may enter the frequency and luminance change ranges associated with PSE risk. Any dynamic lighting effect intended for a public or workplace environment should be checked against Harding-equivalent criteria before specification is finalised.
Automatic test systems for self-testing emergency luminaires typically operate by briefly reducing power to the fitting to simulate a mains failure. Depending on the system design, this can produce a visible change in light output. In most cases this is transient and infrequent, but it is worth confirming with the emergency lighting manufacturer that test-mode operation does not produce output in the PSE risk frequency range.
Windows overlooking roads with heavy traffic, adjacent buildings with dynamic signage, or external LED feature installations can introduce flickering light into workspaces that is beyond the designer's direct control. Where PSE is a known factor for an employee, a workplace risk assessment should consider not only the installed lighting but also the ambient visual environment of their specific workstation.
Epilepsy is recognised as a disability under the Equality Act 2010. This means that employers have a legal duty to make reasonable adjustments where an employee with epilepsy would otherwise be placed at a substantial disadvantage compared with colleagues who do not have the condition. Failing to make reasonable adjustments, once an employer is aware of the condition and its workplace implications, is a breach of the Act and cannot be justified on grounds of cost alone.
The Epilepsy Society's employment guidance confirms that reasonable adjustments for an employee with PSE may specifically include modifications to the lighting environment of their workstation. In practical terms, this could mean specifying a low-flicker luminaire for an individual workspace, relocating a workstation away from a problematic light source, replacing a dimmer system with one that does not produce flicker at the employee's working light level, or screening a window that introduces flickering external light.
Under the Workplace (Health, Safety and Welfare) Regulations 1992, employers also have a duty to provide lighting that does not create risk to the health and safety of employees. HSE guidance document HSG38 Lighting at Work identifies stroboscopic effects and flicker as specific hazards to be addressed in workplace lighting risk assessments. For employers with PSE employees, the intersection of the Equality Act duty and the HSG38 risk framework creates a clear and legally enforceable obligation to act.
An important nuance: employers are not required to conduct a separate risk assessment specifically for disabled employees, but they are required to understand how a condition affects an individual employee and to take appropriate action. Epilepsy Action's guidance confirms that an employee is not legally required to disclose epilepsy at application or interview stage, and that employers should be careful not to make assumptions about capability. The appropriate response when PSE is disclosed is a structured, individual risk assessment followed by documented reasonable adjustments.
For lighting designers and specifiers working on commercial projects, the following practices represent the current standard of care for environments where PSE may be a factor.
Even in a well-specified lighting environment, employers should be prepared to respond appropriately if a colleague with epilepsy has a seizure at work. The Epilepsy Society and Epilepsy Action both provide detailed first aid guidance. Key points include:
An agreed seizure action plan, developed with the individual employee and kept accessible to line managers and colleagues, is the most effective way to ensure an appropriate response. The plan should record the employee's typical seizure type and duration, any known triggers specific to that individual, their medication status, and their preferences for how the incident is managed.
Photosensitive epilepsy is one of the most clearly evidenced lighting-related health risks in the workplace, and the one where the legal and clinical frameworks are most developed. But it sits within a broader landscape of lighting conditions that affect neurological and psychological wellbeing, from migraine and visual stress to neurodiversity and circadian disruption.
The next post in this series addresses that broader landscape directly: Neurodiversity and Lighting Design: Creating Workplaces That Work for Everyone, examining how autistic employees, those with ADHD, and people with sensory processing differences are affected by light and what practical specification responses are available.
For related technical reading on flicker measurement and LED driver quality, see Flicker in LED Lighting: The Hidden Risk Most Offices Ignore. For guidance on glare and visual comfort standards in office environments, see What Is UGR in Lighting? How to Reduce Glare in Offices.