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Return to Ritherdon: Photosensitive Epilepsy
This document is to be used as part of the health and safety risk assessment for the Return to Ritherdon exhibition.
Summary of Assessment
The rate of flashes the lighting system produces is not of a higher enough rate to cause a photosensitive epileptic seizure -- according to the referenced sources below. Based on the live test data, the system produce a flash rate of 1 Hertz through out the course of one day with approximately 8 to 9 hours on usage, on the 23/04/2021. This falls outside the 3-30 Hertz claimed by Epilepsy Society (link below).
Information About Photosensitive Epilepsy
According to Epilepsy Society, a flashing/flicker light between 3-30 hertz can trigger a seizure.
What rate of flashing light can trigger seizures?
Between 3-30 hertz (flashes per second) are the common rates to trigger seizures but this varies from person to person. While some people are sensitive at frequencies up to 60 hertz, sensitivity under 3 hertz is not common.
More information can be found at:
- Epilepsy Society (home page)
- Epilepsy Action
- National Health Service (NHS)
How This Relates to the Return to Ritherdon Project
The project (unnamed at time of writing) this document refers to is one artwork in a much bigger project (called Return to Ritherdon). Within 'unnamed project, is flashing lights. Because of that, the rate of flashing/flickering the artwork produces needs to be reviewed as part of the health and safety risk assessment.
Viewing the Raw Data
This document will only provide a summarised view of the data because of the size of it. If you would like to review the raw data please head to:
- data
- raw-data/results (for the data after it's been processed and used for this risk assessment)
How the Data was Processed/Reviewed
The data was analysed using the code in the Flicker repository. Please review the code/repository there for more information on how the code works -- it is outside the scope of this document.
Breakdown of Data Analysis
The sample of data reviewed for this assessment was taken from the
Light Meter readings produced by factory1 running the software
provided by
Light-Meter,
on the 23/04/2021. Overall, there was 84,294 light readings
taken over the course of between 8 to 9 hours. Also, this was a
live test with readings taken from the intended environment,
under real-world conditions. Of those readings, the number of
readings taken is as follows,
Note: Minutes and Hours are approximate values.
| Readings-per-Second | Seconds | Minutes | Hours |
|---|---|---|---|
| 1 | 5,344 | 89 | 1.5 |
| 2 | 2,955 | 49 | 0.8 |
| 3 | 13,284 | 221 | 3.5 |
| 4 | 8,297 | 138.28 | 2 |
| --------------------- | --------- | --------- | ------- |
| Total | 29,880 | 498 | 8.3 |
Let's say you have the following readings (sample taken from
test-data.csv stored in raw-data,
Note: The '.000000' is a artefact from the code's formatting of the data. You can ignore it. I've only kept it in to keep the data here aligned as close as possible with the raw and computed data data.
| Time-Stamp (YYYY-MM-DD Hr:Min:Sec:MicroSec) | Reading |
|---|---|
| 2021-04-23 07:02:50.000000 | 17 |
| 2021-04-23 07:02:51.000000 | 17 |
| 2021-04-23 07:02:51.000000 | 17 |
| 2021-04-23 07:02:52.000000 | 17 |
| 2021-04-23 07:02:53.000000 | 17 |
| 2021-04-23 07:02:54.000000 | 17 |
| 2021-04-23 07:02:55.000000 | 17 |
| 2021-04-23 07:02:55.000000 | 17 |
Note: I've omitted the Id column from the example above because is
not relevant for this assessment. If you review the raw data, you
will come across that column. More often than not, you can ignore it
if you're not looking to review/work with the code written for main
the system.
If you look at the time-stamp 2021-04-23 07:02:51.000000, you will
see there are two readings recorded. This sample is very small but you
can see the remaining time-stamps have only one reading per each
second intervals. With this in mind, please note there are eight
reading spread across a five-second time-span. That is why the /first/
table above is broken down into time and not just hard frequency
totals. For 5,344 seconds on 23/04/2021, the system took a light meter
reading at one reading-per-second.
According to the sources listed above, 'between 3-30 hertz (flashes per second) are the common rates to trigger seizures'. This means the number of readings to review can be reduced to those operating at four readings-per-second. This is because the lights in this system need to go from an off-state (1) to an on-state (2) and back to an off-state (3). The extra (fourth reading) is needed because there needs to be a starting state (a zero if you will). Below is a sample of a moment when four readings were taken in a one second period,
| Time-Stamp (YYYY-MM-DD Hr:Min:Sec:MicroSec) | Reading | State |
|---|---|---|
| 2021-04-23 09:04:07.000000 | 41 | On |
| 2021-04-23 09:04:07.000000 | 37 | Off |
| 2021-04-23 09:04:07.000000 | 36 | Off |
| 2021-04-23 09:04:07.000000 | 36 | Off |
Note: The light paired with factory1 is set to turn on (in the
gallery gallery1) when the reading is above 39.
In the table above, you will see the light changes state (on to off)
once. For it to reach the noted rate of 'between 3-30 hertz', the
third reading in the table will need to have been above 39 to
trigger the light back on. This would have made the light change from
on to off three times, taking into account the need for a 'starting'
state.
Referring back to the table listing the various rates of readings-per-second (the first table), there are only 8,297 readings (technically it's seconds but 'readings' is easier to process) instead of 84,294. This can be filtered down even more, though. This is because not all four readings-per-second occurrences cause the light to change its current state (I.E. it doesn't go from off to on). With this in mind, the number of entries to review drops to 8 when you list just the times when the light changes its state. They are,
| Time-Stamp | Readings | States | Hertz |
|---|---|---|---|
| 2021-04-23 09:04:07.000000 | 41, 37, 36, 36 | on, on, off, off | 1 |
| 2021-04-23 09:04:11.000000 | 36, 38, 40, 37 | off, on, on, off | 1 |
| 2021-04-23 10:54:05.000000 | 39, 39, 40, 40 | on, on, on, on | 0 |
| 2021-04-23 10:54:07.000000 | 39, 39, 39, 40 | on, on, on, on | 0 |
| 2021-04-23 10:56:46.000000 | 40, 40, 39, 39 | on, on, on, on | 0 |
| 2021-04-23 10:57:13.000000 | 40, 41, 40, 39 | on, on, on, off | 1 |
| 2021-04-23 10:58:13.000000 | 39, 46, 44, 39 | off, on, on, off | 1 |
| 2021-04-23 11:00:11.000000 | 39, 42, 46, 39 | off, on, on, off | 1 |
This tables shows the limitations of the system to trigger a photosensitive epileptic seizure. The system produced at most a flash rate of 1 Hertz over the course of approximately 8 to 9 hours of activity.This is below the '3 to 60 Hertz' claimed by [Epilepsy Society(https://epilepsysociety.org.uk/photosensitive-epilepsy) (more references/links at the top of this file).