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