Photophobia and photosensitive epilepsy:
a guide for theatre lighting designers, engineers, and technicians.
Arnold Wilkins, University of Essex, UK
and
Donald Holder, Donald Holder Lighting
The process of crafting a theatre lighting design requires creative and technical decision-making, involving many factors such as narrative, point of view, music, staging, choreography etc. The decisions potentially impact the physical well-being of the audience. Acute sensitivity to certain lighting conditions may be a real concern for some individuals. Discomfort, even seizures, can be triggered by strobe effects or similar lighting events that break the fourth wall and impact an audience directly.
This guide provides the reader with a deeper understanding of the potential for these unwanted effects.
The human visual system
We see because of the activity of nerves (neurons) that undertake a complex computation. Our visual system has been designed by evolution so that few neurons are firing at any one time to conserve energy. Even so, the brain uses about 20% of the body's energy. The use of energy is minimised by taking advantage of the similarities between the images the eyes see from one moment to the next.
The natural world
Images in the natural world may seem very different one from another, but they have three things in common.
There is little flicker;
There is hardly any monochromatic light, so colour contrast is modest;
There are few regular patterns: the difference in brightness from one point to another depends on the distance between the points.
The coding used by the neurons takes advantage of these features and the visual system operates efficiently with scenes from nature.
In the urban environment the eye sees very un-natural images. Flicker is common on electronic displays and the colour on the screens can be strongly saturated. Striped patterns, rare in nature, are everywhere to be seen: on grills, grids floor coverings, building frontages etc. As a result, the neurons work harder that they would in nature and the oxygen used by the brain increases.
Migraine
About 8% of men and 20% of women experience migraine headache. These people are usually "photophobic" and averse to the visual stimuli that cause strong neural firing, such as flicker, certain strong colours and striped patterns. This is probably because in migraine the neurons in the brain are more excitable than is usually the case.
Photosensitive epilepsy
About 1 in 100 people has epilepsy. Of these people, up to 5% have photosensitive epilepsy, which is more common in females and in the early teens. These people are sensitive when the visual stimulation gives rise to strong rhythmic neural firing, and this happens when they look at flickering lights or certain patterns.
The visual stimuli that trigger migraine headaches and epilepsy are broadly similar. Both disorders occur as a result of strong neural stimulation, but the stimulation has to be rhythmic to cause seizures whereas this is not the case for migraine, so there are subtle differences, particularly as regards stimulus movement.
Strong visual stimulation
Flicker
The visual stimuli that cause strong neural firing are usually those that are easiest to see. We can see flicker most readily at about 16 flashes per second and this is the frequency that is most likely to cause photophobia and seizures. Patients can be sensitive from 3 to about 60 flashes per second but the worst frequencies are in the range 10-20Hz.
The larger the area of the retina receiving flicker, the greater the risk of discomfort or seizures. A flickering light at the centre of gaze is a greater risk than if the same light is seen in the periphery of vision because more of the brain is involved in analysing central vision. Flickering points of light on an actor's costume are unlikely to be a hazard because of their small size but if a spot-light flickers it is a hazard. If the spotlight is directed at the audience it is more hazardous because if members of the audience then close their eyes the light is diffused through the closed eyelids and illuminates the entire retina, greatly increasing the effect of any flicker.
The risk increases with the illuminance and with the contrast between the bright and dark periods. If the contrast (difference between the peak and the time-averaged illuminance divided by the time-averaged luminance) is less than 5% the risk is low, but the risk increases with contrast reaching a maximum at 30% contrast and above.
When considering the use of strobing or flashing light, bear in mind that large variation (or extreme ‘peaks and valleys’) - in both color or intensity can trigger photophobia or photosensitive epilepsy.
A light that keeps a constant illuminance but alternates in colour can also be hazardous but the risk depends on the difference in CIE 1976 UCS chromaticity. The greater the chromaticity difference the greater the risk, so red-blue alternation at 8-45Hz is a hazard whereas red-orange alternation is much less of a risk.
Patterns
About a third of people with photosensitive epilepsy are at risk not just from flicker but also from steadily illuminated repetitive patterns, such as stripes, particularly if large and of high contrast. If you hold your finger at arm's length it covers about one degree subtended at the eye. The patterns that are most likely to cause photophobia and seizures are stripes that have about 3 cycles of the pattern in one degree. About 50% of patients are sensitive in the range 1-8 cycles per degree. The risk increases with pattern size, but if the entire pattern subtends less than 1 degree at the observers eyes it is of little risk.
Patterns of stripes are not a common problem in the theatre, but if a row of spotlights is shone on to a dry ice mist or if high-intensity automated lighting fixtures fitted with a striated template are focused through a heavy concentration of theatrical haze, a pattern of stripes can be created and this could in principle have a spatial frequency in the hazardous range. If the stripes move (or if the pattern is animated) their effect may be enhanced. If the pattern vibrates at 8-45Hz the risk is as great as from a flashing light.
Simulating nature
In the natural world the only stimuli that are likely to trigger photophobia or epilepsy are sunlight interrupted by the leaves of trees or reflected from waves on the surface of water. If these sources of flicker are created in the theatre using lighting fixtures that are fully masked or installed out of audience sightlines, it is best to avoid local flicker at frequencies in the range 8-45Hz.
Conclusion
This guide is not intended to limit imagination or creativity. It may, however, give lighting professionals the ability to recognize potential triggering effects in a theatre production. It is then possible to provide susceptible theatre-goers with the information they need to properly prepare for these specific moments.