The twilight zone

Research into how the eye behaves during twilight could lead to the development of improved car headlights for safer driving and better, more energy-efficient street lighting.

Much is known about how the eye performs in both bright light and low light such as moonlight. However, little is known about the behaviour and performance of the eye during the transition from one to the other.

So the National Physical Laboratory has been measuring the way our eyes work in transitional lighting, with the aim of establishing the first international standards for public lighting in twilight conditions.

The research also aims to allow the development of streetlights that can help drivers spot obstacles, as well as more effective and longer lasting emergency lighting systems.

Streetlights and vehicle headlights are built to deliver optimum illumination in darkness. But this is not always effective, as headlights and streetlighting can combine to create perpetual twilight conditions, making it difficult for drivers to see, identify and react to objects in the road.

Twilight conditions, defined as the stage between light and dark, can also occur during the day because of environmental hazards such as foggy and smoky conditions.

The tests devised by the NPL were designed to explore the way eyes react to night driving conditions. Experiments involved identifying whether a subject could see a target against its background, depending on light levels. The team also examined how quickly the target could be identified and whether a driver could identify what it was, under various conditions.

The eye has four types of receptors; three types of cones, and rods. Their effectiveness varies according to light levels and where light is being focused in the retina. When light levels dim, the sensitivity of the cones decreases. Under lighting conditions such as moonlight, the eye is relying on its rods for most of its vision.

The eye also responds differently depending on the colour of the light reaching it. During the day, a person has full colour vision. However, in low light, vision is mainly black and white – shown by the fact that a flower viewed in moonlight appears to be grey.

The intermediate region of light is known as mesopic level. This is when both the rods and cones are working, but none are at full efficiency. This gives a person some colour vision, but not to the standard they would have during the day.

‘We have been trying to see exactly how eyes respond in this intermediate region,’ said NPL principal research scientist Teresa Goodman.

‘This is hard to do, because the response varies greatly depending on conditions. As part of the project we have been looking at how well people can perform tasks when light levels are in the intermediate region.’

The results have allowed the NPL to produce measurements that show how effective certain lighting is by creating a model showing the visual effect of light sources in the intermediate region.

According to Goodman, this could lead to councils installing different coloured lighting in fixtures such as streetlights.

‘As natural light dims, the eye’s peak sensitivity shifts towards blue light,’ said Goodman. ‘Therefore, lights with a high blue content are better at letting you see. This could also cut the amount of output needed, reducing energy use.’

Only a quarter of journeys take place at twilight, but 40 per cent of all road accidents happen at this time. If car headlamp makers can implement the NPL’s findings, it could lead to safer driving. Goodman said better visibility might be created by using headlights that produce light with two types of content.

‘November, just after the clocks have changed, is the worst time of the year for road deaths,’ said Andrew Howard, head of road safety at the AA Motoring Trust. ‘Lighting isn’t as good as it could be and streetlights create a form of twilight. Also, if drivers can see better it may affect how and when they use the roads and help reduce congestion.’

But Goodman warns that more research will be needed into human behaviour before drivers could benefit, because ‘a light source that makes drivers think they can see better may cause them to drive faster and so become more unsafe’.