National Physical Laboratory

Sight at twilight

NPL is helping to make it safer to travel at twilight when light conditions are most dangerous.

Most public lighting and vehicle lighting systems are built to deliver optimum illumination in darkness, but 40% of road accidents happen at twilight, even though only a quarter of journeys take place at this time.

Sight at twilight


The Challenge

Much is known about how the eye performs in both bright light and low light.

However until recently relatively little was known about the behaviour and performance of the eye during the transition from one to the other - the so-called mesopic region.

The Solution

Research at NPL, in collaboration with vision research laboratories from across Europe, showed how in high light level conditions, the human eye responds best to green light, while at lower light levels our eyes become progressively more sensitive to bluer light.

NPL then worked with other researchers, under the auspices of the International Commission on Illumination (CIE) to use these to develop a new, internationally-accepted measurement scale, which takes account of this change in sensitivity and allows the best lighting for mesopic conditions to be defined.

The Impact

This new measurement scale is now being considered for adoption into national and international standards and policies for the most effective lighting under mesopic or 'twilight' levels, such as those encountered when driving on roads at night.

Knowing how the eye works at these light levels will allow specification of the best forms of lighting for maximum energy efficiency and visibility. The research could lead to new street lighting standards and safer, more effective vehicle headlamps, signal lighting and street lighting.

As a first step towards this goal, NPL has contributed to a guide by the Institution of Lighting Professionals (ILP) entitled 'Lighting for subsidiary roads' (published October 2012) and to an upcoming CIE Technical Report ;The effect of spectral power distribution on lighting for urban and pedestrian areas;.

For further information, please contact Teresa Goodman

Find out more about NPL's research in Environmental and Optical Radiation & Photonics

Last Updated: 19 Nov 2014
Created: 15 Oct 2010

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