Street Lighting Glare: An Investigation using Light Scattering and fMRI Brain Imaging Track Record

Whilst the basis for the disability glare has been understood for a number of years there is some evidence that the current theory may not explain fully the changes in visual performance that relate more to the size and the colour of the glare source.

Currently glare is controlled in main road lighting by Threshold Increment (TI). TI is a function of veiling luminance and road surface luminance and represents the percentage by which the road surface luminance would have to be raised to compensate for the reduced visibility due to the glare.

However, there are reports that larger sources produce less glare than small sources for when equated for intensity of output. Other studies have shown that changing the colour of street lighting from the yellow of high-pressure sodium lamps to white light can improve visibility. It is therefore possible that some part of this improvement may be associated with a reduction in glare.

Discomfort glare is generally considered to be less important in road lighting and it is thought that the strict limits on TI in main road lighting provide adequate control of discomfort glare. However research has shown that discomfort glare may be a problem to motorists by causing fatigue and to pedestrians when providing light to enable facial recognition. This research seeks to use some of the latest tools to investigate glare to develop a better understanding of the subject, so that the design of street lighting and vehicle lights can be improved.

Recently it has become possible to assess scatter in the eye directly instead of depending on visual psychophysical techniques. This technique is critical to the further investigation of disability glare as it gives the researcher the ability to collect together a group of subjects for whom the veiling luminance in any given scene can be calculated precisely. fMRI is another new tool that has recently been developed to the point where it could be useful in the study of glare.

Of key importance to this research is that previous fMRI studies have shown an increase in the blood oxygenation level-dependent activation levels within the visual cortex with increasing stimulus luminance contrast and is regarded as possible candidates for representing the dimension of perceived brightness.

Considering all above this study follows the below objectives and hypotheses: - Light scatter within the eye is a good predictor of the change in visual performance when glare is present - Discomfort glare is a function of source size - Discomfort glare can reduce visual performance - Discomfort glare is a real phenomenon and can be detected in the brain - The role of light source colour and size in the recovery from glare.

This study involves a series of laboratory tests, however, it is also important to check that the findings hold true on streets at night. To that end there will be some tests carried out in conditions representing the tasks in real street conditions and some tests carried out on streets at night.

People

Principle Investigator: Peter Raynham, 

Co-Investigator: Mike Davies

Researcher(s): Navaz Davoodian

Outputs

The following publications have been produced based on the work undertaken for this project:

• Davoudian, N., Raynham, P., & Barrett, E. (2013).  Disability glare: A study in simulated road lighting conditions. Lighting Research and Technology. 
• Raynham, P., & Davoodian, N. (2010). Street Lighting Glare: An Investigation using Light Scattering and fMRI Brain Imaging Track Record. Presented at: ILE Annual Conference.      
• Patterson, E., Bargary, G. & Barbur, J. 2012. Can scattered light improve visual performance? Acta Ophthalmologica, 90, 0-0.               
• Patterson, E., Bargary, G. & Barbur, J. Year. Understanding disability glare: light scatter and retinal illuminance as predictors of sensitivity to contrast. In:  PERCEPTION, 2013. PION LTD 207 BRONDESBURY PARK, LONDON NW2 5JN, ENGLAND, 88-88. 

Impact

Road lighting is a significant undertaking in the UK with 7.1 million lighting points installed consuming around 2.8TWh of electricity per year. The theoretical basis of the current road lighting design methods has recently been challenged. However, any attempt to improve the current design of street lighting for motorists and pedestrians while reducing the electrical power consumed will be conditional on a better understanding of disability and discomfort glare. This is further reinforced by anecdotal evidence that in certain situations street lighting levels could be halved if glare was eliminated. Furthermore the population of night time road users in the UK is changing as the population ages. It was found in the

2001 census that 39.7 % of the population is over 45 and 15.9% is over 65. This is important as current research indicates that older people are significantly more susceptible to glare. The picture is further complicated by introduction of new ophthalmological procedures, such as laser eye surgery, that in general increase higher order aberrations in the eye and in some cases may well make patients more susceptible to glare. 

For further information please contact Peter Raynham