Visual performance under CMH and HPS street lighting at different power and dimming levels

Highly efficient light sources are under development to reduce the power consumption of lighting systems and hence greenhouse-gas emissions in line with the international Kyoto agreement. Preliminary calculations suggested that, within Europe, the expected annual energy savings could be around 10 terawatt hours (TWh), which corresponds to a reduction of at least 5 million tons of carbondioxide (CO2) emissions. Such systems, applied to street lighting, should simultaneously produce benefits in road safety, energy efficiency and the visual quality of outdoor lighting schemes. The earlier European 'NumeLiTe' (Numerical Light for Technology) project, in which TRL participated sponsored by the European Commission, the UK Department for Transport and the UK County Surveyors Society, aimed to prove the feasibility of an optimal and energy efficient outdoor city lighting scheme. This utilised broad spectrum �white� light intended to match the needs of the human visual system better than conventional street lighting, when performing tasks essential to road users. This project was designed in the light of this earlier project to investigate the feasibility of using CMH and HPS lamps of significantly lower power than used in the NumeLiTe project. Moreover, because of the complexity of the human visual system and the road user�s task, evidence was needed to confirm that reduced illumination would not have any adverse effect on road user behaviour and road safety. The results did not show any visual performance benefit from the use of the CMH lighting for traffic streets, but provide some evidence that it may be safe to reduce lighting levels, in traffic streets normally lit to higher levels, to 0.5cd/m2 during the quieter period of the night, as permitted by current standards. Dimming to this lower level, using either type of lamp, should be free of any significant degradation in visual performance, and allow potential energy savings to be realised.