The world in a different light: mesopic lighting, energy use and LEDs

This study questions whether white light is always the best option for street lighting schemes. Two trials conducted on the visual efficiency of outdoor lighting in Europe and USA are considered. A brief description of the anatomy of the human eye is provided. At present all directives and lighting standards are based on the standard photopic lumen. The photopic lumen is a unit which is based upon daylight and daytime lighting levels, but the behaviour of eyes is totally different at night. At night, too much light will reduce the amplification effect of the rods as more light equates to greater cone use. Ideally to conserve energy what is required is more sight with less light. It is suggested hat both visual acuity and brightness can be increased by using scotopically enhanced lighting and that a reducing of energy consumption of 30% could be achieved. Light emitting diodes are important in terms of the benefits of higher scotopic to photopic (S:P) light ratios, because they are the best light source for the efficient manipulation of light frequencies. Consequently, lighting design levels could be achieved with wider column spacings or lower wattage lamps. The significance of these findings are discussed in relation to the standards EN13201 and BS55489 and lighting design software. High S:P light ratios may however be less suitable for old people.