Is white light the right light

This article considers the visual effectiveness of white light, based on recent research at Sheffield Hallam University, UK. Anecdotal claims of the benefits of white light are being investigated. Light measurement is expressed in terms of photopic and scotopic luminance, covering all levels of visible light. Metal halide (MH) lamps, a source of white light, emit radiant power over a broader region of the visible spectrum. Low-pressure sodium (orange) lamps have better photopic luminous efficacy, but at the illuminances specified for street lighting the eye is operating at mesopic conditions, increasing the luminous efficacy of MH lamps. The research under discussion is focused on lighting for pedestrians rather than motorists, investigating brightness and visual performance. Spectral power distribution has an effect on brightness at photopic levels but this is not clearly shown at mesopic levels. Some evidence of spectral effects in mesopic conditions is found, dependent on the nature of the task being undertaken. Research will investigate how lower illuminance permitted for MH lamps affects visual performance.