Spectral considerations for outdoor lighting: Designing for perceived scene brightness

Abstract

Photopic illuminance is the photometric metric used today for specifying parking lot lighting levels. The photopic luminous efficiency function does not represent the spectral sensitivity of the perceived scene brightness of parking lots. Sources with a greater proportion of short-wavelength radiation will be seen as brighter for the same photopic illuminance. Moreover, the psychological benefit of providing people with a sense of safety and security in a parking lot is better correlated with the perceived brightness of the parking lot than with its photopic illuminance. Because photopic illuminance is not predictive of the psychological benefit expected from the parking lot lighting system, electric energy will be unnecessarily wasted if specifications are based upon this metric. Specifying parking lot lighting with a benefit metric based upon perceived scene brightness could reduce electric power requirements as well as the amount of radiant energy reflecting from the pavement and escaping into the night sky. A method of equating brightness for different spectral power distributions is provided.