  Light pollution is one of the most rapidly increasing types of environmental
degradation. To limit this pollution several effective practices have been
defined: shields on lighting fixtures to prevent direct upward light; no over
lighting, i.e. avoid using higher lighting levels than strictly needed for the
task, constraining illumination to the area where and when it is needed.
Nevertheless, even after the best control of the light distribution is reached
and when the proper quantity of light is used, upward light emission remains,
due to reflections from the lit surfaces and atmospheric scatter. The
environmental impact of this "residual light pollution" cannot be neglected and
should be limited too. We propose a new way to limit the effects of this
residual light pollution on wildlife, human health and stellar visibility. We
performed analysis of the spectra of common types of lamps for external use,
including the new LEDs. We evaluated their emissions relative to the spectral
response functions of human eye photoreceptors, in the photopic, scotopic and
melatonin suppressing bands finding that the amount of pollution is strongly
dependent on the spectral characteristics of the lamps, with the more
environmentally friendly lamps being low pressure sodium, followed by high
pressure sodium. Most polluting are the lamps with a strong blue emission, like
white LEDs. Migration from the now widely used sodium lamps to white lamps
(Metal Halide and LEDs) would produce an increase of pollution in the scotopic
and melatonin suppression bands of more than five times the present levels,
supposing the same photopic installed flux. This increase will exacerbate known
and possible unknown effects of light pollution on human health, environment
and on starry sky visibility. We present quantitative criteria to evaluate the
lamps based on their spectral emissions and we suggest regulatory limits.
