Why not 4000 K

Although there is still more to know about spectral sensitivity of human circadian system, in Italy, as a consequence of a Parliamentary inquiry about LED potential hazards in 2011,some decision makers and popular associations have proposed to interdict LED lamps with CCT over 3000K. When dealing with public health, safety is a major concern, however in this case it seems random choice has taken precedence over scientific evidence. Actually defining a good criteria for “circadian” safety could be a major challenge, because: - CCT is not a representative criteria, due to its high ambiguity and to its lack of spectral information; - An approach based on circadian linear models (like Gall’s 2004 sensitivity curve) allows to obtain in an easy way efficiency functions but it does not take into account the circadian human response to light stimuli, which is not linear; - Non-linear models, proposed by Rea in 2005 and 2001 in order to accurately describe the circadian mechanism, are more sophisticated but they are not easy to be applied. The evaluation of the circadian effects is based on the spectral irradiance of light the reaches the eyes, and not on the spectral power distribution of the lighting source. For this reason in exterior environments, assuming negligible spectral inter-reflections effects operated by the surrounding surfaces, it is reasonable to assume that the spectral distribution of irradiance at the eye is the same as the spectral power distribution of the lighting source. In this paper a proposal for an easy way to evaluate the potential “circadian effects” of outdoor light sources is presented: although the model is quite simple, results are in good agreement with those obtained by means of the Rea Non-linear model. With the uncertainties of today's knowledge of the human circadian response, results obtained by evaluating circadian potential effects of the most common light sources used for exterior environments confirm that CCT is an inadequate parameter, especially in the range between 3000K and 4000K. Indeed, owing to the opponent mechanism which involves different receptors on the retina, sources with a CCT of about 4000K appear to have a minor circadian impact compared to sources at 3000K. Consequently, a revision of the current position of decision makers and environmental associations against CCTs greater than 3000K should be reconsidered.