Abstract After decades of intense research, the effects of light on the circadian system have been proved to exist. By studying the relationship between human physiology and light, research has also advanced to the point that there is the belief that these effects will be in future regulated by Standards. Establishing a direct connection among the fundamental radiometric, the traditional photometric and the new circadian approach to lighting becomes a fundamental step for introducing new concepts to lighting practitioners. This theoretical paper has the aim to propose a correlation between the traditional approach to lighting design with the new and still not definitely defined circadian approach by means of a photometric to circadian correlation factor. By this way, once the circadian efficiency function will be experimentally determined, technicians will be able to evaluate also from a numerical point of view the impact of lighting on the circadian system, adding health considerations to the traditional design process.