62|A FONDO efficiency of the lighting installations. I would like to focus this article in the last one, the one that refers to the provision of natural light. As I mentioned in a previous article, every living being that inhabits our planet has evolved following the day- night cycle, regulating their organism according to this cycle. When humans discovered artificial light they extended their activity to time frames they were not designed for. Now, we can find human activity throughout the 24 hours of the day. However, I am interested in highlighting the evolution of human activity during the day, not during the night. If the discovery of artificial light brought about a change in human activities, it could also be said that placing a person inside a closed space could be considered unnatural. Just as other species, human natural evolution has taken place outside, in open spaces. When humans are closed in a limited space, the conditions of his surroundings are broken placing him in a space that is not “natural”. Obviously, we have been living in society, with constructions for a long time, and we could not imagine living any other way but I also imagine that our bodies will take many years to adapt to these “new” living conditions. Our vision has evolved naturally. The day-night cycle is responsible for the way we see today. Consider that around 80% of what we perceive comes from our sight. Our vision is conditioned by natural light so we should pay attention to it. Natural Light Have we ever thought how natural light is? What characteristics does it have? How does it affect us and conditions us? According to Wikipedia “In optimum conditions, in a perfectly clear day and with the sun rays falling almost perpendicular, only three fourths of the energy that comes from outside reaches the earth’s surface. Almost all the ultraviolet radiation and a large part of the infrared are absorbed by the atmosphere. The energy that arrives at sea level is usually 49% infrared, 42% visible light and 9% ultraviolet radiation. On a cloudy day a higher percentage of energy is absorbed, especially infrared”. When trying to define some type of light, we can use various parameters to characterize it: Color Temperature (Ct) It is a value, measured in Kelvin (K), used to detect the tone of light. A light is considered warm if the Ct is lower than 3500 K, neutral if between 3500 K and 5000 K and cold if it is above 5000 K. The tone of the Sun’s Surface is above 6000 K, which indicates that it is a cold light. If we pay attention we will notice that light varies its tone (Ct) throughout the day. At dawn or at dusk, light is very warm, practically red, while at noon, on a sunny day, light is colder; and even more so on a cloudy day. As we have seen, color temperature throughout the day is not constant; it can fluctuate between 2.000 K (dawn and dusk) and 10.000 K on a cloudy day. This variation in color temperature is caused by the atmosphere. As light goes through the layers of the atmosphere it is absorbed, reflected or refracted by it, causing light’s variations in tone. Color rendering index (Ra or CRI) It is defined as the ability of a light source to reveal the colors faithfully. Solar light, when beaming in every Edifici Pujades. MIAS Architects