Light and people
Claiming that “to see we need light” is perhaps now regarded as somewhat self-evident: we are also aware that light is both useful and a necessity. However, it is not only needed to see by – light also plays an indispensable role in our very existence. “No life without light” is a phrase in which the word light can now be replaced by radiation and energy, as forms of life have been found to exist in the absence of visible light but in the presence of other energy sources, such as UV, ultraviolet, radiation and IR, infrared, radiation.
Our biological clocks, i.e. our sleep-wake cycles, are controlled by variations of light. The body’s capability to absorb vitamin D, its metabolism and its immune defence are also controlled by light and possibly by other, so far unestablished, factors.
What is light?
In the eighteenth century, Dr. Samuel Johnson noted that “we all know what light is; but it is not easy to tell what it is”. Today, the recognized explanation says that perceptible light is visually experienced electromagnetic radiation.
It is important to include the distinction “visually experienced”, as man in his natural environment is subject to radiation from the whole of the electromagnetic spectrum. The part of the sun’s radiation spectrum that reaches Earth comprises about 50% light radiation, about 45 % IR radiation and about 5% UV radiation. These figures also indicate the relative proportions of the energy contents of these different types of radiation.
Light, IR and UV radiations
We experience different types of solar radiation in different ways. IR radiation is absorbed by the skin, where cells or receptors register it as heat. When the heat sensation becomes too great, our brain transmits signals to regulate our body temperature by opening pores in the skin and creating a sweat, resulting in a reduction of the sensation. As far as we know, human beings do not have any receptors with which we can experience UV radiation. On the other hand, the effect of an excessive dose of UV radiation can be detected – our skin turns red and we might experience a burning sensation and nausea. Light radiation is absorbed less by the skin than both IR and UV radiation. All our light-sensitive cells or receptors are situated in our eyes and these react in different ways, depending on whether they have a biological or visual function.
Light in buildings
In the wake of these more recent discoveries, it has been shown that both lighting levels and light quality, i.e. the spectral distribution of light, are insufficient from a biological and health point of view. We evolved in a daylight environment in which the radiation intensity outdoors was, and is, far greater than that indoors. Outdoors, the intensity of illumination during the day varies between 500 lux on an overcast day and 100 000 lux on a clear sunny summer day. Indoors, the intensity of illumination is usually between 200 lux and 500 lux. Compared to the conditions outdoors this like very weak twilight.
The amounts of daylight in buildings must be increased considerably. This will mean designing buildings differently, as those built today at best only provide sufficient levels of daylight a few metres in from the facade of the building. An architectural paradigmatic shift is needed, if buildings are to provide good healthy environments in which daylight can function both biologically and visually, while at the same time replacing electrically created light during daylight hours. The problem that has to be solved is how to introduce a sufficient amount of daylight into buildings without having a heating effect that will call for a cooling solution – or how to develop and use energy-efficient light sources that display spectra that are effective both visually and biologically.