Spectral Distribution of Natural Sunlight
“Spectral Distribution of Natural Sunlight" 
Natural sunlight is the light consisting of photons. The sunlight reaching the earth's atmosphere is divided into ultraviolet rays (100 to 400nm), visible rays (380-780nm) and infrared rays (700nm-1mm) according to the wavelength. And DNA, the genetic material of all living organisms, absorbs specific ultraviolet rays in a particular wavelength (fig1). However, the ultraviolet (UVC) (100-280 nm) region is absorbed by the ozone layer in the atmosphere and does not reach the surface.  

Natural sunlight has different spectral distribution over the morning, afternoon and evening (fig2). Especially in the morning and afternoon, it has different spectral distribution including blue light from evening, so that the biorhythm (or circadian rhythm) is adjusted differently for the alertness. And the spectral distribution of typical LED and fluorescent lamp shows that it is very different from the spectral distribution of sunlight (fig3). As such, the biorhythm of human beings has been influenced by sunlight from the beginning. Therefore, it might be necessary for the modern people who are living indoors to be exposed to the sunlight according to morning, afternoon, and evening for the natural biorhythm of human health.  

Sangwook Jung, Ph.D. 
Our Healthcare Committee 

* Next week's topic: The effect of green light on fish growth

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