SOLAR RADIATION AND ITS MEASUREMENT - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 24

The sensitivity of many biological systems typically increases by several orders
of magnitude towards shorter wavelengths while the solar spectrum strongly decreases
in the same spectral range. Therefore, the requirements for accurate UV and light
measurements are very demanding 2 .

2. Solar radiation

Extraterrestrial solar radiation

The Sun is the next star in the sky, producing energy by the fusion of hydrogen
into helium. Solar radiation originates mainly from the Sun's photosphere which has a
temperature of approximately 5800 K. The spectrum can be approximated by that of a
black body radiator having the same temperature (Fig. 2). The spectrum is structured by
many lines (Fraunhofer lines) which originate from absorption and emission processes
in the photosphere, the chromosphere and the corona. The natural variability of the solar
flux in the 280 - 400 nm range is less than 1 % over the well known 11-year and 27-day
cycle in solar activity. In contrast, the variability of the Sun - Earth distance of about
3.4 % leads to a variation of the irradiance of almost 7 % due to the inverse square law
of the irradiance of a point source 3 .

2.5

2.0

1.5

1.0

0.5

0.0

250 500 750 1000 1250 1500 1750 2000 2250 2500

W avelength, λ (nm)

Figure 2. Extraterrestrial solar spectrum (solid line) compared with the black body radiator spectrum
at 5780 K (dotted line).