SOLAR RADIATION AND ITS MEASUREMENT - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 27

downward and the upward irradiance above ground (resulting from diffuse reflection)
strongly depends on the type of surface and the wavelength of the radiation. Typical
albedo values are compiled e.g. in Feister and Grewe 6 . Ground albedo values can range
from a few percent (e.g. over grassland) to approximately 90 percent above freshly
fallen snow. The effect of aerosols and clouds is discussed for example in Seidlitz et al. 2
and Mayer et al. 7 .

3. Radiation measurement

General measurement principle

Terrestrial solar radiation is a highly variable quantity. The risk assessment and
biological UV research therefore require accurate measurements of the instantaneous
radiation environment. Even if the ultraviolet spectral band (280- 400 nm) is of primary
interest in this context the visible part should not be neglected, as many organisms
especially plants regulate metabolic functions by activating their light receptors which
may influence their UV response in turn. Nevertheless the measurement of solar UV-B
radiation requires special efforts. The main reason is the very steep decay of the solar
spectrum towards wavelength shorter than 300 nm due to the strong filtering effect of
stratospheric ozone. Spectral irradiance values drop by 6 orders of magnitude within a
bandwidth of 20 nm! The instruments must therefore, exhibit high sensitivity combined
with a very large dynamical range. Depending on the spectral resolution we distinguish
between spectroradiometry, the method for measuring radiation in narrow wavelength
intervals, and broadband radiometry where the irradiance is integrated over a more or
less wide waveband. Like any physical measurement radiometry is finally a
comparison. In this context the unknown radiation field, e.g. global irradiance is
compared with the emission of a well characterised, calibrated standard lamp. Fig. 5
shows the main steps in the assessment of optical radiation (the term designates
ultraviolet, visible and infrared radiation).

Calibration lamp
supplied by national
metrological institute

Source to be measured
e.g. sun, artificial source.

Radiometer

Figure 5. Comparison of detector readings from a known and an unknown source.