SOLAR RADIATION AND ITS MEASUREMENT - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 32

to determine shape and width of the monochromator's slit function, which characterises
the spectral bandwidth.
Uncertainty Estimation . Spectroradiometric measurements are unfortunately
among the least accurate of all physical measurements, because many rather different
components contribute considerable measurement uncertainties 12 . The main sources of
uncertainty and their approximate contributions are listed in Table1. A total uncertainty
of approximately 15% is a realistic estimation. Absolute spectroradiometric
measurements are tedious and they require strictly applying the rules of good laboratory
practice (GLP) 9,13 , i.e. well maintained equipment and the complete documentation of
the measurement procedures and calibration sources as well as an appropriate error
estimation are obligatory. International instrument intercomparisons are necessary to
keep a high level of expertise and reliability 14 .

Table 1 . Main sources and typical amount of uncertainty in spectroradiometry.

Source of error Typical relative uncertainty

calibration lamp 3 - 4 %

procedure 5 %

input optics cosine error

(depending on angle of incidence)

5 %

radiometer wavelength accuracy 3 - 10 %

other sources linearity 1 %

temperature 1 %

Total 15 %

Broadband measurements

Biological weighting functions . The amount of energy ('radiant exposure') or the
number of photons at a certain wavelength which is necessary to induce a given
biological effect usually depends on the wavelength O of the radiation. This wavelength
dependency is described by the action spectrum s(O). In general, the action spectrum
and thus the sensitivity of a biological system increases with decreasing wavelength in
the UV-B range. Action spectra exist for a large variety of biological systems and
effects, see e.g. Ghetti in this volume 15 .
In the case of monochromatic irradiation at wavelength O, the effectiveness of
the irradiation for the induction of a certain biological effect is described by the product
of the spectral irradiance (or the spectral radiant exposure) and the value of the action
spectrum at O. This multiplication is often called weighting, the product is the weighted
spectral irradiance or the weighted spectral radiant exposure. If the sample is irradiated
with a broadband (polychromatic) source weighting is performed by multiplication of
the action spectrum with the polychromatic source spectrum and integration over the
desired wavelength range. Hereby it is tacitly assumed that photons at different
wavelength contribute independently to the effect under consideration and the action
mechanism is similar for all wavelengths. In order to avoid confusion, it should always