RESPONSE TO UV-B RADIATION: WEIGHTING FUNCTIONS AND ACTION SPECTRA - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 86

limit of this method is given by the potentially large variability of the response in the
range of each differential band, which could significantly alter the shape of the resulting
action spectrum. 25 This limit, however, is present also for monochromatic action
spectroscopy, when using currently available interference filters. The differential
method can be improved by using several irradiance levels for each differential band,
thus reducing statistical errors and allowing a more correct shaping of the action
spectrum. 7

Figure 2. Polychromatic action spectroscopy. Increasingly larger portions of a simulated solar
spectrum are removed by means of different combinations of long-pass filters, in order to determine
an action spectrum of the UV inhibition of photosynthesis in microalgae.

If DAS reduces the problem to the monochromatic situation, the other approach
proposed by Rundel 25 is a deconvolution technique in which it is assumed that the
action spectrum (which in this case can be defined as “biological weighting function”,
BWF 26 ) is an analytical continuous function H(O) and the predicted system response R i
to the i-th irradiation regime E i (O, t) can be expressed as depending on the biologically
active irradiance integrated over the irradiation time:

R i = F[ ³³ O

H(O)E i (O, t)dOdt@

t ,

Once a functional form for H(O) has been chosen in dependence of a given number of
parameters, the latter can be determined by means of iterative calculations to minimize
the differences between each R i and the corresponding measured response. The limit of
this method is that the a priori choice of both the form of H(O) and the functional
relationship F between response and exposure is critical for the correct determination of