A biological weighting function, or action spectrum, takes account of
the wavelength-dependency of biological action;
it is a critical parameter in the assessment of the potential biological
effects of 
-related enhanced ultraviolet radiation
[NAS 1979, NAS 1982,NAS 1984, Coohill 1989].
A number of authors
[Rundel 1983, Caldwell 1968, Caldwell Camp Warner 1986]
have shown that an accurate knowledge of 
is
essential in order to make quantitative estimates of biologically effective
irradiance.
Biological weighting functions have traditionally been determined by
evaluating biological responsiveness to monochromatic radiation with the
objective of identifying potential chromophore targets and elucidating
photobiological mechanisms [Coohill 1991].
Caldwell and co-workers (1986)
[Caldwell Camp Warner Flint 1986]
reviewed evidence suggesting that weighting functions determined using
polychromatic radiation and intact organisms may have more ecological
relevance with respect to assessing the ozone reduction problem.
Work during the 80's
[Ray Smith Baker 1982 assessment, Rundel 1983, Caldwell Camp Warner Flint 1986]
provided biological weighting functions for plant damage by UV that
showed relatively strong dependence on UV-B but also showed a significant
contribution from the UV-A (320-400nm) region.
The last few years have seen significant progress in determining biological
weighting functions for the inhibition of photosynthesis by phytoplankton.
Recent work by Cullen and co-workers
[Cullen Neale Lesser 1992, Neale Lesser Cullen carbon 1994]
used principal component analysis of experimental results to estimate
biological weighting functions which describe the effects of polychromatic
radiation on cells with a relatively high resolution, smoothly-varying
spectral response.
Their work provides 
functions for the inhibition of
phytoplankton photosynthesis by ultraviolet radiation, with results
in absolute units (
).
Other recent estimates, using relatively course broadband spectroscopy,
of 
of natural populations include the work
by Mitchell and co-workers (1990),
[Mitchell 1990 action spectra diffuse sobolev],
by Helbling et al. (1992),
[Helbling Villafane Ferrario Holm-Hansen 1992]
and by Behrenfeld et al. (1993a,b).
[Behrenfeld Hardy Gucinski Hanneman Lee Wones 1993,
Behrenfeld Chapman Hardy Lee 1993]
Figure 1 compares several recently
published action spectra, all normalized to 1.0 at 300nm.
There is now broad agreement among these several workers that the
biological weighting, while highest in the UV-B, also contains a
significant UV-A component.
Although it did not employ completely natural irradiance, the experimental
approach of Cullen et al. provides a practical advantage toward assessing
the effects of ultraviolet radiation and ozone depletion, because it
generates weightings in absolute units, permitting direct comparison of
biological effectiveness between experiments
[Neale Lesser Cullen carbon 1994],
including more accurate comparison of biological effects produced by
artificial sources with different spectral distributions and those due
to actual and/or predicted natural radiation.
It also provides for the development of quantitative models for the
prediction of future impacts of ozone depletion
[Cullen Neale 1994 depletion].
There is a need for accuracy because relatively small changes
in 
can lead to large changes in
biologically effective irradiance.