General Aspects on Action Spectra


In general solar light drives the vital mechanisms in most livings. Solar radiation has, however, also the potential to cause damaging effects, especially the ultraviolet (UV) part of the spectrum. Long-term adoption to the ambient environmental conditions has led to an equilibrium of photoprotection, damage and repair (often light induced too) in the livings to maximise vitality and reproduction. The photoinduced effects can be divided into: vital, damaging, protective and repair. Damaging effects become overwhelming only in some exceptions when photoprotection and the damage counteracting repair mechanisms are overcharged. Damaging effects are caused by overexposure. However too low levels of solar UV radiation can also show negative influences on organisms due to too less stimulation of vital mechanisms (Vitamin D production, photosteering, phototaxis, photomovement, ...) . The equilibrium in organisms can be disturbed by rapid environmental changes, emigration into a new environment, or non-adequate behaviour.

Photobiological effectiveness
An action spectrum describes the spectral effectiveness of a photobiological or photochemical process. It can be given by relative values or as inverted threshold curve. Weighting the spectral irradiance of a source with the action spectrum gives the effectiveness spectrum of the source. An integration over the whole spectral range delivers the biologically effective irradiance (Ebiol). Under clear sky Ebiol depend in first order on the total ozone content of the atmosphere (O3) and the solar height above horizon (sh). The change in Ebiol due to a change of 1% in O3 is often described by the RAF which depends again on O3 and sh. The biologically effective dose Hbiol is gained by taking into account the time of exposure.

Many processes show a continuously direct relation between effect and dose. Some effects stop at a certain level even if the dose is still increasing (e.g. Vitamin D production). Some others occur only if a certain threshold (radiant exposure) dose is exceeded (e.g. Sun burn). For such effects threshold spectra are given. Effects can be distinguished by the latent time (e.g. skin cancer), occurring immediately (e.g. sun burn)or with delay (e.g. pigmentation). Some effects are reversible others are not.

Gaining an action spectrum
Action spectra may be derived in vivo, in vitro or in situ. The light source can either be monochromatic or polychromatic. This can make a difference if an effect is not wavelength additive, but synergetic. This means that a series of irradiation with monochromatic light of different wavelengths do not cause the same effect as irradiation by all at once. In some cases the action spectra can not be derived direct from volunteers. Such effects are then studied indirect in using a model. For the human, for example, effects are studied in animals (e.g. hairless mice) and then transformed to the human by changing or adding an parameter (e.g. skin transmittance,...)

Information from an action spectrum
The action spectrum for formation of photoproducts might be expected to resemble the absorption spectrum of the molecules responsible for these photoproducts (although several conditions need to be fulfilled for this to be case) (Jagger 1985). Nevertheless, comparison of experimentally determined action spectra with the absorption spectra of appropriate molecules can sometimes give insight into the molecule primarily responsible for the effect. For example, in 1928 Gates showed that the bactericidal action of UVR of different wavelengths in Staphylococcus aureus cells closely matched the absorption spectra of nucleotide bases. This observation was confirmed with other unicellular organisms and led to the realisation that nucleic acids have a fundamental role in ultraviolet photobiology. More recently, comparison of erythema action spectra, in patients with possible drug-induced photosensitivity, with the absorption spectra of suspect agents can confirm the diagnosis (Diffey and Farr 1988).

Sources of action spectra
During the past many photobioogical or photochemical effects were found. For a variety of them the corresponding action spectrum could be derived. Many action spectra are published rather schematically by scatter or line plots than in tabled form. Applications are therefore often done with values taken from a plot which may lead to uncertainties especially at logarithmic scaled plots. In many cases effectiveness is given for distinct wavelengths. These points are mostly vanishing when later on shown as line-graphs, etc. (see interpolation or Lost original data) Up to now even peer-reviewed journals do not care on the authors source of an action spectra.

Interpolation, approximation and extrapolation:
Action spectra are derived for certain wavelengths. Model calculations however need relatively high spectral resolution for weighting the spectral irradiance. Therefore another point of concern is interpolation. Linear interpolation may be appropriate for slopes within one magnitude. If the nature of an effect is rather logarithmic then linear interpolation leads to bumps on a logarithmic scaled plot. The same problem occurs by logarithmic interpolation in the linear scale. Both methods are not satisfying.
A variety of action spectra were not derived over the whole UV range. Extrapolation must be done with care. Especially in the UVA may noticeable errors may occur since solar irradiance is magnitudes higher than in the UVB.

History of action spectra:
Some of the action spectra in the UV have already a long history dating back to the first decades of the last century. The action spectrum for the erythema as an example was already standardised by CIE in 1935 following several works done during the 20ties (e.g. Hausser and Vahle 1922).
A continuous work of improvement, technical advance, summarising, or dividing leads to different versions which are often used parallel at the same time. To comprehend earlier studies the availability of a collection of historical action spectra is also of advantage.

Lost original data of action spectra:
Many action spectra (when not published in tabled form) can only be retrieved due digitising the curve/points from an published paper. Especially for older action spectra original data are not available respectively the author can not more be contacted. Sometimes (time expensive) private communication may help.

Measurements of biologically effective radiation:
The proper way to measure biol. effective radiation is using a spectrophotometer with high resolution. This technique is expensive and need operators with high experience. A cheaper and less sophisticated method is to use broadband meter. The spectral sensitivity of broadband meter however does not fit the action spectrum perfectly. Corrections are necessary depending on the atmospheric parameters which change the spectral distribution of sun light (O3, sh, clouds, altitude, ...). Another method is using biodosimeters where the photosensitive layer is essentially the same as working the living organsim. Biodosimetry becomes quite expensive if more than a case study is undertaken.


CIE 1935 Diffey B L and Farr P M 1988 The action spectrum in drug induced photosensitivity, Photochem. Photobiol. 47 49-54
Errera M 1952 Etude photochemique de l'acide désoxyribonucleique I. Mesures énergétiques, Biochim. Biophys. Acta 8 30-7
Gates F L 1928 On nuclear derivatives and the lethal action of ultraviolet light Science 68 478-80
Harm W 1980 Biological Effects of Ultraviolet Radiation (Cambridge: Cambridge University Press)
Jagger J 1985 Solar-UV Actions on Living Cells (New York: Praeger) pp 174-6
Hausser und Vahle, xxxx
Patrick M H and Rahn R O 1976
Photochemistry of DNA and polynucleotides: photoproducts Photochemistry and Photobiology of Nucleic Acids vol II ed S Y Wang (New York: Academic) pp 35-95

Alois W. Schmalwieser: