Polychromatic action spectra

The situation is also complicated by the fact that the response, e.g. the yellowing, in reality is not a function of a single radiation wavelength, a fact that has led to the construction of polychromatic action spectra [5]. Nevertheless, the polychromatic action spectra become very complex and may obscure the individual chromophores. 

Cooley, N.M., Truscott, H.M.F., Holmes, M.G., and Attrige, T.H., Outdoor ultraviolet polychromatic action spectra for growth responses of Beilis perennis and Cynosurus cristatus, ]. Photochem. Photobiol. B Biol., 59, 64, 2000. 

It is a representation of the relative effectiveness of various regions of a polychromatic source spectrum in causing a given type of photoreaction or photodamage. The main difference between the action spectra and activation spectra is that only the latter is dependent upon the spectral irradiance distribution of... 

An action spectrum shows the relationship between the effectiveness of a particular photoprocess and the wavelength of irradiation. They can be conveniently generated from the measurement of photodamage resulting from exposure of samples to near-monochromatic radiation of known irradiance. Action spectra can also be calculated firom source-specific activation spectra, provided the irradiance distribution used in generating the data is accurately known. Accordingly, action spectra have been derived from polychromatic exposure data obtained using a series of cut-on filters [20]. (While no activation... 

It is possible in theory to generate an action spectrum which takes into account the effect of incremental monochromatic radiation in excess of the spectral irradiance of a source spectrum. Narrow-band, near-monochromatic radiation might be superimposed over the polychromatic spectrum from a xenon source to obtain radiation spectrally enhanced in any selected waveband. Using a series of interference filters, a series of cut-on filters, and two filtered xenon sources, it should be possible to generate an action spectrum for polymer degradation under spectrally-altered, UV-enhanced (or even visible-enhanced) source spectra. The effective radiation and dose rate in such an experiment can be written as follows ... 

This assumption is always true for polychromatic-illumination and for monochromatic-illumination experiments in the case of eukaryotic microalgae, according to the well-known action spectrum of photosynthesis. This is not the case for monochromatic illumination of cyanobacteria (Farges et al., 2009). 

In a recent paper, Sutherland described mathematical procedures for calculating the biological effects from exposing samples to polychromatic (especially ultraviolet) radiation. His analysis included a variety of fluence-response functions and presented an approach for predicting total effect and analyzing subtle differences between classical action spectrum analysis and biological weighting functions. 

A dispersed spectrum of a suitable polychromatic source can also be used as a near-monochromatic source to obtain action spectra. A quartz prism or a grating spectrograph is used to disperse the source spectrum and the samples are placed in the appropriate region of the spectrum. A monochromator [26,27] used with a suitable source allows a part of this dispersed spectrum to be isolated at a narrow exit slit to yield a good monochromatic source. 

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