BASIC CONCEPTS OF RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 15

and time of assessment of the endpoint in the animal or human subjects. The physical
factors which influence any reported action spectrum relate to the accuracy of
radiometric and spectroradiometric measurements, the type of light source and the
geometry and spectral bandwidth of each exposure used in the biological experiment.
5. Resolution
While it is obvious that the resolution of the final action spectrum depends upon
the total number of wavelength intervals used during the experiment, it is less obvious
that the spectral bandwidth of a monochromator used will also influence the results. The
use of low-pressure discharge lamps (e.g., the low-pressure, quartz-mercury lamp) or
lasers permit biological exposure to extremely narrow bandwidths; however, the use of
xenon-arc monochromators produce a greater spectral uncertainty in the action
spectrum, because the exposure at each monochromator wavelength setting is actually
the spectral integration over a narrow band of wavelengths. This is determined by the
monochromator's slit function. 2 The narrower the spectral bandwidth at each point, the
more accurate the action spectrum, but also the lowest transmitted power and the longer
the exposure duration at each biological site. Hence the photobiologist must
compromise. The wider the spectral bandwidth, the greater the loss of spectral
resolution and steep curves become shallower. Nevertheless, knowledge of these factors
can permit one to derive a higher-resolution action spectrum by mathematical treatment
(convolution).
6. Action spectroscopy
An action spectrum is the relative spectral response for a photobiological or
photochemical action or reaction. Since an action spectrum is normally determined by
using monochromatic sources to obtain relative exposure doses at each wavelength to
produce the defined effect, the exact nature of the monochromatic source is important.
A radiant exposure at the target surface is measured and the underlying assumption is
that reciprocity of irradiance and time exists (i.e., the Bunsen-Roscoe Law holds). The
action spectrum will differ in situ from that measured on an exterior surface if
intervening molecules do not have a neutral absorption spectrum. For example
erythema, cataract, and retinal effects produced by ultraviolet radiation are mediated by
intervening tissues which absorb some of the energy with shorter wavelengths generally
more attenuated, thereby reshaping the action spectrum.
The spectral bandwidth of the source can affect the resulting action spectrum as
will be shown later. Two types of monochromatic sources are frequently chosen: either
a low-pressure lamp, such as the mercury quartz lamp where very narrow wavelength
lines can be selected by the use of filters or a monochromator, as was traditional in
photobiology of the 1930's, and the use of a xenon-arc monochromator. The advantage
of the low-pressure line source is that the spectral bandwidth of each line is less than
one nanometer. However, the individual emission lines of a lamp are not equally
distributed and may not be near peaks and minima of action spectra. Hence, it is highly
desirable to have a tunable monochromatic source. In recent decades, the high-pressure