BASIC CONCEPTS OF RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 11

predict the effectiveness of the broad spectrum source. The shape of the action spectrum
is determined by a number of factors. Most important is the target molecule itself, the
chromophore. The action spectrum of a pure solution of the chromophore will provide
the fundamental action spectrum; however, other biological factors can alter this
fundamental action spectrum. Optical reflection, absorption or scattering prior to
absorption of photons by the chromophores will frequently shift the peak of the action
spectrum, as in the case of erythema (skin reddening, or “sunburn”). 3-5 The proteins of
the stratum corneum (outermost, horny surface layer) of the skin spectrally filters the
UV photons incident upon the skin and tend to block the transmission of wavelengths
much less than 295 nm, even though the most important chromophore - DNA - has a
spectral absorption peak at a shorter wavelength. 3,6 Chain effects are also possible,
where other biochemistry is initiated by the incident photons. The choice of the
measured endpoint for the effect also affects the action spectrum, and the action
spectrum for erythema shifts to a narrower curve with a longer wavelength peak if
severe, rather than minimal, erythema is the endpoint. The time of assessment of this
biological effect and the degree of severity, the means to measure the effect (e.g., visual
observation, chemical assay, histology, etc.) also generally affect the action spectrum.
Thus we are always left with experimental error and uncertainties. Some of the sources
of uncertainties—the variables—in the determination of action spectra include: physical
measurement errors of the optical radiation, the area of exposure, exposure duration,
distance, the spectral bandwidth (e.g., laser, 1 nm, 5 nm), the number of wavelengths
sampled, individual subject (or anatomical site) variations in sensitivity, etc. In addition,
as well illustrated by erythema, the target tissue may undergo adaptation
(e.g., thickening of the stratum corneum and skin pigmentation) and the type of
assessment (e.g., color, method, time delay, etc) influences the result. Although
erythema was used in this example, the same types of errors can apply to the
determination of plant action spectra.
Unlike most ionizing radiations and radio frequency radiation, UVR-like most
optical radiation—is absorbed very superficially and penetration depth in the skin or
cornea is generally less than 1 mm and for UV-C only a few cell layers. For this reasons
a surface dose concept rather than a volumetric dose is conventionally applied in the
photobiological literature. By contrast, thermal injury is a rate process, dependent upon
the volumetric absorption of energy across the spectrum, and therefore does not follow
the reciprocity rule. Thermal effects show very broad spectral dependence and a
spot-size dependence. Hence, the thresholds for biological injury and human exposure
limits for purely photochemical injury are expressed as a surface exposure dose, known
as radiant exposure. 1-2 The product of the irradiance (or exposure dose rate) E in W/m 2
or W/cm 2 and the exposure duration t is the radiant exposure (or exposure dose)
H expressed in J/m 2 or J/cm 2 , i.e.,
H = E·t (5)
This product always must result in the same radiant exposure or exposure dose
over the total exposure duration to produce a threshold injury. This is termed the Rule of
Reciprocity, or the Bunsen-Roscoe Law. 5 Chemical recombination over long periods
(normally hours) will lead to reciprocity failure, and in biological tissue, photochemical