BASIC CONCEPTS OF RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 13

The S(Ȝ) curve of Figure 6 is an action spectra which is used to spectrally weight
the incident UVR to determine an effective irradiance for comparison with the threshold
value or exposure limit. 13 With modern computer spread-sheet programs, one can
readily develop a method for spectrally weighting a lamp's spectrum by a variety of
photochemical action spectra. The computation may be tedious, but straightforward:
E eff = E Ȝ ·S(Ȝ)·ǻȜ (6)
The exposure limit is then expressed as a permissible effective irradiance E eff or
an effective radiant exposure. One then can compare different sources to determine
relative effectiveness of the same irradiance from several lamps for a given action
spectrum.
Figure 6 . UV Hazard Action Spectrum, S UV (O)
Action spectra: spectral band bandwidth and sampling
Higher resolution requires narrow spectral bandwidths and the use of many
wavelengths, however narrow wavelength bands requires a laser or a low-pressure lamp
and the available wavelengths may not be optimum. In fact, the low-pressure mercury
vapor lamp with quartz envelope is the classic source. Its advantages are that it is
inexpensive and can produce a narrow bandwidth. The disadvantage is that the available
wavelengths are limited, e.g., 254 nm and 280 nm (UV-C); 297 nm, 303 nm, and 313
nm (UV-B); and 335 nm and 365 nm (UV-A). Using a monochromator requires a very
powerful lamp source, and the typical spectral bandwidth may be 5 or even 10 nm. A
wavelength tunable UV laser could be optimum for high resolution except that current
tunable lasers are generally pulsed and reciprocity failure may occur in the nanosecond
time regime. Also, the high spatial coherence produces speckle, the shimmering
granular appearance of diffusely reflected laser light, and may provide uncertainties. In
addition, radiant power may not allow large-spot area irradiation. Nevertheless, careful
laser studies have resulted in refined action spectra for erythema (i.e., “sunburn”). 10
A high-intensity monochromator is frequently employed today to obtain action
spectra. The advantages are that they are continuously tunable from wavelength to
wavelength and they can have a variable bandwidth. However, disadvantages also exist;