BASIC CONCEPTS OF RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 16

xenon-arc has been sufficiently intense that when used with a grating monochromator,
can produce relatively narrow bandwidth monochromatic emission wavelengths.
The problems inherent in the use of a monochromator are not always clearly
evident to the research photobiologist and it is worthwhile to explain that here. The
problems can appear both when the monochromator is used in spectroradiometry of the
source or as part of a monochromatic illumination source. Figure 8 illustrates this point.
The spectral bandwidth of the emitted radiation has a triangular shape when the
monochromator dial is set at a given wavelength. The spectral bandwidth transmitted by
the monochromator is defined at 50% points by what is known as the full-width half-
maximum (FWHM). When a source which is varying in spectral output, such as the sun
or a tungsten-halogen lamp, is used, the transmitted radiation from the monochromator,
when multiplied by the action spectrum provides the relative spectral effectiveness and
the spectral effectiveness can be shifted as shown in the figure. The lower panels
illustrate the use of a xenon arc monochromator where the xenon arc spectrum is
relatively flat in the ultraviolet spectrum, but when the dial is set at 305 nm with the
full-width half-bandwidth of 5 nm, the effective peak wavelength for producing
erythema is actually near 301 nm, although the instrument is set at 305 nm. Hence the
investigator writes down that the relative effectiveness of this narrow band of radiation
is a given value at 305 nm when in fact, that effectiveness value is more characteristic of
a shorter wavelength. The effect of this error is to broaden action spectra as was shown
in Figure 7. This problem has long been recognized, and in deriving occupational
exposure limits for ultraviolet radiation or to correct erythemal or photokeratitis action
spectrum, it was possible to make an adjustment to determine the shifting of the action
spectrum to longer wavelengths. Obviously, a tunable laser would be a desirable light
source except that most current tunable UV lasers are very expensive and are pulsed;
and investigators worry about the potential loss of reciprocity from short-wavelength
pulses. Nevertheless, the laser erythemal action spectrum determined with a laser by
Anders and her associates (1995) clearly demonstrate the value of using monochromatic
sources with a resolution of 1 nm or less. 10
Figure 8. The effect of spectral bandwidth on the effective wavelength of emitted radiation.