Dosimetry UV Modeling and Instruments

Every wavelength causes differing biological effects. Four Action Spectra for different biological targets are shown in Fig. 4.17. It is evident that UV-B is the main culprit for harmful UV consequences, in particular erythema. Convolving a solar UV irradiance spectrum with the specific action spectrum and then integrating

Action spcctra

Action spcctra

Wavelength (ran)

Figure 4.17 Action spectra for several biological targets. Source: Di Menno et al., 2007

Wavelength (ran)

Figure 4.17 Action spectra for several biological targets. Source: Di Menno et al., 2007

it (effective spectrum) over time, the effective dose (ED) is obtained; this is the link to evaluating solar UV risk. The ED integrated over time provides the available environmental dose (AED), i.e., the dose available outdoors during the time considered. Measurements are often expressed in terms of exposure ratios (ER) or fraction of environmental dose falling on a horizontal surface (maximum available dose).

To evaluate the individual dose (ID), (i.e., the dose absorbed by a target exposed outside without considering the time spent indoors), it is necessary know the effective time of exposure. Measurements carried out by ground-based radiometric instruments give the AED, but no information on ID. Moreover, for a human body, not all the parts are exposed to the sun in the same manner, depending on the solar angle view for the surface exposed (i.e., skin or eyes). Therefore a personal dosimeter is an essential requirement for evaluating the ID (Fig. 4.18(a), (b), (c)).

Figure 4.18 (a) Commercial electronic dosimeters; Examples of personal dosimeters; (b) Rafanelli et al. 2002; (c) Kimlin and Parisi, 1999; (d) Electronic dosimeter and dosimeter badge by polysulphone film

Figure 4.18 (a) Commercial electronic dosimeters; Examples of personal dosimeters; (b) Rafanelli et al. 2002; (c) Kimlin and Parisi, 1999; (d) Electronic dosimeter and dosimeter badge by polysulphone film

To describe in detail the real individual habits of sun exposure, long-term studies using personal dosimetry are needed. Furthermore, the dosimeter can be placed on different parts of the body to measure the solar angle view. The dosimetric science for the AED utilizes broadband radiometers in a single site or in network architecture. These instruments are known as biometers and have a spectral response approximating the action spectrum of the phenomena under examination, mostly erythema. They are cheap, but have small differences in construction (e.g., in spectral response). Large differences can occur in the results, thus not permitting a reliable comparison between equipment from different manufacturers (Rafanelli, 2001). On the other hand, the use of more complex instruments (e.g., spectroradiometers), could present serious management and budgetary problems. For these reasons, multichannel broadband radiometers are becoming more widely established for measuring UV irradiance, although not at high resolution, but only in several spectral bands. Multi-channel radiometers are calibrated in physical units, so models reconstructing the continuous UV spectrum are necessary to compute the AED (i.e., Dahlback, 1996; Rafanelli et al., 2000).

Valid examples of multi-channel radiometers are the GUV 511 (Biospherical Inc., USA; Di Menno et al., 2002), with four filters at 305, 320, 340, 380 nm; and the NILU UV Irradiance Meter, NILU Prod. AS, Norway (Dahlback et al., 2007) with five filters at 305, 312, 320, 340, and 380 nm. Both can also perform the PAR measurement. A result of the modeling application for some biological effects is shown in Fig. 4.19. In each plot, the difference between the effective dose computed by spectral and modeled data is less than 8% (Anav et al., 2004).

An alternative approach involves biodosimeters that use simple biological systems (bacteria, spores, and biological cells or molecules) and measure AED directly for the biological effect studied. Biodosimeters are cheap, small devices that require no external power and thus, are widely accepted for both natural and artificial UV dose control. However, biodosimeters have a number of disadvantages. They indicate a damaging effect of UV because UV absorption of a DNA molecule is the basis of a biological response. They require laboratory analysis to evaluate the dose and an in-situ control is not possible. The comparison between different biodosimeters is very hard because they act as "black boxes," the AED is measured in specific biological units and not as comparable physical units, J/m .

A practical personal dosimeter is based on polysulphone badges with plastic film that changes its transmission after UV exposure (Diffey, 1984; McKinlay and Diffey, 1987; Diffey, 1989; Mariutti et al., 2003). This dosimeter (Figs. 4.18(c) and 4.20) also agrees with daily GSUVI data and comparisons of AED measurements do not need to be corrected (Wester, 2006).

As previously stated, to measure the AED, the knowledge of the pass band filter shape is important in order to make comparisons between different instruments, such as multi-channel or broadband radiometers. In the spectral UV region, a very small difference in the shape can produce a large discrepancy in the results of a comparison (Anav et al., 1996; Di Menno et al., 2002). However, the major problem in accurately evaluating irradiance is the presence of clouds in the sky (Mariutti et al., 2002). Thus, the use of spectroradiometers is the best option, but they do involve a longer sampling time.

Figure 4.19 Effective spectral irradiance comparisons between the WL4UV model (lines) and Brewer data (dots) for (from top to bottom): Erythema, DNA, and no melanoma. Data carried out in Rome Italy, May 2, 1996 at SZA = 27.30°.Source: Anav et al., 2004

320 340 360 Wavelength (nm)

Figure 4.19 Effective spectral irradiance comparisons between the WL4UV model (lines) and Brewer data (dots) for (from top to bottom): Erythema, DNA, and no melanoma. Data carried out in Rome Italy, May 2, 1996 at SZA = 27.30°.Source: Anav et al., 2004

Figure 4.20 Personal dosimeter, badges in polysulphone. Source: courtesy of G.F. Mariutti

Modern technology can solve or attenuate the problems. In Di Menno et al. (2007), a new CCD and optical fiber spectrograph (fast spectroradiometer) is shown. The specifications of the instrument are: (1) pass-band range: 250 nm - 400 nm; (2) spectral resolution: 0.25 nm FWHM (improvement of the CCD efficiency making it possible to have a 0.10 nm as FWHM); (3) high signal/noise ratio, very fast acquisition; and (4) automatic management, small size and lightweight with low power consumption. In addition, the use of high quality commercial components and an expert knowledge of the technical specifications of each part, crown it all.

The key to obtaining such a high performance has been the adoption of a new shape for the optical fiber junction to the monochromator box. Commonly, the bush has a circular shape, but in this case, a large part of the beam falls outside the slit at the entrance of the monochromator. By rearranging the bush into a square profile, almost all the beam enters into the monochromator. This solution is shown in Fig. 4.21. With this key feature, the energy inlet increases from 1% to 40%. This allows the measurement of solar UV wavelengths less than 280 nm, which is useful for indoor measurements that are applied to artificial UV sources. The preliminary results of a comparison with a Brewer spectrophotometer, used as a reference, are shown in Fig. 4.22. This allows the application of the UV differential spectral analysis technique to evaluate the concentrations of many compounds present in the atmosphere. Finally, the small dimensions of the instrument allow its use in a configuration suitable for polar night campaigns.

Figure 4.21 Circular and square shapes for optical fiber junction with monocromator box. Source: Di Menno et al., 2007
Bmw 2002 Body
Figure 4.22 Preliminary results of a comparison between Spectrograph vs. Brewer. Source: Di Menno et al., 2007

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