One of the main dosimetric materials for UV exposures has been the polymer polysulphone, which was first employed in the 1970s by Davis et al. (1976a). Despite its immense usefulness, the polysulphone dosimeter is restricted as it is only capable of measuring solar exposures approximately less than ten hours during a clear summer day at a subtropical location before reaching the maximum optical saturation point. Furthermore, the uncertainty of polysulphone increases to 30% for a AA33o between 0.3 and 0.4 (Diffey, 1987). This makes the long-term measurement of UV a difficult task logistically as polysulphone dosimeters would have to be continually replaced on location in order to achieve a continuous stream of measurements. Another dosimeter was formulated in the 1970s, again by Davis et al. (1976b), this time using a polymer called poly 2,6-dimethyl-1,4-phenylene oxide, or just PPO in short. The PPO dosimeter was fabricated using similar methods to polysulphone and was just as easy to use, however instead of having a short responsive lifetime, PPO was capable of receiving a subtropical UV exposure over a period of time no less than five to ten days before complete saturation at the same level of accuracy as its polysulphone counterpart. It can be seen that the potential of the PPO dosimeter was far more substantial than the polysulphone dosimeter; however, in recent years most solar radiation researchers have chosen to use polysulphone.
The PPO dosimeter has recently experienced a revival with a varied amount of research being performed both on it and with it. The optical properties of PPO have been trialled for in-air use and calibrated to the erythemal action spectrum (Lester et al., 2003) and also to short UVA wavelengths (320 nm - 340 nm) by implementation of a Mylar filter (Turnbull and Schouten, 2008), similar to the methodology used by Parisi et al. (2005) when calibrating the prototype phenothiazine dosimeter to the UVA waveband. Figure 7.5 graphically shows how much more erythemal UV solar exposure the PPO dosimeter can handle in comparison to the polysulphone dosimeter. It demonstrates that on a typical summer's day, polysulphone can receive an approximate dosage of 2,500 J/m before reaching its exposure limit at its characteristic sampling wavelength of 330 nm (AA330). Comparatively, the PPO dosimeter can accept close to a further 25,000 J/m before optical saturation at its own particular sampling wavelength of 320 nm (AA320). This is a ten-fold increase in exposure capability during summertime (low solar zenith angle conditions).
The high exposure capability of the PPO dosimeter means that it is also ideal for underwater measurements that would usually be awkward to achieve by using traditional spectroradiometric and radiometric instrumentation. Schouten et al. (2007) tested the PPO dosimeter in a controlled underwater environment using solar UVB simulation focusing on dose response calibration trends, cosine response, interdosimeter variability, dark reaction, UVA/visible wavelength responsivity, and additionally, exposure additivity. The information gathered from this investigation
Figure 7.5 PPO dosimeter (♦) and polysulphone dosimeter (+) calibration to the erythemal action spectrum (CIE, 1987). Polysulphone dosimeter calibration data obtained from Turnbull and Parisi (2005)
showed that PPO was viable for underwater measurements with only a slight decrease in accuracy introduced when compared with in-air measurements, which was caused by watermarking on the PPO film surface. This research also made the important finding that calibrations made in-air could not be used as proxies to calculate underwater exposures.
This initial trial research has since been extended (Schouten et al., 2008) by obtaining calibration regimes at different depths to the real solar UVB spectrum for the PPO dosimeter in four different water types (clear water, sea water, dam water, and creek water) over a wide range of solar zenith angles and under fluctuating ozone conditions. This work found that at shallow depths, calibrations could be transferred from one water type to another with only a relatively small reduction in total uncertainty on the condition that each water type was within a certain spectral transmission (or absorption) range. It was also discovered that PPO calibrations are sensitive to atmospheric ozone variations. This means that if researchers wish to measure UVB with the PPO dosimeter, calibrations would have to be made just before, after, or during the measurement campaign to reduce the response error brought on by ozone attenuation causing changes in the solar spectrum of the UVB wavelengths.
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