POLYSULPHONE FILM THICKNESS AND ITS EFFECTS IN ULTRAVIOLET RADIATION DOSIMETRY    

A.V. Parisi^1*, L.R. Meldrum¹ and M.G. Kimlin¹

¹Centre for Astronomy and Atmospheric Research, Department of Biological and Physical Sciences, University of Southern Queensland, Toowoomba, 4350, Australia. Ph: 61 7 46 312226, FAX: 61 7 46 312721, Email: parisi@usq.edu.au

 
(^* To whom correspondence should be addressed)

ABSTRACT

Polysulphone film thickness and the effects on the dose calibration in ultraviolet radiation dosimetry were investigated. Compared to those obtained with the dose calibration for the 45 mm thick film the erythemal exposures determined from the dose calibrations for the 18, 20 and 30 m m thick film ranged from 33% to -45%. The absolute of the differences averaged to 22%, 37% and 19% for the 18, 20 and 30 mm thick respectively. The variations in the dose response of polysulphone film with different thickness have shown that the dose response is highly dependent on the film thickness and highlighted the importance of employing polysulphone film of consistent and reproducible thickness in ultraviolet photobiology research.

1. Introduction

 Photobiological research to determine the ultraviolet (UV) exposure to humans requires the personal monitoring of UV exposure. The usage of biological and chemical dosimeters has been previously reviewed (Wong and Parisi, 1998, Horneck, 1995). A common technique is to employ a photosensitive film and the UV induced deterioration of the film is calibrated to the measured UV irradiance. A commonly employed film dosimeter material in ultraviolet photobiology research is the polymer polysulphone (Davis et al., 1976, Diffey, 1984).

Polysulphone film has a spectral response (CIE, 1992) that approximates the erythemal response of human skin (CIE, 1987). This film employed in dosimeters of approximate size 3 cm x 3 cm has allowed the monitoring of personal UV exposures during normal daily activities to determine the personal UV exposure for different occupations (for example, Kimlin et al., 1998a, Gies et al., 1995, Airey et al., 1997) and at different locations (Gies et al., 1998, Kimlin et al., 1998b). Additionally, it has allowed the evaluation of the effectiveness of UV protective strategies (for example, Wong et al., 1996). The polysulphone dosimeters allow measurement of UV exposure at multiple sites simultaneously and provide a simple means of measuring integrated UV exposures.

The film employed in the research in the literature is generally of the order of 40 mm thick. This thickness is generally employed as a compromise between ease of handling and matching of the spectral response of the film (CIE, 1992) to the human erythemal action spectrum (CIE, 1987). A polysulphone film of 1 mm thickness on a cellophane substrate has been reported to have a different spectral response to 40 mm thick film (Davis et al., 1981). Consequently, polysulphone film of different thickness may have a different dose response. Variations in film thickness may occur in the film casting process and introduce significant errors in the dose response of the film. No previous research has quantitatively assessed the effect of the polysulphone film thickness in UV radiation dosimetry. This paper investigates the effects on UV exposures measured with polysulphone film of different thickness.
 

2. Materials and Methods

2.1 Film Thickness

The polysulphone film of different thicknesses was produced at the University of Southern Queensland (USQ) Centre for Astronomy and Atmospheric Research. The technique developed with a specifically designed casting table ensures production of consistent and reproducible sheets of polysulphone film. The technique is similar with some variations to that utilised by the original producer (Davis et al., 1976). Polysulphone pellets (Aldrich Chemical Co., Inc. Milwaukee WI 53233 USA) were mixed with chloroform to form a solution and cast on a glass slab optically flat to 1 mm by a moving blade powered by a variable speed DC motor. The height of the blade is adjustable to produce the required thickness.

The average thickness of the film was calculated employing the technique of Davis et al., (1981). This involves weighing a known surface area of the film and employing the density of polysulphone of 1.2 g cm^-3 (Davis et al., 1981). Four film thicknesses of 18, 20, 30 and 45 mm were produced by setting the height of the casting blade above the glass slab to different values. The film was stored in the dark prior to use to prevent UV exposure prior to usage.

2.2 Calibration

The erythemal UV exposure, UV_ery can be expressed by weighting the source spectrum S(l) with the CIE (1987) action spectrum for human erythema, A(l), as follows:

                                                                                                                          (1)

The UV_ery may be measured with a calibrated spectroradiometer measuring S(l) or with a detector that possesses a sensitivity approximating the erythemal action spectrum. In this research, the polysulphone was calibrated against a temperature stabilised calibrated Biometer (model 501, Solar Light Co., Inc. 721 Oak Lane, Philadelphia, PA. 19126) in units of MED or minimum erythemal dose. One MED is the amount of biologically effective UV required to produce barely perceptible erythema after an interval of 8 to 24 hours following UV exposure (Diffey, 1992). The polysulphone was calibrated to the spring solar spectrum in October between approximately 11:00 Australian Eastern Standard Time (EST) and 13:00 EST. The Biometer was calibrated to a spectroradiometer with calibration traceable to the National UV Standard housed at the CSIRO National Measurement Laboratory. From the calibration the unit of one MED on the Biometer was equal to 210 J m^-2.

The film was fabricated into dosimeters of overall approximate size 3 cm x 3 cm with an aperture of approximately 1 cm². A total of 48 dosimeters with 12 each constructed from film of the four thicknesses were employed. The complete batch of dosimeters for the four thicknesses was calibrated at once so that all four thicknesses were exposed to the same source spectrum. A photograph of the UV Biometer and a polysulphone dosimeter are provided in Figure 1. The calibration was performed on a horizontal plane in an unshaded position at Toowoomba (27.5 ^oS), Australia. Following the established procedure (Diffey, 1989), the optical absorbance of the film was measured pre- and post-exposure at 330 nm in a spectrophotometer (Shimadzu Co., Kyoto, Japan) to determine the change in optical absorbance at 330 nm (DA₃₃₀) due to the UV exposure. The absorbance was measured at four sites over the film in order to take into account any minor variations in the film over the dosimeter. The dark reaction was taken into account by allowing a constant time between exposure and read-out and measuring the post-exposure absorbance on the following day.

To see the full size image (47k) click on the picture above

Figure 1 – Photograph of the UV Biometer and a polysulphone dosimeter.

 
3. Results

3.1 Comparison

In Figure 2, the calibration data of the current film of 45 mm thickness is compared to the calibration in the literature. For a film thickness of 40 ± 4 mm, Diffey (1989) obtained a dose calibration of:

            UV_ery = 2000[9(DA₃₃₀)³ + (DA₃₃₀)² + (DA₃₃₀)]                                                                                     (2)

in units of J m^-2. In Figure 2, the factor of 2000 in Equation (2) has been converted to produce units of MED for the solid line in the Figure. The average difference between the calibration data points and the calibration in the literature (Diffey, 1989) compared to the literature calibration is 7%.

The optical absorbances at 330 nm before exposure averaged over the four measurement sites and the twelve dosimeters were 0.530 ± 0.032, 0.530 ± 0.014, 0.543 ± 0.010 and 0.587 ± 0.011 for the film thicknesses of 18, 20, 30 and 45 mm respectively. The error is represented as one standard deviation. There is a statistically significant difference (Student’s t-test) in the optical absorbance at 330 nm between the 45 and the 30, 20 and 18 mm thicknesses (P < 0.05). Similarly, there is a statistically significant difference between the 30 and the 20 and 18 mm thicknesses with no statistical difference between the 20 and 18 mm thicknesses.
 

Figure 2 – Comparison of the calibration data for the 45 m m thick film (· ) to that obtained by previous research (solid line) (Diffey, 1989).
 

3.2 Calibration

The dose calibrations for spring sunshine of the different polysulphone film thicknesses are provided in Table 1. Each of the calibrations have R-squared values between 0.92 and 0.98. The dose calibrations for the 20 mm and the 45 mm thick film are plotted in Figure 3 and show the differences between the calibrations for the two film thicknesses. The dose calibrations are provided to allow comparison of the dose response for the different thicknesses. They are only applicable for the solar UV spectrum encountered in this research as previous research has established that for a different source spectrum, the polysulphone has to be re-calibrated for that spectrum in order to minimise the errors (Wong et al., 1995).

Table 1 – Dose calibrations for spring sunshine of the different polysulphone film thicknesses
 

 
Figure 3 – Calibration data for the 45 mm thick film (x) and the regression curve (1) and the calibration data for the 20 mm thick film (n) and the regression curve (2).

 
The dose calibration equations in Table 1 have been employed for each of the four thicknesses to provide the erythemal exposures for seven different scenarios of DA₃₃₀ from 0.1 to 0.4 in steps of 0.05 as listed in column 1 of Table 2. The erythemal exposures are in the next four columns of Table 2. Different erythemal exposures are produced for the different film thicknesses. For example, for a DA₃₃₀ of 0.2, the erythemal exposure measured is 4.2 MED for the 20 mm film compared to 2.9 MED for the 45 mm film. The final three columns in Table 2 show the percentage differences in the erythemal exposures with the 18, 20, and 30 mm film dose calibrations compared to the 45 mm film dose calibration. The percentage errors range from 33% to –45% compared to the erythemal exposures for the 45 mm film dose calibration.

 
Table 2 – The erythemal exposures for the film thicknesses for different scenarios of change in absorbance and the percentage differences compared to the exposures measured with the 45 mm thick film.
 

4. Discussion

The comparison of the calibration of the 45 mm thick polysulphone film with the polysulphone film calibration in the literature verifies the quality of the current batch of film as it possesses a similar dose calibration to that reported by Diffey (1989). However for different film thicknesses, the dose calibration changes. The percentage differences in the erythemal exposures determined from the dose calibrations for the 18, 20 and 30 mm thick film, compared to those obtained with the dose calibration for the 45 mm thick film ranged from 33% to -45%. The absolute of the differences averaged to 22%, 37% and 19% for the 18, 20 and 30 mm thick film respectively. The variations in the erythemal exposures obtained with polysulphone dosimeters of different thickness have shown that the dose response is highly dependent on the film thickness and highlighted the importance of employing polysulphone film of consistent and reproducible thickness in UV photobiology research. The polysulphone film casting table employed at the USQ with reproducibly settable casting blade heights and casting blade speeds ensures this.

Acknowledgements - The authors would like to thank Ken Mottram, Oliver Kinder and Graeme Holmes in the USQ physics discipline whose technical expertise contributed to this project.
 

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