Scattered ultraviolet radiation underneath a shade-cloth
Wong, C.F.
Photodermatology Photoimmunology and Photomedicine 10(5): 221-224
1994
ISSN/ISBN: 0905-4383 PMID: 7880763 Document Number: 426838
Ultraviolet (UV) radiometers and polysulphone film badges were used to study scattered UV radiation from sunlight underneath a shade-cloth of a size about 70 times 100 cm-2. The transmittance of erythemal UV radiation through the shade-cloth was found to be 0.14 +- 0.03. The results of the measurements were used to derive the shade ratio, defined as the ratio of the erythema UV irradiance or dose in the shadow to that outside the shadow. It was found that the shade ratio is not the same as the transmittance. Its value depends on the height and the orientation of the irradiated object. For a horizontal detector, the shade ratio decreases by more than 20% as the detector height increased from 0.37 m to 1.36 m. At a height of 1.36 m, the shade ratio decreased by about 100% as the horizontal meter was rotated to the vertical direction. The value of the shade-ratio (0.11) approached the value of the transmittance (0.14) as the height (1.53 m) of the horizontal meter approached the height of the shade-cloth (1.7 m). Polysulphone film badges were attached to 10 sites on 2 identical manikins. One manikin was exposed at the centre of the shadow underneath the shade-cloth and the other was exposed in open area for four orientations in both Durham and Brisbane. The experiments performed in Durham were under an overcast sky and those performed in Brisbane were under a clear sky. The results showed that the shade ratio varied from site to site. The mean ratios of the facial dose in the shadow to that outside the shadow were found to be 0.70 +- 0.1 and 0.35 +- 0.10 for Durham and Brisbane measurements respectively. The discrepancies between these values of the shade ratio and the value of the transmittance suggest that the scattered UV radiation underneath the shade structure could increase the shade ratio by more than twice of the transmittance.