menon known as the 'immersion effect'.
Taking solar UV irradiance measurements in real-world underwater environments is hampered by cumbersome spectroradiometric and radiometric measurement equipment, poor instrument response with changes in depth, and unpredictable changes in water quality and transparency as a result of both natural and anthropogenic activity. Shading by nearby trees and plants and the presence of surface waves have made recording reliable estimates for the optical properties of water bodies an extremely difficult proposition (Morrow & Booth, 1997). Morrow and Booth have also noted that underwater UV measurements can be complicated by the fact that total UV becomes a very small signal that has to be measured alongside the much more pronounced visible waveband, as well as by unpredictable features at the surface of the water, including wave-focusing effects such as those reported by Deckert & Michael (2006) and changes in water line elevation caused by evaporation and tidal changes.
From an optical perspective, three main factors influence underwater UV measurements made with all types of UV-measuring instrumentation. The first is that the field of view as seen by any sensor is reduced and hence a smaller amount of radiation is intercepted by the sensor. The second is the change in the local refractive index between the open-air environment in which the sensor was calibrated and the underwater environment where it is employed (Ohde & Siegel, 2003). Thirdly, during a water-based measurement, more light is scattered out of the sensor than in a similar air-based measurement. This is caused by the difference between the refractive indices for air and water at the sensor interface (Hooker & Zibordi, 2005, Zibordi, 2005, Zibordi et al, 2004). This is known as the 'immersion effect'. In order for reliable solar UV measurements to be made underwater, wavelength-dependent immersion factors must be calculated and applied using a strict methodology. However, for the practical exercise presented in this article, it is not necessary to calculate the immersion factor for the recommended UV meter, as only relative measurements will need to be obtained and analysed in order to investigate the Beer-Lambert-Bouguer Law. …