Submersible UV-B spectroradiometer using an acouto-optic tunable fiber

Abstract

Measurements of solar irradiance in the UV-B region are complicated by large (orders of magnitude) variance in radiation fluxes over it. Normally, measurement requires use of double monochromators with high (> 10 6 ) spectral contrast. We have successfully tested a narrow bandwidth (0.3 nm) acousto-optic tunable filter (AOTF) with low (∼ 10 4 ) spectral contrast. The conditions for deployment were use of (i) a double - beam scheme with simultaneous recording of illumination in measuring and reference channels; and (ii) the intrinsic property of the AOTF to be a neutral filter outside of its passband. To apply the necessary contrast the background intensity determined in the reference channel was subtracted from intensity in the measuring channel. Two identical solar-blind photomultipliers in photon-counting regime were used. The integral solar intensity was averaged over the whole period of spectral record in order to shorten the period of time required for measurement at each wavelength. Formulae determining the spectroradiometer sensitivity and resolution, as well as comparison with US commercial instrument (Biospherical Instruments, Inc.) are presented. The testing was undertaken during US-Russian work at Kasitsna Bay, Alaska, in 1995, where additional genetic and cytological studies of the UV-B radiation impact on marine organisms were made.