Hyperspectral portable beam transmissometer for the ultraviolet-visible spectrum

The spectral beam attenuation coefficient is an important optical property of natural water used to quantify light propagation and visibility in the aquatic media, and to study the concentration of the water constituents. Although beam attenuation in the ultraviolet spectral range may be particularly informative, to date, no transmissometer capable of measuring the beam attenuation in the ultraviolet is commercially available. The portable hyperspectral beam transmissometer developed in our lab is capable of measuring across a broad spectral range (300-750 nm) at 2 nm spectral resolution. The transmissometer exhibits a small acceptance angle (0.55 to 0.59° across the spectrum), a well collimated spectral light beam, and precision of ±0.012 m^-1. The attenuation of diverse water samples measured with our transmissometer was found to be significantly similar to that measured with a commercially available transmissometer. Moreover, the attenuation of filtered samples, measured with our transmissometer, was significantly similar to their absorption, measured with a bench-top spectrometer. Testing the transmissometer in the field, the transmission of water samples collected in Lake Malawi, Africa, was measured on site. The magnitude and spectral shape of attenuation were in general agreement with previous reports. All assessment stages confirm the performance, accuracy, and applicability of our transmissometer. The extended spectral range and high spectral resolution of our portable transmissometer make it an excellent tool for studying the characteristics and distribution of dissolved and particulate matter in aquatic media and exploring the constraints imposed on the visibility and visual communication of aquatic organisms known to have ultraviolet photosensitivity.