New Method for Determining Spectral Absorption of Light in the Sea

M. E. Lee^✉, E. B. Shybanov

Marine Hydrophysical Institute of RAS, Sevastopol, Russian Federation

^✉ e-mail: michael.lee.mhi@gmail.com

Abstract

Purpose. The study is purposed at presenting and analyzing a new method for determining light absorption in the sea which for the first time made it possible to redirect almost all the scattered rays from the studied light beam to the photodetector along the path of its propagation in a weakly absorbing medium, as well as at showing that application of a new method providing such an efficient collection of the scattered rays, permits not only to avoid significant errors from a strong influence of scattering upon the results of determining light absorption, but also to give up the necessity in correcting the data by theoretical modeling.

Methods and Results. It is known that sea water is a weakly absorbing light-scattering medium where light propagation is accompanied by its attenuation that is many times stronger due to scattering than due to absorption. Therefore, the determination of light absorption by sea water at a receiving device requires collection of not only the light that has traveled a certain distance in the absorption medium, but also all the light scattered along this path. Previously, a method was proposed for measuring the light absorption in a cylindrical mirror cuvette with a light source at the input and a collector with a photodetector at the output (reflective-tube absorption meter). Somewhat later, a similar method based on the phenomenon of total internal reflection was applied. Since these methods do not provide a complete collection of scattered rays, the data are to be corrected by theoretical modeling. The authors propose a new method for determining spectral absorption of light in a quartz cone cuvette with an external mirror cone. It is shown that the cone cuvette permits to collect most of the scattered rays in the beam passing through the water medium by means of more efficient redirection of these rays from the place of light scattering to the receiver. The rest of the scattered rays that have left the cuvette reach the receiver in the air space between the cuvette and the cone mirror due to multiple reflections from it. As a result, the new method makes it possible to redirect almost all of the scattered light to the receiver and thus to minimize the errors in determining light absorption in a weakly absorbing medium. To quantify the advantages of the new method, the authors have calculated geometric parameters of the scattered light propagation for a quartz cone cuvette in air and for the same cuvette placed inside the external cone mirror.

Conclusions. The combination of a quartz cone cuvette and an external mirror cone in the new method made it possible to collect all the rays scattered in a weakly absorbing medium in the receiver. Thus, it permitted not only to exclude their strong influence upon determining light absorption in the sea, but also to give up correcting the data by theoretical modeling.

Keywords

light absorption, scattering medium, total internal reflection, quartz cone cuvette, cone mirror, scattering angle, ray path

Acknowledgements

The work was carried out within the framework of state assignment FNNN-2024-0012.

Original russian text

Original Russian Text © M. E. Lee, E. B. Shybanov, 2024, published in MORSKOY GIDROFIZICHESKIY ZHURNAL, Vol. 40, Iss. 2, pp. 198–214 (2024)

For citation

Lee, M.E. and Shybanov, E.B., 2024. New Method for Determining Spectral Absorption of Light in the Sea. Physical Oceanography, 31(2), pp. 178-193.

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