Method for Fast Measuring of Seawater-Leaving Radiance Spectra

Abstract

It is noted that the high instrumental accuracy of modern marine spectrophotometers for reflectance measurements is difficult to implement due to large methodological errors caused by the sea surface waves by weather conditions and illumination geometry. A two-beam spectrophotometer based on the use of a multi-channel photomultiplier eliminating the disadvantages is proposed. The design and operation principle of a spectrophotometer, developed on the basis of a compact high-aperture monochromator and a photometric unit, in which the channels of the upwelling radiation and the irradiance of the sea surface are located, are described. In the instrument, instead of absolute measurements, the light fluxes are compared in a differential photometer instead of absolute measurements in order to reduce methodological errors. The use of a differential photometer, which measures the spectral reflectance of water, can significantly increase the measurement accuracy, because does not require absolute calibration of optical channels. The use of a multichannel photomultiplier leads to an additional decrease in methodological errors by reducing the time of measurements of the seawater spectral reflectance and makes it possible to select a sufficient number of spectra in a large volume of data without interference from waves and lighting conditions. The photomultiplier signals are digitized using a compact USB ADC/DAC module with digital input/output functions. Amplified signals from photocathodes are received at the module input, the module output is connected to the USB input of a computer recording signals and controlling the spectrophotometer work.