A group of merchant marines and a Jesuit priest started a scientific revolution aboard the vessel L’Immacolata Concezione during a six week voyage in 1865. The priest, a scholar by the name of Angelo Secchi, with the able assistance of the crew of the ship, repeatedly lowered several weighted disks of different colors and sizes into the Mediterranean and made some fascinating revelations about the color and clarity of the sea water at different locations as a function of such variables as solar elevation and sea state (Secchi, 1866). The measurement of Secchi depth, as it has come to be known, (i.e. the depth, in meters, at which a calibrated white disk of specified diameter and reflectance is no longer visible when lowered and observed on the ship’s sun side) is still, nearly 125 years later, one of the most common measurements of hydrologic optics. Nonetheless, the method of its deployment obviates the very subjective nature of the data. To this end scientists have endeavored to develop evermore accurate optical techniques to address the needs of the many disciplines that are oceanography. The science of optical oceanography has developed over the last half century as an interdisciplinary field characterized by rapid changes in techniques and adaptation of new technologies.

Spinrad, R.W. Optical oceanography: The state of the science. 1988. Oceanography 1(1):22, https://doi.org/10.5670/oceanog.1988.36.

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