Device for determining water transparency. Diameter of the disc - 200-300 mm, painted with white paint. It is released into the water on a cord with marks every 10 cm. Transparency is defined as the depth at which the disc disappears from view. This depth, expressed in meters, is taken as a measure of water transparency. Named after A. Secchi, who measured the transparency of seawater with this method in 1865.

Secchi disk for measuring transparency was first used by Captain Cialdi (Italian Alessandro Cialdi), commander of the Papal Navy on April 20, 1865. The first regular measurements of the transparency of seawater were made in the Pacific Ocean during the Russian circumnavigation expedition on the brig "Rurik" (1815-1818 years) under the command of Lieutenant O.E. Kotzebu by lowering the white saucer on a rope to the depths. At present Secchi disk is used in routine measurements, despite the fact that there are and widely used also electronic devices for measuring water transparency (transmissometers).