The visibility depth or the Secchi depth means that depth from the surface of the sea, where a white disk with a diameter of 30 cm barely disappears from the viewer.
This white disk with a 30 cm diameter is called a Secchi disk. The disk is cheap and simple to make and operate. Individual transparency observations give only restricted data, but by averaging broad observation series the observations can be used both for scientific research and for monitoring the state of the sea.
The term was introduced to optical oceanography in 1865, when the Pope’s scientific assistant, father Pietro Angelo Secchi, made systematic observations about the transparency of seawater in the Mediterranean Sea. It is claimed that he got the idea for the measuring instrument from a porcelain plate that fell into the sea but was still visible from the depth of tens of meters.
The Secchi depth defines quickly and fairly accurate the depth of the lower boundary of the productive layer. The productive layer is the layer from surface towards bottom, where more than 1 % of sunlight of the surface penetrates.