Secchi

Secchi is most often associated with the Secchi disk, a simple instrument used to measure water transparency. The device is named after Pietro Angelo Secchi, an Italian scientist whose work in the 19th century helped advance observational methods in water and light studies.

Pietro Angelo Secchi (1818–1878) was an Italian Jesuit priest and astronomer who contributed to the development of astronomical spectroscopy and the classification of stars. His broader scientific work helped establish systematic approaches to observing celestial and terrestrial phenomena, and the Secchi name has since become linked with a practical method for assessing transparency in water.

The Secchi disk is a circular disk, typically painted with alternating black and white quadrants, attached to a rope or line. It is lowered into water until it disappears from view; the depth at which the disk vanishes is called the Secchi depth and serves as a proxy for water clarity. In some procedures, the depth at which the disk reappears when the rope is pulled back up is also recorded to refine measurements.

The Secchi depth is influenced by lighting conditions, observer eyesight, surface conditions, bottom type, and the concentration of suspended particles or phytoplankton. It provides a simple, low-cost index of transparency that can be tracked over time and across locations, making it a widely used tool in limnology and coastal oceanography.

Today, the Secchi disk remains a foundational instrument for rapid field assessments of water quality, often used alongside modern turbidity sensors and spectrophotometric methods to obtain a fuller picture of aquatic light environments.