Editing Secchi disk (section)

{{Short description|Tool used to measure water transparency/turbidity}}
[[File:Floating Secchi disk.jpg|thumb|A Secchi disk being lowered into the water of a lake]]
[[File:Secchi disk pattern.svg|thumb|upright|The modified Secchi disk design used in fresh water]]
The '''Secchi disk''' (or '''Secchi disc'''), as created in 1865 by [[Angelo Secchi]], is a plain white, circular disk {{convert|30|cm|in|0|abbr=on}} in diameter used to measure water [[transparency (optics)|transparency]] or [[turbidity]] in bodies of water. The disc is mounted on a pole or line and lowered slowly down in the water. The depth at which the disk is no longer visible is taken as a measure of the transparency of the water. This measure is known as the '''Secchi depth''' and is related to water [[turbidity]]. Since its invention, the disk has also been used in a modified, smaller {{convert|20|cm|in|0|abbr=on}} diameter, black-and-white design to measure freshwater transparency.

Similar disks, with a black-and-yellow pattern, are used as [[fiducial marker]]s on vehicles in [[crash test]]s, [[crash test dummy|crash-test dummies]], and other kinetic experiments.