Secchi disk
scientific instrument for measuring turbidity
The Secchi disk (or Secchi disc), as created in 1865 by Angelo Secchi, is a plain white, circular disk 30 cm (12 in) in diameter used to measure water 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 20 cm (8 in) diameter, black-and-white design to measure freshwater transparency.
Similar disks, with a black-and-yellow pattern, are used as fiducial markers on vehicles in crash tests, crash-test dummies, and other kinetic experiments.
History
The original Secchi disk was a plain white disk and was used in the Mediterranean Sea. A plain white, 30 cm (12 in) diameter Secchi disk remains the standard design used in marine studies. In 1899, George C. Whipple modified the original all-white Secchi disk to "...a disc about 8 inches in diameter, divided into quadrants painted alternately black and white like the target of a level-rod...". This modified black-and-white Secchi disk is the standard disk used in limnology (freshwater) investigations.
The public source identifies “Secchi disk” as scientific instrument for measuring turbidity. This brief keeps that definition visible, then builds a research path around Secchi, disk and scientific.
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Stable identifiers, scientific names and standards terminology offer the best bridge between this overview and specialist evidence. The source revision retrieved here is dated Aug 29, 2026. The linked authority identifier is Q575941. None of the 0 selected statements returned an explicit reference. The first chronological checks are 1865 and 1899.
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This entry incorporates text from “Secchi disk” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.