Secchi disk

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.

[1][2] 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...".

[5][6] The Secchi depth is reached when the reflectance equals the intensity of light backscattered from the water.

Submarine photometers can operate at depths of 150 m (492 ft) and can record visible, ultraviolet, and infrared parts of the spectrum.

[10] Secchi disk measurements have been an integral component of Minnesota's and Wisconsin's lake water quality assessment programs for some time; lake residents make periodic measurements and submit their readings to state and local agencies.

[13] The Secchi Disk study was begun in response to a controversial scientific report that suggested the phytoplankton that influences water transparency had declined by 40% in the oceans between the years 1950 and 2008.