Secchi disk
The Secchi disk (or Secchi disc), as created in 1865 by Angelo Secchi, is a plain white, circular disk 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 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. The original Secchi disk was a plain white disk and was used in the Mediterranean Sea. Today, a plain white, 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 now used in limnology (freshwater) investigations. The Secchi depth is reached when the reflectance equals the intensity of light backscattered from the water. 1.7 divided into this depth in metres yields an attenuation coefficient (also called an extinction coefficient), for the available light averaged over the Secchi disk depth. While used as a variable, the extinction coefficient is also used as a variable for turbidity. The light attenuation coefficient, k, can then be used in a form of the Beer–Lambert law, to estimate Iz, the intensity of light at depth z from I0, the intensity of light at the ocean surface. The Secchi disk readings do not provide an exact measure of transparency, as there can be errors because of the sun's glare on the water, or one person may see the disk at one depth, but another person with better eyesight may see it at a greater depth.
