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The Secchi disk (or Secchi disc), was created in 1865 by Angelo Secchi, is a plain white, circular disk 30 cm in diameter. It 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 diameter, black and white design to measure freshwater transparency.

History
The Secchi disk is the oldest "optical instrument" used to measure water transparency of lakes and oceans. Using visual observations to determine transparency of water first arose when Captain Bérard saw a white dish at the bottom of 40 meters on his ship passage from Wallis Island to the Mulgraves. Following this incident, in 1865 Commander Cialdi had read Bérard's account of his observation in his writings and this inspired him to make the first renditions of the Secchi disk to study the transparency of the sea, visibility at the bottom, and wave movement. These disks were made several times by Cialdi, each with different diameters and colors in order to repeat the observation Bérard had made. Cialdi took these studies to Angelo Secchi, and together they created a scientific program aboard the SS L'Immacolata Concezione. What resulted from these experiments was a report written by both Cialdi and Angelo Secchi, and within were Secchi's experiments that explained how size and color of the disk, surface refraction of the sun, cloud cover, and color of the water all affected the depth of visibility. Secchi's work would go on to establish the fundamental experimental procedure for studying water transparency with the Secchi disk The original Secchi disk was a plain white disk and was used in the Mediterranean Sea. Today, a plain white, 30 cm 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.

Secchi depth
The Secchi depth is reached when the reflectance equals the intensity of light backscattered from the water. This depth in meters divided into 1.7 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 just below the water surface.


 * $${I_z \over I_0}= e^{-kz}$$

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. However, it is an inexpensive and straightforward method of measuring water clarity. Because of the potential for variation between users, methods should be standardized as much as possible.

A Secchi disk measurement should always be taken off the shady side of a boat or dock between 9:00 and 15:00. The period for best results is between 10:00 and 14:00. The same observer should take Secchi depth measurements in the same manner every time. One can approach the measurement by lowering the disk beyond a point of disappearance, then raising it and lowering it slightly to set the Secchi depth. Another method is to record the depth at which the disk disappears, lower another few feet, then record the depth at which the disk reappears as it is slowly brought up. The Secchi depth is taken as the average of the two values.

Water transparency is affected by several substances in water that attenuate the light. These are water molecules, particles, and dissolved substances which all absorb and scatter light. Even in pure, clear water light declines exponentially with depth due to the water molecules. Particles in water, such as phytoplankton and silt, are called total suspended solids (TSS) and they cause turbidity. Dissolved organic substances, also known as colored dissolved organic matter (CDOM), color the water much like a tea bag would in a cup of water. These substances influence the color of water bodies, causing the Secchi depths to range from 1cm to 40m in different lakes. The light attenuation coefficient, k, is the equivalent of summarizing kw, light attenuation due to water molecules, kp, light attenuation due to suspended particles, and kc, light attenuation due to color dissolved substances.

k = kw + kp + kc

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. The aggregated longitudinal data are used to reveal general trends in water quality. Similarly, the Indiana Clean Lakes Program trains and relies on volunteers to monitor turbidity in over 80 Indiana lakes using Secchi disks, and uses data submitted by volunteers to monitor lake quality in the state. Another program is the Citizen Lake Awareness and Monitoring (CLAM), which is sponsored by the Water Management Association of Ohio, and it provides an opportunity for Ohio citizens to an active role in watershed management, stream water quality, and aquatic ecology. Each of these programs spread awareness of the importance of our water ecosystems' quality and quantity.

In 2013, a team of marine scientists established the global citizen science Secchi Disk program for seafarers to study the marine phytoplankton. This ongoing citizen science Secchi Disk study combines the traditional plain white, 30 cm diameter marine Secchi Disk with mobile technology to upload Secchi depth data collected from the sea to a central database. The study's first scientific results were published in 2017. The Secchi Disk study was started in response to a controversial scientific report that suggested the phytoplankton that influence water transparency had declined by 40% in the oceans between the years 1950 and 2008.

Secchi disk measurements do not indicate how attenuation changes with depth or particular wavelengths of light. Submarine photometers can operate at depths of 150 m and can record visible, ultraviolet and infrared parts of the spectrum. Turbidimeters have their own light source and can measure transparency with scientific accuracy.