Editing Secchi disk (section)

==Secchi depth==
[[File:Secchi disks.svg|thumb|right|Different kinds of Secchi disks. A marine style on the left and the freshwater version on the right]]
The Secchi depth is reached when the reflectance equals the intensity of light backscattered from the water. 1.7 divided by 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]],
<math display=block>{I_z \over I_0}= e^{-kz},</math>
to estimate ''I''<sub>''z''</sub>, the intensity of light at depth ''z'' from ''I''<sub>0</sub>, the intensity of light at the ocean surface.<ref>{{cite journal |url=https://www.jstor.org/pss/2402029 |title=On the Universality of the Poole and Atkins Secchi Disk: Light Extinction Equation |journal=British Ecological Society |author1=[[Sherwood B. Idso|Idso, Sherwood B.]] |author2=Gilbert, R. Gene |year=1974|volume=11 |issue=1 |pages=399–401 |doi=10.2307/2402029 |jstor=2402029 |bibcode=1974JApEc..11..399I |url-access=subscription }}</ref>

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.<ref>Lind, Owen, T. (1979). ''Handbook of Common Methods in Limnology'' St. Louis: C.V. Mosby Co.</ref> 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 it 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.<ref>Cole, Gerald A. (1994). Textbook of Limnology. 4th ed. Prospect Heights: Waveland Press Inc.</ref>

Secchi disk measurements do not indicate how attenuation changes with depth or particular wavelengths of light. Submarine [[photometer]]s can operate at depths of {{convert|150|m|ft|0|abbr=on}} and can record visible, ultraviolet, and infrared parts of the spectrum. [[Nephelometer|Turbidimeter]]s and [[Transmissometer]]s have their own light sources and can measure transparency with scientific accuracy.<ref>{{cite encyclopedia |url=https://www.britannica.com/technology/undersea-exploration/Acoustic-and-satellite-sensing#ref57193 |title=Undersea exploration |year=2008 |encyclopedia=Encyclopædia Britannica |access-date=1 October 2008|via=Encyclopædia Britannica Online}}</ref>