The Secchi disc is mounted on a pole or line and lowered slowly down into the water. The depth at which the disk is no longer visible (ie when the reflectance equals the intensity of light backscattered from the water) 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. This depth in metres divided into 1.7 yields an attenuation coefficient for the available light averaged over the Secchi disc depth.
Secchi disc 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.
Since its invention, the disc has also been used in a modified, smaller 20cm (8in) diameter, black and white design to measure freshwater transparency. Similar discs with a black-and-yellow pattern, are used as fiducial markers on vehicles in crash tests, crash-test dummies and other kinetic experiments.