It is lowered into the water until the observer can no longer see it. This depth of disappearance is called the Secchi depth. The black serves as a constant background, thus standardizing the contrast. The Secchi depth measures the transparency of water. Transparency can be affected by the color of the water, algae and suspended sediments.
Transparency is an indicator of the impact of human activity on the land surrounding the lake.
Detecting trends in water quality is a primary goal for many lakes' monitoring programs. He was asked by commander Cialdi, head of the Papal Navy, to test a new transparency instrument. In in 1994 at Kent State, Ohio. The purpose is to demonstrate the potential of volunteers to gather environmentally important water quality information to provide a national perspective on water quality, and to increase the numbers and interest of volunteers in environmental monitoring. Measurements are taken during the first week of July.