Transparency of water relates to the depth that light will penetrate water. The transmission of light into a body of water is extremely important since the sun is the primary source of energy for all biological phenomena.
Light is necessary for photosynthesis, a process that produces oxygen.
As light penetrates water, it becomes attenuated and altered in its spectral composition. The Secchi disk is a very simple device about 20 cm in diameter made of metal or plastic. It is based on the work of Father Pietro Angelo Secchi of Rome, Italy who devised a method for studying the transparency of the waters of the Mediterranean Sea in 1865. Secchi was a science advisor to the Pope and head of the Roman Observatory. The Secchi disk provides a means for determining the limit of visibility that is based on contrast.