The Secchi disk, created in 1865 by Pietro Angelo Secchi SJ, is a circular disk used to measure water transparency in oceans and lakes. The disc is mounted on a pole or line, and lowered slowly down in the water. The depth at which the pattern on 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.

The Secchi depth is reached when the reflectance equals the intensity of light backscattered from the water. This depth in metres divided into 1.7 yields an attenuation coefficient (also called an extinction coefficient), for the available light averaged over the Secchi disk depth. The light attenuation coefficient, k, can then be used in a form of the Beer-Lambert law,

to estimate I_z, the intensity of light at depth z from I₀, the intensity of light at the ocean surface.

Secchi disk readings do not provide an exact measure of transparency, as there can be errors due to the sun's glare on the water, or one person may see the disk at one depth, but another, with better eyesight, may see it at a greater depth. However a Secchi disk is an inexpensive and straightforward method of measuring water clarity. Because of the potential for variation between practitioners, methods should be standardized as much as possible.