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,
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.
A Secchi disk measurement should always be taken off the shady side of a boat or dock between 9 a.m. and 3 p.m. (Lind, 1979). According to Cole (1994), the period for best results is between 10 am and 2 pm. 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 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.
Secchi disk measurements have been an integral component of Minnesota's lake water quality assessment programs for some time; lake residents make periodic measurements and submit their readings to state and local agencies. The aggregated longitudinal data are used to reveal general trends in water quality.
Secchi disk measurements give no indication how the attenuation changes with depth or specific wavelengths of light. A submarine photometer can be used to depths of about 150 metres and can directly record the infrared, visible, and ultraviolet portions of the spectrum. Scientifically accurate measurements of turbidity are performed using a turbidimeter. This instrument measures finer gradations of transparency, and has a self-contained light source which can be used at any time of night or day.