The depth at which a Secchi disc disappears from view as it is lowered in water Secchi disc a white disc 20-30 cm in diameter, used as a qualitative way of measuring water clarity. It is lowered from a vessel and viewed from above the surface in full solar illumination to estimate the light attenuation in the water column. This is done empirically by relating the depth at which the disc disappears to the attenuation of light.