The light attenuation coefficient – often shortened to “light attenuation” – describes the decrease in solar irradiance with depth. To calculate this coefficient, light energy is measured at a series of depths from the surface to the depth of 1% illumination. Then, the exponential decline in light is calculated using Beer’s Law with the equation:
where k is the light attenuation coefficient, I is the intensity of light at depth z, and I is the intensity of light at the ocean surface. 
Which translates to:
This measurement can be done for specific colors of light or more broadly for all visible light. The light attenuation coefficient of photosynthetically active radiation (PAR) refers to the decrease in all visible light (400-700 nm) with depth. Light attenuation can be measured as the decrease in downwelling light (Kd) or the decrease in scalar light (Ko) with depth.    Light attenuation is most useful as a measure of the total underwater light energy available to plants, such as phytoplankton and submerged aquatic vegetation.