K d is the diffuse (or downwelling) coefficient of light attenuation (K d), also called simply light attenuation, the v ertical extinction coefficient, or the extinction coefficient. K d describes the rate of decrease of light with depth in water, in units of per meter (m). The “d” stands for downwelling light, which is light coming from above the sensor in a half-hemispherical shape (aka half of a basketball). Scientists sometimes use K d to describe the decrease in the total visible light available for plants in terms of photosynthetically active radiation (PAR) – called “K d (PAR).” In other cases, Kd can describe the decrease in light with depth over a spectrum of colors or wavelengths, usually written as “K d (λ).” At one color (one wavelength) Kd can describe the decrease in light with depth of one color, such as the decrease in blue light at the wavelength 490 nm, written as “K d (490).”