June 2022 – TD 320 OPERATING MANUAL – TURBIDITY SENSOR 4296
Page 91
8.1.1
Light Scatter
The larger a particle is, the more light that will be scattered forward.
Light is scattered and absorbed by particles in the water. Even clear water will have minute light
scatter due to the presence of dissolved particles and molecules. This scatter can be in any
direction, and the intensity in any given direction is dependent on both the light source and the
size of the particle(s). When a particle is much smaller than the beam of light, the scattering is
fairly symmetrical in all directions. The larger the particle becomes, however, the more light that
will be scattered forward (away from the light beam).
Wavelength also affects light scatter. In general, shorter wavelengths will scatter more than longer
wavelengths for the same particle size. Shorter wavelengths are also more susceptible to
absorption by colored molecules in the sample. Near-infrared light is rarely absorbed, so dissolved
colored material will not affect it. This is one of the reasons that turbidity measurements made
through different methods are often not comparable – if the light source is different, the light
scatter will be different. While this is accounted for in the calculations, equivalent results are
guaranteed.
