Also called turbidity coefficient. Attenuation of the flux of optical radiation as it passes through an absorption-free sample.
The variable measured by this second method we call the scatter coefficient s. Like the absorption coefficient a, it is defined in terms of the exponential decrease of the light intensity over a path length z:
In other words, the numerical value of the decadic scatter coefficient corresponds to attenuation down to 1/10 of the incident light. The unit is m-1. The scatter coefficient is a material constant; at a given wavelength, it is always the same for a given suspension. In the case of simple scatter (as opposed to multiple scatter), the figure is proportional to the particle concentration. Generally, the lower the wavelength the lower the scatter coefficient.
Precondition for the application of this method, of course, is that no absorbent substances (color) are present in the medium that would convert the light energy at the wavelength of the incident beam into heat as a result of interaction with the atoms.
In actual practice this method is used for measuring high turbidities, because fine turbidity matter weakens the transmitted light too little and excessively long path lengths would be required. As in the case of scattered light intensity measurement, the instruments are calibrated with the standard suspension formazine. The concentration span utilized in practice ranges from 50 FTU to 20,000 FTU.
As in the case of scattered light intensity measurement, it must be borne in mind here that turbid matter other than formazine has not the same correlation between turbidity value and particle concentration, because the reading is affected by the particle size and the refractive index of the particles in relation to the medium.