At high turbidity levels, straightforward measurement of scattered light intensity without color compensation first displays reduced sensitivity. After a maximum reading has been reached, the reading will decline even though the concentration is rising (curve a). This is caused by the phenomenon of multiple scatter: the particles closer to the light source tend to screen off the incident light, and the scattered light is unable to propagate freely.
But if two beams pass through the medium, both will be attenuated by multiple scatter and there will not be any reversal of the characteristic curve (b). Thus the curve remains unambiguous and no low readings are shown for high concentrations.