A system and a method for monitoring fouling in a fluid stream are provided. A light source generates a light path directed to two distinct portions of a fluid stream, one of which includes a translucent conduit. The conduit experiences fouling on the walls thereof. The light directed towards the conduit is absorbed or scattered to a varying degree as a function of foulant thickness and composition and the wavelength of the light employed. A comparison of foulant contained in the fluid itself may be compared to the foulant in the conduit, and a signal representative thereof can be generated. As a result, dosage of agents added to the fluid stream to control the foulant in the system may be controlled.