Abstract: Optical apparatus for measuring the amount of light reflected and/or scattered by particles suspended in a fluid, such as scrubber carryover appearing as a spray of dispersed water droplets. The amount of light reflected and/or scattered is an indication of the volume and mass of the suspended particles. A beam of light is caused to pass diagonally across a stream of purging gas used to keep the optical components clean and is directed onto the suspended particles at a point outside the purging gas stream. The light reflected back through the purging stream is then measured in intensity to determine the total volume of the particles from which mass may be determined. The invention overcomes the disadvantages of prior art devices in which the purging gas can disrupt the environment in which particle density is to be measured.