Design considerations for particulate instrumentation employing laser light scattering

The design and development of a light scattering instrument that performs in situ and online particle density distribution measurements of polydisperse aerosols is investigated. It was found that off-axis systems are optimally designed for detection of drift particles in a fluctuating ensemble of background particles and that in-line systems operating near the forward lobe collect more radiation with a correspondingly smaller minimum detectable particle. Prototype systems were constructed and measured response characteristics showed good agreement with the Mic theory. Problems encountered with the large, external scattering volume were evident in the form of non-ideal response functions because of inhomogeneities in the spatial intensity distribution of the laser source and variable collection of scattered light. These difficulties were solved by introduction of a light integrating optical system and by proper design of receiver optics to minimize collection or solid angle effects. It was shown that typical particle size distributions can be recovered by appropriate mathematical techniques, even for instruments exhibiting non-ideal response characteristics.