Evaluating a novel liquid-borne particle counter based on interferometry. Comparing detection of nearly index-matched particles

Abstract

Data from the earlier articles in this three-part series show that, in many practical instances, an interferometric counter can detect significantly more particles per liquid volume than can two different dark-field countries, despite the verifiably superior sensitivity (to polystyrene latex spheres) and higher inspected flow rate of the dark-field instruments. In this, the final installment, we show that these increased counts could be due to the previously undetected presence of particles with a refractive index very close to that of the ambient fluid. The interferometric sensor's sensitivity to such particles is superior to that of the dark-field sensors because the interferometer uses phase-measuring techniques for forward-scattered (bright-field) light, whereas the dark-field counters detect light in nonforward directions. We also conclude that optimizing counter sensitivity to PSL spheres does not necessarily optimize performance when measuring particles of unknown optical properties, and that complementary use of both types of sensors can help differentiate particle contaminants in process liquids.