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The Use of Index-Matched Beads in Optical Particle Counters



Dean C. Ripple


Although multiple methods are available for counting and sizing of protein particles over a broad range of sizes, differences in physical properties between common polystyrene-bead reference materials and the actual protein particles of interest result in substantial discrepancies between measured diameters reported by different instruments. In this paper, we explore the use of both silica and poly(methylmethacrylate) bead standards, for bead diameters ranging from 6 µm to 70 µm, in a matrix liquid of approximately matching refractive index. Instrument readings using both flow imaging and light obscuration particle counters illustrate the changes in instrument response that result from measurement of particles of low optical contrast. Both flow imaging and light obscuration gave a reduction in measured particle diameter as the refractive index difference between particle and fluid approached zero, although the reduction in diameter was greater for light obscuration. These shifts in diameter result in errors in reported particle concentration.
Journal of Research (NIST JRES) - 119.029
Report Number


Particles, flow imaging, flow microscopy, light obscuration


Ripple, D. (2015), The Use of Index-Matched Beads in Optical Particle Counters, Journal of Research (NIST JRES), National Institute of Standards and Technology, Gaithersburg, MD, [online], (Accessed June 20, 2024)


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Created January 8, 2015, Updated November 10, 2018