Evaluation of spherical particle sizes with an asymmetric illumination microscope

A polarized microscope system is used to perform goniometric measurements of light scattered by small particles. The light incident angle on a sample of monodispersed latex microspheres is increased sequentially and a microscope objective lens collects scattered light from the samples. Light is only collected at angles greater than the objective lens numerical aperture (NA) so that only light scattered by the spheres is collected. The experimental results were modeled with a Mie theory-based algorithm. Experiments conducted with microspheres of diameter 1.03, 2.03, and 6.4 mum show that, by decreasing the objective lens NA from NA = 0.55 to NA = 0.0548, a more distinguishable scattering pattern is detectable. From these highly shaped curves, we found that the size of a sphere of nominal diameter 2.03 mum was 2.11 +/- 0.06 mum and a 6.4 mum sphere was 6.34 +/- 0.07 mum.