Sub-micron Particle Sizing

submicronSponsors: National Science Foundation

Project Overview: 

There is a great need in many scientific applications to be able to accurately size sub-micron particles. As the particle size decreases to be smaller than the wavelength of light it becomes difficult to obtain images using standard microscopy that can be used to estimate size. This is a classic problem in microscopy and has been studied in a wide range of application areas. To date however, there have been few systems that can size single particles (most typically give size distributions for bulk samples) without requiring knowledge of other system parameters such as refractive index or fluid viscosity. In some applications such as in situ imaging, these parameters are not known a head of time and therefore a new method for sub-micron sizing is needed. Towards this end, the project explores the use of oblique illumination and measurement of the forward and backward scattering from particles in the range of 0.1 to 1 micron.

General Description:

The system is designed around a pair of very high numerical aperture lenses. One lens directs and focuses a blue laser beam to impinge on a very small point in a thin cuvette. As light scatters off particles at that point, the same lens collects a large fraction of the backscattered hemisphere and focuses it on to one CCD sensor. At the same time, a second high NA lens on the opposite side of the sample collects the forward scatter and focuses it on to a second CCD. Each particle scattering event gives rise to two images a forward scatter image and a backward scatter image. These can then be used to estimate the size of the particle using the ratio of forward to backward scatter or estimate the angular scattering function of the particle.


  • Principal Investigator: Jules S. Jaffe
  • Principal Engineer: Liao, Ran
  • Engineering Consultant: Paul L. D. Roberts
  • Graduate Student: Eric Orenstein