A laser-light-scattering method that includes cross-correlation processing of photodetector output signals has been devised for use in measuring the Brownian motions, and thereby indirectly the sizes, of particles suspended in liquids. Older laser-light-scattering methods for determining particle sizes from cross- or autocorrelation of photodetector outputs entail various deficiencies and difficulties, which include the need for precise alignment of two lasers of different wavelengths in a cross-correlation method, inability to obtain useful data at concentrations greater than 0.5 percent in one autocorrelation method, restriction to measurements at shallow depths in another autocorrelation method, and restricted angular range. The present method requires only one laser, can be used at various depths (for example in general liquid vessels or eye lenses), and yields useful data at concentrations >0.5 percent. This method can be used to determine typical particle sizes from 30 Å to 3 µm.

Singly scattered photons are needed for determining particle motions and sizes. As the concentration of particles in solution increases, the proportion of multiply-scattered photons also increases, altering photodetector outputs significantly. In autocorrelation processing, there is no way to distinguish between singly- and multiply-scattered photons arriving at the photodetector; therefore, as concentration increases, interpretation of autocorrelation signals becomes increasingly problematic. In the present method, cross-correlation is used to discriminate against photons multiply scattered from a single illuminating laser beam.

Scattered Laser Light enters two adjacent optical fibers attached to photodetectors. The separation between the receiving tips of the fibers is made smaller than single-scattering speckle but larger than multiple-scattering speckle, so that the cross-correlation (as a function of differential time) of the fluctuations in the outputs of the two photodetectors suppresses contributions by multiple scattering of photons.

The figure depicts a simplified version of the optical configuration of this method. A laser beam is aimed through a vertically oriented cylindrical sample cell containing the particles of interest suspended in a liquid. Optionally, the cell can be placed in a vat of another liquid, the index of refraction of which approximates that of the liquid in the cell. The laser beam is focused to a waist at the middle of the cell or at any other desired depth within the cell. Two optical fibers are positioned with their receiving tips adjacent and aimed toward the beam waist (the nominal scattering volume) to receive light scattered horizontally from the beam axis to a single chosen angle. The receiving tip of one fiber is placed a short distance above the other and the two are located above or below the scattering plane.

Only a narrow waist beam generates the tall speckles which span both fibers. The width of the beam waist (typically ≈80 µm), the separation between the fiber tips (typically ≈250 µm), the distance of the fiber tips from the scattering volume (typically ≈170 mm, as determined by the focus of the cylindrical cell), and the laser wavelength (typically ≈0.5 µm) are chosen so that the relative sizes of time-dependent speckle arising from single and multiple scattering can be used to discriminate against signals from multiple scattering. The proper choice of dimensions is one for which (1) speckle from single scattering of photons from the same scattering volume is large enough to encompass both fiber tips while (2) speckle from multiple scattering, which results from a beam of larger diameter, is significantly smaller than the distance between fiber tips, so that (3) the single-scattering components of the outputs of photodetectors fed by the two fibers are correlated with each other, while (4) the multiple-scattering components of these outputs are not correlated with each other. Thus, cross-correlation of the two photodetector outputs is nearly equivalent to a single-photodetector autocorrelation, and can be inverted to obtain the desired particle-motion and particle-size data.

This work was done by William V. Meyer and Padetha Tin of Ohio Aerospace Institute; David S. Cannell of the University of California, Santa Barbara; James A. Lock and Thomas W. Taylor of the Cleveland State University; and Anthony E. Smart for Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4-8
21000 Brookpark Road
Cleveland
Ohio 44135

Refer to LEW-16517


Photonics Tech Briefs Magazine

This article first appeared in the November, 1999 issue of Photonics Tech Briefs Magazine.

Read more articles from the archives here.