Small Angle Light Scattering


Small Angle Light Scattering (SALS) is a light scattering method  that bridges the gap between standard light scattering techniques such as Dynamic or Static Light Scattering (DLS and SLS) and microscopy. One traditional SALS class of instruments, often referred to as laser diffraction analyzers, specializes in sizing particles in the Mie and Fraunhofer regime. 

Others use a simple SALS device as a rheology add-on to analyze the formation of shear-induced structures during rheological measurements. The SALS technique however offers much more than this, both for static and dynamic light scattering, if one exploits the huge improvements made in laser and digital camera technology over the last 10 years. LS Instruments has developed such a state-of-the-art implementation that employs both dynamic  and static light scattering in the SALS range.


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Limits of DLS and SLS

In the last thirty years DLS and SLS have been widely used to study colloidal, polymer and protein systems in solvents. The technology is based on illuminating a sample with a laser beam and making time- and angle-resolved measurements of the scattered light with a phototube, or more recently, photodiode. Each scattering angle at which detection is performed increases the amount of information gained from the sample. Such a measurement is typically performed by either rotating one detector on a goniometer arm around the sample or fixing several detectors at different scattering angles around the sample. This multi-angle measurement yields information about the size, the structure, the dynamics and even molecular weight of the sample.


One of the main limitations of this analytical technique resides in the difficulty of measuring at angles less than approximately 15° for which small alignment errors are magnified. Due to the fact that light scattering measurements performed at low angles extract information about longer time and length scales, accurate information about slow dynamics and sizes falling in the upper end of the colloidal domain, cannot be obtained with regular DLS and SLS.


SALS Technology 

In the last 15 years several studies published in scientific literature have illustrated a new implementation that overcomes these limitations and that allows measurement at angles down to fractions of a degree, thus the name Ultra Small Angle Light Scattering (USALS) is often used. The most relevant change with respect to classical scattering techniques lies in the adoption of a camera based sensor. Its inherent feature consisting of a two dimensional array of light sensors indeed allows one to overcome the aforementioned alignment problems. Furthermore, in this implementation one measures at small angles which results in both large measured light intensities and slow dynamics.

The increased intensities compensate for the smaller sensitivity of CMOS and/or CCD camera sensors. Additionally, the significantly slower light fluctuations permit the implementation of a software based correlator. Also, the fact that more than one sensor (in this case a pixel) corresponds to a certain scattering angle dramatically improves the speed of acquisition especially for non-ergodic samples.


USALS, ultra small angle light scattering. The optical configuration with two lenses and one beamstop is frequently used in research.

 Fig. 1: Schematic of USALS design


The basic setup which was first illustrated by Ferri [1] is shown in Fig. 1. A collimated laser beam impinges on the sample to be measured. The scattered light and the un-scattered beam then pass through a first lens. At its focal plane a small mirror removes the directly transmitted beam.

The scattered light then passes through a second lens that images each scattering angle to one pixel, or to be more precise to all those pixels belonging to the annulus centered on the virtual laser beam path. This unique feature results in the advantage that the pixel size matches the speckle size, permitting a maximization of both the DLS signal-to-noise ratio and the SLS angular resolution.


One of the main problems of such a setup arises from stray light inevitably present at small angles. Such light mixes with scattered light and gives rise to a heterodyne mixing which invalidates the DLS data analysis procedure used to determine the relevant quantities we want to measure.

In [2] it was shown how such stray light optical mixing could be modeled and that such a model, coupled with a suitable blank measurement, could be successfully used to factor out the heterodyne contribution and allow for the usual DLS data treatment. LSI has implemented this solution in the UltraLab.