Particle Sizing related Static, Dynamic and Electrophoretic LLS Instrumental Experiments

Copyright © 1999, 2002, 2007 Kees Krijnen.


Laser light scattering has provided an important method for particle sizing analysis for at least four decades [REF2]. Classical - static - light scattering (SLS) studies are concerned with the measurement of the intensity of scattered light by particles as a function of the scattered angle. In addition to this kind of study, spectral information can be obtained from the intensity fluctuations of scattered light - dynamic light scattering (DLS). Various forms of the latter type of experiment are known as light beating spectroscopy (LBS), intensity fluctuation spectroscopy (IFS), and photon correlation spectroscopy (PCS). PCS related experiments in laser doppler velocimetry (LDV) permit rates of uniform motion of particles to be measured. A special case of LDV is electrophoretic light scattering (ELS) where very low mobilities are determined.

Today, applications of Digital Signal Processing (DSP) are numerous and still increasing rapidly every day. Typical DSP applications are CD music recording, modems, cellular phones, car ignition, electronic medical instruments, radar systems, etc. All these rely to some extent on digital signal processing [REF1]. DSP is based on representing signals by numbers in a computer (or in specialized digital hardware), and performing various numerical operations on these signals. The history of applied digital signal processing (at least in the electrical engineering world) began around the mid-1960 with the invention of the fast Fourier transform (FFT). However, its rapid development started with the advent of microprocessors in the 1970s. Early DSP systems were designed mainly to replace existing analog circuits, and did little more than mimicking the operation of analog signal processing systems. It was gradually realized that DSP has the potential for performing tasks impractical or even inconceivable to perform by analog means. Today, digital signal processing is a clear winner over analog processing.

Digital correlators have dominated dynamic light scattering experiments since their introduction in 1969. The analog signal from a light detector - generally a photo multiplier tube (PMT) - is digitized and processed further by digital hardware to extract the frequency shift signal from the noise. DSP microprocessors are generally too slow to process the digitized signal as it is produced for digital correlators, which is a digital pulse train of usually several MHz. However, DSP microprocessors can directly digitize the analog signal from a light detector and process it as a spectrum analyser or an analog correlator, or both simultaneously. Actually, before digital correlators became available these were the only techniques scientist could rely on to analyse dynamic light scattering by particles.

Digital correlators are almost entirely built from digital electronics and hardware - today even realized into one VSLI (very large-scale integration) chip. DSP microprocessors and systems are considered as digital hardware. However, their function and performance is totally based on embedded software engineering.

The objective of this web site is to give an instrumental overview - a compilation - of the application of DSP techniques in static and dynamic light scattering experiments. The theory of static, dynamic and electrophoretic laser light scattering is briefly reviewed. The experimental part of this web page starts with electrophoretic light scattering, a special case of dynamic light scattering, by means of spectrum analysis and `analog' autocorrelation. The signal - frequency shift - to be measured is well in the low area of the spectrum, 0-500Hz. No exceptional high DSP speed is required to process and analyze such a signal real-time. The measurements are setup as a phase analysis light scattering (PALS) experiment. The measurement principle of PALS and ELS are exactly the same.