Laser Diffraction 101
Sequoia realizes that for some people, the principle behind laser diffraction seems difficult to understand. The purpose of this article and ‘Laser Diffraction 201’ is to try to explain the principle in a manner that everybody can follow.
The first very important thing to realize is that laser diffraction takes advantage of the fact that a spherical particle of a known size, composition and color scatters light in a manner that is completely understood and can be computed. This is so important to realize that we will re-iterate here: The EXACT shape of the light scattering off of a spherical particle with any size, composition and color can be computed. The second very important thing to realize is that no two particles of different size or composition have the same scattering patterns – they are all unique. If you wish, you can think of the scattering patterns as fingerprints for a particle of a certain size.
The figure below illustrates this. It shows the scattering signature of 3 spherical particles with diameters 20.5, 20 and 7 µm. Note the pronounced difference between the scattering signatures of the 3 particles, even the 20 and 20.5 µm particles.
In the LISST instruments (and most other laser particle sizers), the measurement of light scattering is done by use of ring detectors. That is, each detector is a circular ring of some width. In the figure below, the scattering intensity is plotted against the detector ring number, where each ring essentially is to be thought of as a range of angles. In a separate article we will discuss the rings in more detail, but for the purpose of this article it is sufficient to realize that the 32 detector rings cover 32 ranges of angles over which the light scattering is measured. When particles of more than one size are present (which will almost always be the case), the scattering pattern that is detected by the 32 ring detectors on the LISST is the COMBINED SCATTERING from all the particles in suspension. This is illustrated in the figure below.
In the example with scattering from 4 different particle sizes, the LISST will detect the COMBINED SCATTERING pattern, which is simply the sum of the individual scattering patterns from all the particles present.
The combined scattering pattern detected by the LISST constitutes 32 known measurements. From principles of algebra, with 32 knowns, one can solve for 32 unknowns. THESE UNKNOWNS ARE THE CONCENTRATIONS OF PARTICLES IN 32 SIZE CLASSES. The process of solving for the 32 concentrations is called a mathematical inversion, or simply, INVERSION. The 32 concentrations combine to form a vector, which together is called the particle size distribution, PSD. This process is merely an extension of simple algebra familiar to all from pre-college. Crucially, the inversion does not require any assumptions of the underlying PSD. The next article in this series will elaborate in more detail on this, and start filling in with some math.