Laser Diffraction – Unlimited?

Within the past 20 years, particle size analysis with laser diffraction (LD) has been subject to rapid development, extending the size range stepwise from 1-200 μm to about 0.1 3500 μm. The limits of LD are discussed in terms of light sources, the influence of the beam diameter, special Fourier optics and a new detector design. It is shown that the size range is not only restricted by the wavelength of the laser and the transmission limits of the medium. Its extension is mainly related to improvements in the measurement of the angular intensity distribution. Influences from stability and flow dominate on the coarse side of the measuring range. On the fine side, the spatial extension of aerosols and the resulting demand for extended working distances can be covered only in a parallel laser beam. Extended Fourier optics in combination with an adaptable beam expansion technique and a detector with virtual borders between semicircular elements overcome the existing limits and extend the size range to a lower limit of about 0.05 μm and an upper limit above 10 mm. The sensitivity limit of LD is approaching that of single particle counting techniques. For medical spray and inhaler applications, a 0.1% optical concentration can be converted to particle size distributions even for time-resolved analyses with sample intervals of a few milliseconds. The reproducibility of the sensor, with a standard deviation typically much less than 0.5%, is no longer the limiting factor. The reproducibility of the results is mainly dominated by the reproducibility of sampling, sample splitting, dispersion and the contamination of the optical path. The latter can be improved by the control of flow, especially for in-line and inhaler applications.