Classification of optical particle sizing techniques
Optical particle characterization in flows

The notions of particle size and shape are to be evaluated in terms of the wavelength of the illumination beam and the measurement principle. For spherical particles of diameter D, the value of their size parameter α = πD/λ _e thus strongly conditions their light-scattering properties. With, for wavelengths visible in air, λ ₀ ~ 0.38 – 0.78 µm and those in the fluid medium of index m _e , λ _e = λ ₀ /m _e .

Thus, for $\alpha \ll 1$...