A radical new technique is proposed for in-line process particle measurement at sizes from 0.1micron to 10micron and concentrations up to 50% by volume without interfering in the process. The method involves calculation of optical refractive index of the suspension from critical angle measurements at several wavelengths. This technique requires no light passage through the suspension so its turbidity does not restrict the method. In practice, the critical angle is not clearly marked for turbid suspensions but there is a substantial variation of reflectance in the vicinity of the critical angle.Measuring the suspension refractive index has significant advantages over other widely used non-invasive methods of particle analysis such as spectral extinction and laser diffraction: - The relationship between refractive index and volume fraction is linear for volumetric concentrations up to 50%.- Mie scattering theory can be used to calculate the refractive index of concentrated suspensions of spheres. - Suspension refractive index is sensitive to particle size and size distribution.