the invention relates to methods and apparatus for detecting properties of suspended particles . embodiments disclosed include an optical instrument for detecting properties of a sample , comprising : a sample cell for holding a sample of a particulate dispersion ; a coherent light source configured to illuminate the sample in the sample cell ; a light intensity detector positioned to receive and measure an intensity of light from the coherent light source elastically scattered by the sample in the sample cell ; and a spectral light detector positioned and configured to receive and measure a range of wavelengths of light from the coherent radiation source inelastically scattered by the sample in the sample cell .