Classical, high angle light scattering works only for scattering angles higher than some degrees. Small angle light scattering (SALS) can measure light scattered by wavelengths of some microns; its main problem is the stray light. The new techniques allow an accurate subtraction of the stray light; in particular, for ENFS and SNFS stray light can be evaluated and subtracted point by point.
All SALS instruments must include some device to stop the transmitted beam, like in ONFS. Moreover, the solid state sensors must be accurately positioned with reference to the focus of transmitted beam. In an industrial instrument, the position of both the beam stop and the sensors must be electronically controlled, in order to correct the deformations due to mechanical stress and temperature dilatations. On the contrary, ENFS has an extremely simple and robust structure, which does not require any adjustment. This makes ENFS suited for industrial applications of light scattering measurement, like particle sizing. In Chapter 8, we will show some particle size measurements performed with ENFS.
In Chapter 9, we will show that SNFS can be used to make a measurement of the power spectrum of the non equilibrium fluctuations in a free diffusion process. Such fluctuations have never been observed by SALS, since they are extremely weak and involve mainly low wavevectors. This shows that NFS techniques can be used to obtain measurements not possible with SALS.