The liquid samples we measure are held in a cell; the diameter
must be selected following Eq. (4.1)
for ONFS and ENFS, or Eq. (4.2) for
SNFS.
For homogeneous samples, like colloids, the thickness must be selected in order to have a suitable attenuation of the main beam, about 1%. For ONFS measurements, the thickness of the cell and the volumetric particle density must fulfill Eq. (3.63); generally this condition is spontaneously met.
For ONFS and ENFS measurements, the parallelism between the windows of the cell is not critical, nor the optical quality of them. Since the measured scattered light comes from different regions of the sample, we must provide that it is homogeneous. This implies that the thickness must be uniform, but an optical quality allignment is far beyond what is needed. On the contrary, SNFS requires optical quality windows: the well known ``Foucault test'' sees every deformation of the wavefront, no matter if the associated wavelength is long.
The cells we used are described in detail in Chapters 7 and 9.